The reaction 48Ca + 248Cm → 296116* studied at the GSI-SHIP

Abstract

The synthesis of element 116 in fusion-evaporation reactions of a 48Ca beam with radioactive 248Cm targets was studied at the velocity filter SHIP of GSI in Darmstadt. At excitation energies of the compound nuclei of 40.9MeV, four decay chains were measured, which were assigned to the isotope 292116, and one chain, which was assigned to 293116. Measured cross-sections of (3.4
−1.6+2.7) pb and (0.9
−0.7+2.1), respectively, and decay data of the chains agree with data measured previously at the Flerov Laboratory of Nuclear Reactions in Dubna. As a new result, one α-decay chain was measured, which terminates after four α decays by spontaneous fission. The α energies of the second-to-fourth decay are considerably higher than those measured for the α decays of 289114, 285Cn, and 281Ds and the spontaneous fission half-life is significantly longer than that of 277Hs measured in previous experiments. A possible assignment is discussed in the frame of excited quasiparticle states of nuclei populated in the decay chain from 293116. Also other possible assignments were considered and are discussed. At an excitation energy of 45.0MeV no events were observed resulting in a one-event cross-section limit of 1.6 pb. The technical aspects related with the use of radioactive target material at SHIP are described in detail. The experience gained in this experiment will serve as a basis for future experiments aiming to study still heavier elements at the velocity filter SHIP.

Full text

DOI 10.1140/epja/i2012-12062-1
Regular Article – Experimental Physics
Eur. Phys. J. A (2012) 48:62 THE EUROPEAN
PHYSICAL JOURNAL A
The reaction 48Ca +248Cm →296116∗studied at the GSI-SHIP
S. Hofmann1,2,a,S.Heinz
1,R.Mann
1, J. Maurer
1,3, J. Khuyagbaatar1, D. Ackermann1, S. Antalic4, W. Barth1,
M. Block1,H.G.Burkhard
1, V.F. Comas1,L.Dahl
1,K.Eberhardt
5, J. Gostic6, R.A. Henderson6, J.A. Heredia1,
F.P. Heßberger1,3, J.M. Kenneally6, B. Kindler1, I. Kojouharov1, J.V. Kratz5,R.Lang
1,M.Leino
7, B. Lommel1,
K.J. Moody6,G.M¨unzenberg1,S.L.Nelson
6,K.Nishio
8, A.G. Popeko9, J. Runke5,S.Saro
4, D.A. Shaughnessy6,
M.A. Stoyer6,P.Th¨orle-Pospiech5, K. Tinschert1, N. Trautmann5, J. Uusitalo7, P.A. Wilk6, and A.V. Yeremin9
1GSI Helmholtzzentrum f¨ur Schwerionenforschung, 64291 Darmstadt, Germany
2Institut f¨ur Physik, Goethe-Universit¨at Frankfurt, 60438 Frankfurt, Germany
3Helmholtz-Institut Mainz, Johannes Gutenberg-Universit¨at, 55099 Mainz, Germany
4Department of Nuclear Physics and Biophysics, Comenius University, 84248 Bratislava, Slovakia
5Johannes Gutenberg-Universit¨at Mainz, 55128 Mainz, Germany
6Lawrence Livermore National Laboratory, Livermore, CA 94551, USA
7Department of Physics, University of Jyv¨askyl¨a, 40351 Jyv¨askyl¨a, Finland
8Japan Atomic Energy Agency, Tokai, Ibaraki, 319-1195, Japan
9Joint Institute for Nuclear Research, 141980 Dubna, Russian Federation
Received: 16 January 2012 / Revised: 9 March 2012
Published online: 3 May 2012 – c
Societ`a Italiana di Fisica / Springer-Verlag 2012
Communicated by J. ¨
Ayst¨o
This paper is dedicated to A. Sobiczewski on the occasion of his 80th birthday
Abstract. The synthesis of element 116 in fusion-evaporation reactions of a 48 Ca beam with radioactive
248Cm targets was studied at the velocity filter SHIP of GSI in Darmstadt. At excitation energies of the
compound nuclei of 40.9 MeV, four decay chains were measured, which were assigned to the isotope 292116,
and one chain, which was assigned to 293116. Measured cross-sections of (3.4+2.7
−1.6)pband(0.9+2.1
−0.7) pb,
respectively, and decay data of the chains agree with data measured previously at the Flerov Laboratory
of Nuclear Reactions in Dubna. As a new result, one α-decay chain was measured, which terminates after
four αdecays by spontaneous fission. The αenergies of the second-to-fourth decay are considerably higher
than those measured for the αdecays of 289114, 285 Cn, and 281Ds and the spontaneous fission half-life is
significantly longer than that of 277Hs measured in previous experiments. A possible assignment is discussed
in the frame of excited quasiparticle states of nuclei populated in the decay chain from 293116. Also other
possible assignments were considered and are discussed. At an excitation energy of 45.0MeV no events
were observed resulting in a one-event cross-section limit of 1.6 pb. The technical aspects related with the
use of radioactive target material at SHIP are described in detail. The experience gained in this experiment
will serve as a basis for future experiments aiming to study still heavier elements at the velocity filter SHIP.
1 Introduction
The favorite reaction proposed for producing a superheavy
nucleus (SHN) in the laboratory was the fusion of heavy
isotopes in the reaction 48Ca + 248Cm →296 116∗[1–3].
The main reason was the high neutron richness of both
the projectile and the target nuclei resulting in compound
nuclei (CN) closer to the predicted summit of the super-
heavy island. In addition, high binding energies of the dou-
bly magic 48Ca and of 248 Cm having the stabilizing shell
effect at neutron number N= 152, result in relatively low
excitation energy of the CN and thus lower probability
ae-mail: S.Hofmann@gsi.de
of fission. Thirdly, the reaction is one of the most asym-
metric ones for which target material is available in suf-
ficient quantities. High charge asymmetry between beam
and target nuclei has the advantage of low Coulomb repul-
sion in the entrance channel and thus a higher probability
of fusion.
Beams of 48Ca with sufficient intensities for searching
for SHN in irradiations of curium targets were available at
Berkeley and at Dubna in the middle of the 1970s. After
chemical separation of the expected element 116 isotopes
or their daughter products down to element 112, the sam-
ples were monitored by αand fission-fragment detectors.
For various lifetime ranges with lower limits of about one

Page 2 of 23 Eur. Phys. J. A (2012) 48:62
hour, the experiments resulted in upper cross-section lim-
its of about 100 pb [4–6]. Under the assumption of high
volatility of element 116 and of the α-decay daughter el-
ements 114 and 112, a He-jet apparatus was also used
to study a lifetime range from 100 days down to 1s [7].
Also in this experiment no SHN were observed and cross-
section limits of about 100 pb were derived.
The range of lifetimes experimentally accessible was
considerably extended, both for short and long lifetimes,
when recoil separators and position-sensitive detectors be-
came available for studying fusion-evaporation products at
the end of the 1970s [8,9]. Using the gas-filled separator
SASSY at Berkeley and the velocity filter SHIP at Darm-
stadt the reaction 48Ca + 248 Cm →296116∗was studied
over a wide range of excitation energies [10]. In these ex-
periments upper cross-section limits of 200 pb were de-
termined for the production of element 116 isotopes at a
range of lifetimes from 10 μs to 1 day. In the range from
1 day to 1 year upper limits of 20 pb were reached using
chemical separation [10].
In the following years the experimental set-ups at GSI
were considerably improved with respect to beam inten-
sities and separation and detection efficiencies [11]. An
overall improvement by a factor of ten allowed for study-
ing isotopes produced with cross-sections on a level of
one picobarn, as, e.g., element 112 in the cold-fusion re-
action 70Zn + 208 Pb →277Cn + 1n in 1996 [12, 13]. Reac-
tions based on lead or bismuth targets are generally de-
noted as “cold fusion”, because maximum fusion cross-
sections are measured at the fusion barrier at low excita-
tion energies of less than 20 MeV, whereas reactions based
on actinide targets result in excitation energies of greater
than 35 MeV, for which the term “hot fusion” is in use.
A new search for SHN using hot fusion started at
the Flerov Laboratory of Nuclear Reactions (FLNR) in
Dubna. There, an important prerequisite was the devel-
opment of an efficient ECR ion source for production of
intense beams of 48Ca with low consumption of this ex-
pensive source material. With the energy separator VASI-
LISSA and the gas-filled separator DGFRS equipped with
detector systems similar as the one developed at GSI, a
research program was started in 1998, which was based
on the use of a 48Ca beam and various targets of actinide
isotopes from 238Uto249 Cf. New neutron-rich isotopes
of copernicium, and the new elements from 113 to 118
were synthesized during the following twelve years. In ad-
dition, new isotopes of elements from roentgenium down
to dubnium were identified as daughter products popu-
lated in the relatively long decay chains. An overview of
the results of these experiments is given in [14], the recent
production of element 117 is described in [15].
The most interesting result was that the cross-sections
did not decrease exponentially like in the case of more
neutron-deficient nuclei produced in cold-fusion reactions.
There, the heaviest nucleus, 278113, was synthesized with
a cross-section of only (31+40
−20) fb at RIKEN in Japan [16].
In contrast, the cross-sections for production of more
neutron-rich isotopes in hot-fusion reactions remain rela-
tively constant at values between 0.3 and 5 pb, with max-
imum values for element 114 and 116 [14,15].
Confirmation of hot fusion experiments were success-
fully performed at FLNR itself by a joint FLNR-Swiss
collaboration of nuclear chemists studying the reaction
48Ca + 242 Pu →290114∗by chemical means [17]. The first
successful confirmation experiment in a different labora-
tory was performed at the GSI SHIP. There the isotope
283Cn was synthesized in the reaction 48Ca + 238 U, and
agreement with the FLNR data was obtained both for the
cross-section as well as for the decay properties of 283Cn
and its dominantly fissioning daughter 279Ds [18]. Subse-
quently, synthesis and decay of the isotopes 286114 and
287114 produced in the reaction 48Ca + 242 Pu was verified
in [19] and of 288114 and 289 114 produced in the reaction
48Ca + 244 Pu in [20,21], where even higher cross-sections
of close to 10 pb were measured.
After these promising results, attempts were also un-
dertaken to synthesize element 120. At FLNR, the reac-
tion 58Fe + 244 Pu →302120∗was investigated [22] and at
GSI the reaction 64Ni + 238 U→302120∗[23]. No events
originating from isotopes of element 120 were observed.
Measured upper cross-section limits of 400 fb and 90 fb,
respectively, were reached.
In continuation of experiments at SHIP using actinide
targets we planned to repeat the irradiation of 248Cm tar-
gets with 48Ca ions, however, now at higher beam inten-
sities and using larger area targets than in our 1983 ex-
periment. The main aims of this experiment were, firstly,
testing the use and the safe handling of such large area
radioactive actinide targets at SHIP, secondly, to inde-
pendently confirm decay data of isotopes of element 116,
which were previously published in [24–30], thirdly, to ex-
tend the excitation function to higher energies, fourthly, to
prove transmission data of fusion products through SHIP,
which were obtained from ion optical calculations for these
very asymmetric reactions, and, fifthly, to prepare a basis
for future investigations of SHN using radioactive targets.
The techniques applied in our study are presented in
the experimental part of this paper. Different from us-
ing stable target material, special safety precautions are
needed in the case of highly radioactive targets. The con-
trol of the targets during irradiation is of crucial impor-
tance. Therefore, and because we plan to use actinide tar-
gets also in various experiments in the future, we present
this part of the experiment in greater detail.
Experiences gained with the behavior of the target
material during irradiation are presented in sect. 2.3. In
sect. 3 we present the data measured from the reaction
48Ca + 248 Cm and in sect. 4 we perform a comparison with
previously measured data and theoretical predictions. Fi-
nally, in the conclusions, we present a summary and give
an outlook.
2 Experimental details
2.1 Projectile beam and beam control
Our study of the reaction 48Ca + 248 Cm →296116∗was
performed from June 25 to July 26, 2010. During the first
part ending on July 12, a beam energy of 265.4 MeV was

Eur. Phys. J. A (2012) 48:62 Page 3 of 23
Table 1. Beam and target properties used in the study of the reaction 48 Ca + 248Cm →296 116∗. In columns six and seven beam
energies and excitation energies are given for reactions at the beginning (b), at the center (c) and at the end (e) of the curium
oxide layer. For an estimate of the total target thickness from the measured 248Cm content, we assumed a mixture of CmO2and
Cm2O3, which results in a stoichiometric ratio of curium to oxygen of 1/1.75. Excitation energies were calculated with binding
energies from [31] for projectile and target and from [32] for the CN. Energy-loss values were obtained by extrapolation of data
calculated with the computer code SRIM [33].
Dates Time d(Ti) d(248CmO1.75 )EUNILAC Elab E∗Iproj Dose
in 2010 /days /mg cm−2/mg cm−2/MeV /MeV /MeV /pnA /1018
b-c-e b-c-e
25/06–12/07 16.6 1.05 0.512 265.4 251.4-249.3-247.3 42.6-40.9-39.1 576 5.3
12/07–24/07 11.6 1.05 0.512 270.2 256.3-254.3-252.2 46.7-45.0-43.3 444 2.8
24/07–26/07(a)2.0 1.05 0.512 270.2 256.3-254.3-252.2 46.7-45.0-43.3 185 0.2
26/07–16/08(b)21.0 background
(a)Used for calibration and search for transfer products using SHIP settings for separation of reaction products with velocities from 1.6×vCN to
1.9×vCN.
(b)Background measurement and study of radioactive decays of detector implanted transfer products.
used, in the second part ending on July 24, the beam en-
ergy was increased to 270.2 MeV. Finally, two days were
used for calibrations and measurements of transfer prod-
ucts at 270.2 MeV beam energy. In this case SHIP was
set for products with velocities of 1.7, 1.8, 1.85, 1.9 and
1.95 ×vCN with vCN being the velocity of the CN. A
three weeks period of background measurements followed.
The dates of the irradiations are listed in table 1 together
with target properties, beam energies and projectile doses
reached.
The 48Ca beam was delivered from the ECR ion source
and accelerated through the UNILAC at GSI. Recently
made technical improvements are described in [34,35]. Me-
tallic, isotopically enriched 48Ca (89.5%) and the ECR
oven technique were used. Ions with charge state of 10+
were extracted and accelerated by the high-charge state
injector (RFQ + IH structure) and the Alvarez sections
of the UNILAC. Mean currents of 0.58 and 0.44 pμA
(1 particle μA=6.24 ×1012 particles/s) on target were
used during irradiations at beam energies of 265.4 and
270.2 MeV, respectively. The duty factors amounted to
25.0% (until July 6), 27.5% (until July 11) and 26.0%
(until July 26).
The duty factors are due to beam pulses of 5.0, 5.5 and
5.2 ms width at 50 Hz repetition frequency. The relatively
low duty factors imply, however, for the targets a load of
2.2 and 1.7 pμA during each pulse, respectively. This fea-
ture has to be taken into account when influences of beam
currents on targets are compared with those observed at
the FLNR and RIKEN laboratories, where comparable
average beam intensities are reached with beams of 100%
duty factor.
In this context it is interesting to note that the low
duty factor at the UNILAC is due to the power limita-
tion of the radio frequency tubes of some sections of the
accelerator, whereas the ECR ion source delivers a con-
tinuous beam of 48Ca of 5 pμA at charge state of 10+.
Despite these high currents, the average consumption of
48Ca was only 0.18 mg/h. This value was obtained from
a continuous running time of the ECR source of 40 days,
which started eight days before our experiment. From the
initial filling of the oven with 300mg 48 Ca, only 173 mg
were consumed during this period of 40 days.
The beam energy is adjusted to the requested values
by single resonators. In our case the ions were decelerated
from 5.95 ×AMeV achieved with the Alvarez sections of
the UNILAC to 5.53 ×Aand 5.63 ×AMeV (A= atomic
weight of the projectile). The beam energy was measured
with an accuracy of ±0.2% with a time-of-flight system us-
ing pick-up coils for detection of the beam micro-bunches.
The width of the energy distribution was 0.5% FWHM.
This value is smaller than the energy loss of the beam in
the target layer, see table 1. However, due to the proce-
dure of deceleration sporadically 48Ca ions with energies
of 5.95 ×AMeV passed the accelerator and reached the
target. This contribution was detected by magnetic deflec-
tion of electrons produced with the same velocity as the
beam in interactions with the residual gas in the evacuated
beam line. A maximum amount of 4% of higher-energy
48Ca ions relative to the total intensity was measured.
The detection method using the interaction of the beam
with the residual gas is described in [36]. The apparatus
is placed 3 m in front of the target. It is also used for an
additional permanently accessible energy control having
an accuracy of ±0.3%.
2.2 Targets and target control
The target material was provided by Lawrence Livermore
National Laboratory. An amount of 20 mg 248 Cm was de-
livered in form of curium nitrate to the Institute of Nu-
clear Chemistry of the Johannes Gutenberg-University in
Mainz. There, eight targets were produced by molecular

Page 4 of 23 Eur. Phys. J. A (2012) 48:62
Fig. 1. The upper and the middle part show photos from
one of the eight target segments mounted on the SHIP target
wheel prior and after irradiation with 8.3×1018 48Ca beam
particles, respectively. Each target consists of a 1 mm thick
aluminium frame. Onto this frame a backing foil of titanium
of 2.3 μm thickness is glued. The inner area marks the curium
oxide layer having an average thickness of 0.512 mg/cm2.The
image obtained from the measurement of the target thickness
using scattering of a 30 keV electron beam is shown in the
lower part. This measurement occurs on-line with the target
wheel rotating and without interruption of the irradiation of
the targets with the heavy-ion beam.
plating of the material dissolved in isobutanol. Molecular
plating was performed at a current density of 0.7mA/cm2
and three hours deposition time. Titanium foils served as
backing material. The dimensions of the targets are shown
in fig. 1. A detailed description of the target preparation
is given in [37].
The enrichment of 248Cm was 96.85%. Impurities of
lighter curium isotopes were 0.015% of 247Cm, 3.10% of
246Cm, 0.031% of 245 Cm, and 0.0007% of 244Cm.
The titanium foils were produced by cold rolling in
the GSI target laboratory. Thicknesses of 2.2–2.4 μm
were achieved. The titanium backing foils were glued
onto aluminium frames of 1 mm thickness. Before depo-
sition of the curium material, the titanium backing foils
were tested and their thicknesses were remeasured using
the methods of electron scattering and α-particle energy
loss. The selected foils were pin-hole free and had thick-
nesses of 1.00 mg/cm2(3 foils), 1.05 mg/cm2(1 foil) and
1.09 mg/cm2(4 foils) resulting in energy losses of the 48Ca
beam of 13.3, 13.9, and 14.5 MeV, respectively. We used
the mean value of 14.0MeV for calculating the energy of
the 48Ca ions entering the curium oxide layer.
The amount of 248Cm deposited on the titanium back-
ing was between 0.454 and 0.465mg/cm2. For the further
calculations we used a mean value of 0.460 mg/cm2.The
thickness was determined from the emitted αactivity, and
the homogeneity of the curium layer was determined auto-
radiographically [38]. Both measurements were performed
off-line at the institute in Mainz before the experiment.
The total amount of 248Cm deposited on the eight targets
of the wheel was 10.7 mg.
For determining the energy loss of the beam particles
inside the targets, we have to consider the stoichiometric
ratio between curium and oxygen. Two oxides of curium
are known, CmO2and Cm2O3, which both or a mixture of
both could form the target layer. Therefore, we calculated
the energy loss for both compounds and for the two dif-
ferent beam energies using the computer code SRIM [33].
The energy loss in the 248Cm oxide layer was obtained by
extrapolation of the values of the oxides of 223Fr, 227Ac,
232Th, 231 Pa, and 238U (uranium is the last possible tar-
get available in the SRIM code). For the beam energy
of 251.4 MeV, we obtained energy loss values of 8.18 and
7.87 MeV/(mg/cm2) for the oxides CmO2and Cm2O3,
respectively, and for the beam energy of 256.3 MeV, the
values were 8.13 and 7.82 MeV/(mg/cm2).
In the case of a CmO2layer of 0.519 mg/cm2thick-
ness, the energy loss of 48Ca is 4.25 and 4.22 MeV for the
lower and higher beam energy, respectively. In the case
of a Cm2O3layer of 0.505mg/cm2thickness, the energy
losses amount to 3.97 and 3.95 MeV. The energy losses
for the two different oxides differ only by 0.14MeV in the
middle of the target thickness. Therefore, we used a mean
value of 4.10MeV for the energy loss of the 48 Ca ions from
the beginning to the end of the curium oxide layer hav-
ing a mean thickness of 0.512 mg/cm2. With these values
we calculated the beam energies and excitation energies
of the CN given in table 1.
At GSI, the eight targets were mounted on a standard
SHIP target wheel [39]. The rotation speed of the wheel
(375 rpm) was adjusted in such a way that all eight tar-
gets were irradiated within a period of 160 ms. The wheel
rotated synchronously to the 50 Hz pulse structure of the
beam.
The length of the curium-oxide layer (36 mm) corre-
sponded to the duration of a beam pulse of 5.2 ms consid-
ering the width of the beam spot was 4 mm. The height
of the beam spot was adjusted to the 8 mm height of the
curium layer. The lateral dimensions of the backing foil
were 10 mm larger in order to avoid that tails of the beam
hit the target frame, which would result in unnecessary
heating of the wheel and in increased background of scat-
tered beam particles.
In order to avoid contamination of the vacuum cham-
ber by the radioactive curium, the target wheel is mounted
inside an aluminium housing with only two small aper-
tures (23 mm diameter) for the incoming beam to hit the

Eur. Phys. J. A (2012) 48:62 Page 5 of 23
target and another one for an electron beam used for on-
line target control (see below). The same housing is also
used for transport. Here, the holes are covered and the
housing is completely closed.
Inside of the vacuum chamber, the housing was bolted
to a copper plate, which was cooled during irradiation with
alcohol of −30 ◦C resulting in a temperature of the copper
plate of −27 ◦C. This way an efficient radiative cooling of
the targets was achieved.
Different methods were applied for measuring and
monitoring the target thickness on-line:
i) Continuously and without interruption of the beam,
scattering of 30 keV electrons was used [40]. Here, the
accuracy is ±2%. Two-dimensional thickness information
was obtained by deflecting the electron beam in a radial
direction across the rotating targets. Due to the narrow
width of the electron beam, the lateral resolution is in the
order of 0.5 mm.
ii) The elastically scattered projectiles were registered
and normalized to the beam current. Two scintillation de-
tectors were used, which were mounted at angles of ±30
degrees relative to the beam axis.
iii) Before irradiation and several times during the ex-
periment the beam was stopped, and the αactivity emit-
ted from the curium layer was measured. In this case the
SHIP settings were adjusted to the velocity of the αpar-
ticles, and the αspectra were taken from the rotating tar-
get wheel with the detector system in the focal plane of
SHIP. A two-dimensional energy-versus-time scatter plot
is shown in the upper part of fig. 2.
The spectrum shows the sum of three 20 minutes mea-
surements taken on June 25 and 28 and on July 6. Clearly
visible is the αactivity from the dominant curium iso-
topes. These α-rays are focused on the detector when the
targets move through the spot where the beam hits the
targets during irradiation. The energy broadening of the
curium αlines is due to the relatively thick target layer.
This will be discussed in sect. 2.3.
The narrow αlines occurring in the windows between
the curium activities (marking the beam pauses during ir-
radiation) are due to long-lived αactivities from 208Po and
210Po having energies of 5.11 and 5.30 MeV, respectively,
which were implanted in the stop detector in previous ex-
periments. These activities were useful for controlling the
long-term stability of the electronics.
iv) Subsequent to the measurements of the activity
from the curium targets alone, a 238Pu αsource was moved
into the beam position 120 mm in front of the target. Orig-
inally, in 1975, it was a 37 ×106Bq 242 Cm source with a
small contamination of 244Cm. The source is covered by
a nickel window which degrades the energy of the αpar-
ticles from 5.50 to 4.75 MeV. Similar to the αparticles
from 238Pu also the α’s from the contamination of 244Cm
(5.80 MeV) and 242Cm (6.11 MeV) are degraded by the
source window by about 0.70 MeV to energies of 5.10 and
5.40 MeV, respectively.
With the wheel rotating, the αparticles pass consec-
utively through the layers of backing and curium oxide,
the titanium backing alone and through the empty space
Fig. 2. Energy distribution of αparticles emitted from the
targets (top) and from the targets plus a 238Pu source moved in
front of the targets (bottom). The αenergies were measured as
function of time for complete turns of the wheel lasting 160 ms.
See text for a detailed description.
between the targets. A spectrum taken on August 6, after
the end of the experiment after the targets were irradi-
ated with a beam dose of 8.3×1018 48Ca ions, is shown
in the lower part of fig. 2. Also in this case, the αspectra
were registered in the focal plane of SHIP. Target thick-
nesses were obtained from the energy loss of the αparticles
measured as function of the length of the targets. These
control measurements were repeated in regular intervals
during the experiment for obtaining data as function of
the beam dose.
Intensities of αparticles from curium isotopes of the
target normalized to the intensity of the α’s from the
source result in information on possible losses of target
material. For this purpose time windows were set on the
corresponding periods of the spectrum shown in fig. 2. No
losses were observed within a statistical limit of 0.5%.
v) Finally, about once per week, the target wheel was
stopped and the targets were optically inspected through
glass flanges of the vacuum chamber.

Page 6 of 23 Eur. Phys. J. A (2012) 48:62
During the irradiations, a carbon foil of 30 μg/cm2
thickness was placed at a distance of 125mm downstream
of the target. The foil was glued on a rectangular alu-
minium frame having inner dimensions of 105 mm in hor-
izontal and 45 mm in vertical direction. This foil was
moved periodically with a frequency of 0.004 Hz through
the beam and the emitted reaction products. It is used for
charge equilibration of reaction products after decay of
short-living isomeric states and that of projectiles passing
through cracks in the target which could occur by ther-
mal stress during irradiation. In addition, the placement
of the foil close to the target makes it an efficient collector
of scattered target material. Measuring the αactivity of
the foil after irradiation allowed to determine the amount
of curium material emitted in forward direction. For the
results from a quantitative analysis see sect. 2.3.
In the last part of the experiment, when we used SHIP
settings for transfer products, we observed αlines of 150Dy
and 149Tb. The occurrence of these isotopes could be ex-
plained plausibly by fusion-evaporation reactions with iso-
topes of a target contamination of palladium. Palladium
was used as an electrode in the procedure of molecular
plating. Maximum intensities of the lines were measured
at a velocity setting of 1.9×vCN, with vCN =0.017 ×c,
the velocity of the 296116 CN and cbeing the velocity of
light. The 150Dy and 149 Tb nuclei were observed, because
the velocity setting for the transfer products of 0.032 ×c
was accidentally the same as that of the compound nuclei
of the reaction 48Ca + nat Pd. From the intensities of the
αlines we estimated a contamination of about 4.0 μg/cm2
of natural palladium.
The two αlines at energies of (3966.6±1.9) and
(4235.1±1.7) keV [41], respectively, were useful for energy
calibration. They were not observed during the main ex-
periment with SHIP settings for detection of element 116
ERs. At these settings reaction products different from fu-
sion and evaporation of neutrons or protons are strongly
suppressed. From this we estimate a suppression factor of
at least 5 ×103taking into account the background in the
energy range of the αlines. On the other hand, the sup-
pression of beam particles having a much higher velocity
of 0.11 ×cis significantly higher and dependent on the
position in the focal plane. On the right four strips of the
detector (side closer to the beam axis) having a width of
2 cm, we measured a suppression by a factor of 4.0×10−14
and on the remaining twelve strips (6 cm) the suppression
was 2.3×10−15 .
2.3 Conditioning of the targets
A necessary first step at the beginning of the experiment
was the conditioning of the target material for operation
at high beam intensities. Figure 3a shows the αspectrum
of the curium targets as produced by molecular plating
before irradiation.
A highly resolved αspectrum from the curium tar-
get material is shown in fig. 3d as measured after the
experiment. This spectrum resulted from a thin layer of
curium isotopes deposited on the charge equilibration foil
Fig. 3. Energy distribution of αparticles emitted from the
target at various stages of the experiment: a) targets before
conditioning; b) taken at a beam dose of 0.12 ×1018 projec-
tiles of 48Ca after conditioning with beam intensities increasing
stepwise from 10 up to 800 pnA; c) at the end of the experiment
after irradiation with 8.3×1018 projectiles; d) alpha spectrum
of the activity sputtered onto the charge equilibration foil. In
a) to c) the αparticles were transported through SHIP, in d)
the charge equilibration foil was put at a distance of 155mm
in front of the detector after the experiment. The energy shift
of the peak positions in a) to c) relative to the peaks in d)
is mainly due to the energy loss of αparticles in the target
itself, in the charge equilibration foil and in the foils of the
time-of-flight detectors.
by sputtering. For the measurement the foil was placed
155 mm in front of the detector. Three groups of lines are
observed, which originate from the isotopic composition
of the target as given before. The main lines are at ener-
gies of (5078.38 ±0.25) keV (248Cm), (5385.7±0.9) keV
(246Cm), and (5804.77 ±0.05) keV (244Cm) [41].
For conditioning, the targets were irradiated starting
at a beam current of 10 pnA. The current was increased on
average every 45 min to 20, 40, 80 pnA and then in steps
of 30 pnA up to 410 pnA. After 7 hours of irradiation
at 410 pnA the beam was increased again, every 70 min

Eur. Phys. J. A (2012) 48:62 Page 7 of 23
in steps of 30 pnA up to 560 pnA. For this conditioning,
the beam profile was widened with the use of octupole
magnets [42], so that the whole area of the curium oxide
layer was irradiated on the rotating target wheel.
The target properties changed gradually as observed
by the αspectra measured from the targets before and
during conditioning. The broader distribution of αener-
gies measured before conditioning, as shown in fig. 3a,
became gradually narrower and reached a saturation after
irradiation with 500 pnA and a beam dose of 0.12 ×1018
as illustrated in fig. 3b. The two main groups of αlines
from 248Cm and 246 Cm are well separated.
From the relatively sharp low energy edge we conclude
that a homogenous layer developed. From previous exper-
iments [43], and also from preceding tests with the chemi-
cal homolog Gd2O3, it is concluded that a not well-defined
structure even containing rests of the solvent changed into
a homogenous transparent glass-like layer during irradi-
ation. This layer is highly resistant at increasing beam
doses. An optical inspection of the targets after the exper-
iment supported this conclusion. Further post-irradiation
investigation of a target sector using scanning electron mi-
croscopy (SEM) and ion probe methods are planned to be
performed at LLNL.
Obviously, there exists a relatively large shift and a
broadening of the αlines in figs. 3a) to c) relative to
the lines in d). For the intensive 246Cm line shown in
fig. 3b the shift amounts to 230 keV and the broadening
to 200 keV FWHM. The biggest contribution to these dif-
ferences is due to the energy loss of the αparticles in the
target itself, in the charge equilibration foil, and in the
foils of the time-of-flight (TOF) detectors (see sect. 2.5).
A convolution of the 246Cm αline with the energy loss in
the 0.512 mg/cm2thick target layer resulted in a shift of
86 keV and in a broadening of the line of 136 keV FWHM.
An additional amount of 85 keV is lost by the αparticles
when traversing the charge equilibration foil and the foils
of the TOF detectors. A further contribution arises from
the mechanical change of the targets which become dented
during irradiation (see fig. 1).
On July 9 at a total beam dose of 4.4×1018 we observed
a narrow vertical crack with a size of 0.5mm×3mm in
one of the targets. Four more targets showed similar cracks
on July 12 at a beam dose of 5.7×1018. The size of these
cracks did not change up to the end of the experiment at a
beam dose of 8.3×1018. The cracks were located all at ap-
proximately 19 mm from the beginning of the targets. The
other areas of the targets did not show any traces of the
beam. We therefore conclude that the origin of the damage
from the cracks is related to the measurement of the beam
profile, when the duration of the beam macropulse is re-
duced to 0.1 ms corresponding to a beam spot of 0.65 mm
width on the target. We suppose that this narrow beam
cracked the layers consisting of the titanium backing and
the glass-like curium oxide, a circumstance which has to
be avoided in future experiments.
From the activity collected on the charge equilibra-
tion foil we determined an amount of 46μg by sputtering
up to a beam dose of 6.8×1018 ions when the foil was
replaced. On the second foil we measured an amount of
7.9 μg collected during a beam dose of 1.5×1018 ions. In
this estimate it was assumed that the angular distribution
of the sputtered material is uniform in forward direction.
The collecting charge equilibration foil of 45 mm ×105 mm
is located 125 mm downstream of the target.
The amount of sputtered material normalized to 1018
projectiles is 6.7 and 5.3 μg, respectively. These almost
equal numbers indicate that the sputtering rate is within
error bars independent of the influence of beam dose on
the target. Considering the total amount of 10.7 mg of
248Cm used in the experiment, the losses of material by
sputtering are small, at least in the case of a 48Ca beam
at an energy of 270 MeV.
2.4 Operation of the separator SHIP
The properties and the operation of SHIP are described
in [8,11]. In most of our irradiations described here
an asymmetric setting of the first SHIP quadrupole
triplet was used, which was calculated by a Monte Carlo
method [44]. This setting resulted in an efficiency of 20%
for ERs from 3n and 4n evaporation channels at a target
thickness of 0.512 mg/cm2. Considered in this calculation
is a Gaussian shaped excitation function having an as-
sumed width of 10 MeV FWHM and scattering of the ERs
in the target itself, in the charge equilibration foil and the
foils of the TOF detectors behind SHIP.
Background rates, mainly due to scattered projectiles
having the same velocity as the ERs and energies below
6.5 MeV, depended strongly on the beam profile along the
beam line in front of the target. Using a narrow beam
profile, the total background rate was 75Hz at a beam
current of 600 pnA. However, in this case the width of the
Gaussian profile of 3 mm FWHM in vertical direction was
too narrow for a long-duration irradiation of the targets.
When trying to increase the beam profile to 6 mm
FWHM on target, the background rate increased to
700 Hz. The reason for this higher rate was scattering of
projectiles in the wings of the beam from narrow collima-
tors in the beam line in front of the target. This relatively
high counting rate was lowered to values between 300 and
400 Hz by a 3.5 μm thick Mylar degrader foil placed just
in front of the silicon detector array.
The last dipole magnet of SHIP was adjusted to an an-
gle of 4 degrees relative to the beam direction to suppress
unwanted beam-like particles and maximize ERs implant-
ing into the focal plane detector. At this angle the germa-
nium detectors (see sect. 2.5) are placed enough far from
the straight beam line, which results in a considerable re-
duction of the background of γ- and X-rays emitted from
reactions at the target.
In order to measure the influence of the SHIP qua-
drupole settings on the background rate, we also used the
standard symmetric quadrupole settings [8] for a short
period of two days at the end of the first part of the ex-
periment. While the background rate decreased by 30%,
the calculated efficiency was also decreased by the same
amount. Therefore, we used the asymmetric setting and
accepted a higher background rate.

Page 8 of 23 Eur. Phys. J. A (2012) 48:62
As additional information and with respect to a longer
beam time planned to search for element 120 it should be
mentioned here that the background rate was consider-
ably lower in a first irradiation using a 54Cr beam and the
identical 248Cm targets. This experiment was performed
in May 2011 after removing the narrow collimators from
the beam line. At a beam current of 600 pnA the back-
ground rate from signals between 200 keV and 320 MeV
was only 160 Hz without using any degrader foils in front
of the detectors, even at a beam profile of 6 mm FWHM
in vertical direction.
A promising option for setting the beam profile was
the use of two octupole focussing elements at distances
of 8.91 m and 16.40 m in front of the target [42]. These
elements allow for generation of a rectangular beam pro-
file on the target having the optimal dimensions of 8 mm
and 4 mm vertically and horizontally, respectively. How-
ever, due to the wider beam profile needed inside the oc-
tupoles, the background of scattered projectiles increased
to 2–3 kHz. Nevertheless, these settings were used for con-
ditioning the targets, a process which was described in
sect. 2.3.
Although the application of the octupole magnets was
not fully satisfactory in this experiment, these focussing
elements will allow for optimizing the beam profile on
the target, in order to obtain a nondestructive irradia-
tion of sensitive target material at high beam currents in
the future. A simultaneously low background rate will be
achieved by optimizing the mechanical collimators of the
beam line with ion optical calculations.
2.5 Detector system and energy calibration
In the focal plane of SHIP, event chains consisting of im-
planted ERs and their subsequent αdecay and/or sponta-
neous fission (SF) were identified by the position-and-time
correlation method [9, 11]. In our experiment the main de-
tector was a position-sensitive 16-strip Si PIPS detector
(stop detector) with an active area of 80 mm ×35 mm.
Escaping αparticles or complementary fission fragments
were detected by a “box detector” which covered an area
of 85% of the backward hemisphere. A schematic drawing
of the detector system is shown in fig. 2 of [18]. There,
also the assignment of the numbered strips and segments
is given. All strips and segments are separated into odd
and even ones to avoid cross-talk. The same assignment is
used for the events detected in the present work.
A difference from fig. 2 in [18], is that only the last
two TOF detectors were used in the present experiment
for measuring the ion velocity and for tagging ions which
are implanted into the stop detector. Signals are obtained
from secondary electrons which are emitted when ions pass
through a thin carbon foil covered with magnesium oxide
for easier emission of electrons. The thicknesses of the car-
bon and magnesium oxide layers were 19 and 12.5 μg/cm2
for TOF detector one and 32 and 20 μg/cm2for TOF de-
tector two, respectively.
The electrons are accelerated by a voltage of 3.8kV
applied between the foil and a grid placed 40 mm down-
stream. The grid consists of gilded tungsten wires, 20 μm
in diameter and separated by a distance of 3 mm. The
wires are oriented vertically. One grid reduced the trans-
mission of the TOF detector by 0.67%.
A magnetic field deflects the electrons to the surface
of a stack of two channel plates. Details of the construc-
tion of the TOF detectors are given in [45]. The detectors
are mounted at distances of 425 and 245 mm in front of
the stop detector, in each case measured from the center
between carbon foil and grid. The time resolution is 0.8 ns
FWHM measured for 48Ca beam particles. The time range
of the time-to-amplitude converter (TAC) was set to 1 μs.
The efficiency of the detectors was 99.13 and 99.44%
for TOF detector one and two, respectively, measured
for scattered 48Ca projectiles and target-like nuclei hav-
ing energies >50 MeV. The probability of obtaining a
TOF signal (both detectors fired) was 98.95%. In the anti-
coincidence mode requesting that none of the TOF detec-
tors fired, the number of counts from the stop detector
in the energy range >50 MeV was reduced to 0.38%. In-
cluding the condition that also no signal was derived from
the veto detector (see below), improved this value only
slightly to 0.36%.
Similar data in the energy region of the implanted ERs
from 16 to 26 MeV are slightly worse. The efficiencies of
TOF detectors one and two are 97.03 and 97.24%. The
probability of measuring a TOF signal is 96.77% and the
number of counts measured in the anti-coincidence mode
is 2.25%. In this energy region the inclusion of the veto
detector improved this value to 1.63%.
In the energy range from 8 to 16 MeV, where most of
the αdecays occur, the efficiencies of TOF detector one
and two are 94.80 and 95.35%, respectively. The proba-
bility of getting a TOF signal is 94.18% and the number
of counts in the anti-coincidence mode is 4.03%. This rel-
atively high value is due to background αparticles and
protons emitted from reactions with the titanium back-
ing, which pass SHIP and traverse the stop detector, but
having not triggered the TOF detectors. In this case the
inclusion of the veto detector improves this value to 1.40%.
Obviously, the light particles result in correlated non-
triggering of both TOF detectors, which is already the
case for the energy range from 16 to 26 MeV.
The detector system was completed by a Si veto de-
tector and a Ge clover detector, both mounted behind the
stop detector. The veto detector had the same size and
thickness as the stop detector, however, it was subdivided
in four energy-sensitive segments only. The clover detector
was separated from the SHIP vacuum by a 1 mm thick alu-
minium window. The detector consisted of four Ge crys-
tals, each with a diameter of 50 mm and a length of 70 mm.
Gamma energies were measured in two ranges from 0.02 to
0.8 MeV and 0.1 to 8 MeV for each of the four Ge crystals.
The energy calibration of the Ge detectors was performed
with standard 133Ba, 152 Eu, and 60Co γ-ray sources. En-
ergy resolutions were typically 1.4keV FWHM in the re-
gion of KαX-rays of SHEs having energies of 155.6keV
for Kα1at Z= 110 and 199.6 keV for Kα1at Z= 120 [46].
The time between signals from the Si and the Ge detec-
tors was measured with TACs within a range of 5 μs. The

Eur. Phys. J. A (2012) 48:62 Page 9 of 23
set-up was tested with 5.48 MeV αparticles and coincident
60 keV γ-rays from an 241Am source. In order to reduce
the event rate to be registered, signals from the Ge detec-
tors were measured only when they were in coincidence
with signals from the Si detector array within the time
range of 5 μs.
The energy and the position for each of the 16 strips of
the stop detector were measured in two ranges, one from
0.1 to 16 MeV and from 4 to 320 MeV. The two energy
ranges for each of the 28 segments of the box detector were
from 0.2 to 16 MeV and from 2 to 160 MeV. The energy
resolution of the stop detector strips was 24 keV FWHM at
9MeVα-particle energy, and equal for all strips. From this
value follows a 1σuncertainty of ±10 keV for the energy
of a single measured αparticle due to statistical fluctua-
tions. The final error bar is obtained by quadratic sum-
mation with the uncertainty from the energy calibration
procedure.
In the case of coincident signals from the stop and the
box detector due to αparticles or fission fragments emit-
ted in a backward direction, we performed an individual
energy calibration for the stop detector strip and the box
detector segment involved. This procedure takes into ac-
count the different angle dependent path lengths of the de-
tector dead layers traversed by the emitted particles and
also the different energy resolutions of the box detector
segments. For the combination of strip 6 and segment 17,
which is relevant for one of the αdecays measured here,
we obtained an energy resolution of 64 keV FWHM and a
1σuncertainty of ±27keV. The time between signals from
stop and box detectors was measured with TACs within a
range of 5 μs. The adjustment of amplifiers and electron-
ics for the Si detectors was performed with standard α
sources of 239Pu, 241 Am, 244Cm, and SF fragments from
a252Cf source.
The final energy calibration of the low energy branch
was made with αparticles from nuclei implanted into the
detector. In this case the contribution of the recoiling
daughter nucleus to the detector signal has to be consi-
dered. For example, the recoil energy of the 11.65 MeV
αdecay of 212mPo, which is often used for calibration, is
224 keV, whereas the recoil energy of a SHN with mass
number 292 is only 162 keV for the same αenergy. Al-
though only 28% of the recoil energy contributes to the
signal as shown in [47], the remaining difference in the
given example is still 17 keV, which is outside the ±10 keV
energy uncertainty.
Therefore, we performed a linear calibration fit not
using the pure αenergies, but the accurate values valid
in the case of implanted nuclei. The corrected energies
are Eα(1 + 0.28 ×mα/mrecoil). In order to finally obtain
the pure αenergies of new decays, the fitted energy was
divided by 1 + 0.28 ×mα/mrecoil with mrecoil being the
mass of the daughter nucleus after αdecay. Obviously a
sufficiently well-known mass number assignment of the α
decay has to exist in order to obtain accurate results.
As calibration lines we used αparticles from the
long-lived 208Po (5114.9±1.4 keV) and 210Po (5304.33 ±
0.07 keV) nuclei produced in transfer reactions from pre-
vious experiments. In addition, αlines from transfer prod-
ucts 222Ra, (6559 ±5) keV, 218Rn, (7129.2±1.9) keV, and
214Po, (7686.82 ±0.07) keV were used. The energy values
were taken from the recommendations given in [41].
Useful for a calibration point at relatively high α
energy, where also the αdecays of SHN occur, is the
11.65 MeV decay of the 45.1 s 212mPo isomer, which is pro-
duced in a transfer reaction. However, this αtransition is
known only with an accuracy of ±20 keV [48].
In order to improve the accuracy of this αdecay we
performed a new energy determination using the accu-
rately known αenergies of the transfer products given be-
fore. Sufficiently high intensities were obtained during the
study of transfer products at the end of the main experi-
ment. In this calibration, the accurately known αdecays of
149Tb, (3966.6±1.9) keV and of 150Dy, (4235.1±1.7) keV
were also used [41]. The new energy of (11651 ±5) keV
obtained for the decay of the 45.1 s isomer in 212mPo is
in agreement with the previous value, but the error bar
is considerably reduced. This new value and the ener-
gies of the transfer products as given before were used
for determining the αenergies of the decays measured in
the 48Ca + 248 Cm reaction. Recoil effects were considered
as described before. The α-energy values will be given in
sect. 3.2.
The high energy branch was calibrated with αparti-
cles and the energy of fission fragments from the 252Cf
source. For adapting the energy of this external source to
the energy from detector implanted events, data were used
from previous experiments based on SF of 252No [18].
In our previous study of the reaction 48Ca + 238 U→
286Cn∗it was expected that α-decay chains will end by
SF [18]. Therefore, the signature of SF events from nuclei
implanted close to the surface of the stop detector was
investigated in detail. The main features dealt with the
response of the system in the case of fission fragments
escaping from the stop detector and the emission of γ-
rays in coincidence with SF. The results obtained there
were applied now for determining the energies of SF events
measured in the present experiment.
Here and throughout this work we used the computer
code SRIM [33] for the calculation of energy loss and par-
ticle ranges. Data for isotopes beyond uranium were ob-
tained by extrapolation of values for lighter nuclei.
The detection of correlated events is primarily based
on agreement of the measured positions between implant-
ed ERs and subsequent αdecays or SF. A position reso-
lution of 0.4 mm (FWHM) was determined between ERs
and αparticles fully absorbed in the stop detector. For
ERs and escaping αparticles depositing an energy be-
tween 0.2 and 2MeV in the stop detector, the resolution
was 2.9 mm. Correlation between ERs and SF resulted in
a 1.2 mm position resolution. These values were measured
at high statistics in an irradiation of 206PbS with 48 Ca
ions producing αemitting and spontaneously fissioning
nuclei 252No [18]. The energy dependence of the position
resolution is described in [49].
From each decay, αor SF, we measure and accumu-
late a total of 41 parameters. The most important ones
are energy, position time, and assignment to the detector
strips or segments. For an unambiguous assignment of the

Page 10 of 23 Eur. Phys. J. A (2012) 48:62
strip or segment number two discriminator levels are used,
one at 100 keV and the other at 4 MeV. In this way it
is arranged so that high-energy signals can be assigned
unambiguously, which otherwise resulted in multiple trig-
gers at low discriminator levels, but also low-energy signals
from escaped αparticles down to 100 keV can simultane-
ously be assigned unambiguously. Some of the information
is redundant due to the overlapping energy ranges. The
low energy discriminator level also allows for detection of
conversion electrons, which opens the possibility for spec-
troscopy studies using coincidences with γ-orX-rays.
In addition, beam properties are registered in three pa-
rameters. The sum of this information we call an “event”,
a sequence of correlated events an “event chain” or “decay
chain”. In the case of a SHN the event chain consists of
an implanted nucleus, subsequent αdecays, and usually
SF terminating the chain. All parameters are registered
when they occur within a time window of 5 μs. A dead
time of 16 μs follows, after which the system is ready for
registration of the next event.
3Results
3.1 Cross-sections
A guideline for choosing the beam energy was given by
the data measured previously at FLNR. There, the low-
energy part of the excitation function of the reaction
48Ca + 248 Cm →296116∗was investigated [24–30]. The
highest energy studied resulted in E∗=38.9 MeV. At
this energy six decay chains were measured and assigned
to 292116 resulting in a cross-section of (3.3+2.5
−1.4)pbfor
the 4n channel. Also at the same energy two chains from
the 3n channel were measured resulting in a cross-section
of (1.1+1.7
−0.7) pb. This experiment was performed in April–
May, 2004 [30]. The data are shown in fig. 4 together with
excitation functions for the 2n to 5n evaporation channels
calculated in [50].
At lower beam energies three irradiations were per-
formed in Dubna in June–July and November–December
2000 and January and April–May 2001 [25,27, 30]. At
E∗=30.5 MeV a cross-section limit of 0.9 pb was reached.
At E∗=33.0 MeV, three decay chains were measured
resulting in a cross-section of (0.5+0.5
−0.26) pb, which were
assigned to 293116. At the same excitation energy a cross-
section limit of 0.3 pb was obtained for the 4n channel.
These data are also shown in fig. 4.
In our experiment we investigated the reaction 48Ca +
248Cm →296 116∗at the two higher energies of E∗=40.9
and 45.0 MeV. Measuring times, mean currents and beam
doses are given in table 1.
Theoretical studies proposed an increase of the cross-
section at E∗=40.9MeV [50]. At this energy we de-
tected six decay chains, four of them were assigned to
the 4n channel resulting in a cross-section of (3.4+2.7
−1.6)pb
and one from the other two events to the 3n channel. An
assignment of the other event (chain 1) to the 3n chan-
nel is tentative. Alternative assignments are presented in
Excitation energy / MeV
Cross-section / pb
FLNR
GSI
25 30 35 40 45 50 55
0.1
0.5
5
1
10
3n 4n
2n 5n
Fig. 4. Cross-sections and cross-section limits of the reac-
tion 48Ca + 248 Cm →296116∗measured in [25, 30] and in this
work. The data for synthesis of 293116 (3n channel, triangles)
and 292116 (4n channel, squares) are shown. The experimen-
tal data are compared with results of theoretical calculations
given in [50].
sect. 4. Therefore we present here the cross-section of
(0.9+2.1
−0.7) pb, which is valid for the event definitely as-
signed to 293116. No events were observed in the second
part of the experiment at E∗=45.0 MeV, resulting in a
one-event cross-section limit of 1.6pb (cross-section if one
event would have been observed). Our new cross-section
data are shown together with the previously measured
data in fig. 4.
The decay chains assigned to the 3n and 4n evapora-
tion channels, respectively, are shown in fig. 5. The decay
data given there are best values derived from the discus-
sion presented in sects. 3.2 and 4. Considered were the lit-
erature data on the synthesis of element 114 [20,51] and
116 [29,30] as well as our data from the present work.
Also shown are the decay data of chain 1 measured in our
experiment, whose assignment to 293116 is tentative.
3.2 Decay chains
Decay chains from element 116 isotopes are expected to
terminate by SF. The high-energy signals from SF provide
an easy but nevertheless safe approach for detection of
decay chains. In our case the time structure of the beam
in combination with the electronic signal processing al-
lows for measuring energy spectra in a range from 50 to
320 MeV during the beam pauses almost background free.
Such energy spectra taken during the 15 ms beam pauses
are shown in fig. 6 for the two beam energies correspond-
ing to excitation energies of 40.9 and 45.0 MeV, respec-
tively. From the eight events measured at E∗=40.9MeV
with energies above 100 MeV, six turned out to be corre-
lated with preceding αparticles and implanted ERs. These

Eur. Phys. J. A (2012) 48:62 Page 11 of 23
4n
292
116
288
114
284
Cn
9.927 MeV
0.58 + 0.14 s
TKE = 231 MeV
94 + 21 ms
2
1
10.502 MeV
20 + 96 ms
293
116
289
114
10.029 MeV
0.28 + 1.35 s
9.707 MeV
4.0 + 19.1 s
2
1
3
9.315 MeV
0.25 + 1.18 s
277
Hs
4
TKE = 210 MeV
34 + 164 s
285
Cn
chain 1
9
0.13
0.11
1.8
16
281
Ds
10.625 MeV
13 + 7 ms
3
0.09
15
9.184 MeV
32 + 12 s
3n
10.533 MeV
57 + 46 ms
293
116
289
114
9.818 MeV
1.9 + 0.8 s
2
1
3
8.727 MeV
TKE = 228 MeV
14 + 6 s
277
Hs
4
TKE = 211 MeV
3 + 15 ms
285
Cn
281
Ds
3
17
0.4
7
1
Fig. 5. Decay chains of 292116 (4n channel) and 293116 (3n
channel) measured in the reaction 48Ca + 248 Cm →296116∗.
The given αenergies are from this work (see also figs. 7 and 8)
except for the αenergy of 281Ds, which was taken from [20].
TKE values of 284Cn and 281Ds were taken from [14] and that of
277Hs (3n channel) from [20]. Half-lives T1/2are average values
deduced from the measured lifetimes τlisted in tables 4 and 5
and the literature data [29,30] shown in figs. 9 and 10. Half-
lives measured in the reaction 48Ca + 244 Pu →292114∗[20, 51]
are included. The assignment to 293116 of chain 1 measured in
this work is tentative.
events are numbered according to the time of their regis-
tration from 1 to 6. For the other two SF events and also
the two high-energy events observed at E∗=45.0MeV
no correlated αdecays could be found. We discuss the as-
signment of these events and search for SF events during
the beam pulse and α-decay chains without terminating
by SF at the end of this section.
The assignment of the high-energy signals to SF events
is corroborated by the additional coinciding parameters
as shown in table 2. There, the SF events are grouped
according to their assignment to decay chains from 293116
(event 1 only tentatively and event 6), 292116 (events 2
to 5), and signals which were not correlated to preceding
αdecays (events marked by date).
In agreement with the observations presented in [18],
we measured coincident fragments in the stop and the
box detector in 3 of 10 cases. The time difference (Δt +
t0)stop-box between the two signals is located well within
the range of prompt coincidences at t0,stop-box = (1700 ±
100) ns. Also in agreement is the detection of coincident
Fig. 6. Total number of high-energy events measured in the
beam pauses during irradiations of 248 Cm with 48Ca ions at
beam energies of 265.4 MeV (16.6 days, top) and 270.2 MeV
(11.6 days, bottom). Marked with numbers are the six SF
events terminating decay chains from 292116 (events 2 to 5)
and 293116 (event 1 tentatively, see sect. 4, and event 6). Four
events marked by dates are assigned to transfer products or
unidentified background events. The energies are the uncor-
rected values as given in column 3 of table 2.
γ-rays in 7 of 10 cases, and also in these cases the measured
time difference (Δt +t0)SF-γbetween SF and γdetection
is well within the range of the prompt coincident events
at t0,SF -γ= (1040 ±60) ns. The time signal in the case of
the 55.0 keV γof event 3 was lost due to the higher trigger
level in the faster timing branch.
The total kinetic energy (TKE) of the SF events was
determined as in [18]. For correction of the energy deficits,
values of 35.7 and 24.9MeV were added to the measured
energies in the cases that both fragments were measured in
the stop and the box detector (2 cases) and that both frag-
ments were stopped in the stop detector (4 cases). For the
unassigned SF events a TKE value can not be given with-
out knowing a sufficiently accurate implantation depth.
The properties of the SF events are such that all of
them could be due to decays of isotopes produced in the
reaction 48Ca + 248 Cm. Therefore, we searched for pre-
ceding position-correlated αparticles and implanted ERs.
Such correlated events were found in the cases marked by
1 to 6, out of the total of 10 high-energy events. In ta-
bles 3–5 the most significant parameters of the correlated
ER, αand SF events are presented.
The properties of the implanted ERs are shown in ta-
ble 3. The date and time when the ERs were implanted

Page 12 of 23 Eur. Phys. J. A (2012) 48:62
Table 2. Properties of high-energy events measured during beam pauses in the reaction 48Ca + 248 Cm →296 116∗at beam
energy of 265.4 MeV (E∗=40.9 MeV). The SF events are grouped according to their assignment to decay chains from 293116
(event 6), 292116 (events 2 to 5), and signals which were not correlated to preceding αdecays (events marked by date). The
assignment of event 1 is tentative. At the beam energy of 270.2 MeV (E∗=45.0 MeV), only two events were measured observed
on July 18 and 24. See text for an explanation of the columns.
SF event tMP Estop Ebox TKE (Δt +t0)stop-box Eγ(Δt +t0)SF-γFissioning
no. /ms /MeV; strip no. /MeV; box no. /MeV /ns /keV; Ge no. /ns isotope
1 14.684 185.4; 4 – 210 ±20 – 696.4; 1 984 (277Hs)
6 14.256 176.2; 8 13.3; 5 226 ±20 1691 – – 281Ds
2 10.375 154.0; 4 16.8; 3 207 ±20 1662 180.3; 2 1086 284Cn
3 8.684 195.2; 2 – 220 ±20 – 55.0; 1 – 284 Cn
83.2; 3 1118
4 7.081 193.4; 15 – 218 ±20 – 350.4; 4 1005 284Cn
5 12.948 181.4; 12 – 206 ±20 – 496.1; 4 1004 284 Cn
02/07 7.206 177.9; 15 – – – 97.8; 4 1066 (244mAm)
08/07 19.342 137.3; 4 15.5; 1 – 1677 352.5; 2 1081 –
406.4; 4 1003
18/07 13.444 93.0; 8 – – – – – –
24/07 7.798 161.0; 11 – – – – – –
Table 3. Properties of detector-implanted ER events assigned to 293116 (ER-event 6, upper part) and 292116 (lower part)
measured in the fusion reaction 48Ca + 248 Cm →296116∗at an excitation energy of 40.9 MeV. The assignment of ER-event 1
to 293116 is tentative. See text for an explanation of the columns.
ER event date; time (−Δt +t0)TOF vER/c dMylar EER ytop tb–te(MP) tMP tchain Ntot
no. /ns /% μm /MeV /mm; strip no. /ms /ms /μs/tchain
1 02/07; 01:07 267.4 1.68 3.5 10 6.66; 4 0.150–5.150 0.972 55,973,712 306
6 08/07; 19:54 266.0 1.67 0.0 21 11.08; 8 0.120–5.620 4.326 36,989,930 502
2 02/07; 01:52 265.0 1.66 3.5 9 10.96; 4 0.150–5.150 3.536 1,046,839 5
3 03/07; 20:01 267.0 1.68 0.0 18 28.26; 2 0.150–5.150 3.754 1,504,930 36
4 07/07; 00:10 264.4 1.66 0.0 22 25.05; 15 0.120–5.620 5.484 401,597 7
5 07/07; 09:01 266.6 1.67 0.0 21 27.93; 12 0.120–5.620 5.338 107,610 0
into the stop detector is shown in column 2. The measured
TOF is listed in column 3. The negative sign denotes the
fact that the second TOF detector was used for starting
the time-to-amplitude converter. All values are within a
range of ±1.7 ns around a mean value of 266.0 ns. The vari-
ance corresponds to the time resolution of the detectors.
Using the known velocities of background projectiles hav-
ing the full beam energy, an absolute time calibration was
performed, which resulted in ER velocities around a mean
value of vER =0.0167 ×c(column 4), in good agreement
with the value of 0.0164 ×cestimated from the reaction
kinematics.
During parts of the experiment, degrader foils of 3.5
and 4.5 μm Mylar were used in front of the silicon detector
array, but after the TOF detectors. In the case of chains 1
and 2 the 3.5 μm foil was in use (column 5), which resulted
in a reduction of the measured implantation energy by
about 10 MeV (column 6).
The ER events, the strip number (horizontal position,
see also fig. 1 in [18]), and the vertical position obtained
from the top electrode, which results in a value of 0mm
if the particle is implanted at the bottom and 35 mm if
implanted at the top is shown in column 7 of table 3. A
redundant value was obtained from the bottom electrode,
which in all cases agreed with the one from the top elec-
trode. Both values are given in tables 4 and 5.
The time range of the beam macropulse within the
logic period of 20 ms is listed in column 8. In all cases the

Eur. Phys. J. A (2012) 48:62 Page 13 of 23
time of the events assigned to ER implantations, given
in column 9, overlaps with the width of the beam pulse,
which is in addition to the response of the two TOF de-
tectors another necessary condition for a true ER event.
In column 10, we show the full time durations of the
chains from the implantation of the ERs up to the SF
events. Clearly, the chains assigned to 293116 and to 292116
differ significantly. Finally, in column 11, the total num-
ber of events minus the events of the chain is listed, which
occurred on the given strip independent of the vertical
position during the duration of the chain. This number
reflects the background conditions which are mainly given
by scattered low-energy projectiles having the same ve-
locities as the CN and, therefore, can pass the velocity
filter.
The properties of the correlated αdecays are listed in
table 4 for events assigned to 292116 and in table 5 for
293116, together with data from the implanted ER and
SF events. Chain 6 assigned to 293116 consisted of three
sequential αdecays before the SF. In the case of chain
1fourαdecays were measured. This tentatively assigned
chain is given in table 5 for reasons of comparison. In
the case of 292116 two sequential αdecays were measured
before the SF. For the ER, each of the αdecays and the SF
event of a chain we list the energy, the vertical position
determined from the top and the bottom electrode, the
time difference to the preceding event, and the assigned
isotope. As a matter of course the measured strip number
for the αdecays agrees with that of the implanted ER
andSFevent.Inthecaseofα2of chain 5 (table 4) the
position information from the bottom electrode was lost.
For this escaped αparticle an energy of only 1.17 MeV was
deposited in the stop detector resulting in a position signal
of less than 100 mV, which was below the discriminator
level of the corresponding ADC.
From a total of 15 αdecays, 10 were fully contained in
the stop detector. Their energy could be determined with
an accuracy of ±15 keV. Five αparticles escaped in the
backward direction. Two of them were detected by the box
detector. Their energy was reconstructed from the energy
loss in the stop detector, 1.873 and 1.171 MeV, and the
residual energy in the box detector, 8.757 (segment 21)
and 8.666 MeV (segment 13), for α1and α2of chain 5,
respectively. The energy loss in the inactive surface layers
was considered by using for calibration similar αevents
from isotopes produced in transfer reactions. The uncer-
tainty of the energy of these two events is ±50 keV. The
measured time between signals from the stop and box de-
tectors was (Δt +t0)stop-box of 1619 and 1612 ns, respec-
tively, which agrees with coincident events.
In three cases αparticles escaped. However, from the
deposited energies between 1.4 and 2.7 MeV in the stop
detector, the position and lifetime could still be deter-
mined. Using a relation between αenergies and lifetimes
based on the WKB method, which is discussed in sect. 4,
we determined the αenergies from the measured lifetimes.
The error bars were determined from the upper and lower
limits of the lifetimes valid for a one-event statistics pre-
sented in [52]. The obtained energies for these three cases
plus one from the data given in [51] are marked by the
arrows shown in fig. 7, which presents a comparison of
measured energy data. The ratio of αparticles measured
with full energy in the stop detector, detected as coinci-
dent events in stop and box detector and those of escaped
ones is in agreement with the geometrical conditions of
the detector system.
In one case, namely α2of chain 2, a γ-ray with an en-
ergy of 260.9 keV from Ge crystal 4 was measured within
the coincidence window of 5 μs. However, the measured
time difference of 2450 ns differs considerably from the
time for prompt events which is located at (t+t0)α-γof
1000 to 1100 ns, see also column 8 of table 2. Therefore,
we consider this α-γcoincidence as a chance event.
The parameters band bin column 1 of tables 4 and 5
indicate, if an event occurred during the beam pulse or
during the pause, respectively. Without considering the
first short-lived αdecays of 292116, which have a high
probability to occur in the same macropulse as the cor-
responding ER, we measured three α’s during the pulse
and eight α’s during the pause. This ratio is in agreement
with the 5 and 15 ms durations of pulse and pause.
Due to the difference of chain 1 and 6 listed in table 5,
we turned our attention to the third αdecay, α3, of chain
1 which occurred during the pulse. In a first preliminary
analysis [53], this αevent was considered as being acciden-
tal, despite the fact that its position is well in agreement
with the positions of the other events of the chain. How-
ever, a quantitative statistical analysis resulted in a low
probability for this earlier assumption.
For the analysis we took into account a 55.6 hours pe-
riod of irradiation from June 30, 15:00 h to July 2, 22:00 h
with chain 1 occurring on July 2, 01:07 h. During this pe-
riod the beam intensity was stable at 750 pnA.
In the energy range considered from 9.0 to 10.5 MeV
and within a position window from 6.43 mm (0.2 mm
less then the lowest value measured for α2) to 6.92 mm
(0.2 mm more than the highest value measured for α3), six
events occurred in addition to the three events assigned to
chain 1 (the energy of α1from chain 1 is slightly outside
of the considered energy range).
The six chance events occurred during the pulse, but
had not triggered the TOF detectors. Except for α2and
α4of chain 1, no additional signals were measured during
the beam pauses during the 55.6 hours counting period.
The mean time between the six events is 9.26 h, the events
closest to the chain occurred 1.90 h before and 5.68 h after
occurrence of the chain. The probability that an event oc-
curs within the given large energy window and during the
period of chain 1 between ER and SF, 56 s, is calculated
as ratio of this period to the measured mean distance of
events. The result is 0.17%. That an event occurs between
ER and the last αdecay, 6.56 s, has a probability of 0.02%.
These low probabilities are strong arguments for assigning
α3to a true member of the decay chain.
In four cases high energy SF-like events were measured
during beam pauses, but no preceding αdecays could be
detected within a time window of ten minutes, see dis-
cussion on chance αevents given before. These events are
marked with the date of their registration in table 2. Two

Page 14 of 23 Eur. Phys. J. A (2012) 48:62
Table 4. Properties of four event chains assigned to the de-
cay of 292116 measured in the fusion reaction 48 Ca + 248Cm →
296116∗at a mean excitation energy of 40.9 MeV. Chain num-
bers are given in brackets. The characters band bindicate if
the decay was detected during beam-on or beam-off periods, re-
spectively. Also marked are events for which coincidences with
γ-rays were measured. The coincidence event α2-γ4of chain 2
is interpreted as chance event, see text. An explanation of the
columns is also given in the text.
Chain(no.) Ey
top–ybot Δt Isotope
/MeV /mm /ms
ER(2) 9.0 10.96–24.12 292116
α1(b) 2.715 11.25–23.64 3.3 292116
α2(b, γ4)9.959 ±0.015 11.06–23.97 994 288114
SF(b, γ2) 207 ±20 9.79–24.39 50 284Cn
ER(3) 18.0 28.26–6.70 292116
α1(b)10.625 ±0.015 28.32–6.71 0.076 292116
α2(b)9.896 ±0.015 28.28–6.74 1,236 288114
SF(b, γ1,γ
3) 220 ±20 28.68–6.23 269 284Cn
ER(4) 22.3 25.05–10.04 292 116
α1(b) 1.4 24.37–10.61 28.8 292 116
α2(b) 1.8 24.92–10.01 252 288 114
SF(b, γ4) 218 ±20 27.78–6.40 121 284 Cn
ER(5) 20.8 27.93–7.19 292116
α1(b)10.630 ±0.050 28.80–6.08 11.6 292 116
α2(b)9.837 ±0.050 28.36– – 72 288 114
SF(b, γ4) 206 ±20 28.34–6.29 25 284 Cn
Table 5. Same as table 4, but for the two decay chains con-
sisting of an implanted ER, four, respectively, three αdecays
and SF. Chain 6 is in agreement with literature data given for
293116 [30]. The assignment of chain 1 is tentative, see sect. 4.
Chain(no.) Ey
top–ybot Δt Isotope
/MeV /mm /ms
ER(1) 10.1 6.66–28.32 (293116)
α1(b)10.502 ±0.015 6.78–28.18 29 (293116)
α2(b)10.029 ±0.015 6.63–28.33 406 (289 114)
α3(b)9.707 ±0.015 6.72–28.33 5,766 (285 Cn)
α4(b)9.315 ±0.015 6.71–28.34 356 (281 Ds)
SF(b, γ1) 210 ±20 5.88–28.04 49,417 (277Hs)
ER(6) 21.0 11.08–23.82 293116
α1(b)10.564 ±0.015 10.70–24.27 55 293116
α2(b)9.818 ±0.015 10.74–24.30 118 289114
α3(b)9.184 ±0.015 10.69–24.39 30,857 285Cn
SF(b) 226 ±20 10.53–23.38 5,960 281 Ds
Fig. 7. Comparison of αenergies of event chains measured
in this experiment with literature data for the decay of 292116
produced in the reaction 48 Ca + 248Cm and of 288 114 produced
in the reaction 48Ca + 244 Pu. The chains terminate at 284Cn by
SF. Symbols with bold error bars mark α’s completely stopped
in the stop detector, thin larger error bars mark energies ob-
tained by summing of signals in stop and box detectors. Ar-
rows mark escaped α’s depositing only an energy loss in the
stop detector from which, however, detector position and time
can be deduced. Position of the arrows and their beginning and
end mark the energies determined from the measured lifetimes
including error bars based on a one-event statistics [52]. The
dashed lines mark missing αdecays. The vertical thin lines are
drawn through our measured αenergies for easier comparison
with the literature data.
Fig. 8. Similar to fig. 7, but for event chains starting at 293116
and 289114, respectively. The assignment of chain 1 measured
in this work to 293116 is tentative, see sect. 4, and given here for
reasons of comparison. The different denotations of the error
bars are explained in the caption to fig. 7.

Eur. Phys. J. A (2012) 48:62 Page 15 of 23
events occurred at the lower beam energy and two at the
higher energy, see table 1. The parameters of the events
are also listed. In two cases (events from July 2 and July 8)
the assignment to SF is corroborated by the coincident γ
rays and the detection of a coincident fragment in the box
detector (event from July 8).
A search for a correlated implanted recoil nucleus
was successful in the case of the event from July 2. At
tMP =4.526 ms, 2.680 ms before the SF event, an im-
planted nucleus was measured at an energy of 55.22 MeV
(the 3.5 μm Mylar degrader foil was in use) having a TOF
value (−Δt +t0)TOF of 283.6 ns. The vertical position
in strip no. 15 was at ytop =20.71 mm and that of the
correlated SF event at 20.98mm. From the implantation
energy and the absolute TOF value, we obtained a rough
mass number of 236±17. This value is in good agreement
with fission isomers produced in transfer reactions. With
respect to the measured half-life of (1.9+8.9
−0.8) ms, possible
fissioning isotopes are 240mAm (T1/2=0.94 ms), 242mAm
(14 ms), and 244mAm (0.9 ms). Taking into account the
production cross-section, a proton pick-up from the 3.1%
target impurity of 246Cm and emission of one neutron re-
sulting in 244mAm or a proton pick-up from 248Cm and
subsequent evaporation of three neutrons also resulting in
244mAm is most likely. Due to lack of a calibration proce-
dure for this deeper implanted fissioning nucleus, we can
not determine a sufficiently accurate TKE value.
For the fission event from July 8 a correlated implan-
tation in the range of milliseconds was not measured. The
two closest possible recoils were measured 18 and 46s be-
fore the fission event within a position window of ±2mm
relative to ytop =8.34 mm of the fission event and within
an energy window from 7 to 50 MeV. The recoil data of
Erecoil = 18 and 20 MeV and (−Δt +t0)TOF = 267.2and
269.0 ns, respectively, are in good agreement with data
measured for heavy ERs as listed in table 3 (a degrader
foil was not in use at the time of this event). However,
a statistical analysis shows that at these relatively long
time periods, the absence of correlated αdecays and the
background rate given by the beam current used, the lim-
its of the method searching for correlated recoil-fission
chains without intermediate αdecay are reached. Dur-
ing a measuring time of 3.3 h, we detected 513 possible
recoils within the energy and position windows as given
before and within a TOF window (−Δt +t0)TOF = 252
to 279 ns. The mean distance between detected recoils is
23 s, so that the first two recoils before the fission event
from July 8 are likely due to chance events.
After the main experiment the same stop detector was
used in an investigation of transfer products, see table 1.
The results of these measurements are published else-
where [54]. Due to SHIP settings for transfer products
and elastically scattered target nuclei, a relatively high
number of fissioning nuclei was implanted. Among these
fissioning nuclei we observed also a fraction with a half-life
longer than five days. A detailed analysis of these events is
in progress. It is reasonable that the event from July 8 be-
longs to this group, despite strong suppression using SHIP
settings for separation of ERs.
The two high-energy events observed at the excitation
energy of 40.5 MeV are also likely due to fission events,
because they were detected during the beam pauses. The
absence of coincident γ-rays and second-fission fragments
in part of the data is reasonable taking into account the
results of the study of the detector response on fission
events performed in [18].
A search for correlated implanted recoil nuclei was not
unambiguous. Similarly, as in the case of the event from
July 8, the time difference of observed recoils to the fission
events was in the range of the mean distances between
chance events using the same conditions as given before.
Due to lower background in this part of the experiment,
the mean distance was 50 s and the two possible recoils
next to the fission events were measured at distances of
17 and 33 s in the case of the event from July 18 and 2.4
and 44 s in the case of the event from July 24.
The total number of possible SF events during beam
pulses is determined by the background rate and the anti-
coincidence conditions. At the beam energy of 265.4 MeV
we measured 39,827 events in the energy range from 100
to 200 MeV during beam pulse, 162 had no signals from
either of the two TOF or the veto detectors and, there-
fore, are prospective fission events. The same numbers at
the beam energy of 270.2 MeV are 32634 during pulse and
219 in anti-coincidence. The corresponding beam doses are
given in table 1. For all of these events a search for corre-
lated implanted ERs or preceding αdecays was performed.
However, in no case an event chain was observed with
features which could not be explained by chance events
within the limits as discussed before. Similarly, a search
was performed for correlated α-αevents occurring during
beam pulses or pauses without the condition of a termi-
nating SF event. Except for the six chains listed in tables 4
and 5 and well-known αdecays from transfer products, we
did not observe decay chains which could not be explained
by chance events.
4 Discussion
4.1 Comparison with literature data
In this subsection we compare our results with data
measured previously for the reaction 48Ca + 248 Cm in
Dubna [25,29,30]. In addition, the reaction 48Ca + 244Pu
which results in isotopes of the daughter element 114, was
studied in several experiments at Dubna [24,27,51,55–57]
and in May–June 2009 also at the GSI gas-filled sepa-
rator [20]. A comparison of the measured αenergies is
presented for the decays of 292116 and 288114 in fig. 7 and
for 293116 and 289 114 in fig. 8. The measured lifetimes are
compared in figs. 9 and 10, respectively. The different de-
notations of the error bars in the case of the energy spectra
is explained in the caption to fig. 7.
The comparison reveals that in the case of the even-
even isotope 292116 and its daughter 288114 good agree-
ment exists between all measured energies within error
bars. The two groups of energies for 288114 differing by
63 keV are not yet statistically consolidated. In addition,

Page 16 of 23 Eur. Phys. J. A (2012) 48:62
Fig. 9. Measured lifetimes of individual events assigned to
292116 and its α-decaying daughter 288114. The decay chains
terminate by SF of 284Cn. Shown are data from the reaction
48Ca + 248 Cm measured in this work (first row from top) and
in [29,30] (second row from top) and from the reaction 48Ca+
244Pu measured in [20] and [51] (last two rows). The deduced
mean value of the lifetime τis the center of gravity of the
data points. The corresponding probability density of finding
an event at a certain time is shown by the line. This function
and the error bars were determined according to the study of
error analysis in the case of poor statistics published in [52].
Numbers of the events from our work correspond to the num-
bers of the event chains as given in tables 2 to 4. The units on
the ordinate are arbitrary.
another argument makes it unlikely to interpret the 63 keV
energy difference as being due to rotational properties. As
discussed in [58], the probability for population of a 2+
rotational level by αdecay is only a few percent.
Nevertheless, one can speculate on the requirements
which are necessary for the creation of a rotational level at
such an excitation energy. Using the relations between β2
values and the E2+ energies, which were investigated the-
oretically for deformed heavy nuclei up to 278Cn in [58],
we estimate a β2value of about +0.20 for a 63 keV ro-
tational level in 284112. Calculated β2values based on
the macroscopic-microscopic model are +0.089 for the iso-
topes from 281112 to 285112 [59], and 0.144, 0.123, and
0.095 for 282112, 284 112, and 286112 [60]. However, us-
ing the self-consistent Skyrme-Hartree-Fock-Bogoliubov
method, β2values of +0.19 and +0.14 were calculated for
Fig. 10. Similar to fig. 9, but for the decay chains starting
at 293116 and 289 114, respectively. The assignment of chain 1
measured in this work to 293116 is tentative, see sect. 4, and
given here for reasons of comparison. The lifetimes of isotopes
of this chain are not included in the mean values.
281112 and 285 112 in [61]. This comparison reveals some
uncertainty of the calculations concerning the degree of
nuclear deformation in this region of superheavy elements
located twelve neutrons away from the closed spherical
shell at N= 184. However, the predictions are in agree-
ment with a certain degree of deformation so that, on the
long term, an experimental study of rotational properties
of nuclei in this region of SHN could be successful.

Eur. Phys. J. A (2012) 48:62 Page 17 of 23
Best energy values of the αdecays are determined from
our energy measurements having the smallest error bars.
We obtain for the decay of 292116 a value of (10.625 ±
0.015) MeV and for the decay of 288114 a mean value of
(9.927 ±0.015) MeV.
Good agreement also exists for the results of the life-
time measurements shown in fig. 9, which include the mea-
sured SF lifetimes of the terminating isotopes 284Cn. The
relatively short value of 76 μsforα1of chain 3 is located
two orders of magnitude from the mean value of 18 ms.
However, we do not see a physical reason for population
of a short lived isomer in the even-even nucleus 292116, be-
cause the energy of the decay is well in agreement with the
energies measured in [29]. Therefore, we consider the short
lifetime as being due to a statistical fluctuation. How wide
such fluctuations can be, we can see by the variation of
data points shown in fig. 10b, which cover a range of two
orders of magnitude. The mean lifetime values obtained
from the experimental data are given in the figures.
In this context we would like to mention that also the
occurrence of events during the experiments revealed a
spread of more than two orders of magnitude. At relatively
constant beam intensity the six events listed in table 3
occurred every 66 hours on the average; however, the first
two events occurred within 45 min. As a matter of course
this is also in agreement with a statistical distribution of
the events.
The decay chains from 293116 and 289114 are especially
interesting from the spectroscopic point of view. The pre-
vailing number of energy data shown in fig. 8 and of life-
time data shown in fig. 10 measured in [29,30] and in our
work for the decay chains of 293116 and in [20, 51] for the
decay chains of 289114 are in good agreement. The sys-
tematically higher values of the αenergies given in [20]
are obviously related with not considering recoil effects
using the 11.65 MeV αtransition of 212Po for calibration.
The good agreement of data from both chains, 292116
and 293116, establishes the validity of the data initially
measured in Dubna and represents an independent confir-
mation in two experiments at another laboratory, ref. [20]
and this work. In this respect, the SHIP data are especially
important, because a different type of separator was used,
which suppresses the αxn channels by a factor of five rel-
ative to xn channels. This is an important property for
safely assigning the decay chains to xn channels. Alter-
native production channels of the newly measured decays
in hot fusion reactions by αevaporation or even transfer
reactions and erroneous assignment to xn channels, which
was discussed from time to time, is therefore excluded. En-
ergetically possible pxn channels for almost all of the ER
studied in various reactions so far, can also be ruled out.
These channels would result in odd and even elements,
respectively, dependent on the use of even or odd element
targets, and isotopes having an odd or even number of
neutrons. The systematics of lifetimes of the terminating
fission events known for a relatively large number of nuclei
so far, would be in complete disagreement with the sys-
tematics of fission lifetimes and hindrance factors known
for even, odd and odd-odd nuclei [62]. This statement only
holds for decay chains in regions where sufficiently enough
even and odd isotopes are available for systematic compar-
ison as it is the case for the nuclei studied here.
4.2 Evidence of isomeric states
Although thirteen of the fourteen decay chains assigned to
293116 and 289 114 are in good agreement, chain 1 of our
experiment differs significantly, which deserves special at-
tention. The chain starts with an αdecay of 10.502 MeV,
which is in good agreement with the αdecays assigned
to 293116, see fig. 8. Also the lifetime of 29 ms is in agree-
ment with the mean value of the known decays, see fig. 10.
However, the energies of the subsequent two αdecays are
significantly higher than the literature data and our chain
6, 10.029 MeV instead of 9.818 MeV measured for 289 114
and 9.707 MeV instead of 9.184 MeV measured for 285 Cn.
So far, αdecay of 281Ds was observed only once having
an energy of 8.727 MeV [20], whereas the fourth αdecay
of our chain 1 has an energy of 9.315 MeV.
The lifetimes of αdecays 2, 3, and 4 of chain 1 are
comparable to those of chain 6 and the literature data
within values set by statistical fluctuations. They do not
contradict an assignment of chain 1 to 293116. However,
for the lifetime of the terminating SF event assigned to
277Hs a lifetime of 4.5 ms was measured in [20], whereas
the fission event terminating chain 1 has a lifetime of 49 s.
One reasonable explanation for the different properties
of chain 1 is provided by the single particle energy spec-
trum as calculated and discussed in [61,63–65]. In par-
ticular, for the decay chain from 293116, the one-quasi-
particle excitations were calculated in [61]. The calcula-
tional results, together with the presently known experi-
mental data are shown in fig. 11. Although an assignment
of the measured αtransitions is not directly possible, the
existence of high-spin and low-spin levels near the ground
state allows for construction of isomeric states and even
the possibility of parallel chains decaying preferably be-
tween states of similar angular momentum with similar
lifetimes, which would be in agreement with the experi-
mental observation.
Concerning the SF of 277Hs, it was argued in [20] that
the short half-life of (3+15
−1) ms is due to a more severe drop
of SF half-lives than calculated in [60]. In this theoretical
study SF half-lives of 46 ms and 0.98 ms were calculated
for the neighboring even-even isotopes 276Hs and 278Hs, re-
spectively. Comparing the geometric mean of 6.7 ms of the
two values with our measured value of T1/2= (34+166
−16 )s,
we deduce a hindrance factor of 5100+25000
−2400 . An odd neu-
tron hindrance factor of about three orders of magnitude
is a reasonable value expected from systematics [62]. If
the arguments presented in [20] as given before are cor-
rect, the hindrance factor in our case would be several
orders of magnitude larger. The conclusion in this case
would be that the long half-life is related to a high-spin
isomer, for which the fission barrier is considerably in-
creased compared to an isomer with low spin. This effect,
usually named specialization energy, is due to the strong
rise of high-spin Nilsson energy levels at prolate deforma-
tion. Its influence on the fission barrier was studied in [66].

Page 18 of 23 Eur. Phys. J. A (2012) 48:62
E
2= 0.09
281Ds
13/2–[716]
5/2+[613]
3/2+[611]
1/2+[611]
11/2+[606]
9/2+[604]
9/2+[604]
11/2+[606]
15/2–[707]
3/2+[611]
1/2+[611]
5/2+[602]
7/2+[604]
1/2+[611]
15/2–[707]
5/2+[602]
7/2+[604]
1/2+[611]
15/2–[707]
289114
293116
285Cn
0
200
400
600
800
1000
E/keV 10.47 MeV [59]
9.64 MeV [59]
8.88 MeV [59]
10.71, chain 6
and [20,28,29]
(10.65, chain 1)
9.96, chain 6 and [20,28,29]
(10.17, chain 1)
9.31, chain 6 and [20,28,29]
(9.84, chain 1)
277Hs
3/2+[611]
5/2+[613]
9/2+[604]
1/2+[611]
QĮ,theory = 9.32 MeV [59]
QĮ,exp. = 8.85, [20]
(Q
˞
,exp. = 9.45, chain 1 [this work])
13/2–[716]
E
2= 0.21
E
2= 0.19
E
2= 0.14
E
2= 0.12
Fig. 11. Calculated quasiparticle excitations and ground-state quadrupole deformation of the decay chain starting at the odd-N
nucleus 293 116 [61]. Levels with large differences of angular momentum near the ground state could result in isomeric states
and a complex decay scheme. Therefore, lines connecting the ground states represent the Qαvalues as calculated in [61]. The
intensities of αtransitions will be determined by energies and configurations of the involved levels. Experimental data are also
given, but could not yet be assigned to certain levels.
The short half-life could then be assigned to a low spin iso-
mer. Such an interpretation seems reasonable on the basis
of the calculated level scheme for 277Hs shown in fig. 11.
Predicted Qαvalues for 277Hs are in the range from
8.7 to 9.0 MeV, see fig. 12 and discussion below. The de-
duced half-life is between 69 s and 7.5 s, respectively, for
a transition unhindered by angular momentum (Δl = 0).
This value is three orders of magnitude longer than the
3.1 ms SF half-life measured in [20] and in agreement with
an isomer dominantly decaying by SF.
The calculated half-lives would be also in agreement
with an SF branch of about 50% in the case of a possible
T1/2= 34 s fissioning isomer in 277Hs. In the case of an
angular momentum hindrance of the αdecay, as one would
expect from the discussion given before, the partial αhalf-
life could be significantly longer. At Δl = 4 which would
correspond to a decay from a 9/2+isomer in 277Hs to a
1/2+state in 273Sg, the calculated half-lives are 310 and
33 s for the Qαvalues of 8.7 and 9.0MeV as given before.
In the case of a 13/2−to 1/2+transition with Δl =7the
calculated half-lives are 4,420 and 470 s.
For calculation of the partial αhalf-lives from αen-
ergies we calculated the reduced αwidths for the given
nuclei and normalized these to the width of the Eα=
8.784 MeV, T1/2=0.3μs decay of 212Po. The barrier pene-
trability was calculated using the WKB method (Wentzel-
Kramers-Brillouin, see [67]) with the α-nuclear potential
given in [68]. In comparison with a phenomenological for-
mula presented in [69], the half-lives calculated with the
WKB method agree slightly better with measured values
for nuclei with Z>112. The reason could be that the phe-
nomenological formula was adjusted to mainly deformed
nuclei between Z= 84 and 111, whereas the WKB method
is normalized to the αdecay of 212Po to the spherical
208Pb considered to be an ideal unhindered αtransition.
Therefore, a better agreement of half-lives could also be
a hint that nuclei under discussion here lose deformation
with increasing distance from the deformed region around

Eur. Phys. J. A (2012) 48:62 Page 19 of 23
Z= 108 and N= 162. An important advantage of the
WKB method is that a centrifugal potential can be in-
cluded quite naturally.
Isomeric states in 293116 are not the only possibility for
an assignment of chain 1. In the following we present other
options which contain weak points if compared with the-
oretical predictions; which, however, cannot be excluded
if compared only with known experimental data. We also
want to present this study here, because it shows the rich-
ness of results from nuclear physics experiments in the
region of SHN, which can be expected with sensitive tech-
niques in the future.
The energies and lifetimes of the first two αdecays are
also in agreement with the αdecays of 292116 and 288114.
Such an assignment would require an αor β+branch of
284Cn populating nuclei which are not yet known. The nu-
clei terminating the chain after two further αdecays would
be 276Hs or 276 Bh to which the measured SF half-life of
34 s has to be assigned. According to predictions [60, 70,
71], this is a reasonable value for the odd-odd nucleus, but
seems to be too long for an even-even one. However, taking
into account the rapid change of calculated SF half-lives
of hassium isotopes when only two neutrons are removed,
T1/2(276Hs) = 46 ms and T1/2(274Hs) = 5.8 s [60], a certain
caution is justified.
Four reaction channels which are energetically possi-
ble, result in ER and decay chains not yet known. The
channels 2n, pn, p2n, and p3n would produce the iso-
topes 294116, 294115, 293 115, and 292115, respectively. For
these more neutron rich nuclei located closer to N= 184
increasing fission barriers and long SF half-lives are ex-
pected [60]. As a consequence, αdecay occurs and will
proceed down to the region of hassium and bohrium iso-
topes with neutron numbers 169–171, where the SF prob-
ability dominates. An assignment of our chain 1 to one
of these nuclei cannot be excluded without further ex-
perimental studies, preferably by improving the statistical
significance and tracking the phenomenon as function of
beam energy.
4.3 Systematics of Qαvalues
Whereas fission barriers and deduced SF half-lives are dif-
ficult to calculate, the access to Qαvalues as difference of
masses of neighboring nuclei and deduced partial αhalf-
lives is easier. In the following we compare experimental
Qαvalues of established decay chains with few but repre-
sentative theoretical predictions. In figs. 12 and 13, calcu-
lated Qαvalues are shown over a wide range from element
104 to 122 for the chains passing through 290116, 292116,
293116, and 294 116. Data of this latter decay chain and
those passing through the isotopes 292115 and 293115 were
added in order to follow the trend of data into the direction
of more neutron-rich nuclei and to compare the predictions
with the data measured in our unassigned chain 1. The
more neutron-deficient chain through 290116 was added,
because the experimental data start at the presently heav-
iest element known, Z= 118 [72]. We are also aiming to
obtain a sense for the uncertainties related to predictions
on the stability of isotopes of the so far unknown elements
119 and 120, whose synthesis is presently the aim at the
research centers JINR, RIKEN, and GSI.
Two of the theoretical data shown are based on the
macroscopic-microscopic (MM) model [73–75], one on
the self-consistent mean-field model using the Skyrme-
Hartree-Fock-Bogoliubov (SHFB) method [61, 63], one on
the relativistic mean-field (RMF) model [65], and one on
a semiempirical (SE) shell-model mass equation having
Z= 126 and N= 184 as spherical proton and neutron
shells after the double magic 208Pb [76].
Obviously, the considered range of theoretical Qαval-
ues can be subdivided into three parts concerning the vari-
ations of the predictions. One for elements below darm-
stadtium, one for elements between darmstadtium and
Z= 116, and a third one for elements up to 122. Qualita-
tively, the staggering of the curves does not considerably
change for the six different α-decay chains shown includ-
ing that of the odd element isotopes.
The three regions are also related to different physi-
cal properties of the nuclei. Firstly, consider the region of
well-deformed nuclei below darmstadtium and N<170.
In this region the shape of the nuclei is determined by
stronger binding energy at large deformation due to the
compression of single-particle levels below the energy gaps
at Z= 108 and N= 162 at β2=0.25. The second region
up to element 116 for neutron numbers of the α-decay
chains considered here, is a transitional region of decreas-
ing deformation leading in the direction of the third re-
gion extending up to element 122 and beyond, which is
governed by shell effects of spherical closed shells or sub-
shells.
As far as calculated values are available, good agree-
ment exists for the Qαvalues in the region of deformed
nuclei. There, theoretical calculations could be adjusted
to experimental data which were measured in the past
in cold as well as hot-fusion reactions for isotopes up to
element 113 having slightly lower neutron numbers.
The transitional region covers the predicted shell or
subshell closures at Z= 114 and N= 172. The prominent
feature of the MM models having Z= 114 as a strong
spherical proton shell are large and increasing Qαvalues
for elements at and above 114 and slowly decreasing or
even increasing Qαvalues down to Z= 108, where the
Qαvalues start to decrease again with decreasing neutron
number. The physical reason for this dependence are the
already mentioned strong shell effects for deformed nuclei
at Z= 108 and N= 162 and for spherical nuclei at Z=
114 and N= 184. The shell effects are more pronounced
in the calculation of [59] than in [73,74].
The SHFB model [63] predicts spherical shell closures
at Z= 126 and N= 184. In the region of interest here
from Z= 110 to 120 deformation effects play an impor-
tant role. Large gaps were calculated in the single-particle
spectrum at Z= 120 and N= 172, 178 for oblate shapes
and at Z= 114, 116 and N= 174, 176 for prolate shapes.
Accordingly, the Qαvalues along the decay chains are
more structured and rise less steeply with increasing pro-
ton number than the data of the MM models.

Page 20 of 23 Eur. Phys. J. A (2012) 48:62
Fig. 12. Comparison of measured and calculated Qαvalues
of α-decay chains passing isotopes of element 116 with mass
numbers 290, 292, 293, and 294. Nuclei of these decay chains
belong to the most neutron-rich nuclei which can be produced
in the laboratory. They are of special interest with respect to a
future synthesis of so far unknown elements beyond Z= 118,
see text. Chain 1 from this work is displayed for reasons of
comparison.
Fig. 13. Same as fig. 12, but for α-decay chains passing iso-
topes of element 115 with mass numbers 292 and 293.
The RMF model used in [65] results also in a relatively
strong shell effect at Z= 114 and N= 174 for prolately
deformed nuclei. Accordingly, the Qαvalues are low for
these nuclei.
Finally, the semiempirical model [76] uses Z= 126 and
N= 184 as closed shells. Subshell effects are smoothed,
but nevertheless the Qαvalues are up to element 116 in
good agreement with most of the other results. However,
for nuclei beyond Z= 116 the Qαvalues deviate consider-
ably from the other predictions. They even decrease, when
N= 184 is approached.
Although the experimental Qαvalues are scarce we
notice that the gradient of the experimental data between
elements 114 and 116 is less than in the results of the
MM model [59] and the RMF model [65]. No irregular-
ity is observed when element 114 is crossed in the case of
the chain through 293116. From this experimental obser-
vation we conclude that at neutron numbers 172 to 176
the proton number 114 is not a strong shell or subshell.
Concerning heavier elements beyond Z= 118, the ex-
perimental data is just at the limit which could settle the
question of proton shells at Z= 120 or 126. Increasing Qα
values as predicted by the MM models would rule out shell
closures at 120 and 126. As a consequence the lifetimes of
elements beyond 120 would fall below 1μs which is the
limit of present detection methods. The elements 119 and
120 would be the last ones which could be detected in
the near future. At Z= 120 the 1 μs limit is reached at
Qα=13.3 MeV and at Z= 126 at 14.0MeV.

Eur. Phys. J. A (2012) 48:62 Page 21 of 23
A subshell closure at Z= 120 would result in relatively
long αhalf-lives of element 120. At a Qαvalue of about
11 MeV calculated for 298120 [63], see fig. 12, we obtain a
half-life of 79 ms. In addition, the αhalf-life of element 122
would also be retarded. The stronger trend to lower Qα
values of the semiempirical model would result in αhalf-
lives of 350 ms and 43 s at Qα=10.8 and 10.7 MeV [76]
for isotopes of element 120 and 126 with mass numbers
300 and 310, respectively.
In the region of SHEs, fission barriers are mainly de-
termined by ground-state shell effects. Because Qαvalues
are determined by the difference of binding energies be-
tween parent and daughter nucleus, the gradient of a Qα
systematics reflects the trend of increasing or decreasing
fission barriers. The rapidly increasing Qαvalues of the
MM models for elements above 114 is related to increasing
negative ground-state shell-correction energies and thus
decreasing fission barriers. The opposite trend is valid for
the semiempirical model.
The experimental Qαvalues reveal differences to the
theoretical data of up to 1 MeV, see figs. 12 and 13. Similar
differences must be expected for the ground-state shell-
correction energies and the fission barriers. Fission bar-
riers are an essential part in the calculations of cross-
sections. A rough estimate shows that a 1 MeV increase
of the fission barrier increases the cross-section by one to
two orders of magnitude [77]. Uncertainties of this order
of magnitude, which were revealed by the comparison of
experimental and theoretical Qαvalues, have to be con-
sidered in the discussions on and preparations of experi-
ments aiming to search for new elements. In other words,
sufficiently long beam times have to be provided in or-
der to perform experiments with the perspectives of being
successful.
5 Conclusion
The data presented in this paper are the results of a first
experiment at SHIP using large area, highly radioactive
targets. It was demonstrated that the technical require-
ments for a safe operation of the targets and a neces-
sary monitoring of the target quality during irradiation
with high beam intensities were achieved. The detected
six atoms of element 116 in the reaction 48Ca + 248Cm →
296116∗during a beam time of 17 days and at a cumula-
tive cross-section of 5.1 pb are the results to be compared
with data given in the literature.
Cross-section and properties of four of the six mea-
sured decay chains are in agreement with data measured
previously in the same reaction in Dubna using a gas-
filled separator. These chains were assigned to the isotope
292116. Our results obtained with an evacuated velocity
filter confirm this assignment. Previously discussed pro-
duction of the new isotopes by αxn channels or transfer
reactions can be definitely excluded.
One of our measured decay chains is in agreement with
the results assigned to the isotope 293116, also measured
previously in Dubna. One chain could not be definitely
assigned. It consists of four αdecays and ends by spon-
taneous fission. Although energy and lifetime of the first
αdecay agree with data measured for 293116 in Dubna,
the energies of the second and third αdecay are different.
Also different is the energy of the fourth αdecay and the
half-life of the terminating fission event, which was mea-
sured in one case in the reaction 48Ca + 244 Pu producing
daughter nuclei of element 116 isotopes, in an experiment
at the GSI gas-filled separator. An assignment of this chain
to high-spin isomers, as theoretically predicted, is reason-
able, but tentative, and needs confirmation.
A comparison of the measured Qαvalues with theo-
retical predictions reveals deviations of up to 1 MeV for
decay chains passing through 292116 and 293116 in the
region of neutron numbers between 172 and 177. Accord-
ing to theoretical models, in this region a transition from
spherical shapes to deformed nuclei occurs with decreasing
neutron number and a proton shell or subshell at Z= 114
is crossed. However, the experimental data do not reveal
any discontinuity when proton number 114 is crossed in
the decay chains from element 116 at neutron numbers
near 174. Therefore, we conclude that 114 is not a strong
proton shell at these neutron numbers.
The comparison of the theoretically predicted Qαval-
ues also reveals large deviations for elements beyond 116.
Again, the reason is the predictions of closed shells varies
from Z= 114, 120 or 126 in different models. Therefore,
experimental data on element 120 isotopes are critical for
the verification of existence and strength of potential pro-
ton shells at 120 or 126 and to begin to discriminate be-
tween the models. Depending on these results, the struc-
ture and the stability of nuclei in the region of superheavy
elements can be more accurately estimated. These exper-
iments will also determine if nuclei of elements beyond
120 will have high enough production cross-sections and
if they will have long enough lifetimes for being detectable.
In the present situation, only experimental data can de-
cide about the extension of the island of superheavy nuclei
into the direction of heavier elements.
We would like to thank our colleagues from the ECR ion source
group and the UNILAC accelerator group for excellent perfor-
mance of the 48Ca beam concerning high stability, high cur-
rent, and low material consumption. We are also grateful to
A. Huebner of the GSI target laboratory for the skillful prepa-
ration of the large area titanium backing foils of the targets.
We are also much indebted to our colleagues from the depart-
ment of the scientific-technical infrastructure for providing the
needed hardware for the experiments and the software for the
data analysis. Two of us, SA and SS, were supported by the
Slovak Research and Development Agency (contract APVV-
0105-10) and VEGA (contract 1/0613/11). The work by Liv-
ermore scientists was performed under the auspices of the U.S.
Department of Energy by Lawrence Livermore National Labo-
ratory under Contract DE-AC52-07NA27344. The 248Cm tar-
get material was provided by the U.S. DOE through ORNL.
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