Dominant deuteron acceleration with a high-intensity laser for isotope production and neutron generation

Abstract

Experiments on the interaction of an ultra-short pulse laser with heavy-water, ice-covered copper targets, at an intensity of 2×10^19 W/cm^2, were performed demonstrating the generation of a “pure” deuteron beam with a divergence of 20°, maximum energy of 8 MeV, and a total of 3×10^11 deuterons with energy above 1 MeV—equivalent to a conversion efficiency of 1.5% ± 0.2%. Subsequent experiments on irradiation of a 10B sample with deuterons and neutron generation from d-d reactions in a pitcher-catcher geometry, resulted in the production of ∼ 10^6 atoms of the positron emitter 11C and a neutron flux of (4±1)×10^5 neutrons/sterad, respectively.

Full text

Dominant deuteron acceleration with a high-intensity laser for isotope
production and neutron generation
A. Maksimchuk,
1,a)
A. Raymond,
1
F. Yu ,
1
G. M. Petrov,
2
F. Dollar,
1,b)
L. Willingale,
1
C. Zulick,
1
J. Davis,
2
and K. Krushelnick
1
1
Center for Ultrafast Optical Science, University of Michigan, Ann Arbor, Michigan 48109, USA
2
Naval Research Laboratory, Plasma Physics Division, Washington, DC 20375, USA
(Received 30 January 2013; accepted 3 May 2013; published online 16 May 2013)
Experiments on the interaction of an ultra-short pulse laser with heavy-water, ice-covered copper
targets, at an intensity of 2 1019 W/cm
2
, were performed demonstrating the generation
of a “pure” deuteron beam with a divergence of 20, maximum energy of 8 MeV, and a total of
31011 deuterons with energy above 1 MeV—equivalent to a conversion efficiency of 1.5%
60.2%. Subsequent experiments on irradiation of a 10 Bsample with deuterons and neutron
generation from d-d reactions in a pitcher-catcher geometry, resulted in the production of 106
atoms of the positron emitter 11Cand a neutron flux of ð461Þ105neutrons/sterad, respectively.
V
C2013 AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4807143]
Interactions of ultra-intense lasers with solids are capa-
ble of producing multi-MeV proton
1–3
and ion beams,
4,5
which have applications in charged particle radiography,
6
radiation therapy,
7
isotope production,
8,9
creation of warm
dense matter (WDM),
10
and fast ignitor (FI) research.
11–16
Although protons have been the primary object of attention,
the idea of light ion acceleration is attracting consideration
for the production of WDM and especially for FI studies due
to the fact that light ions can be more efficient than protons
in energy deposition in the compressed core of the inertial
confinement fusion (ICF) targets.
12,17–19
In most experiments conducted to date (without special
prior target cleaning), protons were observed to be the domi-
nant ion species, which suppresses the acceleration of other
ions. These protons are the result of target surface contami-
nation by water vapor and/or hydrocarbons. Efficient accel-
eration of other ions is difficult to achieve because in the
early stage of ion acceleration the space-charge electrostatic
field on the rear target surface accelerates only the outer-
most, proton-rich layer of ions which inhibits the accelera-
tion of the target material ions by shielding them from the
field.
5,20
To realize preferential light ion (deuteron) acceleration,
Hou et al.
21
proposed, instead of target cleaning, to overcoat
the surface with the required contaminants, i.e., with heavy
water in their case. This was accomplished by placing a
small quantity (1 ml) of D
2
O inside the experimental
chamber before sealing and evacuation. Such procedure
resulted in an increased deuteron yield in the direction of tar-
get expansion by a factor of 3–5 times compared to a target
with a deuterated plastic outer layer. Morrison et al.
22
also
followed a similar route but suggested cooling the target to
cryogenic temperatures and injecting a large amount of
heavy water (100 ml) into the experimental chamber to
produce D
2
O ice on the target surface. They minimized re-
deposition of hydrocarbons and H
2
O vapor by placing the
target into a complex cryogenically cooled shroud. While
they were able to substantially increase the deuteron signal
compared to that of protons (99:1), deuterons with maxi-
mum energy of only 3.5 MeV were observed at the focused
intensity of 5 1018 W/cm
2
. Unfortunately, the complexity
of target preparation and large amount of heavy water
required for this experiment are not very practical.
Moreover, the large delay between water injection and the
laser shot (20 s) does not allow for repetitive experiments
and thus demands a different solution to address the issue of
preferential deuteron acceleration with simultaneous sup-
pression of proton acceleration.
In this Letter, we report on dominant high-energy deu-
teron generation and acceleration from a submicron thick
layer of heavy water ice deposited on the front and rear
surfaces of a cryogenically cooled flat metal foil interacting
with an intense laser. The ice deposition was achieved
through heavy water vapor formation inside the experimental
vacuum chamber by the injection of just 90 llofD
2
O
using two nozzles placed at the front and the rear of the tar-
get. It was found that the highest deuteron yield and energies
were realized with the smallest nozzles opening times
(10 ms) and the shortest delays between nozzle spraying
and laser firing (1–2 s), leading to a regime where a “pure”
deuteron beam (with a ratio of deuterons N
d
to protons N
p
up
to 100:1) was produced. At these optimal timings, an imag-
ing of the “pure” deuteron beam using radiochromic film
with a simultaneous measurement of deuteron spectrum was
performed. This allowed a deuteron beam divergence to be
determined and to infer a total number of high-energy deu-
terons as well as the conversion efficiency of the laser energy
into deuterons. With an optimized deuteron beam the experi-
ments on the production of positron active isotope 11 Cfrom
the reaction d-10Band neutron generation from d-d reactions
in a pitcher-catcher geometry were carried out.
The experiments were performed at the Center for
Ultrafast Optical Science of the University of Michigan on a
15 TW hybrid Ti:sapphire/Nd:phosphate glass laser. In these
experiments, the laser delivered up to 6 J, 400 fs pulses at
the fundamental wavelength of 1.053 lm with an energy
amplified spontaneous emission contrast of 10
5
. The
a)
Electronic mail: tolya@umich.edu
b)
Present address: JILA, University of Colorado, Boulder, CO 80309, USA.
0003-6951/2013/102(19)/191117/5/$30.00 V
C2013 AIP Publishing LLC102, 191117-1
APPLIED PHYSICS LETTERS 102, 191117 (2013)
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p-polarized laser beam was focused onto the surface of a
10 lm thick Cu foil sandwiched between two 1-mm thick
perforated brass plates. These plates, which had 3-mm diam-
eter round openings, were mechanically connected to a mas-
sive copper block, which was cryogenically cooled to
165 C using liquid nitrogen. The laser beam was incident
at 22.5and focused to a spot size of 5 lm at the Full-Width
Half-Maximum (FWHM) with an f=2(f¼10.5 cm) dielec-
tric coated off-axis parabolic mirror providing a maximum
focused intensity of 2 1019 W/cm
2
. The vacuum inside
the experimental chamber was 2104Torr and was
improved by approximately one order of magnitude during a
target cool down procedure when the massive copper block
connected to the target holder and the connected copper tub-
ing for the liquid N
2
supply worked as a cold trap precipitat-
ing water vapor.
Inside the vacuum chamber two pulsed valves with 2
mm opening diameter nozzles were installed. The nozzles
were connected by a flexible tubing to the outside heavy
water reservoir, which was pressurized with dry N
2
at a back-
ing pressure of 40 psi. Prior to the laser shot a trigger signal
was sent to Yota One nozzle drivers (Parker Automation) to
initialize the D
2
O spraying inside the experimental chamber
on a paper padding located 10 cm from the target holder. It
was possible to independently adjust the opening times for
both nozzles. When D
2
O was sprayed inside the vacuum,
it boiled, evaporated, and contributed to the formation
of heavy water vapor, which uniformly overcoated the
H-contaminants on the cold target surface, producing a thin
layer of heavy water ice. During this procedure, both sides of
the target may contribute about equally to deuteron accelera-
tion,
23
so one nozzle at the front and the other at rear of the
target have been used to expose both sides to D
2
O vapor. In
order to avoid re-deposition of H-containing contaminants, it
was necessary to fire the laser within a few seconds of the
D
2
O ice formation on a target. We found experimentally that
the use of two nozzles instead of one at the rear allows the
minimization of the time required for the D
2
O ice formation
on both target surfaces and to increase the ratio Nd=Npby
about a factor of two. The thickness of heavy water ice was
measured interferometrically, when heavy water was spayed
from a single nozzle with an opening time of 100 ms, to be
600 nm. This will correspond to an estimated ice layer
thickness of 120 nm when 2 nozzles with opening time of
10 ms were used. During the consecutive shots, more likely,
the heavy water ice starts to grow on the target surface with
some contamination of H
2
O ice and CH due to re-deposition.
Nevertheless, the ice thickness remained smaller that the tar-
get thickness for 20–25 shots produced in a single experi-
mental run. It might be possible to maintain the same D
2
O
ice thickness for the consecutive shots if the target is re-
heated between the shots.
The ion spectra were recorded using a Thomson parab-
ola (TP) spectrometer with a microchannel plate which was
absolutely calibrated against a CR-39 plastic nuclear track
detector. We varied the opening times for the nozzles and
found the optimum opening time for the front and the rear
nozzle to be dt¼10 ms, while the optimum delay time
between the nozzles opening and the laser shot firing was
about Dt¼1:5–2 s. A 10 ms opening time was the shortest
opening time for the nozzle with a stable heavy water output,
which was measured to be 45 61ll. For Dt<1:5 s, a strong
reflection of laser pulse back into the laser system having a
filamentary distribution with peak intensity close to the
damage threshold of optical elements was observed. For this
reason, a study of deuteron production for delays Dt1:5s
was performed. In the case of dt¼10 ms and Dt¼1:5–2s,
it was possible to increase the ratio Nd=Npup to about 100:1,
producing deuterons with maximum energies of 8 MeV. This
ratio was confirmed directly by counting the number of
tracks in the deuteron and proton traces when CR-39 was
used as a detector for the TP spectrometer. Fig. 1shows deu-
teron and proton spectra for Dt¼2 s. While deuterons were
the dominant species for energies below 4.0 MeV, their num-
bers became comparable to protons at around 4.5 MeV and
even lower at 5.0 MeV. The presence of protons can be
attributed to impurities in the heavy water used in the experi-
ment, which has 0.2% of H
2
O, and/or contamination of the
target surface by the residual water vapor left inside the vac-
uum chamber. The inset in Fig. 1shows typical ion traces
produced on the TP spectrometer with a very strong deuteron
and a weak proton trace. One should also notice the presence
of oxygen ion traces up to a charge of 6
þ
for the target with
D
2
O ice on the surface instead of the typical carbon ion
traces from the CH contaminants. At larger delays at Dt>2
s, the maximum deuteron energy degrades very quickly
along with the number of deuterons, which possessed the
highest energies, while the low energy component of deu-
teron spectra (E
d
¼1–3 MeV) remained almost unchanged
(Fig. 2). The ratio of D to H signals also decreased due to re-
deposition of H-contaminants over the D
2
O ice layers and
became 1:1 for Dt10 s.
For the optimum delay time of Dt¼1:5 s the “pure”
deuteron beam was imaged using radiochromic film (RCF)
MD-55, covered in a 15-lm Al filter with a simultaneous
measurement of deuteron spectrum (Fig. 3(a)). From this
experiment, it was found that deuteron beam had a FWHM
divergence of 20 degrees (Fig. 3(b)), and that the total num-
ber of deuterons in a beam with energy above 1 MeV was
31011. This corresponds to a conversion efficiency of laser
energy into deuterons of about 1.5% 60.2%.
FIG. 1. Experimental spectra for deuterons and protons for delay Dt¼2s
with Nd=Np100:1; the inset shows the ion traces produced on TP spec-
trometer (the brightness of the TP image was enhanced to make visible a
weak proton trace).
191117-2 Maksimchuk et al. Appl. Phys. Lett. 102, 191117 (2013)
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Numerical simulations using a two dimensional electro-
magnetic PIC code,
24,25
with ionization package, were per-
formed to calculate the proton and deuteron spectra from a
thin, 5 lmAlfoil(q¼2.7 g/cm
3
) covered with a D
2
O contam-
inant layer (q¼1.1 g/cm
3
) with thickness 0.5 lm located on
either the front or rear target surface. The D
2
Oismixedwith
5% protons to simulate the presence of native contaminants.
The laser radiation is linearly polarized and normally incident
on the target, having peak intensity, duration, and a spot size
identical to the experimental ones. The simulation box is a
square with dimensions 100 100 lm
2
, the cell size is 20 nm,
and the number of computational particles is 10
7
.Thetarget
is sufficiently wide (98 lm) to avoid fringe and “mass limited
target” effects. A pre-plasma is added on the irradiated side of
the target to account for the impact of a nanosecond pre-pulse.
It consists of a layer with fast exponential drop near the target
surface and a shoulder of low-density plasma. The first layer
has a thickness of L
1
¼0.5 lm at 1/e level. The shoulder is a
plasma starting at the target with density 2 n
crit
and linearly
decreasing to zero at the vacuum-preplasma interface some
L
2
¼10 lm away. The pre-plasma profile is given by the
expression neðxÞ=ncrit ¼55expðjxj=L1Þþ2ð1jxj=L2Þfor
x0. The pre-plasma profile simulates the overall trend
observed experimentally and in simulations.
26
The electron
density of the flat top part of the Al target, located at x 0, is
initialized with density 55 n
crit
, corresponding to Al average
charge 
Z¼1, but during the course of the simulations due to
collisional and optical field ionization it reaches 500 n
crit
,
where ncrit ffi1.1 10
21
cm
3
is the critical electron density
for laser wavelength k¼1lm.
Simulation results are shown in Fig. 4. The high-energy
tail of proton and deuteron spectra from rear-side accelera-
tion (RSA), due to Target Normal Sheath Acceleration
(TNSA),
3,27
extends to energy 7 MeV. The maximum deu-
teron energy is comparable to that of protons and the number
of deuterons exceeds that of protons by nearly one order of
magnitude. The calculated overall conversion efficiency of
laser energy into deuterons is a factor of four higher than the
conversion efficiency into protons. We conclude that with
the introduction of artificial contaminants the ion accelera-
tion from the rear side of the foil is dominated by deuterons.
The spectra of protons and deuterons from the front-side
acceleration (FSA), due to skin-layer pondermotive accelera-
tion,
1,2,28,29
show similar features, except that it is more
Maxwellian-like (Fig. 4). As with RSA, the ion acceleration
is dominated by deuterons, whose number exceeds the num-
ber of protons by about one order of magnitude. The simula-
tions illustrate that when the natural contaminants (H
2
O) are
replaced by artificial ones (D
2
O), the ion acceleration from
either the front or rear of the target is dominated by the spe-
cies of the new material. Another interesting observation is
that even if the artificial contaminants contain small amount
of hydrogen, the protons do not impede the deuteron acceler-
ation. This is true for both FSA and RSA. The simulations
support the experimental observations that other ions such as
deuterons can be preferentially accelerated.
Some important applications using “pure” deuteron
beams, such as short-lived radio-isotope production and neu-
tron generation, were tested. Similar experiments have been
conducted before with a layer of a deuterated (CD) plastic
deposited on the target surfaces
9,30,31
in a pitcher-catcher ge-
ometry in which it was found that the H-contaminants sub-
stantially inhibited deuteron acceleration.
20
At the optimum delays Dt, the deuteron beam was used
to activate a 10Bsample (enrichment of 90%) producing
FIG. 2. Number of deuterons in three energy intervals (1-3 MeV—squares;
3-5 MeV—circles; 5-8 MeV—triangles) for different delay periods between
spraying and the laser pulse.
FIG. 3. Deuteron beam image with E ⲏ2.5 MeV on RCF (a); the lineout of
deuteron beam image (b).
FIG. 4. Simulated spectra of deuterons and protons from FSA (a) and RSA
(b) integrated over angular distribution.
191117-3 Maksimchuk et al. Appl. Phys. Lett. 102, 191117 (2013)
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atoms of the positron emitter 11 C. The radioisotope 11Chas a
half-life of t1=2¼20:32 m and is used in nuclear medicine
for positron-emission tomography to produce a three-
dimensional image or picture of functional processes in the
body. In the experiment, a cylindrically shaped 10 Bsample
10 mm in diameter and 5 mm thick was positioned 10 mm
behind the target and parallel to its surface. The sample had
a 1-mm hole in the middle to monitor the ion spectra using
the TP spectrometer. The yield of 11Cwas measured using a
high-purity germanium (HPGe) detector model GEM55-P4-
83 from ORTEC by counting the number of crays emitted
when positrons annihilate. The HPGe detector was abso-
lutely calibrated using a 22Na source with an activity of
78 nCi, showing 8.3% detection efficiency with a geometri-
cal factor taken into account. Figure 5and the inset present a
typical high-resolution c-rays spectrum measured from a 10 B
sample activated with the high-energy deuteron beam. Note
a strong peak at 0.511 MeV due to positron annihilation. To
increase the accuracy of the measurements, 6 laser shots
were fired on the target, producing energetic deuterons with
similar spectra. Knowing the delays between the laser shots,
the time when the counting started, and assuming that the
measured yield N0¼0:083 N0(where N
0
is a true yield)
of 11Cis the same for each shot, it is possible to calculate the
measured number Nof 11Catoms when the HPGe counting
started: N¼N0ðPexpðtilnð2Þ=t1=2ÞÞ¼1:61 N0.By
taking the measurements of c-rays emitted at 0.511 MeV and
plotting them as a function of time, a total measured number
of 11Catoms produced right after the last laser irradiation
was found. This allowed calculating of the true number of
11Catoms produced per single laser shot N
0
¼9.8105(Fig.
5(b)), which corresponds to an induced radioactivity of 20
nCi - one order of magnitude higher than was produced with
the same laser
9
but using deposited layers of CD plastic.
With an optimized deuteron beam an experiment on
neutron generation from the d-d reaction in a pitcher-catcher
geometry,
9,30–32
where ion production and neutron genera-
tion targets are separated, was carried out. The catcher was a
sheet of a 0.5 mm thick deuterated polystyrene placed in the
target normal direction and intercepting the whole d-beam.
The neutron spectra were measured with a time-of-flight
(TOF) diagnostic in the laser propagation direction (22.5to
the target normal) using a Hamamatsu photo-multiplier tube
(PMT) assembly (H2431-50) coupled with an acrylic light
guide to a 15 cm diameter, 2.5 cm thick, EJ-204 plastic scin-
tillator from Eljen Technology. The detector was 2.8 m away
from the interaction region and placed in a 10 cm thick lead
house. Figures 6(a) and 6(b) show typical scope traces taken
with a LeCroy 104MXi digital oscilloscope without and with
borated plastic in front of the PMT assembly. This demon-
strates univocally strong neutron generation compared to
results in a similar geometry with the same laser but with the
layered CD plastic targets.
31
In fact a calculated total neutron
yield of (461Þ105n/sterad inferred from the neutron
spectrum (Fig. 6) was even greater than the yield from the
bulk CD targets.
31
The total neutron yield was confirmed
with simultaneous measurements by bubble detectors BD-
PND (Bubble Technology Industries), which showed the
yield dNn=dX¼ð964Þ105n/sterad.
In conclusion, we experimentally demonstrated prefer-
ential deuteron acceleration from D
2
O ice covered copper
targets using a simple technique of heavy water vapor depo-
sition onto a cryogenically cooled target. This technique
allows for repeatable and reproducible results and showed
for a laser intensity of 2 1019 W/cm
2
the generation of
31011 deuterons with energies above 1 MeV, correspond-
ing to a conversion efficiency of 1.5%. Deuterons with
maximum energy of 8 MeV were observed. Subsequent
experiments on irradiation of a 10 Bsample with deuterons
and neutron generation from d-d reactions in a pitcher-
catcher geometry, resulted in the production of 10
6
atoms
of the positron emitter 11Cand a neutron flux of 4105
neutrons/sterad, correspondingly. The performed studies
may also have important implications for the light ion FI
research and for the production of WDM.
This study was supported by Defense Threat Reduction
Agency (DTRA) and the Office of Naval Research (ONR).
The authors would like to thank the University of Michigan
Neutron Science Laboratory for use of the D-D generator for
detectors calibration.
1
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191117-4 Maksimchuk et al. Appl. Phys. Lett. 102, 191117 (2013)
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