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SciPost Phys. Proc. 12, 031 (2023)
Characterisation of low background CaWO4crystals
for CRESST-III
Angelina Kinast1⋆, G. Angloher2, S. Banik3,4, G. Benato5, A. Bento2,6 , A. Bertolini2,
R. Breier7, C. Bucci5, J. Burkhart3, L. Canonica2, A. D’Addabbo5, S. Di Lorenzo5,
L. Einfalt3,4, A. Erb1,8 , F. v. Feilitzsch1, N. Ferreiro Iachellini2, S. Fichtinger3, D. Fuchs2,
A. Fuss3,4, A. Garai2, V. M. Ghete3, S. Gerster9, P. Gorla5, P. V. Guillaumon5, S. Gupta3,
D. Hauff2, M. Ješkovský7, J. Jochum9, M. Kaznacheeva1, H. Kluck3, H. Kraus10,
A. Langenkämper1,2, M. Mancuso2, L. Marini5,11 , L. Meyer9, Valentyna Mokina3,
A. Nilima2, M. Olmi5, T. Ortmann1, C. Pagliarone5,12, L. Pattavina1,5, F. Petricca2,
W. Potzel1, P. Povinec7, F. Pröbst2, F. Pucci2, F. Reindl3,4, J. Rothe1, K. Schäffner2,
J. Schieck3,4, D. Schmiedmayer3,4 , S. Schönert1, C. Schwertner3,4, M. Stahlberg2,
L. Stodolsky2, C. Strandhagen9, R. Strauss1, I. Usherov9, F. Wagner3, M. Willers1
and V. Zema2
⋆
angelina.kinast@tum.de
14th International Conference on Identification of Dark Matter
Vienna, Austria, 18-22 July 2022
doi:10.21468/SciPostPhysProc.12
Abstract
The CRESST-III experiment aims at the direct detection of dark matter particles via their
elastic scattering off nuclei in a scintillating CaWO4target crystal. For many years CaWO4
crystals have successfully been produced in-house at Technische Universität München
with a focus on high radiopurity. To further improve the CaWO4crystals, an extensive
chemical purification of the raw materials has been performed and the crystal TUM93
was produced from this powder. We present results from an α-decay rate analysis per-
formed on 344 days of data collected in the ongoing CRESST-III data-taking campaign.
The α-decay rate could significantly be reduced.
Copyright A. Kinast et al.
This work is licensed under the Creative Commons
Attribution 4.0 International License.
Published by the SciPost Foundation.
Received 03-10-2022
Accepted 02-05-2023
Published 04-07-2023 Check for
updates
doi:10.21468/SciPostPhysProc.12.031
1Physik-Department, Technische Universität München, D-85747 Garching, Germany
2Max-Planck-Institut für Physik, D-80805 München, Germany
3Institut für Hochenergiephysik der Österreichischen Akademie der Wissenschaften,
A-1050 Wien, Austria
4Atominstitut, Technische Universität Wien, A-1020 Wien, Austria
5INFN, Laboratori Nazionali del Gran Sasso, I-67100 Assergi, Italy
6LIBPhys-UC, Departamento de Fisica, Universidade de Coimbra,
P3004 516 Coimbra, Portugal
031.1
SciPost Phys. Proc. 12, 031 (2023)
7Comenius University, Faculty of Mathematics, Physics and Informatics,
84248 Bratislava, Slovakia
8Walther-Meißner-Institut für Tieftemperaturforschung, D-85748 Garching, Germany
9Eberhard-Karls-Universität Tübingen, D-72076 Tübingen, Germany
10 Department of Physics, University of Oxford, Oxford OX1 3RH, United Kingdom
11 GSSI-Gran Sasso Science Institute, I-67100 L’Aquila, Italy
12 Dipartimento di Ingegneria Civile e Meccanica, Università degli Studi di Cassino
e del Lazio Meridionale, I-03043 Cassino, Italy
1 Introduction
CRESST-III (Cryogenic Rare Event Search with Superconducting Thermometers) [1]aims at
the direct detection of dark matter (DM) using cryogenic calorimeters. The standard CRESST-
III module consists of a scintillating 24 g CaWO4single crystal as a target. It is operated at
≈10 mK temperature and is equipped with a transition edge sensor (TES) read out by a SQUID
(Superconducting QUantum Interference Device) for a precise measurement of the energy
deposited by a particle interaction within the crystal. In addition to the CaWO4crystal, a light
detector (also equipped with a TES) is read out in coincidence. This enables discrimination
between electromagnetic interactions (background-like events), α-decays (background events,
less relative scintillation light) and nuclear recoils (signal-like events, least relative scintillation
light) due to the different relative fraction of scintillation light produced. CRESST-III detectors
reach thresholds as low as 30.1 eV, allowing a very sensitive measurement of particle recoil
energies [1].
One key point for the excellent performance of these detectors is the quality of the target
crystals, including a high radiopurity of the CaWO4material, to minimise backgrounds result-
ing from natural decay chains. Especially β-decays can cause events in the region of interest
for DM searches. To assure a high quality of the CaWO4crystals, they have been produced in-
house at Technische Universität München (TUM) for many years [2]. In this way, every step of
the production is controlled and optimised. The crystal TUM40 operated in CRESST-II showed
an excellent performance and a lower background compared to commercially purchased crys-
tals operated in the same CRESST run [3].
To further improve the radiopurity, an extensive chemical purification of the raw materials
and the CaWO4powder has been developed at TUM. HPGe screening of the powder shows
promising results for an improved radiopurity, however, the sensitivity of this method is limited
and only limits on the radiopurity could be stated [4]. From this purified powder, the crystal
TUM93 has been produced in 2019. In total three CRESST-III target crystals were cut from
the ingot and mounted into CRESST-III modules named TUM93A, TUM93B and TUM93C. The
crystal TUM93A was cut from the top of the ingot and is, due to segregation effects during
crystal growth, expected to be the most radiopure crystal among the three detector crystals [5].
All modules are currently being operated in the ongoing CRESST-III data-taking campaign
started in November 2020. A radiopurity analysis focusing on α-decays detected in ≈344 days
of this data-taking campaign is presented in this work. For this analysis, a new approach for
energy reconstruction has been developed and is presented in the following.
031.2
SciPost Phys. Proc. 12, 031 (2023)
WP
∆T
∆R
NormalC.
SuperC.
Max pulse height
Temperature
Resistance
0
TC
Figure 1: Working principle of a TES. The TES is heated into its transition in the so-
called working point (WP). A particle interaction results in a temperature increase ∆T
which in turn results in a resistance increase ∆R. The maximum resistance increase
is defined by the resistance difference between the normal conducting resistance and
the WP resistance.
2 Analysis
The output of both the phonon detector (PD) and the light detector (LD) are recorded with
a continuous data acquisition to enable a dead-time free stream of data which is further pro-
cessed offline. In this way, the analysis can be adapted to the specific need of e.g. the low-
energy DM analysis or, as in this case, the analysis of α-decays with energies of several MeV.
Still, the reconstruction of such highly energetic events with CRESST-III detectors and stan-
dard analysis approaches is not possible, due to the optimisation of the detectors to lowest
energies.
One reason for this is the working principle of the TES used for the signal readout of both
the PD and the LD. A TES is a thin W-film operated at a temperature between the superconduct-
ing and normal conducting phase (see Figure 1). Energy deposition in the crystal heats the TES
(∆T) and results in a resistance change (∆R) proportional to the energy deposition. To max-
imise this resistance change, and lower the detector threshold, a steep transition is required.
When the energy deposited in the crystal heats the TES completely into its normal conducting
phase (like for α-decays), a maximum resistance change and in turn a maximum pulse height
is observed which stays constant until the TES cools back into its transition region. In addition,
such high energy depositions cause a fast rise in the resistance which cannot be followed by
the SQUID electronics, which is losing magnetic flux quanta and changes the absolute baseline
voltage of the stream. Figure 2(left) shows an example of an α-event recorded in the detector
TUM93A. The pulse is flat at the top as the TES is in its fully normal conducting state and
the baseline level is lower at the end of the pulse compared to the baseline level before the
pulse due to the flux quantum loss (FQL). These pulses cannot be reconstructed with standard
pulse reconstruction methods as they cannot handle the FQLs. Hence, the new reconstruction
method was developed which uses the length of the flat part of the pulse (its saturation time),
which is determined by the time the pulse needs to reach 90 % of its maximum voltage. The
saturation time is indicated by the blue line and is used to reconstruct the energy deposited
in the crystal, as it gives a measure of how long the TES needs to come back to its operating
temperature. Together with a correction for the SQUID FQLs in which the difference between
031.3
SciPost Phys. Proc. 12, 031 (2023)
e−/γ-band
α-band
saturation time
flux quantum loss
Figure 2: Left: Typical event recorded by the PD for an α-decay in the CaWO4crystal.
The pulse has a changing baseline level due to flux quantum losses in the SQUID. In
addition, the pulse is flat at the top as the TES is completely normal conducting in
this time period. Right: Calibrated scatter plot for the data set of TUM93A. For both,
the LD and the PD, the reconstruction was performed using the saturation time. Two
bands are visible, the e−/γ-band on the left and the α-band on the right.
the baseline level before and after the pulse is determined, the energy of α-decay pulses can
be reconstructed in both the PD and the LD.
In the next step some data selection criteria are applied to the data: Coincidences with the
muon veto and the artificial heat pulses sent to the detector for stabilisation and monitoring
are excluded. In addition, electronic artefacts like SQUID-resets are removed from the data
set and events with too slow a change in resistance are excluded from the data set to prevent
the wrong reconstruction of too low energetic pulses. No additional data selection criteria are
applied to avoid the possibility of removing α-decay events from the data. The resulting scatter
plot of the reconstructed energy in the LD against the reconstructed energy in the PD is shown
in Figure 2(right). The e−/γ-band, also reconstructed with the saturation time method, is
visible as the steep band on the left, as the relative light output is higher for electromagnetic
interactions. The α-decay band is nicely separated from the electromagnetic background.
The α-spectrum is calibrated using four lines present in the data selected from a wide energy
range. As a cross-check the end of the e−/γ-band at 2.6 MeV is used. The 180W decay line at
2.52 MeV, 226Ra at 4.88 MeV, the 210Po surface background line at 5.30 MeV and the 218Po line
at 6.11 MeV are fitted by an exponential function as the saturation time has an exponential
dependence on the deposited energy. The pulse model on which this assumption is based is
published in [6]. The measurement time is corrected for dead times caused by muon veto
coincidences and the artificial heat pulses sent to the detector for its stabilisation.
3 Results
The calibrated α-spectra for the detectors TUM93A (6.53 kg·d exposure), TUM93B (6.89 kg·d
exposure) and TUM93C (6.87 kg·d exposure) are shown in Figure 3. Prominent features are
the 180W decay at 2.52 MeV and the two 210Po lines at 5.41 MeV (full energy detected by the
crystal) and at 5.30 MeV for decays where the daughter nucleus escapes from the surface of
the crystal and does not deposit energy in it. The strong presence of both peaks compared to
other energy areas of the spectra hints towards surface contamination of the CaWO4crystals
with 222Rn and with 210Pb, which decayed to 210Po. A background model is currently being
developed for a more detailed study of the spectra of all three crystals.
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SciPost Phys. Proc. 12, 031 (2023)
2345678910
Energy [MeV]
0.0
0.8
1.5
2.3
3.1
3.8
4.6
Counts/20keV/kg/day
TUM93A
2345678910
Energy [MeV]
0.0
0.7
1.5
2.2
2.9
3.6
4.4
Counts/20keV/kg/day
TUM93B
0.00
1.46
2.91
4.37
5.82
7.28
Counts/20keV/kg/day
TUM93C
2345678910
Energy [MeV]
0.0
0.5
Po210 ext
Po210 int
W180
Figure 3: Final α-spectra for all TUM93 detectors. All detectors feature two promi-
nent lines at 5.30 MeV and 5.41 MeV. Both result from the decay of 210 Po which hints
toward surface contamination with 222Rn. At 2.52 MeV the α-decay of 180W is visi-
ble.
Even though the spectra seem to be dominated by surface contamination, a conservative
α-decay rate from natural decay chains in the TUM93 crystals was calculated by summing up
all events in the energy region from 3 MeV up to 10MeV, shown in Table 1.
The rate difference in the three crystals, even though they were cut from the same ingot,
has two origins. First, during crystal growth impurities are less likely to be built into the crystal
lattice compared to the crystal atoms. Hence, the impurity concentration in the melt increases
and in turn also along the growth axis in the crystal. This process is called segregation. In
addition, the high presence of the 5.30 MeV 210Po line indicates a comparably high surface
contamination which can be different for each detector crystal. The highest observed rate in
TUM93B could also hint toward a mix-up of the crystals TUM93B and TUM93C during detector
mounting.
Comparing these conservative limits to the α-activity of e.g. the crystal TUM40, which
was studied in detail in [3,7]with an α-decay rate from natural decay chains of 3.080 mBq
kg
this yields a minimum impurity reduction factor of >5.97 for TUM93A, >3.18 for TUM93B
031.5
SciPost Phys. Proc. 12, 031 (2023)
Table 1: Conservative α-decay rate of isotopes of the three natural decay chains
(238U, 235U, 232Th) in an energy range of 3 MeV to 10 MeV. All events are assumed
to be of intrinsic origin even though there are hints that the two main contributions
are from surface contamination with 210Po.
Detector α-Activity µBq
kg
TUM93A 516 ±62
TUM93B 919 ±79
TUM93C 761 ±76
and >3.85 for TUM93C. These results show a significant impact of the chemical purification
on the α-decay rate in TUM93. The e−/γ-band activity and the activity of single α-decaying
isotopes are currently being studied with the help of simulations.
Acknowledgements
Funding information This work has been funded by the Deutsche Forschungsgemeinschaft
(DFG, German Research Foundation) under Germany’s Excellence Strategy - EXC 2094 -
390783311 and through the Sonderforschungsbereich (Collaborative Research Center)
SFB1258 ‘Neutrinos and Dark Matter in Astro- and Particle Physics’, by the BMBF 05A20WO1
and 05A20VTA and by the Austrian science fund (FWF): I5420-N, W1252-N27. FW was sup-
ported through the Austrian research promotion agency (FFG), project ML4CPD. SG was sup-
ported through the FWF project STRONG-DM (FG1). The Bratislava group acknowledges a
partial support provided by the Slovak Research and Development Agency (project APVV-15-
0576).
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