- Email: [email protected]
Detector calibration measurements in CRESST
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 559 (2006) 372–374 www.elsevier.com/locate/nima
Detector calibration measurements in CRESST W. Westphala,, C. Coppia, F.v. Feilitzscha, C. Isailaa, T. Jagemannb, J. Jochumb, J. Ko¨niga, T. Lachenmaiera, J.-C. Lanfranchia, W. Potzela, W. Raua, M. Starka, D. Wernickea,c a Technische Universita¨t Mu¨nchen, Physik Department E15, James-Franck-StraX e, D-85748 Garching, Germany Eberhard Karls Universita¨t Tu¨bingen, Physikalisches Institut I, Auf der Morgenstelle 14, D-72076 Tu¨bingen, Germany c VeriCold Technologies GmbH, BahnhofstraX e 21, D-85737 Ismaning, Germany
b
Available online 27 December 2005
Abstract The CRESST dark matter experiment uses the simultaneous measurement of the scintillation light and the heat signal of a CaWO4 crystal to discriminate between background electron recoil and nuclear recoil events. At the Technical University of Munich calibration measurements have been performed to characterize the detectors. These measurements include the determination of the light output and scintillation time constants of CaWO4 at temperatures below 50 mK. The setup used in these measurements consist of a CaWO4 crystal, which is mounted in a reflective housing together with a silicon light detector carrying an Ir/Au transition edge sensor (TES) evaporated directly onto it. r 2005 Elsevier B.V. All rights reserved. PACS: 29.40.Mc; 95.35.þd Keywords: CRESST; Dark matter; Cryogenic detectors; Scintillation detectors
1. Introduction
2. Detector concept and requirements
In the CRESST experiment [1] the different light output for electron and nuclear recoils is used for background rejection. Harmful backgrounds that remain are neutrons and recoils from alpha decays as they fall into the nuclear recoil region. For the understanding of the former a neutron scattering experiment at the accelerator laboratory in Garching is being installed [2]. For the latter an experiment with an alpha source inside the detector housing has been performed [3]. For such experiments even the background rate is much higher than could be handled by the actual dark matter detectors. In order to be able to cope with this count rate a modified version of the CRESST detector optimized for this application has been developed.
The detectors used have been described in [4]. The light detector consists of a 20 20 0:5 mm3 silicon or germanium crystal equipped with an Ir/Au TES and is located next to a CaWO4 crystal in a reflective housing. While the phonon detector is already sensitive and fast enough for our calibration measurements, the sensitivity in the light channel still needs improvement. This sensitivity depends on the efficiency of the light detector itself, but also on the amount of light that is emitted from the CaWO4. Therefore measurements of the light output of several crystals at the operation temperature of the detectors have been performed. Since the present light detectors cannot detect single scintillation light photons, a good detector should integrate the light signal. For optimal integration the decay time of the detector pulses must not be faster than the decay time of the scintillation light. In the experiments described below, this decay time has been measured precisely at low temperatures.
Corresponding author.
E-mail address: [email protected] (W. Westphal). 0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.12.013
ARTICLE IN PRESS W. Westphal et al. / Nuclear Instruments and Methods in Physics Research A 559 (2006) 372–374
For the measurements of the light output, the energy and the intrinsic time constants of the light detector is calibrated using a 55Fe source installed in the cryostat. For a crystal/light detector combination with known light collection this setup can also be used to test new materials for the reflective housing. The light collection is defined as the absolute amount of light energy per electron recoil energy in CaWO4 that is detected in the light detector. Note that this is not exactly the total light output of the crystal as it also includes, for example, losses from the reflectivity of the housing and the absorption efficiency of the light detector. The direct-hit X-ray events and the scintillation light events can easily be discriminated using their pulse shapes (see Fig. 1). There is no need for a coincidence measurement with the phonon detector on CaWO4, so also crystals that do not have a TES yet can be characterized. The energy content of the events is determined via the integral of the pulses. Table 1 summarizes the results of the light collection measured for several crystals. While these values are not absolute values for the light output, they still allow a comparison of the crystals. It is known that heating the crystals under vacuum can reduce the light output. Still, while producing the Ir/Au films for the detectors the crystals have to be heated up to 480 1C. For the crystal ‘‘Elisa’’ the temperature was ramped up faster than usual in order to minimize the exposure to high temperatures. The measurement of the light collection before and after detector fabrication of this crystal demonstrates that this procedure was successful in maintaining the good light output.
1.2 Direct Hit Event Scintillation Light Event
Crystal
Light collection [%]
‘‘Doreen’’ ‘‘Elisa’’ before processing ‘‘Elisa’’ after processing ‘‘Babsi’’
0:8 0:1 1:4 0:1 1:4 0:1 1:2 0:2
Crystal ‘‘Doreen’’ was measured after the sensor fabrication. ‘‘Elisa’’ was measured before and after the detector fabrication process. ‘‘Babsi’’ does not have a TES yet. All crystals were purchased from the same supplier and went through equal polishing processes. They all have one surface roughened to avoid light trapping.
2.5 Deconvoluted Data Bi-Exponential Fit 2
1.5
1
0.5
0
0
0.5
1
1.5
2
2.5
3
3.5
4
Time [ms] Fig. 2. Light curve of the crystal ‘‘Elisa’’ obtained via numerical deconvolution of the pulses shown in Fig. 1. The apparently strong deviation for small times is consistent with the overall fluctuations in the deconvoluted curve.
For obtaining the pulse shape f light of the original scintillation light signal a numerical deconvolution of the pulse shapes of the light pulse f lp and the direct hit signal f direct is performed using fast Fourier-transformation. Fig. 2 shows the light curve obtained in this way from the pulse templates in Fig. 1. The light curve can be well fitted with a bi-exponential decay a t=t1 1 a t=t2 f light ðtÞ ¼ C e þ e t1 t2
1
Normalized Amplitude
Table 1 Light collection measured for different crystals at temperatures below 50 mK
Amplitude [a.u.]
3. Measurements and results
373
0.8 0.6 0.4 0.2 0 -0.2
-1
0
1
2
3
4
Time [ms] Fig. 1. Pulse shape templates of ‘‘direct-hit’’ events from 5.9 keV X-rays of an 55Fe source hitting the light detector and scintillation light events from the 1170 keV gamma line of a 60Co source for the CaWO4 crystal ‘‘Elisa’’.
with the parameters a ¼ 0:75, t1 ¼ 0:39 ms and t2 ¼ 2:6 ms. The values are in agreement with the results given in [5]. This is different from measurements at higher temperatures, where faster decay times where determined (e.g. [6]). Measurements with an Am–Be neutron source show different pulse shapes in the light detector for electron and nuclear recoil events, where the electron recoils have a slower decay time (see Fig. 3). Unfortunately in these measurements there are no direct light detector hit events
ARTICLE IN PRESS W. Westphal et al. / Nuclear Instruments and Methods in Physics Research A 559 (2006) 372–374
374
times. Furthermore, no neutron calibration been performed there yet.
1.2 Electron Recoil Event Nuclear Recoil Event
1
Normalized Amplitude
4. Conclusion and outlook 0.8
0.4
Measurements of the light output and the scintillation time constant of several CaWO4 crystals at very low temperatures were performed. The different scintillation decay time constants for electron and nuclear recoil events need further investigation.
0.2
Acknowledgements
0.6
0 -0.2
-1
0
1
2
3
4
This work has been supported in part by the DFG via SFB-375, EEC network program HPRN-CT2002-00322, the ILIAS-Project (RII3-CT-2004-506222), VIDMAN (VH-VI-033) and the Maier–Leibnitz Labor Garching.
Time [ms] Fig. 3. Templates of light detector pulses for electron and nuclear recoil events. Both templates were taken from events with the same light energies.
as reference, so the decay times of the scintillation light could not be extracted. This difference in decay times is hard to observe in the CRESST measurements at Gran Sasso, as the light detector used there has longer decay
References [1] G. Angloher, et al., Astropart. Phys. 23 (2005) 325 arXiv:astro-ph/ 0408006. [2] C. Coppi, et al., these proceedings. [3] M. Stark, et al., in preparation; M. Stark, Ph.D. Thesis, Technische Universita¨t Mu¨nchen, 2005. [4] M. Stark, et al., Nucl. Instr. and Meth. A 545 (2005) 738. [5] P.C.F. Di Stefano, et al., J. Appl. Phys. 94 (10) (2003) 6887 arXiv:physics/0307042. [6] G. Blasse, G. Bokkers, J. Solid State Chem. 49 (1983) 126.
Nuclear Instruments and Methods in Physics Research A 559 (2006) 372–374 www.elsevier.com/locate/nima
Detector calibration measurements in CRESST W. Westphala,, C. Coppia, F.v. Feilitzscha, C. Isailaa, T. Jagemannb, J. Jochumb, J. Ko¨niga, T. Lachenmaiera, J.-C. Lanfranchia, W. Potzela, W. Raua, M. Starka, D. Wernickea,c a Technische Universita¨t Mu¨nchen, Physik Department E15, James-Franck-StraX e, D-85748 Garching, Germany Eberhard Karls Universita¨t Tu¨bingen, Physikalisches Institut I, Auf der Morgenstelle 14, D-72076 Tu¨bingen, Germany c VeriCold Technologies GmbH, BahnhofstraX e 21, D-85737 Ismaning, Germany
b
Available online 27 December 2005
Abstract The CRESST dark matter experiment uses the simultaneous measurement of the scintillation light and the heat signal of a CaWO4 crystal to discriminate between background electron recoil and nuclear recoil events. At the Technical University of Munich calibration measurements have been performed to characterize the detectors. These measurements include the determination of the light output and scintillation time constants of CaWO4 at temperatures below 50 mK. The setup used in these measurements consist of a CaWO4 crystal, which is mounted in a reflective housing together with a silicon light detector carrying an Ir/Au transition edge sensor (TES) evaporated directly onto it. r 2005 Elsevier B.V. All rights reserved. PACS: 29.40.Mc; 95.35.þd Keywords: CRESST; Dark matter; Cryogenic detectors; Scintillation detectors
1. Introduction
2. Detector concept and requirements
In the CRESST experiment [1] the different light output for electron and nuclear recoils is used for background rejection. Harmful backgrounds that remain are neutrons and recoils from alpha decays as they fall into the nuclear recoil region. For the understanding of the former a neutron scattering experiment at the accelerator laboratory in Garching is being installed [2]. For the latter an experiment with an alpha source inside the detector housing has been performed [3]. For such experiments even the background rate is much higher than could be handled by the actual dark matter detectors. In order to be able to cope with this count rate a modified version of the CRESST detector optimized for this application has been developed.
The detectors used have been described in [4]. The light detector consists of a 20 20 0:5 mm3 silicon or germanium crystal equipped with an Ir/Au TES and is located next to a CaWO4 crystal in a reflective housing. While the phonon detector is already sensitive and fast enough for our calibration measurements, the sensitivity in the light channel still needs improvement. This sensitivity depends on the efficiency of the light detector itself, but also on the amount of light that is emitted from the CaWO4. Therefore measurements of the light output of several crystals at the operation temperature of the detectors have been performed. Since the present light detectors cannot detect single scintillation light photons, a good detector should integrate the light signal. For optimal integration the decay time of the detector pulses must not be faster than the decay time of the scintillation light. In the experiments described below, this decay time has been measured precisely at low temperatures.
Corresponding author.
E-mail address: [email protected] (W. Westphal). 0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.12.013
ARTICLE IN PRESS W. Westphal et al. / Nuclear Instruments and Methods in Physics Research A 559 (2006) 372–374
For the measurements of the light output, the energy and the intrinsic time constants of the light detector is calibrated using a 55Fe source installed in the cryostat. For a crystal/light detector combination with known light collection this setup can also be used to test new materials for the reflective housing. The light collection is defined as the absolute amount of light energy per electron recoil energy in CaWO4 that is detected in the light detector. Note that this is not exactly the total light output of the crystal as it also includes, for example, losses from the reflectivity of the housing and the absorption efficiency of the light detector. The direct-hit X-ray events and the scintillation light events can easily be discriminated using their pulse shapes (see Fig. 1). There is no need for a coincidence measurement with the phonon detector on CaWO4, so also crystals that do not have a TES yet can be characterized. The energy content of the events is determined via the integral of the pulses. Table 1 summarizes the results of the light collection measured for several crystals. While these values are not absolute values for the light output, they still allow a comparison of the crystals. It is known that heating the crystals under vacuum can reduce the light output. Still, while producing the Ir/Au films for the detectors the crystals have to be heated up to 480 1C. For the crystal ‘‘Elisa’’ the temperature was ramped up faster than usual in order to minimize the exposure to high temperatures. The measurement of the light collection before and after detector fabrication of this crystal demonstrates that this procedure was successful in maintaining the good light output.
1.2 Direct Hit Event Scintillation Light Event
Crystal
Light collection [%]
‘‘Doreen’’ ‘‘Elisa’’ before processing ‘‘Elisa’’ after processing ‘‘Babsi’’
0:8 0:1 1:4 0:1 1:4 0:1 1:2 0:2
Crystal ‘‘Doreen’’ was measured after the sensor fabrication. ‘‘Elisa’’ was measured before and after the detector fabrication process. ‘‘Babsi’’ does not have a TES yet. All crystals were purchased from the same supplier and went through equal polishing processes. They all have one surface roughened to avoid light trapping.
2.5 Deconvoluted Data Bi-Exponential Fit 2
1.5
1
0.5
0
0
0.5
1
1.5
2
2.5
3
3.5
4
Time [ms] Fig. 2. Light curve of the crystal ‘‘Elisa’’ obtained via numerical deconvolution of the pulses shown in Fig. 1. The apparently strong deviation for small times is consistent with the overall fluctuations in the deconvoluted curve.
For obtaining the pulse shape f light of the original scintillation light signal a numerical deconvolution of the pulse shapes of the light pulse f lp and the direct hit signal f direct is performed using fast Fourier-transformation. Fig. 2 shows the light curve obtained in this way from the pulse templates in Fig. 1. The light curve can be well fitted with a bi-exponential decay a t=t1 1 a t=t2 f light ðtÞ ¼ C e þ e t1 t2
1
Normalized Amplitude
Table 1 Light collection measured for different crystals at temperatures below 50 mK
Amplitude [a.u.]
3. Measurements and results
373
0.8 0.6 0.4 0.2 0 -0.2
-1
0
1
2
3
4
Time [ms] Fig. 1. Pulse shape templates of ‘‘direct-hit’’ events from 5.9 keV X-rays of an 55Fe source hitting the light detector and scintillation light events from the 1170 keV gamma line of a 60Co source for the CaWO4 crystal ‘‘Elisa’’.
with the parameters a ¼ 0:75, t1 ¼ 0:39 ms and t2 ¼ 2:6 ms. The values are in agreement with the results given in [5]. This is different from measurements at higher temperatures, where faster decay times where determined (e.g. [6]). Measurements with an Am–Be neutron source show different pulse shapes in the light detector for electron and nuclear recoil events, where the electron recoils have a slower decay time (see Fig. 3). Unfortunately in these measurements there are no direct light detector hit events
ARTICLE IN PRESS W. Westphal et al. / Nuclear Instruments and Methods in Physics Research A 559 (2006) 372–374
374
times. Furthermore, no neutron calibration been performed there yet.
1.2 Electron Recoil Event Nuclear Recoil Event
1
Normalized Amplitude
4. Conclusion and outlook 0.8
0.4
Measurements of the light output and the scintillation time constant of several CaWO4 crystals at very low temperatures were performed. The different scintillation decay time constants for electron and nuclear recoil events need further investigation.
0.2
Acknowledgements
0.6
0 -0.2
-1
0
1
2
3
4
This work has been supported in part by the DFG via SFB-375, EEC network program HPRN-CT2002-00322, the ILIAS-Project (RII3-CT-2004-506222), VIDMAN (VH-VI-033) and the Maier–Leibnitz Labor Garching.
Time [ms] Fig. 3. Templates of light detector pulses for electron and nuclear recoil events. Both templates were taken from events with the same light energies.
as reference, so the decay times of the scintillation light could not be extracted. This difference in decay times is hard to observe in the CRESST measurements at Gran Sasso, as the light detector used there has longer decay
References [1] G. Angloher, et al., Astropart. Phys. 23 (2005) 325 arXiv:astro-ph/ 0408006. [2] C. Coppi, et al., these proceedings. [3] M. Stark, et al., in preparation; M. Stark, Ph.D. Thesis, Technische Universita¨t Mu¨nchen, 2005. [4] M. Stark, et al., Nucl. Instr. and Meth. A 545 (2005) 738. [5] P.C.F. Di Stefano, et al., J. Appl. Phys. 94 (10) (2003) 6887 arXiv:physics/0307042. [6] G. Blasse, G. Bokkers, J. Solid State Chem. 49 (1983) 126.