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Munich cryogenic detector development for direct Dark Matter search
I | tl[ll I at'-,1"i 'J; i'l,'l[t~,'ll +! Nuclear Physics B (Proc. Suppl.) 35 (1994) 172-174 North-Holland
PROCEEDINGS SUPPLEMENTS
Munich cryogenic detector development for direct Dark Matter search A. Nucciotti a • p. Colling a, S. Cooper a, D. D u m m e r a?, F. v. Feilitzsch 5, p. Ferger a, G. Forster b, M. Frank ~, H.-J. Gebauer ~, K. Hallatschek b, E. Kellner b, U. Nagel at, F. PrSbst a, A. Rulofs a, W. Seidel a L. Stodolsky a aMax-Planck-Institut ffir Physik, F6hringer Ring 6, D-80805 Mfinchen, G e r m a n y bTechnische Universit£t Miinchen, Physik-Dept. E15, D-85747 Garching, G e r m a n y We are developing massive cryogenic detectors with low energy thresholds and high resolution for use in a direct Dark Matter search experiment. Our detector consists of a superconducting phase transition thermometer evaporated onto the surface of an absorber crystal. An energy resolution of 220 eV FWHM (for 6 keV X-rays) has been reached with a 31 g sapphire crystal using a proximity-effect thermometer. The latest results for sapphire detectors using tungsten superconducting phase transition thermometers, whose development is just beginning, are also presented. The planned further development and use of such detectors in a search for Dark Matter particles (WIMPs) is discussed, showing their advantages for low WIMP masses.
1. I N T R O D U C T I O N We have been developing massive cryogenic calorimeters with very good energy resolution and low energy threshold for use in a search for Dark Matter particles (WIMPs) via their elastic scattering on nuclei [1,2]. Cryogenic calorimeters have the advantage of being fully sensitive to lowenergy nuclear recoils. We have recently applied for space in the Gran Sasso National Laboratories where as a first step a 1 kg sapphire detector (in one or a few pieces) with an energy threshold of 0.5 keV would be installed. This first stage experiment will provide a good sensitivity to W I M P s with masses below 10 GeV and spin-dependent interactions. Later larger masses (up to 100kg total) and other target materials will be used. 2. D E T E C T O R
DEVELOPMENT
A cryogenic calorimeter consists of an absorber, in which the particle interaction takes place, and a t h e r m o m e t e r attached to the absorber. The energy deposited in the absorber by a particle in*Present Address: Dipartimento di Fisica dell'Universit~ dl Milano, Milano, 1-20133, Italy tpresent Address: lZl~diometric Physics Division, NIST, Gaithersburg, MD 20899, USA !tOn leave from the Institute of Chemical Physics and Biophysics, EE0100 Tallinn, Estonia
teraction is measured via the corresponding temperature rise. Since heat capacities decrease towards lower temperatures, one can gain in sensitivity by running a calorimeter at low temperature (T < 100inK). The critical part of a calorimeter is the thermometer. We are developing superconducting phase transition (SPT) thermometers, which consist of a film of superconducting material evaporated onto the surface of an absorber. In the region around the critical t e m p e r a t u r e To, the resistance of the S P T thermometer goes from the normal-conducting value to zero in the superconducting state, showing a strong t e m p e r a t u r e dependence. Since the operating t e m p e r a t u r e of the calorimeter is determined by the critical temperature Tc of the thermometer, S P T thermometers with low Tc are needed to achieve high sensitivity for the calorimeter. This has been pursued in two ways: 1. films of iridium overlaid with gold, using the proximity effect to reduce the Tc of the bilayer below that of pure iridium (112 mK), and 2. films of pure tungsten (To = 15 mK). I r / A u proximity-effect thermometers with critical temperatures as low as 33inK and narrow transitions have been produced. Using an I r / A u thermometer with % = 45 m K evaporated onto a
0920-5632/94/$07.00 © 1994 - Elsevier Science R.V. All fights reserved. SSDI 0920-5632(94)(X)453-3
173
A. Nucciotti et al./Munich cryogenic detector development for direct dark matter search
8O
701 /
601
Mn
Ka
50. 40~'~ 30-
AEFwHM=220 eV
20:
Mn K#
10: 0 20'00
4000
6000
B000
10000
Energy [eV]
Figure 1. Energy spectrum measured using a 31 g sapphire crystal with a Ir/Au SPT thermometer while irradiating the calorimeter with a 55Fe Xray source. 31 g sapphire (a-A12Oz) absorber, an energy resolution of 220 eV (FWHM) for collimated 5.9 keY X-rays has been obtained (Fig. 1). We have also succeeded in evaporating epitaxial tungsten films on sapphire which have transition temperatures Tc near the 15 mK value of the bulk material. A first calorimeter with a tungsten thermometer evaporated onto a 4 g sapphire crystal gives an energy resolution of 75 eV (FWHM) for 1.5 keV X-rays, as shown in Fig. 2. The detector was operated with a threshold of about 100eV.
IBOOi
'
'
,
'
jAL~,
'
'
'
'
'
V
'
'
'
Mn
'
From our understanding of the present detectors, we expect to be able soon to build a detector with a threshold of 0.5keV, a FWHM resolution of 0.2keV, and a mass of 100g in one piece. Further development is in progress to make detectors with the same sensitivity but masses up to 1 kg. Our crystal sizes are at present limited to about 30 g by the background rates in our laboratory, not by the detector performance. Significant further progress in resolution and detector mass will probably only be reached in an underground lowbackground setup. 3. P L A N N E D
EXPERIMENTAL
SETUP
For a Dark Matter search experiment it is of extreme importance to reach the lowest possible radioactive background. To avoid background events induced by cosmic rays the whole experiment needs to be located in an underground laboratory. In addition it will be carefully shielded against the natural radioactivity of the surroundings. Because not all materials needed for building a dilution refrigerator are radiopure, it is advantageous to divide the experimental setup into two parts: one containing the dilution refrigerator and the other one containing the detector surrounded by very low background materials and shielding (Fig. 3). The cooling power of the refrigerator is transferred into the cold box by a cold finger. The space for experiments will be about 30 liters. The line of sight into the shielding will be blocked by an additional shield.
1200 1000
AEF~
= 75
eV
mixing chamber co[d finger
6O0
[ead shields copper shield
~00 20
thermal radiation 0
1000
2000
3000
/+000
5000
6000
7000
"
"
Energy [eV ]
Figure 2. Energy spectrum after optimal filtering measured using 4g sapphire crystal with a tungsten SPT thermometer while irradiating the calorimeter with a fluorescence X-ray source.
shields space for experiment
~//////////////////////////A Figure 3. Sketch of layout of the cold box.
A. Nucciotti et al./Munich cryogenic detector development for direct dark matter search
174
The materials of the cold box and of the detector itself are the most critical. Low radioactivity copper can be used for the cold box shielding, for the thermal radiation shields, for the flanges and for the screws. The cold box will contain no stainless steel, indium seals or cryogenic liquids. Any other materials that are needed will be carefully tested for radiopurity. Monte Carlo simulations to understand the effect of the internal contaminations of the detector are in progress and a simulation of the cold box shielding will be started soon. Sapphire crystals from different suppliers who use different crystal growth methods are being tested. First results using neutron activation analysis to determine their radioactive contaminations found no uranium, thorium, or potassium contaminations above our present detection limits (K< 1800ng/g, Th< 0.002ng/g, U< 0.02ng/g). More analyses with different techniques are necessary to improve the sensitivity, in particular for potassium. 4. S E N S I T I V I T Y
TO WIMPS
~- 10 "2
9 10 -3 a_ 2~ b
I po
"., ..................
5. C O N C L U S I O N S A sapphire detector with an energy threshold of 0.5 keV and with a total mass of 1 kg (in one or few pieces) is feasible. This first stage experiment carried out in a suitable low background environment could extend the search for WIMPs in the mass range below 10 GeV. The cryostat design and materials selection are in progress. REFERENCES
Fig. 4 shows a comparison of the estimated sensitivity limits for our experiment and two planned experiments for the cross section of a WIMP scattering on a proton with a spindependent interaction [2]. A constant background of 1 count/kg/keV/day is assumed for a NaI detector [3] (100kg.y, Eth = 16keV), a 73Ge detec-
10 .4
tor with 99% background rejection [4] (0.5 kg.y, Eth = 14.5keV), and an A1203 detector (lkg.y, Eth = 0.5keV). This plot shows that already 1 kg of sapphire can give a significant contribution to the search for WIMPs, complementary to the other planned experiments. A positive signature for dark matter could be given by the observation of the expected 10% annual modulation of the recoil spectrum. Since our detector allows the use of different materials as absorbers, recoil spectra can also be compared using targets with different nuclear spin or nuclear mass.
.....
101 10 2 Mass [GeV/c 2]
Figure 4. Estimated sensitivity limits.
10 3
1. M. Frank et al., Fifth International Workshop on Low Temperature Detectors, 29 July - 3 Aug., 1993, Berkeley, California, to be published in J. Low Temp. Phys., Nov. 1993; U. Nagel et al., ibid; P. Colling et al., ibid; G. Forster et al., ibid. 2. S. Cooper et al.,"Proposal to the Gran Sasso Laboratory for a Dark Matter Experiment using Cryogenic Detectors", MPI preprint MPIPhE/93-29 (1993). 3. C. Bacci et al. (BRS coll.), Phys. Lett. B 293 (1992) 460. 4. D.O. Caldwell in Low Temperature Detectors for Neutrinos and Dark Matter IV, Oxford 1991, Editions Frontitres, p.387.
PROCEEDINGS SUPPLEMENTS
Munich cryogenic detector development for direct Dark Matter search A. Nucciotti a • p. Colling a, S. Cooper a, D. D u m m e r a?, F. v. Feilitzsch 5, p. Ferger a, G. Forster b, M. Frank ~, H.-J. Gebauer ~, K. Hallatschek b, E. Kellner b, U. Nagel at, F. PrSbst a, A. Rulofs a, W. Seidel a L. Stodolsky a aMax-Planck-Institut ffir Physik, F6hringer Ring 6, D-80805 Mfinchen, G e r m a n y bTechnische Universit£t Miinchen, Physik-Dept. E15, D-85747 Garching, G e r m a n y We are developing massive cryogenic detectors with low energy thresholds and high resolution for use in a direct Dark Matter search experiment. Our detector consists of a superconducting phase transition thermometer evaporated onto the surface of an absorber crystal. An energy resolution of 220 eV FWHM (for 6 keV X-rays) has been reached with a 31 g sapphire crystal using a proximity-effect thermometer. The latest results for sapphire detectors using tungsten superconducting phase transition thermometers, whose development is just beginning, are also presented. The planned further development and use of such detectors in a search for Dark Matter particles (WIMPs) is discussed, showing their advantages for low WIMP masses.
1. I N T R O D U C T I O N We have been developing massive cryogenic calorimeters with very good energy resolution and low energy threshold for use in a search for Dark Matter particles (WIMPs) via their elastic scattering on nuclei [1,2]. Cryogenic calorimeters have the advantage of being fully sensitive to lowenergy nuclear recoils. We have recently applied for space in the Gran Sasso National Laboratories where as a first step a 1 kg sapphire detector (in one or a few pieces) with an energy threshold of 0.5 keV would be installed. This first stage experiment will provide a good sensitivity to W I M P s with masses below 10 GeV and spin-dependent interactions. Later larger masses (up to 100kg total) and other target materials will be used. 2. D E T E C T O R
DEVELOPMENT
A cryogenic calorimeter consists of an absorber, in which the particle interaction takes place, and a t h e r m o m e t e r attached to the absorber. The energy deposited in the absorber by a particle in*Present Address: Dipartimento di Fisica dell'Universit~ dl Milano, Milano, 1-20133, Italy tpresent Address: lZl~diometric Physics Division, NIST, Gaithersburg, MD 20899, USA !tOn leave from the Institute of Chemical Physics and Biophysics, EE0100 Tallinn, Estonia
teraction is measured via the corresponding temperature rise. Since heat capacities decrease towards lower temperatures, one can gain in sensitivity by running a calorimeter at low temperature (T < 100inK). The critical part of a calorimeter is the thermometer. We are developing superconducting phase transition (SPT) thermometers, which consist of a film of superconducting material evaporated onto the surface of an absorber. In the region around the critical t e m p e r a t u r e To, the resistance of the S P T thermometer goes from the normal-conducting value to zero in the superconducting state, showing a strong t e m p e r a t u r e dependence. Since the operating t e m p e r a t u r e of the calorimeter is determined by the critical temperature Tc of the thermometer, S P T thermometers with low Tc are needed to achieve high sensitivity for the calorimeter. This has been pursued in two ways: 1. films of iridium overlaid with gold, using the proximity effect to reduce the Tc of the bilayer below that of pure iridium (112 mK), and 2. films of pure tungsten (To = 15 mK). I r / A u proximity-effect thermometers with critical temperatures as low as 33inK and narrow transitions have been produced. Using an I r / A u thermometer with % = 45 m K evaporated onto a
0920-5632/94/$07.00 © 1994 - Elsevier Science R.V. All fights reserved. SSDI 0920-5632(94)(X)453-3
173
A. Nucciotti et al./Munich cryogenic detector development for direct dark matter search
8O
701 /
601
Mn
Ka
50. 40~'~ 30-
AEFwHM=220 eV
20:
Mn K#
10: 0 20'00
4000
6000
B000
10000
Energy [eV]
Figure 1. Energy spectrum measured using a 31 g sapphire crystal with a Ir/Au SPT thermometer while irradiating the calorimeter with a 55Fe Xray source. 31 g sapphire (a-A12Oz) absorber, an energy resolution of 220 eV (FWHM) for collimated 5.9 keY X-rays has been obtained (Fig. 1). We have also succeeded in evaporating epitaxial tungsten films on sapphire which have transition temperatures Tc near the 15 mK value of the bulk material. A first calorimeter with a tungsten thermometer evaporated onto a 4 g sapphire crystal gives an energy resolution of 75 eV (FWHM) for 1.5 keV X-rays, as shown in Fig. 2. The detector was operated with a threshold of about 100eV.
IBOOi
'
'
,
'
jAL~,
'
'
'
'
'
V
'
'
'
Mn
'
From our understanding of the present detectors, we expect to be able soon to build a detector with a threshold of 0.5keV, a FWHM resolution of 0.2keV, and a mass of 100g in one piece. Further development is in progress to make detectors with the same sensitivity but masses up to 1 kg. Our crystal sizes are at present limited to about 30 g by the background rates in our laboratory, not by the detector performance. Significant further progress in resolution and detector mass will probably only be reached in an underground lowbackground setup. 3. P L A N N E D
EXPERIMENTAL
SETUP
For a Dark Matter search experiment it is of extreme importance to reach the lowest possible radioactive background. To avoid background events induced by cosmic rays the whole experiment needs to be located in an underground laboratory. In addition it will be carefully shielded against the natural radioactivity of the surroundings. Because not all materials needed for building a dilution refrigerator are radiopure, it is advantageous to divide the experimental setup into two parts: one containing the dilution refrigerator and the other one containing the detector surrounded by very low background materials and shielding (Fig. 3). The cooling power of the refrigerator is transferred into the cold box by a cold finger. The space for experiments will be about 30 liters. The line of sight into the shielding will be blocked by an additional shield.
1200 1000
AEF~
= 75
eV
mixing chamber co[d finger
6O0
[ead shields copper shield
~00 20
thermal radiation 0
1000
2000
3000
/+000
5000
6000
7000
"
"
Energy [eV ]
Figure 2. Energy spectrum after optimal filtering measured using 4g sapphire crystal with a tungsten SPT thermometer while irradiating the calorimeter with a fluorescence X-ray source.
shields space for experiment
~//////////////////////////A Figure 3. Sketch of layout of the cold box.
A. Nucciotti et al./Munich cryogenic detector development for direct dark matter search
174
The materials of the cold box and of the detector itself are the most critical. Low radioactivity copper can be used for the cold box shielding, for the thermal radiation shields, for the flanges and for the screws. The cold box will contain no stainless steel, indium seals or cryogenic liquids. Any other materials that are needed will be carefully tested for radiopurity. Monte Carlo simulations to understand the effect of the internal contaminations of the detector are in progress and a simulation of the cold box shielding will be started soon. Sapphire crystals from different suppliers who use different crystal growth methods are being tested. First results using neutron activation analysis to determine their radioactive contaminations found no uranium, thorium, or potassium contaminations above our present detection limits (K< 1800ng/g, Th< 0.002ng/g, U< 0.02ng/g). More analyses with different techniques are necessary to improve the sensitivity, in particular for potassium. 4. S E N S I T I V I T Y
TO WIMPS
~- 10 "2
9 10 -3 a_ 2~ b
I po
"., ..................
5. C O N C L U S I O N S A sapphire detector with an energy threshold of 0.5 keV and with a total mass of 1 kg (in one or few pieces) is feasible. This first stage experiment carried out in a suitable low background environment could extend the search for WIMPs in the mass range below 10 GeV. The cryostat design and materials selection are in progress. REFERENCES
Fig. 4 shows a comparison of the estimated sensitivity limits for our experiment and two planned experiments for the cross section of a WIMP scattering on a proton with a spindependent interaction [2]. A constant background of 1 count/kg/keV/day is assumed for a NaI detector [3] (100kg.y, Eth = 16keV), a 73Ge detec-
10 .4
tor with 99% background rejection [4] (0.5 kg.y, Eth = 14.5keV), and an A1203 detector (lkg.y, Eth = 0.5keV). This plot shows that already 1 kg of sapphire can give a significant contribution to the search for WIMPs, complementary to the other planned experiments. A positive signature for dark matter could be given by the observation of the expected 10% annual modulation of the recoil spectrum. Since our detector allows the use of different materials as absorbers, recoil spectra can also be compared using targets with different nuclear spin or nuclear mass.
.....
101 10 2 Mass [GeV/c 2]
Figure 4. Estimated sensitivity limits.
10 3
1. M. Frank et al., Fifth International Workshop on Low Temperature Detectors, 29 July - 3 Aug., 1993, Berkeley, California, to be published in J. Low Temp. Phys., Nov. 1993; U. Nagel et al., ibid; P. Colling et al., ibid; G. Forster et al., ibid. 2. S. Cooper et al.,"Proposal to the Gran Sasso Laboratory for a Dark Matter Experiment using Cryogenic Detectors", MPI preprint MPIPhE/93-29 (1993). 3. C. Bacci et al. (BRS coll.), Phys. Lett. B 293 (1992) 460. 4. D.O. Caldwell in Low Temperature Detectors for Neutrinos and Dark Matter IV, Oxford 1991, Editions Frontitres, p.387.