- Email: [email protected]
Science and technology of large-mass bolometer arrays
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 520 (2004) 151–155
Science and technology of large-mass bolometer arrays Chiara Brofferioa,b,* a
Dipartimento di Fisica, Universita" di Milano-Bicocca, Piazza della Scienza 3, Milano 20126, Italy b INFN, Sezione di Milano, Via Celoria 16, Milano 20133, Italy
Abstract After almost 20 years of R&D, cryogenic detectors are now officially inserted in the list of detectors for fundamental and applied physics. One of the main research fields where bolometers are conveniently exploitable is in the search of rare events, like Double Beta Decay, Dark Matter or Axions. The first requirement that had to be fulfilled for this application was to progress from the mg range of the first absorbers towards the kg range of nowadays detectors, together with the development of the array technique, that climbed from the first four detector style up to the present 60 and, possibly, the future 1000 channel array. The increase in mass and in number of channels must go on together with crucial scientific and technological achievements, some of which are discussed in this paper. r 2003 Elsevier B.V. All rights reserved. PACS: 07.57.Kp; 07.20.Mc; 29.90.+r Keywords: Bolometers; Underground detectors; Array technology
1. Introduction Low-temperature Phonon-mediated detectors (PMD) of particles are devices naturally tailored to search for rare events in hot topics of nonaccelerator- and astro-particle physics. This is due to the high energy sensitivity (which translates into low threshold and/or high resolution), to the flexibility in material choice, and to the possibility to reject some type of background if other excitations are detected simultaneously with phonons. That is why the most promising nextgeneration experiments searching for Neutrinoless *Address for Correspondence: Dipartimento di Fisica, Universit"a di Milano-Bicocca, Piazza della Scienza 3, Milano 20126, Italy. Tel.: +39-02-6448-2426; fax: +39-02-6448-2463. E-mail address: [email protected] (C. Brofferio).
Double Beta Decay (0n-DBD) and Dark Matter (DM) are often based on this kind of technology. However, PMDs have also a natural limitation: their size. Present technology does not allow to realize detectors larger than about 5 cm (linear dimension) or heavier than about 1 kg. On the contrary, the sensitivity of 0n-DBD and DM experiments scales as the square root of the total active mass (since a signal must be detected in competition with a Poisson-fluctuating background): next generation experiments will require total masses of the order of 10–1000 kg. The only way to achieve this size range with PMDs is the multiplication of the sensitive channels, i.e. the realization of arrays with typically 50–1000 detectors. The state of the art in this field is shown in Table 1.
0168-9002/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2003.11.280
ARTICLE IN PRESS C. Brofferio / Nuclear Instruments and Methods in Physics Research A 520 (2004) 151–155
152
Table 1 Development of arrays of PMDs: the state of the art Group
Main physics
Method
CDMS [1] CRESST [2] CUORE [3]
Dark matter Dark matter Double beta decay Dark matter
Ioniz.+phonons ZIP sensors 6 Light+phonons TES sensors 4 Heat only NTD Ge 62
1.2 kg 1 kg 41 kg
42 33 (+33) 1000
8.4 10 kg 750 kg
Ioniz.+heat NTD Ge
1 kg
18 (120)
6 (38) kg
EDELWEISS [4]
In some case, mass is not the only added value of detector arrays. The possibility to operate the array elements in temporal (anti)-coincidence can provide further information on the registered events. In extreme cases, the array of detectors aims at being a single high-granularity segmented device. In this paper, we will not focus on the physics and technology of the single-array element, for which we refer to specific papers. Rather, we would like to examine the points which must specifically be addressed when dealing with a multi-element large-mass array, taking into account also the typical requirements of rare event searches: *
* *
*
* * * *
cryogenics, with emphasis on the cooling down of large masses; control of vibrational noise; control of radioactivity from detectors and cryostat; reproducibility of the single element performances; detector assembly in clean conditions; mechanical structure of the array; read-out and electronics; efficiency of the duty-cycle.
Of course, many of these items are strictly interconnected. All the collaborations which have realized or are planning to realize arrays are dealing with the above list to smaller or larger extent.
2. Cryogenics, shielding and vibrations When 50–1000 PMDs have to be cooled down to 10–50 mK, an experimental volume of the order of
N. of detectors Sensitive mass (present) (present)
3
N. of detectors Sensitive mass (future) (future)
0.1 m3 (presently, CRESST and CUORICINO for instance) through 1 m3 (in the future, CUORE) must be available for the detectors. Therefore, a high-power dilution unit must be combined with a large cryostat (placed in an underground lab), taking into account that the residual unavoidable radioactivity of the dilution unit and of the dewar around it must be shielded by typically 20 cm of high-purity lead. This non-trivial task is accomplished by the various groups with more or less innovative approaches: (1) a traditional structure with an internal lead shielding between the dilution unit and the detectors (CUORICINO/CUORE); (2) a standard unit with a long cold finger to keep the experimental volume at some distance from the mixing chamber, reducing the mass of the lead shielding inside the cryostat (CRESST); (3) a lateral ‘‘ice box’’, following the typical Gediode approach, that guarantees an efficient shielding of the experimental room, even if the cryostat design is in this case not at all trivial (CDMS); (4) finally, a ‘‘reverse cryostat’’, with the mixing chamber on the top and the detectors above it, no more ‘‘hanging’’ from the dilution unit, but still to be shielded from it with internal lead (Edelweiss). The traditional structure of point (1) has the advantage that it can be easily matched to an antivibrational mounting. The detector can be suspended by one or more helical springs (as already done in the CRESST and CUORICINO experiments), realizing a low pass mechanical filter which
ARTICLE IN PRESS C. Brofferio / Nuclear Instruments and Methods in Physics Research A 520 (2004) 151–155
suppresses the acoustical noise generated by the cryogenic apparatus. An annoying source of vibrational noise is represented by the 1 K bath. This problem can be substantially solved if the helium picked up from the main helium bath is driven superfluid before being injected in the 1 K bath, by means of a proper heat exchanger. Very good results were obtained in CUORICINO following this approach. Another important point in cryogenics is the cooling down procedure. The large amount of copper in detector holders, normally rich with molecular hydrogen inclusions, introduces a parasitic power due to ortho- to para-hydrogen conversion, responsible for long coolingdown times and long term temperature drifts. Annealing of copper under vacuum can help solving this problem. Another approach consists of keeping the detector around 20 K for a few days during the cooling down. At this temperature the ortho- to para-hydrogen conversion is sped up since the hydrogen inclusions are still in the gaseous state. The outcome of this technique was spectacular in the CUORICINO case: the array was cooled down from 1.5 K to less than 10 mK in less than 2 days and no temperature drifts were observed in the subsequent period. When ton-scale arrays are realized, the time for cooling down from room temperature to 4.2 K will be significant. In order to speed it up, forced liquid helium re-circulation methods should be developed, on the basis of the experience gathered by the gravitational antenna groups.
3. Materials, clean assembly and reproducibility of the performances The dominant material in detector holders is normally copper, which joins the properties to be achievable with very high radio-purity and to be ideal for low-temperature heat-sinking, due to the high thermal conductivity. Small amounts of Teflons are also present: normally they are used to hold the crystals, assuring a firm mechanical support (Teflons elasticity, which is preserved at
153
cryogenic temperatures, helps in that) and a low thermal conduction. Again, Teflons radio-purity can be very high. Therefore, the Teflons-copper combination is the key-feature of many detector holders. Fabrication and assembly of detectors must at the same time assure the reproducibility of detector performance and avoid detector contamination with radioactive impurities. The detector assembly must be performed in special clean environments with low radon concentration. In addition, for large array construction it should be organized on a semi-industrial basis in order to reduce fabrication time and to improve repeatability. In case of TES sensors the transition temperatures of the W films should exhibit a low spread, which is not a straightforward task. On the contrary, Neutron Transmutation Doped (NTD) Ge thermistors are intrinsically well reproducible, due to the large attenuation length of thermal neutrons in germanium with respect to the crystal size. The coupling of the NTD sensor to the main crystal is more critical. The use of glue, even with sophisticated control systems has proven to give a large spread in the thermal conductance of the detector–sensor interface, up to an order of magnitude, in spite of all the efforts made to develop a reproducible procedure. The film deposition technique should provide instead a more reproducible coupling to the crystal. The thermal coupling of the detector to the heat bath can be controlled by means of specially devoted metal wires (CRESST) or realized simply by the copper–Teflons–crystal series (CUORICINO). The last method is not very reproducible and produces pulses with large spread (a factor 2) in the decay time. CUORICINO, with its 62 elements, is by far the largest array ever realized. As anticipated, a significant spread is present in pulse shape and height. However, the most important parameter in detector behavior for DBD search, i.e. the energy resolution, is high (around 8 keV FWHM at 2615 keV) and reasonably uniform, making CUORICINO an extremely successful device for its physics goal.
ARTICLE IN PRESS 154
C. Brofferio / Nuclear Instruments and Methods in Physics Research A 520 (2004) 151–155
4. Mechanical structure, read-out and electronics All the arrays up to now realized have a common element: to have a tower-like structure. Detectors are stacked 1 by 1 (CDMS and EDELWEISS) or 4 4 (CUORICINO). This often has an obvious explanation in the conventional cryogenic systems, in which the experimental space is a long vertical cylinder. As a consequence, future next generation arrays are often conceived as a set of parallel towers, multiplying the present structure. This is the case of CUORE (25 CUORICINO-like towers), CDMSII (7 CDMS-I towers) and CRESST-II (12 towers). CUORE case is clearly distinct from the others as far as the arrangement of array elements is concerned: they are set so as to increase as much as possible the mutual detector visibility and to minimize inert materials between elements. The goal is to identify alpha or beta particles emitted at the surface and sharing energy between two detectors. In this sense, CUORE is a single segmented device, while the other arrays are mainly just a collection of individual detectors. Even in the latter case however, fast neutron events in coincidence between two detectors will play (and have already played in CDMS-I) an important role to get information on this type of background. Wiring is a not negligible business in large-array construction. In CUORICINO case, about 150 wires were drawn along the cryostat with five thermalization points, for a total of about 1500 solders. Other groups are developing special multipole strips and connectors to speed up the wiring, having always in mind the material selection for low radioactivity. The heat load determined by the wires at each thermal stage must be estimated and controlled in order not to affect detector base temperature. As far as the electronics of the phonon channel is concerned, there is a big difference between the TES and the NTD Ge thermistor read-out. In the former case, SQUIDs, expensive and delicate, are the front end elements. Methods of multiplexing are under study for the reduction of the readout channels and/or of the SQUID number itself.
In the latter, more conventional voltage amplifiers are foreseen, and an individual read-out chain for each element is conceivable both from the practical and economical point of view. At the present design stage, CUORE foresees a read-out entirely based on room temperature conventional electronics.
5. Efficiency of the duty cycle 0n-DBD and DM searches require years of lifetime in order to achieve significant results. Therefore, the duty cycle of the detector arrays has to be very efficient. Many factors may conspire to reduce the measuring time, given the delicate nature of PMDs and of the cryogenic apparatuses. In the case of Mi-DBD and CUORICINO, several stops were due to any sort of trivial reasons (failure of mixture circulation pump, air leak in the mixture circuit, failure of the helium liquefier, problems with the cooling water circulation for compressors etc.) and to less ordinary facts, like not fully understood noise bursts of electric and vibrational origin. Considerable efforts to overcome all these problems have lead now to a duty cycle of better than 70%, which however includes also the calibration time. Similar results have been obtained at the best by other collaborations, like CDMS-I. During data taking, detector stability must be checked, and possible instabilities due to temperature drifts corrected. In the case of Mi-DBD and CUORICINO (by far the cryogenic arrays with the longest data taking periods), the instabilities are corrected by means of a heater thermally coupled to the main crystal which injects a fixed amount of energy in the detector. The best correction is achieved by correlating the amplitude of the thermal pulse generated by the heater with the detector instantaneous temperature, measured by the DC level of the detector output immediately before the heater pulse. This correlation allows to correct the pulse amplitude of every pulse once the corresponding baseline DC level is known. Similar methods are adopted by other groups.
ARTICLE IN PRESS C. Brofferio / Nuclear Instruments and Methods in Physics Research A 520 (2004) 151–155
References [1] D. Abrams, et al., CDMS Coll., Phys. Rev. D 66 (2002) 122003 references therein. [2] C. Bucci, et al., Update of the Proposal to the LNGS for a Second Phase of the CRESST Dark Matter Search, MPI preprint MPI PhE/2001–02.
155
[3] C. Arnaboldi, et al, The CUORICINO Coll., First results on neutrinoless double beta decay of Te-130 with the calorimetric CUORICINO experiment, to be published in Phys. Lett. B. [4] S. Marnieros, et al., EDELWEISS Coll., Latest results from the EDELWEISS Dark Matter Experiment, this Proceedings, and references therein.
Nuclear Instruments and Methods in Physics Research A 520 (2004) 151–155
Science and technology of large-mass bolometer arrays Chiara Brofferioa,b,* a
Dipartimento di Fisica, Universita" di Milano-Bicocca, Piazza della Scienza 3, Milano 20126, Italy b INFN, Sezione di Milano, Via Celoria 16, Milano 20133, Italy
Abstract After almost 20 years of R&D, cryogenic detectors are now officially inserted in the list of detectors for fundamental and applied physics. One of the main research fields where bolometers are conveniently exploitable is in the search of rare events, like Double Beta Decay, Dark Matter or Axions. The first requirement that had to be fulfilled for this application was to progress from the mg range of the first absorbers towards the kg range of nowadays detectors, together with the development of the array technique, that climbed from the first four detector style up to the present 60 and, possibly, the future 1000 channel array. The increase in mass and in number of channels must go on together with crucial scientific and technological achievements, some of which are discussed in this paper. r 2003 Elsevier B.V. All rights reserved. PACS: 07.57.Kp; 07.20.Mc; 29.90.+r Keywords: Bolometers; Underground detectors; Array technology
1. Introduction Low-temperature Phonon-mediated detectors (PMD) of particles are devices naturally tailored to search for rare events in hot topics of nonaccelerator- and astro-particle physics. This is due to the high energy sensitivity (which translates into low threshold and/or high resolution), to the flexibility in material choice, and to the possibility to reject some type of background if other excitations are detected simultaneously with phonons. That is why the most promising nextgeneration experiments searching for Neutrinoless *Address for Correspondence: Dipartimento di Fisica, Universit"a di Milano-Bicocca, Piazza della Scienza 3, Milano 20126, Italy. Tel.: +39-02-6448-2426; fax: +39-02-6448-2463. E-mail address: [email protected] (C. Brofferio).
Double Beta Decay (0n-DBD) and Dark Matter (DM) are often based on this kind of technology. However, PMDs have also a natural limitation: their size. Present technology does not allow to realize detectors larger than about 5 cm (linear dimension) or heavier than about 1 kg. On the contrary, the sensitivity of 0n-DBD and DM experiments scales as the square root of the total active mass (since a signal must be detected in competition with a Poisson-fluctuating background): next generation experiments will require total masses of the order of 10–1000 kg. The only way to achieve this size range with PMDs is the multiplication of the sensitive channels, i.e. the realization of arrays with typically 50–1000 detectors. The state of the art in this field is shown in Table 1.
0168-9002/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2003.11.280
ARTICLE IN PRESS C. Brofferio / Nuclear Instruments and Methods in Physics Research A 520 (2004) 151–155
152
Table 1 Development of arrays of PMDs: the state of the art Group
Main physics
Method
CDMS [1] CRESST [2] CUORE [3]
Dark matter Dark matter Double beta decay Dark matter
Ioniz.+phonons ZIP sensors 6 Light+phonons TES sensors 4 Heat only NTD Ge 62
1.2 kg 1 kg 41 kg
42 33 (+33) 1000
8.4 10 kg 750 kg
Ioniz.+heat NTD Ge
1 kg
18 (120)
6 (38) kg
EDELWEISS [4]
In some case, mass is not the only added value of detector arrays. The possibility to operate the array elements in temporal (anti)-coincidence can provide further information on the registered events. In extreme cases, the array of detectors aims at being a single high-granularity segmented device. In this paper, we will not focus on the physics and technology of the single-array element, for which we refer to specific papers. Rather, we would like to examine the points which must specifically be addressed when dealing with a multi-element large-mass array, taking into account also the typical requirements of rare event searches: *
* *
*
* * * *
cryogenics, with emphasis on the cooling down of large masses; control of vibrational noise; control of radioactivity from detectors and cryostat; reproducibility of the single element performances; detector assembly in clean conditions; mechanical structure of the array; read-out and electronics; efficiency of the duty-cycle.
Of course, many of these items are strictly interconnected. All the collaborations which have realized or are planning to realize arrays are dealing with the above list to smaller or larger extent.
2. Cryogenics, shielding and vibrations When 50–1000 PMDs have to be cooled down to 10–50 mK, an experimental volume of the order of
N. of detectors Sensitive mass (present) (present)
3
N. of detectors Sensitive mass (future) (future)
0.1 m3 (presently, CRESST and CUORICINO for instance) through 1 m3 (in the future, CUORE) must be available for the detectors. Therefore, a high-power dilution unit must be combined with a large cryostat (placed in an underground lab), taking into account that the residual unavoidable radioactivity of the dilution unit and of the dewar around it must be shielded by typically 20 cm of high-purity lead. This non-trivial task is accomplished by the various groups with more or less innovative approaches: (1) a traditional structure with an internal lead shielding between the dilution unit and the detectors (CUORICINO/CUORE); (2) a standard unit with a long cold finger to keep the experimental volume at some distance from the mixing chamber, reducing the mass of the lead shielding inside the cryostat (CRESST); (3) a lateral ‘‘ice box’’, following the typical Gediode approach, that guarantees an efficient shielding of the experimental room, even if the cryostat design is in this case not at all trivial (CDMS); (4) finally, a ‘‘reverse cryostat’’, with the mixing chamber on the top and the detectors above it, no more ‘‘hanging’’ from the dilution unit, but still to be shielded from it with internal lead (Edelweiss). The traditional structure of point (1) has the advantage that it can be easily matched to an antivibrational mounting. The detector can be suspended by one or more helical springs (as already done in the CRESST and CUORICINO experiments), realizing a low pass mechanical filter which
ARTICLE IN PRESS C. Brofferio / Nuclear Instruments and Methods in Physics Research A 520 (2004) 151–155
suppresses the acoustical noise generated by the cryogenic apparatus. An annoying source of vibrational noise is represented by the 1 K bath. This problem can be substantially solved if the helium picked up from the main helium bath is driven superfluid before being injected in the 1 K bath, by means of a proper heat exchanger. Very good results were obtained in CUORICINO following this approach. Another important point in cryogenics is the cooling down procedure. The large amount of copper in detector holders, normally rich with molecular hydrogen inclusions, introduces a parasitic power due to ortho- to para-hydrogen conversion, responsible for long coolingdown times and long term temperature drifts. Annealing of copper under vacuum can help solving this problem. Another approach consists of keeping the detector around 20 K for a few days during the cooling down. At this temperature the ortho- to para-hydrogen conversion is sped up since the hydrogen inclusions are still in the gaseous state. The outcome of this technique was spectacular in the CUORICINO case: the array was cooled down from 1.5 K to less than 10 mK in less than 2 days and no temperature drifts were observed in the subsequent period. When ton-scale arrays are realized, the time for cooling down from room temperature to 4.2 K will be significant. In order to speed it up, forced liquid helium re-circulation methods should be developed, on the basis of the experience gathered by the gravitational antenna groups.
3. Materials, clean assembly and reproducibility of the performances The dominant material in detector holders is normally copper, which joins the properties to be achievable with very high radio-purity and to be ideal for low-temperature heat-sinking, due to the high thermal conductivity. Small amounts of Teflons are also present: normally they are used to hold the crystals, assuring a firm mechanical support (Teflons elasticity, which is preserved at
153
cryogenic temperatures, helps in that) and a low thermal conduction. Again, Teflons radio-purity can be very high. Therefore, the Teflons-copper combination is the key-feature of many detector holders. Fabrication and assembly of detectors must at the same time assure the reproducibility of detector performance and avoid detector contamination with radioactive impurities. The detector assembly must be performed in special clean environments with low radon concentration. In addition, for large array construction it should be organized on a semi-industrial basis in order to reduce fabrication time and to improve repeatability. In case of TES sensors the transition temperatures of the W films should exhibit a low spread, which is not a straightforward task. On the contrary, Neutron Transmutation Doped (NTD) Ge thermistors are intrinsically well reproducible, due to the large attenuation length of thermal neutrons in germanium with respect to the crystal size. The coupling of the NTD sensor to the main crystal is more critical. The use of glue, even with sophisticated control systems has proven to give a large spread in the thermal conductance of the detector–sensor interface, up to an order of magnitude, in spite of all the efforts made to develop a reproducible procedure. The film deposition technique should provide instead a more reproducible coupling to the crystal. The thermal coupling of the detector to the heat bath can be controlled by means of specially devoted metal wires (CRESST) or realized simply by the copper–Teflons–crystal series (CUORICINO). The last method is not very reproducible and produces pulses with large spread (a factor 2) in the decay time. CUORICINO, with its 62 elements, is by far the largest array ever realized. As anticipated, a significant spread is present in pulse shape and height. However, the most important parameter in detector behavior for DBD search, i.e. the energy resolution, is high (around 8 keV FWHM at 2615 keV) and reasonably uniform, making CUORICINO an extremely successful device for its physics goal.
ARTICLE IN PRESS 154
C. Brofferio / Nuclear Instruments and Methods in Physics Research A 520 (2004) 151–155
4. Mechanical structure, read-out and electronics All the arrays up to now realized have a common element: to have a tower-like structure. Detectors are stacked 1 by 1 (CDMS and EDELWEISS) or 4 4 (CUORICINO). This often has an obvious explanation in the conventional cryogenic systems, in which the experimental space is a long vertical cylinder. As a consequence, future next generation arrays are often conceived as a set of parallel towers, multiplying the present structure. This is the case of CUORE (25 CUORICINO-like towers), CDMSII (7 CDMS-I towers) and CRESST-II (12 towers). CUORE case is clearly distinct from the others as far as the arrangement of array elements is concerned: they are set so as to increase as much as possible the mutual detector visibility and to minimize inert materials between elements. The goal is to identify alpha or beta particles emitted at the surface and sharing energy between two detectors. In this sense, CUORE is a single segmented device, while the other arrays are mainly just a collection of individual detectors. Even in the latter case however, fast neutron events in coincidence between two detectors will play (and have already played in CDMS-I) an important role to get information on this type of background. Wiring is a not negligible business in large-array construction. In CUORICINO case, about 150 wires were drawn along the cryostat with five thermalization points, for a total of about 1500 solders. Other groups are developing special multipole strips and connectors to speed up the wiring, having always in mind the material selection for low radioactivity. The heat load determined by the wires at each thermal stage must be estimated and controlled in order not to affect detector base temperature. As far as the electronics of the phonon channel is concerned, there is a big difference between the TES and the NTD Ge thermistor read-out. In the former case, SQUIDs, expensive and delicate, are the front end elements. Methods of multiplexing are under study for the reduction of the readout channels and/or of the SQUID number itself.
In the latter, more conventional voltage amplifiers are foreseen, and an individual read-out chain for each element is conceivable both from the practical and economical point of view. At the present design stage, CUORE foresees a read-out entirely based on room temperature conventional electronics.
5. Efficiency of the duty cycle 0n-DBD and DM searches require years of lifetime in order to achieve significant results. Therefore, the duty cycle of the detector arrays has to be very efficient. Many factors may conspire to reduce the measuring time, given the delicate nature of PMDs and of the cryogenic apparatuses. In the case of Mi-DBD and CUORICINO, several stops were due to any sort of trivial reasons (failure of mixture circulation pump, air leak in the mixture circuit, failure of the helium liquefier, problems with the cooling water circulation for compressors etc.) and to less ordinary facts, like not fully understood noise bursts of electric and vibrational origin. Considerable efforts to overcome all these problems have lead now to a duty cycle of better than 70%, which however includes also the calibration time. Similar results have been obtained at the best by other collaborations, like CDMS-I. During data taking, detector stability must be checked, and possible instabilities due to temperature drifts corrected. In the case of Mi-DBD and CUORICINO (by far the cryogenic arrays with the longest data taking periods), the instabilities are corrected by means of a heater thermally coupled to the main crystal which injects a fixed amount of energy in the detector. The best correction is achieved by correlating the amplitude of the thermal pulse generated by the heater with the detector instantaneous temperature, measured by the DC level of the detector output immediately before the heater pulse. This correlation allows to correct the pulse amplitude of every pulse once the corresponding baseline DC level is known. Similar methods are adopted by other groups.
ARTICLE IN PRESS C. Brofferio / Nuclear Instruments and Methods in Physics Research A 520 (2004) 151–155
References [1] D. Abrams, et al., CDMS Coll., Phys. Rev. D 66 (2002) 122003 references therein. [2] C. Bucci, et al., Update of the Proposal to the LNGS for a Second Phase of the CRESST Dark Matter Search, MPI preprint MPI PhE/2001–02.
155
[3] C. Arnaboldi, et al, The CUORICINO Coll., First results on neutrinoless double beta decay of Te-130 with the calorimetric CUORICINO experiment, to be published in Phys. Lett. B. [4] S. Marnieros, et al., EDELWEISS Coll., Latest results from the EDELWEISS Dark Matter Experiment, this Proceedings, and references therein.