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Scintillating bolometers: a powerful instrument for low background experiments
Nuclear Physics B (Proc. Suppl.) 215 (2011) 262–264 www.elsevier.com/locate/npbps
Scintillating bolometers: a powerful instrument for low background experiments L. Gironiab a
Universit`a degli Studi di Milano-Bicocca, Italy
b
Istituto Nazionale Fisica Nucleare, Milano-Bicocca, Italy
Thanks to their intrinsic characteristics, bolometers are suitable detectors for fundamental physics experiments, like neutrinoless Double Beta Decay and Dark Matter searches, and for radioactivity measurements. In particular great attention to such applications has been motivated by the usage of bolometers with combined read out: heat plus scintillation, heat plus ionization. In the case of heat plus scintillation we can combine the very high energy resolution of a bolometer with the very good particle discrimination capabilities of a scintillating detector.
1. Detectors for rare events searches Considerable interest has always been payed to searches for extremely rare events, from the particle physics community, since it could give an insight into the existence of new fundamental physics. The goal of such experiments can be different: from fundamental physics searches, such as neutrinoless Double Beta Decay (0νDBD), Dark Matter (DM) identification and electron stability, to the study of radioactive sources such as rare alpha decays or the study of very low surface contaminations. Experiments to search rare processes have to satisfy some requirements in order to reach good sensitivities: first of all low background but also very good energy resolution and, if possible, big target mass. Low background In experiments without discrimination capabilities any energy released in the detector can produce an unwanted background. For these detectors therefore there are a lot of sources of background, that can be divided into two main categories: internal contaminations and external ones. Background due to internal contaminations can be reduced with an accurate selection of building materials while the reduction of external contaminations requires well designed shieldings. 0920-5632/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2011.04.025
Thanks to the work done in several years of research and development very low background levels have been achieved: the 0.18 c/keV/Kg/y reached in Cuoricino [1], an experiment for 0νDBD located in the underground laboratory of Gran Sasso (LNGS), is an example. However, this very low level of background is not enough for rare events searches, so new techniques, based on a combined read out which allows discrimination capabilities, are under development. Detector material It is straightforward that the more material is used as detector the larger is the probability to observe the searched process. However, also the kind of material used as detector is very important. Just an example to explain: in neutrinoless Double Beta Decay a peak, at the Q value of the transition, is searched. The half live of this process depends on the effective neutrino mass and on the nuclear factor of merit and then on the isotope used. Moreover, other experimental characteristics depend on the choice of the isotope candidate for the 0νDBD, such as the natural abundance of the isotope and the energy of the transition Q. The transition energy is very important not only for the phase space of the transition but also from a background point of view [2]. Similar considerations can be made for the experiments looking for Dark Matter where the
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cross section for non-relativistic WIMPs depends on the nature of the couplings [3]. Energy resolution Energy resolution is a very important requirement especially for experiments that search for a well defined peak in the energy spectrum as in the 0νDBD, in the studies of rare alpha decays and in the studies of very low surface contaminations. Indeed if we don’t have good energy resolution it is very difficult to discriminate the searched peak from the background. Moreover, in experiments for 0νDBD this requirement is much more important because of the two neutrino Double Beta Decay (2νDBD): an isotope candidate to neutrinoless Double Beta Decay will decay also with two neutrino with a continuum spectrum up to the Q value of the transition. Then, without a good energy resolution, the searched peak will be covered by the continuum spectrum of the tail of the 2νDBD. It is possible to divide the most used detectors for rare events searches, that satisfy these requirements, into three categories: solid state detectors, phonon detectors and scintillators. Each of them has its own advantages but also some disadvantages. Detectors based on semiconductors have a very good energy resolution, but only few materials can be used. Phonon detectors, based on the measurement of the increase of the temperature due to the interaction of a particle, have similar energy resolution and allows a wide choice of absorber materials. However these detectors are very slow and until recently did not allow discrimination of the interacting particles. Instead light detectors are very fast detectors, have a very good discrimination capability but bad energy resolution. In order to increase sensitivities of the experiments, many detectors have been build trying to combine different informations. Scintillating bolometers for instance allow to combine information from heat and light channel. 2. Scintillating Bolometers A scintillating bolometer is, in principle, a very simple device [4]. It is composed by a calorimet-
Figure 1. Operating principle of scintillating bolometers. The release of energy inside a scintillating crystal follows two channels: light production and thermal excitation.
ric mass or absorber (a scintillating crystal coupled with a thermometer) and a Light Detector, optically coupled to it, and able to measure the emitted photons (see Fig. 1). The driving idea of this hybrid detector is to combine the two information available: the heat (i.e. the fraction of the energy released in the crystal absorber which is converted into phonons) and the emitted scintillation light (i.e. the small fraction of the energy which is converted into photons). Thanks to the different scintillation yield (or scintillation Quenching Factor) of different particles (namely β - γ, α and neutrons) they can be very efficiently discriminated. Thanks to the possibility of using different materials as scintillating crystals this technique can be used for low background experiments to study a wide range of processes, from 0νDBD to rare radioactive sources. Thin and dark slabs (usually of Ge, Si or sapphire) are typically used as light detectors. The idea to use a bolometer as light detector was first developed by C. Bobin et al. [5] and then optimized [6,7] for Dark Matter searches. In the last years a lot of different crystals were tested for 0νDBD. Some of these gave excellent results such as CdWO4 and ZnSe. CdWO4 has shown very good discrimination of
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L. Gironi / Nuclear Physics B (Proc. Suppl.) 215 (2011) 262–264
different particles (β - γ and α) thanks to the different scintillation yield [8]. This crystal allows to study the 0νDBD of 116 Cd with a Q-value of 2.8 MeV and a natural abundance of 7.5%. Moreover it allows the study of rare radioactive decays such as the rare alpha decay of 180 W. ZnSe is an excellent candidate to study the 0νDBD due to the favorable isotope content in it (a high isotopic abundance, ease enrichment, high Q value). However, this crystal showed a very peculiar feature: a quenching factor greater than 1 [9]. In part this may limit the ability of discrimination of interacting particles due to possible problems of collection of light. However, it was observed that the thermal pulses in the LD produced by the light emitted from this crystal has a different shape depending on the interacting particles. The combination of two things would make for excellent power of discrimination. A feasibility study for an experiment searching the 0νDBD of the 100 Mo (i.a.=9.6%, Qvalue= 3.03 MeV) with scintillating bolometers was tested, among others, with a CaMoO4 crystal. The sensitivity that can be reached by this crystal is limited and this is due to the presence of the continuum due to the 2νDBD of 48 Ca that decays with a Q-value of 4.27 MeV. However, this crystal showed a very interesting feature observed later also in other scintillating bolometers: the capability to discriminate interacting particles by the shape of the thermal signal [10]. This feature is very interesting because it could allow to realize a bolometric experiment able to discriminate interacting particles without the need to detect even the scintillation light. As mentioned before, scintillating bolometer were developed first for Dark Matter searches. CRESST [11] uses scintillating CaWO4 crystals as target and, to detect the small fraction of the interaction energy emitted from the crystals in the form of scintillation light, a separate cryogenic light detector is used. It consists of a sapphire wafer with a 1 µm silicon layer on one side, acting as photon absorber. Similar to the absorber crystals, the light detectors have a thin evaporated tungsten film which is operated as a transition edge sensor to read out the light signal. Other crystals, used as scintillating bolome-
ters mainly for Dark Matter searches [12], have achieved excellent results on the study of rare events. For example, using a BGO crystal the rare α decay of 209 Bi with an half-life of (1.9 ± 0.2) × 1019 yr was observed for the first time [13]. Thanks to the very high light yield of this crystal it can also be used to study surface contaminations of materials with high sensitivity. Indeed bolometers are fully active detectors that can reach large dimensions. It will therefore be possible to couple the materials to be studied to the large area surfaces of the bolometer absorber. This will reach sensitivity orders of magnitude larger than the classical Surface Barrier Detectors for alpha surface contamination studies. 3. Conclusions Scintillating bolometers offer a powerful technique for fundamental physics experiments, like 0νDBD and Dark Matter searches, and they are excellent detectors for measurements of radioactive sources. Some of the great achievements of different research groups developing this technique were presented. REFERENCES 1. C.Arnaboldi et al., Phys. Rev. C 78 (2008) 035502 2. S. Pirro et al., Physics of Atomic Nuclei 69 (2006) 2109 3. Particle Data Group, Dark Matter Reviews 4. A. Alessandrello et al., Phys. Lett. B 420 (1998) 109 5. C. Bobin et al., Nucl. Instr. Meth. A 386 (1994) 453 6. G. Angloher et al., Astropart. Phys. 23 (2005) 325 7. S. Cebrian et al., Phys. Lett. B 563 (2003) 48 8. C. Arnaboldi et al., accepted by Astroparticle Physics, arXiv:1005.1239 9. C. Arnaboldi et al., submitted to Astroparticle Physics, arXiv:1006.2721 10. L.Gironi, Nucl. Instr. Meth. A 617 (2010) 478 11. J. Schmaler et al., stabiarXiv:0912.3689v1 12. N. Coron et al., J. Phys. Conf. Ser. 203 (2010) 13. P. de Marcillac, Nature 422 (2003) 876
Scintillating bolometers: a powerful instrument for low background experiments L. Gironiab a
Universit`a degli Studi di Milano-Bicocca, Italy
b
Istituto Nazionale Fisica Nucleare, Milano-Bicocca, Italy
Thanks to their intrinsic characteristics, bolometers are suitable detectors for fundamental physics experiments, like neutrinoless Double Beta Decay and Dark Matter searches, and for radioactivity measurements. In particular great attention to such applications has been motivated by the usage of bolometers with combined read out: heat plus scintillation, heat plus ionization. In the case of heat plus scintillation we can combine the very high energy resolution of a bolometer with the very good particle discrimination capabilities of a scintillating detector.
1. Detectors for rare events searches Considerable interest has always been payed to searches for extremely rare events, from the particle physics community, since it could give an insight into the existence of new fundamental physics. The goal of such experiments can be different: from fundamental physics searches, such as neutrinoless Double Beta Decay (0νDBD), Dark Matter (DM) identification and electron stability, to the study of radioactive sources such as rare alpha decays or the study of very low surface contaminations. Experiments to search rare processes have to satisfy some requirements in order to reach good sensitivities: first of all low background but also very good energy resolution and, if possible, big target mass. Low background In experiments without discrimination capabilities any energy released in the detector can produce an unwanted background. For these detectors therefore there are a lot of sources of background, that can be divided into two main categories: internal contaminations and external ones. Background due to internal contaminations can be reduced with an accurate selection of building materials while the reduction of external contaminations requires well designed shieldings. 0920-5632/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2011.04.025
Thanks to the work done in several years of research and development very low background levels have been achieved: the 0.18 c/keV/Kg/y reached in Cuoricino [1], an experiment for 0νDBD located in the underground laboratory of Gran Sasso (LNGS), is an example. However, this very low level of background is not enough for rare events searches, so new techniques, based on a combined read out which allows discrimination capabilities, are under development. Detector material It is straightforward that the more material is used as detector the larger is the probability to observe the searched process. However, also the kind of material used as detector is very important. Just an example to explain: in neutrinoless Double Beta Decay a peak, at the Q value of the transition, is searched. The half live of this process depends on the effective neutrino mass and on the nuclear factor of merit and then on the isotope used. Moreover, other experimental characteristics depend on the choice of the isotope candidate for the 0νDBD, such as the natural abundance of the isotope and the energy of the transition Q. The transition energy is very important not only for the phase space of the transition but also from a background point of view [2]. Similar considerations can be made for the experiments looking for Dark Matter where the
L. Gironi / Nuclear Physics B (Proc. Suppl.) 215 (2011) 262–264
263
cross section for non-relativistic WIMPs depends on the nature of the couplings [3]. Energy resolution Energy resolution is a very important requirement especially for experiments that search for a well defined peak in the energy spectrum as in the 0νDBD, in the studies of rare alpha decays and in the studies of very low surface contaminations. Indeed if we don’t have good energy resolution it is very difficult to discriminate the searched peak from the background. Moreover, in experiments for 0νDBD this requirement is much more important because of the two neutrino Double Beta Decay (2νDBD): an isotope candidate to neutrinoless Double Beta Decay will decay also with two neutrino with a continuum spectrum up to the Q value of the transition. Then, without a good energy resolution, the searched peak will be covered by the continuum spectrum of the tail of the 2νDBD. It is possible to divide the most used detectors for rare events searches, that satisfy these requirements, into three categories: solid state detectors, phonon detectors and scintillators. Each of them has its own advantages but also some disadvantages. Detectors based on semiconductors have a very good energy resolution, but only few materials can be used. Phonon detectors, based on the measurement of the increase of the temperature due to the interaction of a particle, have similar energy resolution and allows a wide choice of absorber materials. However these detectors are very slow and until recently did not allow discrimination of the interacting particles. Instead light detectors are very fast detectors, have a very good discrimination capability but bad energy resolution. In order to increase sensitivities of the experiments, many detectors have been build trying to combine different informations. Scintillating bolometers for instance allow to combine information from heat and light channel. 2. Scintillating Bolometers A scintillating bolometer is, in principle, a very simple device [4]. It is composed by a calorimet-
Figure 1. Operating principle of scintillating bolometers. The release of energy inside a scintillating crystal follows two channels: light production and thermal excitation.
ric mass or absorber (a scintillating crystal coupled with a thermometer) and a Light Detector, optically coupled to it, and able to measure the emitted photons (see Fig. 1). The driving idea of this hybrid detector is to combine the two information available: the heat (i.e. the fraction of the energy released in the crystal absorber which is converted into phonons) and the emitted scintillation light (i.e. the small fraction of the energy which is converted into photons). Thanks to the different scintillation yield (or scintillation Quenching Factor) of different particles (namely β - γ, α and neutrons) they can be very efficiently discriminated. Thanks to the possibility of using different materials as scintillating crystals this technique can be used for low background experiments to study a wide range of processes, from 0νDBD to rare radioactive sources. Thin and dark slabs (usually of Ge, Si or sapphire) are typically used as light detectors. The idea to use a bolometer as light detector was first developed by C. Bobin et al. [5] and then optimized [6,7] for Dark Matter searches. In the last years a lot of different crystals were tested for 0νDBD. Some of these gave excellent results such as CdWO4 and ZnSe. CdWO4 has shown very good discrimination of
264
L. Gironi / Nuclear Physics B (Proc. Suppl.) 215 (2011) 262–264
different particles (β - γ and α) thanks to the different scintillation yield [8]. This crystal allows to study the 0νDBD of 116 Cd with a Q-value of 2.8 MeV and a natural abundance of 7.5%. Moreover it allows the study of rare radioactive decays such as the rare alpha decay of 180 W. ZnSe is an excellent candidate to study the 0νDBD due to the favorable isotope content in it (a high isotopic abundance, ease enrichment, high Q value). However, this crystal showed a very peculiar feature: a quenching factor greater than 1 [9]. In part this may limit the ability of discrimination of interacting particles due to possible problems of collection of light. However, it was observed that the thermal pulses in the LD produced by the light emitted from this crystal has a different shape depending on the interacting particles. The combination of two things would make for excellent power of discrimination. A feasibility study for an experiment searching the 0νDBD of the 100 Mo (i.a.=9.6%, Qvalue= 3.03 MeV) with scintillating bolometers was tested, among others, with a CaMoO4 crystal. The sensitivity that can be reached by this crystal is limited and this is due to the presence of the continuum due to the 2νDBD of 48 Ca that decays with a Q-value of 4.27 MeV. However, this crystal showed a very interesting feature observed later also in other scintillating bolometers: the capability to discriminate interacting particles by the shape of the thermal signal [10]. This feature is very interesting because it could allow to realize a bolometric experiment able to discriminate interacting particles without the need to detect even the scintillation light. As mentioned before, scintillating bolometer were developed first for Dark Matter searches. CRESST [11] uses scintillating CaWO4 crystals as target and, to detect the small fraction of the interaction energy emitted from the crystals in the form of scintillation light, a separate cryogenic light detector is used. It consists of a sapphire wafer with a 1 µm silicon layer on one side, acting as photon absorber. Similar to the absorber crystals, the light detectors have a thin evaporated tungsten film which is operated as a transition edge sensor to read out the light signal. Other crystals, used as scintillating bolome-
ters mainly for Dark Matter searches [12], have achieved excellent results on the study of rare events. For example, using a BGO crystal the rare α decay of 209 Bi with an half-life of (1.9 ± 0.2) × 1019 yr was observed for the first time [13]. Thanks to the very high light yield of this crystal it can also be used to study surface contaminations of materials with high sensitivity. Indeed bolometers are fully active detectors that can reach large dimensions. It will therefore be possible to couple the materials to be studied to the large area surfaces of the bolometer absorber. This will reach sensitivity orders of magnitude larger than the classical Surface Barrier Detectors for alpha surface contamination studies. 3. Conclusions Scintillating bolometers offer a powerful technique for fundamental physics experiments, like 0νDBD and Dark Matter searches, and they are excellent detectors for measurements of radioactive sources. Some of the great achievements of different research groups developing this technique were presented. REFERENCES 1. C.Arnaboldi et al., Phys. Rev. C 78 (2008) 035502 2. S. Pirro et al., Physics of Atomic Nuclei 69 (2006) 2109 3. Particle Data Group, Dark Matter Reviews 4. A. Alessandrello et al., Phys. Lett. B 420 (1998) 109 5. C. Bobin et al., Nucl. Instr. Meth. A 386 (1994) 453 6. G. Angloher et al., Astropart. Phys. 23 (2005) 325 7. S. Cebrian et al., Phys. Lett. B 563 (2003) 48 8. C. Arnaboldi et al., accepted by Astroparticle Physics, arXiv:1005.1239 9. C. Arnaboldi et al., submitted to Astroparticle Physics, arXiv:1006.2721 10. L.Gironi, Nucl. Instr. Meth. A 617 (2010) 478 11. J. Schmaler et al., stabiarXiv:0912.3689v1 12. N. Coron et al., J. Phys. Conf. Ser. 203 (2010) 13. P. de Marcillac, Nature 422 (2003) 876