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Development of new bolometers for rare events with background active discrimination
Progress in Particle and Nuclear Physics 57 (2006) 269–271 www.elsevier.com/locate/ppnp
Review
Development of new bolometers for rare events with background active discrimination C. Nones a,b,∗ , L. Foggetta c , A. Giuliani a,c , M. Pedretti a,c , C. Salvioni a,c , S. Sangiorgio a,c a INFN, Sezione di Milano, Italy b Dipartimento di Fisica dell’Universit`a di Milano-Bicocca, Italy c Dipartimento di Fisica e Matematica dell’Universit`a dell’Insubria, Italy
Accepted 18 November 2005
Abstract We present an innovation in the structure of a bolometer which overcomes one of the drawbacks of the calorimetric mode. In this work we prove that studying the dynamics of the heat flow in the detector we can obtain information on the particle impact point and we are able to develop a new technique for an active background discrimination, in particular in view of the CUORE experiment. c 2005 Elsevier B.V. All rights reserved. Keywords: Neutrino mass; Double beta decay; Low background; Bolometers; Cryogenics
1. Introduction CUORE (Cryogenic Underground Observatory for Rare Events) [1] will be a next generation Double Beta Decay experiment based on TeO2 macrobolometers and it will be installed in the Laboratori Nazionali del Gran Sasso. It will extend the sensitivity to the effective neutrino mass to the inverted hierarchy region of the neutrino mass pattern (∼50 meV). One of the major challenges for CUORE is the radical improvement of the background coming from α and β particles, emitted close to the surface of the detector or of the materials which surround it, that contribute with a continuum spectrum which can extend down to a region relevant for double beta decay search for 130 Te. ∗ Corresponding author at: INFN, Sezione di Milano, Italy.
E-mail address: [email protected] (C. Nones). c 2005 Elsevier B.V. All rights reserved. 0146-6410/$ - see front matter doi:10.1016/j.ppnp.2005.11.022
270
C. Nones et al. / Progress in Particle and Nuclear Physics 57 (2006) 269–271
2. Detectors: The idea and the experimental set-up The basic idea that lies behind the possibility to identify the origin of an event is quite simple: it consists of the active shielding of the main bolometer by means of thin foils of ultrapure materials. The original idea is that shielded bolometers are not independent from the main bolometer but are attached to it to form a single composite bolometer. On each shield, a thermistor is attached for temperature reading: shields are thus bolometers with an absorber of proper size and shape. The origin of the events can be clearly determined by comparing the pulse amplitudes coming from different detector elements. If an α particle comes from outside the bolometer, it interacts with an active layer releasing there all its energy (surface event). As a consequence, the temperature will rise and there will be a signal on both the layer thermistor and the TeO2 one. Because of the small heat capacity of the layer, its thermistor signal will be much higher and faster than that of the TeO2 thermistor. On the other hand, an event inside the TeO2 crystal (bulk event) will lead to pulses with similar amplitudes and shapes on both thermistors. In this work we present results obtained with different materials as active shields. The main property of shields is that they must have a small heat capacity compared to that of the main absorber. The first material that one has in mind is germanium which has a small heat capacity and can be grown at a very high purity level. The main problem of using Ge shields is the high cost for the production of such crystals. In view of the CUORE experiment, which will consist of 988 bolometers, we had to find a material with similar properties as germanium; we moved from germanium to silicon which has a reasonable cost but a worse level of achievable purity. To avoid the introduction of new materials in the CUORE set-up, we decided to test also TeO2 shields which have the same thermal contractions of the main absorber but we found out that they are very fragile to handle. We performed several tests [2] at the Cryogenic Laboratory of the Insubria University (Como, Italy) starting in April 2004; we used a main absorber of TeO2 with a rectangular shape (2 ×2 ×0.5 cm or 2 ×2 ×0.76 cm) while we tested different shields such as Ge crystals (1.5×1.5 cm surface and 500 µm thickness), Si crystals (1.5×1.5 cm surface and 300 µm thickness) and TeO2 crystals (2 × 2 cm surface and 500 µm thickness). Shields were coupled to the main absorber by means of four epoxy spots (Ge), two Ge stand-off (height: 50 µm), placed in a central position (Si) or four Ge stand-off glued at the corner of each large face (TeO2 ). Stand-off were coupled to both crystal and slabs through a thin layer of epoxy glue. Neutron transmutation doped (NTD) Ge thermistors (3 × 1.5 × 1 mm in all cases) were glued on the main absorbers and on the auxiliary bolometers with six epoxy spots, except for the NTDs on the TeO2 shields which were glued with a single central epoxy spot. The coupling of the main absorber to the heat bath was realized by means of 8 and 4 PTFE blocks. We used an external α source to test our technique. The detectors were cooled down in separate runs in a dilution refrigerator; the detector thermistors were dc biased through a voltage supply and a 20 G room-temperature load resistance. Typical operation temperatures were between 25 and 28 mK. The DC voltage across the thermistor were typically ∼25 mV. Voltage pulses were read-out by a dc-coupled low noise differential voltage amplifier, followed by a filtering single-ended stage. The front end electronics was located at room temperature. The signals were acquired by a 12 bit transient recorder, collecting 1024 voltage points for each pulse, and registered for off-line analysis. The set-up was not optimized for high energy resolution measurements, since the main purpose of the experiment was to verify and to understand the detector sensitivity to surface events. 3. Experimental results In all the three runs we have obtained a good separation between surface and bulk events.
C. Nones et al. / Progress in Particle and Nuclear Physics 57 (2006) 269–271
271
The two classes of events are well separated and the four main α lines of the source are clearly appreciable. We can also explain the presence of additional populations in the region between the two bands due to cosmic muons crossing both the TeO2 absorber and the layer, giving rise to mixed events, with most of the energy deposited in the main absorber but with a small simultaneous energy release in the layer. A powerful separation between the two event types can be achieved also by pulse shape analysis. Looking at the rise time distribution for pulses from the slab thermistor, fast surface events are well separated by the slow bulk events and a selection of the fast events identifies clearly the surface event band in the scatter plot. References [1] C. Arnaboldi et al., Phys. Rev. B 584 (2004) 260. [2] L. Foggetta et al., Phys. Rev. Lett. 86 (2005) 134106.
Review
Development of new bolometers for rare events with background active discrimination C. Nones a,b,∗ , L. Foggetta c , A. Giuliani a,c , M. Pedretti a,c , C. Salvioni a,c , S. Sangiorgio a,c a INFN, Sezione di Milano, Italy b Dipartimento di Fisica dell’Universit`a di Milano-Bicocca, Italy c Dipartimento di Fisica e Matematica dell’Universit`a dell’Insubria, Italy
Accepted 18 November 2005
Abstract We present an innovation in the structure of a bolometer which overcomes one of the drawbacks of the calorimetric mode. In this work we prove that studying the dynamics of the heat flow in the detector we can obtain information on the particle impact point and we are able to develop a new technique for an active background discrimination, in particular in view of the CUORE experiment. c 2005 Elsevier B.V. All rights reserved. Keywords: Neutrino mass; Double beta decay; Low background; Bolometers; Cryogenics
1. Introduction CUORE (Cryogenic Underground Observatory for Rare Events) [1] will be a next generation Double Beta Decay experiment based on TeO2 macrobolometers and it will be installed in the Laboratori Nazionali del Gran Sasso. It will extend the sensitivity to the effective neutrino mass to the inverted hierarchy region of the neutrino mass pattern (∼50 meV). One of the major challenges for CUORE is the radical improvement of the background coming from α and β particles, emitted close to the surface of the detector or of the materials which surround it, that contribute with a continuum spectrum which can extend down to a region relevant for double beta decay search for 130 Te. ∗ Corresponding author at: INFN, Sezione di Milano, Italy.
E-mail address: [email protected] (C. Nones). c 2005 Elsevier B.V. All rights reserved. 0146-6410/$ - see front matter doi:10.1016/j.ppnp.2005.11.022
270
C. Nones et al. / Progress in Particle and Nuclear Physics 57 (2006) 269–271
2. Detectors: The idea and the experimental set-up The basic idea that lies behind the possibility to identify the origin of an event is quite simple: it consists of the active shielding of the main bolometer by means of thin foils of ultrapure materials. The original idea is that shielded bolometers are not independent from the main bolometer but are attached to it to form a single composite bolometer. On each shield, a thermistor is attached for temperature reading: shields are thus bolometers with an absorber of proper size and shape. The origin of the events can be clearly determined by comparing the pulse amplitudes coming from different detector elements. If an α particle comes from outside the bolometer, it interacts with an active layer releasing there all its energy (surface event). As a consequence, the temperature will rise and there will be a signal on both the layer thermistor and the TeO2 one. Because of the small heat capacity of the layer, its thermistor signal will be much higher and faster than that of the TeO2 thermistor. On the other hand, an event inside the TeO2 crystal (bulk event) will lead to pulses with similar amplitudes and shapes on both thermistors. In this work we present results obtained with different materials as active shields. The main property of shields is that they must have a small heat capacity compared to that of the main absorber. The first material that one has in mind is germanium which has a small heat capacity and can be grown at a very high purity level. The main problem of using Ge shields is the high cost for the production of such crystals. In view of the CUORE experiment, which will consist of 988 bolometers, we had to find a material with similar properties as germanium; we moved from germanium to silicon which has a reasonable cost but a worse level of achievable purity. To avoid the introduction of new materials in the CUORE set-up, we decided to test also TeO2 shields which have the same thermal contractions of the main absorber but we found out that they are very fragile to handle. We performed several tests [2] at the Cryogenic Laboratory of the Insubria University (Como, Italy) starting in April 2004; we used a main absorber of TeO2 with a rectangular shape (2 ×2 ×0.5 cm or 2 ×2 ×0.76 cm) while we tested different shields such as Ge crystals (1.5×1.5 cm surface and 500 µm thickness), Si crystals (1.5×1.5 cm surface and 300 µm thickness) and TeO2 crystals (2 × 2 cm surface and 500 µm thickness). Shields were coupled to the main absorber by means of four epoxy spots (Ge), two Ge stand-off (height: 50 µm), placed in a central position (Si) or four Ge stand-off glued at the corner of each large face (TeO2 ). Stand-off were coupled to both crystal and slabs through a thin layer of epoxy glue. Neutron transmutation doped (NTD) Ge thermistors (3 × 1.5 × 1 mm in all cases) were glued on the main absorbers and on the auxiliary bolometers with six epoxy spots, except for the NTDs on the TeO2 shields which were glued with a single central epoxy spot. The coupling of the main absorber to the heat bath was realized by means of 8 and 4 PTFE blocks. We used an external α source to test our technique. The detectors were cooled down in separate runs in a dilution refrigerator; the detector thermistors were dc biased through a voltage supply and a 20 G room-temperature load resistance. Typical operation temperatures were between 25 and 28 mK. The DC voltage across the thermistor were typically ∼25 mV. Voltage pulses were read-out by a dc-coupled low noise differential voltage amplifier, followed by a filtering single-ended stage. The front end electronics was located at room temperature. The signals were acquired by a 12 bit transient recorder, collecting 1024 voltage points for each pulse, and registered for off-line analysis. The set-up was not optimized for high energy resolution measurements, since the main purpose of the experiment was to verify and to understand the detector sensitivity to surface events. 3. Experimental results In all the three runs we have obtained a good separation between surface and bulk events.
C. Nones et al. / Progress in Particle and Nuclear Physics 57 (2006) 269–271
271
The two classes of events are well separated and the four main α lines of the source are clearly appreciable. We can also explain the presence of additional populations in the region between the two bands due to cosmic muons crossing both the TeO2 absorber and the layer, giving rise to mixed events, with most of the energy deposited in the main absorber but with a small simultaneous energy release in the layer. A powerful separation between the two event types can be achieved also by pulse shape analysis. Looking at the rise time distribution for pulses from the slab thermistor, fast surface events are well separated by the slow bulk events and a selection of the fast events identifies clearly the surface event band in the scatter plot. References [1] C. Arnaboldi et al., Phys. Rev. B 584 (2004) 260. [2] L. Foggetta et al., Phys. Rev. Lett. 86 (2005) 134106.