C Arnaboldi

C Brofferio

O Cremonesi

L Gironi
email: luca.gironi@mib.infn.it

M Pavan

G Pessina

S Pirro

E Previtali

A novel technique of particle identification with bolometric detectors

Keywords: Bolometers, Scintillators, Pulse shape discrimination (PSD) PACS: 23.40B, 95.35.+d, 07.57.K, 29.40M, 66.70.-f

We report in this paper the proofs that the pulse shape analysis can be used in some bolometers to identify the nature of the interacting particle. Indeed, while detailed analyses of the signal time development in purely thermal detectors have not produced so far interesting results, similar analyses on bolometers built with scintillating crystals seem to show that it is possible to distinguish between an electron or γ-ray and an α particle interaction. This information can be used to eliminate background events from the recorded data in many rare process studies, especially Neutrinoless Double Beta decay search. Results of pulse shape analysis of signals from a number of bolometers with absorbers of different composition (CaMoO 4 , ZnMoO 4 , MgMoO 4 and ZnSe) are presented and the pulse shape discrimination capability of such detectors is discussed.

Rare event searches

Rare event studies, such as the search for Neutrinoless Double Beta decay (ββ(0ν)) [1] or the identification of Weakly Interacting Massive Particle (WIMP) interactions with ordinary matter [2], are of extreme interest in astroparticle physics, since they would imply new physics beyond the Standard Model. In both cases, as in all the rare event studies, spurious events are a limiting factor to the reachable sensitivity of the experiment.

Unfortunately natural radioactive background is often present in the detector itself or in the materials surrounding it, no matter how much one can try to reduce it with shieldings, selection of materials and complicated purification techniques. In order to handle the residual unavoidable background, all the envisaged approaches require both a good energy resolution (which always helps in the comprehension of the different structures of an energy spectrum) and the capability to identify the nature of the projectile that interacted with the detector. Indeed, the searched event has always well defined signatures helping to distinguish it from background, for instance two electrons with a fixed sum energy in the case of the ββ(0ν) .

Bolometers [START_REF] Booth | [END_REF] are based on the detection of phonons produced after an energy release by an interacting particle and can have both an excellent energy resolution and extremely low energy threshold with respect to conventional detectors. They can be fabricated from a wide variety of materials, provided they have a low enough heat capacity at low temperatures, which is the only requirement really unavoidable to build a working bolometer. The latter is a priceless feature for experiments that aim at detectors containing particular atomic or nuclear species to optimize the detection efficiency. If other excitations (such 29 as ionization charge carriers or scintillation photons) are col- 51

∆T (t) = E C e -t τ ( 2
)
52

In order to obtain a measurable temperature rise the heat ca-53 pacity of the absorber must be very small: this is the reason 54 why bolometers need to be operated at cryogenic temperatures 55 (of the order of 10-100 mK).

56

Preprint submitted to Atroparticle Physics February 26, 2011 A real bolometer is somewhat more complicated than the naive description presented above. It is made of different elements and it is therefore represented by more than one heat capacity and heat conductance. As such, the time development of the thermal pulse is characterized by various time constants.

In principle, if the bolometer performs as an ideal calorimeter and if the conversion of the energy into heat deposited by the particle is instantaneous (as assumed in equation 2), then the device is insensitive to the nature of the interacting particle. Although this situation is generally very far from reality, it is however true that the small differences are difficult to detect and the goal has been so far achieved only relying on complicated solutions. Among these are scintillating bolometers.

The concept of a scintillating bolometer is very simple: a bolometer coupled to a light detector [4]. The first must consist of a scintillating absorber thermally linked to a phonon sensor while the latter can be any device able to measure the emitted photons. The driving idea of this hybrid detector is to combine the two available informations, the heat and the scintillation light, to distinguish the nature of the interacting particles, exploiting the different scintillation yield of β/γ, α and neutrons.

Dark Matter as well as ββ(0ν) searches can benefit of this capability of tagging the different particles, and more generally this technique can be exploited in any research where background suppression or identification is important.

Dark matter experiments look for a very rare signal generally hidden in a huge background. The signal is a nuclear recoil with an energy of few keV (or less) induced by the scattering of a WIMP off a target nucleus. Experiments like CDMS [5],

Edelweiss [6] or CRESST [7] clearly show that in such energy region the background is dominated by β/γ interactions. A second source of background are α decays, contributing through energy degraded α's and nuclear recoils. The capability to distinguish a nuclear recoil -candidate for a WIMP interactionfrom α or β/γ clearly allows to improve drastically the experimental sensitivity.

A similar approach was proposed also for applications in ββ(0ν) searches [4]. More recently, such a possibility has been demonstrated to be viable for a number of candidate nuclei [8].

In this case the major interest is the identification of α interactions. Indeed the other important source of background, namely γ-rays, is virtually indistinguishable from the ββ(0ν) signal.

The suggested way to eliminate the problem of γ-rays con- [9,10,11] and as a limiting factor for the experiment CUORE [10,12]. Both the experiments search for the ββ(0ν) of 130 Te whose transition energy is at 2527 keV, In the following section we report the results obtained with 191 the pulse shape analysis on some of the tested crystals. The basic concept is to build a filter that, when applied to the 266 raw-pulse, produces -as output -a pulse with the best signal to response in scintillating bolometers [14]. For the molybdates 306 that are here reported this heat quenching factor is lower than 307 few percent while for ZnSe it is a little bit higher, about 10 per-308 cent for ∼6 MeV alpha particles [16]. This miscalibration of 309 the α band however does not imply an appreciable change in 310 the discrimination confidence level described below.

311

In the case of the light signal the energy calibration is not 312 needed and we therefore present its value in arbitrary units. explained by the partition of energy in the two channels with different decay constants. In particular, as shown by [28,29] the scintillation produced in molybdates by α and β/γ particles presents few decay-time constants with different relative intensities. Also a strong temperature dependence of the averaged decay-time of the light pulses in these same crystals was reported [30,31]. For this reason -while at room temperature the averaged decay-time of molybdate or tungstate scintillators is of the order of tens of µs (thus almost instantaneous in the heat pulse timescale) -at low temperatures it increases to hundreds of µs and is therefore comparable to the typical rise-time of the heat signal of our scintillating bolometers.

In the following sections we analyse the results obtained with different crystals, each being a possible candidate for a ββ(0ν) experiment. We quote for each crystal the discrimination power achieved -in the ββ(0ν) region -between α and β/γ particles on the basis of the light/heat ratio or simply on the basis of the pulse shape of the heat signal. We discuss in more detail the case of CaMoO 4 , briefly summarizing the results obtained for other crystals.

CaMoO 4

Recently CaMoO 4 has been intensively studied, for its possible application as a scintillating bolometer for ββ(0ν) and Dark

Matter experiments [8,34,35]. This crystal contains two isotopes that could undergo ββ(0ν) : 48 Ca (Q ββ =4.27 MeV) and 100 Mo (Q ββ =3.03 MeV). Actually, while the large content of 100 Mo makes this crystal very attractive, the presence of 48 Ca is a problem. Indeed, the natural isotopic abundance of 48 Ca (a.i.=0.19%) is too low to study the ββ(0ν) without enrichment, which is extremely difficult, and at the same time, it is too high to study the ββ(0ν) of 100 Mo, since the background due to the ββ(2ν) of 48 Ca in the ββ(0ν) region of 100 Mo will limit rise axis of events in the 2-4 MeV regions due to β/γ events (in green on the right) and α events (in red on the left). The separation here corresponds to 6.5 sigma. These same events are reported, with identical colors, in Fig. 1.

the reachable sensitivity for the latter isotope. A possible so-397 lution to this problem was suggested by Annenkov et al. [36] 398 who proposed an experiment with CaMoO 4 depleted in 48 Ca. Since our main goal was to study the efficiency of α particle rejection in the ββ(0ν) region (i.e. at about 3 MeV), a second source (B) was also used. This was obtained contaminating an Al stripe with a 238 U liquid solution and later covering the stripe with an alluminated Mylar film (6 µm thick). Thus the source produced a continuous spectrum of α particles, extending from about 3 MeV down to 0.

In Fig. 1 we show the Light vs. Heat scatter plot, collected while the crystal was exposed to an external 232 Th source. The total live time of this measurement was ∼43 h. The FWHM energy resolution, on the heat signal, ranges from 2.7 keV at 243 keV to 8.7 keV at 2615 keV on β/γ events and is about 10 keV on 5 MeV α's. The light yield of the crystal, evaluated calibrating the light detector with a 55 Fe source, is 1.87 keV/MeV (for more details on the procedure used for the evaluation of the light yield see references [14,16]).

The two separate bands -clearly visible in the Light vs. Heat scatter plot -are ascribed to β/γ's (upper band) and α's (lower band). The upper band is dominated by the 232 Th source γ's (plus the environmental γ's). The lower band is due to α's from source A and from Uranium and Thorium internal contamination of the crystal (these are the monochromatic lines above 4 MeV) and source B (the continuum counting rate below 4 MeV). Above 8 MeV we observe a group of events ascribed to α+β summed signals due to internal contamination in Bismuth and Polonium. Indeed, due to the long rise-time of this device (∼5 ms), the beta decay of 214 Bi or 212 Bi (respectively of 238 U and 232 Th chains) followed immediately by α decay of 214 Po (τ=164 µs) and 212 Po (τ=298 µs ), may lead to a pile up on the rise-time of the thermal pulses that can hardly be recognized as a double signal. The two decays produce therefore a single pulse, with an energy that is the sum of the two.

The discrimination between the α and the β/γ populations, provided by the scintillation signal, can be evaluated by measuring the difference between the average amplitude of the light signal produced by the two kinds of particles (Light β/γ and Light α ), considering a group of events releasing the same energy in the scintillating crystal. This difference is then compared with the width of the two distributions (σ β/γ and σ α ).

The discrimination confidence level D Light can be then defined as:

D Light = Light β/γ -Light α σ 2 β/γ + σ 2 α (4)
To evaluate this discrimination power in the ββ(0ν) region we selected events belonging to the 2615 keV γ-line full energy peak and compared their light pulse distribution with that of events of similar energy in the α band (Fig. 2 left panel). D light results to be 12.6 sigma. The reason for using events with the same energy to evaluate D light is in the energy dependence of the distance (and width) of the α and β/γ bands, that induce an energy dependence of the discrimination confidence level. In the energy range where both the bands are populated (i.e. between 1 and 3 MeV), D Light (E) appears to be linearly decreasing with energy. Its extrapolated value at E=0 being 3 sigma. As already anticipated, we discovered that the heat signal

483

We will refer in the following to these two groups of events as to In other words the ∼150 µs difference in rise-time of α and β/γ events cannot be appreciated when the pulse amplitude is too low and, therefore, the noise modifies appreciably the signal shape. Finally, although in this measurement the rise-time is the most efficient shape parameter for α event discrimination, good discrimination levels were observed also in other parameters such as the decay-time and the TVR. The possibility to increase the discrimination power by combining the information from different shape parameters or the fit on the rise-time is under study.

We note that either in the Light vs. Heat scatter plot as in the τ lin rise vs. Heat one we can see some outliers that could indicate a possible failure of the particle identification technique.

However such events can be accounted for if one considers the following two effects. If a γ-ray interacts both in the light detector and in the scintillator the light signal that is read-out has a wrong amplitude since it is not only ascribed to scintillating photons but also to a direct γ interaction in the Ge wafer. The measurement is performed in a high rate condition, therefore we expect to have a number of pile-up events in each of the two detectors, this leads to an erroneous evaluation of the pulse amplitude and pulse rise-time. where the τ 1 and τ 2 parameters are obtained by fitting the decay 567 of the thermal signals in the raw-pulse acquired for each event. Heat (fig. 6) clear difference between events due to the interac-599 tion of α particles and events due to β/γ events.

600

The discrimination between the two populations provided by this crystal can be found in [16].

627 Also in this case, the RDC lin vs. Heat (fig. 8) showed a 628 difference between events due to the interaction of α particles

Conclusion 632

The possibility to discriminate the nature of the particle inter-

30 lected

 30 in addition to phonons, bolometers have already shown 31 to be able to discriminate nuclear recoils from electron recoils, 32 or α particles from β particles and γ-rays. In this paper we will 33 report on the possibility to obtain similar results just by pulse 34 shape analysis, without the requirement of a double readout for 35 phonons and ionization or scintillation light.

36 2 .47

 2 Bolometric Technique and Scintillating Bolometers 37 Bolometers can be essentially sketched as a two-component 38 object: an energy absorber in which the energy deposited by a 39 particle is converted into phonons, and a sensor that converts 40 thermal excitations into a readable signal. The absorber must 41 be coupled to a constant temperature bath by means of a weak 42 thermal conductance. 43 Denoting by C the heat capacity of the bolometer, the temper-44 ature variation induced by an energy release E in the absorber 45 The accumulated heat flows then to the heat sink through the 48 thermal link and the absorber returns to the base temperature 49 with a time constant τ = C/G, where G is the thermal conduc-50 tance of the link:

173

  by a reflector to properly collect the scintillation light (therefore cannot be faced to a sample whose radioactive emission has to 175 be identified). The devices here discussed overcome these two 176 difficulties. Traditionally the devices used in this field are Si 177 surface barrier detectors. For low counting rates, large area low 178 background detectors are needed. Today Si surface barriers de-179 tectors with an active area of about 10 cm 2 , a typical energy 180 resolution of about 25-30 keV FWHM, and counting rates of 181 the order 0.05 count/h/cm 2 between 3 and 8 MeV are available 182 [19]. A bolometer like those here discussed can easily reach a 183 much larger active area, has a typical energy resolution of 10 184 keV and a background counting rate in the 3-8 MeV region that 185 can be as low as 0.001 count/h/cm 2 . Thanks to the particle iden-186 tification technique discussed in this paper, it can distinguish an 187 α emission from a β/γ one 1 and finally can reject the β/γ back-188 ground extending its measurement field to energies by far lower 189 than 3 MeV.190

192 3 .

 3 photons converting their energy into heat. This was realised us-207

217A

  silicon resistance, glued on the crystals, was used to pro-218 duce a calibrated heat pulse in order to monitor the thermal gain 219 of the bolometer. This is indeed subject to variation upon tem-220 perature drifts of the cryostat that can spoil the energy resolu-221 tion. In most cases this temperature drift could be re-corrected 222 off-line on the basis of the measured thermal gain variation 223 [23, 24].

224 3 . 1 .

 31 Read-out and DAQ 225 The read-out [25] of the thermistor was performed via a 226 preamplifier stage, a second stage of amplification and an 227 antialiasing filter (a 6 pole roll-off active Bessel filter 120 228 db/decade [26]) located in a small Faraday cage. The ADC was 229 a NI USB-6225 device (16 bit 40 differential input channels). 230 For each triggered signal the entire waveform (raw-pulse) is 231 sampled, digitized and acquired for the off-line analysis. Since 232 all the relevant parameters (including the amplitude) of the trig-233 gered signals are evaluated off-line, a particular care has to be 234 dedicated to the optimization of the signal filtering and digiti-235 zation. In the case of the scintillating bolometer the large heat 236 capacity of the absorber, coupled to the finite conductance of 237 the crystal-glue-thermistor interface results in quite slow sig-238 nals, characterised by a rise-time of the order of few ms 2 and a 239 decay-time of hundreds of ms (determined by the crystal heat 240 capacity and by its thermal conductance toward the heat sink). 241 Consequently the sampling rate typically used for the signal is 242 1-4 kHz, over a time window of 200-2000 ms. The Bessel fil-243 ter acts mainly as antialising, to avoid spurious contributions 244 in the sampled signal. Generally it is preferred to fix its cut-245 off frequency at the lowest value that does not deteriorate the 246 signal to noise ratio (i.e. to obtain the best results in terms of 247 energy resolution). This results to be a frequency of the order 248 of 10 Hz, which is by far lower than what needed for antialias-249 ing purposes. In the studies here presented the Bessel cut-off 250 frequency was fixed at 120 Hz in order to exploit the maximum 251 available information in the signal bandwidth.

252 3 . 2 .

 32 Analysis techniques 253Off-line analysis aims at determining the pulse amplitude and 254 energy together with several pulse shape parameters associated system. Starting from these quantities the physical informations 257 that are relevant for the scientific goals can be extracted.258The first step of the analysis consists in the correct evalua-259 tion of the pulse amplitude. Since thermal pulses are super-260 imposed to stochastic noise, a simple maximum-minimum al-261 gorithm would not give the better achievable resolution. We 262 therefore use the Optimum Filter (OF) approach[27]. This al-263 gorithm has proven to provide the best estimate of the pulse 264 amplitude and, as a consequence, the best energy resolution.

  265

  267noise ratio. The filtered pulse is then used to evaluate the signal 268 amplitude. It can be proven that in the frequency domain the 269 OF transfer function H( ω ) is given by 270 H(ω) = K S * (ω) N(ω) e -jωt M (3) 271 where S(ω) is the Fourier transform of the ideal thermal sig-272 nal (reference pulse in the absence of noise), N(ω) is the noise 273 power spectrum, t M is the delay of the current pulse with respect 274 to the reference pulse and K is a proper normalising factor usu-275 ally chosen in order to obtain the correct event energy. 276 The role of the optimum filter is to weight the frequency com-277 ponents of the signal in order to suppress those frequencies that 278 are more affected by noise. It can be seen from eq. 3 that, in 279 order to build the filter, the shape of the reference pulse S(ω) 280 and the noise power spectrum N(ω) must be known. S(ω) is 281 usually estimated by averaging a large number of recorded raw-282 pulses, so that the noise associated with each of them averages 283 to zero. N(ω) is obtained according to the Wiener-Khintchine 284 theorem by acquiring many detector baselines in absence of 285 thermal pulses and averaging the corresponding noise power 286 spectra. 287 Once the pulse amplitude has been evaluated, gain instability 288 corrections are applied to data. Due to the dependence of the 289 detector response on the working temperature, the same amount 290 of released energy can produce thermal pulses of different am-291 plitudes. Gain instabilities are corrected monitoring the time 292 behaviour of thermal pulses of fixed energy, generated every 293 few minutes across a Si heater resistor attached on the crystal 294 absorber [23, 24]. Finally the amplitude to energy conversion 295 (calibration) is determined by measuring the pulse amplitudes 296 corresponding to fixed calibration lines. In the measurements 297 here reported the signal of the scintillating bolometer (we will 298 refer to this signal as to the heat or thermal signal) has been cal-299 ibrated on the basis of the full energy peaks visible in the spec-300 trum collected when the detector was exposed to an (external to 301 the cryostat) 232 Th source. These peaks have a nominal energy 302 of: 511, 583, 911, 968 and 2615 keV. Below 511 keV and above 303 2615 keV the energy calibration is extrapolated. However, the 304 heat response for α particles is slightly different from the β/γ 305

319

  tered pulse A(t). They are the root mean square differences 320 between the current signal A(t) and the reference pulse after 321 OF filtering A 0 (t)= H(t) ⊗ S(t). In more detail, the filtered re-322 sponse function A 0 (t) is synchronized with the filtered signal 323 A(t), making their maxima to coincide, then the least square dif-324 ferences of the two functions are evaluated on the right (TVR) 325 and left (TVL) side of the maximum on a proper time interval 326 which is usually chosen depending on the shape of the OF sig-327 nals. Although these two parameters do not have a direct phys-328 ical meaning, however they are very sensitive (even in noisy 329 conditions) to any difference between the shape of the analyzed 330 pulse and the response function. Consequently, they are used 331 either to reject fake triggered signals (e.g. spikes) or to iden-332 tify variations in the pulse shape with respect to the reference 333 response function (and this will be our case).

334 4 .Figure 1 :

 41 Figure 1: CaMoO 4 crystal faced to two α sources. Left panel: scatter plot of Light vs. Heat. The energy calibration is performed on the γ peaks. In red (light in the bottom curve) events in the 2-4 MeV region due to α particles while in green (light in the top distribution) events due to β/γ in the same region. Right panel: Scatter plot of τ lin rise vs. Heat in CaMoO 4 crystal obtained for the same events. In green (light in the upper curve) events in the 2-4 MeV region due to β/γ particles and in red α events.

Figure 2 :

 2 Figure 2: Evaluation of the β/γ and α discrimination power in CaMoO 4 . Left panel: projection on the Light axis of the 208 Tl γ-line (2615 keV) (in green on the right) and of α events (in red on the left) that released a similar energy in the CaMoO 4 crystal. The separation corresponds to 12.6 sigma. Right panel: projection on the τ linrise axis of events in the 2-4 MeV regions due to β/γ events (in green on the right) and α events (in red on the left). The separation here corresponds to 6.5 sigma. These same events are reported, with identical colors, in Fig.1.

399

  Despite these possible problems, we discovered that CaMoO 4 400 is an extremely interesting crystal because of its capability to 401 discriminate β/γ from α, thanks to the different shape of the 402 thermal pulses. 403 The sample we used was a cylindric CaMoO 4 crystal with 404 a mass of 158 g (h = 40mm, ∅ = 35mm). The crystal was 405 faced to two α sources. Source A was obtained by implantation 406 of 224 Ra in an Al reflecting stripe. The shallow implantation 407 depth allows to reduce to a minimum the energy released by 408 the α's in the Al substrate so that monochromatic α lines (those 409 produced in the decay chain of 224 Ra to the stable 208 Pb isotope) 410 can be observed in the scintillating bolometer. Besides these α 411 particles -all with energies above 5 MeV -the source emits a 412 β with a maximum energy of 5 MeV, due to the decay of 208 Tl.

Figure 3 :

 3 Figure 3: CaMoO 4 crystal, comparison of β/γ's and α's pulse shape. Top panel: thermal pulse obtained by averaging signals belonging to the 2615 keV γ-line (black continuous line) and signals due to α particles that release a similar energy (red dotted line). Bottom panel: difference, in percent of the maximum amplitude, between the two averaged pulses.

467 6 476

 6 shape is enough to discriminate β/γ's from α's. This is shown 468 in the right panel of Fig. 1 where we report the (thermal pulse) 469 τ lin rise vs. Heat scatter plot for the same events shown in the left 470 panel. Two separate bands, ascribed to β/γ and α events can be 471 identified. The former with an average rise-time of ∼5.8 ms, 472 the latter with ∼5.6 ms. In this plot the weak energy (Heat) 473 dependence of τ rise observed for the two populations was cor-474 rected by fitting their rise-time distributions with two lines hav-475 ing the same slope. To do this we used β/γ events in the 0.5-2.MeV range and α events in the 1.5-7 MeV range. The τ rise is 477 then linearized re-defining it as τ lin rise = τ rise -slope × E with 478 slope=0.0075 ms/MeV. 479 In order to emphasize the correspondence in the identifica-480 tion of β/γ and α events, signals selected in the Light vs. Heat 481 scatter plot as having an energy between 2 and 4 MeV and be-482 longing to the β/γ or α bands, are reported in different colors.

484 2 - 4 Figure 4 :

 244 Figure 4: ZnMoO 4 crystal. Scatter plot of Light vs. Heat (left panel) and scatter plot of RDC vs. Heat (right panel). In green events in the 2-4 MeV regions due to β/γ particles and in red those due to α's.

Figure 5 :

 5 Figure5: Evaluation of the β/γ and α discrimination power in ZnMoO 4 . Projection on the RDC lin axis (after linearization of the scatter plot of Fig.4, right panel) of events in the 2-4 MeV region due to β/γ events (in green) and α events (in red).

Figure 6 :

 6 Figure 6: MgMoO 4 crystal: scatter plot of RDC lin vs. Heat (top panel) and projection of 2-4 MeV events on the RDC lin axis (bottom panel). In green events in the 2-4 MeV region due to β/γ particles and in red those due to α's.

  is a ββ emitter with an isotopic abundance of 9.2% and 610 a Q-value of (2995.5 ± 2.7) keV. It has always been considered 611 a good candidate for ββ(0ν) studies because of its high transi-612 tion energy and the favorable nuclear factor of merit. For these 613 reasons in the last years an R&D work was carried out in which 614 we have extensively studied the performances of ZnSe detectors 615 in different conditions. For our studies we have used different 616 ZnSe crystals. Characteristics of measurements done and ob-617 tained results are reported in detail in [16]. Here we report the 618 discrimination power on the RDC obtained with the Huge ZnSe 619 crystal (h = 50mm, ∅ = 40mm, 337g). 620 In Fig. 7 the Light vs. Heat scatter plot of a measurement of 621 ∼70 h of live time is shown. In order to have a high number 622 of α counts in the 2-3 MeV region also in these measurements 623 a degraded 238 U source was placed in front of the crystal. An 624 explanation of the Quenching Factor larger than one (i.e. the α 625 band lies above the β/γ band) and other anomalies observed in 626

629Figure 7 :

 7 Figure 7: ZnSe Crystal: scatter plot of Light vs. Heat. In green events in the 2-4 MeV region due to β/γ particles and in red those due to α's.

642 ZnMoO 4 ,

 6424 633acting in a bolometric detector, simply on the base of the shape 634 of the thermal pulse, is now definitely proved and opens new 635 possibilities for the application of these devices in the field of 636 rare events searches. In particular, the high rejection capabil-637 ity that could allow to completely rule out the α background in 638 ββ(0ν) experiments was demonstrated. Unfortunately, based on 639 the results obtained so far, the applicability of this technique to 640 Dark Matter searches cannot yet be proved directly. This fea-641 ture was observed in different scintillating crystals (CaMoO 4 , MgMoO 4 and ZnSe) and new tests are under prepara-643 tion in order to investigate if a similar behavior can be observed 644 also in other compounds. A discrimination confidence level 645 of ∼6.5 sigma was obtained both for CaMoO 4 and ZnMoO 4 646 crystals in the 2-4 MeV energy region. Discrimination con-647 fidence levels reached with MgMoO 4 (D RDC =1.8σ) and ZnSe 648 (D RDC =2.2σ) are not so high but they indicate that even with 649 these crystals the discrimination based on pulse shape analysis 650 is possible. New techniques aiming at improving the discrimi-651 nation power of the pulse shape analysis are being studied.

653

  This work was funded and developed under the Bolux exper-654 iment of INFN. Thanks are due to E. Tatananni, A. Rotilio, A. 655 Corsi and B. Romualdi for continuous and constructive help in 656 the overall setup construction. Finally, we are especially grate-657 ful to Maurizio Perego for his invaluable help in the develop-658 ment and improvement of the Data Acquisition software.

Table 1 :

 1 Double beta decay isotopes with endpoint energies above the 208 Tl line.

		Isotope Q ββ [MeV] natural abundance
		116 Cd	2.80	7.5 %
		82 Se	3.00	9.2 %
		100 Mo	3.03	9.6 %
		96 Zr	3.35	2.8 %
		150 Nd	3.37	5.6 %
		48 Ca	4.27	0.19 %
	114	therefore in a region where γ background (mainly due to Comp-
	115	ton events produced by 2615 keV photons) can be still impor-
	116	tant.
	117	The α contribution to the background in the ββ(0ν) region
	118	(i.e. at about 3 MeV) is the following. In the natural chains
	119	we have various nuclei that decay emitting an α particle with
	120	an energy between 4 and 8 MeV, their energy is quite higher
	121	than most ββ(0ν) Q-values. However, if the radioactive nucleus
	122	is located at a depth of a few µm inside a material facing the
	123	detector, the α particle looses a fraction of its energy before
	124	reaching the detector and its energy spectrum looks as a contin-
	125	uum between 0 and 4-8 MeV [13]. A similar mechanism holds
	126	in the case of surface contaminations on the bolometer.
	127	This radioactive source plays a role in almost all detectors
	128	but it turns out to be particularly dangerous for fully active de-
	129	tectors, as in the case of bolometers. It can be efficiently iden-
	130	tified and removed with active background suppression tech-
	131	nique such as that conceived with scintillating bolometers.
	132	The development of a hybrid detector, able to discriminate
	133	α particles and optimized for ββ(0ν) searches, was the main
	134	purpose of our studies on scintillating bolometers. We tested
	135	several devices, differing mainly in the scintillating crystal ma-
	136	terial and size, to study their thermal response, light yield
	137	and radio purity. The results obtained so far are reported in
	138	[14, 15, 16, 17]. On our way, we discovered an extremely inter-
	139	esting feature of some of the tested crystals: the different pulse
	140	shape of the thermal signals produced by α's and by β/γ's. This
	141	feature opens the possibility of realizing a bolometric experi-
	142	ment that can discriminate among different particles, without
	143	the need of a light detector coupled to each bolometer. In the
	144	case of a huge, multi-detector array, such as CUORE [12] and
	145	EURECA [18], the benefits of employing this technique would
	146	be impressive:
	147	• more ease during assembly because the single element of
	148	the array would be a quite simpler device.
	149	• fewer readout channels, with not only an evident reduc-
	150	tion of cost and work, but also a cryogenic benefit (in a
	151	cryogenic experiment particular care should be devoted to
	152	reduce any thermal link between room temperature and the
	153	bolometers working at few mK: the heat load of the read-
	154	out channels must be taken into account and their reduc-
	155	tion is always a good solution).

170

α particles with energies lower than 2615 keV where the β/γ 171 induced background dominates the detector counting rate). On 172 the other hand, a scintillating bolometer has to be surrounded

313

  Besides the amplitude, few other characteristic parameters of 314 the pulse are computed by the off-line analysis. Some of them 315 are: τ rise and τ decay , TVL and TVR. The rise-time (τ rise ) and the 316 decay-time (τ decay ) are determined on the recorded raw-pulse as 317 (t 90% -t 10% ) and (t 30% -t 90% ) respectively. TVR (Test Value Right)

	318
	and TVL (Test Value Left) are computed on the optimally fil-

The minimum observable signal rise-time is limited by the integration on the parasitic capacitance of the signal wires that connect the NTD thermistor to the front-end electronics.

It was observed that the best discrimination power is obtained 569 by the ratio of the two decay constants (RDC):

571

The scatter plot of RDC vs. Heat is reported in the right panel of The two peaks (β/γ and α) are then fitted with a gaussian (Fig.