Scintillating Bolometers for Double Beta Decay Stefano Pirro - Milano Background Limitation for Simple bolometers Principles of operation of scintillating bolometers Some “history” Cd-Mo-Ca based crystals ZnSe crystals Conclusions CSNII, Napoli 1 Dicembre 2009 Stefano Pirro - CSNII - 1 Dic 2009

Surface and environmental contamination -region -region CUORICINO  Background Environmental “underground” HPGE Background: 238U and 232Th trace contaminations 76Ge 116Cd 130Te 100Mo 82Se Furthermore a not negligible part of the background can arise from high energy neutrons from -spallation Stefano Pirro - CSNII - 1 Dic 2009

A serious problem : Surface contaminations CUORICINO TO CUORE Bkg in DBD region Continuum 210Po α line Sum energy: 2527 keV 3600 keV 5407 keV Stefano Pirro - CSNII - 1 Dic 2009

Background identification : Bulk contaminations BI-Po Thanks to Bi-Po’s and Bi-Tl’s internal contaminations do not play a significant role Does not hold for 234Pa (Q=2195 keV) Stefano Pirro - CSNII - 1 Dic 2009

Stefano Pirro - CSNII - 1 Dic 2009 Some “Old” History The first measurement of light and heat in a bolometer was performed in 1992 by the Milano group that demonstrated the possibility to discriminate -particles in a 1 cm3 CaF2 . But this technique, using a silicon PD at low temperatures showed several difficulties Radioactivity induced by the PD itself Cold stage charge preamplifier inside the cryostat Relatively small surface area of the PD For these reasons (and other…) the technique was abandoned In 2003 we started to develop thin absorbers detectors in order to measure  surface radioactivity Since the early beginning it was clear that these devices were extremely sensitive (FWHM hundreds of eV) allowing, therefore, the measure of scintillating light Stefano Pirro - CSNII - 1 Dic 2009

Principles of operation =C/G T=E/C A Bolometric Light Detector is fully active a particle detector The time response of a BLD is the same of a standard bolometer O (ms) The QE of a BLD is, probably, comparable with that one of PD’s but it is not easy to measure it Normally PURE (undoped) Ge or Si crystal are used as absorbers Stefano Pirro - CSNII - 1 Dic 2009

Light Detectors - Performances Our Light detectors are generally Pure Germanium disks (thickness 0.3-1 mm) . The Performances of a LD are normally evaluated through the Energy resolution on the 55Fe doublet (5.9 & 6.5 keV X-Ray) Ge (Ø = 66 mm) FWHM=550 eV Ge (Ø = 35 mm) FWHM=250 eV Stefano Pirro - CSNII - 1 Dic 2009

Stefano Pirro - CSNII - 1 Dic 2009 First Detector The first Scintillating detector was mounted in March 2004 It was constituted by a 1 cm3 PbMoO4 crystal, simply glued on a copper frame, facing a 36 mm diam Ge Crystal, acting a LD Thermistor Heater Reflecting cavity Stefano Pirro - CSNII - 1 Dic 2009

First Detector – Results (1) Light On Ge 210Pb  210Bi (47 keV) +(15keV) +  (61 keV) 210Bi  210Po (1.17 MeV) 210Po 206Pb (5,3 MeV) Heat in PbMoO4 S. Pirro - Cryoscint - Oxford, 7 Dec 2004 Stefano Pirro - CSNII - 1 Dic 2009

First Detector – Results (2) Light signal on Ge detector 47 keV on PbMoO4 Our “simple” Light detector setup is extremely sensitive S. Pirro CryoScint - Oxford, 7 Dec 2004 Stefano Pirro - CSNII - 1 Dic 2009

First “LARGE” mass high RESOLUTION scintillating bolometer for DBD A very impressive result was indeed obtained as a by-product. The idea was to measure the thermal neutrons flux inside the CUORE R&D cryostat using a CdWO4 crystal, via the (113Cd,n) reaction. 6.3 cm dia 1 mm thick Ge 3x3x2 cm3 CdWO4 (140g) Qbb(116Cd)=2805 keV S. Pirro - Scintillating Double Beta bolometers NANP05 Dubna 23 June 2005 Stefano Pirro - CSNII - 1 Dic 2009

Stefano Pirro - CSNII - 1 Dic 2009 Setup Mixing Chamber Stainless steel wire Light Detectors First damping mass (Roman Lead) Very “un-clean” setup” Cuore Radioctivity test run (II Damping stage) S. Pirro - Scintillating Double Beta bolometers NANP05 Dubna 23 June 2005 Stefano Pirro - CSNII - 1 Dic 2009

Background measurement During April 2005 a background measurement was performed with the RAD detector 417 h live time measurement S. Pirro - Scintillating Double Beta bolometers NANP05 Dubna 23 June 2005 Stefano Pirro - CSNII - 1 Dic 2009

Scintillating bolometers and ”official” funding The activity was officially funded by INFN through the BoLux (R&D) Experiment* 2007-2008 and by EC Ilias (FP6) The Bolux group (1.5 –1.8 FTE) was originally formed by INFN-Mi-Bicocca In 2007 I asked for an EC Starting grant to perform a small Experiment based on CdWO4 but the project was badly rejected - “essential weaknesses in the proposal “ BoLux was “extended” for 2009 with the join of Roma 1 (total 2.6 FTE) A “PRIN” was presented to MIUR in 2009 and is still under evaluation The CUPIDO R&D* project (INFN) was accepted by CSN5 for 2010 (total 3.4 FTE) but was hardly cut by referees. The activity of Scintillating bolometers is inserted in ILIAS-NEXT (FP7) proposal with a request of money of 300 k€ ……. Lucifer was approved by ERC (Ferroni) * Responsible: Stefano Pirro Stefano Pirro - CSNII - 1 Dic 2009

Summary of (almost) all the measured crystals Good Scintillation light Poor Scintillation light No Scintillation light ZnSe PbMoO4 SrMoO4 CdMoO4 CdWO4 CaF2 CaMoO4 ZnMoO4 Li2MoO4 ZrO2 MgMoO4 TeO2 Stefano Pirro - CSNII - 1 Dic 2009

Results on the first array of CdWO4 crystals (1) 4 3x3x3 cm3 (215 g each) CdWO4 1 common LD facing the 4 crystals CdWO4 – 3x3x6 Stefano Pirro - CSNII - 1 Dic 2009

Results on the first array of CdWO4 crystals (2) CUORE test crystal Large light detector Internal Roman lead shield 4 CdWO4 3x3x3 cm3 crystal array CdWO4 3x3x6 cm3 Small light detector Stefano Pirro - CSNII - 1 Dic 2009

Results on the first array of CdWO4 crystals (3) The data on the single 420 g 3x3x6 cm3 crystal are presented. The obtained scatter plot is shown it corresponds to 1066 hours of background measurement 44 days background Background-Free area 2615 keV 208Tl  The MC simulation predicts a background level of 10-4 c/keV/kg/y in the region of interest Stefano Pirro - CSNII - 1 Dic 2009

Results on “large” crystals – CaMoO4 CaMoO4 is not a “perfect candidate” for future DBD Experiment since it contains 48Ca But this compound did show an extreme interesting feature Rise time of the CaMoO4 crystal – no light detection [ms] [au] Decay time of the CaMoO4 crystal – no light detection 147Sm- 2310 keV CaMoO4 Bolometers permits alpha discrimination (99,7%) without Light detection !!!! Stefano Pirro - CSNII - 1 Dic 2009

ZnMoO4 – A promising Molibdate A 22 g ZnMoO4 crystal was grown by Institute for Scintillation Materials (Kharkov, Ukraine) In collaboration with by Institute for Nuclear Research (Kiev, Ukraine) 226Ra, 222Rn, 218Po, 214Bi-214Po (56 mBq/kg) 210Pb (360 mBq/kg) Stefano Pirro - CSNII - 1 Dic 2009

ZnSe – an extremely interesting compound ZnSe crystal was/is an extremely puzzling and interesting material. ZnSe crystal has a huge scintillation output but the emission spectra is close to the absorption spectra (1 mm) It normally doped with Te in order to increase the overall light output (we tested UNDOPED crystals)  4 cm dia, 1.7 cm height, 120 g 2 cm dia 3 cm height, 39 g Stefano Pirro - CSNII - 1 Dic 2009

ZnSe – a puzzling scintillation In the first run (2007) we observed a “very” strange scatter plot Stefano Pirro - CSNII - 1 Dic 2009

Stefano Pirro - CSNII - 1 Dic 2009 ZnSe – Light Pulse Moreover a “good” surprise arise from the time development of the Scintillation Signal The scintillation signal has a difference in the O(10ms) range. This effect is absolutely unexpected, even at very low temperatures Stefano Pirro - CSNII - 1 Dic 2009

ZnSe – Light Detector PulseShapeAnalysis (1)  Ionizing particles / Looking at the coincidences between Heat in ZnSe and “Light” in the light detector, three population appears Stefano Pirro - CSNII - 1 Dic 2009

ZnSe – Light PSA (2) /  337 g “new” ZnSe Crystal First Results on a 4 cm  5 cm height 337 g ZnSe Crystal 337 g “new” ZnSe Crystal Light Calibration with 232Th and a smeared  source 210Po / Smeared  source  Beta region Rejection > 99.5 % Stefano Pirro - CSNII - 1 Dic 2009

 rejection > 97 % without light detection ZnSe – Heat PSA This compound shows another very interesting feature: ’s show different thermal pulse development /   rejection > 97 % without light detection Stefano Pirro - CSNII - 1 Dic 2009

Stefano Pirro - CSNII - 1 Dic 2009 Crystal Summary Reproducibility Radiopurity Availability /n rejection 116Cd CdWO4 82Se ZnSe 100Mo …. ZnMoO4 Stefano Pirro - CSNII - 1 Dic 2009

Stefano Pirro - CSNII - 1 Dic 2009 Conclusions 1 Developed a new powerful technique to be used for future DBD Experiments We tested several types of scintillating crystals with interesting  emitters (100Mo,116Cd, 82Se, 48Ca) Within them CdWO4 is “ready to use” The “outsider” ZnSe is now, probably, the best candidate , even if some more tests are needed Molibdates needs more R&D both for radioactivity and scintillation light Stefano Pirro - CSNII - 1 Dic 2009

Stefano Pirro - CSNII - 1 Dic 2009 Conclusions 2 Different nuclei HAVE to be investigated by DBD experiments DBD Detectors with Q  >2615 keV can reach background levels  2 orders of magnitude smaller with respect to the present (funded) experiments 130Te 76Ge 100Mo 116Cd Gerda CUORE If we think about possible “ III generation experiments” the key point will be (if we consider solved the background issue….) Energy Resolution ( 1 %) Scintillating Bolometers are the only detectors that can fulfill all these 3 requirements Stefano Pirro - CSNII - 1 Dic 2009