Double beta decay searches with enriched and scintillating
Double beta decay searches with enriched and scintillating bolometers Stefano Pirro - Milano - Bicocca The Future of Neutrino Mass Measurements - Nu. Mass 2010 INT Seattle February 8 -11, 2010 Stefano Pirro – Nu. Mass 2010
Background Issues (1) Stefano Pirro – Nu. Mass 2010
Background Issues (2) XX Internal trace contaminations (238 U and 232 Th) Surface contaminations Surrounding trace contaminations (238 U and 232 Th) Neutron induced background: direct and through (n, n’) Long living - Emitters 106 Ru 106 Rh 110 m. Ag (Q=3. 5 Me. V) (Fission) (Q=2. 9 Me. V) (fast n activation) Stefano Pirro – Nu. Mass 2010
Surface and Bulk contaminations -region 76 Ge 130 Te 116 Cd 100 Mo 82 Se 136 Xe CUORICINO Background Environmental “underground” Background: 238 U and 232 Th trace contaminations Furthermore a not negligible part of the background can arise from high energy neutrons from -spallation Stefano Pirro – Nu. Mass 2010
Some History The first measurement of light and heat in a bolometer was performed in 1992 by the Milano group But this technique, using a silicon PD at low temperatures showed several difficulties Ø Radioactivity induced by the PD itself Ø Cold stage charge preamplifier inside the cryostat Ø Relatively small surface area of the PD For these reasons the technique was abandoned In 2004 we started to develop bolometer as LD (as CRESST and Rosebud Experiments) The activity was then funded by INFN through the Bo. Lux (R&D) Experiment* 2007 -2009 and by EC The CUPIDO R&D* project (INFN) funded for 2010 will go on with R&D on this technique. * Responsible: Stefano Pirro – Nu. Mass 2010
Principles of operation =C/G T= E/C Stefano Pirro – Nu. Mass 2010
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 55 Fe doublet (5. 9 & 6. 5 ke. V X-Ray) Ge (Ø = 66 mm) FWHM=550 e. V Ge (Ø = 35 mm) FWHM=250 e. V Stefano Pirro – Nu. Mass 2010
Summary of (almost) all the measured crystals Good Scintillation light Poor Scintillation light No Scintillation light Pb. Mo. O 4 Zr. O 2 Mg. Mo. O 4 Zn. Se Li 2 Mo. O 4 Te. O 2 Cd. Mo. O 4 Sr. Mo. O 4 Cd. WO 4 Ca. F 2 Ca. Mo. O 4 Zn. Mo. O 4 Stefano Pirro – Nu. Mass 2010
Undoped 48 Ca. F 2 In 2007 we tested a Ca. F 2 crystal. The light output was “rather poor” but definitively enough to discriminate alpha’s Calibration (232 Th) on a 3 x 3 x 3 cm 3 PURE Ca. F 2 crystal There was a lack of an actual calibration due to the “lightness” of the compound; nevertheless the Stefano Pirro – Nu. Mass 2010 Signal/noise ratio of the Ca. F 2 was excellent.
Results on the first array of 116 Cd. WO 4 crystals (1) 4 3 x 3 x 3 cm 3 (215 g each) Cd. WO 4 1 common LD facing the 4 crystals Cd. WO 4 – 3 x 3 x 6 Stefano Pirro – Nu. Mass 2010
Results on the first array of 116 Cd. WO 4 crystals (2) 44 days background un d. F re ea re a The data on the single 420 g 3 x 3 x 6 cm 3 crystal is presented here. The obtained scatter plot is shown it corresponds to 1066 hours of background measurement Ba ck g ro 2615 ke. V 208 Tl The MC simulation predicts a background level of 10 -4 c/ke. V/kg/y in the region of interest Stefano Pirro – Nu. Mass 2010
Zn 100 Mo. O 4 – A promising Molibdate A 22 g Zn. Mo. O 4 crystal was grown by Institute for Scintillation Materials (Kharkov, Ukraine) In collaboration with by Institute for Nuclear Research (Kiev, Ukraine) 226 Ra, 222 Rn, 218 Po, 214 Bi-214 Po (56 m. Bq/kg) 210 Pb (360 m. Bq/kg) Stefano Pirro – Nu. Mass 2010
Zn 82 Se – an extremely Puzzling compound (1) Ionizing particles / Ln (Light) Looking at the coincidences between Heat in Zn. Se and “Light” in the light detector, three population appears Stefano Pirro – Nu. Mass 2010
Zn 82 Se – an extremely promising compound (2) First Results on a 4 cm 5 cm height 337 g Zn. Se Crystal 337 g “new” Zn. Se Crystal 210 Po / s Re n>9 o i t c e j 9. 5 % egion Beta r Light S are e m d rce u o Calibration with 232 Th and a smeared source Stefano Pirro – Nu. Mass 2010
“Beyond” Scintillating bolometers are extremely powerful in order to discriminate background. Nevertheless, they require a double readout that implies a dedicated technical and construction effort. This is the “price” one has to pay in order to really “knock down” background. It would be nice to obtain the same w/o this effort… Stefano Pirro – Nu. Mass 2010
Results on “large” crystals – 48 Ca 100 Mo. O 4 Ca. Mo. O 4 is not a “perfect candidate” for future DBD Experiment since it contains 48 Ca But this compound did show an extreme interesting feature [ms] The results are obtained on a ~160 g crystal 147 Sm- 2310 ke. V Rise time Decay timeofofthe the. Ca. Mo. O 4 4 crystal––no nolightdetection [au] Ca. Mo. O 4 Bolometers permits alpha discrimination (99, 8%) without Light detection Stefano Pirro – Nu. Mass 2010
Zn 82 Se – an extremely promising compound (3) This compound shows another very interesting feature: ’s show different thermal pulse development / rejection > 97 % without light detection Stefano Pirro – Nu. Mass 2010
Zn 100 Mo. O 4 – A promising Molibdate (2) Light- Heat scatter plot No Light information - PSA This seems a characteristic of Molibdate crystals (also observed on other molibdates) Stefano Pirro – Nu. Mass 2010
Conclusions - I ü We tested several types of scintillating crystals with interesting emitters (100 Mo, 116 Cd, 82 Se, 48 Ca) ü Within them Cd. WO 4 is “ready to use” ü The “outsider” Zn. Se is now, probably, the best candidate , even if some more tests are needed ü Molibdates need more R&D both for radioactivity and scintillation light ü This technique is the only one that can be used for several interesting DBD emitters with excellent energy resolution (0. 3 1 % FWHM) ü Simulations show that a background level of 10 -4 c/ke. V/kg/y can be “easily” reached without too much “restrictions” on internal radioactivity. ü Within few months we plan to test 2 small arrays of Cd. WO 4 and Zn. Se crystals ( O(kg) ) in order to completely test the technique (unexpected “surprises” can always happen) Ø Particle Identification through Pulse Shape Analysis could result in a new, unexpected, ally Stefano Pirro – Nu. Mass 2010
Conclusions-2 Different nuclei HAVE to be investigated by DBD experiments CUORE 76 Ge 116 Cd DBD Detectors with Q >2615 ke. V can reach background levels 2 orders 100 Mo of magnitude smaller with respect to the others 130 Te 82 Se Gerda-Majorana If we think about possible “ III generation experiments” the key point will be (if we consider solved the background issue…. ) Energy Resolution ( 1 %) SNO+ 136 Xe EXO Super-Nemo Scintillating Bolometers are the only detectors that can fulfill all these 3 requirements Stefano Pirro – Nu. Mass 2010
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