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A search for neutrinoless double beta decay of 130Te with a bolometric detector
Nuclear Physics B (Proc. Suppl.) 28A (1992) 229-232 North-Holland
A SEARCH FOR NEUTRINOLESS DOUBLE BETA DECAY OF '3°Te WITH A BOLOMETRIC DETECTOR A. Alessandrello, C. Brofferio+, D.V. Camin, 0. Cremonesi, E. Fiorini, G. Gervasîo, A. Giuliani, M. Pavan, G. Pessina, E. Prevîtali and L. Zanotti Dipartimento di Fisica dell'Universita' di Milano, and Sezione di Milano dell'INFN, I-20133 Milano, Italy Presently at the Laboratori Nazionali del Gran Sasso, L' Aquila, Italy A thermal detector consisting of a 34 g Te0a crystal has been operated at about 20 mK in a specially constructed low activity dilution refrigerator installed in the Gran Sasso Underground Laboratory. A spectrum of the thermal pulses collected in more than thousand hours of effective running time shows no evidence for neutrinoless double beta decay of'3°Te . The corresponding lower limit on the process lifetime is two orders of magnitude more stringent than those obtained for the same nucleus with counter techniques and ofthe same order as the values yielded by geochemical searches. tes as possible, in particular those with larger A, where the above mentioned suppressions should 1. INTRODUCTION lower. Geochemical experiments on DBDY41 are based Double beta decay (DBD) is a spontaneous raon the search for an anomalous aboundance ofthe dioactive process where a nucleus (AZ) decays into (A,Z-2) isotope in rocks with an abundant content the isobar (A,Z+2) with the contemporary emission of the nucleus (AZ). Due to the very long (geologiof two electrons'). The process where two antineucal)"running time ofthe experiment they are chaneutrino DBD) is a trinos are also emitted (two racterized by high sensistivities but don't allow to second order transition allowed by the standard discriminate between neutrinoless and two neutrielectroweak theory. Since the nuclear recoiling no channels. Evidence for DBD of82Se and 13°Te has energy is negligible, the two electrons and the two been obtained by Heidelberjel and Missourim antineutrinos share the total transition energy E.; groups; while, however, 82Se rates are comparable, of the electron energies the spectrum of the sum there is some disagreement on the '3°Te halflifetiis continuous and presents a bump at about one mes: (16 .3 ± 1.4) -102°, and (? t 2) -102° for the Heithird ofEo. In neutrinoless DBD, whose existence delberg and Missouri groups respectively . Evidence would imply lepton number non-conservation, a for DBD of'28Te presented by Missouri, is not sharp peak would appear in this spectrum in coreiconfirmed by other groups. spondence to Eo. A further lepton number violaDirect, or instrumental, experiments havebeen boson, ting DBD where a massless Goldstone so far carried out with ionization detectors; evidenMajoron, would be emitted'] will not be named for two neutrino DBD of82Se, "GGe and'°°Mo has ce considered here. (some disagreement exists for the last been found") The uncertainties in the prediction of neutrinowhile lower limits for neutrinoless nucleus rates) less and two neutrino DBD rates are mainly due to DBD of76Gel'6 -nand 13 %181 exceed 1024 and 101 difficulties in properly evaluating nuclear matrix respectively. years for specielements . Recent calculations performed The use of thermal detectors to search for rare fic DBD candidates and taking into account preprocessest' °l as well as for X-ray spectroscop3P 11 viously neglected ground state effects, seem to has been suggested since 1984, many thermal agree for the two neutrino channeeM; a strong supthereafter been implemented"2-I'll. In devices have pression found for the neutrinoless DBD by the approach the particle is revealed by the bolometric Caltech groupe is, however, not confirmed by other measuring the temperature rise of a crystal, which authorsl'r31. There is therefore a general consensus is proportional to the ratio between the energy lost on the need to investigate as many DBD candida0920-5cî32/92/$05 .00 0 1992 - Elsevier Science Publishers B .V
All rights reserved .
A. Alessandrello et aL lA search for neutrinoless double beta decay of t 3°Te
230
by the particle in form ofheat and the heat capacity. For a diamagnetic and dielectric crystal operated at low temperatures, the latter is proportional to the cube ofthe operating to Debye temperature ratio. At tens ofmillikelvin the heat capacity can therefore become so small that even the tiny heat delivered by the particle to the crystal can be revealed and measured by a suitable thermometer in thermal contact with it. 2. THE DETECTOR Various tellurium detectors for gray spectroscopy and searches in DBD have been developed by our groupP81. After operating a crystal ofpure tel lurium[19', which we found however too brittle at low temperatures, further measurements were carried out with TeO2 crystals, a material which presents much better thermal and mechanical properties. After a-ray and gray spectroscopy on the intrinsic radioactivity various TeO2 samples, a 34 g (20x20x15 mms) crystal has been chosen for the present experiment. It is suspended, by means of 16 tips, in a frame ofOxygen Free High Conductivity (OFHC) copper fastened to the heat sink ofthe dilution refirigerator. The gold plated tips exert on the crystal an adjustable pressure by means of springs. This allows to compensate the different contractions of the crystal and frame at low temperatures. All materials in contact or surrounding the crystal have
been previously tested against intrinsic radioactivity with our gray spectrometer located in the Gran Sasso Underground Laboratory. The thermal pulses ofthe crystal are detected and measured by means of a Neutron Transmutation Doped (NTD) thermistor, kept in thermal contact with vacuum grease . The thermistor is biased using a battery and a load resistor. In order to attain a high detector sensitivity a large resistance thermistor has been chosen . Since this might imply an excessive signal integration on the parasitic capacitance of the detector-preamplifier link, with the consequent reduction in the signal-to noise ratio, a low capacitance link to the cryogenic voltage-sensitive preamplifiers based on GaAs MESFETs and operated at 4 W211 has been adopted. The detector was mounted and operated in the underground laboratory ofGran sasso, at a depth of3500 m.w.e, where the background ofcharged cosmic rays and offast neutrons is reduced by 6 and 4 orders ofmagnitude respectivelym]. The Gran Sasso dilution refrigerator was specially built, in collaboration with Oxford Instruments, with previously tested low radioactivity materials (Fig. l) . The entire dilution refrigerator is shielded against local radioactivity by a layer oflow activity lead with minimum thickness of 10 cm. The content of radon is reduced by surrounding the refrigerator with a plastic bag and fluxing it with nitrogen. To keep electromagnetic interference from local sources to its minimum, the refrigerator and the biasing and post-amplifier circuits are located inside a Faraday cage. All control and power supplies wires enter through LC low-pass filters while BIN feedthroughs are used for the signals. The 2
10 ._ 0 .0
Y ,am.
. ,&o
taon.
arm.
-ton
aao .
fm,vv NIVJ
Fig. l Gran Sasso Underground Laboratory experimental setup.
Fig-2 A calibration spectrum obtained with a 2"rh source.
A. Alessandreflo et al. lA searchja neutrinoless double beta decay of 13°Te
mm thick stainless steel Faraday cage walls are covered externally with sound absorbing and antivibrating materials, and with sound absorbing material in the interior for minimum reverberation. The detector base temperature in the Gran Sasso was 18.3 mK, rising to 20 .5 mK by biasing the detector with 1 .7 mV through a 17.3 MW resistor . The corresponding thermistor resistance was 1.6 M.Q. The detector response stability was
s0. .
n 30 .
22M.
2600 .
MM .
Ida
aeo0.
Em-4v N.Vi
a00.
M
s=
smo.
Fig. 3 Background spectrum obtained in 1051.4 hours of effective running time . Non linearity effects are apparent. routinely checked by means of s°Co and "qh source illuminating the detector through a small openable window in the lead shield. A calibration spectrum with the 232Th source, showing single and double escapes ofthe 21171 2614 .5 keV line, is reported in Fig.2. The average FWH1Vi resolution above 1 MeV is 40 keV. The voltage pulses corresponding to 2615 keV yrays are ofabout 0.1 mV, corresponding to an apparent heat capacity of2.2 nJ/K. This figure is about one order of magnitude larger than the value obtained by extrapolating to lower temperatures the existing data on TeO21231 on the basis ofthe Debye law. Measurements carried out on the thermistor by exposing it to y-ray, show that its contribution to the total heat capacity of the bolometer is negligible (<.1 nJ/KJ. A background spectrum, totalling 1051.4 hours effective running time is shown in Fig.3. of A residual even iftiny contamination background due to232Th is shown by the presence ofthe
2111T12615 keV line. The large above 5 V shown in Fig-3 is due to a-decay of" o, a active contaminant which is normally present in tellurium compoundswk We would like to stress that the energy of the peak corresponds to the, transition energy, namely the sum of the energies of the alpha particle and of the recoilingnucleus. A non-linear fit to various y peaks from and 'Co calibration runshas been in fact carried out on the basis of a thermal model with a Ta dence of the thermal capacity, where a is afree parameter. By extrapolation to higher energies we obtain a value of 5480 t 50 keV in agreement with the figure of5407.6 for the transition energy of 2iopo. Its rate (0,38 t 0.02 counts h-1) corresponds to a contamination of3 mBqkg' in TeO., a low figure with respect to other materials 041. It does not contribute substantially to the background in the DBD energy region since the a particles are emitted inside the detector and therefore not degraded by energy loss . Below and above a peak there is indication ofdifferent, even iflower, a activities, but statistics does not allow to attribute them to the J or 2"rh chars. By attributingall these counts to contaminations of the 23OU or22*Th chains in secular equilibrium inside the crystal we obtain upper limits of0.36 and 0.45 mBq kgl (90% C.L.) respectively. 3. CONCLUSIONS No peak appear in the region of u°Te neutrinoless DBD, where the background counting rate is (9+2)- 10-3 counts keV1h-lkgl. Contributions to this background from a and ß decays generated inside the crystal should be negligible on the basis ofthe above mentioned limits on its mU and 21Th contaminations. Relevant contributions from residual activities of NTD thermistor after neutron irradiation can also be excluded, since the pulses generated inside the thermistor have structures definitely different from the ones generated inside the crystal"" . The only conclusion is that background origin is external to the detector . This is, however, not surprising; materials with considerable activities like stainless steel, silver compounds and electronic components are still present in the interior ofthe dilution refrigerator and, moreover, it's very difficult to use low intrinsic radioactivity materials (like electrolytic lead) with poor mechanical properties for the shielding of a large set-up like a powerful dilution
232
A. Alessandrefto et aL lA search forneutrinoless double beta decay of ts°Te
refrigerator. The antimony charged lead used in the present measurement, though selected among various samples, has still some radioactivity. By applying a maximum likelihood analysis to our energy spectrum (taking into account non linearity effects), we extract lower limits of8.6-10P° and 4.4-10P° years at the 68 and 90% confidence level for neutrinoless DBD of 'ne. These limits are ofthe same order as the values obtained by geochemical experiments and two orders ofmagnitude more stringent than those obtained by a previous experiment carried out with a CdTe detectorMl REFERENCES 1. For recently published reviews see T. Tomoda, Rep. Prog. Phys. 54 (1991) 53; MX Moe, Nuclear Phys. B (Prec. Suppl.) 19 (1991) 158; E. Fiorini, Nucl. Phys. News, l (1991) 17; A.Faessler, J. Phys. G: Nucl. Part. Phys. 17 (1991) S15; D.O.Caldwell, J. Phys. G: Nucl. Part. Phys. 17 (1991) 137; H.V. IOapdor, J. Phys. G: Nvcl. Part. Phys. 17 (1991) S295; 2. J.Engel, P.Vogel and M .R.Zirnbauer, Phys.Lett. 225B (1989) 5 3. A.Faessler et al., Phys.Rev.C 43 (1991) R21 4. T.Kirsten, Upcoming experiments and plans in low energy neutrino physic3, in: Neutrino 88 (Boston, June 1988), ed. by J.Schneps. 5. WJ.Lin, O.K anuel, and D.S.Muangnoicharoen, Nucl.Phys . A 481 (1988) 484 6. D.Caldwell et al., J.Phys.G: Nucl.Part.Phys.17 (1991) 7. F.T.Avignone et al., Phys.Lett. 256B(1991)559 8. H.T.Wong et al., Phys.Rev.Lett. 67(1991)1218 9. G.F.De1PAntonio and E.Fiorini, Suppl.Nuovo Cimento 17(1960)132 10. E. Fiorini and T.O . Niinikoski, Nucl. Instr. and Methods 224 (1984) 83; 11. S.H. Moseley, J.C. Mather, and D. cCammon, J. Appl. Phys. 56(1984) 1257; 12. E. Fiorini, Physica B167 (1991) 388; 13. Proceedings of"Low Temperature DetectorsforNeutrinos andDark Matter I", ed . K Pretzl (Springer and Verlag,1988); 14. Proceedings of"Low Temperature Detectors for Neutrinos and Dark Matter II", eds . D. Gonzales-Mestres and L. Perret-Gallix (Ed. Frontiers, 1989); 15. Proceedings of"Low Temperature Detectors for Neutrinos and Dark Matter III", eds. L. Brogia
to, D.V. Camin, E. Fiorini (ed. Frontieres, 1990)"; 16. Proceedings of"Superconducting and Low Temperature Particle Detectors7, eds. G. Waysand and G. Chardin (Elseviere Sci. Pu., 1989); 17. G.R.Dick et al., Phys.Lett. 2458 (1990) 186 18. A. Alessandrello, C. Brofferio, D.V. Camin, O. Cremonesi, E.Fiorini, G. Gervasio, A. Giuliani, M. Pavan, G. Pessina, E.Previtali and L. Zanotti: Gamma ray spectroscopy with high-Z thermal detectors, Nucl. Instr. and Method A (in press), also for previous references; 19. A.Alessandrello, C. Brofferio, D.V. Camin, O. Cremonesi, E.Fiorini, A. Giuliani, and G. Pessina, Phys. Lett. 247B (1990) 442 ; 20. B. Sadoulet, N. Wang, F.C. Wellstood, E.E. Haller, and J. Beeman, Phys. Rev. B41(1990) 3761; 21. A. Alessandrello, C. Brofferio, D.V. Camin, AGiuliani, G.Pessina, and E. Previtali, Nucl. Instrum. and Meth. A295 (1990) 405 ; 22. L. Zanotti, J. Phys. G: Nucl. Part. Phys. 17 (1991) S373; 23. G.K White, S.J. Collocott, and J. Collins, J. Phys.: Condens. Matter 2, 7715 (1990); 24. E. Bellotti et al., Nuovo Cimento 95A (1986) 1; 25. A. Alessandrello et al., Nucl. Instr. and Method B61(1991)106; 26. L.W. itchell and P.H.Fisher, Phys.Rev. C38 (1988) 895
A SEARCH FOR NEUTRINOLESS DOUBLE BETA DECAY OF '3°Te WITH A BOLOMETRIC DETECTOR A. Alessandrello, C. Brofferio+, D.V. Camin, 0. Cremonesi, E. Fiorini, G. Gervasîo, A. Giuliani, M. Pavan, G. Pessina, E. Prevîtali and L. Zanotti Dipartimento di Fisica dell'Universita' di Milano, and Sezione di Milano dell'INFN, I-20133 Milano, Italy Presently at the Laboratori Nazionali del Gran Sasso, L' Aquila, Italy A thermal detector consisting of a 34 g Te0a crystal has been operated at about 20 mK in a specially constructed low activity dilution refrigerator installed in the Gran Sasso Underground Laboratory. A spectrum of the thermal pulses collected in more than thousand hours of effective running time shows no evidence for neutrinoless double beta decay of'3°Te . The corresponding lower limit on the process lifetime is two orders of magnitude more stringent than those obtained for the same nucleus with counter techniques and ofthe same order as the values yielded by geochemical searches. tes as possible, in particular those with larger A, where the above mentioned suppressions should 1. INTRODUCTION lower. Geochemical experiments on DBDY41 are based Double beta decay (DBD) is a spontaneous raon the search for an anomalous aboundance ofthe dioactive process where a nucleus (AZ) decays into (A,Z-2) isotope in rocks with an abundant content the isobar (A,Z+2) with the contemporary emission of the nucleus (AZ). Due to the very long (geologiof two electrons'). The process where two antineucal)"running time ofthe experiment they are chaneutrino DBD) is a trinos are also emitted (two racterized by high sensistivities but don't allow to second order transition allowed by the standard discriminate between neutrinoless and two neutrielectroweak theory. Since the nuclear recoiling no channels. Evidence for DBD of82Se and 13°Te has energy is negligible, the two electrons and the two been obtained by Heidelberjel and Missourim antineutrinos share the total transition energy E.; groups; while, however, 82Se rates are comparable, of the electron energies the spectrum of the sum there is some disagreement on the '3°Te halflifetiis continuous and presents a bump at about one mes: (16 .3 ± 1.4) -102°, and (? t 2) -102° for the Heithird ofEo. In neutrinoless DBD, whose existence delberg and Missouri groups respectively . Evidence would imply lepton number non-conservation, a for DBD of'28Te presented by Missouri, is not sharp peak would appear in this spectrum in coreiconfirmed by other groups. spondence to Eo. A further lepton number violaDirect, or instrumental, experiments havebeen boson, ting DBD where a massless Goldstone so far carried out with ionization detectors; evidenMajoron, would be emitted'] will not be named for two neutrino DBD of82Se, "GGe and'°°Mo has ce considered here. (some disagreement exists for the last been found") The uncertainties in the prediction of neutrinowhile lower limits for neutrinoless nucleus rates) less and two neutrino DBD rates are mainly due to DBD of76Gel'6 -nand 13 %181 exceed 1024 and 101 difficulties in properly evaluating nuclear matrix respectively. years for specielements . Recent calculations performed The use of thermal detectors to search for rare fic DBD candidates and taking into account preprocessest' °l as well as for X-ray spectroscop3P 11 viously neglected ground state effects, seem to has been suggested since 1984, many thermal agree for the two neutrino channeeM; a strong supthereafter been implemented"2-I'll. In devices have pression found for the neutrinoless DBD by the approach the particle is revealed by the bolometric Caltech groupe is, however, not confirmed by other measuring the temperature rise of a crystal, which authorsl'r31. There is therefore a general consensus is proportional to the ratio between the energy lost on the need to investigate as many DBD candida0920-5cî32/92/$05 .00 0 1992 - Elsevier Science Publishers B .V
All rights reserved .
A. Alessandrello et aL lA search for neutrinoless double beta decay of t 3°Te
230
by the particle in form ofheat and the heat capacity. For a diamagnetic and dielectric crystal operated at low temperatures, the latter is proportional to the cube ofthe operating to Debye temperature ratio. At tens ofmillikelvin the heat capacity can therefore become so small that even the tiny heat delivered by the particle to the crystal can be revealed and measured by a suitable thermometer in thermal contact with it. 2. THE DETECTOR Various tellurium detectors for gray spectroscopy and searches in DBD have been developed by our groupP81. After operating a crystal ofpure tel lurium[19', which we found however too brittle at low temperatures, further measurements were carried out with TeO2 crystals, a material which presents much better thermal and mechanical properties. After a-ray and gray spectroscopy on the intrinsic radioactivity various TeO2 samples, a 34 g (20x20x15 mms) crystal has been chosen for the present experiment. It is suspended, by means of 16 tips, in a frame ofOxygen Free High Conductivity (OFHC) copper fastened to the heat sink ofthe dilution refirigerator. The gold plated tips exert on the crystal an adjustable pressure by means of springs. This allows to compensate the different contractions of the crystal and frame at low temperatures. All materials in contact or surrounding the crystal have
been previously tested against intrinsic radioactivity with our gray spectrometer located in the Gran Sasso Underground Laboratory. The thermal pulses ofthe crystal are detected and measured by means of a Neutron Transmutation Doped (NTD) thermistor, kept in thermal contact with vacuum grease . The thermistor is biased using a battery and a load resistor. In order to attain a high detector sensitivity a large resistance thermistor has been chosen . Since this might imply an excessive signal integration on the parasitic capacitance of the detector-preamplifier link, with the consequent reduction in the signal-to noise ratio, a low capacitance link to the cryogenic voltage-sensitive preamplifiers based on GaAs MESFETs and operated at 4 W211 has been adopted. The detector was mounted and operated in the underground laboratory ofGran sasso, at a depth of3500 m.w.e, where the background ofcharged cosmic rays and offast neutrons is reduced by 6 and 4 orders ofmagnitude respectivelym]. The Gran Sasso dilution refrigerator was specially built, in collaboration with Oxford Instruments, with previously tested low radioactivity materials (Fig. l) . The entire dilution refrigerator is shielded against local radioactivity by a layer oflow activity lead with minimum thickness of 10 cm. The content of radon is reduced by surrounding the refrigerator with a plastic bag and fluxing it with nitrogen. To keep electromagnetic interference from local sources to its minimum, the refrigerator and the biasing and post-amplifier circuits are located inside a Faraday cage. All control and power supplies wires enter through LC low-pass filters while BIN feedthroughs are used for the signals. The 2
10 ._ 0 .0
Y ,am.
. ,&o
taon.
arm.
-ton
aao .
fm,vv NIVJ
Fig. l Gran Sasso Underground Laboratory experimental setup.
Fig-2 A calibration spectrum obtained with a 2"rh source.
A. Alessandreflo et al. lA searchja neutrinoless double beta decay of 13°Te
mm thick stainless steel Faraday cage walls are covered externally with sound absorbing and antivibrating materials, and with sound absorbing material in the interior for minimum reverberation. The detector base temperature in the Gran Sasso was 18.3 mK, rising to 20 .5 mK by biasing the detector with 1 .7 mV through a 17.3 MW resistor . The corresponding thermistor resistance was 1.6 M.Q. The detector response stability was
s0. .
n 30 .
22M.
2600 .
MM .
Ida
aeo0.
Em-4v N.Vi
a00.
M
s=
smo.
Fig. 3 Background spectrum obtained in 1051.4 hours of effective running time . Non linearity effects are apparent. routinely checked by means of s°Co and "qh source illuminating the detector through a small openable window in the lead shield. A calibration spectrum with the 232Th source, showing single and double escapes ofthe 21171 2614 .5 keV line, is reported in Fig.2. The average FWH1Vi resolution above 1 MeV is 40 keV. The voltage pulses corresponding to 2615 keV yrays are ofabout 0.1 mV, corresponding to an apparent heat capacity of2.2 nJ/K. This figure is about one order of magnitude larger than the value obtained by extrapolating to lower temperatures the existing data on TeO21231 on the basis ofthe Debye law. Measurements carried out on the thermistor by exposing it to y-ray, show that its contribution to the total heat capacity of the bolometer is negligible (<.1 nJ/KJ. A background spectrum, totalling 1051.4 hours effective running time is shown in Fig.3. of A residual even iftiny contamination background due to232Th is shown by the presence ofthe
2111T12615 keV line. The large above 5 V shown in Fig-3 is due to a-decay of" o, a active contaminant which is normally present in tellurium compoundswk We would like to stress that the energy of the peak corresponds to the, transition energy, namely the sum of the energies of the alpha particle and of the recoilingnucleus. A non-linear fit to various y peaks from and 'Co calibration runshas been in fact carried out on the basis of a thermal model with a Ta dence of the thermal capacity, where a is afree parameter. By extrapolation to higher energies we obtain a value of 5480 t 50 keV in agreement with the figure of5407.6 for the transition energy of 2iopo. Its rate (0,38 t 0.02 counts h-1) corresponds to a contamination of3 mBqkg' in TeO., a low figure with respect to other materials 041. It does not contribute substantially to the background in the DBD energy region since the a particles are emitted inside the detector and therefore not degraded by energy loss . Below and above a peak there is indication ofdifferent, even iflower, a activities, but statistics does not allow to attribute them to the J or 2"rh chars. By attributingall these counts to contaminations of the 23OU or22*Th chains in secular equilibrium inside the crystal we obtain upper limits of0.36 and 0.45 mBq kgl (90% C.L.) respectively. 3. CONCLUSIONS No peak appear in the region of u°Te neutrinoless DBD, where the background counting rate is (9+2)- 10-3 counts keV1h-lkgl. Contributions to this background from a and ß decays generated inside the crystal should be negligible on the basis ofthe above mentioned limits on its mU and 21Th contaminations. Relevant contributions from residual activities of NTD thermistor after neutron irradiation can also be excluded, since the pulses generated inside the thermistor have structures definitely different from the ones generated inside the crystal"" . The only conclusion is that background origin is external to the detector . This is, however, not surprising; materials with considerable activities like stainless steel, silver compounds and electronic components are still present in the interior ofthe dilution refrigerator and, moreover, it's very difficult to use low intrinsic radioactivity materials (like electrolytic lead) with poor mechanical properties for the shielding of a large set-up like a powerful dilution
232
A. Alessandrefto et aL lA search forneutrinoless double beta decay of ts°Te
refrigerator. The antimony charged lead used in the present measurement, though selected among various samples, has still some radioactivity. By applying a maximum likelihood analysis to our energy spectrum (taking into account non linearity effects), we extract lower limits of8.6-10P° and 4.4-10P° years at the 68 and 90% confidence level for neutrinoless DBD of 'ne. These limits are ofthe same order as the values obtained by geochemical experiments and two orders ofmagnitude more stringent than those obtained by a previous experiment carried out with a CdTe detectorMl REFERENCES 1. For recently published reviews see T. Tomoda, Rep. Prog. Phys. 54 (1991) 53; MX Moe, Nuclear Phys. B (Prec. Suppl.) 19 (1991) 158; E. Fiorini, Nucl. Phys. News, l (1991) 17; A.Faessler, J. Phys. G: Nucl. Part. Phys. 17 (1991) S15; D.O.Caldwell, J. Phys. G: Nucl. Part. Phys. 17 (1991) 137; H.V. IOapdor, J. Phys. G: Nvcl. Part. Phys. 17 (1991) S295; 2. J.Engel, P.Vogel and M .R.Zirnbauer, Phys.Lett. 225B (1989) 5 3. A.Faessler et al., Phys.Rev.C 43 (1991) R21 4. T.Kirsten, Upcoming experiments and plans in low energy neutrino physic3, in: Neutrino 88 (Boston, June 1988), ed. by J.Schneps. 5. WJ.Lin, O.K anuel, and D.S.Muangnoicharoen, Nucl.Phys . A 481 (1988) 484 6. D.Caldwell et al., J.Phys.G: Nucl.Part.Phys.17 (1991) 7. F.T.Avignone et al., Phys.Lett. 256B(1991)559 8. H.T.Wong et al., Phys.Rev.Lett. 67(1991)1218 9. G.F.De1PAntonio and E.Fiorini, Suppl.Nuovo Cimento 17(1960)132 10. E. Fiorini and T.O . Niinikoski, Nucl. Instr. and Methods 224 (1984) 83; 11. S.H. Moseley, J.C. Mather, and D. cCammon, J. Appl. Phys. 56(1984) 1257; 12. E. Fiorini, Physica B167 (1991) 388; 13. Proceedings of"Low Temperature DetectorsforNeutrinos andDark Matter I", ed . K Pretzl (Springer and Verlag,1988); 14. Proceedings of"Low Temperature Detectors for Neutrinos and Dark Matter II", eds . D. Gonzales-Mestres and L. Perret-Gallix (Ed. Frontiers, 1989); 15. Proceedings of"Low Temperature Detectors for Neutrinos and Dark Matter III", eds. L. Brogia
to, D.V. Camin, E. Fiorini (ed. Frontieres, 1990)"; 16. Proceedings of"Superconducting and Low Temperature Particle Detectors7, eds. G. Waysand and G. Chardin (Elseviere Sci. Pu., 1989); 17. G.R.Dick et al., Phys.Lett. 2458 (1990) 186 18. A. Alessandrello, C. Brofferio, D.V. Camin, O. Cremonesi, E.Fiorini, G. Gervasio, A. Giuliani, M. Pavan, G. Pessina, E.Previtali and L. Zanotti: Gamma ray spectroscopy with high-Z thermal detectors, Nucl. Instr. and Method A (in press), also for previous references; 19. A.Alessandrello, C. Brofferio, D.V. Camin, O. Cremonesi, E.Fiorini, A. Giuliani, and G. Pessina, Phys. Lett. 247B (1990) 442 ; 20. B. Sadoulet, N. Wang, F.C. Wellstood, E.E. Haller, and J. Beeman, Phys. Rev. B41(1990) 3761; 21. A. Alessandrello, C. Brofferio, D.V. Camin, AGiuliani, G.Pessina, and E. Previtali, Nucl. Instrum. and Meth. A295 (1990) 405 ; 22. L. Zanotti, J. Phys. G: Nucl. Part. Phys. 17 (1991) S373; 23. G.K White, S.J. Collocott, and J. Collins, J. Phys.: Condens. Matter 2, 7715 (1990); 24. E. Bellotti et al., Nuovo Cimento 95A (1986) 1; 25. A. Alessandrello et al., Nucl. Instr. and Method B61(1991)106; 26. L.W. itchell and P.H.Fisher, Phys.Rev. C38 (1988) 895