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Development of a thermal scintillating detector for double beta decay of 48Ca
Nuclear Physics B (Proc. Suppl .) 28A (1992) 233-235 North-Holland
DEVELOPMENT OF A THERMAL SCINTILLATING DETECTOR FOR DOUBLE BETA DECAY OF48Ca A. Alessandrello, +V. Bashkirov, *C. Brofferio D. V. Camin, O. Cremonesi, E. Fiorini, G. Gervasio, A. Giuliani, M. Paean, G. L. Pessina, E. Previtali and L. Zanotti Dâpartimento di Fisica dell Università di Milano and Sezione di Milano delrINFN, I--20133 Moscow Engineering Ph~rsical Institute Moscow, USSR Laboratori Nazionaü de1 Gran Sasso, LAquila, Italy
'
,
Among double beta candidates "Ca stands out for its 4.271 MeV transition energy, well above most ofthe contribution ofnatural y and p radioactivity, but extremely near to the energy released in the a decay of mU (4.274 MeV including nucleus recoil). A CaF$(Eu) detector with loth thermal pulse and scintillation light readout would give very good discrimination againstthis very dangerous source ofbackground . We tested CaF2 crystals with 0.01 to 0.07% Eu doping, in the range of temperature between 300 K and 20 mK The result shows that detection ofthe scintillation light from alpha particles of 5.4 MeV with a silicon photodiode is possible down to 20 mK with high signaUnoise ratio, and that such doping levels do not affect the performance of CaF2 as a thermal detector 1. INTRODUCTION
48Ca has the highest known transition energy between pß active nuclei ; this circumstance makes it a candidate ofexceptional interest in the search for pp decay, despite its quite low natural isotopic abundance (0.187 %). A large transition energy, clearly, gives a strong increase in the expected decay rate, due to the larger phase space available. Limiting the discussion to the case ofthe m,-driven neutrinoless decay, the phase space of48Ca is, including Coulomb corrections, one order of magnitude larger than that of"Ge"l. Many theoretical calculations assign to '*Ca a rather low value for the nuclear matrix element, obtaining for the value ofthe `Nuclear Factor of Merit' of48Ca, defined as NFM =mé t~"vf T& values around 10-'4 y-1121~ while the same value for 'sGe is calculated to be of the order of 10,14 + 10-13 y 113.41. A very recent worklsl based on the QRPA approach, however gives for 48Ca a NFM of 2.3x10-13 y'1, still larger than that of"Ge. The experimental researches on 48Ca are limited by the availability of enriched material, and are mostly due to the Columbia University group worksls1, which set for the 2-neutrino decay a 0920-5632/92/$05.00 0 1992 - Elsevier Science Publishers B.V
lower timit of T2, z 3.6 x 101' years (68% C.L.). A recent investigation performed by the Bejing University and theWorld Labor with large scintillators ofnatural Ca2 has set for the neutrinoless decay a lower limit of To. 2:9.5 x 1021 years (76% C.L.). 2. BACKGROUND AND ENRICHMENT
An advantage in working with 4gCa is the fact the double beta events are expected to fall in a region ofthe energy spectrum where the background is expected to be very low. The hardest y rays emitted from the natural radioactive nuclei are those belonging to "T1, of2614.7 keV energy, while p emissions from the same sources do not exceed the energy of3270 keV (MBi); most of others pp candidates have transition energies that fall in the region ofthe natural y and p radioactivity spectrum (fig-1) Natural alpha radioactivity occurs at energies similar to those ofdouble beta decay of4$Ca, and is thus a potentially dangerous source of spurious counts; CaF2 scintillatios are insensitive to this background, thanks to the large ßJ0 ratio of CaF2(Eu), while bolometric detectors are equally sensitive to p, a, and nuclear recoilst81.
All rights reserved.
A. Alessandreüo et aL l1?evelopment ofa thermal schuillating detector
234
Particularly dangerous is the a activity of218U, which releases a total energy of4274 keV, only 3 keV apart from ßß decay energy of4BCa. Despite the very high sensitivity and energy resolution of bolometric detectors'91 a contamination ofa nucleus of2 "U over 1014 nuclei of a would give a signal hardly distinguishable from a ßßo decay with an half-life of5 x10P y. The Moscow Engineering Physical Institute group has developed the techniques necessary to realize high purity CaF$(Eu) monocrystals enriched to 90% in 48Ca and masses up to 70 g. Radiopurity measurements performed on these crystals have set an upper limit of 10-9 atoms/ atom to U and Th contaminations.
"Do
um
.=
an
.1: Transition energies ofsome pp emitters superimposed to a typical 7 background
The aim ofthis study is the development ofa scintillating lometer, that should operate at a temperature of 10+20 mK, with the ability to reject. a induced events looking at the ratio between the thermal pulse and the scintillation light signal. The interest of this technique is obviously not limited to double beta decay, but can extend also to dark matter experiments, where a discrimination between electrons and nucleus recoils could be obtained in much the same way. Such detectors have been object ofspeculation in the pastno' but, to our knowledge, there have not been experimental studies in this field so far. 3. EXPERIMENTAL
Due to the lack of experimental data on the scintillation ofCaFZ(Eu) at very low temperature, a preliminary measurement was necessary to exclude that this could fade out or became too long at the planned operating point. A measurement ofthe heat capacity of CaF2 (Eu) at the same temperature was also needed since, being europium paramagnetic, doubts
could be cast on the performance of"such substance as a thermal detector. We tested crystals ofcylindrical shape,10 mm height an 10 mm base diameter, approximately. Eu concentration ranged from 0.0796 in mass to pure CaF2. Light detection in the scintillating bolometer will be performed with PIN silicon photodiodes. This choice is due to the necessity ofhaving the detector in tight contact with the crystal, as the signal expected is very low. Photodiodes are also well suited for this application, thanks to their very low intrinsic radioactivity and their negligible heat capacity at very low temperature (®IX8 _ 645 ICJ; a photodiode can be left in contact with the detector without the need of a thermal-insulating light guide, the only side effect should be a small deterioration in energy resolution due to fluctuations in the heat generated by the migration ofthe photoelectrons in the bias electric field. This effect is limited to few keV for a signal of 104 electrons in typical conditions. Even ifwe found no fundamental reasons for which a photodiode should lose its sensitivity at very low temperature, a check on this subject was also necessary, thus the photodiode technique has been chosen in this preliminary test also. In Fig. 2 the set-up used for scintillation measurements is shown: the crystal was coated with a PTFE reflector and a particles from a u'Am source were allowed to hit one side through s0MK-100K
Fig. 2 Detector used for measures of scintillation down to 20 MK
a small hole. A photodiode, glued on the opposite side, was connected to a GaAs charge-sensitive amplifier that could operate in the 100 K-1K temperature range. The photodiode had a surface of 100 mm~ and a capacitance of 45 pF in total depletion condition. Q. E. at CaF2 emission peak is about 50%, limited by the sapphire window. The amplifier had a 100 pF input stage, its ri-
A. Alessandrello et al I Development ofa thermal xihfiffating detector
setime was 30 ns and its rms noise level (at 1K temperature) was 55 e- with Cd = 0 pF and 100 e, with Cd =100 PF . The measurements down to 10 K were performed in a small 4 He circulation cryostat, while the lowest part of the temperature range has been explored with a dilution refrigerator. Scintillation induced by a particles of5.4 MeV
2W -
23'5
4. THE DETECTION OFyRAYS A thermal detector has been realized with a 0.01% doped crystal and a "une mistor: It reached a Temperature 63 larger than our how, probably due to a non perfectly designed mounting set-up. The detector has been tested with source, in the spectrum Compton escape peak of the 2615 keV line are c ble (fig. 5). Energy resolution was limited to 55 keV (FWIN), because ofthe high o nature. The height ofthe pulses (30 tLV at 1500 keV) wass consistent with theheat pected ex!icm the Dbye- law, in i "g thatthe effect ofdoping is indeed negligible, at least at this level of sensitivity and temperature.
um am 10M 14M P u.. e.whcd.sr e Fig. 3 : Scintillation spectra at various temperatures from a " 0.0396 doped crystal àm
has been observerd with high signal/noise ratio down to the lowest temperature . In the selected geometry the photodiode was hit not only by the scintillation light generated in the crystal, but also by the y rays of 60 keV emitted by the source. These produced the small peak visible in the right part ofthe spectrum that has been used to calibrate the response ofthe photodiode . Energy resolution was about 10% (FWHM), a figure consistent with the measured signal/noise ratio. The dependence of the scintillation yield is shown in fig 4. We would like also to stress that, even ifwe did not measure directly the decay time ofthe scintillation, this shows that it did not exceed too much the 10 gs shaping time of our electronic chain. ! . ..
. . . . .....
. . . . . . ..... . . ..... . . . . ....... . . .......... . . . . . . . . . . . .. . . ..... . 4. . . . . ..... . . . . ....
... . . . .... . .
®i. . . . . ., . .' .s . . .,... .. .. . ....... . .. ...... . . . ..... . . . ..y... . . . . ... . . ....... . . . . . ....¢ . . . ...... . ...... . . ... . . . . .... .. . . ......} . . . . . . .... . . . . ..... . .
0.01
0.
Tàq> ao
Fig. 4 : Dependence of the scintillation signal from the temperature
REFERENCES 1 M. Doi, T. Kotani and E. Takasugi, Progr thecr phys 83, (1985); 2 Haxton and Stephenson, Progr part nucl phys, 12,409,(1984) ; 3 K Muto, E. Bender and H.V. Klapdor, Z Phys, A334,187, (1989); 4 J. Engel, P. Vogel and M. R. Zinnbauer, Phys Rev, C37, 731, (1988) ; 5 J. Suhonen, Univ. of Jyvtisylâ preprint, 25, (1991) 6 M. Bardin et al: Nucl Phys, A158,337-363, (1970) 7 Ke You et al: Phys lett, B265, 53, (1991) 8 A. Alessandrello et al: "A search for neutrinoless double beta decay on 13°Te with bolometric detectors, this volume; 9 A. Alessandrello et al: "A Thermal ' resolution a and yray spectrometer for searches on ßß decay and dark mattee Submitted for publication on Phys. Lett.(1991) 10 L. Gonzales-Mestres and A.. Perret ntièproc. ofTAUD 89 conference, Editions res, Gif-surYvette Cedex - France (1989)
DEVELOPMENT OF A THERMAL SCINTILLATING DETECTOR FOR DOUBLE BETA DECAY OF48Ca A. Alessandrello, +V. Bashkirov, *C. Brofferio D. V. Camin, O. Cremonesi, E. Fiorini, G. Gervasio, A. Giuliani, M. Paean, G. L. Pessina, E. Previtali and L. Zanotti Dâpartimento di Fisica dell Università di Milano and Sezione di Milano delrINFN, I--20133 Moscow Engineering Ph~rsical Institute Moscow, USSR Laboratori Nazionaü de1 Gran Sasso, LAquila, Italy
'
,
Among double beta candidates "Ca stands out for its 4.271 MeV transition energy, well above most ofthe contribution ofnatural y and p radioactivity, but extremely near to the energy released in the a decay of mU (4.274 MeV including nucleus recoil). A CaF$(Eu) detector with loth thermal pulse and scintillation light readout would give very good discrimination againstthis very dangerous source ofbackground . We tested CaF2 crystals with 0.01 to 0.07% Eu doping, in the range of temperature between 300 K and 20 mK The result shows that detection ofthe scintillation light from alpha particles of 5.4 MeV with a silicon photodiode is possible down to 20 mK with high signaUnoise ratio, and that such doping levels do not affect the performance of CaF2 as a thermal detector 1. INTRODUCTION
48Ca has the highest known transition energy between pß active nuclei ; this circumstance makes it a candidate ofexceptional interest in the search for pp decay, despite its quite low natural isotopic abundance (0.187 %). A large transition energy, clearly, gives a strong increase in the expected decay rate, due to the larger phase space available. Limiting the discussion to the case ofthe m,-driven neutrinoless decay, the phase space of48Ca is, including Coulomb corrections, one order of magnitude larger than that of"Ge"l. Many theoretical calculations assign to '*Ca a rather low value for the nuclear matrix element, obtaining for the value ofthe `Nuclear Factor of Merit' of48Ca, defined as NFM =mé t~"vf T& values around 10-'4 y-1121~ while the same value for 'sGe is calculated to be of the order of 10,14 + 10-13 y 113.41. A very recent worklsl based on the QRPA approach, however gives for 48Ca a NFM of 2.3x10-13 y'1, still larger than that of"Ge. The experimental researches on 48Ca are limited by the availability of enriched material, and are mostly due to the Columbia University group worksls1, which set for the 2-neutrino decay a 0920-5632/92/$05.00 0 1992 - Elsevier Science Publishers B.V
lower timit of T2, z 3.6 x 101' years (68% C.L.). A recent investigation performed by the Bejing University and theWorld Labor with large scintillators ofnatural Ca2 has set for the neutrinoless decay a lower limit of To. 2:9.5 x 1021 years (76% C.L.). 2. BACKGROUND AND ENRICHMENT
An advantage in working with 4gCa is the fact the double beta events are expected to fall in a region ofthe energy spectrum where the background is expected to be very low. The hardest y rays emitted from the natural radioactive nuclei are those belonging to "T1, of2614.7 keV energy, while p emissions from the same sources do not exceed the energy of3270 keV (MBi); most of others pp candidates have transition energies that fall in the region ofthe natural y and p radioactivity spectrum (fig-1) Natural alpha radioactivity occurs at energies similar to those ofdouble beta decay of4$Ca, and is thus a potentially dangerous source of spurious counts; CaF2 scintillatios are insensitive to this background, thanks to the large ßJ0 ratio of CaF2(Eu), while bolometric detectors are equally sensitive to p, a, and nuclear recoilst81.
All rights reserved.
A. Alessandreüo et aL l1?evelopment ofa thermal schuillating detector
234
Particularly dangerous is the a activity of218U, which releases a total energy of4274 keV, only 3 keV apart from ßß decay energy of4BCa. Despite the very high sensitivity and energy resolution of bolometric detectors'91 a contamination ofa nucleus of2 "U over 1014 nuclei of a would give a signal hardly distinguishable from a ßßo decay with an half-life of5 x10P y. The Moscow Engineering Physical Institute group has developed the techniques necessary to realize high purity CaF$(Eu) monocrystals enriched to 90% in 48Ca and masses up to 70 g. Radiopurity measurements performed on these crystals have set an upper limit of 10-9 atoms/ atom to U and Th contaminations.
"Do
um
.=
an
.1: Transition energies ofsome pp emitters superimposed to a typical 7 background
The aim ofthis study is the development ofa scintillating lometer, that should operate at a temperature of 10+20 mK, with the ability to reject. a induced events looking at the ratio between the thermal pulse and the scintillation light signal. The interest of this technique is obviously not limited to double beta decay, but can extend also to dark matter experiments, where a discrimination between electrons and nucleus recoils could be obtained in much the same way. Such detectors have been object ofspeculation in the pastno' but, to our knowledge, there have not been experimental studies in this field so far. 3. EXPERIMENTAL
Due to the lack of experimental data on the scintillation ofCaFZ(Eu) at very low temperature, a preliminary measurement was necessary to exclude that this could fade out or became too long at the planned operating point. A measurement ofthe heat capacity of CaF2 (Eu) at the same temperature was also needed since, being europium paramagnetic, doubts
could be cast on the performance of"such substance as a thermal detector. We tested crystals ofcylindrical shape,10 mm height an 10 mm base diameter, approximately. Eu concentration ranged from 0.0796 in mass to pure CaF2. Light detection in the scintillating bolometer will be performed with PIN silicon photodiodes. This choice is due to the necessity ofhaving the detector in tight contact with the crystal, as the signal expected is very low. Photodiodes are also well suited for this application, thanks to their very low intrinsic radioactivity and their negligible heat capacity at very low temperature (®IX8 _ 645 ICJ; a photodiode can be left in contact with the detector without the need of a thermal-insulating light guide, the only side effect should be a small deterioration in energy resolution due to fluctuations in the heat generated by the migration ofthe photoelectrons in the bias electric field. This effect is limited to few keV for a signal of 104 electrons in typical conditions. Even ifwe found no fundamental reasons for which a photodiode should lose its sensitivity at very low temperature, a check on this subject was also necessary, thus the photodiode technique has been chosen in this preliminary test also. In Fig. 2 the set-up used for scintillation measurements is shown: the crystal was coated with a PTFE reflector and a particles from a u'Am source were allowed to hit one side through s0MK-100K
Fig. 2 Detector used for measures of scintillation down to 20 MK
a small hole. A photodiode, glued on the opposite side, was connected to a GaAs charge-sensitive amplifier that could operate in the 100 K-1K temperature range. The photodiode had a surface of 100 mm~ and a capacitance of 45 pF in total depletion condition. Q. E. at CaF2 emission peak is about 50%, limited by the sapphire window. The amplifier had a 100 pF input stage, its ri-
A. Alessandrello et al I Development ofa thermal xihfiffating detector
setime was 30 ns and its rms noise level (at 1K temperature) was 55 e- with Cd = 0 pF and 100 e, with Cd =100 PF . The measurements down to 10 K were performed in a small 4 He circulation cryostat, while the lowest part of the temperature range has been explored with a dilution refrigerator. Scintillation induced by a particles of5.4 MeV
2W -
23'5
4. THE DETECTION OFyRAYS A thermal detector has been realized with a 0.01% doped crystal and a "une mistor: It reached a Temperature 63 larger than our how, probably due to a non perfectly designed mounting set-up. The detector has been tested with source, in the spectrum Compton escape peak of the 2615 keV line are c ble (fig. 5). Energy resolution was limited to 55 keV (FWIN), because ofthe high o nature. The height ofthe pulses (30 tLV at 1500 keV) wass consistent with theheat pected ex!icm the Dbye- law, in i "g thatthe effect ofdoping is indeed negligible, at least at this level of sensitivity and temperature.
um am 10M 14M P u.. e.whcd.sr e Fig. 3 : Scintillation spectra at various temperatures from a " 0.0396 doped crystal àm
has been observerd with high signal/noise ratio down to the lowest temperature . In the selected geometry the photodiode was hit not only by the scintillation light generated in the crystal, but also by the y rays of 60 keV emitted by the source. These produced the small peak visible in the right part ofthe spectrum that has been used to calibrate the response ofthe photodiode . Energy resolution was about 10% (FWHM), a figure consistent with the measured signal/noise ratio. The dependence of the scintillation yield is shown in fig 4. We would like also to stress that, even ifwe did not measure directly the decay time ofthe scintillation, this shows that it did not exceed too much the 10 gs shaping time of our electronic chain. ! . ..
. . . . .....
. . . . . . ..... . . ..... . . . . ....... . . .......... . . . . . . . . . . . .. . . ..... . 4. . . . . ..... . . . . ....
... . . . .... . .
®i. . . . . ., . .' .s . . .,... .. .. . ....... . .. ...... . . . ..... . . . ..y... . . . . ... . . ....... . . . . . ....¢ . . . ...... . ...... . . ... . . . . .... .. . . ......} . . . . . . .... . . . . ..... . .
0.01
0.
Tàq> ao
Fig. 4 : Dependence of the scintillation signal from the temperature
REFERENCES 1 M. Doi, T. Kotani and E. Takasugi, Progr thecr phys 83, (1985); 2 Haxton and Stephenson, Progr part nucl phys, 12,409,(1984) ; 3 K Muto, E. Bender and H.V. Klapdor, Z Phys, A334,187, (1989); 4 J. Engel, P. Vogel and M. R. Zinnbauer, Phys Rev, C37, 731, (1988) ; 5 J. Suhonen, Univ. of Jyvtisylâ preprint, 25, (1991) 6 M. Bardin et al: Nucl Phys, A158,337-363, (1970) 7 Ke You et al: Phys lett, B265, 53, (1991) 8 A. Alessandrello et al: "A search for neutrinoless double beta decay on 13°Te with bolometric detectors, this volume; 9 A. Alessandrello et al: "A Thermal ' resolution a and yray spectrometer for searches on ßß decay and dark mattee Submitted for publication on Phys. Lett.(1991) 10 L. Gonzales-Mestres and A.. Perret ntièproc. ofTAUD 89 conference, Editions res, Gif-surYvette Cedex - France (1989)