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Double beta decay study of 48Ca by CaF2 scintillator
ELSEVIER
Nuclear Physics A721 (2003) 525c-528~ www.elsevier.com/locatelnpe
Double beta decay study of 48Ca by CaFz scintillator Ogawa, I.“, R. Hazama*, S. Ajimura”, K. Matsuoka”, N. Kudomib, K. Kumeb, H. Ohsumi”, K. Fushimid, N. Suzuki”, T. Nittaa, H. Miyawaki”, S. Shiomi”, Y. Tanakaa, Y. Ishikawa&, M. ItamuraB, K. Kishimoto”, A. Katsuki”, H. Sakai”, D. Yokoyamaa, S. Umehara”, S. Tomii’, K. Mukaida”, S. Yoshida”, H. Ejirib and T. Kishimoto” “Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan bResearch Center for Nuclear Physics, Osaka University, Ibaraki, Osaka 567-0047, Japan CFaculty of Culture and Education, Saga University, Honjo, Saga 840-8502, Japan dFaculty of Integ r a ted Arts and Science, The University of Tokushima, Tokushima 770-8502, Japan A CaFs scintillation detector system (ELEGANT VI) is developed to search for neutrinoless double beta decay (OVDBD) of 48Ca. No events were observed around the Q-value energy region after the analysis of 4.23 kg yr data. To derive the lower limit for the half-life, the expected number of background events was estimated by a Monte Carlo simulation using the measured activities of 214Bi and 220Rn inside CaFs crystals. A new lower limit is obtained to be 1.8 x 1O22yr at the 90 % C.L. 1. Introduction
We have been studying the double beta decays of 48Ca. The Q-value of 48Ca + 48Ti is the highest (4.27 MeV) among potential double beta decay nuclei. The Q-value is far above energies of y-rays from natural radioactivities (maximum 2.615 MeV from “‘Tl decay), therefore we can naturally expect small background in the energy region we are interested in. It also means an advantage of large phase space factor. For the OvDBD measurement, small background from DBD with 2 neutrino emission (SVDBD) is also expected. Most stringent lower limit for the half-life of OvDBD of 48Ca, qT2 > 9.5 x 1021yr (76 % C.L.), was obtained by Beijing group [l] using 37 kg of CaFs scintillation crystals. 2. Detector
system
We are studying the double beta decay of 48Ca with a CaF2 scintillation detector system (ELEGANT VI)[2] at the underground laboratory (Oto Cosmo Observatory). This is the experiment with an “active source” (source = detector) and the detection efficiency for the DBD events is high. This detector system is also used for spin-coupled dark matter search [a]. Since both measurements aim at searching for ultra-rare events, background reduction/rejection is crucial. A schematic drawing of the ELEGANT VI is shown in 0375-9474/03/$ - see front matter 0 2003 Elsevier Science B.V. All doi:lO.l016/S0375-9474(03)01115-l
rightsreserved
I. Ogawa et al. /Nuclear Physics A721 (2003) 52Sc-528~
526~
Fig. 1. It consists of passive shields (C-G scintillators (B) .
in Fig. l), CaFs detectors
cross-seetlonal view
(A) and CsI(T1)
Side view
Figure 1. A schematic drawing of ELEGANT VI. (A)25 segmented CaFs detectors. The detectors are numbered from 1 (bottom left in cross-sectional view) through 5 (bottom (C)Cd sheet of 0.6 m m thick. (D)Pb right) to 25 (top right). (B)38 CsI(T1) scintillators. shield of 10 cm thick. (E)OFHC Cu shield of 5 cm thick. (F)LiH-loaded paraffin of 15 m m thick. (G)air-tight box inside of which is purged by pure Nz gas. The whole system is surrounded by HsBOs-loaded water tanks.
For further rejection of backgrounds, the scintillators were designed to achieve 47r active shielding system. 38 CsI(T1) scintillators are used as veto counters for 25 CaFs detectors. The CaFs detectors have active lightguides against backgrounds from PMT sides which are not covered by CsI(T1) scintillators. Segmentation of the CaFs detectors also enables us to reject the background events which spread in two or more detectors; a true event is mostly confined to a single detector. All the materials used in the detector system have been carefully selected for low radioactivity by low background Ge detector. After the event selection by 47r active shielding system, backgrounds around 3 - 5 MeV range are most probably from internal radioactive contamination. Since Q-values and half-lives of natural radioactivities are well known, following decays are relevant: 214Bi
Qa = 3.27 MeV
in the uranium “‘Tl 212Bi
) 214P~(Tl,2 = 164.3 px)
Qa= 7.83MeV , 210Pb,
chain, and
Qp = 5.00 MeV Qp = 2.25 MeV
, “‘Pb, ,
212Po(Tl,2 = 0.299 psec)
(2) Qa = 8.g5MeV , 208Pb,
(3)
in the thorium chain. In order to estimate the activities of nuclei (214Bi in the uranium and 220Rn in the thorium chain) inside the CaFs(Eu) scintillation crystals, a delayed coincidence method was used. Since two crystals had much higher contaminations than others, we did not
I. Ogawa et al, /Nuclear Physics A721 (2003) S25c-528~
527e
use these crystals in the following analysis. Measured total activities in the 23 CaFz(Eu) crystals were 214Bi - (1.11 %O.Ol) x 10m3Bq/kg and azoRn - (9.81 f0.15) x 10e5 Bq/kg. 3. Results
(a)
Energy(keV)
09
Energy(keV)
Figure 2. (a)A comparison of Monte Carlo simulations from the internal radioactivities (dashed line) and the ~vDBD of 48Ca (dotted line) with experimental data (solid line) of a statistical significance of 4.23 kg yr. The arrow indicates the OvDBD energy window. (b)Simulated background spectra for each decay - dotted line : decay (l), dashed line : decay (2) and solid line : decay (3).
The energy spectrum of the 23 crystals with a statistical significance of 4.23 kg yr is shown in Fig. 2 (a). No events are observed in the OvDBD energy window which is a 30 peak interval centered at 4.27 MeV. In order to derive the lower limit of the half-life for the OVDBD, we need to estimate background events in this energy window. We considered two possible background sources, ~vDBD from 48Ca and the decays (l), (2) and (3) f rom internal radioactivities. The known half-life of the 2vDBD was used to simulate an expected spectrum in Fig. 2 (a). Contribution from the ~vDBD of 48Ca in the OvDBD energy window was found to be negligible. Background events from internal radioactivities was estimated by a Monte Carlo simulation based on GEANT 3.21 code. Total simulated energy spectrum from the decays (l), (2) and (3) is plotted in Fig. 2 (a) with experimental data. Also shown in Fig. 2(b) are simulated background spectra for each decay. The expected number of background events in the OvDBD energy window is 2.63 f 0.13. Following the procedure of Ref. [3], we extracted a half-life limit for the OvDBD of 48Ca. The resulting lower limit of half-life (for the O+ + O+ transition) is[4] T$
2
8.6 x 1O22year (68 % C.L.)
528c
I. Ogawa et al. /Nuclear Physics A721 (2003) 525c-528~ 2
1.8 x 1O22 year (90 % C.L.).
This result can be converted to the upper limit for the effective Majorana neutrino mass (m,) neglecting right-handed currents. Depending on the theoretical nuclear matrix elements (see Table 8 & 9 in Ref.[5]), the upper bound at the 90 % C.L. is (m,) < (6.3 - 39.4) eV. 4. Conclusion
and outlook
The most stringent limit on the half-life of the neutrino-less double beta decay of 48Ca (48Ca -+48Ti) has been obtained by using the CaFz scintillation detector system with a total statistical significance of 4.23 kg yr. The number of observed events in the signal window was 0 while the expected background events was 2.6. A Monte Carlo simulation using the results from the measurement of activities indicated that the main backgrounds around 3 - 5 MeV range were from the sequential decays of the daughter nuclei of U and Th. These events can be rejected using the pulse shape recorded in the gate time window. A measurement is underway with flash ADC to record the pulse shape of the events with energy above -2 MeV. We expect that further operation enables us to obtain a finite value on the half-life of ~vDBD and improved limit on the OVDBD. For further improvements, especially to reach the mass region suggested by the oscillation experiments, we need to develop a large scale detector system. We propose a new detector system, CANDLES, where 0(102 - 103) kg of CaFz crystals are immersed in a liquid scintillator which acts as an active shield to veto backgrounds. Scintillation light is viewed by large PMTs. Using the experience gained from ELEGANT VI, a prototype of CANDLES is being developed. We expect that CANDLES will enable us to make a considerable improvement on the DBD of *‘Ca. REFERENCES 1. 2.
3. 4. 5.
K. You et al., Phys. Lett. B 265 (1991) 53. R. Hazama et al., Proc. of the 4th Int. Conf. on Weak and Electromagnetic Interactions in Nuclei (WEIN 95), Osaka, Japan, June 1995, World Scientific, Singapore P. 635; R. Hazama et al., Proc. of the XIV Int. Conf. on Particles and Nuclei (PANIC 96), CEBAF, USA, May 1996, World Scientific, Singapore p. 477; R. Hazama et al., Proc. of the Int. Workshop on the Identification of Dark Matter (IDM 96), September 1996, Sheffield, UK, World Scientific, Singapore p.397; T. Kishimoto et al., Proc. of the 2nd RESCEU Int. Symp. on Dark Matter in the Universe and its Direct Detection, November 1996, Tokyo, Universal Academy Press Inc., Tokyo, p. 71; I. Ogawa et al., Nucl. Phys. A663-664 (2000) 869c; T. Kishimoto et al., Proc. of the Identification of the Dark Matters (IDM2000), September 2000, York, UK; R. Hazama et al., Nucl. Instr. Meth. in Phys. Res. A 482 (2002) 297. G.J. Feldman and R.D. Cousins, Phys. Rev. D 57 (1998) 3873. I. Ogawa, et al., submitted to Phys. Lett. B. J. Suhonen and 0. Civitarese, Phys. Rep. 300 (1998) 123.
Nuclear Physics A721 (2003) 525c-528~ www.elsevier.com/locatelnpe
Double beta decay study of 48Ca by CaFz scintillator Ogawa, I.“, R. Hazama*, S. Ajimura”, K. Matsuoka”, N. Kudomib, K. Kumeb, H. Ohsumi”, K. Fushimid, N. Suzuki”, T. Nittaa, H. Miyawaki”, S. Shiomi”, Y. Tanakaa, Y. Ishikawa&, M. ItamuraB, K. Kishimoto”, A. Katsuki”, H. Sakai”, D. Yokoyamaa, S. Umehara”, S. Tomii’, K. Mukaida”, S. Yoshida”, H. Ejirib and T. Kishimoto” “Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan bResearch Center for Nuclear Physics, Osaka University, Ibaraki, Osaka 567-0047, Japan CFaculty of Culture and Education, Saga University, Honjo, Saga 840-8502, Japan dFaculty of Integ r a ted Arts and Science, The University of Tokushima, Tokushima 770-8502, Japan A CaFs scintillation detector system (ELEGANT VI) is developed to search for neutrinoless double beta decay (OVDBD) of 48Ca. No events were observed around the Q-value energy region after the analysis of 4.23 kg yr data. To derive the lower limit for the half-life, the expected number of background events was estimated by a Monte Carlo simulation using the measured activities of 214Bi and 220Rn inside CaFs crystals. A new lower limit is obtained to be 1.8 x 1O22yr at the 90 % C.L. 1. Introduction
We have been studying the double beta decays of 48Ca. The Q-value of 48Ca + 48Ti is the highest (4.27 MeV) among potential double beta decay nuclei. The Q-value is far above energies of y-rays from natural radioactivities (maximum 2.615 MeV from “‘Tl decay), therefore we can naturally expect small background in the energy region we are interested in. It also means an advantage of large phase space factor. For the OvDBD measurement, small background from DBD with 2 neutrino emission (SVDBD) is also expected. Most stringent lower limit for the half-life of OvDBD of 48Ca, qT2 > 9.5 x 1021yr (76 % C.L.), was obtained by Beijing group [l] using 37 kg of CaFs scintillation crystals. 2. Detector
system
We are studying the double beta decay of 48Ca with a CaF2 scintillation detector system (ELEGANT VI)[2] at the underground laboratory (Oto Cosmo Observatory). This is the experiment with an “active source” (source = detector) and the detection efficiency for the DBD events is high. This detector system is also used for spin-coupled dark matter search [a]. Since both measurements aim at searching for ultra-rare events, background reduction/rejection is crucial. A schematic drawing of the ELEGANT VI is shown in 0375-9474/03/$ - see front matter 0 2003 Elsevier Science B.V. All doi:lO.l016/S0375-9474(03)01115-l
rightsreserved
I. Ogawa et al. /Nuclear Physics A721 (2003) 52Sc-528~
526~
Fig. 1. It consists of passive shields (C-G scintillators (B) .
in Fig. l), CaFs detectors
cross-seetlonal view
(A) and CsI(T1)
Side view
Figure 1. A schematic drawing of ELEGANT VI. (A)25 segmented CaFs detectors. The detectors are numbered from 1 (bottom left in cross-sectional view) through 5 (bottom (C)Cd sheet of 0.6 m m thick. (D)Pb right) to 25 (top right). (B)38 CsI(T1) scintillators. shield of 10 cm thick. (E)OFHC Cu shield of 5 cm thick. (F)LiH-loaded paraffin of 15 m m thick. (G)air-tight box inside of which is purged by pure Nz gas. The whole system is surrounded by HsBOs-loaded water tanks.
For further rejection of backgrounds, the scintillators were designed to achieve 47r active shielding system. 38 CsI(T1) scintillators are used as veto counters for 25 CaFs detectors. The CaFs detectors have active lightguides against backgrounds from PMT sides which are not covered by CsI(T1) scintillators. Segmentation of the CaFs detectors also enables us to reject the background events which spread in two or more detectors; a true event is mostly confined to a single detector. All the materials used in the detector system have been carefully selected for low radioactivity by low background Ge detector. After the event selection by 47r active shielding system, backgrounds around 3 - 5 MeV range are most probably from internal radioactive contamination. Since Q-values and half-lives of natural radioactivities are well known, following decays are relevant: 214Bi
Qa = 3.27 MeV
in the uranium “‘Tl 212Bi
) 214P~(Tl,2 = 164.3 px)
Qa= 7.83MeV , 210Pb,
chain, and
Qp = 5.00 MeV Qp = 2.25 MeV
, “‘Pb, ,
212Po(Tl,2 = 0.299 psec)
(2) Qa = 8.g5MeV , 208Pb,
(3)
in the thorium chain. In order to estimate the activities of nuclei (214Bi in the uranium and 220Rn in the thorium chain) inside the CaFs(Eu) scintillation crystals, a delayed coincidence method was used. Since two crystals had much higher contaminations than others, we did not
I. Ogawa et al, /Nuclear Physics A721 (2003) S25c-528~
527e
use these crystals in the following analysis. Measured total activities in the 23 CaFz(Eu) crystals were 214Bi - (1.11 %O.Ol) x 10m3Bq/kg and azoRn - (9.81 f0.15) x 10e5 Bq/kg. 3. Results
(a)
Energy(keV)
09
Energy(keV)
Figure 2. (a)A comparison of Monte Carlo simulations from the internal radioactivities (dashed line) and the ~vDBD of 48Ca (dotted line) with experimental data (solid line) of a statistical significance of 4.23 kg yr. The arrow indicates the OvDBD energy window. (b)Simulated background spectra for each decay - dotted line : decay (l), dashed line : decay (2) and solid line : decay (3).
The energy spectrum of the 23 crystals with a statistical significance of 4.23 kg yr is shown in Fig. 2 (a). No events are observed in the OvDBD energy window which is a 30 peak interval centered at 4.27 MeV. In order to derive the lower limit of the half-life for the OVDBD, we need to estimate background events in this energy window. We considered two possible background sources, ~vDBD from 48Ca and the decays (l), (2) and (3) f rom internal radioactivities. The known half-life of the 2vDBD was used to simulate an expected spectrum in Fig. 2 (a). Contribution from the ~vDBD of 48Ca in the OvDBD energy window was found to be negligible. Background events from internal radioactivities was estimated by a Monte Carlo simulation based on GEANT 3.21 code. Total simulated energy spectrum from the decays (l), (2) and (3) is plotted in Fig. 2 (a) with experimental data. Also shown in Fig. 2(b) are simulated background spectra for each decay. The expected number of background events in the OvDBD energy window is 2.63 f 0.13. Following the procedure of Ref. [3], we extracted a half-life limit for the OvDBD of 48Ca. The resulting lower limit of half-life (for the O+ + O+ transition) is[4] T$
2
8.6 x 1O22year (68 % C.L.)
528c
I. Ogawa et al. /Nuclear Physics A721 (2003) 525c-528~ 2
1.8 x 1O22 year (90 % C.L.).
This result can be converted to the upper limit for the effective Majorana neutrino mass (m,) neglecting right-handed currents. Depending on the theoretical nuclear matrix elements (see Table 8 & 9 in Ref.[5]), the upper bound at the 90 % C.L. is (m,) < (6.3 - 39.4) eV. 4. Conclusion
and outlook
The most stringent limit on the half-life of the neutrino-less double beta decay of 48Ca (48Ca -+48Ti) has been obtained by using the CaFz scintillation detector system with a total statistical significance of 4.23 kg yr. The number of observed events in the signal window was 0 while the expected background events was 2.6. A Monte Carlo simulation using the results from the measurement of activities indicated that the main backgrounds around 3 - 5 MeV range were from the sequential decays of the daughter nuclei of U and Th. These events can be rejected using the pulse shape recorded in the gate time window. A measurement is underway with flash ADC to record the pulse shape of the events with energy above -2 MeV. We expect that further operation enables us to obtain a finite value on the half-life of ~vDBD and improved limit on the OVDBD. For further improvements, especially to reach the mass region suggested by the oscillation experiments, we need to develop a large scale detector system. We propose a new detector system, CANDLES, where 0(102 - 103) kg of CaFz crystals are immersed in a liquid scintillator which acts as an active shield to veto backgrounds. Scintillation light is viewed by large PMTs. Using the experience gained from ELEGANT VI, a prototype of CANDLES is being developed. We expect that CANDLES will enable us to make a considerable improvement on the DBD of *‘Ca. REFERENCES 1. 2.
3. 4. 5.
K. You et al., Phys. Lett. B 265 (1991) 53. R. Hazama et al., Proc. of the 4th Int. Conf. on Weak and Electromagnetic Interactions in Nuclei (WEIN 95), Osaka, Japan, June 1995, World Scientific, Singapore P. 635; R. Hazama et al., Proc. of the XIV Int. Conf. on Particles and Nuclei (PANIC 96), CEBAF, USA, May 1996, World Scientific, Singapore p. 477; R. Hazama et al., Proc. of the Int. Workshop on the Identification of Dark Matter (IDM 96), September 1996, Sheffield, UK, World Scientific, Singapore p.397; T. Kishimoto et al., Proc. of the 2nd RESCEU Int. Symp. on Dark Matter in the Universe and its Direct Detection, November 1996, Tokyo, Universal Academy Press Inc., Tokyo, p. 71; I. Ogawa et al., Nucl. Phys. A663-664 (2000) 869c; T. Kishimoto et al., Proc. of the Identification of the Dark Matters (IDM2000), September 2000, York, UK; R. Hazama et al., Nucl. Instr. Meth. in Phys. Res. A 482 (2002) 297. G.J. Feldman and R.D. Cousins, Phys. Rev. D 57 (1998) 3873. I. Ogawa, et al., submitted to Phys. Lett. B. J. Suhonen and 0. Civitarese, Phys. Rep. 300 (1998) 123.