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Low background study of the neutrinoless double beta decay of 76-GE and upper limit for neutrino mass
Volume 138B, number 4
PHYSICS LETTERS
19 April 1984
LOW BACKGROUND STUDY OF THE NEUTRINOLESS DOUBLE BETA DECAY OF 76-Ge AND UPPER LIMIT FOR NEUTRINO MASS A. FORSTER, H. KWON, J.K. MARKEY, F. BOEHM and H.E. HENRIKSON California Institute of Technology, Pasadena, CA 91125, USA Received 16 January 1984
AGe detector system was built possessing significantly reduced background, from natural radioactivities, notably 208TI (2.6 MeV line). After a 2600 h measuring period a lower limit for the half life of the neutrinoless double beta decay in 76-Ge of T(1/2, ou) > 1.7 × 1022 yr (lo) was obtained, corrresponding to a neutrino mass upper limit of 22 eV.
Neutrinoless double beta decay provides a sensitive probe for the study o f the fundamental properties of weak interaction [1 ]. The positive identification of this process would constitute evidence for non-conservation of lepton number and would signal the existence o f Majorana neutrinos. These neutrinos may be massive and, in addition, may be coupled to right-handed weak currents. So far neutrinoless double beta decay has not been observed and upper limits for a Majorana neutrino mass in the range o f 5 to 100 eV have been quoted in the literature [2]. The most sensitive laboratory results have been obtained from the study o f the double beta decay of 76-Ge. This nucleus appears to be a promising candidate for further measurements with high sensitivity. A high purity coaxial Ge detector serves as a source (natural abundance o f 76-Ge is 7.76%) as well as the detector. The total energy of the decay electrons, e 0 ; 2039.6 -+ 0.9 eV [3] is deposited in the Ge detector and is expected to appear as a monoenergetic peak in the energy spectrum. The main advantage of the Ge detector is its energy resolution which improves the signal-to-noise ratio in the presence o f background radiation, such as natural radioactivities, gamma rays resulting from neutron induced nuclear reactions, or cosmic ray related events. For the present experiment a coaxial high purity Ge detector with 90 cm 3 fiducial volume was fabricated by Princeton Gamma-Tech according to our plans and specifications o f construction materials. 0.370-2693/84/$ 03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publish ing Division)
Oxygen-flee, high conductivity electrolytic (OFHC) copper used for the cryostat and the crystal mounting have been carefully chosen and tested to minimize natural radioisotope contamination. In place o f the usual molecular sieve serving as a cryopump, an ion pump was installed for maintaining the insulation vacuum. The energy resolution was measured to be 2.0 keV at 1.3 MeV over short periods of time, and was broadened to 2.6 keV for the measurement reported here extending over 2600 h. The detector is installed in a subbasement room at Caltech. A total o f one meter concrete overburden (equivalent to about 3 meter water), provides a modest shielding against charged cosmic ray primaries. The detector system is surrounded by 15 cm of electrolytic copper followed b y 15 cm lead. An air tight container surrounds the shielding and prevents contamination from radon emanating from the surrounding concrete walls and the floor. Plastic scintillation veto counters, 0.8 cm thick, completely surround the system as illustrated in fig. 1 and provide an active shield from charged cosmic ray particles. The data acquisition system consists o f two circuits, one handling the energy pulses of the Ge detector and the other generating the cosmic ray veto signal [4]. The veto pulses trigger a fast real time clock which measures the time of arrival of the last veto pulse with respect to a pulse in the Ge detector. The veto time was 14/Js. Care was exercised to eliminate electronic noise from the main power distribution and to main301
Volume 138B, number 4
PHYSICS LETTERS VFTO DETECTORS TIGHT CONTAINER 3ETECTOR 3HIELDING SHIELDING -O DETECTORS
'PORT TABLE N2 DEWAR ~vOR
~Pb
SHIELDING
Fig. 1. Detector set up.
tain a constant (+- 1 K) temperature in the laboratory. The overall drift of the electronics was found to be one channel (0.5 keV) over a three-month period. Gain shift stabilization was not used. A 260 h run was devoted to studying the background energy spectrum up to 10 MeV with a 1024 channel resolution. Fig. 2 shows these spectra with and without veto. The veto condition reduces the background rates by a factor 10 to 500, depending on energy. (Veto times longer than 14 ~s did not significantly improve the background.) The figure also I00.0
IO.O
xlO
CiT (No Veto)
ce
0.1
O.OI
I,--
0.001
0
2
4 6 B Energy (MeV)
MILAN (Mont Blanc]
I0
Fig. 2. High energy spectra in the Ge detector. 302
12
19 April 1984
contains a recent background spectrum by Bellotti et al. [5] from a similarly sized Ge detector in the Mont Blanc tunnel (5000 m.w.e.). While the muon flux in the tunnel is reduced compared to the surface by a factor of 107 the background above 3 MeV in Bellotti's spectrum is lowered by a factor o f only 20. As to the origin o f our background, we find strong evidence for neutron induced reactions in Ge and Cu. The appearance o f two gamma lines in our energy spectrum at 139.7 keV and 198.2 keV correspond to deexcitation of low energy excited states in 75-Ge and 71-Ge, respectively, and can be attributed to neutron capture in 74-Ge and 70-Ge. Furthermore, the presence of gamma transitions in 72-Ge and 74-Ge as well as in 63-Cu and 65-Cu is evidence for (n, n') reactions in Ge and Cu. In particular, our spectrum shows a line at 1481.7 keV (0.009 c/h) arising from the deexcitation in 65-Cu. As this line is very close to the peak expected from the 0 - 2 double beta decay in 76-Ge at 1480.5 keV [3], the search for this branch is not very sensitive in our experiment. The neutron energy spectrum which thus extends to several MeV is most likely due to capture of muons in the detector and its surrounding shieldings, as well as in the building walls. In the energy spectrum up to 10 MeV, we detect a drop of count rate above 8 MeV, most likely produced by neutron capture in Cu which is the main construction and shielding material close to the Ge crystal. A similar drop can be seen in Bellotti's spectrum at about 6 MeV consistent with the assumption of neutron capture in Hg, the main shielding material close to their detector. While the principal source of neutrons in work can be attributed to muon capture, the remaining source o f neutrons at the tunnel location presumably is the spontaneous fission o f 238-uranium in the surrounding rock. Following the describe(t high energy, low resolution run, a high resolution run was conducted for a period o f 2600 h. The energy spectrum between 0 and 3 MeV digitized into 8192 channels is shown in fig. 3a. Above 500 keV the integral rate was 25 c/h. Portions o f this high resolution run around the decay energy e 0 and near the 2.6 MeV 208-T1 peak are shown in figs. 3b and 3c. The rate of the 2.6 MeV line in a 3.4 keV window is 0.009 +- 0.003 c/h, a factor of 20 lower than the corresponding rate of 0.18 c/h reported in ref. [5]. The Compton background from the 2.6 MeV gamma near the decay energy e 0 =
Volume 138B, number 4
PHYSICS LETTERS CHANNEL
2c:.1 5
NUMBER
4:2 } (
6 i44
8192
50 }
a % t -- 75,~e
0 0
2 -- 716e
tO &J
5-2'2pb 5 --214pb
2- ~(:'0
(D
z., 7, 8, ! , 4 -
CO
6-e*
214~
2]Plllbilstloln
7
Z
2,;,{3
0 -- 288Ac
9,
12, 15 -
C}
6°Co
u)
O.5
I.C
i.5
2.0
2,5
3,O
ENERGY (MeV) 5200
5240 5260 5280 5300
5220
i~
5320 5340
(2059.6±O.9)keV
B£ (Or) 0 + 4 0
S o
T
('4
1
5560 5580 5400 ÷
b
c-
5 O
C)
2000
2010
2020
2030
2040
2050
2060
6740
6760
6780
(2614.47±0.50)
[
6800
6820
6840
keV
8°STI
6860
6880
gy interval at 2.0396 MeV, and its standard deviation x/~, we derive a limit for the half life of neutfinoless double beta decay of T(1/2,ou) > 1.7 X 1022 yr. In comparison, Bellotti et al. [5], obtain after 6800 h counting a value N = 37 corresponding to a limit T(1/2, or) > 3.7 X 1022 yr. A consistent reanalysis of the result of Avignone et al. [6] furnishes T(1/2, ou) > 1.3 X 1022 yr. Following the analyses of Doi et al. (1) and Haxton et al. [1] our result furnishes limits for the Majorana neutrino mass o f m u < 22 eV and m u < 9 eV, respectively. It is clear that our background conditions and thus our sensitivity can be further improved by moving our detector system to a neutron shielded underground location. One of us (AF) wishes to thank the Alexander yon Humboldt Foundation for a fellowship. Many stimulating discussions with Petr Vogel are gratefully acknowledged. We also wish to thank C.A. Barnes for the loan of a large quantity of low background lead shielding blocks. The work was supported by the US Department of Energy under Contract DE-AT0381 ER40002.
2070
ENERGY (keV) 6Z20
19 April 1984
6900
6920
Note added. With additional data for a total of 3820 h the lifetime limit now is T1/2(ov ) > 1.9 X 1022 yr.
C
References
:£
4
b~ CP
2590
2600
2610
2620
2630
2640
2650
2660
ENERGY (keV) Fig. 3. High resolution Oe detector spectra. (a) T o t a l energy
range from 0 to 3 MeV. (b) Portion of spectrum near total decay energy eo = 2.0396 MeV. (c) Portion of spectrum near the 208-T1 line at 2.6145 MeV. 2.0396 MeV is reduced accordingly. In the present experiment the total background rate (averaged over a 100 keV interval) was 0.0016 -+ 0.0001 c/h keV and is presumably due to neutron capture gamma rays. From the observed number of background counts, N = 14, for a 2600 h counting period in a 3 keV ener-
[1 ] See for example S.P. Rosen, Neutrino 81, Vol. II (Univ. of Hawaii, 1982) p. 76; M. Doi et al., Prog. Theor. Phys. 69 (1983) 602; H. Primakoff and S.P. Rosen, Ann. Rev. Nucl. Sci. 31 (1981) 145. [2] F. Boehm, Neutrino mass from oscillation and double beta decay experiments, APS Ann. Meet. Div. Part. Fields (Blacksburg, VA 1983), AlP Conf. Proc. 1984; F. Boehm, Status of lepton conservation and neutrino mass, Surveys in high energy physics, Vol. 3 (Harwood, Academic Publishers, London, 1983) p. 195 ; D. Bryman and C. Picciotto, Rev. Mod. Phys. 50 (1978) 11. [3] A. Wapstra, private communication, to be published. [4] A. Forster et al., Proc. Conf. Low energy tests of conservation laws in particle physics (Blacksburg, VA, 1983), AIP Conf. Proc. (1984). [5 ] E. Bellotti et al., New results on Ge double beta decay and electron stability, Proc. Intern. Conf. on High energy physics (Brighton, July 1983); see also E. Bellotti et al., Phys. Lett. 124B (1983) 435. [6] F.T. Avignone et al., Phys. Rev. Lett. 50 (1983) 721. 303
PHYSICS LETTERS
19 April 1984
LOW BACKGROUND STUDY OF THE NEUTRINOLESS DOUBLE BETA DECAY OF 76-Ge AND UPPER LIMIT FOR NEUTRINO MASS A. FORSTER, H. KWON, J.K. MARKEY, F. BOEHM and H.E. HENRIKSON California Institute of Technology, Pasadena, CA 91125, USA Received 16 January 1984
AGe detector system was built possessing significantly reduced background, from natural radioactivities, notably 208TI (2.6 MeV line). After a 2600 h measuring period a lower limit for the half life of the neutrinoless double beta decay in 76-Ge of T(1/2, ou) > 1.7 × 1022 yr (lo) was obtained, corrresponding to a neutrino mass upper limit of 22 eV.
Neutrinoless double beta decay provides a sensitive probe for the study o f the fundamental properties of weak interaction [1 ]. The positive identification of this process would constitute evidence for non-conservation of lepton number and would signal the existence o f Majorana neutrinos. These neutrinos may be massive and, in addition, may be coupled to right-handed weak currents. So far neutrinoless double beta decay has not been observed and upper limits for a Majorana neutrino mass in the range o f 5 to 100 eV have been quoted in the literature [2]. The most sensitive laboratory results have been obtained from the study o f the double beta decay of 76-Ge. This nucleus appears to be a promising candidate for further measurements with high sensitivity. A high purity coaxial Ge detector serves as a source (natural abundance o f 76-Ge is 7.76%) as well as the detector. The total energy of the decay electrons, e 0 ; 2039.6 -+ 0.9 eV [3] is deposited in the Ge detector and is expected to appear as a monoenergetic peak in the energy spectrum. The main advantage of the Ge detector is its energy resolution which improves the signal-to-noise ratio in the presence o f background radiation, such as natural radioactivities, gamma rays resulting from neutron induced nuclear reactions, or cosmic ray related events. For the present experiment a coaxial high purity Ge detector with 90 cm 3 fiducial volume was fabricated by Princeton Gamma-Tech according to our plans and specifications o f construction materials. 0.370-2693/84/$ 03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publish ing Division)
Oxygen-flee, high conductivity electrolytic (OFHC) copper used for the cryostat and the crystal mounting have been carefully chosen and tested to minimize natural radioisotope contamination. In place o f the usual molecular sieve serving as a cryopump, an ion pump was installed for maintaining the insulation vacuum. The energy resolution was measured to be 2.0 keV at 1.3 MeV over short periods of time, and was broadened to 2.6 keV for the measurement reported here extending over 2600 h. The detector is installed in a subbasement room at Caltech. A total o f one meter concrete overburden (equivalent to about 3 meter water), provides a modest shielding against charged cosmic ray primaries. The detector system is surrounded by 15 cm of electrolytic copper followed b y 15 cm lead. An air tight container surrounds the shielding and prevents contamination from radon emanating from the surrounding concrete walls and the floor. Plastic scintillation veto counters, 0.8 cm thick, completely surround the system as illustrated in fig. 1 and provide an active shield from charged cosmic ray particles. The data acquisition system consists o f two circuits, one handling the energy pulses of the Ge detector and the other generating the cosmic ray veto signal [4]. The veto pulses trigger a fast real time clock which measures the time of arrival of the last veto pulse with respect to a pulse in the Ge detector. The veto time was 14/Js. Care was exercised to eliminate electronic noise from the main power distribution and to main301
Volume 138B, number 4
PHYSICS LETTERS VFTO DETECTORS TIGHT CONTAINER 3ETECTOR 3HIELDING SHIELDING -O DETECTORS
'PORT TABLE N2 DEWAR ~vOR
~Pb
SHIELDING
Fig. 1. Detector set up.
tain a constant (+- 1 K) temperature in the laboratory. The overall drift of the electronics was found to be one channel (0.5 keV) over a three-month period. Gain shift stabilization was not used. A 260 h run was devoted to studying the background energy spectrum up to 10 MeV with a 1024 channel resolution. Fig. 2 shows these spectra with and without veto. The veto condition reduces the background rates by a factor 10 to 500, depending on energy. (Veto times longer than 14 ~s did not significantly improve the background.) The figure also I00.0
IO.O
xlO
CiT (No Veto)
ce
0.1
O.OI
I,--
0.001
0
2
4 6 B Energy (MeV)
MILAN (Mont Blanc]
I0
Fig. 2. High energy spectra in the Ge detector. 302
12
19 April 1984
contains a recent background spectrum by Bellotti et al. [5] from a similarly sized Ge detector in the Mont Blanc tunnel (5000 m.w.e.). While the muon flux in the tunnel is reduced compared to the surface by a factor of 107 the background above 3 MeV in Bellotti's spectrum is lowered by a factor o f only 20. As to the origin o f our background, we find strong evidence for neutron induced reactions in Ge and Cu. The appearance o f two gamma lines in our energy spectrum at 139.7 keV and 198.2 keV correspond to deexcitation of low energy excited states in 75-Ge and 71-Ge, respectively, and can be attributed to neutron capture in 74-Ge and 70-Ge. Furthermore, the presence of gamma transitions in 72-Ge and 74-Ge as well as in 63-Cu and 65-Cu is evidence for (n, n') reactions in Ge and Cu. In particular, our spectrum shows a line at 1481.7 keV (0.009 c/h) arising from the deexcitation in 65-Cu. As this line is very close to the peak expected from the 0 - 2 double beta decay in 76-Ge at 1480.5 keV [3], the search for this branch is not very sensitive in our experiment. The neutron energy spectrum which thus extends to several MeV is most likely due to capture of muons in the detector and its surrounding shieldings, as well as in the building walls. In the energy spectrum up to 10 MeV, we detect a drop of count rate above 8 MeV, most likely produced by neutron capture in Cu which is the main construction and shielding material close to the Ge crystal. A similar drop can be seen in Bellotti's spectrum at about 6 MeV consistent with the assumption of neutron capture in Hg, the main shielding material close to their detector. While the principal source of neutrons in work can be attributed to muon capture, the remaining source o f neutrons at the tunnel location presumably is the spontaneous fission o f 238-uranium in the surrounding rock. Following the describe(t high energy, low resolution run, a high resolution run was conducted for a period o f 2600 h. The energy spectrum between 0 and 3 MeV digitized into 8192 channels is shown in fig. 3a. Above 500 keV the integral rate was 25 c/h. Portions o f this high resolution run around the decay energy e 0 and near the 2.6 MeV 208-T1 peak are shown in figs. 3b and 3c. The rate of the 2.6 MeV line in a 3.4 keV window is 0.009 +- 0.003 c/h, a factor of 20 lower than the corresponding rate of 0.18 c/h reported in ref. [5]. The Compton background from the 2.6 MeV gamma near the decay energy e 0 =
Volume 138B, number 4
PHYSICS LETTERS CHANNEL
2c:.1 5
NUMBER
4:2 } (
6 i44
8192
50 }
a % t -- 75,~e
0 0
2 -- 716e
tO &J
5-2'2pb 5 --214pb
2- ~(:'0
(D
z., 7, 8, ! , 4 -
CO
6-e*
214~
2]Plllbilstloln
7
Z
2,;,{3
0 -- 288Ac
9,
12, 15 -
C}
6°Co
u)
O.5
I.C
i.5
2.0
2,5
3,O
ENERGY (MeV) 5200
5240 5260 5280 5300
5220
i~
5320 5340
(2059.6±O.9)keV
B£ (Or) 0 + 4 0
S o
T
('4
1
5560 5580 5400 ÷
b
c-
5 O
C)
2000
2010
2020
2030
2040
2050
2060
6740
6760
6780
(2614.47±0.50)
[
6800
6820
6840
keV
8°STI
6860
6880
gy interval at 2.0396 MeV, and its standard deviation x/~, we derive a limit for the half life of neutfinoless double beta decay of T(1/2,ou) > 1.7 X 1022 yr. In comparison, Bellotti et al. [5], obtain after 6800 h counting a value N = 37 corresponding to a limit T(1/2, or) > 3.7 X 1022 yr. A consistent reanalysis of the result of Avignone et al. [6] furnishes T(1/2, ou) > 1.3 X 1022 yr. Following the analyses of Doi et al. (1) and Haxton et al. [1] our result furnishes limits for the Majorana neutrino mass o f m u < 22 eV and m u < 9 eV, respectively. It is clear that our background conditions and thus our sensitivity can be further improved by moving our detector system to a neutron shielded underground location. One of us (AF) wishes to thank the Alexander yon Humboldt Foundation for a fellowship. Many stimulating discussions with Petr Vogel are gratefully acknowledged. We also wish to thank C.A. Barnes for the loan of a large quantity of low background lead shielding blocks. The work was supported by the US Department of Energy under Contract DE-AT0381 ER40002.
2070
ENERGY (keV) 6Z20
19 April 1984
6900
6920
Note added. With additional data for a total of 3820 h the lifetime limit now is T1/2(ov ) > 1.9 X 1022 yr.
C
References
:£
4
b~ CP
2590
2600
2610
2620
2630
2640
2650
2660
ENERGY (keV) Fig. 3. High resolution Oe detector spectra. (a) T o t a l energy
range from 0 to 3 MeV. (b) Portion of spectrum near total decay energy eo = 2.0396 MeV. (c) Portion of spectrum near the 208-T1 line at 2.6145 MeV. 2.0396 MeV is reduced accordingly. In the present experiment the total background rate (averaged over a 100 keV interval) was 0.0016 -+ 0.0001 c/h keV and is presumably due to neutron capture gamma rays. From the observed number of background counts, N = 14, for a 2600 h counting period in a 3 keV ener-
[1 ] See for example S.P. Rosen, Neutrino 81, Vol. II (Univ. of Hawaii, 1982) p. 76; M. Doi et al., Prog. Theor. Phys. 69 (1983) 602; H. Primakoff and S.P. Rosen, Ann. Rev. Nucl. Sci. 31 (1981) 145. [2] F. Boehm, Neutrino mass from oscillation and double beta decay experiments, APS Ann. Meet. Div. Part. Fields (Blacksburg, VA 1983), AlP Conf. Proc. 1984; F. Boehm, Status of lepton conservation and neutrino mass, Surveys in high energy physics, Vol. 3 (Harwood, Academic Publishers, London, 1983) p. 195 ; D. Bryman and C. Picciotto, Rev. Mod. Phys. 50 (1978) 11. [3] A. Wapstra, private communication, to be published. [4] A. Forster et al., Proc. Conf. Low energy tests of conservation laws in particle physics (Blacksburg, VA, 1983), AIP Conf. Proc. (1984). [5 ] E. Bellotti et al., New results on Ge double beta decay and electron stability, Proc. Intern. Conf. on High energy physics (Brighton, July 1983); see also E. Bellotti et al., Phys. Lett. 124B (1983) 435. [6] F.T. Avignone et al., Phys. Rev. Lett. 50 (1983) 721. 303