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Double beta-decay half-life of 82Se
Volume 163B, number 1,2,3,4
PHYSICS LETTERS
21 November 1985
DOUBLE BETA-DECAY HALF-LIFE OF S2Se K. M A R T I and S.V.S. M U R T Y Chemistry Department, B-OI7, University of California, San Diego, La Jolla, CA 92093, USA Received 5 July 1985 We report the detection of 82KrBB from the double beta-decay of S2Se in a troilite inclusion of the Cape York meteorite. The calculated half-life is compatible with the umangite result, but incompatible with the cloud-chamber value. The recommended S2Se half-life of 7"1/: = (1.2 _+0.3) × 102o yr, does not suggest a violation of lepton number conservation.
Double beta-decay (titi) processes received attention because they may provide sensitive tests for lepton-number conservation and for the neutrino restmass (e.g. ref. [1]). For many eveneven nuclei, first-order ti-decay is energetically forbidden, but second-order titi processes are allowed. The neutrinoless titi decay (Majorana) is expected to be about a million times more probable than the two-neutrino titi decay (Dirac), but is forbidden by lepton-number conservation and by the rh~ = 0 (restmass of the electron-neutrino) requirement. If there is a violation of lepton-number conservation, or if rh~ ~ 0, the titi rates would be substantially larger because of the Majorana decay process. Evidence for a measurable decay rate in 128Te [2] was interpreted as evidence for a non-zero neutrino restmass [3], but a more recent investigation of the titi ratio of 128Te and 13°Te did not support this positive evidence and sets a limit of rh~ < 5.6 eV [4]. Two different experimental approaches to the study of tiff processes were used. The "geochemical" approach refers to isotopic studies on ancient terrestrial ores with high concentrations of the parent nucleus searching for excesses of daughter nuclides produced by titi over the age of the ores. The instrumental approaches attempt to detect the electrons as they are emitted. For 82Se --~ 82KF both approaches were taken. The mass-spectrometric determination of excess 8EKE in selenium ores provided clear evidence for tiff-decay of 82Se [5,6], but the inferred half-lives are one order of magnitude larger than that
obtained by a cloud-chamber experiment [7]. Since the calculated lifetime is in agreement with the latter, it was generally presumed that some 8EKE was lost from the ore samples in metamorphic processes [1]. We report the identification in a meteorite of 82KF due to titi decay of 82Se which accumulated over 4.5 Ga, and we derive a "cosmochemical" half-life of 82Se. This is the first detection of tiff-decay in a meteorite. We have determined by mass spectrometry the isotopic abundances of Kr in samples of the metal and of a troilite (FeS) inclusion of the iron meteorite Cape York. Interior samples of this very large meteorite are well suited for studies of rare nuclear reaction products, since other components, consisting of trapped gases and of cosmic ray proton-induced spallation products, are quite low. Our studies of in-situ produced Kr and Xe components on Se and Te [8] revealed the presence of excess 82Kr which cannot be attributed to nuclear processes other than tiff-decay. In table 1 we list the measured isotopic abundances in two samples of troilite, and the in-situ produced excesses of 8°Kr, 82Kr and S3Kr. The 83Kr is entirely#_due to neutron capture by 82Se (n, T)83Se ~ 83Kr, and some fractions of 8°Kr and 82Kr are produced in neutron reactions on Br. The Se content is 65 + 10 ppm [9] (average of 5 measurements), while the Br content is not known. However, the measured cross-section ratio 080/082 for thermal and epithermal neutrons [10] permits us to calculate the maximum contribution to 82Kr by assigning all S°Kr to this component. On the
0370-2693/85/$ 03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
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Volume 163B, number 1,2,3,4
PHYSICS LETTERS
21 November 1985
Table 1 Measured Kx isotope ratios and excesses. Uncertainties in isotope ratios are 95% confidence limits. The corrected excesses for 82Kr are obtained by subtracting a spallation contribution (calculated from the measured 3BAr= 5.8 × 10- to c c / g and the ratio (83 Kr/38Ar)spall. = 1.77 × 10 -5 in Costilla Peak, an iron meteorite of the same group as Cape York [11]}. The remainder of the excess 80Kr is assigned to the Br(n, ),) contribution. The upper and lower limits are obtained by assigning the total 80Kr excess to either Br(n, ~,) products or to the spallation component, respectively (see text). Excesses in M a t / g (10 6 a t o m s / g )
Isotope Isotope ratios
sample 1
8°Kr 82Kr 83Kr 84 Kr 86Kr
sample 1 (1.803 g)
sample 2 (0.704 g)
0.069 0.292 1.58 ~- 1.00 0.304
0.054 + 0.007 _ 0.009 0.229 + 0.684+0.047 -= 1.00 0.297 + 0.014
+ 0.014 _ 0.016 + +0.05 + 0.024
measured
corrected for spallation and Br(n, -/) contributions
measured
0.67 2.07 31.8 =--0 -0.02
-= 0 1-7Q+0.48 L..o_0.66 31.56_+1.51 -~ 0 - 0 . 0 2 + 0.54
0.83 + 0.39 1.52 + 0.51 26.94+2.63 -= 0 - 0 . 4 2 _+ 0.82
_+ 0.31 _ 0.38 + +1.2 + 0.54
other hand, some fraction of S°Kr and 82Kr is probably due to cosmic ray spallation reactions on elements heavier than Kr (such as Mo). Our calculations based on observed 3BAr produced from Fe [8] indicate that this component may not be negligible. We use measured cross-section ratios 080//082 in iron meteorites [11] to assess maximum contributions to S2Kr from spallation reactions, including also (p, n) reactions on Se. The Rb concentration ( < 0.6 ppb) determined in an adjacent sample [12] in order to evaluate a possible S~Rb(n, a) contribution, is found to be negligible. The amounts of S2Kr, which cannot be accounted for by any or all of the above competing processes, are 1.78 and 1.20 M a t / g respectively for the two troilite samples (1 Mat = 10 6 atoms). Fig. 1 shows the observed Kr isotope ratios 82Kr/84Kr and 83Kr/S4Kr, as obtained in the stepwise release experiment performed at three different temperatures. The filled symbols are the measured data, and the open symbols represent lower limits for the l l 0 0 ° C fractions which were obtained by applying maximum possible corrections to S2Kr for nuclear processes other than tiff-decay. Fig. 1 shows that excesses on S2Kr and S3Kr are linearly correlated, as is expected if both processes relate to the same element Se, which occurs in solid solution with S in the troilite. However, S2Kr from tiff-decay accumulated over the age of the Cape York meteorite, while SaKr was produced by 72
sample 2
-
-
corrected for spallation and Br(n, ~,) contributions --- 0 1 20 +0.60 0.88 •
26.72+2.80 --- 0 - 0 . 4 2 ± 0.82
secondary neutrons more recently, during the time interval of exposure to cosmic radiation. The linear correlation shows that the system was not disturbed. The correlation also reveals that the different a2Kr and S3Kr excesses in the two samples, at least in part, reflect variable Se abundances, since both excesses are proportional to the Se concentration. There are some signatures of extinct radioactivities [8] which indicate that the meteorite is as old as the solar system. On the other hand, some troilite inclusions in other iron meteorites indicate early disturbed 39Ar-a°Ar ages [13]. We adopt a 4.5 + 0.1 Gyr age for our troilite samples. In our calculations, we implicitly assume that tiff rates of 8°Se and contributions to 8°Kr and S2Kr by neutrino capture in 8°'82Se a r e negligible. The first assumption is supported by the data on five ore samples [5], since all show 82K-F but not S°Kr excesses. The average excess of the two samples is 82K.rsfl --~ 1.49 +0.40 M a t / g , -0.55 where the uncertainties correspond to the more precise relative uncertainties of sample Number 1. We quadratically add 15% errors in both Kr and Se concentrations and 2% to the age of Cape York and obtain
T1/2 =
In 2 (82se/a2Kr) T
(0.97 + 0 . 3 6 ] )< 1020 yr 0.45 ] L as the tiff half-life of S2Se. =
-
Volume 163B, number 1,2,3,4
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PHYSICS LETTERS
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83Kr/e4Kr Fig. 1. The filled symbols in the figure show the measured ratios 82Kr/84 Kr versus 83Kr/84 Kr; totals are also given as circled data points. The digits refer to the temperature (hundreds of °C) steps in the stepwise release experiment. Open symbols are explained in the text.
This meteoritic half-life is one order of magnitude longer than the cloud-chamber result [7], which indicates that only --- 10% of the observed cloud-chamber events are actually due to tiffdecay. A comparison with geochemical data reveals agreement within error limits with the umangite result [5], but a 2.5 times smaller value than that reported for the tellurobismuthite from Boliden [6]. In the latter case, there is evidence that this system has not remained closed, since a comparison of observed and calculated neutron capture effects in 82Se and 13°Te indicate a 83K.r//131Xe ratio which cannot be accounted for by either thermal or epithermal neutrons. The umangite data [5] are based on the determination of the relative abundance of S6Krf, due to 238U fission and of 82Kr~ from 82Se. This Kr isotopic ratio should be little affected by system openings, except by a possible incorporation of trapped gas during mineral formation, and the close agreement with the meteorite result is reassuring. We recommend an average value of T1/2 = (1.2 + 0.3) × 10 20
yr for the tiff half-life which overlaps both the Cape York and umangite data. Since this observed 82Se half-life is close to 10 20 yr, there is no indication of a violation of lepton-number conservation. Recent theoretical estimates [1,3,14] are in the range of 10195 ± 1 and do not permit to rule out the possibility of small violations. An experimental lower limit for the neutrinoless decay mode is > 3.1 × 10 21 yr [15]. V.F. Buchwald provided the Cape York troilite sample. We thank C.S. Maclsaac and G.W. Lugrnair for the Rb determination. This work was supported by NASA Grant N A G 9-41.
References [1] H. Primakoff and S. Rosen, Ann. Rev. Nucl. Sci. 31 (1981) 145. [2] E. Hennecke, O. Manuel and Sabu, Phys. Rev. C l l (1975) 1378; E. Hennecke, Phys. Rev. C17 (1978) 1168.
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PHYSICS LETTERS
[3] M. Doi, T. Kotani, H. Nishiura, K. Okuda and E. Takasugi, Phys. Lett. 103B (1981) 219. [4] T. Kirsten, H. Richter and E. Jessberger, Phys. Rev. Lett. 50 (1983) 474. [5] T. Kirsten and H.W. Miiller, Earth and Planet. Sci. Lett. 6 (1969) 271. [6] B. Srinivasan, E. Alexander, R. Beaty, D. Sinclair and O. Manuel, Econ. Geology 68 (1973) 252. [7] M.K. Moe and D. Lowenthal, Phys. Rev. C22 (1980) 2186. [8] K. Marti, Live 1291-129Xe Dating, Proc. Workshop on Cosmogenic nuclides (Los Alamos, NM, 1985); S.V.S. Murty and K. Marti, Geochim. Cosmochim. Acta (1985), to be submitted.
74
' 21 November 1985
[9] K.P. Jochum, H. Hintenberger and V.F. Buchwald, Meteoritics 10 (1975) 419. [10] K. Marti and G.W. Lugmair, Proc. 2nd Lunar Sci. Conf. Vol.2 (1971) p. 1591. [11] J.M. Munk, Earth Planet. Sci. Lett. 2 (1967) 301. [12] G.W. Lugmair, private communication. [13] S. Niemeyer, Geochim. Cosmochim. Acta 43 (1979) 1829. [14] W. Haxton, G. Stephenson and D. Strottman, Phys. Rev. D25 (1982) 2360. [15] B.T. Cleveland, W.R. Leo, C.S. Wu, L.R. Kasday, A.M. Rushton, P.J. Gollon and J.D. Ullman, Phys. Rev. Lett. 35 (1975) 757.
PHYSICS LETTERS
21 November 1985
DOUBLE BETA-DECAY HALF-LIFE OF S2Se K. M A R T I and S.V.S. M U R T Y Chemistry Department, B-OI7, University of California, San Diego, La Jolla, CA 92093, USA Received 5 July 1985 We report the detection of 82KrBB from the double beta-decay of S2Se in a troilite inclusion of the Cape York meteorite. The calculated half-life is compatible with the umangite result, but incompatible with the cloud-chamber value. The recommended S2Se half-life of 7"1/: = (1.2 _+0.3) × 102o yr, does not suggest a violation of lepton number conservation.
Double beta-decay (titi) processes received attention because they may provide sensitive tests for lepton-number conservation and for the neutrino restmass (e.g. ref. [1]). For many eveneven nuclei, first-order ti-decay is energetically forbidden, but second-order titi processes are allowed. The neutrinoless titi decay (Majorana) is expected to be about a million times more probable than the two-neutrino titi decay (Dirac), but is forbidden by lepton-number conservation and by the rh~ = 0 (restmass of the electron-neutrino) requirement. If there is a violation of lepton-number conservation, or if rh~ ~ 0, the titi rates would be substantially larger because of the Majorana decay process. Evidence for a measurable decay rate in 128Te [2] was interpreted as evidence for a non-zero neutrino restmass [3], but a more recent investigation of the titi ratio of 128Te and 13°Te did not support this positive evidence and sets a limit of rh~ < 5.6 eV [4]. Two different experimental approaches to the study of tiff processes were used. The "geochemical" approach refers to isotopic studies on ancient terrestrial ores with high concentrations of the parent nucleus searching for excesses of daughter nuclides produced by titi over the age of the ores. The instrumental approaches attempt to detect the electrons as they are emitted. For 82Se --~ 82KF both approaches were taken. The mass-spectrometric determination of excess 8EKE in selenium ores provided clear evidence for tiff-decay of 82Se [5,6], but the inferred half-lives are one order of magnitude larger than that
obtained by a cloud-chamber experiment [7]. Since the calculated lifetime is in agreement with the latter, it was generally presumed that some 8EKE was lost from the ore samples in metamorphic processes [1]. We report the identification in a meteorite of 82KF due to titi decay of 82Se which accumulated over 4.5 Ga, and we derive a "cosmochemical" half-life of 82Se. This is the first detection of tiff-decay in a meteorite. We have determined by mass spectrometry the isotopic abundances of Kr in samples of the metal and of a troilite (FeS) inclusion of the iron meteorite Cape York. Interior samples of this very large meteorite are well suited for studies of rare nuclear reaction products, since other components, consisting of trapped gases and of cosmic ray proton-induced spallation products, are quite low. Our studies of in-situ produced Kr and Xe components on Se and Te [8] revealed the presence of excess 82Kr which cannot be attributed to nuclear processes other than tiff-decay. In table 1 we list the measured isotopic abundances in two samples of troilite, and the in-situ produced excesses of 8°Kr, 82Kr and S3Kr. The 83Kr is entirely#_due to neutron capture by 82Se (n, T)83Se ~ 83Kr, and some fractions of 8°Kr and 82Kr are produced in neutron reactions on Br. The Se content is 65 + 10 ppm [9] (average of 5 measurements), while the Br content is not known. However, the measured cross-section ratio 080/082 for thermal and epithermal neutrons [10] permits us to calculate the maximum contribution to 82Kr by assigning all S°Kr to this component. On the
0370-2693/85/$ 03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
71
Volume 163B, number 1,2,3,4
PHYSICS LETTERS
21 November 1985
Table 1 Measured Kx isotope ratios and excesses. Uncertainties in isotope ratios are 95% confidence limits. The corrected excesses for 82Kr are obtained by subtracting a spallation contribution (calculated from the measured 3BAr= 5.8 × 10- to c c / g and the ratio (83 Kr/38Ar)spall. = 1.77 × 10 -5 in Costilla Peak, an iron meteorite of the same group as Cape York [11]}. The remainder of the excess 80Kr is assigned to the Br(n, ),) contribution. The upper and lower limits are obtained by assigning the total 80Kr excess to either Br(n, ~,) products or to the spallation component, respectively (see text). Excesses in M a t / g (10 6 a t o m s / g )
Isotope Isotope ratios
sample 1
8°Kr 82Kr 83Kr 84 Kr 86Kr
sample 1 (1.803 g)
sample 2 (0.704 g)
0.069 0.292 1.58 ~- 1.00 0.304
0.054 + 0.007 _ 0.009 0.229 + 0.684+0.047 -= 1.00 0.297 + 0.014
+ 0.014 _ 0.016 + +0.05 + 0.024
measured
corrected for spallation and Br(n, -/) contributions
measured
0.67 2.07 31.8 =--0 -0.02
-= 0 1-7Q+0.48 L..o_0.66 31.56_+1.51 -~ 0 - 0 . 0 2 + 0.54
0.83 + 0.39 1.52 + 0.51 26.94+2.63 -= 0 - 0 . 4 2 _+ 0.82
_+ 0.31 _ 0.38 + +1.2 + 0.54
other hand, some fraction of S°Kr and 82Kr is probably due to cosmic ray spallation reactions on elements heavier than Kr (such as Mo). Our calculations based on observed 3BAr produced from Fe [8] indicate that this component may not be negligible. We use measured cross-section ratios 080//082 in iron meteorites [11] to assess maximum contributions to S2Kr from spallation reactions, including also (p, n) reactions on Se. The Rb concentration ( < 0.6 ppb) determined in an adjacent sample [12] in order to evaluate a possible S~Rb(n, a) contribution, is found to be negligible. The amounts of S2Kr, which cannot be accounted for by any or all of the above competing processes, are 1.78 and 1.20 M a t / g respectively for the two troilite samples (1 Mat = 10 6 atoms). Fig. 1 shows the observed Kr isotope ratios 82Kr/84Kr and 83Kr/S4Kr, as obtained in the stepwise release experiment performed at three different temperatures. The filled symbols are the measured data, and the open symbols represent lower limits for the l l 0 0 ° C fractions which were obtained by applying maximum possible corrections to S2Kr for nuclear processes other than tiff-decay. Fig. 1 shows that excesses on S2Kr and S3Kr are linearly correlated, as is expected if both processes relate to the same element Se, which occurs in solid solution with S in the troilite. However, S2Kr from tiff-decay accumulated over the age of the Cape York meteorite, while SaKr was produced by 72
sample 2
-
-
corrected for spallation and Br(n, ~,) contributions --- 0 1 20 +0.60 0.88 •
26.72+2.80 --- 0 - 0 . 4 2 ± 0.82
secondary neutrons more recently, during the time interval of exposure to cosmic radiation. The linear correlation shows that the system was not disturbed. The correlation also reveals that the different a2Kr and S3Kr excesses in the two samples, at least in part, reflect variable Se abundances, since both excesses are proportional to the Se concentration. There are some signatures of extinct radioactivities [8] which indicate that the meteorite is as old as the solar system. On the other hand, some troilite inclusions in other iron meteorites indicate early disturbed 39Ar-a°Ar ages [13]. We adopt a 4.5 + 0.1 Gyr age for our troilite samples. In our calculations, we implicitly assume that tiff rates of 8°Se and contributions to 8°Kr and S2Kr by neutrino capture in 8°'82Se a r e negligible. The first assumption is supported by the data on five ore samples [5], since all show 82K-F but not S°Kr excesses. The average excess of the two samples is 82K.rsfl --~ 1.49 +0.40 M a t / g , -0.55 where the uncertainties correspond to the more precise relative uncertainties of sample Number 1. We quadratically add 15% errors in both Kr and Se concentrations and 2% to the age of Cape York and obtain
T1/2 =
In 2 (82se/a2Kr) T
(0.97 + 0 . 3 6 ] )< 1020 yr 0.45 ] L as the tiff half-life of S2Se. =
-
Volume 163B, number 1,2,3,4
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83Kr/e4Kr Fig. 1. The filled symbols in the figure show the measured ratios 82Kr/84 Kr versus 83Kr/84 Kr; totals are also given as circled data points. The digits refer to the temperature (hundreds of °C) steps in the stepwise release experiment. Open symbols are explained in the text.
This meteoritic half-life is one order of magnitude longer than the cloud-chamber result [7], which indicates that only --- 10% of the observed cloud-chamber events are actually due to tiffdecay. A comparison with geochemical data reveals agreement within error limits with the umangite result [5], but a 2.5 times smaller value than that reported for the tellurobismuthite from Boliden [6]. In the latter case, there is evidence that this system has not remained closed, since a comparison of observed and calculated neutron capture effects in 82Se and 13°Te indicate a 83K.r//131Xe ratio which cannot be accounted for by either thermal or epithermal neutrons. The umangite data [5] are based on the determination of the relative abundance of S6Krf, due to 238U fission and of 82Kr~ from 82Se. This Kr isotopic ratio should be little affected by system openings, except by a possible incorporation of trapped gas during mineral formation, and the close agreement with the meteorite result is reassuring. We recommend an average value of T1/2 = (1.2 + 0.3) × 10 20
yr for the tiff half-life which overlaps both the Cape York and umangite data. Since this observed 82Se half-life is close to 10 20 yr, there is no indication of a violation of lepton-number conservation. Recent theoretical estimates [1,3,14] are in the range of 10195 ± 1 and do not permit to rule out the possibility of small violations. An experimental lower limit for the neutrinoless decay mode is > 3.1 × 10 21 yr [15]. V.F. Buchwald provided the Cape York troilite sample. We thank C.S. Maclsaac and G.W. Lugrnair for the Rb determination. This work was supported by NASA Grant N A G 9-41.
References [1] H. Primakoff and S. Rosen, Ann. Rev. Nucl. Sci. 31 (1981) 145. [2] E. Hennecke, O. Manuel and Sabu, Phys. Rev. C l l (1975) 1378; E. Hennecke, Phys. Rev. C17 (1978) 1168.
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Volume 163B, number 1,2,3,4
PHYSICS LETTERS
[3] M. Doi, T. Kotani, H. Nishiura, K. Okuda and E. Takasugi, Phys. Lett. 103B (1981) 219. [4] T. Kirsten, H. Richter and E. Jessberger, Phys. Rev. Lett. 50 (1983) 474. [5] T. Kirsten and H.W. Miiller, Earth and Planet. Sci. Lett. 6 (1969) 271. [6] B. Srinivasan, E. Alexander, R. Beaty, D. Sinclair and O. Manuel, Econ. Geology 68 (1973) 252. [7] M.K. Moe and D. Lowenthal, Phys. Rev. C22 (1980) 2186. [8] K. Marti, Live 1291-129Xe Dating, Proc. Workshop on Cosmogenic nuclides (Los Alamos, NM, 1985); S.V.S. Murty and K. Marti, Geochim. Cosmochim. Acta (1985), to be submitted.
74
' 21 November 1985
[9] K.P. Jochum, H. Hintenberger and V.F. Buchwald, Meteoritics 10 (1975) 419. [10] K. Marti and G.W. Lugmair, Proc. 2nd Lunar Sci. Conf. Vol.2 (1971) p. 1591. [11] J.M. Munk, Earth Planet. Sci. Lett. 2 (1967) 301. [12] G.W. Lugmair, private communication. [13] S. Niemeyer, Geochim. Cosmochim. Acta 43 (1979) 1829. [14] W. Haxton, G. Stephenson and D. Strottman, Phys. Rev. D25 (1982) 2360. [15] B.T. Cleveland, W.R. Leo, C.S. Wu, L.R. Kasday, A.M. Rushton, P.J. Gollon and J.D. Ullman, Phys. Rev. Lett. 35 (1975) 757.