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Two neutrino double-beta decay of 100Mo to the first excited 0+ state in 100Ru
23 February
1995
PHYSICS Pbyaics
Letters
LETTERS
B
B 345 ( 199.5) J08-413
Two neutrino double-beta decay of ‘mM~ to the first excited O+ state in ‘(‘ORu A.S. Barabasha,F.T. Aviqone ID b, J.I. Collar b, C.K. Guerard b, R.J. Arthur ‘, R.L. Brodzinski ‘, H.S. Miley ‘, J.H. Reeves‘, J.R. Meier d, K. Ruddick d, V.I. Umatov e b Department ’ School
’ Inxtirure of Theoretical and Experimental of Physics and Astronomy, University
of
Physics,
of South
117259 Carolina.
Moscow, Russia Columbia, SC 29208,
’ Pacific Norfhwest Laboratory’. Richland. WA 99352, USA Physics and Astronomy, University of Minnesota, Minneapolis. MN 554550112. ’ Lebedev Physical Instituie. 142092 Moscow, Russia
LISA USA
Received 1 August 1994; revised manuscript received 9 December 1994 Editor: J.P. Schiffer
Abstract Double-beta decay of ‘(“‘MO to the O+ excited state at 1130.29 keV in ““%I has been observed. A 956 g sample of molybdenum powder enriched to 98.468% 1°0Mo was counted in a “Marinelli” geometry with a well-shielded, ultralow-background getmanium detector. The cascade ga,nma rays at 539.53 and 590.76 keV in ‘%I were observed. The resulting half-life is (6.1: t:f ) X lo’-” yr at the 68% confidence limit in disagreement with a recently published limit.
1. Introduction
Neutrinoless double-beta (/j/3) decay experiments hold the promise of the discovery of elementary particle physics beeyc;ld the standard model. Such experiments, however, depend on calculated nuclear matrix element9 for their in:crprctittion cf this exotic proccegs. TLe phase-space fac:or
tisct
ier SrienLe
B.V.
SSDIO370.2692(94!~~01657-7
the literature [ I-41, and the recent experimental status has also been reviewed [ 5-81, The ordinary allowed second-order weak decay, 2 v Ppdecay, obeys the equation [T:,f*]-‘=GZ’(EoZ)
lM*“I*,
(1)
where rhc quantity G*‘~E,J) is a kinematical factor depending on electron and neutrino wave timztions, and M* ” i:; rhe nuclear matrix element. While M”’ and M” are pot identical, the measurement of M*’ can provide a valuable test .of the nuclear model used to calculate AI”. The matrix elements MO” and M*” have been calculated with the nuclear shell model 191 and with a variety of schematic models. most notably the F;t$yicle Random Phase Approximation (QRPA)
A.S. Barabash
et al. / Ph~G.s
Direct measurements of T T;;. and hence 1M2 “1, have been reported for “‘Se [ 131, %e [ 14-17 1, ‘%40 [ 1g-201, and ““Nd [ 21.22 1. All of these measurcmerits involved the P&decay fro-a the 0 ’ ground state of the parent to the O+ ground state of the daughter nuclide. The three measurements of the BP-decay of ‘r’r’Mo from its O+ ground state to the O+ ground state in ‘O”Ru resulted in the following half-jives: (1.16?::%) X 1019 yr [ 181, (l.lS_‘$g) X lOi yr [ 191, and (9.5f0.4 [stat.] L-0.9 [syst.]) X 10’s yr IW. In earlier work [23], it was shown that the three isotops “‘%r, ‘@‘MO, and ‘.‘oNd probably have Q-values large enough to allow direct observation of the gamma rays following BEdecay to the first excited O+ states of the daughter nuclei. The mezsurcment described in this communication involves the p/IIdecay transition from the O+ ground state of “‘“MO to the first excited O+ state in ‘%u at 1130.29 ksV. The phase-space factors can be calculated by numerically integrating E!q. ( 16) in Ref. [ 21, using a relativistic Fermi function. Assuming lM*“l is the same for both the ground state and excited state transition, the ratio of decay rates is the ratio of phase-space integrals, whirh differ by their end-point energies. Numerical integration yields a ratio of 55. Usirlg the half-life for the ground state transition of Refs. [ 18,191, the predicted half-life is T,,2(Oi +O: ) =6.4+:$ X 10” yr.
131e counting facility used in this experiment is located in the Soudan mine in Minnesota at a depth of 2090 mwe. The system consists of an intrinsic germanium detector having a volume of = 1!4 cd and an energy resolution of 2.3 keV FWHM at 1333 keV assembled in a cryostat of low-background copper components housed in a large bulk shield of ordinary lead which has an inner liner of 5 cm thick, = 150 yr old lead recovered from a German ship that sank in the North Sea. The old lead liner is used to shield the detector from the radiations of “‘Pb and its progeny present in the contemporary lead. The bulk shield is atmospherically sealed and pressurized with the nitrogen boil-off gas from the detector dewar to mitigate radon and its progeny in the shield.
Letters
B 345 (1995)
4OP-413
409
Cu Cryostat Fig. 1. Geometry of shield, Mtinelli beaker. and germtium detector.
The sample consisted of 956 g of powdered molybdenum metal enriched to (98.468&0.009)% ““‘MO contained in a lucite Marinelli beaker (a right circular cylinder with a hole for inserting the detector in one end, as shown m Fig. 1). Since the sample nearly surrounds the detectl>r, this configuration is only suitable for detecting sin& gamma-ray events. The photopeaks of intclest are the 590.76 and 539.53 keV lines corresponding to the de-rxcitation gamma rays through !he 0: -12: + 0: cascade.
3. Results The energy spectrum obtained ‘7 415.43 d of coun!ing is shown in the region of interest at 1 keV per channel in Fig. 2. Radioactive impurities identified in the sample include primordial potassium, thorium, uranium, and radium as well as traces of ‘.WCsand ‘37Cs. The 511 keV line from positron, annihilation, the 583.19 keV line from 208Tl(2’“Th), the 569.32 and 604.71 keV lines from ‘34Cs, and the 609.32 keV line from 2’4Bi(226Ra) are shown in the figure and provide an internal check on the resolution and energy calibration of the system. The energy regions of the spectrum corresponding to the expected peaks from the /3P decay de-excitation gammarayq wer? analyzed by mul-
410
A.S. Barabash
er al. / Plyics
Fig. 2. Energy
spectrum
tiple independent techniques. The results are 66-t22 counts for the net area under the 539.53 keV peak and 67 + 19 counts for the 590.76 keV peak, where the uncertainties represent one standard deviation ( 1a). The enriched molybdenum powder was removed from the Marinelli beaker and the empty beaker was counted in the same configuration for 103.01 d to verify tha! the observed positive peak areas were due to the molybdenum. No positive net counts were observed in the background spectrum. An upper limit of 5 counts in either peak at the 68% confidence limit was derived from the background data. It is also necessary to show that the positive peak areas are not engendered by some other mechanism in the molybdenum sample. Since the peak areas are of the same intensity, it is extremely unlikely that they could originate from any other source than the expected cascade in nx)Ru.The only other plausible mechanisms for feeding this cascade are from (n, n’) and (n, p) reactions on a ‘OORuimpurity in the sample. If these reactions occurred, an analogous reaction in the germanium detector would also have been observed. There arc = 2 X 10” atoms of 7JGe in the detector. Fast neutron excitation of this ‘*Ge would result in a 595.8 keV gamma ray. The integral production rates of these hypothesized reactions are not known, but let’s assume the reactions on ‘“Ru are no more than a factor of 50 more prolific than those on 74Ge. Since the 595.8 keV gamma ray would be internal to the germanium detector, it would have approximately a factor of 50 higher efhciency for detection than those from the molybdenum sample, and the total number of each of the gamma rays observed would be the same. The measured teeRu
Letters
B 34-7 (1995)
in the region
-108-113
of interest.
concentration in the sample is < 0.35%, corresponding to <2.0X 102’ atoms. There are < 13 net counts at 595.8 keV. Therefore, there are CO.13 counts in the photopeaks of interest from neutron reactions on ‘~‘Ru. Anothe, calculation demonstrating insufficient neutron flux to produce the potentially interferingreactions is based on the neutron capture of ‘O”Mo which could be observed by the 306.86 keV gamma ray from the decay of “‘Tc. There are < 15 net counts as.306.86 keV. The thermal neutron flux would be expected to be no less than the fast flux in this scenario. Normalizing for the ratios of number of target atoms, cross sections, detector efficiencies, and branching ratios, there are < 0.003 counts in the photopeaks of interest from neutron reactions on ““‘Ru. The absolute counting efficiencies were determined by replacing the enriched ‘n”Mo metal powder in the Marinelli beaker with 975.4 g of natural molybdenum metal powder homogeneously contaminated with 1.753 Bq/kg ?f az6Ra.Data were acquired for 25.04 d, and the net arcas under the peaks at 295.22, 35 1.99, 1764.5 1, and 2204.12 keV were used to construct an absolute energy versus efficiency curve for single gamma rays from the enriched molybdenum sample. From this curve, the measured efficiency of the 539.53 keV peak would be 0.01182 and of the 590.76 keV peak would be 0.01107 if they were emitted as single, independent events. Hcwever, these gamma rays are a coincident cascade with a strong angular correlation which results in true efficiencies that am lower than the above values, Furthermore, it is not possible to experimentally duplicate the counting situation to measure the exact efficiencies. Consequently, two Monte Carlo
Table 1 Raw data accumulated irl an elapsed time of 415.43 d in the energy region of interest E(keV)
0
I
2
3
SIO 520 530 540 550 560 570 580 590
193 93 91
200 100 90 77 74 84 104 123 91
144 96 88 74 91 89 69 197 65
Iii 107 86 99 80 96 83 221 74
110 84 77 II5 80 89
-
simulations were performed: one treated the gamma rays as single events, and the other modeled the coincidence cascade and angular correlation. Although the calculated values were not expected to be absolutely correct (the calculated singles efficiencies were 0.01164 and 0.01104 for the 539.53 and 590.76 keV peaks, respectively!), the ratio of calculated coincidence to single effic.‘encies (0.8497 for the 539.53 keV peak and 0.8419 for the 590.76 keV peak) should reflect the actual ratio very closely. Therefore, the above measured singles efficiencies were multiplied by these calculated ratios to arrive at efficiencies for the 539.53 keV peak of 0.01004 and for the 590.76 keV peak of 0.009320 which were applied to the experimental data. In our earlier preliminary report [24], the Monte Carlo calculated efficiencies were significantly higher than the above values. The discrepancy has been traced to an error in the detector/sample geometry use: in those earlier calculations. The half-life can be calculated according to the formula T ,,2 = (In 2)Nt/dN,
(2)
where N is the number of “‘OMo atoms. 5,669 X 102$ t is the time of counting, 1.137 yr; and dN is the number of “%I0 decays. Therefore, at the 68% confidence level, the half-life based on the 539.53 keV peak is T ,,2= (6.8?::$) X 10’” yr, and based on tht 590.76 keVpeakit’isT,12=(6.2?:::)X1020yr. Both gamma rays of interest are from pure electric quadrupole transitions with very nearly the same internal conversion crr‘fficients; hence they have almost identical intensities. Adding these two energy regions of the spectrum improves the statistical accuracy of the
4
5
6
7
8
9
124 87 89 R2 p2 92 73 166 7.5
92 85 8.5 80 14 66 71 89 73
90 7s 86 84 78 76 76 57 70
101 73 95 86 91 98 69 63 58
loo 105 Sd 97 87 104 76 63 62
I10 87 118 94 80 124 78 34 81
data, and dividing the net area under the combined photopeak by the average efficiency leads to the more precise value of the (0,: 40: ) P@-decay half-life: Tllz = (6.1_+1::) X 102” yr.
(3) The precise value of the half-life derived from these data depends slightly on the method chosen for integrating the photopeaks. The actual data acquired in 1.137 yr of counting time are given in Table 1 in the energy region of interest. Readers are invited to determine for themselves the strength c.’ argument supporting a positive observation and the half-life. Griffiths and Vogel’s [ 251 matrix element calculation based on our preliminary half-life fbr decay to the first excited O+ s:ate should be amended to IME; 1= G.30+:::: MeV - ’ based on the measured half-life reported here. Their corresponding value for the ground state transition based on the experimental I.?lf-lives reported in Refs. [ 18,191 is JM& 1= 0.30?::0,: MeV - ‘. An important conclusion of Ref. [ 25 ] is that both matrix elements are dominated by transitions through the 1 + ground state of the intermediate nucleus, ‘9~. Based on that assumption, the relevant QRPA calculated matrix element for the ground state transition is 1M&T I = 0.25 MeV - ’ and that for the decay to :he first O+ excited state is I&Z& 1 =0.24 MeV - ‘. These calculated matrix elements are similar, as are the experimental values, and their absolute values are only about 20% smaller. Independent calculations recently reported by Suhonen and Civitarese [26] support the conclusions of Griffiths and Vogel [ 251. Our experiment. of course, does not distinguish betwl;en 2 v and 0 v @&decay. However, Alston-Gamjost et al. [ 271 have reported Ty& > 4.4 X 10” yr for
AS. Barabash
412
Table 2 Half-lives
( X IOmM yr) for pp-decay Transition
of ‘“‘MO Level
et al. /Physics
to selected
excited
Lerters
B 345 (1995)
levels in ‘O”Ru at the 90%
408413
confidence
limit
This work
Ref. [28]
Ref. (291
Ref. 1301
> 16 6 . *+x71.7 z-13 > 13
>5 > 12 >6
> I.5 10.8 > 0.65
> 0.85 > 0.26 > 0.35 >3.21
(keV) o+ 32; o---o; 0+--+2; 0+-o;
53953 1130.29 1362.06 1740.7
neutrinoless PPdecay to the ground state of ‘OORu. This implies 0 v P&decay constitutes < 0.026% of the ground state transitions, and it is reasonable to assume a negligible contribution ot 0 u events in our data. Double-betadecay of ‘wo is energetically possible to a host of excited states in ‘O”Izu, and half-lives or limits can also be obtained from the data for each of these possible transitions. The measured half-lives or limits for decay to the first four J= 0,2 levels in ‘OORu are given in Table 2. Results from this work are compared to literature values for the same transitions [ 28301. Recently, the NEMO Collaboration [ 281 published the results of a similar experiment in which they did not observe these two gamma rays. While their sample size and counting efficiencies were similar to ours, their background was higher by a factor of = 2, and their data collection time was shorter by a factor of =4. Under similar counting conditions our experimental results would also have been null. The inconsistency between the two experimental results implies the NEMO Collaboration pla, ed a too conservative limit on the number of double-betadecay events which could have been in their data. Similar experiments are being contemplated for measuring the double-beta decay of %Zr, ‘16Cd, and ‘qd to the excited states of their daughters. Some constraints on the half-life of ‘16Cd have already been determined by Norman and Meekhof [ 3 11, using a similar technique, and by Barabash, Kopylov, and Cherekhovsky [ 301.
National Science Foundation under grant PHY8805401. The authors wish to thank P. Vogel and WC. Haxton for their continued interest and advice.
References [ I ] H. Primakoff (1981) 145. [2]
and 5 P. Rosen,
Ann.
W.C. Haxton and J.G. Stevenson, (1984) 409. [ 31 M. Doi. T. Kotani and E. Tak;lsugi. 83 (1985) 1.
Rev. Prog.
Nucl.
Part. Nucl.
Prog. Theor.
The portions of this .vork conducted in the United States were supported by the U.S. Department of Energy under contract DE-ACO6-76RL0 1830 and the
Phys.
Phys.
12
Suppl.
[4] T. Tomoda. Rep. Pmg. Phys. 54 (1991) 53. [ 5) F.T. Avignone 111 and R.L. Erudzinski. Prog. Part. Nucl. Phys. 21 (1988) 99. ]6] A. Morales, in: Theoretical and Phenomenologicd Aspects of Underground Physics, eds. 4. Bottino and P. Monacelli (Editions Frontieres. Gif-sur-‘Yvette, 1989) pp. 97-148. ]7] M.K. Moe, Nucl. Phys. B (Pnc. Suppl.) 19 (1991) 158. [ 81 M.K. Moe, Intern. J. Mod. Phys. E 2 ( 1993) 507. [9]
A.G. Williams and WC. Haxton, in: Intersections Between Particle and Nuclear Physics, ed. GM. Bunce (American Institute of Physics, New York, 1988) p. 924. [ IO] J. Engle. P. Vogel and M.R. Zimbauer, Phys. Rev. C 37 ( 1988) 731. ] I I] K. Muto and H.V. Klapdor, Phys. Lett. B 201 ( 1988) 420; K. Muto, E. Bender and H.V. Klapdor. Z. Phys. A 339 (1991) 435. [ 121 C. fivitarese, A. Faessler. J. Suhonen and S.R. Wu, J. Phys. G 17 (1991) 943. 1131 S.R. EllirHt, A.A. Hahn and M.K. Moe, Phys. Rev. Lett. S9 (1987) 2020. ] 141 A.A. Vacenko, I.V. Kirpichnikov. V.A. Kuznetsov. A.S. St-tin. A.G. Djanyan. V.S. Pogosov, S.P. Shachysisyan and A.G - amanyan, Mod. Phys. L&t. A 5 ( 1990) 1299. ] 151 H.S. n4iley.F.T. Avignone 111. R.L. Bmdzinski. J.H. Reeves, Phys. Rev. Lett. 65 ( 1990) 3092.
Acknowledgments
Part. Sci. 31
[Ia]
J.I. Collar
and
F.T. Avignone 111, R.L. Brodzinski. C.K. Guemrd. I.V. Ki@chnikov. H.S. Miley. V.S. Pogosov, J.H. Reeves, AS. Stamstin and A.G. Tamanyan. Phys. L&t. B 256 ( 1991) 559. [ 171 M. Beck, F. Bensch. J. Bockholt. G. Heusser. M. Hirsch. H.V. Klapdor-Kleingrothaus. B. Maier. F. Petry. A. Piepke. H. Sacker, M. V6llinger. K. Zuber. A. Balysh. S.T. Belyaev. A.
AS. Bambaslt
et al. /Physics
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4M-413
413
Demehin. A. Gurov. I. Kondratenko. V.I. Lebedev and A. Miiller, Phys. Rev. Lett. 70 (1993) 2853. [ !8] S.R. Elliott, M.K. Moe, M.A. Nelson and M.A. Vient. J. Phys. G 17(1991JSl45. [ 191 H. Ejiri, K. Fushimi. M. Kawasaki, H. Kinoshifa. H. Ohsumi. K. Okada. H. Sano, T. Shima. E. Tabasugi. J. Tanaka ,tnd T. Watanabe, J. Phys. G 17 (1991) SISS.
Fackler. G. Fontaine and J. Tnin Thanh VSn (Editions Frontietes. Gif-sur-Yvette. 1991) p. 77. 12.51 A. Griffiths and P. Vogel, Phys. Rev. C 46 (1992) 181. 1261 J. Suhonen and 0. Civitarese. Phys. Rev. C 49 ( 1994) 3055.
1201 NEMO Collab., D. Dassie et al.. preprint, submitted to Nucl. Phys. (1994). 1211 V.A. Artem’ev, E.V. Bnkhman. O.Ya. Zel’dovich, A.K. Karelin, V.V. Kirichenko, O.M. Kozodaeva, V.A. Lyubimov.
A.J. Caffrey. Phys. Rev. Lett. 71 ( 1993) 831. : 28 1 D. Bldm, J. Busto, I.E. Campagne, D. Dassit. F. Hubert. M.C. Isaac. C. Izac. S. Julian, V.N. Kouts, B.N. Kropivyansky. D. Lalanne. T. Lamhamdi. F. Laplanche, F. Leccia, I. Linck, C.
A.I. Mitin. V.N. Paramokhin. T.N. Tsvetkova, S.I. Vasil’ev. A.A. Klimenko. S.B. Osetrov, A.A. Pomnnskii and A.A. Smol’nikov, JETP Lett. 58 (1993) 262. 1221 S.R. Elliott. M.K. !&e. M.A. Nelson and M.A. Vient, Nucl. Phys. B (Proc. Suppl.) 31 (1993) 68. 1231 AS. Barabash. Pis’xaZh. Eksp. Teor. Fiz. 51 (1990) 181. 1241 A..% Batabash, F.T. Avignone Ill. C.K. Guerard. R.L. Brodzinski. H.S. Massive Neutrinos
Miley, J.H. Reeves and V.I. Umatov. in: Tests of Fundamental Symmetries, eds. 0.
1271 M. Alston-Gamjost. B.L. Dougherty, R.W. Kennev. R.D. Tripp, J.M. Ktivicich, H.W. Nicholson. C.S. Sutton, B.D. Dieterle. SD. Foltz, C.P. Lervitt. R. A. Reeder, J.D. Baker and
Longuemate, P. Mennrath, F. Natchez. Szklarz. V.I. Tretyak and Yu.G. Zdesenko. (1992) 506. [ 291 N. Kudomi, H. Ejiri. K. Nagata.
K. Okada,
F. Scheibling, G. Phys. Len. B 275 T. Shibata. T. Shima
and J. Tanaka, Phys. Rev. C 46 ( 1992) R2132. 1301 AS. Barabash. A.V. Kopylov and V.i. Cherekhovsky. Phys. Lat. B 249 ( 1990) 186. 1311 E.B. Norman and D.M. hleekhof. Phys Leu. B 19~5 (1987) L26.
1995
PHYSICS Pbyaics
Letters
LETTERS
B
B 345 ( 199.5) J08-413
Two neutrino double-beta decay of ‘mM~ to the first excited O+ state in ‘(‘ORu A.S. Barabasha,F.T. Aviqone ID b, J.I. Collar b, C.K. Guerard b, R.J. Arthur ‘, R.L. Brodzinski ‘, H.S. Miley ‘, J.H. Reeves‘, J.R. Meier d, K. Ruddick d, V.I. Umatov e b Department ’ School
’ Inxtirure of Theoretical and Experimental of Physics and Astronomy, University
of
Physics,
of South
117259 Carolina.
Moscow, Russia Columbia, SC 29208,
’ Pacific Norfhwest Laboratory’. Richland. WA 99352, USA Physics and Astronomy, University of Minnesota, Minneapolis. MN 554550112. ’ Lebedev Physical Instituie. 142092 Moscow, Russia
LISA USA
Received 1 August 1994; revised manuscript received 9 December 1994 Editor: J.P. Schiffer
Abstract Double-beta decay of ‘(“‘MO to the O+ excited state at 1130.29 keV in ““%I has been observed. A 956 g sample of molybdenum powder enriched to 98.468% 1°0Mo was counted in a “Marinelli” geometry with a well-shielded, ultralow-background getmanium detector. The cascade ga,nma rays at 539.53 and 590.76 keV in ‘%I were observed. The resulting half-life is (6.1: t:f ) X lo’-” yr at the 68% confidence limit in disagreement with a recently published limit.
1. Introduction
Neutrinoless double-beta (/j/3) decay experiments hold the promise of the discovery of elementary particle physics beeyc;ld the standard model. Such experiments, however, depend on calculated nuclear matrix element9 for their in:crprctittion cf this exotic proccegs. TLe phase-space fac:or
tisct
ier SrienLe
B.V.
SSDIO370.2692(94!~~01657-7
the literature [ I-41, and the recent experimental status has also been reviewed [ 5-81, The ordinary allowed second-order weak decay, 2 v Ppdecay, obeys the equation [T:,f*]-‘=GZ’(EoZ)
lM*“I*,
(1)
where rhc quantity G*‘~E,J) is a kinematical factor depending on electron and neutrino wave timztions, and M* ” i:; rhe nuclear matrix element. While M”’ and M” are pot identical, the measurement of M*’ can provide a valuable test .of the nuclear model used to calculate AI”. The matrix elements MO” and M*” have been calculated with the nuclear shell model 191 and with a variety of schematic models. most notably the F;t$yicle Random Phase Approximation (QRPA)
A.S. Barabash
et al. / Ph~G.s
Direct measurements of T T;;. and hence 1M2 “1, have been reported for “‘Se [ 131, %e [ 14-17 1, ‘%40 [ 1g-201, and ““Nd [ 21.22 1. All of these measurcmerits involved the P&decay fro-a the 0 ’ ground state of the parent to the O+ ground state of the daughter nuclide. The three measurements of the BP-decay of ‘r’r’Mo from its O+ ground state to the O+ ground state in ‘O”Ru resulted in the following half-jives: (1.16?::%) X 1019 yr [ 181, (l.lS_‘$g) X lOi yr [ 191, and (9.5f0.4 [stat.] L-0.9 [syst.]) X 10’s yr IW. In earlier work [23], it was shown that the three isotops “‘%r, ‘@‘MO, and ‘.‘oNd probably have Q-values large enough to allow direct observation of the gamma rays following BEdecay to the first excited O+ states of the daughter nuclei. The mezsurcment described in this communication involves the p/IIdecay transition from the O+ ground state of “‘“MO to the first excited O+ state in ‘%u at 1130.29 ksV. The phase-space factors can be calculated by numerically integrating E!q. ( 16) in Ref. [ 21, using a relativistic Fermi function. Assuming lM*“l is the same for both the ground state and excited state transition, the ratio of decay rates is the ratio of phase-space integrals, whirh differ by their end-point energies. Numerical integration yields a ratio of 55. Usirlg the half-life for the ground state transition of Refs. [ 18,191, the predicted half-life is T,,2(Oi +O: ) =6.4+:$ X 10” yr.
131e counting facility used in this experiment is located in the Soudan mine in Minnesota at a depth of 2090 mwe. The system consists of an intrinsic germanium detector having a volume of = 1!4 cd and an energy resolution of 2.3 keV FWHM at 1333 keV assembled in a cryostat of low-background copper components housed in a large bulk shield of ordinary lead which has an inner liner of 5 cm thick, = 150 yr old lead recovered from a German ship that sank in the North Sea. The old lead liner is used to shield the detector from the radiations of “‘Pb and its progeny present in the contemporary lead. The bulk shield is atmospherically sealed and pressurized with the nitrogen boil-off gas from the detector dewar to mitigate radon and its progeny in the shield.
Letters
B 345 (1995)
4OP-413
409
Cu Cryostat Fig. 1. Geometry of shield, Mtinelli beaker. and germtium detector.
The sample consisted of 956 g of powdered molybdenum metal enriched to (98.468&0.009)% ““‘MO contained in a lucite Marinelli beaker (a right circular cylinder with a hole for inserting the detector in one end, as shown m Fig. 1). Since the sample nearly surrounds the detectl>r, this configuration is only suitable for detecting sin& gamma-ray events. The photopeaks of intclest are the 590.76 and 539.53 keV lines corresponding to the de-rxcitation gamma rays through !he 0: -12: + 0: cascade.
3. Results The energy spectrum obtained ‘7 415.43 d of coun!ing is shown in the region of interest at 1 keV per channel in Fig. 2. Radioactive impurities identified in the sample include primordial potassium, thorium, uranium, and radium as well as traces of ‘.WCsand ‘37Cs. The 511 keV line from positron, annihilation, the 583.19 keV line from 208Tl(2’“Th), the 569.32 and 604.71 keV lines from ‘34Cs, and the 609.32 keV line from 2’4Bi(226Ra) are shown in the figure and provide an internal check on the resolution and energy calibration of the system. The energy regions of the spectrum corresponding to the expected peaks from the /3P decay de-excitation gammarayq wer? analyzed by mul-
410
A.S. Barabash
er al. / Plyics
Fig. 2. Energy
spectrum
tiple independent techniques. The results are 66-t22 counts for the net area under the 539.53 keV peak and 67 + 19 counts for the 590.76 keV peak, where the uncertainties represent one standard deviation ( 1a). The enriched molybdenum powder was removed from the Marinelli beaker and the empty beaker was counted in the same configuration for 103.01 d to verify tha! the observed positive peak areas were due to the molybdenum. No positive net counts were observed in the background spectrum. An upper limit of 5 counts in either peak at the 68% confidence limit was derived from the background data. It is also necessary to show that the positive peak areas are not engendered by some other mechanism in the molybdenum sample. Since the peak areas are of the same intensity, it is extremely unlikely that they could originate from any other source than the expected cascade in nx)Ru.The only other plausible mechanisms for feeding this cascade are from (n, n’) and (n, p) reactions on a ‘OORuimpurity in the sample. If these reactions occurred, an analogous reaction in the germanium detector would also have been observed. There arc = 2 X 10” atoms of 7JGe in the detector. Fast neutron excitation of this ‘*Ge would result in a 595.8 keV gamma ray. The integral production rates of these hypothesized reactions are not known, but let’s assume the reactions on ‘“Ru are no more than a factor of 50 more prolific than those on 74Ge. Since the 595.8 keV gamma ray would be internal to the germanium detector, it would have approximately a factor of 50 higher efhciency for detection than those from the molybdenum sample, and the total number of each of the gamma rays observed would be the same. The measured teeRu
Letters
B 34-7 (1995)
in the region
-108-113
of interest.
concentration in the sample is < 0.35%, corresponding to <2.0X 102’ atoms. There are < 13 net counts at 595.8 keV. Therefore, there are CO.13 counts in the photopeaks of interest from neutron reactions on ‘~‘Ru. Anothe, calculation demonstrating insufficient neutron flux to produce the potentially interferingreactions is based on the neutron capture of ‘O”Mo which could be observed by the 306.86 keV gamma ray from the decay of “‘Tc. There are < 15 net counts as.306.86 keV. The thermal neutron flux would be expected to be no less than the fast flux in this scenario. Normalizing for the ratios of number of target atoms, cross sections, detector efficiencies, and branching ratios, there are < 0.003 counts in the photopeaks of interest from neutron reactions on ““‘Ru. The absolute counting efficiencies were determined by replacing the enriched ‘n”Mo metal powder in the Marinelli beaker with 975.4 g of natural molybdenum metal powder homogeneously contaminated with 1.753 Bq/kg ?f az6Ra.Data were acquired for 25.04 d, and the net arcas under the peaks at 295.22, 35 1.99, 1764.5 1, and 2204.12 keV were used to construct an absolute energy versus efficiency curve for single gamma rays from the enriched molybdenum sample. From this curve, the measured efficiency of the 539.53 keV peak would be 0.01182 and of the 590.76 keV peak would be 0.01107 if they were emitted as single, independent events. Hcwever, these gamma rays are a coincident cascade with a strong angular correlation which results in true efficiencies that am lower than the above values, Furthermore, it is not possible to experimentally duplicate the counting situation to measure the exact efficiencies. Consequently, two Monte Carlo
Table 1 Raw data accumulated irl an elapsed time of 415.43 d in the energy region of interest E(keV)
0
I
2
3
SIO 520 530 540 550 560 570 580 590
193 93 91
200 100 90 77 74 84 104 123 91
144 96 88 74 91 89 69 197 65
Iii 107 86 99 80 96 83 221 74
110 84 77 II5 80 89
-
simulations were performed: one treated the gamma rays as single events, and the other modeled the coincidence cascade and angular correlation. Although the calculated values were not expected to be absolutely correct (the calculated singles efficiencies were 0.01164 and 0.01104 for the 539.53 and 590.76 keV peaks, respectively!), the ratio of calculated coincidence to single effic.‘encies (0.8497 for the 539.53 keV peak and 0.8419 for the 590.76 keV peak) should reflect the actual ratio very closely. Therefore, the above measured singles efficiencies were multiplied by these calculated ratios to arrive at efficiencies for the 539.53 keV peak of 0.01004 and for the 590.76 keV peak of 0.009320 which were applied to the experimental data. In our earlier preliminary report [24], the Monte Carlo calculated efficiencies were significantly higher than the above values. The discrepancy has been traced to an error in the detector/sample geometry use: in those earlier calculations. The half-life can be calculated according to the formula T ,,2 = (In 2)Nt/dN,
(2)
where N is the number of “‘OMo atoms. 5,669 X 102$ t is the time of counting, 1.137 yr; and dN is the number of “%I0 decays. Therefore, at the 68% confidence level, the half-life based on the 539.53 keV peak is T ,,2= (6.8?::$) X 10’” yr, and based on tht 590.76 keVpeakit’isT,12=(6.2?:::)X1020yr. Both gamma rays of interest are from pure electric quadrupole transitions with very nearly the same internal conversion crr‘fficients; hence they have almost identical intensities. Adding these two energy regions of the spectrum improves the statistical accuracy of the
4
5
6
7
8
9
124 87 89 R2 p2 92 73 166 7.5
92 85 8.5 80 14 66 71 89 73
90 7s 86 84 78 76 76 57 70
101 73 95 86 91 98 69 63 58
loo 105 Sd 97 87 104 76 63 62
I10 87 118 94 80 124 78 34 81
data, and dividing the net area under the combined photopeak by the average efficiency leads to the more precise value of the (0,: 40: ) P@-decay half-life: Tllz = (6.1_+1::) X 102” yr.
(3) The precise value of the half-life derived from these data depends slightly on the method chosen for integrating the photopeaks. The actual data acquired in 1.137 yr of counting time are given in Table 1 in the energy region of interest. Readers are invited to determine for themselves the strength c.’ argument supporting a positive observation and the half-life. Griffiths and Vogel’s [ 251 matrix element calculation based on our preliminary half-life fbr decay to the first excited O+ s:ate should be amended to IME; 1= G.30+:::: MeV - ’ based on the measured half-life reported here. Their corresponding value for the ground state transition based on the experimental I.?lf-lives reported in Refs. [ 18,191 is JM& 1= 0.30?::0,: MeV - ‘. An important conclusion of Ref. [ 25 ] is that both matrix elements are dominated by transitions through the 1 + ground state of the intermediate nucleus, ‘9~. Based on that assumption, the relevant QRPA calculated matrix element for the ground state transition is 1M&T I = 0.25 MeV - ’ and that for the decay to :he first O+ excited state is I&Z& 1 =0.24 MeV - ‘. These calculated matrix elements are similar, as are the experimental values, and their absolute values are only about 20% smaller. Independent calculations recently reported by Suhonen and Civitarese [26] support the conclusions of Griffiths and Vogel [ 251. Our experiment. of course, does not distinguish betwl;en 2 v and 0 v @&decay. However, Alston-Gamjost et al. [ 271 have reported Ty& > 4.4 X 10” yr for
AS. Barabash
412
Table 2 Half-lives
( X IOmM yr) for pp-decay Transition
of ‘“‘MO Level
et al. /Physics
to selected
excited
Lerters
B 345 (1995)
levels in ‘O”Ru at the 90%
408413
confidence
limit
This work
Ref. [28]
Ref. (291
Ref. 1301
> 16 6 . *+x71.7 z-13 > 13
>5 > 12 >6
> I.5 10.8 > 0.65
> 0.85 > 0.26 > 0.35 >3.21
(keV) o+ 32; o---o; 0+--+2; 0+-o;
53953 1130.29 1362.06 1740.7
neutrinoless PPdecay to the ground state of ‘OORu. This implies 0 v P&decay constitutes < 0.026% of the ground state transitions, and it is reasonable to assume a negligible contribution ot 0 u events in our data. Double-betadecay of ‘wo is energetically possible to a host of excited states in ‘O”Izu, and half-lives or limits can also be obtained from the data for each of these possible transitions. The measured half-lives or limits for decay to the first four J= 0,2 levels in ‘OORu are given in Table 2. Results from this work are compared to literature values for the same transitions [ 28301. Recently, the NEMO Collaboration [ 281 published the results of a similar experiment in which they did not observe these two gamma rays. While their sample size and counting efficiencies were similar to ours, their background was higher by a factor of = 2, and their data collection time was shorter by a factor of =4. Under similar counting conditions our experimental results would also have been null. The inconsistency between the two experimental results implies the NEMO Collaboration pla, ed a too conservative limit on the number of double-betadecay events which could have been in their data. Similar experiments are being contemplated for measuring the double-beta decay of %Zr, ‘16Cd, and ‘qd to the excited states of their daughters. Some constraints on the half-life of ‘16Cd have already been determined by Norman and Meekhof [ 3 11, using a similar technique, and by Barabash, Kopylov, and Cherekhovsky [ 301.
National Science Foundation under grant PHY8805401. The authors wish to thank P. Vogel and WC. Haxton for their continued interest and advice.
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