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A measurement of the double beta decay half-life of 48Ca
UCLEAR PHYSICE
PROCEEDINGS SUPPLEMENTS ELqEV1ER
Nuclear Physics B (Proc. Suppl.) 48 (1996) 213-215
A Measurement of the double beta decay half-life of 48Ca A. Balysh a, A. De Silva b, V. I. Lebedeva, K. Lou e, M. K. Moe b, M. A. Nelson b *, A. Piepke e, A. Pronskiy a, M. A. Vient b, P. Vogelc t aKurchatov Institute, Kurchatov Square, 123182 Moscow, Russia bDepartment of Physics and Astronomy, University of California, Irvine Irvine, CA 92717-4575, USA Cphysics Department, California Institute of Technology Pasadena, California 91125, USA *SCa is the lightest of the many double beta decay nuclei, and the only one simple enough to be treated exactly in the shell model without truncation. Thus a flf32v measurement of this isotope provides a unique test of the nuclear physics involved in tiff matrix element calculations. Enriched 4SCa sources of two different thicknesses have been exposed in a time projection chamber, and yield a preliminary T ~ 2 --- (5.5+_a1:~)x 10~a y, in agreement with shell model calculations. 1. I N T R O D U C T I O N A recent shell mode[ calculation of the 4SCa ~/~2~ matrix element has been carried out for the complete fp shell with a hamiltonian constrained by the spectroscopy of the mass number 48 system [1]. The predicted half-life is very close to the experimental limit of T2~2 > 3.6 x 1019 y by C. S. Wu and her collaborators [2], and does not exceed 102° y if the spectroscopic levels are held within 2 a of experimental values. Thus, an increase of a factor of 3 in the experimental limit would begin to cause serious problems for the theory [3]. To test this fundamental application of the shell model to/~/~ decay, a collaboration was formed to measure 4SCa in the UC Irvine time projection chamber (TPC). 48Ca has the highest energy release of all ~ nuclei (Q~# = 4.3 MeV), which places the sumenergy spectrum in a very favorable position visa-vis the background. This singular advantage is largely offset, however, by the difficulty of re*Present address: Dynamics Technology, Inc., 21311 Hawthorne Blvd., Suite 300, Torrance, CA 90503-5610 ?This work was supported in part by the US Department of Energy under contracts DE-FG03-91ER40679 and DEFG03-88ER40397. 0920-5632/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved. PII: S0920-5632(96)00242-3
moving chemically-similar contaminations such as a°Sr and Z2eRa and by the miniscule 0.187% natural isotopic abundance. 2. T H E E X P E R I M E N T A L
SET UP
2.1. T h e i s o t o p e s a m p l e The Kurchatov Institute supplied 77 g of finely powdered CaCOa enriched to 73% in 4SCa. Mass spectroscopic analysis [4] placed upper limits for U and Th each at 0.8 ppb, which would easily give an experimental ~f~ sensitivity of 102o years if the decay chains were in equilibrium and there were no other serious high Q-value impurities. 2.2. T h e thick s o u r c e We assumed a-priori the equilibrium scenario and made a relatively thick source to maximize the mass of 4SCa. The CaCO3 powder was injected into an octagonal glass box by compressed argon. The powder settled uniformly over a 4 pm Mylar film at the bottom of the box, and was subsequently fixed with a light misting of Formvar. Two such deposits, each of 2427 cm 2 area, were placed face-to-face for insertion into the TPC. This source consisted of 42.2 g of CaCO3 (18.5 mg/cm 2 overall thickness, including the Mylar
214
A. Balysh et al./Nuclear Physics B (Proc. Suppl.) 48 (1996) 213 215
and Formvar). A calibrated 0.5 pCi 2°7Bi conversion source was placed in a 3 cm square void in the CaCO3 deposit at the center of the octagon. 2.3. T h e t h i n source For reasons discussed below, the thick source was replaced part way through the experiment with one containing only 10.3 g of CaC03, with a total thickness of 5.4 mg/cm 2. 2.4. T P C The TPC has an 82 c m x 82 cm x 20 cm sensitive volume, with the/~/~ source as the central electrode. The gas is 93% He, 7% propane at atmospheric pressure, in a magnetic field of 1.2 kG. Electron energy is determined from the radius and pitch of the helix fit to the track. The chamber remains live for 1 ms following each trigger to record the 164 ps 214po alpha particle tagging false events from 214Bi. The TPC was constructed of low background materials, and is surrounded by Pb shielding and cosmic-ray veto counters. It operates under a minimum of 72 m of rock in a tunnel at the Hoover Dam. Details of the apparatus have been described elsewhere [5]. 3. T H E DATA Data were recorded on site with 8 mm magnetic tape, and subsequently run through stripping software to select e- and 2e- events. Twoelectron events were individually scanned by a physicist. All unambiguous negative pairs emitted from opposite sides of the source with a common point of origin were fitted with helices, and the parameters written to a/3/3 candidate file or a 214Bi file, depending on whether a 214po alpha particle appeared at the vertex within the following millisecond. Far more numerous were lone electrons from the 4SCa deposit. These were fitted automatically by software, and also written to a parameter file. 4. R E S U L T S The 4SCa was considerably hotter than predicted by the mass-spectroscopic upper limits for U and Th. Bi-Po activity indicated that secular equilibrium was broken at radium by factors
of about 30 and 20 for the U and Th chains respectively. The lone electron spectrum, however, showed 9°Sr and its daughter 0Oy to be responsible for most of the ,,~ 270 mBq activity. 9°St has little energy, but 9oy (Qz = 2.3 MeV) can intrude on much of the 4SCa j3/~2~ spectral range. Fortunately, 90y is nearly a pure beta emitter, with only a very weak conversion line near 1.7 MeV. Its main contribution to/3/3 background is through MSller scattering, a weaker process than the troublesome beta + internal-conversion pairs in the primordial decay chains. The rate of /~/~2~ candidate events from the thick source exceeds the Wu limit, and the spectral shape bears little resemblance to ~ / ~ . Faced with a similar situation, the Wu group focused on the high-energy end of the sum spectrum where the high Q ~ for 4SCa results in the best ratio of signal to background. They set their limit by assigning all counts above 2.2 MeV sum energy to/~/~2~. The same approach with our data gives a limit only slightly higher. In principle, the TPC can reject ~14Bi background that the streamer chamber used 25 years ago by Madam Wu could not, because of TPC sensitivity to the 214po alpha particle tag. But with the thick source, much of this advantage is lost since only ,-,1/4 of the alpha particles escape the calcium. Excessive MSUer scattering of the abundant 9Oy beta particles is another thicksource background. For these reasons we elected after 2440 h of running, to substitute a thin source with only 1/4 of the CaCO3 mass. The thin source gave a factor of 2 improvement in signal-to-background, but with 4001 h of exposure the half-life limit did not improve. This was the first hint that the actual half-life was close at hand. Both sources were pressed for additional information. The onset of improved signal-tobackground with increasing energy is sharper in the singles spectrum of the/~/~2~ candidates than in the sum spectrum. Therefore, a singles threshold replaced the sum threshold in subsequent analysis. The untagged 2~4Bi 2e- contribution was calculated as the tagged count multiplied by (1 - Pa)/P,~, where Pa is the probability that the 214po alpha particle will escape the source.
A. Balysh et al./Nuclear Physics B (Proc. Suppl.) 48 (1996) 213-215
215
2.5
1.5
0.5
0
0
'
i
"
2
"
3
" .......
41 . . . . . . . . .
5
Sum Energy (MeV)
Figure 1. Kurie plot of residual f/~ candidates. The intercept is consistent with 4SCa (Qf~ -- 4.3 MeV).
These background spectra were subtracted from the data, and the residual spectrum was rendered as a Kurie plot using the Primakoff - Rosen approximation, but with the singles threshold accounted for (Fig. 1). The intercept near 4.3 MeV is in good agreement with Q ~ for 4SCa. The process was carried out for both thick and thin sources and a variety of singles thresholds, with similar results. The flagged 214Bi 2e- events also give a respectable looking "Kurie plot" but the energy intercept is distinctly lower at 3.2 MeV. A Kurie plot for 2°STl from an earlier 22°Rn injection does not form a straight line. The corresponding 4SCa half-lives from thick and thin sources are plotted against singles threshold in Fig. 2. A preliminary estimate of T12~2 is ,(5 •5+3'5~ -1.5/ X 1019 y. 5. C O N C L U S I O N S
Pa was determined independently from the fraction of lone electrons in the energy range 2.5 3.2 MeV that were accompanied by 214po alphas. (Lone electrons in this range were assumed to be purely 214Bi, an assumption well supported by beta spectrum fits.) The MSller 2e- contribution was calculated from the lone electron spectrum by Monte Carlo, and a small 2°aTl 2e- component was determined from observed 212Bi - 212Po events and their known ratio to 2°STl 2e- events measured from a 22°Rn injection. ..-..1021
Although the 4SCa was heavily contaminated, backgrounds were well defined by associated alpha activity and the lone electron spectrum. The residual data give half-life values that are consistent between sources of two different thicknesses, and among various singles energy thresholds. The corresponding Kurie plots all point to the 4SCa Q value to the exclusion of known backgrounds. A preliminary half-life value supports the shell model calculations. Details of the analysis will be published elsewhere. REFERENCES
I
10~
i
I i ........
500
'
i .........
400
a ........
,I .........
500 600 700 Singles Energy Threshold (keY)
Figure 2. Half-life vs. singles threshold for thick (solid) and thin (dashed) sources.
1. A. Poves, R. P. Bahukutumbi, K. Langanke, and P. Vogel, Phys. Lett. B361 (1995) 1. 2. R . K . Bardin, et al., Nucl. Phys. A158 (1970) 337. 3. Other authors manage to suppress/~f/zv arbitrarily, but do not say whether their hamiltonian is consistent with A=48 spectroscopy. See K. Muto, E. Bender, and H. V. KlapdorKieingrothaus, Z. Phys. A339 (1991) 435. 4. ICP-MS Analysis by Elemental Research Inc., Vancouver, B.C., Canada. 5. S . R . Elliott, A. A. Hahn, and M. K. Moe, Nucl. Instrum. Meth. in Phys. Res., A273 (1988) 226.
PROCEEDINGS SUPPLEMENTS ELqEV1ER
Nuclear Physics B (Proc. Suppl.) 48 (1996) 213-215
A Measurement of the double beta decay half-life of 48Ca A. Balysh a, A. De Silva b, V. I. Lebedeva, K. Lou e, M. K. Moe b, M. A. Nelson b *, A. Piepke e, A. Pronskiy a, M. A. Vient b, P. Vogelc t aKurchatov Institute, Kurchatov Square, 123182 Moscow, Russia bDepartment of Physics and Astronomy, University of California, Irvine Irvine, CA 92717-4575, USA Cphysics Department, California Institute of Technology Pasadena, California 91125, USA *SCa is the lightest of the many double beta decay nuclei, and the only one simple enough to be treated exactly in the shell model without truncation. Thus a flf32v measurement of this isotope provides a unique test of the nuclear physics involved in tiff matrix element calculations. Enriched 4SCa sources of two different thicknesses have been exposed in a time projection chamber, and yield a preliminary T ~ 2 --- (5.5+_a1:~)x 10~a y, in agreement with shell model calculations. 1. I N T R O D U C T I O N A recent shell mode[ calculation of the 4SCa ~/~2~ matrix element has been carried out for the complete fp shell with a hamiltonian constrained by the spectroscopy of the mass number 48 system [1]. The predicted half-life is very close to the experimental limit of T2~2 > 3.6 x 1019 y by C. S. Wu and her collaborators [2], and does not exceed 102° y if the spectroscopic levels are held within 2 a of experimental values. Thus, an increase of a factor of 3 in the experimental limit would begin to cause serious problems for the theory [3]. To test this fundamental application of the shell model to/~/~ decay, a collaboration was formed to measure 4SCa in the UC Irvine time projection chamber (TPC). 48Ca has the highest energy release of all ~ nuclei (Q~# = 4.3 MeV), which places the sumenergy spectrum in a very favorable position visa-vis the background. This singular advantage is largely offset, however, by the difficulty of re*Present address: Dynamics Technology, Inc., 21311 Hawthorne Blvd., Suite 300, Torrance, CA 90503-5610 ?This work was supported in part by the US Department of Energy under contracts DE-FG03-91ER40679 and DEFG03-88ER40397. 0920-5632/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved. PII: S0920-5632(96)00242-3
moving chemically-similar contaminations such as a°Sr and Z2eRa and by the miniscule 0.187% natural isotopic abundance. 2. T H E E X P E R I M E N T A L
SET UP
2.1. T h e i s o t o p e s a m p l e The Kurchatov Institute supplied 77 g of finely powdered CaCOa enriched to 73% in 4SCa. Mass spectroscopic analysis [4] placed upper limits for U and Th each at 0.8 ppb, which would easily give an experimental ~f~ sensitivity of 102o years if the decay chains were in equilibrium and there were no other serious high Q-value impurities. 2.2. T h e thick s o u r c e We assumed a-priori the equilibrium scenario and made a relatively thick source to maximize the mass of 4SCa. The CaCO3 powder was injected into an octagonal glass box by compressed argon. The powder settled uniformly over a 4 pm Mylar film at the bottom of the box, and was subsequently fixed with a light misting of Formvar. Two such deposits, each of 2427 cm 2 area, were placed face-to-face for insertion into the TPC. This source consisted of 42.2 g of CaCO3 (18.5 mg/cm 2 overall thickness, including the Mylar
214
A. Balysh et al./Nuclear Physics B (Proc. Suppl.) 48 (1996) 213 215
and Formvar). A calibrated 0.5 pCi 2°7Bi conversion source was placed in a 3 cm square void in the CaCO3 deposit at the center of the octagon. 2.3. T h e t h i n source For reasons discussed below, the thick source was replaced part way through the experiment with one containing only 10.3 g of CaC03, with a total thickness of 5.4 mg/cm 2. 2.4. T P C The TPC has an 82 c m x 82 cm x 20 cm sensitive volume, with the/~/~ source as the central electrode. The gas is 93% He, 7% propane at atmospheric pressure, in a magnetic field of 1.2 kG. Electron energy is determined from the radius and pitch of the helix fit to the track. The chamber remains live for 1 ms following each trigger to record the 164 ps 214po alpha particle tagging false events from 214Bi. The TPC was constructed of low background materials, and is surrounded by Pb shielding and cosmic-ray veto counters. It operates under a minimum of 72 m of rock in a tunnel at the Hoover Dam. Details of the apparatus have been described elsewhere [5]. 3. T H E DATA Data were recorded on site with 8 mm magnetic tape, and subsequently run through stripping software to select e- and 2e- events. Twoelectron events were individually scanned by a physicist. All unambiguous negative pairs emitted from opposite sides of the source with a common point of origin were fitted with helices, and the parameters written to a/3/3 candidate file or a 214Bi file, depending on whether a 214po alpha particle appeared at the vertex within the following millisecond. Far more numerous were lone electrons from the 4SCa deposit. These were fitted automatically by software, and also written to a parameter file. 4. R E S U L T S The 4SCa was considerably hotter than predicted by the mass-spectroscopic upper limits for U and Th. Bi-Po activity indicated that secular equilibrium was broken at radium by factors
of about 30 and 20 for the U and Th chains respectively. The lone electron spectrum, however, showed 9°Sr and its daughter 0Oy to be responsible for most of the ,,~ 270 mBq activity. 9°St has little energy, but 9oy (Qz = 2.3 MeV) can intrude on much of the 4SCa j3/~2~ spectral range. Fortunately, 90y is nearly a pure beta emitter, with only a very weak conversion line near 1.7 MeV. Its main contribution to/3/3 background is through MSller scattering, a weaker process than the troublesome beta + internal-conversion pairs in the primordial decay chains. The rate of /~/~2~ candidate events from the thick source exceeds the Wu limit, and the spectral shape bears little resemblance to ~ / ~ . Faced with a similar situation, the Wu group focused on the high-energy end of the sum spectrum where the high Q ~ for 4SCa results in the best ratio of signal to background. They set their limit by assigning all counts above 2.2 MeV sum energy to/~/~2~. The same approach with our data gives a limit only slightly higher. In principle, the TPC can reject ~14Bi background that the streamer chamber used 25 years ago by Madam Wu could not, because of TPC sensitivity to the 214po alpha particle tag. But with the thick source, much of this advantage is lost since only ,-,1/4 of the alpha particles escape the calcium. Excessive MSUer scattering of the abundant 9Oy beta particles is another thicksource background. For these reasons we elected after 2440 h of running, to substitute a thin source with only 1/4 of the CaCO3 mass. The thin source gave a factor of 2 improvement in signal-to-background, but with 4001 h of exposure the half-life limit did not improve. This was the first hint that the actual half-life was close at hand. Both sources were pressed for additional information. The onset of improved signal-tobackground with increasing energy is sharper in the singles spectrum of the/~/~2~ candidates than in the sum spectrum. Therefore, a singles threshold replaced the sum threshold in subsequent analysis. The untagged 2~4Bi 2e- contribution was calculated as the tagged count multiplied by (1 - Pa)/P,~, where Pa is the probability that the 214po alpha particle will escape the source.
A. Balysh et al./Nuclear Physics B (Proc. Suppl.) 48 (1996) 213-215
215
2.5
1.5
0.5
0
0
'
i
"
2
"
3
" .......
41 . . . . . . . . .
5
Sum Energy (MeV)
Figure 1. Kurie plot of residual f/~ candidates. The intercept is consistent with 4SCa (Qf~ -- 4.3 MeV).
These background spectra were subtracted from the data, and the residual spectrum was rendered as a Kurie plot using the Primakoff - Rosen approximation, but with the singles threshold accounted for (Fig. 1). The intercept near 4.3 MeV is in good agreement with Q ~ for 4SCa. The process was carried out for both thick and thin sources and a variety of singles thresholds, with similar results. The flagged 214Bi 2e- events also give a respectable looking "Kurie plot" but the energy intercept is distinctly lower at 3.2 MeV. A Kurie plot for 2°STl from an earlier 22°Rn injection does not form a straight line. The corresponding 4SCa half-lives from thick and thin sources are plotted against singles threshold in Fig. 2. A preliminary estimate of T12~2 is ,(5 •5+3'5~ -1.5/ X 1019 y. 5. C O N C L U S I O N S
Pa was determined independently from the fraction of lone electrons in the energy range 2.5 3.2 MeV that were accompanied by 214po alphas. (Lone electrons in this range were assumed to be purely 214Bi, an assumption well supported by beta spectrum fits.) The MSller 2e- contribution was calculated from the lone electron spectrum by Monte Carlo, and a small 2°aTl 2e- component was determined from observed 212Bi - 212Po events and their known ratio to 2°STl 2e- events measured from a 22°Rn injection. ..-..1021
Although the 4SCa was heavily contaminated, backgrounds were well defined by associated alpha activity and the lone electron spectrum. The residual data give half-life values that are consistent between sources of two different thicknesses, and among various singles energy thresholds. The corresponding Kurie plots all point to the 4SCa Q value to the exclusion of known backgrounds. A preliminary half-life value supports the shell model calculations. Details of the analysis will be published elsewhere. REFERENCES
I
10~
i
I i ........
500
'
i .........
400
a ........
,I .........
500 600 700 Singles Energy Threshold (keY)
Figure 2. Half-life vs. singles threshold for thick (solid) and thin (dashed) sources.
1. A. Poves, R. P. Bahukutumbi, K. Langanke, and P. Vogel, Phys. Lett. B361 (1995) 1. 2. R . K . Bardin, et al., Nucl. Phys. A158 (1970) 337. 3. Other authors manage to suppress/~f/zv arbitrarily, but do not say whether their hamiltonian is consistent with A=48 spectroscopy. See K. Muto, E. Bender, and H. V. KlapdorKieingrothaus, Z. Phys. A339 (1991) 435. 4. ICP-MS Analysis by Elemental Research Inc., Vancouver, B.C., Canada. 5. S . R . Elliott, A. A. Hahn, and M. K. Moe, Nucl. Instrum. Meth. in Phys. Res., A273 (1988) 226.