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Experimental study of 19Ne decay and its significance to the study of neutral weak currents
Volume 51B, number 1
PHYSICS LETTERS
8 July 1974
E X P E R I M E N T A L S T U D Y O F 19Ne D E C A Y A N D I T S S I G N I F I C A N C E TO THE STUDY OF NEUTRAL WEAK CURRENTS* F.M. MANN and R.W. KAVANAGH California Institute of Technology, Pasadena, Calif. 91109, USA Received 24 May 1974 An upper limit of 0.003% for the beta decay of 19Ne to the 1.55 MeV level in 19F has been determined. Its significance for a recently suggested experiment to study neutral weak currents is discussed. Donnelly et al. [ 1] have recently suggested that neutral weak currents could be studied through the use of inelastic neutrino scattering on nuclei. One of their suggested experiments is -19F(~e, ~ ) 19F*(1.55 MeV) (,y) 19 F*(0.20 MeV) (,y)19F, which they predict would have a production cross section for reactor-energy~e of 6.3 X 10 - 4 4 B cm2/ nucleon, where B is the branching for the fl decayof the ground state of 19Ne to the 1.55 MeV level in 19F. Using the single-particle Nilsson model with a K = 1/2 band and four adjustable parameters, Donnelly et al. calculated B to be (1.0 -+ 0.9) X 10 - 4 , implying a logft = ~ n+ 1.03 • They note that the validity of this J-v_0. model is rather well established through the extensive intermediate-coupling calculations of Benson and Flowers [2] and that the model accurately predicts the K = 1/2 band energies, the M1 ,y- decay rate from the 1.55 MeV level to the 0.20 MeV level, and the ground-state magnetic moment of 19Ne. However, the Gamow-Teller/3-decay matrix element needed to f'md B is not directly tested by these successes and could be considerably smaller than Donnelly et al. predict. In fact, Wildenthal [3], using the manyparticle shell model, which has also met with considerable success, has calculated B = 3 X 10- lO, implying logft = 10.5. Assuming B = 10 - 4 and proposing a hexagonal array of 300 cylinders of BaF2(Eu ) to detect the 'y-ray cascade from the 1.55 MeV level, Donnelly et al. predict 0.7 coincident events/day with a signal-to-noise * Supported in part by the National Science Foundation [GP-28027].
ratio of 1 in the neutrino flux from the Savannah River Reactor. Using the value of B predicted by Wildenthal, the count rate would decrease to 2 × 10 -6 coincident events/day and a signal-to-noise ratio of 3 X 10 - 6 , making the experiment unfeasible. In view of these drastically different predictions, the Nilsson model predicting a typical allowed strength, and the many-particle shell model predicting a branch several orders of magnitude weaker, we report here an attempt to measure the strength of the/3 branch to the 1.55 MeV level. A thick target o f P b F 2 was bombarded by 11 MeV protons from the CIT-ONR tandem accelerator. The target was retained inside a beryllium rabbit by a tantalum foil and was transported by an air shuttle after bombardment to a remote 73 cm 3 Ge(Li) detector heavily shielded with lead. The rabbit was separated from the detector by 2 cm of lucite in order to stop the emitted positrons with a minimum of bremsstrahlung, and by 5 cm of lead to attenuate the 511 keV annihilation radiation much more severely than the 1357.0 +--0.2 keV "y-ray resulting from the decay of the 1.55 MeV level. Two successive counting periods of 17.4 s (the half-life of 19Ne) were used to verify the lifetimes of the observed ,y-rays. After the counting periods, the rabbit was transferred back for another bombardment. The relevant portion of the spectrum for the first 17.4 s counting period is shown in the figure. The 511 keV annihilation peak stands out clearly. The peak at 1291 keV is the double-escape peak of the 2313 keV ,y ray following the decay of 140 produced from 14N(p, n). The photopeak at 1434 keV is from 52Mn* decays produced from 52Cr(p, n). The nitrogen and chromium contaminants result from parts of the rabbit system. 49
Volume 51B, number 1
PHYSICS LETTERS
8 July 1974
511
1291
40K 1200 30 K 800
~ Z 20K 0
400
IOK
r~-
460
-a
530
¥ RAY
ENERGY (keY)
Fig. 1. Partial "/-ray spectrum for 19Ne03+)lgF. The peaks at 1291 and 1434 keV are from contaminant decays. The expected decay to the 1554 keV level would produce a 1357 keV photopeak at the arrow. Using detector efficiencies determined by use of 22 Na and 56Co calibration sources, an upper limit (2o confidence level) for B is 3 X 1 0 - 5 , implying a l o g f t greater than 5.6. For comparison, if the 1357keV peak were as large as the 1434 keV photopeak shown in the figure, t h e n B would equal (1.1 + 0.2) X 10 - 4 . Further work is planned to increase the experimental sensitivity. Using the present experimental limit for B, and retaining the other arguments of Donnelly et al., the expected coincident counting rate for 19F(~-e,~e)
50
19F*(77) 19F would be less than 0.2/day with a signal-to-noise ratio less than 0.3.
References [1] T.W. Donnelly, et al. Phys. Lett. 49B (1974) 8. [2] H.G. Benson and B.H. Flowers, Nucl. Phys. A126 (1969) 305. [3] Cited in W.A. Langford and B.H. Wildenthal, Phys. Rev. C7 (1973) 688.
PHYSICS LETTERS
8 July 1974
E X P E R I M E N T A L S T U D Y O F 19Ne D E C A Y A N D I T S S I G N I F I C A N C E TO THE STUDY OF NEUTRAL WEAK CURRENTS* F.M. MANN and R.W. KAVANAGH California Institute of Technology, Pasadena, Calif. 91109, USA Received 24 May 1974 An upper limit of 0.003% for the beta decay of 19Ne to the 1.55 MeV level in 19F has been determined. Its significance for a recently suggested experiment to study neutral weak currents is discussed. Donnelly et al. [ 1] have recently suggested that neutral weak currents could be studied through the use of inelastic neutrino scattering on nuclei. One of their suggested experiments is -19F(~e, ~ ) 19F*(1.55 MeV) (,y) 19 F*(0.20 MeV) (,y)19F, which they predict would have a production cross section for reactor-energy~e of 6.3 X 10 - 4 4 B cm2/ nucleon, where B is the branching for the fl decayof the ground state of 19Ne to the 1.55 MeV level in 19F. Using the single-particle Nilsson model with a K = 1/2 band and four adjustable parameters, Donnelly et al. calculated B to be (1.0 -+ 0.9) X 10 - 4 , implying a logft = ~ n+ 1.03 • They note that the validity of this J-v_0. model is rather well established through the extensive intermediate-coupling calculations of Benson and Flowers [2] and that the model accurately predicts the K = 1/2 band energies, the M1 ,y- decay rate from the 1.55 MeV level to the 0.20 MeV level, and the ground-state magnetic moment of 19Ne. However, the Gamow-Teller/3-decay matrix element needed to f'md B is not directly tested by these successes and could be considerably smaller than Donnelly et al. predict. In fact, Wildenthal [3], using the manyparticle shell model, which has also met with considerable success, has calculated B = 3 X 10- lO, implying logft = 10.5. Assuming B = 10 - 4 and proposing a hexagonal array of 300 cylinders of BaF2(Eu ) to detect the 'y-ray cascade from the 1.55 MeV level, Donnelly et al. predict 0.7 coincident events/day with a signal-to-noise * Supported in part by the National Science Foundation [GP-28027].
ratio of 1 in the neutrino flux from the Savannah River Reactor. Using the value of B predicted by Wildenthal, the count rate would decrease to 2 × 10 -6 coincident events/day and a signal-to-noise ratio of 3 X 10 - 6 , making the experiment unfeasible. In view of these drastically different predictions, the Nilsson model predicting a typical allowed strength, and the many-particle shell model predicting a branch several orders of magnitude weaker, we report here an attempt to measure the strength of the/3 branch to the 1.55 MeV level. A thick target o f P b F 2 was bombarded by 11 MeV protons from the CIT-ONR tandem accelerator. The target was retained inside a beryllium rabbit by a tantalum foil and was transported by an air shuttle after bombardment to a remote 73 cm 3 Ge(Li) detector heavily shielded with lead. The rabbit was separated from the detector by 2 cm of lucite in order to stop the emitted positrons with a minimum of bremsstrahlung, and by 5 cm of lead to attenuate the 511 keV annihilation radiation much more severely than the 1357.0 +--0.2 keV "y-ray resulting from the decay of the 1.55 MeV level. Two successive counting periods of 17.4 s (the half-life of 19Ne) were used to verify the lifetimes of the observed ,y-rays. After the counting periods, the rabbit was transferred back for another bombardment. The relevant portion of the spectrum for the first 17.4 s counting period is shown in the figure. The 511 keV annihilation peak stands out clearly. The peak at 1291 keV is the double-escape peak of the 2313 keV ,y ray following the decay of 140 produced from 14N(p, n). The photopeak at 1434 keV is from 52Mn* decays produced from 52Cr(p, n). The nitrogen and chromium contaminants result from parts of the rabbit system. 49
Volume 51B, number 1
PHYSICS LETTERS
8 July 1974
511
1291
40K 1200 30 K 800
~ Z 20K 0
400
IOK
r~-
460
-a
530
¥ RAY
ENERGY (keY)
Fig. 1. Partial "/-ray spectrum for 19Ne03+)lgF. The peaks at 1291 and 1434 keV are from contaminant decays. The expected decay to the 1554 keV level would produce a 1357 keV photopeak at the arrow. Using detector efficiencies determined by use of 22 Na and 56Co calibration sources, an upper limit (2o confidence level) for B is 3 X 1 0 - 5 , implying a l o g f t greater than 5.6. For comparison, if the 1357keV peak were as large as the 1434 keV photopeak shown in the figure, t h e n B would equal (1.1 + 0.2) X 10 - 4 . Further work is planned to increase the experimental sensitivity. Using the present experimental limit for B, and retaining the other arguments of Donnelly et al., the expected coincident counting rate for 19F(~-e,~e)
50
19F*(77) 19F would be less than 0.2/day with a signal-to-noise ratio less than 0.3.
References [1] T.W. Donnelly, et al. Phys. Lett. 49B (1974) 8. [2] H.G. Benson and B.H. Flowers, Nucl. Phys. A126 (1969) 305. [3] Cited in W.A. Langford and B.H. Wildenthal, Phys. Rev. C7 (1973) 688.