Measurement of 107Ag(alpha,gamma)111In cross sections
Lawrence Berkeley National Laboratory(2004)
摘要
Measurement of 107 Ag(α,γ) 111 In Cross Sections Coral M. Baglin, Eric B. Norman # , Ruth-Mary Larimer ∗ , Gregory A. Rech Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA Abstract. Cross sections have been measured for the 107 Ag(α,γ) 111 In reaction at several α-particle energies between 7.8 MeV and 11.9 MeV. This reaction is of interest because it can provide a check on calculations of low-energy (α,γ) cross sections required for stellar nucleosynthesis predictions. Stacks of natural Ag foils of 1 μm thickness and 99.97% purity were bombarded with 4 He + beams. Following irradiation, the yields of the 171-keV and 245-keV photons produced in the 2.805 day electron-capture decay of the 111 In product nucleus were measured off-line. The Ag foils were interleaved with 99.6% purity, 6 μm thick natural Ti foils so that known cross sections for the 48 Ti(α,n) reaction could be used to check the accuracy of the beam current integration. For any given beam energy, beam energy degradation in the foils resulted in lower effective bombarding energies for successive foils in the stack, enabling measurements to be made for several energies per irradiation. The measured cross sections are compared with published statistical-model calculations. INTRODUCTION Surprisingly few measurements of (α,γ) cross sections exist at energies near the Gamow window for p process nuclei. A measurement several years ago by Somorjai et al. [1] of 144 Sm(α,γ) cross sections yielded values which were almost an order of magnitude lower than expected from statistical model predictions on which stellar nucleosynthesis calculations rely heavily. More recently, measurements of 96 Ru(α,γ) cross sections [3] gave values roughly half those expected from statistical model calculations performed using the recent NON-SMOKER code [2]. Hopefully, measurements of (α,γ) cross sections for additional nuclides in this mass region may establish trends and indicate whether refinements of the theoretical treatment of the α channel in the NON-SMOKER calculations are called for. EXPERIMENTAL DETAILS Beam 100 nA to 1 μA beams of 4 He + ions from the Lawrence Berkeley National Laboratory’s 88” cyclotron were used to bombard stacks of Ag and Ti foils. Each stack of foils was mounted on a water- cooled, electrically-insulated Cu Faraday cup with magnetic electron suppression just upstream of the targets. Irradiation times ranged from 1 hour to 4 hours and charge integrations varied from 600 μC to 6600 μC. The extracted beam energies used were 8.5 MeV, 8.63 MeV, 9.0 MeV, 11.05 MeV and 12.0 MeV. However, after energy degradation in the foils, the energies at the center of the Ag foils ranged from 7.8 MeV to 11.9 MeV and those at the center of the Ti foils ranged from 6.6 MeV to 11.16 MeV. Present Address: Lawrence Livermore National Laboratory Deceased
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statistical model,physics,cross section
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