A study of the Li 7(He 3, n)B 9 reaction by time-of-flight techniques

Abstract

The Li 7(He 3, n)B 9 reaction has been investigated at incident energies from 5.2 to 12.5 MeV, using a pulsed-beam time-of-flight spectrometer in conjunction with the ONR-CIT tandem accelerator. The spectrometer includes a pre-acceleration beam-bunching system, which increases the average target current by a factor of approximately 10 for protons and deuterons, and 4 for He 3 and alpha beams. Previously unreported B 9 levels were found at excitation energies of 12.06±0.06, 14.01±0.07, 14.670±0.016 and 16.024±0.025 MeV, with widths of 800±200, 390±110, <45 and 180±16 keV, respectively. At 12.5 MeV bombarding energy, covering the range of excitation of B 9 up to 17.9 MeV, two additional neutron groups correesponding to known B 9 states at 17.19 and 17.63 MeV were observed. The neutron groups were superimposed on an intense background continuum, attributable to processes leading to the final configuration 2α+p+n. An angular distribution, at 8.7 MeV incident energy, of the neutrons leading to the 14.67 MeV state is roughly isotropic in the centre-of-mass system, and the total cross section is 8 mb. A T = 3 2 assignment for the 14.67 MeV state is suggested by its narrow width, and by recent studies of the Li 7(t, p) 9 and Li 7(He 3, p)Be 9 reations. A comparison of the Li 7(He 3, p) Be 9 and Li 7(He 3, n)B 9 reactions shows that the cross sections for formation of the 14.67 MeV B 9 state and the analogous Be 9 state are approxinately equal, if barrier-penetration effects in the outgoing channels are removed.