The ¹⁰B(n,0)⁷Li and ¹⁰B(n,1)⁷Li Alpha-Particle Angular Distributions for En < 1 MeV

The ¹⁰B(n,0)⁷Li and ¹⁰B(n,1)⁷Li angular distributions have been measured at the Geel Electron Linear Accelerator time-of-flight spectrometer in the incident neutron energy range from 0.1 keV to 1 MeV. A twin Frisch-grid ionization chamber has been used with two very thin 94% ¹⁰B-enriched samples mounted back-to-back on the common cathode. With this type of charged-particle detector, it is possible to measure the angular distribution of the alpha particles in a nearly 2x2 solid angle, with a clear separation of the alpha-particle yields from both reaction channels: -emission to the ⁷Li ground state {0} and to its first excited state {1}. Hence, for the first time nearly the full solid angle at all incident neutron energies investigated has been covered. A strong angular anisotropy was observed and is discussed in the frame of the compound nucleus reaction mechanism. The alpha-particle center-of-mass angular distributions have been used to calculate the branching ratio ¹⁰B(n,0)/¹⁰B(n,1). Both data sets had a strong impact at the International Atomic Energy Agency Coordinated Research Project “Improvement of the Standard Cross Sections for Light Elements” and the corresponding evaluation of the standards data file for this reaction.