Experiments carried out on the detection of slow neutrons from the ($\alpha , n$) reaction in ${\mathrm{Be}}^9$ show an increasing number of low-energy neutrons beginning at an alpha particle energy of 4.3 Mev, and extending to 5.6 Mev. The energies and angular distribution of these neutrons for $E_{\alpha }=4.92\mathrm{} \mathrm{and} 5.11$ Mev have been measured using the polyethylene sphere-moderated neutron spectrometer. The results indicate a peaking in the forward direction and an energy distribution consistent with their origin from inelastic alpha-particle scattering leaving ${\mathrm{Be}}^9$ excited to the level at 1.75 Mev, which subsequently breaks up with neutron emission. At an alpha-particle energy of 5.11 Mev, the mean energy of these neutrons and the differential cross sections are: 0°(0.33 Mev, 22 mb/sr); 70°(0.21 Mev, 8 mb/sr); and 120°(0.07 Mev, 2 mb/sr). The neutron spectrum of a Pu-Be source has been determined to have a large number of low-energy neutrons with a peak intensity at 0.3 Mev.

• Received 18 December 1961

DOI:https://doi.org/10.1103/PhysRev.126.1794