A detailed study of the fragments produced from helium when bombarded by 90-Mev neutrons was made in a 22-inch cloud chamber filled with helium gas and operated in a pulsed magnetic field of 22 000 gauss. The particles were identified by curvature and relative ionization, or by change of curvature with range when they stopped in the chamber. The energy of the incident neutron could be calculated from measured quantities for most modes of disintegration. The total number of events, for incident neutrons above 40 Mev, was normalized to the interpolated $n$-${\mathrm{He}}^4$ total cross section of 1.9×${10}^{-25\mathrm{}}$ ${\mathrm{cm}}^2$; thus absolute cross sections were established for the various disintegration processes. Energy dependences were compared and energy and angular distributions have been determined.

• Received 11 August 1952

DOI:https://doi.org/10.1103/PhysRev.89.508