Fast neutron capture with a white neutron source

A system has been developed at the Los Alamos National Laboratory to measure gamma‐rays following fast neutron reactions. The neutron beam is produced by bombarding a thick tantalum target with the 800 MeV proton beam from the LAMPF accelerator. Incident neutron energies, from 1 to over 200 MeV, are determined by their times of flight over a 7.6‐m flight path. The gamma‐rays are detected in five 7.6×7.6‐cm cylindrical bismuth germanate (BGO) detectors which span an angular range from 45° to 145° in the reaction plane. With this system it is possible to simultaneously measure the cross section and angular distribution of gamma‐rays as a function of neutron energy. The results for the cross section of the 12C(n,n’γ=4.44 MeV) reaction at 90° and 125° show good agreement with previous measurements, while the complete angular distributions show the need for a large a4 coefficient which was not previously observed. Preliminary results for the 12C(n,n’γ=15.1 MeV) reaction have also been obtained. The data obtain...