Some recent experiments with a collimated monoenergetic fast neutron beam

A collimated monoenergetic neutron beam was employed in the energy range 2-10 MeV to measure absolute cross sections for the following reactions: 40Ca(n,p), 40Ca(n,α), 40Ca(n,n'); 23Na(n,p), 23Na(n,α) using a CaF2(Eu) and a NaI(Tl) scintillator, respectively. A neutron beam with the stated properties is produced using the associated particle method. Neutrons from the 2H(d,n)3He reaction are detected in coincidence with the associated 3He ions. The method has several advantages which are discussed. In the experiment with the NaI(Tl) scintillator the (n,p) and (n,α) reactions were distinguished with pulse-shape discrimination. A new method of measuring the lifetime of 40Ca(3.35) developed in these experiments yield a value for the monopole matrix element of 2.6 +- 0.1 fm2. Qualitatively, the excitation functions follow the predictions of the statistical model. The initial rapid increase of the cross section with increasing energy is followed by a reduced cross section. Thus the increased penetrability of the intermediate-compound-nucleus formed in the reaction competes with the increasing number of exit channels. (author)