Neutron Cross Section Measurement Using the Associated Particle Technique

The associated particle technique is applied to the D(d.n) He3 reaction, in order to produce a tagged neutron beam of accurately known energy, flux, and direction. The incident deuteron beam is obtained from a 400 Kv positive ion Van de Graaff accelerator. A description is given of the design of a uniform field sector magnet and other equipment associated with the stabilization and calibration of the energy of the incident deuteron beam. A versatile n-He3 coincidence system is described. The use of a silicon surface barrier detector with a thin nickel foil window enables complete resolution of the He3 peak with consequent improved neutron flux determination. The tagged neutron beam is used to measure the absolute neutron cross sections of the K39 (n,p) A39 and K39 (n, alpha) Cl36 reactions at a neutron energy of 2.46 Mev. The results obtained, (95 plus-minus 4) mb and (6.2 plus-minus 1) mb respectively, are compared with values obtained by other workers, and with theoretical predictions.