The Energy Spectrum of the 350 MeV Neutron Beam from the Liverpool Synchrocyclotron

A knowledge of the energy spectrum of the neutron beam from the 156-inch cyclotron was required for cross section measurements. The spectrum of the recoil protons produced in n-p collisions was measured with a counter telescope containing a copper absorber of variable thickness. A calibration run was made in the external proton beam to measure the transmission of the absorber as a function of proton energy. It was found that the shape of the spectrum was consistent with the assumption that the neutrons are produced in `elastic' proton-neutron collisions in the beryllium target. The shape is then determined by the amplitude of radial oscillations in the cyclotron and the energy lost by the protons in their multiple traversals of the target.