Absolute neutron flux determination in fast neutron spectra

A flux detector system was developed in order to determine the fast neutron flux between 1 and 30 MeV obtained with the Karlsruhe isochronous cyclotron. The counter system represents a telescope-like proton recoil device using solid radiators and gas scintillation transmission counters. Flux determination is accomplished separately between about 1 to 6 MeV and approximately 5 to 30 MeV, with a small energy interval of overlap. Below 6 MeV, recoil protons are detected in a single gas scintillation chamber viewed by three photomultipliers requiring a fast coincidence. Above this range, high energy recoil protons are identified by coincidences in three adjacent chambers and by their specific energy losses. Since the transmission of the entire flux counter system is better than 99 percent, it can be used for simultaneous flux measurements in partial cross section determination. The accuracy for the determination of the neutron flux is about 5 percent