Prompt neutron detection at Hermes-III

Summary form only given. Pulsed bremsstrahlung from the Hermes-III generator produces photofission for research in Intense Pulsed Active Detection (IPAD)². Hermes-III is operated in three modes producing bremsstrahlung with different endpoint energies: 8, 12, and 16 MeV, all with 30 ns x-ray pulse width. The electron-beam current increases with output voltage, from 300 kA at 8 MV to 600 kA at 16 MV. Previous experiments demonstrated detection of delayed neutrons from fissions induced in a depleted uranium (DU) plate. The work reported here focuses on detecting prompt fission neutrons, which are about 100 times more numerous than delayed neutrons. Neutrons are detected using ³He tubes surrounded by two different thicknesses of polyethylene (PE) to moderate the neutrons. A thin layer of cadmium absorbs neutrons with energy less than 1 eV. The “low-energy” detector has 2.5 cm PE and its response peaks at 6 keV. The “high-energy” detector has 12 cm PE and its response peaks at 1.6 MeV. MCNPX calculations for Hermes-III experiments at 8 MV indicate the high-energy detector is capable of detecting prompt fission neutrons (from DU) while effectively excluding photoneutrons produced in the environment or from a lead target. The detector electronics produce pulses that are counted as a function of time after a Hermes-III shot. Scattered x-rays make the detector signals useless for the first 10 microseconds, the time range when prompt neutrons first reach the detectors. But the thick PE disperses the prompt neutrons so they are recorded in the ³He gas at later times. Prompt neutrons are detected between 20 and 500 microseconds, in agreement with MCNPX predictions. When Hermes is operated in the 8-MV mode, the photoneutron signal from the background (no target) or a lead target is zero for times less than 500 microseconds. In the 12-MV mode, photoneutron signals are not negligible, but the presence of fission ne- trons can be detected by comparing pulse counts from the two detectors, essentially identifying higher energy neutrons than expected from nonfissionable materials. At 16 MV, photoneutron energies are comparable to the prompt fission neutron energies and detection of fission neutrons by this method is more difficult.