Photofission experiments on the HERMES-III generator

The HERMES-III generator is used for research in photofission and induced background. This work follows experiments using the 8-MV, 200 kA, 40 ns bremsstrahlung pulse from the Mercury generator to induce photofission in a variety of targets.² HERMES is operated in two modes: half-machine at 8 MV and 300 kA, similar to Mercury, and full-machine at 16 MV and 600 kA. The objective of the research is to understand radiation background in the environment as the bremsstrahlung endpoint is increased. Standard electrical and X-ray diagnostics are used to characterize the bremsstrahlung source.The x-ray beam is apertured by a 46 cm square hole in a 25 cm thick steel panel, and by two 10 cm thick lead doors, located 12 m from the x- ray source. Targets are located 6 m beyond the aperture. Targets investigated include depleted uranium (DU), lead, borated polyethylene, steel and water.Neutrons are diagnosed using a rhodium counter (time integrated), two ³He tubes and two plastic scintillators coupled to photomultiplier tubes (time resolved). The detectors are located varying distances from the target. More neutron pulses are recorded from the DU target than from the lead target, possibly evidence of photofission. Far more neutrons are detected in the 16-MV mode than the 8- MV mode, as expected. Many pulses are recorded for about 20 ms from all target types. The neutron signals persist for several seconds from the DU target, evidence of delayed neutrons from photofission.