Personnel Neutron Dosimeter Developments

Personnel neutron dosimeter studies at Battelle- Northwest have developed two approaches which provide substantially improved performance compared with the commonly used NTA emulsion dosimeters. A third approach utilizing thermoluminescent dosimeters has not met with equal success. Plastics, glasses, and minerals used in conjunction with fissionable foils provide the basis for a full energy-spectrum neutron dosimeter. Theoretical and experimental studies indicate that fast neutron doses as low as 10 mrads and thermal neutron doses of 0.1 mrads can be measured. These solid-state nuclear-track detectors were fabricated with fission foils of metallic 235U, 20-fold depleted 238U, and neptunium oxide. Exposure studies of Ra-Be, Pu-Be and PuF4 neutron sources and monoenergetic fast neutrons obtained from a 2-MeV positive-ion accelerator characterized the energy and dose response of this device. Comparison studies with solid-state nuclear-track detectors and NTA emulsions were made in both laboratory and field exposure situations. The solid-state nuclear-track detectors permitted dose evaluation in several neutron energy groups in mixed gamma and neutron radiation fields. This new neutron dosimeter principle used with a single 237Np foil will provide substantially improved neutron dosimetry in mixed radiation fields compared with the commonly used NTA emulsion dosimeters. An activation foil principle is also being developed for application as a full spectrum dosimeter. This dosimeter uses the human body as a moderator for incident fast neutrons and scatters some of the resulting thermal neutrons back to a rhodium activation foil. Radioactive decay of the rhodium foil exposes a normal beta-gamma film packet placed next to the foil in the dosimeter. With an appropriate filter system design, photon radiation interference is eliminated and the neutron-produced film darkening can, by calibration, be related to the incident fast-neutron dose. Laboratory and field tests of neutron dosimeters of this type provided a detection limit of about 10 mrads and demonstrated the ability to measure neutron dose in the presence of gamma and X-radiations. These dosimeter films may be mechanically read by conventional beta-gamma film densitometers. (author)