Characterization of neutron detectors in high gamma-ray environments

In pursuing its Safeguards obligations the IAEA measures fission neutrons in high gamma-radiation fields of spent fuel. In order to characterize IAEA-owned detectors the response of two neutron detectors (provided by IAEA) with high counting efficiency and low gamma sensitivity was investigated in neutron and gamma fields as well as in mixed neutron/gamma fields. Furthermore, the optimum moderator thickness (polyethylene moderator, PE) was calculated for maximum detection efficiency with response to fission spectrum neutrons. These calculations were performed using the MCNP code. The relevant data of the He-3 detectors (e.g. wall thickness, wall material, length etc.) were taken into account as well as two different source geometries: a point source and a line source. The gamma measurements were performed mainly with an intense Co-60 source. Neutrons were produced by Cf-252 and Am/Be isotopic sources and for even higher intensities with a sealed tube 14 MeV neutron generator. Pulse height spectra have been measured. Whereas at low gamma dose rates the multichannel pulse height spectrum shows a clear separation of the neutron and gamma ray signals, in high gamma radiation fields pile-up events may prevent a clear separation of the neutron events. For different gamma dose rates the quality of separation of neutron and gamma signals has been recorded. The results of the MCNP calculations for the PE moderator are compared with actual measurements. Results of the measurements of the characteristics of these He-3 neutron tubes and an evaluation regarding their application in high gamma-ray environments are discussed.