The Measurement of Neutron Source Terms Using the Nested Neutron Spectrometer

The Canadian Nuclear Safety Commission (CNSC) regulates many different types of accelerator facilities across Canada. To fulfil its mandate, the CNSC evaluates the shielding design of proposed facilities prior to issuing a licence to construct each facility. For complex facilities, Monte Carlo simulations are used to validate the applicant's shielding design. Once facilities become operational, inspections are performed to verify compliance with applicable CNSC regulatory safety requirements. These inspections include verification of both photon and neutron dose rates in the facility. The neutron source term has a major impact on the shielding design and resulting dose rates, but often this information is not available in any of the literature for a particular type of accelerator, so on-site measurements of the spectrum may be required. Traditionally, Bonner Spheres Systems (BSS) with activation foils and TLDs are used for this type of measurement. Recently, the CNSC has tested and used a new instrument called the Nested Neutron Spectrometer (NNS). It operates under the same principle as a Bonner Sphere Spectrometer; that is, the spectral information is based on the neutron energy response of a thermal neutron detector at the centre of moderating bodies. However, the NNS has modular cylindrical shells which make it lighter than the BSS, and easier to use in the field. The NSS can also be operated in ion chamber mode in order to measure high intensity neutron fluence rates, such as those found inside the vaults of accelerators. Finally, the NNS is accompanied by a user-friendly WindowsTM-based unfolding software. The results can be obtained in a very short period of time (within an hour) and immediately used in Monte Carlo simulations, benchmarking is in a very well agreement with the doses around the facilities, NNS is versatile and a useful tool in the assessment of neutron shielding facilities. (author)