CALIBRATION OF NEUTRON REFERENCE FIELDS

Neutron reference fields at the Los Alamos Central Health Physics Calibration Facility are traceable to the National Institute for Standards and Technology (NIST) through a primary calibration of the source emission rates. The source Q value allows for the calculation of the true fluence rate and dose equivalent rate under ideal conditions. In practice, corrections are required for the influence of factors such as room return, scatter from positioning tables and fixtures, source anisotropy, and spectral degradation due to source rabbits and guide tubes. At Los Alamos, Monte Carlo methods have been implemented to calculate the magnitude of these factors directly and independently of each other. Detailed models of the calibration facility, source capsule, and transfer systems are used in the calculations, which are based on facility and engineering drawings. Shadow cones are sometimes used to estimate the instrument response due to room return. However, the shadow cone technique cannot be used at all distances, may perturb the scattered neutron distribution, and does not fully account for scatter from positioning fixtures. Simulations have been performed for all of the standard fields: bare 252Cf, D2Omoderated 252Cf, 241AmBe, and polyethylene-moderated 252Cf. It is shown that room return may easily account for over 20% of the dose rate - even under nominally free-in-air conditions - and that positioning fixtures may be a significant scatter source term. In addition, the source anisotropy effect is as large as 7% in the case of a 10-curie 241AmBe source. All of these modifying factors have been taken into account in the determination of the true dose rate for instrument and dosimeter irradiations. Since detailed energy response functions are rarely available, it is usually not possible to back out the instrument or dosimeter response to the direct component of the field. The reported irradiation includes both the direct and scattered components of the field.