Abstract
Prompt gamma neutron activation analysis (PGNAA) is the most direct method of measuring total-body nitrogen. In combination with internal hydrogen standardisation, it is possible to reduce the dependence on body habitus. The uniformity of activation and detection, however, cannot be optimised sufficiently to eliminate the dependence entirely, and so further corrections are essential. The availability of the powerful Monte Carlo code MCNP(4A) has allowed a more accurate analysis of the activation facility, and yields corrections for body habitus and superficial fat layers. The accuracy of the correction is retained as the source-to-skin distance is reduced, although the activation uniformity is thereby degraded. This allows the use of a
252Cf source with lower activity and hence reduces the running cost of the facility.