Dosimetry of a nearly monoenergetic 6 to 7 MeV photon source by NaI(Tl) scintillation spectrometry

Abstract The dosimetry of a nearly-monoenergetic 6–7 MeV photon source developed at the National Bureau of Standards (NBS) for radiation protection instrument calibration has been carried out by NaI(Tl) scintillation spectrometry. This approach uses calculated 3 in. × 3 in. NaI(Tl) detector-response functions that have been shown to be reasonably accurate up to 20 MeV. A least-squares fit of the appropriate response functions to a selected region of the pulse-height distribution determines the primary 6–7 MeV photon fluence. The uncertainty in the fluence determination is based on the χ 2 of the fit, the statistics of the data, and the uncertainty in the response functions. The air kerma delivery due to the primary photons at a reference point in the photon field was calculated from the primary photon fluence. The uncertainty in the determination of air kerma delivery for primary photons was less than 5% (1 std. dev.). The primary high-energy photon contribution to the detector response was subtracted from the data and the remaining distribution due to lower-energy photons was evaluated by spectrum unfolding analyses. The spectrum-unfolded results indicate that a contribution of approximately 12% of the total air kerma was mostly from a continuous distribution of photons extending up to 4.5 MeV.