Validation and uncertainty of DRF's for a 1"×2" NaI collimated detector for radioisotope holdup measurements
Detector response functions (DRFs) have become the subject of increasing scientific interest for the last thirty years in several industrial applications of radiation detection. These applications include gamma-ray, prompt gamma-ray, and X-ray spectrometry for elemental analysis and location as applied to mining, radio-tracing in medicine, and holdup source characterization. However, it is difficult to create a rigorous mathematical formulation of the DRF. Therefore, semi-empirical and stochastic approaches have been developed instead to construct detector and application specific DRFs. A DRF can be considered to be a function that converts the energy-dependent incidence of incoming source particles onto a detector into a detector response spectrum that simulates the experimental response. The DRF can also be used in the reverse sense in an inverse problem setting, as a step in the process of predicting the physical characteristics of an unknown source (e.g. the holdup problem and library spectra). Much of the recent work on DRFs has been performed by Gardner. He has developed accurate DRF models through semi-empirical curve fitting and Monte Carlo simulation. His DRFs for 3" X 3" NaI detectors have been developed with the experimental measurements taken by Heath and for 6"x6" bare (unshielded) NaI(Tl) detector data with single-energy sources centered on the detector face at a distance of 10 cm. There was agreement with the Heath benchmark detector measurements within two Poisson standard deviations of the measured data. DRFs have also been developed for other detectors such as high purity Germanium detectors and Si(Li) for the same geometries as NaI. Other than a few artifacts such as backscatter peaks, there was good agreement between simulated responses and the experimental data.