Automated construction of detector models for efficiency interpolation in gamma-ray spectrometry.
A method was developed for automated construction of detector models in gamma-ray spectrometry, which can be used in Monte Carlo calculations of efficiency calibration curves. Full-energy peak efficiencies were first measured for different gamma-ray energies and for a given sample-detector arrangement and then calculated by the Monte Carlo method. For these calculations a detector model was employed along with a computer algorithm, which seeks agreement between the experimental efficiencies and the calculated ones by automatically determining the parameters of the model. The resulting agreement of the calculated data with the experimental one was within the relative uncertainty of the latter (3-4%) and the parameters of the detector models obtained were close to the values specified by the manufacturers. The detector models thus constructed can then be used for interpolation of calibration curves.
MONTE CARLO SIMULATION OF A CLEARANCE BOX MONITOR USED FOR NUCLEAR POWER PLANT DECOMMISSIONING
When decommissioning a nuclear facility it is important to be able to estimate activity levels of potentially radioactive samples and compare with clearance values defined by regulatory authorities. This paper presents a method of calibrating a clearance box monitor based on practical experimental measurements and Monte Carlo simulations. Adjusting the simulation for experimental data obtained using a simple point source permits the computation of absolute calibration factors for more complex geometries with an accuracy of a bit more than 20%. The uncertainty of the calibration factor can be improved to about 10% when the simulation is used relatively, in direct comparison with a measurement performed in the same geometry but with another nuclide. The simulation can also be used to validate the experimental calibration procedure when the sample is supposed to be homogeneous but the calibration factor is derived from a plate phantom. For more realistic geometries, like a small gravel dumpster, Monte Carlo simulation shows that the calibration factor obtained with a larger homogeneous phantom is correct within about 20%, if sample density is taken as the influencing parameter. Finally, simulation can be used to estimate the effect of a contamination hotspot. The research supporting this paper shows that activity could be largely underestimated in the event of a centrally-located hotspot and overestimated for a peripherally-located hotspot if the sample is assumed to be homogeneously contaminated. This demonstrates the usefulness of being able to complement experimental methods with Monte Carlo simulations in order to estimate calibration factors that cannot be directly measured because of a lack of available material or specific geometries.
Monte-Carlo calculation of photofractions and intrinsic efficiencies of cylindrical NaI(Tl) scintillation detectors
Abstract This paper describes a Monte-Carlo calculation of efficiencies and photofractions of NaI(Tl) detectors for gamma-rays between 0.2 and 10.0 MeV. The crystals considered are right circular cylinders ranging in size from 3″ diameter × 2″ length to 5″ diameter × 6″ length. The results may be used to determine the optimum crystal dimensions for given crystal volume and gamma-ray energy. The influence of collimators of different spertures is investigated. Except two characteristic cases, however, results are given for uncollimated radiation only.