Towards a Flexible Monte Carlo Calibration of a Whole Body Counter Spectrometer System - Highlighting the Need of Increased Model Complexity for Large Plastic Scintillators

Sammanfattning: Whole body counters, WBC, can be used for fast and accurate screening and follow-ups in the case of an emergency situation involving ionizing radiation given that the WBC calibration method is fast and versatile. A Monte Carlo model of a WBC system could meet these requirements under the condition that the model is an accurate description of the WBC system. The aim of the licentiate thesis was to develop and verify a Monte Carlo model, using the code MCNPX 2.6.0, of a WBC spectrometer system consisting of four large plastic scintillators (NE 102A, each measuring 91.5 × 76.0 × 24.5 cm3), in a nearly 3π geometry. The model was verified by comparing the calculated and measured total efficiency for nuclides in the energy range 662-1333 keV. This was done for 77 source positions per nuclide. For each source position additional calculations were performed to investigate the effect of an erroneous source placement where the source was placed ±2 cm from the correct position. The measured spectra had a very dominating and fluctuating noise in lower channels, which therefore had to be discriminated and a method for implementing this in the calculations is presented. The effect on energy resolution due to non-linear light yield and light losses was investigated. MCNPX was used to determine Birks’ constant !", which was then used to investigate the non-linear light yield using Birks’ equation. The effect of light losses was observed by comparing calculated energy deposition pulse height spectra with measured spectra for two different source positions; if no light losses occurred two nearly identical calculated spectra for two positions would lead to two nearly identical measured spectra. The results showed a good agreement between calculated and measured total efficiencies except close to the detector edges and for cascade-emitting nuclides due to true coincidence summing effects. The calculations with erroneous source placements revealed that the deviations from calculated and measured results could not solely be explained by an erroneous source position. Birks’ constant was found to be 9.6 mg·cm-2 MeV-1 and no effect on energy resolution due to light yield non-linearities was observed. The effects of light losses were observed since the energy resolution in measured spectra showed a source position dependence. It was concluded that the Monte Carl model is sufficient but would benefit from a more complex model description where processes, such as light transport, occurring after energy deposition are implemented.

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