Thesis Abstracts 2003
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Simulation of the Response Function of Bubble Detectors by Monte Carlo Methods
By: M. Pierre (Partial Fulfilment of the Requirements for the Degree of Doctor of Philosophy in Nuclear Engineering)
Supervisors: Dr. B.J. Lewis and Dr. L.G.I. Bennett
Abstract
As part of the National Aeronautic and Space Administration (NASA) Atmospheric Ionizing Radiation (AIR) Mission, the Royal Military College of Canada was involved in measuring the cosmic radiation at high altitude using bubble detectors, manufactured by Bubble Technology Industries. In order to calibrate these detectors for the high-altitude cosmic radiation field, an independent Monte Carlo transport computer code for Bubble Detectors (MCBD) was developed to simulate the fluence response R for neutron energies ranging from 100 keV to 150 MeV. The simulation of neutron interactions with the detector was described by transport theory, so that the energy loss could be evaluated.
The transport equation was solved with the development of an independent Monte Carlo treatment, which tracked the neutrons throughout the detector. The Monte Carlo treatment was implemented as an Object Oriented Program. The subsequent recoil energy of the ions in the superheated fluid was recorded and R was calculated based on the number of ions having a sufficient threshold energy (determined to be at least 0.028 MeV).
In order to verify the MCBD program, transport calculations were performed and compared to MCNP version 4B, where the two approaches proved to be in good agreement. The R function, as predicted by MCBD, also compared well with earlier accelerator experiments for the detector using mono-directional and mono-energetic beams.
The predicted R function was used with neutron spectra collected at commercial aircraft altitudes (Goldhagen) to calculate a new dose equivalent response RH, for the bubble detector. Again, this calibration factor compared well with a previously-proposed experimental evaluation, in support of the validation of the MCBD code.
In particular, the predicted response function was used to provide a new bubble detector dose equivalent response for the high-altitude AIR experiments. The neutron dose equivalent for these scientific flights was evaluated using this new calibration factor. A comparison was also made to a previous analysis (i.e., using an old calibration factor) where the difference was less than 20%. Although direct comparison with the results obtained by other detectors can not be made, the trend indicated by the bubble detectors proved to be in agreement with all other detectors.
