published_or_final_version / Physics / Master / Master of Philosophy

Thesis--M. Phil., University of Hong Kong.

Thesis (M.S.)--University of Michigan, 1977. / Project completed January 1978. Degree awarded December 1977.

Thesis (M.S.)--University of Michigan, 1961. / Degree awarded in Aug. 1961. Project completed in July 1962.

Thesis (Ph.D)--Mechanical Engineering, Georgia Institute of Technology, 2010. / Committee Co-Chair: Hertel, Nolan; Committee Co-Chair: Kahn, Bernd; Committee Member: Kulp, David; Committee Member: Lee, Eva; Committee Member: Wang, Chris. Part of the SMARTech Electronic Thesis and Dissertation Collection.

The method of measuring the energy of a charged particle by means of the total amount of ionisation it produces in a gas, through which it passes, is well established. Gas filled devices may be subdivided into (a) current ionisation chambers and (b) pulse ionisation chambers, proportional counters and Geiger counters. Proportional counters in a very crude form were used by Rutherford and Geiger in 1908. Soon afterwards the Geiger counter was developed, which after considerable improvement by Geiger and Müller in 1928, became the well known Geiger-Müller counter. In fact the three gas filled devices mentioned in (b) can be achieved with the same counter by using it with different applied voltages. If the applied voltage on a typical cylindrical counter is varied and the output pulse height is measured for a fixed number of ion pairs liberated in the gas, a characteristic curve of the form shown in Fig.(1) will be obtained. With voltages between say v₁ and v₂, the field strength is just sufficient to collect all the ion-pairs and no new ion-pairs are liberated, so the pulse height will remain constant. This is the region of pulse ionization chambers where the gas gain is unity. As the voltage is further increased to between v₂ and v₃ both elastic and inelastic collisions take place and the pulse height increases exponentially.

Tissue equivalent proportional counters are frequently used to measure dose and dose equivalent in mixed radiation fields that include neutrons; however, detectors simulating sites 1?m in diameter underestimate the quality factor, Q, for low energy neutrons because the recoil protons do not cross the detectors. Proportional counters simulating different site-sizes can be used to get a better neutron dose equivalent measurement since the range and stopping power of protons generated by neutrons in the tissue-equivalent walls depend on the energy of the primary neutrons. The differences in the spectra measured by different size detectors will provide additional information on the incident neutron energy. Monte Carlo N-particle extended (MCNPX) code was used to simulate neutron transportation in proportional counters of different simulated tissue diameter. These Monte Carlo results were tested using two solid walled tissue equivalent proportional counters, 2mm and 10mm in diameter, simulating tissue volumes 0.1?m and 0.5?m in diameter, housed in a single vacuum chamber. Both detectors are built with 3mm thick tissue equivalent plastic (A-150) walls and propane gas inside for dose measurement. Using these two detectors, the spectra were compared to determine the underestimation of y for large detector, and thereby obtain more information of the incident neutron particles. Based on the MCNPX simulation and experimental results, we can see that the smaller detector produces a larger average lineal energy than the larger detector, which means the larger detector (0.5?m diameter tissue equivalent size) underestimates the Q value for the low energy neutron, therefore underestimates the effective dose. These results confirm the results of the typical analysis of lineal energy as a function of site size.

This thesis presents an independent analysis of the data from 3-He-filled proportional counters from the third phase of the Sudbury Neutrino Observatory (SNO) data. These counters were deployed in SNO's heavy water to independently detect neutrons produced by the neutral current interaction of 8-B solar neutrinos with deuterium. Previously published results from this phase were based on a spectral analysis of the energy deposited in the proportional counters. The work in this thesis introduces a new observable based on the time-profile of the ionization in the counters. The inclusion of this observable in a maximum-likelihood fit increases the potential to distinguish neutrons from backgrounds which are primarily due to alpha-decays. The combination of this new observable with the energy deposited in the counters results in a more accurate determination of the number of neutrons. The analysis presented in this thesis was limited to one third of the data from the proportional counters, uniformly distributed in time. This limitation was imposed to reconcile different time-lines between the submission of this thesis, a thorough review of this work by the SNO Collaboration and results from an independent analysis that is still underway. Analysis of this reduced data set determined that 398 +/- 29 (stat.) +/- 9 (sys.) neutrons were detected in this reduced data-set. The number compares well to the previous analysis of the data, based only on a spectral analysis of the deposited energy, which determined that 410 +/- 44 (stat.) +/- 9 (sys.) were detected in the same time period. The analysis presented here has led to a substantial increase in the statistical accuracy. Assuming that the statistical accuracy will increase when the full data set is analyzed, the results from this thesis would bring the uncertainty in the 8-B solar neutrino flux to down 6.8% from 8.5% in the previously published results. The work from the thesis is intended to be included in a future analysis of the SNO data and will result in a more accurate measurement of the total flux of solar neutrinos from 8-B as well as reduce the uncertainty in the $\theta_{12}$ neutrino oscillation mixing angle. / Thesis (Ph.D, Physics, Engineering Physics and Astronomy) -- Queen's University, 2009-09-16 15:56:28.195

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