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The Effect of High Dose Rate on Tissue Equivalent Proportional Counter Measurements in Mixed Neutron-Gamma Fields

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The Effect of High Dose Rate on Tissue Equivalent Proportional Counter Measurements in Mixed Neutron-Gamma Fields The Effect of High Dose Rate on Tissue Equivalent Proportional Counter Measurements in Mixed Neutron-Gamma Fields by Nael Qashua A Thesis Submitted in Partial Fulfillment Of the Requirements of the Degree of Master of Applied Science in Nuclear Engineering Faculty of Energy Systems and Nuclear Science University of Ontario Institute of Technology April 2010 © Nael Qashua, 2010 Abstract Tissue equivalent proportional counters (TEPCs) are commonly used for radiation monitoring in areas where a mixture of neutron and photon radiations may be present, such as those commonly encountered in nuclear power plants. In such radiation fields, the dose rate of each component can vary drastically from extremely low to very high. Among these possible combinations of radiation fields with very different dose rates, a mixed field of an intense photon and a weak neutron dose component is the more commonly encountered. This study describes the measurement of lineal energy spectra carried out with a 5.1 cm (2 inch) diameter spherical TEPC simulating a 2 µm diameter tissue site in low energy (33 – 330 keV neutrons) mixed photon-neutron fields with varying dose rates generated by the McMaster University 1.25 MV double stage Tandetron accelerator. The Tandetron accelerator facility was employed to produce neutrons using thick 7Li targets via the 7Li(p, n)7Be reaction. A continuous spectrum of neutrons is generated at any selected proton beam energy which is very narrow at beam energies very close to the threshold of the reaction 1.88 MeV and becomes wider as the proton beam energy moves further away from the threshold energy of the reaction. Dose rates which resulted in dead times as high as 75% for the data acquisition system were employed to study the effect of dose rate on the measured quality factors, microdosimetric averages ( and ) absorbed dose and dose equivalent. The dose rate at a given beam energy was varied by changing the accelerator beam current. A variety of mixed neutron gamma fields was generated using neutron beams with mean energies extending approximately from 33 keV to 330 keV ii with the 7Li target using proton beam energies ranging from 1.89 to 2.5 MeV. In direct beams, 478 keV photons which are produced in the 7Li target via inelastic scattering interaction 7Li(p, pγ)7Li dominate the low LET component of the mixed field of radiation. When a 2 cm thick polyethylene moderator was inserted between the neutron producing target and the counter, the low LET component of the mixed radiation field also contained 2.20 MeV gamma rays originating from 1H(n, γ)2H capture interactions in the moderator. We have observed that high dose rates due to both photons and neutrons in a mixed field of radiation result in pile up of pulses and distort the lineal energy spectrum measured under these conditions. The pile up effect and hence the distortion in the lineal energy spectrum becomes prominent with dose rates which result in dead times larger than 25% for the high LET radiation component. In intense neutron fields, which may amount to 75% dead time, a 50% or even larger increase in values for the measured microsdosimetric averages and the neutron quality factor was observed. This study demonstrates that moderate dose rates which do not result in dead times of more than 20- 25% due to either of the component radiations or due to both components of mixed field radiation generate results which are acceptable for operational health physics mixed neutron-gamma radiation monitoring using tissue equivalent proportional counters. iii Acknowledgments I would like to express my sincere gratitude and appreciation to my supervisor; Dr. A. Waker for the unique opportunity he gave me to work in this area of research, his support, guidance throughout my work. During my time at McMaster University I have received help from numerous people. I am very grateful for the assistance and advice in all experiments I receive from Dr. I. Aslam. I‟d like to thank S. McMaster, manager of McMaster University Accelerator Laboratory for helping me with many details. Thanks to Justin for turning the Tandetron Accelerator beam so many times. I owe a special note of gratitude to Dr. R. Machrafi for assisting me with miscellaneous tasks in fundamentals of radiation techniques. On a personal note, I thank my, loving wife and kids, Rihaf, Zein and Lauren, to whom this work is dedicated, for their encouragement and for complaining so very little about my absence. My parents, Mahmud and Rafika, I thank for helping me in so many ways. I appreciate the financial support of this research through University Network of Excellence in Nuclear Engineering UNENE industrial research. iv Table of Contents Introduction ......................................................................................................................... 1 1.1 Thesis Objectives ................................................................................................. 2 1.2 Previous Studies with TEPCs in Radiation Protection Dosimetry ....................... 3 1.3 Outline of Investigation and Thesis ..................................................................... 4 Background to Theory and Methods................................................................................... 5 2.1 Mixed Field Neutron-Gamma Radiation Dosimetry ............................................ 5 2.2 Radiation Protection Dosimetry ........................................................................... 7 2.2.1 Ambient Dose Equivalent ............................................................................. 8 2.2.2 Quality Factor and Linear Energy Transfer .................................................. 8 2.3 Proportional Counters ........................................................................................ 10 2.3.1 Tissue Equivalence ..................................................................................... 13 2.3.2 Simulation of Unit Density of Microdosimetric Tissue Equivalent Volumes 14 2.4 Determination of Ambient Dose Equivalent with Tissue Equivalent Proportional Counter .......................................................................................................................... 16 2.4.1 Determination of Absorbed Dose ............................................................... 20 2.4.2 Determination of Quality Factor ................................................................. 21 2.4.3 Derivation of Dose Equivalent .................................................................... 22 2.4.4 Moments of Single Event Lineal Energy Distribtion ................................. 23 2.4.5 Derivation of Statistical Uncertainty .......................................................... 24 Materials and Methods ...................................................................................................... 25 3.1 Experiment Set-Up ............................................................................................. 25 3.2 Principle of TEPC Calibration ........................................................................... 27 3.3 Accelerator Based Neutron Source .................................................................... 31 Results and Discussion ..................................................................................................... 33 4.1 Precision of TEPC Measurements ..................................................................... 33 4.1.1 Precision of TEPC Measurements for Given Operating Parameters Ep and Ip 34 4.1.2 Precision of TEPC Measurements Due to Varying Ep at a given Ip ........... 37 v 4.1.3 Precision of TEPC Measurements Due to Varying Ip at a given Ep ........... 42 4.2 Effect of High Dose Rate on TEPC Microdosimetric Measurements in Mixed Photon and Neutron Fields ............................................................................................ 46 4.2.1 Dose Rates for Mixed Field Photons and Neutrons from Accelerator Based Neutron Source .......................................................................................................... 46 4.2.2 TEPC Measurements in an Intense Mixed Photon and Neutron Field ....... 48 4.2.3 TEPC Measurements in Intense Photon Field and Weak Neutron Field .... 59 4.2.4 Influence of Pile up Rejection Circuit on TEPC Measurements ................ 62 4.3 Possible Alternative Strategy to Improve Response of TEPCs in High Dose Rate Environment .................................................................................................................. 63 Conclusions and Future Work .......................................................................................... 66 5.1 Conclusions ........................................................................................................ 67 5.2 Recommendation for Future Plans ..................................................................... 68 vi List of Tables Table 1 Reproducibility of 5.1 cm (2 inch) TEPC measurements at Ep = 1.89 MeV and Ip = 400 µA with dose and dose rates normalized to beam current of 100 μA………………………….…………………………………..……36 Table 2 Reproducibility of 2 inch (5.1 cm) TEPC measurements at Ep = 1.89 MeV and Ip = 400 µA with dose and dose equivalent rates normalized to beam current of 100 μA………………………………………………………...39 Table 3 Reproducibility of 2 inch (5.1 cm) TEPC measurements at Ep = 1.90 MeV and Ip = 400 µA with dose and dose equivalent rates normalized to beam current of 100 μA……………………………………………...…………40 Table 4 Reproducibility of 2 inch (5.1cm) TEPC measurements at Ep =
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