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In radiological protection, models are used to assess radiation risk by means of extrapolation from high dose and dose rate to low dose and dose rate. In this thesis five main biophysical models of radiation action have been evaluated, appraised and inter-compared. The five models are lethal and potentially lethal (LPL) by Curtis, pairwise lesion interaction (PLI) by Harder, cellular track structure (CTS) by Katz, hit size effectiveness (HSE) by Bond and Varma and track core (TC) by Watt. Each model has been developed based on certain underlying mechanisms or phenomena, to permit interpretation and prediction on the induction of a specified biological endpoint such as cell reproductive death, chromosome aberrations and mutations. Biological systems of interest are, for example, mammalian cells containing deoxyribonucleic acid (DNA). Evidence is mounting that double strand breaks in the DNA are the critical lesions for various biological end points. To proceed with this work the TC model has been chosen. Cancer induction by ionising radiation is the stochastic effect of prime concern in radiological protection. Cancer induction cannot be avoided entirely but its frequency of occurrence may be reduced to acceptable level by lowering the amount of radiation received. The methods of assessment developed by ICRP, in terms of the cancer risk coefficients, are presented in this thesis. In the conventional (legal) system of dosimetry, radiation is quantified by the amount of energy absorbed per unit mass of tissue. Quality factors, superseded by radiation weighting factors, are needed to account for the quality dependence on radiation type. As an alternative, a new dosimetry system is proposed here which is based on the mean free path for primary ionisation along particle tracks and the integral fluence generated by the radiation field, whether directly or indirectly ionising radiation. From the study of cellular data, the mean free path for primary ionisation along particle tracks (lambda) emerges as a parameter which best unifies biological damage data. Radiation effect is found to depend, not on the energy transferred but to depend mainly on the frequency and spatial correlation of interactions. Maximum effect occurs when lambda is equal to lambda0 (2 nanometre, nm). The term 'Absolute Biological Effectiveness' (ABE) is introduced as a parameter which indicates the probability to induce a specified effect, per unit incident fluence. In this endeavour, only direct effects are considered in deriving ABE values for various radiations. However other factors such as indirect effects, inter-track action, repair processes and radiation rate, can be incorporated later if required, in the derivation of ABE. ABE values for photons up to 60Co i.e 1253 keV and neutrons up to 105 keV, have been calculated and presented in this thesis. An attempt has been made to re-express the cancer risk coefficients, derived by ICRP, in the new dosimetry system, in terms of the ABE (Absolute Biological Effectiveness). The hypothesis put forward in this thesis is that the induction of a specified biological-end-point in a biological system due to ionising radiations, is determined not by the amount of energy absorbed per unit mass (dose), but rather by the number of events (ionizations) spatially correlated, along the primary radiation track. Based on this hypothesis, a new unified dosimetry system, independent of radiation type, is proposed. Suggestions are made for possible measuring instruments which have the equivalent response characteristics, namely maximum efficiency of detection for the mean free path Success in devising such types of instrument would ensure the practicability of the new dosimetry system, in operational radiological protection.

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A research report submitted to the Faculty of Health Sciences, University of the Witwatersrand, Johannesburg, in partial fulfillment of the requirements for the degree of Master of Medicine in Diagnostic Radiology Johannesburg 2019 / Introduction: Neonates in a neonatal ICU or high care unit are a high-risk population. Besides a vulnerability to medical and surgical conditions, which often require radiological investigation, they are also at risk from the effects of radiation used in imaging. These risks increase with radiation dose. Numerous studies have assessed the dose due to primary radiation, however few have assessed the secondary radiation dose, and none have quantified the dose over time. Aim: To quantify the secondary radiation dose in our neonatal high care unit in order to determine if additional protective measures from secondary radiation are necessary. Method: A prospective analytic study was undertaken using multiple thermoluminescent devices in a cubicle of a neonatal high care unit, and control dosimeters outside the unit. Dosimeters were deployed for a 4 week period. Simultaneously, data was collected on patient numbers, and the X-rays performed in the unit. Results were compared to reference ranges for primary and secondary radiation (2-3 mSv per annum). Results: The average secondary radiation dose was 0.108mGy (p=0.6553) over 4 weeks, less than the expected background radiation dose of 0.17 – 0.25mGy. There was a large number of patients moving through the unit during the study period (89), with an average of 14 patients in the unit at a time, however this did not result a large number of X-ray exposures. Twenty one percent of patients were in the unit for less than a day, and 49 % were admitted for less than 3 days. Sixteen patients (18%) had X-ray investigations, with a total of 21 investigations and 30 exposures. Thirty percent of primary radiation dose was due to repeat exposures. Patients receiving X-rays had an average of 2 X-ray examinations (range: 1 to 4 studies) performed, with an average Entrance Skin Dose of 196.7µSv (0.197mGy) – range 77 to 554µSv (0.077mGy to 0.554mGy). There was no statistically significant difference between weeks or zones (p=0.1060 and p=0.8237 respectively), and differences in primary radiation doses was likely due to chance. Conclusion: Additional measures to protect patients in the unit from secondary radiation are unnecessary. There was a low probability of patients having a radiological investigation in the neonatal high care unit, and secondary radiation doses were not measurably higher than background radiation. / TL (2019)

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science. Johannesburg, 2013. / In this study a dose assessment is used to demonstrate conformance to national and international dose limits for workers and meets the Necsa ALARA goals for a radiological repair task. The dose assessment methodology is based on international standards, principles and criteria and involves the process of determining radiological dose, through the use of exposure scenarios, bioassay results, monitoring data, source term information, and pathway analysis. The radiological task is the replacement of the leaking steam coil on the radioactive effluent evaporator facility at Necsa. The effluent treatment facility, its operation, the origin of the radioactive effluent and hazards associated with the leaking coil are discussed. The dose assessment is supported by measurement of actual radiological conditions in the area where the task will be performed using suitable and calibrated instrumentation. The assumptions were limited to the physical phenomena associated with the behaviour of materials and available from national and international studies. The importance of proper planning of all the tasks associated with the replacement task as well as sources of inaccuracy and uncertainty associated with the calculated doses are discussed. The results of the assessment are evaluated in terms of ALARA, namely the safety fundamental principles of justification, optimisation and limitation of facilities and activities. Other dose reduction options, such as personal protective clothing and equipment, were considered to show that the doses conform to the ALARA objectives of Necsa and other operation optimisation measures.

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published_or_final_version / Radioisotope / Master / Master of Philosophy