Hot particle dosimetry using a wall-less tissue equivalent proportional counter
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Highly radioactive specks of irradiated nuclear fuel or activated material, colloquially known as 'hot particles', can enter the workplace or external environment from nuclear power plant operations such as refurbishment and decommissioning or from a severe nuclear accident. If deposited on the skin, ingested or inhaled, hot particles can pose a significant radiological hazard. Typically, hot particles are beta particle or alpha particle emitters and deposit their energy over very small distances in tissue making dose determinations through measurement or computation to the target tissues at risk such as epithelial cells in lung and gut and basal cells in skin difficult. Furthermore, current methods fail to determine changes that might occur in radiation quality as the charged particles lose energy and their stopping power increases with depth of penetration providing challenges in assessing the biological outcome of an exposure. In this work, an apparatus that is not commercially available was constructed so absorbed dose and quantities that can be used to quantify radiation quality could be measured as a function of tissue depth. The apparatus consisted of a custom fabricated wall-less tissue equivalent proportional counter and a mechanism for increasing the beta or alpha particle source to counter distance by distances equivalent to tens of micrometers of unit density tissue. Monte Carlo computational models of the experimental apparatus were also made using the electron transport code PENELOPE for several beta particle emitting sources acting as a proxy for hot particles. Results from both experiments and Monte Carlo simulations for low energy beta emitters showed steep dose gradients as dose rates decreased to almost zero over tissue-like distances of a few tens of microns depending on the energy spectrum of the beta particles. An increase in dose-mean lineal energy indicative of increased biological effectiveness was also observed, experimentally and computationally, as source to target tissue distances increased. Taken together, the experimental and computational methods described in this work have proved the principle of using microdosimetric methods for the direct determination of hot particle dose and potential biological effectiveness and concludes with recommendations for further research to promote the development of equipment suitable for nuclear power plant field deployment.