Dosimetric characterization of a miniature x-ray source for biophysics studies

Abstract

Tritium is a low energy beta emitter that is found in abundance at CANDU nuclear power plants and is a cause of concern since it accounts for roughly thirty three percent of the total occupational exposure of nuclear workers. According to current standards, the radiation weighting factor for all electrons irrespective of their energies is 1. However, there is some evidence suggesting that this factor for low energy electrons could be more than 1. Due to the complexities associated with handling tritium in a liquid or gaseous state alternate arrangements to simulate tritium radiation are investigated. Therefore, in this project it is proposed to use a miniature X-Ray source (Mini-X) to simulate tritium radiation and characterise these low energy X-Ray beams with the aim of investigating the relative biological effectiveness of low energy electrons. The scope of this study is to design and characterize low energy X-Ray beams, suitable for biophysical experiments using Gafchromic’s XR-QA2 radiochromic film and Standard Imaging’s A-20 ionization chamber. To safely carry out the characterization experiments a dual layered safety shielding was constructed and parameters including beam dimensions, air-kerma and dose to tissue values from the Mini-X were explored. Lastly, a cross calibration curve between an optically stimulated luminescent dosimeter (nanoDots) and A-20 ionization chamber was established, with an aim to use nanoDots for a proposed future biophysical experiment. It was found that the Mini-X produced a flat beam profile at distances up to 12.7 cm from the source. In addition, air-kerma values of 11.56 and 6.442 cGy/s could be obtained at 2.54 cm and 5.08 cm respectively with operating parameters of 20 keV and 75 μA. It was also established that there is no substantial difference between air-kerma and dose to tissue values for the X-Ray beams studied. For future work, a biophysical experiment would be carried out using fish eggs where the impact of the Mini-X X-Ray beams on egg hatchability and embryonic mortality would be measured.