Measurement of Potential Alpha Energy Concentration of Radon and Thoron Daughters Using an Electret Dosemeter

Abstract

An electret dosemeter consisting of an electret loaded into a 60 ml chamber has been used earlier for quantitative measurement of gamma radiation. The chamber was modified to hold an air sampling filter paper at the bottom and air was sampled through it. It was shown that nearly 20% of the energy of alpha radiation emitting from the filter paper was dissipated in the volume of the chamber. This energy in turn produced ion pairs which were collected by the electret resulting in a measurable change in charge status of the electret. By taking an air sample of known volume and allowing the registration of alpha energy, not only during sampling but also for three hours after sampling, a direct measure of the potential alpha energy concentration of radon daughters could be calculated in easily interpretable working level units. By proper choice of the thickness of the electret and the total flow, it was possible to measure radon daughter concentrations down to milliworking level units. The field measurements carried out by this method compared well with those obtained by established methods. The procedure was particularly useful for measuring time-averaged concentrations needed for effective survey of a mine area or for personal dosimetry. For thoron decay products, it was shown that the alpha energy concentration measured during sampling and up to a period of 7.7 hours post sampling, time being reckoned from the mid-time of sampling, yielded 33% of the potential alpha energy concentration due to thoron daughters. An air sample collected and measured in this way registered all of radon daughter alpha potential energy and one third of thoron daughter alpha potential energy, thus providing an equivalent radon daughter concentration. This procedure is suitable for evaluating any mixture of daughter products without consideration of whether they originated from radon or thoron.