Electret gamma/x-ray low level dosimeter

Abstract

A small enclosed ion chamber with an electrostatically charged electret attached to the inner chamber wall. Filtered holes in the chamber permits radon in the ambient air in the chamber to enter or exit by diffusion. Positive or negative ions (depending on the polarity of the electret charge) formed in the chamber air by the passing of gamma radiation or X-rays move to and collect on the electret surface by virtue of its electrostatic attraction to ions of opposite polarity. These ions neutralize the ions of opposite polarity on the electret and cause a measurable reduction in its surface voltage which reduction is proportional to the dose of gamma and/or X-ray incidence on the chamber. The reduction of the electret surface voltage is used to calculate the dose of gamma or X-ray radiation incident upon the chamber during a known exposure period. In one embodiment, a sealed radioactive-gas-tight flexible-wall container, such as a bag, surrounds the chamber to prevent any ambient radon from entering the ion chamber. In addition, an absorbent such as activated charcoal can be included in the bag to remove any radon captured within the sealed chamber. Removal of this radon removes its undesirable contribution to the gamma/X-ray dose signal and thereby enhances the accuracy of the monitor.