An electret dosimeter charged by radiation-induced ionizations in air /

An electret radiation dosimeter for long-term personnel monitoring is described. The design of this prototype (a modified parallel-plate ionization chamber) and the associated isothermal electret charging technique are presented. In the charging process, an external voltage causes ions created in air by the passage of radiation to move towards, and become trapped on, a dielectric (e.g., Mylar, Teflon) that covers the measuring electrode, forming an electret. Once the external voltage is removed, the field across the sensitive volume is produced by the electret charge, such that during subsequent irradiation, ions opposite in sign to those on the electret surface are attracted to the electret thus depleting the charge layer in an amount proportional to the exposure. Further irradiation releases the remaining charge on the electret which is measured with an electrometer. This technique allows the electret to be charged, used in the field, and discharged in situ, without dismantling the dosimeter as is required with other electret dosimeters relying on corona charging or other forming methods. Calibration, energy dependence, exposure range, and guard-ring effects of the dosimeter are discussed. This electret dosimeter may prove to be a viable alternative to film dosimeters and TLDs, and is inherently superior because the measuring medium is air