Feasibility study of an electret dosimetry technique

Characteristics of a radiation charged electret dosimeter are described. The dosimeter is based on a parallel-plate ionization chamber with the exception that the collecting electrode is covered by a thin polymer, Teflon or Mylar. During the charging of the dosimeter, ions produced in the sensitive volume by an external radiation source drift in the externally applied electric field and become trapped on the polymer surface forming an electret. Once the external supply is removed, a field across the sensitive volume is produced by the electret charge, such that during any subsequent irradiation, ions opposite in sign to those on the electret surface are attracted to the electret and deplete the charge layer in an amount proportional to the air kerma. The remaining charge on the electret is read by an electrometer, through further irradiation. This technique allows the dosimeter to be simultaneously charged and calibrated, used in the field, simultaneously discharged and read, and reused again in situ without dismantling the dosimeter. Various parameters, however, were considered by direct discharge, rather than by initial exposure and subsequent measurement. Calibration, energy dependence, air kerma range, and reproducibility are discussed, and guard-ring effects on the linearity of the chamber are presented. Measurements of natural charge decay on the electrets are introduced.