An Evaluation of Hydrogen as a TEPC Counting Gas in Radiation Protection Microdosimetry

Abstract

Tissue-equivalent proportional counters (TEPC) filled with conventional tissue-equivalent gases have a significantly low neutron dose equivalent response in the neutron energy region between thermal and about 200 keV. Theoretical modelling for monoenergetic neutron fields suggests that using pure hydrogen as a microdosimetric counting gas would improve the dose equivalent response. Average neutron dose equivalent response calculations have been done for selected realistic broad energy neutron fields and are found generally to support this suggestion. Gas gain investigations performed with conventional tissue-equivalent gases and hydrogen indicate that hydrogen has only a limited region of applied voltage, up to a reduced electric field at the anode of about 600 V.cm −1 . torr −1 , where the counter operates in the proportional mode. Conventional tissue-equivalent gases can be used up to about 3000 V.cm −1 . torr −1 . Microdosimetric measurements performed with hydrogen as the counting gas nevertheless demonstrate that it is suitable for measuring event-size spectra and carrying out quantitative dosimetry. The non-tissue-equivalent nature of hydrogen, however, does present some difficulties in calibration and interpretation of the measured spectra. Taking into account the non-tissue-equivalence and the poor gas gain characteristics of hydrogen the advantage of using this counting gas, at least in its unadulterated form, appears marginal. However, if TEPC ambient dose equivalent responses are found to be lower than that predicted by the theoretical model used here for workplace neutron fields, then it may become an attractive option to use hydrogen. In this case effort will be required to improve the gas gain characteristic of hydrogen by investigating the effect of small admixtures of quenching gases