AN EVALUATION OF HYDROGEN AS A TEPC COUNTING GAS IN RADIATION PROTECTION MICRODOSIMETRY

Tissue-equivalent proportional counters (TEPC) filled with conventiona l tissue-equivalent gases have a significantly low neutron dose equiva lent response in the neutron energy region between thermal and about 2 00 keV. Theoretical modelling for monoenergetic neutron fields suggest s that using pure hydrogen as a microdosimetric counting gas would imp rove the dose equivalent response. Average neutron dose equivalent res ponse 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 gas es and hydrogen indicate that hydrogen has only a limited region of ap plied voltage, up tw a reduced electric field at the anode of about 60 0 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 measuri ng event-size spectra and carrying out quantitative dosimetry. The non -tissue-equivalent nature of hydrogen, however, does present some diff iculties in calibration and interpretation of the measured spectra. Ta king into account the non-tissue-equivalence and the poor gas gain cha racteristics of hydrogen the advantage of using this counting gas, at least in its unadulterated form, appears marginal. However, if TEPC am bient dose equivalent responses are found to be lower than that predic ted by the theoretical model used here for workplace neutron fields, t hen it may become an attractive option to use hydrogen. In this case e ffort will be required to improve the gas gain characteristic of hydro gen by investigating the effect of small admixtures of quenching gases .