Use of high pressure ionisation chambers and a comparison with TEPCS [tissue-equivalent proportional counters] for obtaining microdosimetric information in unspecified radiation fields

Improved and detailed theoretical treatment of initial recombination of ions in tissue-equivalent (TE) high pressure ionisation chambers has recently demonstrated the potential of such instruments to measure accurately radiation quality average parameters in unspecified radiation fields. This paper presents an experimental testing of the high pressure ionisation chamber method and a comparison with low pressure proportional counters (TEPC). The restricted dose mean LET (L-bar500.D) of 22 to 150 kVp diagnostic X-rays is measured with the high pressure ionisation chamber and compared to y-barD values derived with the proportional counter. At the higher X-ray energies (>60 kVp), both methods show that the L-bar500,D and the y-barD remain relatively constant as a function of energy. At the lower X-ray energies (<30 kVp), the high pressure chamber records significantly elevated values of the L-bar500,D. The low pressure proportional counter does not record this increase. This difference is partly attributed to the effective filtering of the lower energy X-rays at the proportional counter metal wall and partly to limitations arising from the definition of y-barD. The mean quality factor Q-bar and the microdosimetric parameter y-bar0,D are derived for d(4)-Be neutrons (in air and in water) and for p(4)-Be neutrons (in air). The results are compared with proportional counter data at the same points and agreement to within 10 to 15% is found. (author)