Experimental microdosimetry in high energy radiation fields
Spurny, F.; Bednar, J.; Vlcek, B. Bottollier-Depois, J.-F. Molokanov, A.G.
Proceedings of the 10th international congress of the International Radiation Protection Association on harmonization of radiation, human life and the ecosystem Tokyo (Japan) 14-19 May 2000, No. P-3b-164;16 refs., 10 figs., 2 tabs
To determine microdosimetric characteristics in the beams and fields of high energy panicles with the goal, also, to compare the classical method of experimental microdosimetry, a tissue equivalent low pressure proportional counter (TEPC) with the linear energy transfer (LET) spectrometer based on a chemically etched polyallyldiglycolcarbonate as a track etched detector (TED). To test the use of TED LET spectrometer in the conditions, where the use or TEPC is not possible (high energy charged particle beams at high dose rates). The results obtained with the TEPC NAUSICAA were used in this work to compare them with other data. This TEPC measures directly the linear energy in the interval between 0.15 and 1500 keV/mum in tissue, the low gas pressure (propan based TE mixture) permits to simulate a tissue element of about 3 mum. It can be used in the fields with instantaneous dose equivalent rates between 1 muSv/hour and 1 mSv/ hour. TED LET spectrometer developed to determine LET spectra between 10 and 700 keV/mum in tissue. Primarily, track-to-bulk etch rate ratios are determined through the track parameters measurements, the spectra of these ratios are convened to LET spectra using the calibration curve established by means of heavy charge panicles. The critical volume of thi spectrometer is supposed to be a few nm. There is no limit of use for the dose rate, the background tracks limit the lowest threshold to about 1 mSv, the overlapping of tracks (the highest one) to 100 mSv. Both experimental microdosimetry methods have been used in on board aircraft radiation fields, in on-Earth high energy radiation reference fields, and in the beams of protons with energies up to 300 MeV (Dubna, Moscow, Loma Linda). First, it should be emphasized, that in all high energy radiation fields studied, we concentrated our analysis on the region, where both methods overlap, i.e. between 10 and 1000 keV/mum in tissue. It should be also stressed, that the events observed in this region correspond to the interactions of secondary panicles created by primary radiation are close to the sensitive volume of the equipment. The comparison of both methods was performed in the field on board aircraft and in high energy reference fields. it was found that the microdosimetric distributions observed agreed at least qualitatively, a quantitative agreement of integral dosimetric values was found as well. The measurements in proton beams were performed in several points along the Bragg curve. Actually, we were able to observe the influence of primary beam contamination due to the filtration as well as due to the secondary particle created during the penetration of beams in the phantome. The relevance of such data for the radiotherapy application of high energy protons is evident. It was proved that both method can give relevant and useful information on the microdosimetric distributions in complex beams and fields of high energy panicles. The further development of this approach is in progress in our laboratories.