Microdosimetry in the Build-up Region of Gamma Ray Beams
| Autor: | A. Tilikidis, A. Brahme, L. Lindborg |
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| Rok vydání: | 1990 |
| Předmět: | |
| Zdroj: | Radiation Protection Dosimetry. 31:227-233 |
| ISSN: | 1742-3406 0144-8420 |
| Popis: | A microdosimetric system based on a spherical wall-less proportional counter has been developed for measurements under non-equilibrium conditions. The wall-less counter reduces the differences in the slowing down and scattering properties between detector and medium present in ordinary ionisation chamber measurements, and the low gas pressure minimises the perturbation of the sample electron fluence. Measurements have been performed in the build-up region of both ordinary (but free of contaminating electrons) and clean (i.e. without backscattering and with filtration of the contaminating photons and free of contaminating electrons) gamma ray beams for a simulated target diameter of 2 µm. The results showed fast radiation quality variations in the build-up region of ordinary gamma ray beams due to the rapid build-up of low energy contaminating photons from the source and collimators. The dose mean lineal energy in the first millimetre of depth in an ordinary 60Co beam, 50 mm in diameter is reduced by as much as 34% compared with the value at the dose maximum. Separation of the dose components showed that 5% of this reduction is due to the backscattering while the introduction of a low energy photon filter between the source and the counter could only explain 4% of the reduction. Absorbed dose calculations showed that the surface dose for this ordinary 60Co beam is 11% and that backscattering corresponds to 14% of this dose and the contaminating photons to 6%. |
| Databáze: | OpenAIRE |
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