High energy (greater than or equal to 50 MeV) protons can be considered, when their ionization losses stopping power is taken into account, as the radiation with low (< 5 keV/mum) linear energy transfer (LET). However, their energy is already sufficient to create, through the nuclear reactions in irradiated matter, secondary particles with much higher LET. This phenomenon can modify the characteristics of the energy transfer process due to these particles, which should be taken into account when such particles are used for radiobiology studies and/or for radiotherapy. The importance of these secondary particles was studied experimentally by means of the LET spectrometer based on a chemically etched track detector, in which the tracks of primary protons are not revealed. The studies were performed in proton beams available at the Joint Institute of Nuclear Research (JINR) at Dubna, Russia, with protons of primary energies of about 200 MeV and 1 GeV. The microdosimetric distribution of secondary particles mentioned are presented and compared and their contribution to primary proton ionization losses absorbed dose is estimated. This contribution increases relatively with the proton energy, and also changes with the depth of penetrated material. The importance of this phenomenon to some applications is discussed. (C) 2001 Elsevier Science Ltd. All rights reserved. [References: 12]
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