RECOMBINATION INDEX OF RADIATION QUALITY OF MEDICAL HIGH-ENERGY NEUTRON BEAMS

Recombination index of radiation quality (RIQ) was determined in high energy neutron beams from the 660 MeV phasotron of the Joint Institute for Nuclear Research, Dubna (JINR). Measurements were performed with a high pressure ion chamber at two collecting voltages and compared wi th calculations considering spectra of all secondary and tertiary char ged particles. The neutron beams produced on beryllium, copper and lea d targets were investigated. Only minor differences between RIQ values for these beams were observed. A high decrease in RIQ, from 6 to 3.5, has been observed in the first 5 cm depth in a water phantom, followe d by a rather small progressive decrease towards 3.3 +/- 0.3 for depth s larger than 10 cm. Laterally small changes have been observed at a g iven depth between values on the beam axis and outside the border of t he beam.