COMPARISON OF VARIOUS DOSE QUANTITIES IN TISSUE AND TISSUE SUBSTITUTES AT NEUTRON ENERGIES BETWEEN 20 MEV AND 100 MEV

Using kerma factor data for H, C, N, and O in the neutron energy range from 20 MeV to 100 MeV provided by recent calculations and evaluation s, the absorbed dose conversion factor alpha(t,p) from A-150 tissue su bstitute plastic (p) to ICRU standard tissue (t) was calculated. The r esults show a constant value of alpha(t,p) = 0.92 with uncertainties o f about 5% from 20 MeV to 50 MeV and of 7% above 50 MeV. Dose distribu tions from measurements with low pressure proportional counters are us ed to determine the mean quality factors in C and O. The dose distribu tions and the quality factors exhibit strong similarities for C and O. Combined with mean quality factors of A-150 plastic and with evaluate d kerma factors, the ratio beta(t,p) of dose equivalent in ICRU tissue to that in A-150 plastic was calculated. The result indicates that, d ue to the excess of C in A-150 plastic, the TEPC may overestimate the ambient dose equivalent by about 15% for neutron energies above 20 MeV . These findings confirm the suitability of the TEPC as dose equivalen t meter for radiation protection and also suggest that for neutron the rapy dosimetry the excess of C in A-150 plastic should be taken into a ccount by using alpha(t,p) = 0.92 at neutron energies above 20 MeV.