✍ Scribed by Helmut Paul; Oksana Geithner; Oliver Jäkel
No coin nor oath required. For personal study only.
For the purpose of dosimetry in radiation therapy, one uses air filled ionization chambers and converts the reading to absorbed dose in water which is the standard reference medium. For this conversion, an accurate knowledge of the stopping powers of water and air is required. Since experimental data in the required energy range are almost non-existent, one has to rely on semi-empirical or theory-based stopping power tables. For the example of carbon ions of 400 MeV/nucleon, we calculate the stopping power ratio S w,air as a function of depth in water, using Monte Carlo program SHIELD-HIT v.2 that takes the production of secondary particles by nuclear reactions into account. We find that the result is very nearly the same as the simple ratio of stopping powers of water and air, obtained using the simplest Bethe theory, and that this ratio is essentially determined by the mean ionization potentials I water and I air .
We find that the I values for water and air used in Reports 49 and 73 of the International Commission on Radiation Units and Measurements (ICRU) differ from each other, which leads to a peculiar shape of the curve S w,air if the tables from Report 49 are used for p and a fragments and those from Report 73 for the rest. If a single set of I values is used instead, the curve is monotonely rising, and the absolute value is smaller. In view of newer published results, we suggest that the values I water and I air should probably be increased from those given in the ICRU Reports. However, at the energy considered, all our results for S w,air fit into the range 1.13 ± 0.02 determined in the IAEA Code of Practice.
📜 SIMILAR VOLUMES