Microdosimetric distributions for nanometer-size targets in water irradiated with 60Co gamma rays: frequency distributions of total number of effective primary events produced in a target.

Microdosimetric distribution/Target theory/Nanometer-size target/Calculation method/60Co ƒÁ rays A calculation method of frequency distributions for the microdosimetric quantity J (number of effec tive primary events produced in a target), hj, is presented. This method is briefly compared with other approaches developed by Kellerer and Chmelevsky, and Wilson and Paretzke for numerical calculations of microdosimetric quantities. The hj distributions are calculated for nanometer-size targets in water irradiated with 60Co ƒÁ rays as a practical application. It was found that the hJ distribution remarkably depends on two parameters, DƒÐ0 and 1/W', where D is the absorbed dose, ƒÐ0 is the mean geometrical cross section of the target, 1 is the mean target thickness, and W' is the mean energy to produce an effective primary event in the target. Our hJ distributions are compared with those in the conventional target theory. Finally, the quantity z' and the frequency distribution for z', hz', which correspond to the specific energy z and the probability density f(z) defined in ICRU Report 36, are derived from J and the hJ distribution, respectively. The hz' distribution depends on the values of DƒÐ0 and 1/W' and the frequency mean of z', z', is equal to the absorbed dose D.