Microdosimetric Distributions for Nanometer-size Targets in Water Irradiated with ^<60>Co Gamma Rays: Frequency Distributions of Total Number of Effective Primary Events Produced in a Target

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A calculation method of frequency distributions for the microdosimetric quantity J (number of effective 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 h_j distributions are calculated for nanometer-size targets in water irradiated with ^<60>Co γ rays as a practical application. It was found that the h_j distribution remarkably depends on two parameters, D_<σ_0> and 1^^-/W', where D is the absorbed dose, σ_0 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 h_j distributions are compared with those in the conventional target theory. Finally, the quantity z' and the frequency distribution for z', 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 D_<σ_0> and 1/W' and the frequency mean of z', z'^^-, is equal to the absorbed dose D.
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