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According to the definition of ambient dose equivalent, the geometry of the problem was very simple. A 30 cm diameter sphere of unit density tissue and composition as specified by ICRU (H, 10.1%; C, 11.1%; N, 2.6%; O, 76.2%; %-compositions are given by weight) was exposed to a parallel particle beam uniformely expanded over its front surface. The medium between the source and the ICRU sphere was assumed to be vacuum.

In order to obtain the depth-dose distributions along the principal axis of the sphere, the energy deposited has been scored as a function of the depth and radius in an R-Z binning cylindrical structure. Different grids have been selected according to the depth: 0.2 cm longitudinal bins have been considered up to 2 cm, and 1 cm longitudinal bins for larger depths. The radial bin was taken to be 1 cm.

The determination of the dose equivalents has been carried out taking the quality factor to be a function of linear energy transfer L, as shown in eq. 2. Therefore the energies deposited per unit mass have been directly multiplied by the quality factor appropriate to the linear energy transfer of the charged particle imparting energy to the matter.

The values of the ambient dose equivalent have been averaged over the depth 0.95-1.05 cm or 0.9-1.1 cm according to the incident energy.

Once the ambient dose equivalent (H^*(ε)) as a function of particle energy for various kinds of radiation was computed, the fluence-to-ambient dose equivalent conversion coefficients (f_H^*(ε)) were calculated in terms of ambient dose equivalent per unit of fluence (Sv · cm²):

where Φ(ε) is the fluence of primary particle of energy ε.

Calculation results are presented in section 4.

Giuseppe Battistoni; INFN, Milano
Stefan Roesler; CERN, Geneva

Last updated: 10th of October, 2008