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Detector composition: c) atomic composition
Atomic properties of the detector also have an effect on the interaction cross sections and influence the response of the
detector. These are:
• The atomic number Z (Z-dependence in photo-electric effect, pair production)
• The I-value (mean excitation energy or potential related to stopping power, linearly increasing with Z)
• The density effect parameter
(reduction in the collision stopping power)
Bouchard et al, Med. Phys. 42 (10), 6048-61, Oct 2015
Monte Carlo simulations of cavity dose response to 1.25MeV photon pencil beams in cavity of identical
electron density (water), but different atomic composition
Water-density air
Water-density silicon
Cavity:
5mm radius,
2mm thick
Cavity:
2mm radius,
2mm thick
Atomic composition perturbation effects significantly smaller in comparison to density perturbation effects
Edge of cavity
Edge of cavity