ESTRO course Dose Modelling and Verification 2017

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)

Monte Carlo simulations of cavity dose response to 1.25MeV photon pencil beams in cavity of identical electron density (water), but different atomic composition

Edge of cavity

Edge of cavity

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

Bouchard et al, Med. Phys. 42 (10), 6048-61, Oct 2015