Table 1-2. Approximate attenuation of photons with a layer of water equal to the maximum
range of secondary charged particles28
Primary Radiation Energy Maximum Secondary Photon Attenuation (%i) inl
(MeV) Electrons Range (cm) Maximum Electron Range
0.1
0.01387 0.23%
1.0
0.43 3.00%
4.933 10.04%
9.305 15.%
1.5.2 High Energy Beam Dose Calculations
Based on the previous discussion, as the energy of the ionizing radiation increases, the
penetration power of the secondary charged particles increases more rapidly than the penetration
power of the primary radiation in a given volume. This leads to CPE failure, caused by the small
number of charged particles produced at deeper depths in volume V, relative to the number of
charged particle produced at initial points along the primary radiation field direction. This is
indicative of significant attenuation of the primary indirectly ionizing radiation.
As a result of this phenomenon, higher numbers of charged particles will be generated
close to the entrance compared to the rest of the volume, leading to CPE failure considering dose
in that volume. The degree of this failure becomes progressively large for higher energies, so that
D no longer equals to Kc A more detailed discussion on this topic is presented in chapter 4.
1.5.3 Limits of Charged Particle Equilibrium
Assuming that a maximum energy is transferred from a photon to an electron, we can use
this assumption to investigate a CPE requirement for a water phantom region, as defined earlier.
Considering the continuous slowing down approximation range of the electron (REsn), and
average range of the maximum energy electrons as function of energy.
Ross =IT dTL (1-13)