THE EGS5 CODE SYSTEM

Figure 2.5: Feynman diagram for single photon e + e − annihilation.
is assumed to have energy ˘k = m. The gamma angular distribution is taken to be isotropic the
photons moving in opposite directions.
It should be noted that a positron may annihilate and give off only one photon via the process
shown in Figure 2.5. Messel and Crawford[103] make the point that the ratio of one photon
annihilation to two photon annihilation is small until higher energies are reached, at which point
the absolute value of the cross section is small. Thus, EGS ignores one photon positron annihilation.
Note that it is also possible to have annihilation accompanied by three or more photons, but these
interactions are even less likely than one photon annihilation and are not considered in EGS.
2.13 Continuous Electron Energy Loss
Recall the distinction made earlier between collisions contributing to discrete and to continuous
energy losses in electron and positron scattering. In discrete collisions, secondary particles with
energies above their cutoff energies are created and subsequently transported. Continuous energy
losses result from interactions in which the energy transfered to the secondary particles is below
the discrete transport energy thresholds. The resulting secondary particles, which are either soft
bremsstrahlung photons or low energy atomic electrons. The mean total energy loss per unit length
in these continuous loss collisions is thus given by
−
( ) dE±
dx Total
Continuous
= −
( ) dE±
dx Soft
Bremsstrahlung
−
( ) dE±
dx Sub-Cutoff
Atomic Electrons
(2.253)
where ± denotes positive or negative electrons. The first term on the right-hand side of Equation
2.253 is the same for electrons and positrons (to the accuracy with which we treat bremsstrahlung)
and is given by Equation 2.148.
The second term may be expressed as the integral of the differential cross section for transferring
a specified amount of energy, T , to an atomic electron, times the amount of the energy transfer
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