Question

Consider photons of ionising energy incident on a small volume of a medium and which interact in the medium. By referring to the resulting interactions and product particles, explain the difference between the terma, kerma, collision kerma, rad iative kerma, and the absorbed dose in the medium. Relate each to the incident particle fluence.

Answer 1

Here,

Kerma K =Kinetic Energy Released per unit Mass

Energy of photons is imparted to matter in a two stage process. In the first stage the photon energy transfers to electrons and in the second stage the electrons transfer energy to the medium through ionisations and atomic excitations. kerma quantifies the first stage, where the energy is transferred from indirectly ionising radiation to directly ionising radiation

And it is defined as a

Where dE is the sum of the initial kinetic energies of all the charged particles liberated by uncharged particles in a mass dm.

Total kerma can be split into two parts collisional kerma and radiative kerma K_rad. Collisional kerma K_col leads to the production of electrons that dissipate their energy as ionisation near electron tracks in the medium. Radiative kerma K_rad leads to the production of bremsstrahlung as the charged particles are decelerated in the medium.

Unit: J kg-1 , and the special name for the unit of kerma is gray (Gy) .

Absorbed dose D :

It is given as

Where d ? is the mean energy imparted to matter of mass dm. Energy imparted is the energy incident minus the energy leaving the mass; minus the energy released in nuclear transformations .

Electrons travel through the medium and deposit energy along their tracks. Therefore the absorption of energy described by absorbed dose does not take place at the same location as the transfer of energy described by kerma. Unit: J kg-1. The absorbing medium should always be specified. Special name for the unit of absorbed dose is gray (Gy).
Generally, the transfer of energy (kerma) from the photon beam to charged particles at a particular location does not lead to the absorption of energy by the medium (absorbed dose) at the same location. This is due to the non-zero (finite) range of the secondary electrons released through photon interactions.

In general, however, the ratio of dose and collisional kerma is often denoted as:

Under the conditions that (1) radiative photons escape the volume of interest and (2) secondary electrons are absorbed on the spot (or there is charged particle equilibrium of secondary electrons). The absorbed dose to medium Dmed is related to the electron fluence ? in the medium as follows

The s/rho term is the unrestricted mass collisional stopping power of the medium at the energy of the electron.