Physics and Engineering of Radiation Detection

618 Chapter 11. Dosimetry and Radiation Protection

η< 1 η=1 η> 1

Ab

sor

be

d^

D

ose

Depth

Figure 11.2.1: Plot of absorbed dose

with respect to the depth of the target

material.

contributes to the dose and all the radiative energy transfers result in photons

escaping from the target material.

η>1: As the radiation penetrates deeper, the probability of radiative loss pho-

tons being absorbed inside the medium increases, mainly due to their broader

energy spectrum. Hence the dose can actually become higher than the collision

Kerma.

E.2 Cema

One limitation of Kerma is that it is defined only for uncharged particles, such as
photons. For charged particles, another quantity called Cema has been introduced.
It is analogous to Kerma in definition and is mathematically written as

C=

dEcon

dM

, (11.2.31)

wheredEconvis the energy lost by charged particles in a material of massdM.The
SI units of Cema areJ/kgorGray.

E.3 Terma..............................

In chapter 2 we discussed different mechanisms of energy transfer when radiation
passes through matter. We saw that the absorption of radiation may lead to several
different types of excitations as well. Some of these excitations my not lead to the
release of kinetic energy and therefore would not be included in the definition of
Kerma. To overcome this shortcoming, another quantity called Terma has been
defined. Terma is the acronym of total energy released in a medium per unit mass
and can be evaluated from

T=

dEtotal

dM

. (11.2.32)

wheredEtotalis the total energy transferred by the incident radiation to the medium
having massdM. Just like Kerma and Cema, the usual units for Terma are also
J/kgofGray.