Physics and Engineering of Radiation Detection

11.2. Quantities Related to Dosimetry 615

Table 11.2.4: New and old units of quantities related to radiation dose and their
conversion factors.

Old Unit New Unit Conversion

Name Symbol Name Symbol Factor

Radiation Absorbed Dose rad Gray Gy 1 Gy= 100rad

Rad Equivalent for Man rem Sievert Sv 1 Sv= 100rem

Curie Ci Bacquerel Bq 1 Bq=2. 7 × 10 −^11 Ci

11.2.EKerma,Cema,andTerma....................

In this section we will describe three quantities, Kerma, Cema, and Terma, that are
in common use in the field of dosimetry due to their effectiveness in characterizing
the macroscopic effects of radiation.

E.1 Kerma..............................

Kerma is an acronym of kinetic energy released in a medium per unit mass. It is
defined as the total kinetic energy of all the charged particles liberated by uncharged
particles per unit mass of the target material. Mathematically, it is written as the
quotient of the charged particle’s kinetic energyEkinand the mass of the material
dm,thatis

K=

dEkin

dm

. (11.2.18)

Kerma is generally measured in the same units that are used for absorbed dose,
that is,J/kgorGray.NotethatheredEkinis the kinetic energy of the charges
produces as a result of radiation interaction. This energy is not necessarily equal to
the energy transferred by the incident radiation since some of the energy can also
go into other processes, such as radiative and radiationless transitions. The other
point to note here is that the mass elementdmshould be very small.
The above definition of Kerma is actually a simplified form of the actual micro-
scopic definition, which takes the form

K= lim

V→ 0

Ekin

ρ V

. (11.2.19)

A point worth noting here is that the condition of the volume element shrinking to
zero ensures that the deposition of energy by radiative transfers gets ignored. In
this way Kerma is actually an approximation of absorbed dose.
Kerma isnotindependent of the type of the target material and therefore must
always be defined with respect to the medium. For example, the energy released in
air per unit mass of air should be written asair Kerma.
The energy lost by the incident radiation has two main components: collisional
loss and radiative loss.Kcan therefore be divided intoKcolandKradcorresponding