Physics and Engineering of Radiation Detection

616 Chapter 11. Dosimetry and Radiation Protection

to collisional and radiative losses respectively. The total Kerma would then be equal
to their sum, that is
K=Kcol+Krad. (11.2.20)

It is customary to denote the energy lost through radiative processes as a fraction of
the radiative to total Kerma. This fraction is generally known asradiative fraction
and is given by

g=

Krad

K

. (11.2.21)

Note that this ratiogis mainly a function of the energy loss through Bremsstrahlung,
which in most situations can be neglected, that isg≈0. The above equation implies
that the total Kerma can be evaluated from

K=

Kcol

1 −g

. (11.2.22)

Earlier in the chapter we introduced the term energy fluence, which has the dimen-
sions of energy per unit area. If we multiply this quantity with the mass energy
transfer coefficientμm,tr, the resultant will have the dimensions of Kerma. In fact,
this is how total Kerma is often computed, that is

K=μm,trΨ, (11.2.23)

where Ψ is the energy fluence. Note that here the use of mass energy transfer
coefficients ensures that we obtain total Kerma. If one wishes to evaluate only
collision Kerma, the relevant parameter to use would be mass absorption coefficient
μm,col.
Kcol=μm,colΨ (11.2.24)
There also exists a direct relationship between air Kerma and the exposure we
introduced earlier in the chapter. The reader would recall that the exposure deter-
mines the amount of charge (either electrons or ions) produced per unit mass of air
while collision Kerma represents the kinetic energy released per unit mass due to
collisions. Now, since this energy goes into creation of charge pairs therefore the two
terms should be directly related. To derive this relationship we first note that the
total charge produced in air can be written as

dQ=eN=e

dEcol

Wair

, (11.2.25)

whereNis the total number of charge pairs created,dEcolis the collision energy
loss, andWairis the energy needed to create an electron ion pair in air. Dividing
both sides of the above equation by the mass elementdm,weget

dQ

dm

=

e

Wair

dEcol

dm

(11.2.26)

or X =

e

Wair

Kcol,air, (11.2.27)

which is our required relation between exposure and collision Kerma in air. Now,
since the factorWair/e≈ 33 .85 is constant for air, therefore we can also write

Kcol,air≈ 33. 85 X. (11.2.28)