Physics and Engineering of Radiation Detection

11.2. Quantities Related to Dosimetry 611

Solution:

The energy fluence rate for photons is given by

Ψr = E ̄

dN

dtda

=

hc

̄λ

dN

dtda

,

whereλ ̄is the average wavelength of the photons. Substituting the given

values in this equation gives

Ψr =

(

6. 63 × 10 −^34

)(

2. 99 × 108

)

0. 12 × 10 −^9

1. 5 × 104

(1) (1. 8 × 10 −^4 )

=1. 37 × 10 −^7 Jm−^2 s−^1

The integrated flux can be computed as follows.

Φ=

dN

da

=

1. 5 × 104

1. 8 × 10 −^4

=8. 33 × 107 m−^2

=8. 33 × 103 cm−^2

11.2.D Exposure and Absorbed Dose - Mathematical Definitions

We have already defined the terms exposure and absorbed dose. Here we will look
at their proper mathematical definitions.
ExposureX is defined as the chargedQproduced by heavy charged particles
(ions) per unit massdMof dry air when all the electrons liberated by the incident
photonsare completely stopped. Mathematically we can write this as

X=

dQ

dM

. (11.2.12)

This definition of exposure has a fundamental problem that it is valid only for
photons in dry air. Of course the intention for defining it in this way was to stan-
dardize the quantity and to make available a yardstick for comparison. However
since the ways photons interact with target materials are very different from other
particles, such as neutrons orα-particles, therefore exposure has not been found to
be very useful in characterizing damage due to other types of radiation.

Example:

Estimate how much energy deposition of photons per unit mass of dry air is

equivalent to 1R.