Physics and Engineering of Radiation Detection

612 Chapter 11. Dosimetry and Radiation Protection

Solution:

We know that

1 R=2. 58 × 10 −^4 C/kg.

To determine the number of charge pairs perkgcorresponding to 1R,we

simply divide the right hand side of the above equivalence by the unit electrical

charge, that is

N =

Q

e

=

2. 58 × 10 −^4

1. 602 × 10 −^19

=1. 61 × 1015 charge pairs perkg. (11.2.13)

The production of charge pairs is related directly to the energy depositedE

and the energy needed to produce a charge pair, through the relation

N=

E

W

.

TheW-value for most gases including air is approximately 34eV.Usingthis

and the value ofNas calculated above we can determine the required energy

equivalent to 1Ras follows.

E = NW

=

(

1. 61 × 1015

)

(34)

=5. 47 × 1016 eV kg−^1

=5. 47 × 107 MeV g−^1

In laboratory environments it is often desired to estimate the exposure expected
from a known radioactive source. Intuitively thinking we can say that the exposure
from a radioisotope is proportional to the following quantities.

Source Activity: The more the activity the larger number of decays and

hence higher exposure. Source activity is given byλdNwithλdandNbeing

the sample’s decay constant and the number of radioactive atoms in the sample

respectively.

Inverse of Distance Squared:Exposure decreases with increasing distance

from the source. According to the inverse square law, the flux of radiation

varies by inverse of the square of the distance from a point source. Since

exposure is directly related to flux, it will also be inversely proportional tor^2 ,

the distance between the source and the point of measurement. A point worth

mentioning here is that a source can be considered a point source if the point

of measurement is much larger than its mean radius.

Exposure Time:Radiation exposure increases with increasing exposure time.