Physics and Engineering of Radiation Detection

26 Chapter 1. Properties and Sources of Radiation

1 Ci =3. 7 × 1010 disintegrations/sec

1 mCi =10−^3 Ci=3. 7 × 107 disintegrations/sec

1 μCi =10−^6 Ci=3. 7 × 104 disintegrations/sec

1 Bq = 1 disintegration/sec

1 Bq =2. 703 × 10 −^11 Ci

1.4 Activation.................................

It is possible to induce radioactivity into materials by letting them interact with
radiation. The process is known as activation and is extensively used to produce
radioactive particle sources and activation detectors. The radiation emitted by the
activated material is generally referred to asresidual radiation. Most of the activated
materials emitγandβparticles but, as we will see later, it is possible to activate
materials that emit heavier particles.
To activate the material, it must be irradiated. As soon as the irradiation starts,
the material starts decaying. This means that both the processes, irradiation and
decay, are happening at the same time. The rate of decay would, of course, depend
on the half life of the activated material. LetRactbe the rate of activation in the
sample. The rate of change in the number ofactivatedatomsNin the material is
then given by
dN
dt

=Ract−λdN, (1.4.1)

where the second term on the right hand side represents the rate of decay.λdis the
decay constant of the activated material. Integration of the above equation yields

N(t)=

Ract

λd

(

1 −e−λdt

)

, (1.4.2)

where we have used the boundary condition: att=0,N= 0. We can use the
above equation to compute the activityAof the material at any timet. For that we
multiply both sides of the equation byλdand recall thatλdN≡A. Hence we have

A=Ract

(

1 −e−λdt

)

. (1.4.3)

Note that the above equation is valid for as long as irradiation is in process at a
constantrate. In activation detectors, a thin foil of an activation material is placed
in the radiation field for a time longer compared to the half life of the activated
material. The foil is then removed and placed in a setup to detect the decaying
particles. The count of decaying particles is used to determine the activation rate
and thus the radiation field.
The activation rateRactin the above equations depends on the radiation flux^6
as well as the activation cross section of the material. In general, it has energy

(^6) Radiation or particle flux represents the number of particles passing through a unit area per unit time.
We will learn more about this quantity in the next chapter.