Physics and Engineering of Radiation Detection

2.3. Interaction of Photons with Matter 93

Energy (keV)

~E −255γ Eγ

Compton Edge

Photopeak

Backscatter Peak

Events per Energy Bin

Compton Continuum

Figure 2.3.7: Typicalγ-ray spectrum obtained by a detector surrounded

by heavy shielding (such as lead).

Herer 0 =2. 818 × 10 −^13 cmis the classical electron radius andα=hν/mc^2 with
νandmbeing the wavelength of the incident photon and the electron rest mass
respectively.

Example:

A photon incident on an atom scatters off at an angle of 55^0 with an energy

of 150keV. Determine the initial energy of the photon and the energy of the

scattered electron.

Solution:

The wavelength of the scattered photon is

λs =

hc

Es

=

(

6. 62 × 10 −^34

)(

2. 99 × 108

)

(150× 103 )(1. 6 × 10 −^19 )

=8. 247 × 10 −^12 m.

The wavelength of the incident photon can be computed with the help of

equation 2.3.10 as follows.

λi = λs−

h

m 0 c

[1−cosθ]

=8. 247 × 10 −^12 −

6. 62 × 10 −^34

(9. 11 × 10 −^31 )(2. 99 × 108 )

[1−cos 55^0 ]

=5. 870 × 10 −^12 m.