1000 Solved Problems in Modern Physics

7.1 Basic Concepts and Formulae 379

At low Photon energy

σph∝Z^5 /(hν)^7 /^2 (7.45)

μph=Nσph (7.46)

I=I 0 exp (−μphx) (7.47)

Low photon energy can be measured from the observation of absorption edges.

E(eV)= 13 .6(Z−σ)^2 /n^2 (7.48)

wheren=1 for the K-series andn=2 for the L-series etc,σis the screening
constant andZis the atomic number of the absorber.
The photoelectric absorption also follows the exponential law.

Pair production

At incident photon energies greater than 2mc^2 (1.02 MeV), the electron–positron
pair production becomes important.

hν=T−+T++ 2 mc^2 (7.49)

μp∝Z^2 (7.50)

μtotal=μc+μph+μβ (7.51)

Importance of the three processes is shown in Fig. 7.9

Fig. 7.9Importance of the
three processes at increasing
photon energy