TAP 511- 4: Absorbing radiations
Shielding from x-rays and gamma rays
intensity I
penetrating
thickness x
fraction absorbed  extra thickness
dI/I = –µ dx
intensity decreases exponentially
I
= exp (–µx)
I0
ln 2
half-thickness x1/2 = µ
exponential decrease
of intensity with
thickness
Ways photons are absorbed or lose energy
thickness x
1. ionise atom or put atom in higher energy level
2. scattering from electrons
half-thickness
of absorber
Absorption by successive thicknesses
3. electron–positron pair production (important at high energies)
Half-thicknesses
photon energy
200 keV
x-ray
concrete
25 mm
intensity reduced to one half by each block
1 MeV
 ray
50 mm
Here shielding is related to the absorption of a constant fraction of the radiation in each
thickness of absorber traversed.
lead
0.5 mm
10 mm
External reference
This activity is taken from Advancing Physics chapter 18, 10 O (O stands for
‘O.H.T’.)