Half Value Layer

Half Value Layer

The half value layer expresses the thickness of absorbing material needed for reduction of the incident radiation intensity by a factor of two. There are two main features of the half value layer:

  • The half value layer decreases as the atomic number of the absorber increases. For example 35 m of air is needed to reduce the intensity of a 100 keV gamma ray beam by a factor of two whereas just 0.12 mm of lead can do the same thing.
  • The half value layer for all materials increases with the energy of the gamma rays. For example from 0.26 cm for iron at 100 keV to about 1.06 cm at 500 keV.

half value layer

The half value layer expresses the thickness of absorbing material needed for reduction of the incident radiation intensity by a factor of two. With half value layer it is easy to perform simple calculations.
Source: www.nde-ed.org

Table of Half Value Layers (in cm) for a different materials at gamma ray energies of 100, 200 and 500 keV.

Absorber100 keV200 keV500 keV
Air3555 cm4359 cm6189 cm
Water4.15 cm5.1 cm7.15 cm
Carbon2.07 cm2.53 cm3.54 cm
Aluminium1.59 cm2.14 cm3.05 cm
Iron0.26 cm0.64 cm1.06 cm
Copper0.18 cm0.53 cm0.95 cm
Lead 0.012 cm 0.068 cm 0.42 cm

See previous:

Pair Production

See above:

Gamma Ray Attenuation

See next:

Mass Attenuation Coefficient