Half-value layer
As a half-value layer or half- thickness is defined as the thickness of the irradiated material, in which electromagnetic radiation such as gamma or X- radiation, the radiation intensity - reduced by half - and in particular the dose rate. The half- thickness depends on how the absorption coefficient of the specific properties of the material and the photon energy of the incident radiation.
The half- thickness can only serve to approximate sizing easier shields as diverse physical effects (for example, dose build-up, scattering, Skyshine effects) are not considered. Precise calculations are performed for example with Monte Carlo simulations or transportation bills.
The term tenth value thickness be considered as analogous; traversing this thickness reduces the intensity of one tenth of the original value.
Calculation
In contrast to α - and β - γ - radiation radiation does not reach the maximum. The intensity of the γ - radiation is only weakened when passing through matter.
The ratio of the dose rate, which is calculated without screening in the beam path, and the dose rate of the unscattered radiation in the same place with the thickness of shielding material is referred to as (material) attenuation factor of the unscattered radiation:
For the reciprocal attenuation factor, the formula is
Herein, the attenuation coefficient. For the half-value layer thickness is considered, by definition,
Thus, the half- value layer of the attenuation coefficients obtained by
Or vice versa
For the dose rate behind a shield with an arbitrary thickness is thus obtained