Half-Value Layer Calculation The thickness of any given material where 50% of the incident energy has been attenuated is know as the half-value layer (HVL). The HVL is inversely proportional to the attenuation coefficient (m) and the two values are related by the following equation. Since m is normally given in units of cm^{-1}, the HVL is commonly expressed in units of cm. This relationship comes from the intensity attenuation equation. It can be seen that if an incident energy of 1 and a transmitted energy is 0.5 (1/2 the incident energy) is plugged into the equation, the thickness (x) multiplied by m must equal 0.693 (since the number 0.693 is the exponent value that give a value of 0.5). X in this case is the half-value layer. Example Calculation What is the HVL for a material with a attenuation coefficient of 0.4/cm? |