Half-value layer

Patent Pending A material's half-value layer (HVL), or half-value thickness, is the thickness of the material at which the intensity of radiation entering it is reduced by one half.[1] HVL can also be expressed in terms of air kerma rate (AKR), rather than intensity: the half-value layer is the thickness of specified material that, "attenuates the beam of radiation to an extent such that the AKR is reduced to one-half of its original value. In this definition the contribution of all scattered radiation, other than any [...] present initially in the beam concerned, is deemed to be excluded."[2] Rather than AKR, measurements of air kerma, exposure, or exposure rate can be used to determine half value layer, as long as it is given in the description.