Half-value layer (HVL) is the width of a material required to reduce the air kerma of an x-ray or gamma-ray to half its original value. This applies to narrow beam geometry only as broad-beam geometry will experience a large degree of scatter, which will underestimate the degree of attenuation.
It is an indirect measure of photon energy or the hardness of a beam. HVL is measured in millimetres of aluminium.
It is related to linear attenuation coefficient (μ) with following formula:
- HVL = 0.693 / μ
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