Half value layer

Dr Jeremy Jones and Dr Henry Knipe et al.

Half value layer (HVL) is the width of a material required to reduce the air kerma of a x-ray or gamma-ray to half its original value. This applies for 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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Article Information

rID: 22271
Section: Physics
Tag: cases
Synonyms or Alternate Spellings:
  • HVL

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