Reference · Secure facilities & shielding
Half-value layer (HVL)
Half-value layer
The half-value layer (HVL) is the thickness of a specified material that attenuates radiation to half its original intensity. It is the core quantity for sizing radiation shielding: each added HVL halves the dose again.
What it is
HVL (also called half-thickness) is defined for a given radiation energy and material. Because attenuation is roughly exponential, two half-value layers reduce intensity to a quarter, three to an eighth, and so on.
Why it exists
It gives physicists and designers a simple, standardized way to translate a required dose reduction into a material thickness for a barrier.
Who it applies to
Anyone specifying or verifying radiation shielding — medical physicists and the contractors who build lead- and concrete-shielded rooms to their designs.
Frequently asked
What is a half-value layer?
A half-value layer is the thickness of a material that cuts radiation intensity in half. Because attenuation is exponential, adding another half-value layer halves the remaining intensity again — two layers give one-quarter, three give one-eighth.
Related terms
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