Half-value layer converter
Convert between a material’s attenuation coefficient and the thickness that halves or cuts to a tenth an X-ray or gamma beam.
That's about the same as lead (Co-60). Cutting the beam a thousandfold takes about 11.96 cm of it.
What it means
How strong a shield is that?
Narrow-beam attenuation. Real shields let through more because of scattered radiation (build-up); design shielding to the relevant standards.
Every unit
That shielding in every unit
Learn
Three things worth knowing
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HVL × 3.3 = TVL
A tenth-value layer is 3.32 half-value layers. Ten HVLs cut a beam about a thousandfold.
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Measuring X-ray quality
Radiologists in the 1920s described X-ray beams by how much aluminium or copper halved them; HVL still defines beam quality.
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Not a straight line
Doubling the shield does not stop the beam: it halves what was left again.
Shortcuts
Popular shielding conversions
Background
About half-value layers
X-rays and gamma rays fade exponentially through matter: I = I₀ e^(−μx). The half-value layer (HVL), ln 2 ÷ μ, is the thickness that halves the beam; two HVLs leave a quarter, ten leave about a thousandth. The tenth-value layer cuts it to 10 %.
Cobalt-60 gamma rays need about 1.2 cm of lead or 6 cm of concrete per halving; diagnostic X-rays only 0.25 mm of lead.