Radiography
Shielding: half-value and tenth-value layers
Transmission through a given thickness, or the thickness needed for a given reduction, from a half-value or tenth-value layer.
For reference only — verify against your procedure and the governing code.
mR/h, mrem/h or µSv/h — the shielded rate comes out in the same unit.
10 for one tenth, 2 for half.
Result
- Transmission
- 25 %
- Number of layers
- 2
- Thickness for the reduction
- 42.19mm
- Equivalent single-slope TVL
- 42.19mm
Published half-value layers (starting points)
- ASTM E94/E94M-17 Table 1 (typical)
- ASTM E94/E94M-17 Table 1 (typical)
- NRC Training Center health physics review, slide 14 (rule of thumb) — converted from the source's in.
- NRC Training Center health physics review, slide 14 (rule of thumb) — converted from the source's in.
Formula
- Transmission = (1/2)^(x / HVL) = (1/10)^(x / TVL)
- x = HVL · log₂(I₀ / I) = TVL · log₁₀(I₀ / I)
- Shielded rate = unshielded rate · transmission
Units. Half-value layer, tenth-value layer and thickness in mm or in. The rate is in whatever unit you enter it (mR/h, mrem/h or µSv/h): the shield only scales it. Presets given by their source in inches only are converted, and say so.
Assumptions and limits
- A single attenuation slope (narrow-beam behaviour). Real shields with scatter build-up transmit more than this predicts, especially thick ones.
- Published HVLs differ with beam geometry: ASTM E94 Table 1 lists 0.35 in. of steel for Ir-192, while the NRC training material uses 0.5 in. as a rule of thumb. Use your licence's or RSO's values.
Sources
- ASTM E94/E94M-17, Standard Guide for Radiographic Examination Using Industrial Radiographic Film — §14.4.2 Eq (1) and its worked example; Table 1 (typical steel HVL).
- U.S. NRC Training Center, "Industrial Radiography Health Physics Review" (NRC ADAMS ML12146A259) — gamma constants (slide 23), inverse square law (slide 24), half-value-layer rules of thumb (slide 14), 1 Ci = 37 GBq (slide 17).
More radiography calculators
For reference only — verify against your procedure and the governing code.
Standards and documents are cited by their publishers' names to identify them. TRACE is not affiliated with, endorsed by or sponsored by ASTM International, ASME, the U.S. Nuclear Regulatory Commission, NIST or the Laboratoire National Henri Becquerel. Formulas are paraphrased and cited by clause; get the documents themselves from their publishers.