Calculators / Dose and shielding
Co-60 shielding: half-value layers and required thickness
Co-60’s two gammas at 1173 and 1332 keV make it the reference hard-to-shield source. Its unshielded rate is 13.1 mSv/h H*(10) per curie at 1 m. Buildup matters here: a broad-beam half-value layer is thicker than the textbook narrow-beam one, and the gap grows with depth.
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- 1 Ci @ 1 m, H*(10)
- 13.1 mSv/h
- 1 Ci @ 1 m, EXPOSURE
- 1.29 R/h
- LEAD HVL (BROAD)
- 1.56 cm
- CONCRETE TVL (BROAD)
- 30.5 cm
Half- and tenth-value layers
| MATERIAL | HVL NARROW | HVL BROAD | TVL NARROW | TVL BROAD |
|---|---|---|---|---|
| Lead | 1.03 cm | 1.56 cm | 3.44 cm | 4.65 cm |
| Tungsten | 0.642 cm | 1.02 cm | 2.14 cm | 2.98 cm |
| Steel (iron) | 1.65 cm | 3.58 cm | 5.48 cm | 9.24 cm |
| Concrete | 5.2 cm | 12.4 cm | 17.3 cm | 30.5 cm |
| Water | 11 cm | 28.5 cm | 36.5 cm | 67.4 cm |
Narrow beam counts only unscattered photons, e−μx. Broad beam adds the scattered ones through the buildup factor B, and is the one that matters for a shield. Because B grows with depth, a TVL is more than 3.32 HVLs.
The buildup here is the ANSI/ANS-6.4.3 point-isotropic factor for each material, an infinite-medium value: it overstates the scatter that reaches a point behind a finite wall. A slab Monte Carlo in the app’s audit puts the true dose 1 m behind 30 cm of concrete about 1.4× lower, and behind 10 cm of lead about 1.2× lower, so a design based on these values errs on the thick side. The values are also first layers, from zero thickness; deeper layers are thinner.
Worked examples
Lead. A 1 Ci (37 GBq) source read at 1 m:
to 25 µSv/h 11.6 cm of lead to 2.5 µSv/h 15.5 cm of lead
Concrete. The same source, with the wall’s far face at 3 m, needs 68.3 cm of ordinary concrete for 2.5 µSv/h.
Both thicknesses are solved in the app’s Shield mode and checked by putting the answer back through the dose calculation.
Assumptions
- Point source, a single slab between it and the point of interest, no scatter from walls or floor.
- ANSI/ANS-6.4.3 buildup, each material with its own coefficients, applied with the attenuation coefficient without coherent scattering as the standard requires. Infinite-medium factors applied to a slab are conservative behind a wall.
Sources
- Gamma lines: IAEA decay_rads (ENSDF). Attenuation: NIST SRD 126.
- Buildup: ANSI/ANS-6.4.3 (Trubey, NUREG/CR-5740, 1991), with μ without coherent scattering from NIST XCOM. H*(10): ICRP 74 Table A.21.
Numbers on this page computed by the physics code of Nuclear Toolkit 1.4.1, the same code as the app. How it is checked.