Calculators / Dose and shielding
Cs-137 dose rate and shielding
Cs-137 emits no gamma of its own: the 661.657 keV line (85.1 per 100 decays) comes from its daughter Ba-137m, always present at equilibrium. Values below sum every line of the ENSDF spectrum, as the Dose tab does.
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- EXPOSURE, 1 Ci @ 1 m
- 326 mR/h
- H*(10), 1 GBq @ 1 m
- 92.8 µSv/h
- Γ AIR KERMA
- 0.0771 µGy·m²/(MBq·h)
- Γ H*(10)
- 0.0928 µSv·m²/(MBq·h)
The formula
K̇(r) = A / (4π r²) · Σᵢ yᵢ Eᵢ (μen/ρ)air(Eᵢ) air kerma rate Ḣ*(10) = A / (4π r²) · Σᵢ yᵢ Eᵢ (μen/ρ)air(Eᵢ) · h*(Eᵢ) h* = H*(10)/Ka, ICRP 74 shielded: each line × B(μx) · e^(−μx), capped at 1
Dose rate against distance
| DISTANCE | 1 Ci: H*(10) | 1 Ci: EXPOSURE | 1 GBq: H*(10) |
|---|---|---|---|
| 0.3 m | 38.2 mSv/h | 3.62 R/h | 1.03 mSv/h |
| 0.5 m | 13.7 mSv/h | 1.3 R/h | 371 µSv/h |
| 1 m | 3.44 mSv/h | 326 mR/h | 92.8 µSv/h |
| 2 m | 859 µSv/h | 81.5 mR/h | 23.2 µSv/h |
| 5 m | 137 µSv/h | 13 mR/h | 3.71 µSv/h |
| 10 m | 34.4 µSv/h | 3.26 mR/h | 928 nSv/h |
Half- and tenth-value layers
Broad-beam values for H*(10), with ANSI/ANS-6.4.3 buildup: the thickness that halves, or cuts to a tenth, the dose rate behind a slab.
| MATERIAL | HVL | TVL |
|---|---|---|
| Lead | 0.768 cm | 2.35 cm |
| Steel (iron) | 2.88 cm | 7.13 cm |
| Concrete | 10.9 cm | 24.9 cm |
| Water | 26.6 cm | 57.3 cm |
Worked example
A 100 mCi (3.7 GBq) Cs-137 source, 1 m away, behind 2 cm of lead:
unshielded 344 µSv/h 2 cm lead 49.7 µSv/h (transmission 0.145)
Assumptions
- Point source, no self-absorption or encapsulation, vacuum path (the app’s default). With its air option on, the rates at these distances change by under 2 %.
- Photons below 20 keV are excluded (the Γδ convention, δ = 20 keV); the Ba K x-rays near 32 keV are kept.
- Buildup uses the ANSI/ANS-6.4.3 point-isotropic factors for each material, which are infinite-medium values applied to a slab. That overstates the dose behind a wall: a slab Monte Carlo in the app’s audit puts the true dose 1.5–1.7× lower 1 m behind these concrete and water walls, and 1.1–1.2× lower behind lead. The layers above therefore err on the thick side.
- These are first layers, measured from zero thickness. Deeper into a shield each further layer is thinner, which is why broad-beam tables quoting deep-penetration values list smaller numbers.
Sources
- Gamma spectrum: IAEA decay_rads (ENSDF).
- Attenuation and energy absorption: NIST SRD 126 (Hubbell & Seltzer).
- H*(10)/Ka: ICRP Publication 74, Table A.21. Buildup: ANSI/ANS-6.4.3 (Trubey, NUREG/CR-5740, 1991), with μ without coherent scattering from NIST XCOM.
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.
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