Calculators / Dose and shielding
Pb-212 dose rate: why the progeny matter
Pb-212 on its own is a modest gamma emitter. Within hours its progeny grow in, and Tl-208’s 2614 keV gamma then carries most of the dose. A calculation on the parent alone reads about 8× low, and the shield it suggests is far too thin.
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Parent alone against parent with progeny
| SOURCE | Γ H*(10) µSv·m²/(MBq·h) | 1 GBq @ 1 m | LEAD TO 2.5 µSv/h |
|---|---|---|---|
| Pb-212 alone | 0.0262 | 26.2 µSv/h | 0.338 cm |
| Pb-212 + progeny | 0.21 | 210 µSv/h | 10.1 cm |
Transient equilibrium
A daughter shorter-lived than its parent settles at a fixed multiple of the parent’s activity — not equal to it, as in secular equilibrium:
A_d / A_p → λ_d / (λ_d − λ_p) Bi-212: 1.105 × A(Pb-212) Tl-208: 0.399 × A(Pb-212) (35.94 % branch of Bi-212)
Starting from freshly purified Pb-212, Bi-212 is within 1 % of that ratio after 7.4 hours. Any source older than that — which is every source that has been shipped — should be calculated with its progeny.
Other parents with the same trap
Any parent whose aged progeny raise its dose by more than half has a “+ progeny” entry in the Dose tab, enforced by the test suite. The full list: Zr-95, Mo-99, Ba-131, Ba-140, Ce-144, Ra-223, Ra-224, Ra-226, Th-228, Th-232, U-232, U-238, Np-237, Pb-214, Pb-212, Bi-212, Ra-228. Choosing the bare parent shows a warning that points to them.
Assumptions
- Progeny at their equilibrium activity ratio, computed from IAEA half-lives and branching — the aged-source case.
- Point source, ANSI/ANS-6.4.3 buildup, photons below 20 keV excluded.
Sources
- Gamma lines and branching: IAEA decay_rads (ENSDF), NUBASE2020.
- Equilibrium: Lamarsh & Baratta §2. Attenuation: NIST SRD 126.
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.