Hold-time calculation for the Ac-225 chain
Ac-225 (t½ ≈ 9.9 d) decays through a chain of short-lived progeny that ends at Pb-209 (t½ ≈ 3.25 h). Decay-in-storage under 10 CFR §35.92 requires holding for ≥10 half-lives of the longest-lived component and confirming release by survey with instrumentation able to detect the radiation present. For Ac-225 streams the parent governs the calendar hold; the daughter gamma from Bi-213 (~440 keV) governs the survey methodology.
Inputs to the hold-time decision
10 half-lives of the parent
≥10 × 9.9 days ≈ 99 days as the regulatory floor for Ac-225 streams. This is the calendar minimum before a release survey is even attempted — not the release date itself.
Progeny equilibrium and Pb-209
Once secular equilibrium is reached, daughter activity tracks parent activity. Pb-209 (t½ ≈ 3.25 h) is the longest-lived progeny and decays out within a day of the parent reaching background. The chain does not extend the calendar hold meaningfully — but it dictates survey methodology.
Initial activity and background
Higher initial activity per container, or lower local background, extends the time to reach the survey threshold. A hot-lab dose-draw container will take longer to clear than a post-administration glove bin even with the same calendar age.
Survey threshold in your written procedure
The release threshold (typically ≤ background + a defined margin on both alpha and gamma channels) is what actually determines release date. Document the threshold, instrument, and counting time in your written procedure — not just the calendar rule.
Common misconceptions
What people assume
Hold time = 10 × 9.9 d and you're done. If the gamma survey clears, the container is safe to release. All streams need the same hold time. Pb-209's 3.25 h adds days to the hold.
What actually governs release
Calendar floor AND survey result both required. Alpha survey clears independently — gamma alone is insufficient. Higher-activity streams need longer than the floor. Pb-209 decays out within hours once parent reaches background.
One phrasing that tends to hold up well in inspection: 'whichever is later — calendar floor OR a documented release survey clearing both alpha and gamma channels.' Including that in the written procedure avoids most common release-survey disputes.
- 10 CFR §35.92 — Decay-in-storage.
- NCRP Report No. 155 — Management of Radionuclide Therapy Patients.