Contamination monitoring
Standard Geiger-Mueller pancake probes used for Lu-177 do not detect alpha radiation. Alpha contamination monitoring requires zinc sulfide (ZnS) scintillation detectors, wipe tests counted on a low-background alpha counter, and routine surface-survey discipline in every restricted area.
Three monitoring modalities
Direct frisking
ZnS scintillation probes for shoes, gloves, lab coats, and surfaces after every dose draw or administration. Probe-to-surface distance matters — alphas travel only centimeters in air, so the probe face must be held within roughly 1 cm of the surface to detect.
Wipe (smear) tests
Routine smear surveys of work surfaces, dose calibrator wells, hoods, and patient areas, counted on a dedicated alpha or alpha/beta low-background counter. Document per RSO procedure with action levels appropriate to the work area (restricted, unrestricted, public).
Air sampling
Programs handling open Ac-225 should evaluate the need for periodic air sampling in the hot lab — particularly during any compounding, evaporation, or operation that could aerosolize. Document the basis for the air-sampling decision either way.
Survey frequencies (typical starting point)
Daily / after-use
Direct frisking of all surfaces touched during dose handling. Survey of the dose calibrator, L-block, and any transport container immediately after use. Documented on the day's hot-lab log.
Weekly / monthly
Comprehensive wipe-test survey of the hot lab, administration room, restricted corridors, and patient-restroom surfaces. Trending over time helps spot procedural drift before it becomes a contamination event.
A clean Geiger reading after handling Ac-225 doesn't tell you much on its own. Pairing beta/gamma frisking (which catches the daughter chain) with an alpha-specific survey on critical surfaces helps avoid a common false-negative — a pancake-probe frisk that reads clean while alpha activity remains on the bench.
- 10 CFR §20.1501 — General surveys and monitoring requirements.
- ANSI N323A — Radiation Protection Instrumentation Test and Calibration, Portable Survey Instruments.