Medical Physicist's Notes · Area Gamma Monitor
Six typical deployment locations,two thresholds at each.
Area gamma monitor deployment is decided by the radiation-safety officer in the site survey — placement rationale, threshold values, AERB dossier line. This post walks six typical locations at a cyclotron / PET / theranostic facility, the rationale for each, indicative threshold values, and what each unit contributes to the AERB inspection dossier.
Six typical locations
Where to place area gamma monitors
| Location | Why | Threshold (indicative) |
|---|---|---|
| Cyclotron vault entry (corridor side) | First-line read of the field operator walks into. Two thresholds gate the entry-decision window. | Warning ~1 µSv/h · Action ~5 µSv/h (site-dependent) |
| Hot-lab clean-room boundary | Confirms contained activity is not spilling into the corridor; documents radiopharmacist workplace dose. | Warning ~0.5 µSv/h · Action ~2 µSv/h |
| Theranostic ward corridor | Patient rooms generate localised dose-rate field. Documents compliance with public-area dose limits. | Site-specific; based on annual public-dose budget |
| Decay-store / waste-storage area | Catches mislabelled vial or damaged shielding container before contamination incident. | Warning ~2 µSv/h · Action ~10 µSv/h (waste-area allowance higher) |
| Facility perimeter (outdoor posts) | Documents the public-area dose-rate consistent with the AERB facility licence; demonstrates compliance. | Background + small margin (typically < 0.1 µSv/h) |
| Hot-cell exhaust adjacency | Documents the dose-rate at the operator side of the hot cell during a high-activity production run. | Site-specific; coupled to the production-run dose budget |
Source: AERB Safety Code for Medical Cyclotron Facilities; ICRP Publication 75; site-survey practice. Threshold values are indicative; final values from the site survey + AERB facility licence.
Threshold setting
How the warning + action thresholds are derived
- Threshold settings are not universal — every site survey produces site-specific values keyed to the AERB facility licence, the workplace occupancy and the public-area dose-budget.
- The warning threshold is set at the level that triggers operator investigation but not evacuation. The action threshold is set at the level that disallows entry / triggers area evacuation / triggers interlock-driven beam-off.
- For a cyclotron facility, the vault-entry warning threshold is typically about 5× the routine corridor background; the action threshold is typically about 5× the warning threshold. Final values come out of the site survey.
- Outdoor perimeter monitors live close to background. The "warning" threshold is set at background + a margin that catches a real release event but doesn't trip on weather-driven background variation.
- Threshold change events are logged on the device and streamed to the RMS server. AERB inspection reads the threshold-change log; an undocumented threshold change is an inspection finding.
AERB dossier
What each monitor contributes to inspection
- Type-test certificate (manufacturer)
- Calibration certificate (in-country traceable; renewal cycle per AERB requirements)
- Site-survey report with monitor placement rationale + threshold derivation
- Commissioning record (initial baseline read; threshold acceptance test)
- 180-day local archive accessible from the device; RMS-server archive for long-term trending
- Annual review record: monitor still in service, calibration current, threshold still appropriate