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