7040 A. Counting Room
Because the quality of low-level radioactivity measurements improves when background radiation levels are low and stable, facility designers should carefully evaluate the counting room’s location and design with facility-specific contributors to background radiation in mind. As a result, the counting room’s location, design, and construction may vary significantly from one laboratory to the next. For example, locating a counting room in a lower story of a facility may provide more shielding from cosmic radiation, while locating it in a higher story may better limit terrestrial radiation. Avoid transporting, handling, or storing materials containing even moderate levels of radioactivity (e.g., samples, sample residuals, radioactive standards, or wastes) in or near the instrument area. Select building materials for walls, floor, and ceiling that minimize background radiation from such radionuclides as postassium-40 (40K) and uranium and thorium chain radionuclides. Also, construct exposed surfaces (floors, walls, ceilings, countertops, etc.) from materials that are easy to clean if contamination occurs. Provide enough room in the instrument area to allow sufficient access for required periodic maintenance.
An appropriate heating, ventilation, and air-conditioning (HVAC) system should be provided to maximize instrument stability. In many cases, a humidity-control system will also prove valuable for instrument stability. As with all electronic equipment, follow the instrument manufacturer’s recommendations for operating temperature and humidity as closely as possible.
Consider that not all counting instruments have voltage-regulating or power-conditioning circuitry suitable for controlling minor fluctuations in line voltage. Additionally, most power lines in laboratories (even new construction) are not designed to control power fluctuations for the building. Voltage fluctuations can cause changes in background and in the instrument’s resolution and energy calibration. The effect of unusual fluctuations can be minimized using an auxiliary voltage regulator/transformer. The risk of losing analytical data due to power interruptions can be minimized by using uninterruptable power supplies (UPS).
Although a clean room is not required to control contamination, the counting room should be adequately segregated from the area(s) where samples are physically and chemically processed to minimize exposure to dust and fumes, thereby minimizing contamination and protecting the instrumentation’s electrical stability. Minimize the migration of radioactive contamination by locating the counting room in an area with minimal traffic flow and regulate the HVAC system so air flows from areas of least to greatest potential contamination (i.e., from counting areas to chemical-separation areas and then to sample-preparation areas). Implementing measures to minimize fluctuation in radon levels will significantly improve the quality of low-level measurements that the facility can perform.