Answer to Question #13443 Submitted to "Ask the Experts"

How critical is location of health physics counting system instruments such as gamma products Mini "T" or Ludlum 3030E used in regular contamination surveys? The question arose when these instruments were moved from one location to another in a facility. What steps are necessary to relocate such instruments, if a need arises?

Where one elects to locate instrumentation used in performing health-physics related measurements may be influenced by a number of factors. These include such considerations as the radiation background characteristics in the prospective location, the physical characteristics of the location (e.g., strength of load-bearing floors), the potential for radioactive contamination in the location, the human traffic patterns in the location, convenience with respect to egress from your area and ingress to areas frequented by staff, and the influence of other decision makers. I shall attempt to address some of the considerations that apply in taking these factors into account.

Clearly, the radiation background may be an important factor, especially if it is high or if it is subject to appreciable variability, since these factors affect one's ability to make accurate and precise measurements. It is desirable to select an area far enough removed from source storage and/or use areas to minimize effects from these. Measurements should be made in prospective areas in advance of any move to evaluate the background characteristics, both magnitude and variability and, in some instances, the types of radiation contributing to the background. Both of the counting systems you mention are alpha-beta systems that use ZnS (alpha) in combination with a plastic scintillator (beta) to do simultaneous alpha and beta counting. Elevated ambient backgrounds will likely have a larger effect on the beta counting than on the alpha systems because the discriminator levels in the alpha detectors are set high enough to reject most of the beta/electron/gamma-electron induced pulses; naturally, effects of elevated backgrounds should be checked for both the alpha and beta channels. Another consideration that may be a concern is radon. This can be problematic, especially in some basement areas in which good ventilation may not exist and where radon entry pathways into the building may exist. Even non-basement areas in some buildings may be subject to excessive buildup of radon if ventilation is inadequate. Elevated radon levels can affect backgrounds for both alpha and beta counting.

If some of your instrumentation includes (or may include in the future) heavy shielding, such as is typical for gamma spectrometry systems, you must pay attention to the load-bearing characteristics of the floors in the areas of interest. The two systems you describe would likely require no special structural support, since they incorporate relatively little shielding.

It is preferable that any area to be used for counting not be subject to likely contamination. Locations close to work areas where unsealed radioactive material is being used, especially if such areas have had past histories of spills and consequent area contamination, are undesirable in order to avoid tracking of radioactive material into your space. Clearly, it is desirable to select a location that is not subject to excessive non-health physics personnel traffic in order to minimize the potential for introducing contamination into your work area.

It is obviously also desirable that the selected area be reasonably convenient for access, egress, and use. The selected area must be able to accommodate all of the current and likely future instrumentation and other gear that may be involved with your operations. It is desirable that your staff be able to get to other areas requiring monitoring and to be able to return with test wipes, etc., fairly quickly, especially when responding to emergency situations. Depending on the size of your facility and the numbers and distributions of authorized users, some inconvenience may have to be accepted in such matters.

Finally, you must deal with the judgment and decision of the administrative person or persons who will often make the final judgment about what location you will occupy. Unfortunately, such individuals are not always influenced most strongly by what is most appropriate and technically most sound. Despite this, in my experience, it is always worth establishing contact with such individuals early on to provide them with reliable and justifiable rationales for the preferences you might have. Since your move has apparently already taken place, this point is currently moot, but it is something that should be kept in mind for future possible moves or modifications of your space.

All of the above considerations should be dealt with as one plans for a move, and some time may be required to obtain information necessary to make the proper decision. For example, the radiation background should be evaluated over a long enough period of time to be able to observe possible variations in background, depending on what activities are being conducted in the facility. If there is reason to suspect that radon may be a problem, you will need time to make meaningful measurements, which may sometimes extend over several months. Since you would already be familiar with where radioactivity is being used you should be able to make reasonable fast decisions about potential contamination and traffic patterns with respect to the proposed area. If you are not familiar with the structural characteristics of the building, you may have to consult with a building engineer to determine what the floor-bearing characteristics are if you have plans to move heavily shielded instrumentation into the area. You may also have to arrange for trained riggers to move such equipment if such workers are not onsite.

While other items may arise in relation to other specific needs associated with a new location, the above discussion relates to items that are often of concern. I hope the discussion is helpful.

George Chabot, PhD, CHP