Answer to Question #1151 Submitted to "Ask the Experts"

Q

Is there an inexpensive detector/instrument (less than $200) to warn workers of elevated levels of radioactivity while conducting site visits at locations expected to be contaminated by uranium and other mine tailings wastes (overseas locations with unreliable monitoring records)?

A

One would expect uranium, and to a lesser extent thorium, series radionuclides in mine tailings waste at these sites. Thus, there will be a fair amount of medium- to high-energy beta/gamma activity in conjunction with numerous alpha emitters. The best detector to have would be a "pancake" mica window-style Geiger-Mueller (GM) probe with appropriate electronic scaler/rate meter. If held close to a surface, this probe will detect alpha, beta, and gamma radiations to varying degrees. Even at some distance, it will detect the beta/gamma activity. There are many manufacturers of such equipment which can be found by reviewing "Affiliate Member" links on the Health Physics Society Web site. Unfortunately most survey meters with a pancake probe or internal pancake GM tube on the bottom of the instrument will run about $300 to $600. However, there are thousands of old CDV700-type civil defense meters out in hands of state agencies and the general public through surplus (for example, online auctions). Most are still functional and can be had for free or less than $25. Note: not worth a dime more. The standard small area thin metal wall GM probe on these instruments may detect higher energy betas and gamma radiation, but will be significantly less sensitive than a pancake GM probe. Also, there is normally one audio indication on the CDV700. Thus, you have to watch the meter deflection. The good news is there are two Affiliates that provide replacement pancake probes for the CDV700 for less than $200. If cost is a concern, this might be a way to go. But if you do go this route, I'd bring a couple back-up CDV700 meters and if one fails, one can just swap the probe and be back in business. The other nice feature of the CDV700—there is an uncalibrated uranium check source on the side of the instrument. If one obtained a small calibrated and traceable beta source (for example, Tc-99), an efficiency could be determined for each probe. That information coupled with the known 15 cm square pancake window area, will allow calculation of a correction factor, to ultimately provide a final dpm/100 cm sq. David J. Allard, CHP