Answer to Question #628 Submitted to "Ask the Experts"

Q

Our CDV-777 kits are about to expire. I desire to replace them with something more accurate, yet still very "user friendly" for my fire department. Our primary objective would be to set a safe perimeter and call for expert help. Our incidents include everything from soil compaction measuring devices to medicinal treatments for cancer patients who may be involved in traffic accidents, etc. Can you recommend instruments (both wide area hand-held detectors and personal dosimeters) that meet our objective?

A

This a very timely question in that FEMA is no longer supporting these old CDV-777 kits through upgrades of instruments and many state instrument calibration facilities are closing. These items have had a significant impact on local emergency response organizations. The CDV-777 kits contained the CDV-700 Geiger-Mueller (GM) counter, CDV-715 ion chamber (IC), and quartz fiber direct-reading dosimeters (DRD) and charger. In the short term, there are plenty of these instruments at state facilities, and they can be had for the shipping cost. They can also be found on a regular basis for little money at online auction sites. Hundreds of thousands of these meters were made for the Civil Defense program during the Cold War. When calibrated to ¹³⁷Cs or ⁶⁰Co, the CDV-700 GM survey meter is adequate for a rough (i.e., +/- 20%) low range gamma dose rate measurement for mid- to high-energy gamma radiation (i.e., 200 to 1,000 keV). It will over-respond (i.e., give a conservative reading) at low energies (e.g., the 60 keV gamma from ²⁴¹Am in a soil compaction or moisture gauge). The wall thickness on the GM tube is ~ 40 mg/cm^2, and thus it has limited utility for contamination monitoring of loose radioactivity even when the probe is open. One nice feature that it does have is a small, exempt-quantity uranium check source on the side of the case to verify function. The CDV-715 survey meter IC is adequate for high-range gamma dose rate measurements only. Its energy response is similar to the GM (flat at mid- to high-gamma energies) but it drops off at low energies because of the steel case. If the batteries are good on either and one fails to respond, don't waste your money on repair. The various styles of DRDs FEMA supplied are quite robust and should last for a long time. They should be discarded if found to drift with no radiation field, or if they don't respond when calibrated. For annual calibrations (if the state doesn't perform this function), there are many local vendors that provide this type of service. Contact your state radiation control program, see the Conference of Radiation Control Program Directors Web site, or call a local hospital or nuclear power plant's radiation safety office to learn whom they might use. With the objective to set a safe perimeter until assistance arrives, the local emergency responder should limit his exposure (and the public's) to less than a 0.02 mSv/hr (2 mR/hr) gamma field with no contamination present if medical intervention is not needed. If medical intervention is needed, then that should always be the priority. Responders should also limit the spread of contamination if possible. The replacement of the old CDV-700 is a good idea if the budget permits. If not, some vendors have replacement "pancake" GM probes that will plug into the cable connection on the CDV-700 for less money. There are many vendors who manufacture GM survey meters with "pancake" probes. See our affiliates Web page for affiliates that build and supply such instruments. Many also have full emergency response kits to replace the CVD-777. The "pancake" GM probe is perhaps the best single probe for field measurements. It has a thin mica window ~ 2 mg/cm^2 that will detect alpha and beta contamination, and when measuring gamma radiation through the front, it is responsive to very low energies. However, as with the CDV-700 metal wall GM tube, the "pancake" will (conservatively) over-respond at the lower energies if not compensated with a special filter. Most "pancake" and other GM probes and survey meters should easily permit calibration up to a dose rate of 0.5 to 1 mSv/hr (50 to 100 mR/hr). At these and higher dose rates, the modern meter will not saturate and drop to a "zero" reading. Some old GM meters may do this in a very high radiation field. If one needs or wants a higher range IC, these are also available from many suppliers. Newer design ICs may also have better low-energy response. With respect to the DRDs, they come in low-range (0-200 mR) and high-range. The low-range is no doubt the best for emergency responders. I believe they are still being manufactured by at least one domestic and perhaps another overseas vendor. The dosimeters and required chargers may be purchased through several suppliers. With regard to chargers, I would recommend the piezoelectric hand type charger with no batteries to maintain. If the quantities are few or money is not an issue, many vendors provide digital display electronic dosimeters for personal monitoring. Some use GM tubes, while others use solid state devices. The above noted HPS URL provides links to information for all these suppliers and manufacturers. Most DRDs and electronic dosimeters have a fairly flat energy response at mid- to high-gamma energies. Finally, there are ANSI standards that cover the testing and calibration of all these devices. Manufacturers and calibration services should conform to these standards. If you check with members of the radiation safety staff at a high-volume user of these devices (e.g., large medical center or nuclear power plant), they should be able to provide an idea of whose equipment is the easiest to use, the most reliable, and the most robust—three attributes necessary for emergency response equipment. For training, one should contact the state radiation control or emergency response program. FEMA, NRC, and DOE all have training materials available that would be helpful in this area, and the FEMA: Preparedness—Radiological Emergency Preparedness Program Web page would be a good place to start. David Allard, CHP