Two Letters Re: Test Sources for Radiation Meters--The Cesium Source Already in Your Home

Sunday, Apr 9, 2006

Mr. Rawles,
I'm very suspicious of the information from "Ole Rad" that you could test a Civil Defense field survey meter with the radiation that comes from a smoke
detector. Several things in his post don't add up:
1) Smoke detectors use Americium-241 as their source and the radiation at 1 meter distance is "less than 1/1000th of that from background radiation" (source: http://www.arpansa.gov.au/is_smkdt.htm).
2) The CDV-777-2 is the radiation detection kit which contains a field survey meter, dosimeters, and a dosimeter charger. The kit might also contain a CDV-700 geiger counter, but that has it's own beta check source on the side. Also, the CDV-700 uses 4 D cells, while a CDV-715 or 717 uses 1 D cell. The CDV-720 uses 2 D cells.
3) It takes a minimum of 0.1 R/h (or 100 mR/h) to defect the needle to the "1" position on the meter for a CDV-715, 717, or 720 field survey meter set
to the lowest range (meter reading x 0.1 R/h). A 1 microCurie Cesium-137 source emits about 2 milli-R/h at the surface. Thus, it would take 50 of these sources together to produce 0.1 R/h. A low-level source can be used with a field survey meter, but it requires a special pancake probe instead of the ion chamber. Regards, - A.C.

JWR Replies: In my estimation, Ole Rad's advice only applies to Geiger counters with a low (highly sensitive) range.


Jim: There has been some confusion lately about the surplus civil defense radiation gear. Here is a quick rundown on what you might find.
Survey Meters
CDV-715, CDV-717, CDV-720 - These are what's known as a "high range" meter. They use a device called an ion chamber to measure life- threatening levels of radiation. They were intended to be distributed to fallout shelters in the event of a nuclear war, so that radiation levels outside could be monitored and reported. This type of unit WILL NOT detect low levels of radiation, such as that from a "dirty" bomb, a radium-dial clock, smoke detector or tritium gun sight. About the only way to make the needle move much on one of these is to expose it to a large gamma source (such as in a calibration lab or cancer treatment facility). You may be able to test one of these by exposing it to a doctor or dentist's X-ray machine. Set the meter on the lowest range and see what happens.
Most of these have a self-check circuit that can let you know if the basic electronics are functional. If you are serious about keeping one or more of these around for a real emergency, you should definitely get it calibrated and serviced. The KI4U folks can get this done, and there are several other facilities that will calibrate these meters. Expect to pay $20-75 for a meter in good shape, and possibly another $100+ for calibration.
CDV-700 - This is a true Geiger counter. You can easily spot these by the "hot dog" shaped probe attached to it via a cable. These are pretty sensitive, and will pick up small radioactive items, such as radium-dial watches. Tritium gun sights are just too weak to be detected by any common detector. Also, these have small test source affixed to the side that you can use to instantly test if the meter is working. These usually sell for $100+, and would be much more appropriate for detecting fallout from a dirty bomb, nuke plant accident, etc. These were designed for checking people, food, etc for small amounts of contamination.
Along with the Civil Defense surplus, there are a number of newly made Geiger counters, usually from Russian companies. Harbor Freight sometimes has one called the Quartex, and there is another one called the RKSB-104. You can often find these on eBay.
Dosimeters
These are small, yellow sticks that look like a big crayon. Unlike a survey meter or geiger counter, these do not instantly show you how much radiation they are being exposed to at the moment. Rather, you wear them around, and they let you know how much total dosage you received over a day, month, etc. Like the survey meters, these come in high and low-range models. The good ones are made by Bendix. Avoid the other brands, unless they are of new commercial manufacture (such as Dosimeter Corp).
CDV-741,742 - High range (0-100 or 0-200 RADS). Useful after a nuclear war, not useful for much else.
CDV-138 - Low range (0-200 Millirads). Useful for working around an X- ray machine, checking if you got exposed from a small source or accidental leak. Much more rare than the other type.
The dosimeters must be charged before they are useful. Look for a CDV-750 or similar charger. You can test dosimeters by charging them up (this sets the needle to zero) and then leaving them sitting for a couple of weeks. If some of them rapidly leak down to zero, they are bad. Otherwise, these items have a very long useful life. They have no batteries, and only need the charger to put a static electricity charge into a small piece of fiber. The static charge leaks off it when exposed to radiation.
Here are links to more than you ever wanted to know about Civil Defense gear:
http://www.civildefensemuseum.com/
http://groups.yahoo.com/group/CDV700CLUB/
Thanks, - JN


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