Buying a radiation detection instrument.
Written by George Dowell.
Html and grapics by Tracy Albert.
What IS and what IS NOT a Geiger Counter.
Let's clear the air right away on this topic. Many newcomers to this fascinating hobby are introduced to their first instrument by a "bargain" buy on eBay or elsewhere. Not that there is anything wrong with this at all, but many if not most advertisers are either uninformed or untruthful. Know what you are getting, and what to expect from it once you have it. This summary may help.
***A Geiger Counter by definition must have a Geiger-Mueller tube detector element as the probe. From here on out we'll call that component a GM or G-M tube. The G-M tube detector itself comes in 3 different physical configurations, namely:
1) The thin wall tube, commonly referred to as the "hot-dog", Good sensitivity to Beta and Gamma radiation. Rugged. Inexpensive. Made from metal or glass. A type 6993 metal tube is the standard tube included on any CDV-700. Easily picks up radiation from a Radium watch dial. Material thickness of the walls will block all Alpha particles.
2) End-window tube. As the name implies, it is a similar tube to the hot dog, usually a little larger diameter, but fitted with an end-window of very thin Mica or Mylar. By doing this, Alpha radiation can also be detected, and the larger gas volume makes it more sensitive to Beta and Gamma as a bonus. Maybe 10 times more sensitive than the "hot-hog". The trade off is the tube is that it is more fragile because of the very thin window, and more expensive.
3) Pancake tube. Actually this element looks more like a hockey puck. Usually 2" or so in diameter and 1/2" thick. The entire face is a thin-window, and this type probe is the most sensitive of the G-M family. It will let in Alpha, Beta, and Gamma radiation, with a sensitivity out of this world. Even though they are also fragile, usually the housing or "handle" in which they are installed gives them some protection. A screen is almost always used in front of the element to keep it from coming into physical contact with any object, which would surely damage the window material. 100 or so times more sensitive than the "hot-dog". Be careful if buying a used element. They are very susceptible to damage in shipping.
To buy a new GM tube http://www.lndinc.com/ is the best source.
All G-M tubes share some common attributes.
A) They need a high voltage DC charge in order to work. This is generated within the counter and is impressed directly upon the wire going to the probe. BE CAREFUL! Military surplus units usually use 500 or 700 Volts. Older non-solid-state commercial units can use 500 to 900 Volts. Modern commercial units use 900 Volts almost exclusively. Any G-M tube will work on any Geiger counter, as long as the operating voltage is compatible. The FIRST modification to a CDV-700 unit should be to add a BNC probe connector.
B) All are filled with a gas mixture, at low pressure. This gas is usually Neon, and a Halogen gas called a Quench gas.
C) When radiation penetrates into the tube, it causes the gas to ionize, and the high voltage is essentially shorted to ground between the two electrodes ( one being the body of the tube, the other a wire or other conductive structure inside). When this happens a very large pulse is produced, and that event is the "click" you eventually hear in the earphone. All pulses are the same, no matter what triggers them. Only a fraction (<1%)of the radiations will produce this click, with the vast majority passing right through the tube, or being absorbed by the walls. D) Because there is a certain time needed between pulses for the quench gas to extinguish the spark, G-M tubes have a definite limit on the upper level of how many pulses can be detected in a given minute. Rephrasing this important point: A G-M tube will become saturated in a high radiation field, and will actually stop responding, giving a low or zero reading. For a detailed explanation of the above, see:
http://www.wpi.edu/Academics/Depts/ME/Nuclear/Reactor/Labs/R-ion.html
OK now we have the basics, we can move on. Re-read point C) above. With only 1% efficiency, we can see that a G-M tube, sensitive as they may be, can only give a certain reading from a given radiation sample. As a matter of fact each of the 3 types of G-M tube will give a different reading, according to it's sensitivity. Gamma radiations from different sources have different energy levels. These energy levels are analogous to "frequency" in other types of electromagnetic radiations, like light, and radio. It would be helpful if the probe detector would somehow give a different height pulse for each different energy level it encounters. In the case of the G-M tube, this is not possible.
Now in order to achieve more sensitivity and to produce a pulse of varying height, we must move up to the next level of detector probe. This class device class is called "Scintillators". A Scintillator works on an entirely different principle. We will explore them in detail in a separate article, but in brief, they work by using a special material NaI(Tl) that will emit a light pulse whenever it encounters gamma radiation. This material varies depending on it's intended use, but they all produce 1 photon for each particle or ray they encounter. Fixed to this material is a super sensitive electric eye tube, called a photomultiplier. When it sees the photon, it will amplify it as much as 1 million times and produce an electrical pulse output. Now the good news : Not only is the Scintillator almost 100% efficient, the pulses it gives off vary in height according to the energy level of the radiation. In more advanced counters, this variable height allows you to discriminate between the different energy levels and perform what is know as Gamma Spectroscopy. Identification of specific isotopes is possible with such a setup. Ratemeters or counters that are specifically designed to work with Scintillators will have a variable high voltage control. However good results can be obtained with the fixed 900 Volts from a CDV-700 or similar, if modifications are made to the metering circuits. We will go into this later in another article.
Click on highlighted text to see a picture.
OK now go back and re-read point D) above. Because the G-M tube will saturate, it is useless in a very high radiation field ( such as would be encountered after a Nuclear Bomb blast). There are other devices that ONLY work in a high field but have NO sensitivity to low radiation. These are the ION-CHAMBER class. Included are the Civil Defense CDV-710, 715, 720 etc. Basically anything EXCEPT a CDV 700 is probably an ION- CHAMBER. Now the bad news. These work on an entirely different principle than the G-M series, and can not be upgraded to work with a G-M tube. In addition there is virtually no way to test these "survey meters", because it would take a dangerously high radiation field to make them respond even a little. Bottom line, if you have a CD 715, it is a conversation piece, cold-war curiosity, civil defense collectable, door stop, but it IS NOT A GEIGER COUNTER. Of the all yellow Civil Defense items available, only the CDV-700 is a Geiger Counter.
To contact George or Tracy regarding Geiger counters.
http://groups.yahoo.com/group/GeigerCounterEnthusiasts/
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