FAQ - Geiger counter tips and techniques.

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Richard Hull
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FAQ - Geiger counter tips and techniques.

Post by Richard Hull » Thu Aug 14, 2003 3:25 pm

Geiger counters, as purchased, whether new or used are limited in their capabilities. In spite of this, they are among the easiest to use and most sensitive of all the radiation detectors.

A geiger counter is not a device as much as it is a detector hooked to a bunch of electronics.

The detector is the key and its use is related to its application.

The electronics is another issue and can limit or expand the value of the detector.

Types of geiger tubes********************************
1. Alpha/Beta/Gamma
2. Beta/Gamma
3. Gamma

Type 1. Alpha-Beta-Gamma

This is always a mica windowed tube. It is the most sensitive of all geiger tubes and, in general, the most sensitve nuclear detector you can obtain. It MAY NOT be the best detector for a number of applications, however.

This tube takes on only two real forms; a long tubed detector and a pancake detector.

The long tube mica windowed detector is a good compromise in efficiency for detection of all particles encontered. It is, therefore, the tube of choice for the best general survey and laboratory geiger counters. We will get into reasons shortly.

The Pancake detector is the absolute finest of the geiger detectors in that it is the tube with the largest sensitive area. Thus, very weak sources are easily located and counted. It is one of the best alpha detectors readily available.

Now, as to efficiencies.......

Both the long tube and the pancake are just about 100% efficient in counting all alphas that penetrate their windows. The key portion of the foregoing is "....that penetrate their windows". Alphas have a very short range in air (<3-7cm). They require a certain energy just to penetrate the mica window. Therefore, to be absolutely sure you collect all the alphas you MUST be at point-blank range from the source to the window!

There are two problems here; contamination of the window and damage to the detector. Alpha sources can be nasty and loose particles can often drift up or be ejected from the source and attach to the mica window, thus raising the tube's background count forever. (you can't brush, touch or clean a mica windowed tube.) The mica window is so thin that the slightest touch or impact can blow the window into hundreds of fine pieces. Thus, an irregular or angular source, like a rock, can do this in a heartbeat if jammed too close to the window. Fine mesh, metallic screens are often placed over the mica window in the tube mount to preclude or help prevent this. This now relegates the tube to never being 100% efficient for external alpha and a correction factor must be determined for the alphas stopped dead by the screen.

Beta radiation is only moderately detected by the mica windowed GM counter based on its average energy. The efficiency is thus widely variable for beta detection in a mica windowed system. Thus, little real hard data about the beta radiation can be gleaned from a geiger detector. Higher energy betas will just not traverse enough gas in the short pancake detector and only the weakest of beta will be counted (~10kev - 100kev) and even here not all will be counted.

The long tube mica windowed geiger tube is a much better beta counter due to the longer gas path and will count many more of the higher energy betas than its pancake cousin. It will count almost all of the lower energy betas.

Gamma radiation is barely seen by a mica windowed geiger detector, especially the pancake. Efficiencies here are in the lower single digits (~ 2-5%)based on the Gamma energy. Gamma requires compton scattered electrons in the counter gas to be detected, for the most part, with some photo electrons thrown in for good measure. Due to the great penetrating power of gamma rays and the rarefaction of gas in a geiger tube, few gammas that enter the tube interact with the gas to produce the demanded charged particles needed for avalanche. Note this tube is a great detector of x-rays under 60kev due to the low energy and wide spectrum of the radiation coupled with the torent of particles often seen from actual x-ray sources.

Type 2. The Beta-Gamma counter

This geiger tube, typified by the ubiquitous Victoreen 1B85 tube. Is a rugged work horse and is used in vast numbers of survey instruments but rarely in lab instruments. The tube is all aluminum with the shell being very thin. (<.2mm thickness)

This tube is totaly incapable of alpha counting due to the metal shell which gives the detector its robust nature. The stronger beta particles slip right throught the thin shell and are readily counted with an efficiency only slightly higher than the mica windowed counter, but rarely exceeding 15-25%.

Gamma radiation is detected only slightly better with this tube mainly due to the slight increase in pressure and the metal-photon reactions just off the inner walls of the metal shell throwing compton and photo electrons into the gas. It is also a moderately good detector of medium energy x-rays.

Type 3. The gamma only geiger detector.

This is a relatively rare bird as it suffers from the natural insensitivity of the geiger device in general to photons.

Such detectors are most often made from heavy steel or stainless steel walled tubes. This warrants zero detection of alpha and even very intense blasts of high energy beta particles. The beauty of the tube is that it can be calibrated by shell thickness to have a limited ideal gamma or x-ray energy detection region. None of these tubes are found on any common instruments either in survey gear or lab gear. More often they are part of a "built-in" radiation monitoring system on X-ray machines and industrial gamma thickness gauge systems. In spite of this, GM counters are NOT the device of choice to give accurate x-ray exposure. Only ionizaation chamber counters can do this.

Limitations of gieger detectors **************************

Other than the weakness of the mica windows, the non-alpha capabilites of two of the types, the real limits in geiger work is the count rate.

The fact that there is a gas in the tube and that total ionization and breakdown of the tube in its noraml gas amplifcation mode of operation is the very method of operation, demands that an upper limit on counting be set due to de-ionization and stabilization time of the gas. (quenching time)

In addition, their is period at the end of the discharge that the tube is insensitive to a new discharge event and this is called the dead time.

This time is dependant on the tube geometry, fill gas type and pressure and other mechanical concerns.

The upshot here is that the very finest, and normally halogen quenched tubes found for sale today, have a quench and dead time on the order of 60 microseconds. This effectively limits the best of the best geiger systems to no more than 16,000 cps or just under a million counts per minute. Thus, a .5 microcurie source would be at the upper limit of saturating the counter. In addition the counts would never come equally spaced as in this ideal example. Allowing for pulse "pile up" or lack of it, one might not realistically rely on GM counts in excess of 400,000 cpm in the best systems. This slugishness and limited count ability limits geiger counters to relatively weak sources. (still, I wouldn't like be around a WEAK 400,000 cpm source for too long.)

The Electronics hooked to a GM detector is all important*******

The normal survey meter, as found on E-bay, is not a good instrument for quantitive data collection. It is more a study in low bid, price point, economics than an example of how to make a geiger system. The electronics are always slap-dash and cheapo. The full potential of the detector head (GM tube) is rarely developed in these instruments. To assure stable operation over wide battery voltage ranges, deliberately long time constants and integration periods are chosen. These often leave the top end of the counter below 100,000 cpm. (most often below 50,000 cpm).

Specific laboratory instruments with pancake probes are often much more accurate and can count a good bit higher when coupled with correction factors. However, to squeeze the maximum out of a geiger system you can roll your own using NIM components and warrant a solid quarter million CPM rate for those ultra hot sources. Regardless, a fine data collection GM system, especially one designed to count activated material will absolutely have to have a digital rate meter/timer-counter readout and not an analog meter. Such meters are for field use (mineral collecting, contamination detection, etc.) and quick guestimates of activity or sniffing out x-ray leaks around HV operated gear and systems.

Making counts......Count...........**********************

I would advise for GM work with activated materials a pancake counter that is home brewed.

Recipe.............

Take a fresh or known good 2" LND 7311 pancake tube and apply a 3.3megohm limited 900 volts to it. Place a 1 kohm resistor btween the case body or shell to ground. The tube output will now be a DC pulse of .25-1.0 volt that is very short (~10us) for each count or particle detected. This output will appear across the 1 kohm resistor. Feed this to an SCA (single channel amplifier.) Set the lower level detect to about .1 volts and the upper level detect to about twice whatever a scope shows as the average amplitude of your GM tube's output pulse to be. NOTE some SCAs use Delta (greek letter on dial) and have no upper level detect pot. Set the delta pot to the difference between the lower level detect setting and the 2X pulse hegiht figure found on your scope. The SCA will now output a whopping 5 volt clean and square pulse for each GM tube pulse into the SCA.

This clean, low impedance pulse can now be fed directly to any number of inputs of other modules, but is most often now run to a counter timer module which advances one counter per pulse received over a controlled and timed interval. This is the best and most precise lab grade GM system you might ask for. By doing the above you will have a precision lab grade system that is pretty immune to low energy white noise on the lines and to terawatt pulse energies in and about the lab. If the equipment survives, the pulse count will be fairly reliable.

If the above is not available then at least feed the phone jack output of you little survey meter into an external digital counter. (Frequency counters often have a totalize or count mode)

There you have it..... A solution to successful application of geiger tubes and how to squeeze that last ounce of performance from what you have.

You should now also have an idea about the limitations of the geiger counter and detector head, itself, in general.

Sidebar....................

Some of you may get the impression I am down on the simple E-bay GM survey meter. FAR FROM IT, I assure you! I have, at my home and in my lab over 37 fully functional GM counters and survey meters of the lowest form to the most expensive. (pancake, long tube, and beta-gamma).

By comparison, I have only two plus ultra NIM GM setups. I use the counter that fits the occassion and pupose suitable to its talents or strong points.


Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

davidtrimmell
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Re: FAQ - Geiger counter tips and techniques.

Post by davidtrimmell » Mon Aug 18, 2003 6:00 am

Richard a really good FAQ. I have a couple things to comment on.
First, with 2Pi geometry (pancake GM tube) 50% theoretical max detection efficiency is possible. Only with liquid scintillation can one actually approach 100%. Even with 4Pi HPGe systems are much lower. But as long as long as you Calibrate then you know exactly what you have!
Second, I believe betas are going to interact most in a "pancake" detector. Alphas are defiantly very detectable, but not as reliably detected with the gm as with the PM tube interfaced to ZnSulfide Al Mylar detector.

But it is important that folks understand calibration (included with calibration is determination of detector efficiency) and, of course geometry. If you can get a good source with emissions in mid-range of what you want to measure then you can create a jig, place source in it measure with detector (this does need to be a scaler, and not count rate meter) measure at least 10 times (with both source and background) and average (simple method*). Then If (yes another if!) you have Lab or Collage with Rad lab around contact the RSO and tell them that you have Exempt source (let him know that it is a professional source and not a homemade potentially leaky one!) and are a amateur and would like to know if they would let you bring it there for them to measure (in your jig) with their "calibrated" detector. If that can happen then you’re in bis with just a few calculations. At least get within a order of magnitude or so!

Recommended reading: NCRP Report #58

* I recommend looking up a good text on counting statistics. The following websites gives you the idea:

http://www.eas.asu.edu/~holbert/eee460/ ... istics.pdf

http://www.kronjaeger.com/hv-old/radio/ ... h/exp2.htm

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Re: FAQ - Geiger counter tips and techniques.

Post by ChrisSmolinski » Mon Aug 18, 2003 4:34 pm

An excellent article, thanks Richard.

One question - wouldn't a pancake GM tube be fairly efficient for betas, even higher energy? As I recall, it just takes one interaction in the gas for a pulse to be produced, and I would think that even a high energy beta will make a few interactions along the way, even in a shallow pancake. Plus I guess there is the possibility of interactions at the inner surface producing electrons (or low energy photons) that go back into the gas.

Or have I overlooked something?

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Richard Hull
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Re: FAQ - Geiger counter tips and techniques.

Post by Richard Hull » Mon Aug 18, 2003 6:05 pm

Chris and Dave.... thanks for your input and Dave for the counting statistics papers adjunct to my original post. All would-be nuclear metrologists here should print these pages out and keep them at hand. It is one thing to take readings and another to have them mean something. I have stressed this throughout this list's existence.

Efficiency of the counter is hyper-critical and unless you know this key factor, then you are really just quoting relative numbers for your counting system and not absolute numbers.

Relative numbers are fine for your records and lab notes, but once you start passing the data along, it can be misleading unless you state whether they are relative or absolute.

This is why a paper, be it a simple lab report or a formal scientific paper should include great detail about the primary instruments used to take the data.

Beta particles have rather tremendous ranges in any gas at any pressure compared to alpha particles. Always in excess of 10X or more for 100kev and hotter betas. There are far less ionizations per unit travel as well.

All mica windowed GM tubes are good beta detectors. Pancake detectors are rarely 0.50" deep.....A very shallow volume, indeed. The rarified gas makes electron interactions in the gas less likely per unit volume traversed at any energy than in air. For a given entrance window diameter. The longer mica windowed tubes will certainly afford a greater opportunity for beta reaction with the gas to create avalache. This is for beta particles that enter more or less normal to the plane of the window.

Pancakes seem to do good at beta due to most pancake detectors having two inch diameters as opposed to most long tubed mica counters having 1" or LESS diameter windows. The square law for detector entrance area really helps the pancake seem better.

The pancake is merely a contrivance for getting a lot of GM detector area into a small volume and offers not one bit of advantage over the long tubed geiger of similar aperture. The long tube, however offers increased volume for detection of more energetic particles and always does better at beta and gamma than a pancake of equal aperture. This is due to increased gas volume for beta and increased wall area for gamma. 2" diameter "long" tubes are almost never encountered.

It is true also that any really hot beta will slam into the rear wall of the shallow pancake and almost certainly blast out a number of electrons back into the gas. This is equally true of the long tube.

I personally prefer the pancake strictly because of the compactness and huge window or detection area.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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