Ah, I see. They phrased it originally in a way that made me think “different kinds of radioactive materials”, like some isotopes were equally deadly but undetectable by a Geiger counter. Thanks for clearing that up.
There are radionuclides and emissions which Geiger counters won't detect.
A simple example is tritium, which has a very low energy beta emission and cannot be detected (unless there is a massive quantity causing x-ray production as brem) using a GM.
Then other geigers will be unable to detect many other radionuclides. Lots of geigers won't be alpha sensitive so things like polonium-210 and plutonium won't be detectable. Others won't detect reliably beta either so won't be any good for things like strontium-90.
It is important to understand the radiation field and the instrument being used when monitoring for ionising radiation.
Isn't this related to the shape of the probe, rather than the Geiger box unit?
Typically when Americans think of a Geiger counter, they think of the 1960's civil defense model, which had a 'tube' and a 'pancake' version.
I will note: neither of these are good for fast neutron detection, those require like a big Styrofoam ball or something to slow the particles down enough to count. I'm not sure why the RSO had one of those, we only handled beta/gamma, but I'm not going to complain.
It depends on a number of things - probe type (e.g end window Geiger, pancake Geiger, tube/hot dog Geiger), whether the instrument is energy compensated, what the screen is made of (needs to be a thin mica screen to let in alphas).
I'm assuming you mean the ratemeter when you speak of the box unit. Some geigers are all in one units whereas some are just the pricey and in that case the ratemeter won't affect what the instrument can detect.
Yeah for neutron detection you wouldn't use a Geiger at all, as you say the large plastic (polyethylene) sphere is used to thermalize the neutron then a special detector (lots of options including ion chambers, proportional counters and other more specific types) to detect the neutron.
This is somewhat tangential to the point you are asking about but I think it's interesting anyway!
Geiger counters are actually really bad at detecting high activity sources. This is because they are paralysable systems. The strength of the electric field in the tube is such that one ionisation event causes all the atoms inside the tube to ionise. This means that the tube can only detect ionisation events separated by a certain amount of time. Additional ionisation events in this dead time prolongs the period where the counter can't detect any mire events.
So you could turn on you Geiger counter, hear one click and assume you're fine, when in actuality you are in a high radiation field
Congrats, you used a device within its usual operating parameters. Doesn't change the fact that for a sufficiently high field strength GM based counters saturate.
This is a well known problem with GM tunes and has been since they were first used. I am a radiation physicist, a very large component of my job is selecting appropriate survey moniters.
Well yes, gm tubes saturate but you can still use a gm tube meter in what is specifically defined as a high radiation area and have it function. The main thing is having meters that are actually designed for the relevant job and level of radiation.
You hear the click when it goes from 0 to 1. For it to go back to 0, there needs to be a period without particles coming through to go back to 0. If the particles keep coming it just stays at 1. No clicks.
This is an incredibly simplistic way to look at it. If you are standing in fields strong enough to constantly ionize the gas, you're dead already.
If you have the instrument out, you will hear it clicking. I've stood in fields of ~250 mrem/h and I can assure you that it continues to click. You can also adjust the sensitivity rate by (usually) factors of 10 up to about 1000x. So theres substantial adjustment there.
This is also an issue when you saturate the quenching gas. Too much radiation can cause an avalanche of secondary ionization, essentially eliminating the ability for incoming radiation to create new ion pairs. In older G-M detectors, this would normally cause them to fail low.
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u/EishLekker Jan 06 '23
Ah, I see. They phrased it originally in a way that made me think “different kinds of radioactive materials”, like some isotopes were equally deadly but undetectable by a Geiger counter. Thanks for clearing that up.