GM counter and alpha particles

[Vilius]

Hi,
I am an undergraduate electronics engineer building a GM counter for my final assessment. I read a lot of theory about GM counters as well as the nuclear physics theories in general, but there is one thing I can not find an answer to. Why do none of the GM counters detect alpha particles? I have looked through so many different models (both pancake and end-window types) and I did not find a single one capable of detecting alpha particles. I know that alpha has the smallest penetrating capability, but at the same time, it has the largest charge and inertia as well (due to a comparably highest mass of all radioactive particles.) Thank you in advance for any observations.

 

[Alex A]

Any GM tube with a thin window (usually mica) should detect alpha particles. You will find there will be a minimum energy and that will depend on how thick the window is. A good tube to look at is the LND712. It's small and typically quite expensive, but it's the classic. Anything with a similar window thickness around 2mg/sqcm should do the same job.

 

[Rive]

Why do none of the GM counters detect alpha particles?

'None' is a bit of a stretch (as it was already stated above), but in general, most GM detectors (and the 'most' here comes from the fact that most of them are made cheap) does not have the right type of GM tube and cover to provide adequate window for them.

Though there are other detectors which does not have window at all

 

[Vilius]

And what about neutrons? A popular explanation is that the neutrons only have mass with no charge, so they can not ionize the gas inside a GM tube, but gamma photons also do not have a charge, however, gamma rays are detected...
And the last thing. GM counter datasheets often provide ,,maximum detectable beta particle /gamma ray energy" I can understand, why low energy particles/rays do not get detected, but what is the physics behind maximum detectable energy?

Thank you in advance

 

[Alex A]

Can you point to a datasheet with a maximum energy limit? On a proper sheet you will get a response curve, which may tail off at high energies, because these interact less with a given amount of matter.

Gamma and X-ray photons are not charged, but they interact by the Photoelectric effect or the Compton effect, and so produce high energy electrons which are charged and ionising - they leave a trail of charged pairs in their wake. A GM tube may only detect 1% of the gamma rays that go through it depending on the energy and construction, and making the metal shell thicker usually results in a tube that is a better gamma detector.

Neutrons are a similar case in that they can for example knock protons out of a hydrogen containing material which is then an ionising radiation and can be detected by a scintillator or a GM tube. This is how neutrons were discovered - a block of paraffin placed in front of a GM tube increased the counts which suggested something previously invisible was being detected. Slow neutrons are often detected by having materials that produce nuclear fragments of high energy, and usually in tubes that operate below the Geiger plateau (and so the pulse size depends on the number of ion pairs produced, a GM tube produces the same large pulse for every event). Nuclear fragments tend to drop a lot of energy in a small path length, producing a big pulse. This enables energy discrimination, which is important, because you want to know what you are detecting. Neutron detection is a huge field and there are radically different ways of doing this, including weirdly simple ways like wrapping silver foil around a GM tube. All methods have strengths and weaknesses.

 

[Vilius]

I am using an obsolete Philips 18504 or ZP1400 (one tube has 2 names, really strange, but it is what it is...)
So here is a picture from the datasheet.

,,A GM tube may only detect 1% of the gamma rays that go through it depending on the energy and construction, and making the metal shell thicker usually results in a tube that is a better gamma detector."

Can you explain the reason for that?

About the neutrons, does it mean that neutrons can only ionise hydrogen?

It started as a simple question, but now I see that it went really far... I am really curious :)

Thank you again

 

[Astronuc]

And what about neutrons? A popular explanation is that the neutrons only have mass with no charge, so they can not ionize the gas inside a GM tube, but gamma photons also do not have a charge, however, gamma rays are detected..

Neutrons would be detected in a gas, e.g, ¹⁰BF₃ or ³He, where the ¹⁰B and ³He absorb a neutron and fission, for former undergoing an (n,α) reaction and the latter, an (n,p) reaction. Also, depending on the neutron energy, one might surround the tube with paraffin or other hydrogenous material to thermalize the neutrons.

Edit/update: One could also coated a Geiger counter or proportional counter tube with a strong neutron absorber, e.g, Hf, or B₄C, with B enriched in ¹⁰B, or HfB₂, or even ²³⁵U. The last option is a fission chamber, which uses fission reactions to interact with neutrons.

Otherwise, one might use so-called Self-Powered Neutron Detectors
https://www.oecd-nea.org/science/rsd/ic96/4-2.pdf

 

[Alex A]

For a detector to be useful, the important question is not how many events it is detecting, or missing, but how many are out there and how accurately that can be estimated. That table gives information on what the tube can and can't detect and how much you would need to adjust the numbers to get the true answer.

GM tubes are usually made of thin materials so to be sensitive to beta rays etc. Gamma rays don't interact well, especially at high energy, so most will sail through the tube like it isn't there.

Hydrogen isn't a great way to detect neutrons (people that work with proton recoil counters may disagree), and putting wax in front of a GM tube with a window is highly inefficient, but a proton and a neutron have similar masses so when a neutron does hit a proton the maximum kinetic energy transfer can happen. If you are going to play marbles, you want it to hit another marble. A bowling ball would probably barely move, and you wouldn't expect a GM tube to detect neutrons normally.