Understanding Counts in a Geiger Counter: A Mechanistic Explanation

[ananthu]

One thing is not clear to me. When alpha or beta or gamma radiation enter into geiger counter, it ionizes the gas and ionization current produces counts in the detector. What exactly this counts indicate? Does it show the number of alpha or beta particles enter into the tube? Since even if one mill gram of a sample will contain millions of atoms, at any instant what will be the probability of number of atoms that decay by emitting alpha or beta particles? What is the mechanism of counting the number of alpha or beta particles by the counter? Also in the case of gamma rays, how is the count done, since they are only electromagnetic waves but not particles? Can anybody explain?

 

[dauto]

Each particle produces its own current response which is individually identifiable. Gamma rays are particles and they will ionize the gas in the chamber causing a response as well.

 

[SteamKing]

Each particle produces its own current response which is individually identifiable. Gamma rays are particles and they will ionize the gas in the chamber causing a response as well.

Gamma rays are extremely short wavelength EM radiation.

http://en.wikipedia.org/wiki/Gamma_ray

Gamma rays cause ionization of matter by interacting with the electrons of atoms. An electron receives part of the energy of the incident gamma ray, causing the electron to be ejected from the atom and ionizing it.

 

[mfb]

Since even if one mill gram of a sample will contain millions of atoms, at any instant what will be the probability of number of atoms that decay by emitting alpha or beta particles?

If the decay rate is too large (typically: 1000 particles hitting the detector per second), the Geiger counter cannot measure the rate any more. It always needs some time to recover after a detection, and with those large rates it will just go like hit->recovery->hit->...

For typical sizes of Geiger counters, the background rate is less than 1 particle per second, so you have a wide range where you can measure radioactivity.

What is the mechanism of counting the number of alpha or beta particles by the counter?

Just count. Every count is a detected decay.

 

[dlgoff]

Since even if one mill gram of a sample will contain millions of atoms, at any instant what will be the probability of number of atoms that decay by emitting alpha or beta particles?

With really large/"hot" samples, you can get a count rate by measuring the sample's total mass then remove and measure the mass of a small (e.g. ~1%) portion that will give you a count rate without saturating your Geiger tube. It then becomes a simple proportion calculation to find the "total" rate.

 

[ananthu]

Just count. Every count is a detected decay.

Thank you all for detailed answers. In this connection, I think I can take the privilege of placing some more questions.

I have read that more than two or more alpha or beta particles or one alpha and one beta particle will not be emitted by an atom simultaneously. They will be always followed by one after other. Does this rule prohibit several thousand atoms of a sample from emitting their own alpha or beta particle simultaneously?

What I mean is, is it not possible for, say, a ten thousand atoms out of ten million atoms present in a given sample eject ten thousand alpha particles at the same time? In that case, the ionization current produced inside the tube should be only sum total effect of all the particles hitting the gas at the same time. Then how the counter can count the number of particles correctly? Also how will it apply to the gamma rays? Will it not come continuously at least for a short period as a wave after each alpha or beta emission?

Also, what most puzzles me is the question, 'where do all the alpha and beta particles go after they are released?' Will they not further ionize the air molecules present in the atmosphere? After all, from the time of birth of the earth, millions of Uranium atoms should have emitted the alpha or beta particles!

Further discussion from the experts, I hope, will throw more light on this topic.

 

[dlgoff]

Thank you all for detailed answers. In this connection, I think I can take the privilege of placing some more questions.

I have read that more than two or more alpha or beta particles or one alpha and one beta particle will not be emitted by an atom simultaneously. They will be always followed by one after other. Does this rule prohibit several thousand atoms of a sample from emitting their own alpha or beta particle simultaneously?

What I mean is, is it not possible for, say, a ten thousand atoms out of ten million atoms present in a given sample eject ten thousand alpha particles at the same time? In that case, the ionization current produced inside the tube should be only sum total effect of all the particles hitting the gas at the same time. Then how the counter can count the number of particles correctly? Also how will it apply to the gamma rays? Will it not come continuously at least for a short period as a wave after each alpha or beta emission?

Also, what most puzzles me is the question, 'where do all the alpha and beta particles go after they are released?' Will they not further ionize the air molecules present in the atmosphere? After all, from the time of birth of the earth, millions of Uranium atoms should have emitted the alpha or beta particles!

Further discussion from the experts, I hope, will throw more light on this topic.

I don't think you can expect to get all these questions answered here without doing some reading on the subject. I can give you some links that may answer some but more importantly, teach you.

Radioactivity
Gamma Radioactivity
Radioactive Half-Life
Radioactive Decay Paths
Radiation Detection

 

[dauto]

Simultaneous emissions are highly unlikely and can be disregarded unless you are talking about a very hot source. dlgoff explained what to do about hot sources.

No, the gamma ray is not continuously emitted. keep in mind that electromagnetic radiation is quantized into individual photons which will hit the Geiger one at a time.

The alpha and beta particles will lose energy over time due to collisions and won't be ionizing radiation anymore. The alpha particle is just a helium nucleus. That's the main source of helium in our planet. Next time you're looking at a helium balloon think of it as an alpha particle balloon. The beta particle is just an electron (positron for the reversed beta decay particle).

 

[mfb]

I have read that more than two or more alpha or beta particles or one alpha and one beta particle will not be emitted by an atom simultaneously.

They can still emit that in quick succession.

Does this rule prohibit several thousand atoms of a sample from emitting their own alpha or beta particle simultaneously?

No. All decays are independent of each other.

Also, what most puzzles me is the question, 'where do all the alpha and beta particles go after they are released?' Will they not further ionize the air molecules present in the atmosphere? After all, from the time of birth of the earth, millions of Uranium atoms should have emitted the alpha or beta particles!

They do, until they lost all their energy. Without strong electric fields, the electrons and ions recombine after a while.

 

[MathMatt]

Gamma rays are particles and they will ionize the gas in the chamber causing a response as well.

\gamma intercation typically happens in the anode of the ionization chamber with considerably less interaction in the gas due to the higher Z of the anode. This is why G.M. tubes are very inefficient at detecting \gamma radiation.