Charge collection principles for gaseous detectors

[abotiz]

Hi,

I am reading the great Knoll book about radiation detection and measurement, and I don't fully understand the difference in charge collection for gaseous detectors. Is it the collection of electrons or positive ions in the detectors, a) Ion chamber, b) Proportional counters, c) GM tubes

This is what I do know,

1) When charged particles are subjected to an electric field, they move to the respective electrodes. This motion constitutes to a current which you measure. Electrons move faster then ions, however, positive ions motion contributes to a higher current (due to their mass?)

This is what I think is true,

2) Ion chambers, in current mode you collect both (electrons and positive ions) because the production of ion pairs reaches an equilibrium. In pulse mode you want to approximate the current measured to the energy depostited, so you want a fast counting system, therefore you only measure the current from the electrons.

3) In Proportional counters, almost all charge carriers (positive ions and electrons) are created near the anode, and the largest contribution to the puls is the drift of the positive ions due to their larger mass??

4) In GM tubes, Iam not sure, do you measure the total current created from both electrons and positive ions from the entire tube?

Another question I have is, how do you ONLY measure the current from positive ions and not electrons, vice versa ? Current, viewed from Ohms law does not separate electrons and positive ions. When Knoll says e.g. the signal measured is mostly due to the drift of the positive ions, is this because the integration time of the circuit is chosen in such way which makes the statement true?

Thank you very much!

 

[mfb]

positive ions motion contributes to a higher current (due to their mass?)

The amount of charge from the initial track is the same for both, as the initial gas is neutral. I would not expect a different current, unless you use charge amplification at the electrodes (but then it depends on the geometry).

3) In Proportional counters, almost all charge carriers (positive ions and electrons) are created near the anode, and the largest contribution to the puls is the drift of the positive ions due to their larger mass??

The anode gets significant current peaks when the electrons hit it.

4) In GM tubes, Iam not sure, do you measure the total current created from both electrons and positive ions from the entire tube?

You can measure the total current, but the precise value does not matter anyway.

Another question I have is, how do you ONLY measure the current from positive ions and not electrons, vice versa ?

With a careful choice of capacitances, you can separate them.