Charge Sharing Phenomenon between Collector and Guard Ring in GARFIELD Ionization Chamber Simulation

[pedro]

I’m simulating an ionization chamber with GARFIELD, where the electric field is imported from COMSOL. An electron-ion pair is generated inside the chamber, with the anode on the top and the bottom electrode consisting of a collector and a guard ring. In theory, electrons should drift to the anode, while positive ions should only reach either the collector or the guard ring — meaning one of the two bottom electrodes should register a charge of e (the elementary charge of an electron) and the other zero. However, in actual simulations, both the collector and guard ring show non-zero charge signals, with their sum always equaling exactly e.

[Danilo]

Dear @pedro,

Let me add in the loop @hschindl .

Best,
D

[hschindl]

Hi @pedro, can you provide a minimal working example that reproduces the behaviour you observed?

[Piet]

Dear @pedro

If you have obtained the signals on both collector and guard ring, it means you have calculated the weighting fields for both electrodes, which is the correct way to proceed. Now the induced signal is obtained through the multiplication of the moving charge with its velocity (hence taking into account its travelled path) and the weighting field. Hence a non-zero weighting field of your guard electrode results also for an ion that has its endpoint on the collector for a very small but non-zero signal on the guard electrode. Sum of both equals the total ion charge. So at first sight I do not see a problem in what you have described above, and to me this seems the expected behaviour. In reality maybe you do not see the very small signal induced on the guard ring, because you are not looking for it and your guard ring is just connected to ground.

greets
Piet