Positioning of Capacitance in Ionization Chamber's Equivalent Circuit

[Simobartz]

TL;DR Summary

Positioning of Capacitance in Ionization Chamber's Equivalent Circuit: Why Not to the Left of the Voltage Generator?

I'm delving into the topic of ionization chambers, but as someone without a background in electrical engineering, I'm finding the equivalent circuits a bit challenging to comprehend. Specifically, I'm puzzled by the placement of the chamber's capacitance and any parallel capacitance in the equivalent circuit. My understanding suggests that if the capacitor represents the ion chamber's capacitance, it should be positioned to the left of the voltage generator. However, as you can see in the attached picture, it is placed on the right. Why is it placed in this way?

note: in the picture C represents the capacitance of the chamber plus any parallel capacitance

 

[Alex A]

The impedance of the battery is taken as zero, so in math terms it's the same and when thinking about the RC constant of the output pulse it makes more sense to show them close together.

There is similar weirdness with AC analysis. If a signal line has a capacitor to Vcc, and another to ground these are in parallel in the AC small signal equivalent circuit. This is equivalent to assuming Vcc and Gnd are the same place.

 

[Simobartz]

the problem is that, if the chamber is on the right, the voltage difference at equilibrium across it is zero. While, on the left it is Vo. And the voltage difference is an important property for an ionization chamber. So, what do you mean when you say that they are mathematically equivalent?

 

[berkeman]

the problem is that, if the chamber is on the right, the voltage difference at equilibrium across it is zero. While, on the left it is Vo. And the voltage difference is an important property for an ionization chamber. So, what do you mean when you say that they are mathematically equivalent?

Can you post a link to a datasheet for a typical ionization chamber that you are considering? (hopefully with application circuits included in the datasheet) Thanks.

 

[Alex A]

You cannot put the chamber on the right of the circuit as it is. You can put the Capacitor on the left instead. This would change the DC voltage over it, but it's not a real capacitor, it's a lumped quantity most of which is from the chamber but also any cable capacitance.

If the capacitance is across the output then it's a meaningful quantity you want to know. For example the noise developed by the amplifier usually depends on the capacitance of the detector.

Most real ion chambers have a more complicated diagram because the current being measured is very small. Conduction through and over the insulators becomes significant so 'guard electrodes' are placed between the high voltage terminals and the collection electrode. These are grounded and the current between them and the high voltage terminal is not measured. Current leaked between two insulated electrodes both close to ground potential is much smaller (many orders of magnitude) than between a high voltage electrode and ground.

Ion chambers intended for spectroscopy can have an extra grid inside, called a 'Frisch grid' which makes the measured charge from different positions in the volume more equal.

All these arrangements still just result in a single equivalent C value at the output.