# Detecting resonance Q factor with a change in dielectric properties.

1. Feb 17, 2012

### DragonPetter

I have seen some examples where a system has a resonance frequency, and, when the system changes, its Q factor also changes, and so the resonance frequency shifts, and this shift can be measured to detect the change in the system. An example I can think of is very sensitive mass measurement instruments, where addition of material changes the damping factor of a vibrating plate of a capacitor/piezoelectric surface.

I have a few questions with regards to this method, if anyone has any background or ideas on this:

1. Could a change in dielectric - say its permitivity changes - create a shift in frequency of an AC signal on capacitor and be detected?

2. How does ionizing radiation change a dielectric? I understand, under the right conditions(high voltage), it can create ion pairs that I would think has to change the dielectric properties in some way.

3. Would measuring a frequency/phase shift or Q factor change on an ion chamber from dielectric changes in an AC field have more, less, or about the same sensitivity as measuring the charge collected from ion pairs in a DC field? I think that this would offer a lot more noise and leakage immunity and improved bandwidth from conventional extremely low current measurements.

I am trying to think of different/non-traditional ways to use ion chambers, and just am curious about general opinions or thoughts on this.

2. Feb 17, 2012

### DragonPetter

Update:
I have access to a radiation field and an empty ion chamber (like a big air capacitor), and measured the capacitance as I put it into the field, without any DC bias other than what the capacitance meter might be applying. I see the capacitance increase from 120pF to 135pF and then drop back down to 120pF after about 10 seconds. I think this is most likely not really a change in capacitance, but rather current flow from the ionization. I can't really explain why the capacitance drops back down (or why current flow stops). I think possibly the chamber has been quenched.

3. Feb 17, 2012

### yungman

I cannot answer most of the question either I am not familiar or I don't quite understand your question.
I can answer (1). Assume it is LC resonance circuit, yes, if you change the permittivity of the dielectric in the capacitor, you change the capacitance and it will change the resonance frequency as:

$$f_c=\frac{1}{2\pi \sqrt{LC}}$$

Like you said about the mass measurement where different weight cause change in capacitance.

I have no idea on (2), you might post in Classical Physics section here.

In (3), are you comparing this with current collected by a Faraday Cup and use a transconductance amp to convert into voltage for measuring? I don't have enough information. It all depend on the current to be detected. If you have higher current close to 1uA, speed is not slow. As you can see, the down side of using the resonance, the frequency change is proportional to the square root of the capacitance change. I don't have enough information to even comment. You have to either provide more info or do calculation yourself.