Gamma spectroscopy- Fine Gain Coarse Gain factors

  1. I'm currently doing a spectroscopy lab where you use a scintillator and Cs-137 to obtain a Cs-137 spectrum. I calibrated this spectrum using the known gamma energy for Cs-137 at 662 Kev at the photo peak. I am now trying to use this calibration for a Co-60 spectrum. The problem is that when I obtained my Cs-137 Spectrum I had the coarse gain at x16 and fine gain at x5. When I was taking data for the Co-60 I had to adjust the coarse gain to x8 and fine gain to x3.5. I understand the coarse gain is half that of the Cs-137 but how would I factor in the fine gain? For example for Cs-137 the photo peak was on channel 935 at 662Kev. So if I adjust the gain by half then 662Kev should correspond to channel 467.5 for the Co-60 spectrum. How does the fine gain adjust the factor?
  2. jcsd
  3. Hi,

    I know this is a little late for a reply, but perhaps someone else is wondering the same thing.

    This is good question, and Iam not sure there really is correct answer regarding fine gain.
    I once heared from a wise man, that coarse gain is what you would expect to be, a pure amplification, and I would say you are correct on using half the coarse gain, nothing major should affect the spectrum.

    However, the wise man told me that, fine gain somehow takes a percentage from the input signal and passes this on to the coarse gain, this would mean that messing with fine gain should be done carefully.

    E.g. the less you take from a signal the less noise you would amplify, so I am pretty sure you would receive different values for the FWHM depending on what fine gain value you use. So even if you "fine tune" the fine gain to match a certain channel, I would think that the spectrum would not be the same, because you have messed with the amount of signal you are using.

    However, this depends on what kind of information you are wanting for from the spectrum.

    If you simply just want to find out the calibration factor, I would guess you could view like this:

    Now that the coarse gain is half, this means the correction factor should be 0.5-something.

    The fine gain would be the last remaining digits, like 87.

    So the correction factor would be 0.587 to divide or multiply (not sure which one, too tired :P) with the calibration value.

    It would be like two simple equations where you solve the parameter "fine gain".

    I could of course be wrong :)
  4. 604
    Science Advisor

    Old thread is old!

    This of course assumes that the amplifier is "perfect". Which is not something you can assume, well, ever. Halving the gain will almost certainly not put the peak exactly half way down the spectrum. In reality, you'd have to pick your one gain setting for the entire experiment - pick it such that you can see the highest energy peak, and then calibrate the spectrum.

    Also, by the way, using only one point (the 662 keV line for 137Cs) to calibrate an energy spectrum is extremely incorrect. The OP should have taken uncalibrated spectra for several sources, and then made a decent calibration.
  5. Well, it was a "lab" (not so serious) and he was using a scintillator, which is known for it's linearity. So I would not say its extremely incorrect, but a good approximation :)
  6. 604
    Science Advisor

    Huh. I actually teach a Compton scattering lab for undergrads, and I'd fail someone so hard if they handed me something with a one point calibration - the point of labs is to learn good lab skills, after all.

    ETA: And you really won't get the positions of the peaks to particularly good accuracy.
  7. Well, he/she wanted to calibrate with Cs and Co, there was just an unexpected problem.

    So do I :) and I would commend the student for trying to solve the problem by using the amplifier gain.

    However, a better option would be to redo the calibration.
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