Shaping time on my amplifier alters the gamma energy registered?

  1. Hi,

    My understanding of shaping time on amplifiers is that its the amount of time the capacitor has to collect charge. I would say that the longer the time the better energy resolution you would have due to that all the charge created from the photon is collected. However, if the time is long enough, that would mean that another electrons energy created from another photon interaction could add up. Following this, the amount of counts (or pulses I can seperate) decreases. Almost like the effects of dead time on a GM-tube.

    But when I tried this to prove that this was the case, I got somewhat different results.

    Sure I registered more counts with shorter shaping time, 0.5usec, when compared to 12usec. And the FWHM was better with a longer shaping time.

    But the energy for the peak was affacted.

    With 0.5usec the peak was at 345keV (kilo electron volts) and with 12usec the peak was at 280keV. My guess was that with longer shape time the photons would add up, so the peak would be like 600keV.

    Any ideas?
    Thank you very much!

    Iam working with a scintillation detector btw.
     
  2. jcsd
  3. mfb

    Staff: Mentor

    How did you modify this time?
    How did you get your energy value?

    I would expect some calibration issue.
     
  4. I have what some calls a NIM-Bin which supplies electricity to some units, I used a High Voltage supply and amplifier wich can be seen in the attached image.

    I got my energy through an MCA connected to a software on my laptop, a gamma spectrometry software.

    Thank you
     

    Attached Files:

  5. 1. What type of radioactive source you used for this exp?
    2. I assume your scintillation detector (NaI) is connected to a charge preamplifier and you are using traditional energy spectroscopy (MCA)-Pulse height analysis method where the measured energy is directly proportional to the pulse amplifier amplitude.
    3. Is these two measurement (0.5u & 12 us) are using the same amplifier gain setup? & Is your MCA system is properly calibrated? It look wired coz 1 would expect the peak energy would not varied that much or the 0.5us shaping time would produce lower energy peak since your amplifier will reached peak too soon.
     
  6. Hi,

    1) Almost two years ago, I cant recall the radioactive source used for this setup.

    2) Yes

    3) Everything is the same, except for the shaping time.

    I know that longer shapping time should increase the centroid channel for the peak, maybe 10 keV (for NaI due to PM-tube increases the deviation) at most due to complete charge collection or addition of non relevant event from another interaction from different photon. But is there something else that might affect this?

    Maybe someone who has more or less the same setup could try this?

    Thank you
     
  7. mfb

    Staff: Mentor

    The calculation of the energy might make some assumptions about the time-distribution of your signals. If this assumption is wrong (e. g. if it assumes a lower peak and a larger tail), a shorter time can lead to a higher reconstructed energy.
     
  8. Thank you,

    However I did not understand your post compelety about the time aspect time. Are you talking about Wilkinson-type ADC?

    I just set up the same experiment today, this time with Cs-137, and pretty much the same result with a different NaI, preamp and new Nim-Bin ( with HV and Amplifier).
    Energy 662keV with shaping time 4µs,
    With shaping time 0.5µs the peak almost went outside the spectrum, energy around 2.5MeV
    For 12µs the full energy abpsorption peak was around 80keV.

    I can add that the dead time of the system was not high, around 10%.

    Thank you
     
  9. mfb

    Staff: Mentor

    As long as we don't know how your software calculates the energy values, it is just speculation.
     
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