I am quite new in nuclear physics instrumentation. So my question may seem silly.

We are performing timing experiment in our lab to obtain better time resolution of NaI(Tl) detectors in conjunction with its relevant electronics. Modules are-

Na22 source; NaI(Tl) followed by preamp (gain set at X6, means 6 times gain); timing SCA (timing single channel analyser); TAC (time to amplitude converter); MCA (multi-channel analyser).

Now the reason we avoid using Amplifier is that, it will blur timing signal. So basically we are feeding preamp siganl of NaI(Tl) directly to timing SCA. Problem is then, it is very hard to set windows (LLD and ULD) of timing SCA so that both can trigger only when $511~\rm keV$ pass by.

Any ideas?


enter image description here

We have 22Na source which will emit two prompt (no time gap) 511 keV gamma rays deu to electron positron annihilation. Although in figure amplifier is shown but now we are trying the same experiment without Amplifier. With amplifier we got timing resolution of 13 ns approx. Here timing SCAs will act in window mode and their window will be fixed at appropriate position so that both only discriminate 511 keV photons (there are other gammas also see spectrum below). And one SCA will produce a start logical pulse and other SCA will produce a stop pulse for TAC. The duration between start and stop pulse will be used as discharge time of a capacitor placed inside TAC. Finally that charge will be used as signal to MCA. MCA signal graph will look like this (see below). From that we can get time resolution using calibration slope.

Now without ampilfier signal will be of low voltage. Hence there will be two ways. One- use preamp of NaI(Tl) as amplifier (changing jumper setting). Two- increase PMT volatage. I have tried both of them. But with no success. Everytime I am getting signals in MCA all over the place which means SCAs are not catching 511s.

Any suggesstion?

Thanks in advance.

enter image description here enter image description here

  • 1
    $\begingroup$ -1. Please clarify what your acronyms stand for. $\endgroup$
    – Chris
    Commented Feb 18, 2018 at 11:56
  • $\begingroup$ Would Electrical Engineering be a better home for this question? (If so, please don't cross-post - flag for a moderator to migrate instead.) $\endgroup$ Commented Feb 21, 2018 at 20:39
  • 1
    $\begingroup$ This seems clearly to be an experimental problem in nuclear physics! Some better wording is warranted. $\endgroup$
    – freecharly
    Commented Feb 21, 2018 at 20:49
  • 1
    $\begingroup$ This depends a lot on the specific instruments you are using, and if they are matched. Could you indicate the specific units you have hooked up? $\endgroup$
    – Jon Custer
    Commented Feb 21, 2018 at 21:08
  • $\begingroup$ @JonCuster 1. NaI(Tl) ORTEC Model No.-2M2/2, ORTEC Scinti Pack Model 256 PMT base, Apmplifier ORTEC 545A, Timing SCA 551, TAC 516, MCA Canberra. $\endgroup$ Commented Feb 21, 2018 at 23:21

1 Answer 1


It is not quite clear to me what you are trying to do. Why not just turn up the voltage of the PMT? But one solution may be to look at the signals on an oscilloscope. For example, here is a screen shot that I put some annotation on:

enter image description here

It is measuring the time for muon decay in the lower trace with a plastic scintillator. The oscilloscope also shows the signal from a NaI detector positioned above it. As you can see, the NaI detector is not as fast as the plastic scintillator.

(And the oscilloscope is again live on the web here, now with 5 liters of scintillator liquid).

Another thing to do is coincidence spectroscopy using the stereo sound-card input of a computer. Time resolution is low then (limited by the audio range). There are free software from different sources. A very simple way to hook things up is just a passive low-pass filter.

  • $\begingroup$ @aranyak So you are trying to improve on the 13 ns timing resolution. Very difficult to make suggestions. Maybe use constant-fraction discriminators (but that would not help that much if you are always looking at 511 keV anyway. Check noise levels, improve electronic shielding. $\endgroup$
    – user137289
    Commented Feb 18, 2018 at 20:30

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