A transimpedance amplifier will work.
I am digitizing the signal straight from the output of the PMT.

 

A transimpedance amplifier will work.
I am digitizing the signal straight from the output of the PMT.

So you are using a current-to-digital ADC? Didn't even know those existed before now

 

The purpose of anti-aliasing filter is to remove frequencies above the Nyquist limit when performing frequency analysis. You are not doing frequency analysis. You are doing pulse shape analysis, pulse counting and pulse height analysis.

A LP-filter will simply smooth out the signal pulses.

If your input is a delta-function you will end up with a pulse-width approx. equal to the rise time + fall time. In other words, a delta-function gets convolved with the response function of the system.

The pulse width will be determined by the decay time in the scintillator and the collection time in the PMT.

I am doing exactly what you are doing. I am detecting gamma rays using a PMT with NaI scintillator, digitizing straight into an ADC with no preamp. Then I collect energy spectrum after processing the pulse height.

So because the LYSO crystals used have a decay time of 36 ns, the bandwidth based on the scintillator alone will be a maximum of 27.78 MHz?

 

So you are using a current-to-digital ADC? Didn't even know those existed before now

No. A current pulse will look like a voltage across a resistive load.

 

So because the LYSO crystals used have a decay time of 36 ns, the bandwidth based on the scintillator alone will be a maximum of 27.78 MHz?

No. Bandwidth is not the inverse of pulse-width.
Bandwidth is related to rise and fall times but it is not the inverse of rise and fall time.
Bandwidth gives the max frequency component of a signal.

As a rough guide, BW x Rise Time = 0.35

 

No. Bandwidth is not the inverse of pulse-width.
Bandwidth is related to rise and fall times but it is not the inverse of rise and fall time.
Bandwidth gives the max frequency component of a signal.

As a rough guide, BW x Rise Time = 0.35

Sorry, what I meant to ask was that since the decay time of the scintillator is 36 ns, then the pulses will have a width of at least 36 ns between them, making the frequency of pulses be lower than 27.78 MHz? Or can pulses appear during the decay time?

 

Pulses are totally random and can occur at any time.
When a new pulse appears on the decay tail of a previous pulse you get "pulse pileup".
Pulse pileup, recognition and rejection becomes critical at high count rates.

 

Pulses are totally random and can occur at any time.
When a new pulse appears on the decay tail of a previous pulse you get "pulse pileup".
Pulse pileup, recognition and rejection becomes critical at high count rates.

So if i understand this correctly; It is possible for pulses to appear at a frequency above 250 MHz, but this is rather rare and when it happens it doesn't really matter, the only thing that happens is that the signals get undersampled?

 

So if i understand this correctly; It is possible for pulses to appear at a frequency above 250 MHz, but this is rather rare and when it happens it doesn't really matter, the only thing that happens is that the signals get undersampled?

No. This is a real phenomenon that happens all the time in different situations.
It is not a problem of being undersampled.
The problem is two many events happening at the same time and it overloads the detection system.

Imagine you are standing on the side of a multi-lane expressway and your job is to count the number of cars passing each second. When traffic is light there is no problem counting cars. Come rush hour and you will have a hell of a time counting cars across 4 or more lanes.

That is what happens at high count rates. Too many events happening all at the same time. Increasing the sampling rate does not alleviate the problem because the system is unable to differentiate one pulse from the next.

 

The PMT can be operated in two different modes:

1) Pulse counting mode
2) Current mode

When rates are low enough, one can count individual events.

When pulse pileup overwhelms the ability to count individual events, one can resort to current mode by measuring the average current output of the PMT.

 

The PMT can be operated in two different modes:

1) Pulse counting mode
2) Current mode

When rates are low enough, one can count individual events.

When pulse pileup overwhelms the ability to count individual events, one can resort to current mode by measuring the average current output of the PMT.

I will try and read up on the theory behind these modes and how to best design the system. However, I believe I'll have to stick with the system we've designed following certain design criteria we received because sadly there isn't much time left before I have to deliver the Bachelor thesis... I can, however, use this theory in my thesis and point to a better solution for the system. But most of all, this was rather interesting, and it is worth looking further into!
Thank you so much for taking the time to explain this to me! I also have to apologize for taking up so much of your time