Re: fadc dynamic range of FCAL

[gen (Eugene Chudakov) <gen@jlab.org>]

Let me make a few comments to the Beni's message.
 
Beni wrote:

>in the center from the front. The range from 9GeV down to 30 MeV is 300
>so the signal amplitude expected from this 30MeV photon would be 6mV.
>such a small signal will probably drown in the noise no matter how many
>bits the adc has.

The esimate seems reasonable.
This calorimeter must measure amplitudes equivalent to 30MeV, 
with an accuracy of 20% or so, at least for the position measurement of
relatively important 0.5 GeV showers.
With PMTs and integrating ADCs people routinely measure 6mV signals. 
At Hall A we  use LeCroy 1881 with 50fC/bin. A 6mV signal 30ns long, 
on 50 Ohm, goes to the bin #70. 
The noise is typically 10 bins, with a 150ns gate.
 
One has to suppress the 60Hz noise by either having only
one 50 Ohm coupling on the line PMT-->ADC (the others should have much higher 
impedances), or by using AC filters. In order to suppress the high frequency
 noise the signal has to be integrated (>50ns ?). 

If one bin is equivalent to 30MeV*0.2=6MeV, 9 GeV corresponds to the bin 
#1500. 10-bit fADC is a bit short.

Eugene
>
>Hi All,
>
>I come back to the issue of the dynamic range the FCAL has to  cover.
>By this I mean the range in FCAL signal amplitudes the fADC has to handle .
>I am not talking about how many bits the adc is or should be using for 
>digitization.
>
>Let me assume that I want to see at maximum a 9GeV photon in the FCAL
>that will not saturate the fADC. lets say the signal amplitude will be 
>-1.8V.
>Now I know that cosmic rays that pass perpendicular through a calo block
>generate as many photons as a 30 MeV photon that would hit the calo block
>in the center from the front. The range from 9GeV down to 30 MeV is 300
>so the signal amplitude expected from this 30MeV photon would be 6mV.
>such a small signal will probably drown in the noise no matter how many
>bits the adc has.
>The problem is the dynamic range of 300 we need to cover if we want no 
>saturation
>at 9GeV and we want to see a very low energy photon like 30MeV.
>So the question is what is more important low photons or no saturation. 
>So here my
>thoughts on this issue.
>
>A) We want to see photons with as low as possible an energy.
>    -> we have to set the HV such that cosmics signals are clearly separated
>         from the pedestal. This means signal amplitudes in the order of 
>20-40mV
>         This would allow us to see photons of the order of 30 MeV energy.
>         (From MC simulation we know that cosmics passing perpendicular 
>to the calo block
>          will generate about the same amount of photo-electrons as a 30 
>MeV photon hitting
>          the center front of the calo block.)
>    -> however in this way a 9 GeV photon would generate about a 12V 
>signal in the central block
>         and the fADC will clearly saturate. (I assume here the base can 
>handle the current
>         needed for such a large signal). the response of the calo block 
>is linear. the amount
>         of photon-electrons is directly proportional to the deposited 
>energy which is directly
>         proportional to the incident photon energy.
>
>    => there might be 2 possible solution to this problem assuming the 
>base can
>          handle these large amplitude signals.
>          1) about 30% of the energy is deposited in the neighouring 
>blocks. So the
>              surrounding blocks of the one hit by the 9GeV photon will 
>see photons
>              causing signal amplitudes in the order of 1V which will 
>not saturate
>              the fADC and these signals could be used to estimate the 
>signal amplitude
>              in the central block.
>              This is of course rather crude since most of the time the 
>photon does not hit
>               a calo block in the center and there might be more than 
>one block saturating
>              and an algorithm to reconstruct the real photon energy 
>might be very difficult
>              to find and implement.
>
>          2) we split the signal before the fADC input by a ratio 1:7 
>and delay the signal with
>              the large amplitude by say 150ns and combine the two 
>signals again before
>              the input into the fADC. So for each event we would have 
>two signals in the fADC
>              the "low-gain" signal at latency time X and the second 
>"high-gain" signal at latency time
>              X+150ns.
>              This would not work I guess if the rate in a single 
>channel is high enough to cause
>              significant pile-up or the PMT-base can not handle these 
>high amplitude signals.
>         
>B) we set the low energy photon threshold and hence the gain using the 
>pmt HV depending
>    on the physics topic of the run. Lets say the physics we are after 
>gives rise to a high
>    photon limit of 3GeV. Then we could set the gain of the PMTs such 
>that 3GeV is full range and
>    a 30MeV photon would then generate a signal of about  18mV.
>    Or lets say we have a physics topic of looking only for high photon 
>energies in the FCAL.
>    Then we set the HV such the high energies will not saturate.
>    In this scenario we would be forced to select the FCAL gain 
>according to the physics program
>    and we might lose potential data we did not even know we might have 
>had as our understanding
>    and insights grow over the course of the experiment.
>
>
>My view of the problem is:
>1) from a physics point of view we want to have the FCAL threshold as 
>low as possible to
>    measure photons as low as possible.
>
>2) if this low threshold means 30MeV photons to be seen we talk about a 
>factor of 300
>    in dynamic range for the highest photons, if we do not want saturation.
>
>3) even if the threshold is higher lets say 150MeV and we do not want 
>saturation at 9GeV
>    we have to cover a dynamic range of 60. Also this is already tricky.
>
>I am looking forward to your comments and thoughts. If I am wrong about 
>this and it is
>not a problem I would be really happy ;-)
>
>cheers,
>Beni
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