Re: fadc dynamic range of FCAL

[Elton Smith <elton@jlab.org>]

Hi Beni,

My feeling is that we need to measure the lower energies accurately. I do
not see a large downside to having non-linearities at the high end
(controlled saturation) which can be taken out in software. It would be
best if this can be achieved in the voltage divider so that the ADC is
unaffected.

Cheers, Elton.




Elton Smith
Jefferson Lab MS 12H5
12000 Jefferson Ave
Suite # 16
Newport News, VA 23606
elton@jlab.org
(757) 269-7625
(757) 269-6331 fax

On Tue, 22 Jan 2008, beni zihlmann wrote:

> 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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