[Hallc_running] Suggestion to shorten NPS readout interval from 400 to 200 nsec

[Carlos Munoz Camacho]

Dear Peter, all,

   Thank you for looking into ways to decrease the data rate for the
upcoming kinematic settings. Malek and I have discussed this proposal of
reducing the FADC readout window, but we don't think we should do it.

As it has been shown, waveform analysis is crucial to improve the energy
resolution of the NPS calorimeter, which is in turn the key to the good
missing mass resolution needed for exclusive channels such as DVCS. It is
incorrect to say that we only analyze events in a 100 ns time window; we
are currently fitting all 110 samples (i.e. 440 ns). Attached is a sample
(normalized) waveform.

The pulse itself extends over ~100 ns (25 samples). In order to remove
pile-up before the coincidence pulse, we need enough samples to fit a pulse
arriving before it.

There are also channel-to-channel variations in the coincidence time of +/-
20 ns (5 samples) and variations due to the calorimeter distance as a
function of the kinematic setting.

We also need enough time window to compute accidentals.

While we could scrap *a few* samples at the end (and possibly at the
beginning) of the current readout window, this is probably not worth the
trouble as it won't significantly change things (20-30% at most).

We are open to discussing other ways of reducing the data rate without
compromising the experiment's feasibility.

Best,
Carlos


On Fri, Mar 8, 2024 at 11:45 PM Peter Bosted via Hallc_running <
hallc_running at jlab.org> wrote:

> Background: we have been generally running considerably less beam current
> than in the NPS and SIDIS proposals (mostly planned for 30 muA). As a
> result, we are getting anywhere from 2 to 10 times less events than
> we hoped for.
>
> There are several factors that limit which current we use:
> a) keep anode current in average of NPS columns 0 and 1 below 30 muA
> b) keep data rate low enough that no crate exceeds 80 MB/sec. Since the
> crates all have roughly the same rates, we need to be well below
> 400 MB/sec to avoid this happening to any of the 4 and a half VME crates
> (one of the 5 is only half-populated). Last night we did a 30 minute
> run at 300 MB/sec with no trips. I think there is general agreement
> that keeping the rate below 200 MB/sec is acceptable.
> c) keeping the trigger rate low eneough to avoid significant computer dead
> time corrections, as well as exceeding the maximum transfer rate of data
> from the hall to the mass storage system.
>
> For most of the settings for the rest of the experiment, factor c) will
> be the limit that we reach before factors a) and b). Since the event size
> (and hence computer live time and transfer rates) are largely determined
> by writing out the NPS FADC data, we can gain up to a factor of two
> in what current we run by reducing the readout time for the FADC.
>
> At present, we have a readout window of 400 nsec.
>
> We only analyze events in a 100 nsec time window.
>
> But we need to readout over an interval longer than 100 nsec in order
> to catch the long tails of some pulses.
>
> A resonable compromise as far as I can tell would be to shorten the
> readout interval from 400 to 200 nsec.
>
> To keep the window centered on the coincidence time peak, we would
> reduce the time by 75 nsec on the back end, and 125 on the front end,
> as far as I understand.
>
> I propose that the experts get together on Tuesday after the Moller run
> and implement this change, after taking one run with the 400 nsec
> window, so we can make sure thaat no good data is being lost. As
> far as I understand, the experts include Alex, Wassim, Ben, and Sanguang.
>
> As I understand, only the config file(s) need to be changed: no knobs
> on the crates need to be turned.
>
>
> Prof. Peter Bosted
> email: bosted at jlab.org
> phone: (808) 315-1297 (cell)
> P.O. Box 6254, Ocean View, HI 96737
>
> _______________________________________________
> Hallc_running mailing list
> Hallc_running at jlab.org
> https://mailman.jlab.org/mailman/listinfo/hallc_running
>
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