> Finally, Chris convinced me to throw in Ioannis' suggestion for a fix: Ioannis
> suggested to get rid of the zero-suppresion and use all memory (currently
> split between ADC and TDC words) for ADC words. This will double the
> avaiable time window for digitization, in which case a common stop at
> ~160usec(?) will cover most of our acceptance for slow particles. Ed, would
> something like that be doable?
If we did this, what would the impact on the average amount of data
readout be? I worry that it would be quite a bit. I really don't like
this basic concept since it isn't clearly needed and limits
acceptance. We don't have to do everything that somebody thinks up to
help the problem.
>
> Capacitor in the Fanouts
> ========================
> Work-bench tests we've performed during the last two weeks have indicated
> strange behaviour of the fanout channel that was modified with the 1mF cap.
> This involves reduced pulse amplitude and 2mv p-p oscillations; none of
> them were observed before. We believe both have to do with the soldering
> of the cap and not with a systematic effect. As a cross check we plan to
> check ASAP the modified channels currently in MACRO.
The soldering!? Then why can't you just retouch the solder and get
rid of the problem? I don't understand this suggestion at all. Is it
just the usual grounding problems instead?
> People looking at WFD data: notice that channels 3B01-0,3B02-0,3B03-0,3B04-0
> have already been modified with the 1mF cap on the fanout.
> In addressing the issue of variable WFD stopping scheme, Chris and Ioannis
> looked quite extensively in the maximum pulse output of our PMTs.
> Their measurements show that our PMT bases are not capable of providing
> pulses at the level of ~4V and above for more than ~5usecs. This is due to
> the capacitors at the last dynode stages which can not supply the necessary
> charge in order to meet the peak anode current. The PMT output at that level
> is consistent with loss of gain (effectively you lose the last few dynodes
> stages). They've doubled the capacitor value in the last dynode chain and
> they thus managed to see the maximum charge delivered by the PMT almost
> doubled.
I agree that the bases saturate at a lower level than 10V x 10 us. Do
remember though that the horizontal tanks have two tubes/bases and
therefore will put out twice the current which you have measured for
the single tube. That has always been my thinking in the 10 V x 10 us
business and we will always have half the saturated signal from
vertical tanks as from horizontal tanks.
>
> Capacitor in the Disc. Circuitry & SPE's Disc. Efficiency
> =========================================================
> We performed a number of test-bench measurements over a grid of pulse
> width/pulse amplitude values in order to fine tune this capacitor value.
> In all of these measurements the LED/PMT set up in the dark box was used
> to create real-like (with p.e. stats) PMT pulses. In the table below you
> may find the MINIMUM PMT pulse height below which we were starting losing
> parts (on the WFD discriminator) of the input pulse. The PMT signals were
> observed on the digital scope and the *mean* measured the pulse is reported.
>
> 0.05uF 0.1uF 0.2uF 1uF (original design)
> -----------------------------------------------------
> 5usec 30mV 15mV 15mV 12mV
> 10usec 70mV 25mV 18mV 13mV
> 20usec >200mV 50mV 18mV 15mV
> 30usec >200mV 100mV 45mV 20mV
>
> Notice that interruptions in the WFD discriminator signal does not necessarily
> imply complete loss of input signal (due to signal pre/post-sampling).
> The situation is significantly improved with the 0.1uF capacitor. Larger
> capacitor values (>=0.2uF) can do even better BUT they increase the time over
> which the negative discriminator stays "on" to values close to our hardware
> limit (~100usec). If we really want to increase this capacitor we have to go
> to >>1uF (40uF?) capacitors that will do stay "on" longer BUT the overall
> overshoot amplitude will never exceed threshold.
>
> It was rather a surprise for us the fact that a 30mV (mean) PMT pulse
> followed by the (0.01+0.05)*125 uFohm=7.5usec discriminator RC circuit
> couldn't stay above threshold for its entire 5usec duration. This led
> us to the question of EFFICIENCY in discriminating spe's by the WFD
> discrimination circuitry. We have performed multiple tests that demonstrated
> that at our nominal 4mV/spe gain, spe's are not discriminated with 100%
> efficiency. It is true that spe's are described with distributions
> both in height and width. Our measurement show that over the entire
> spe spectrum, 100% efficiency is achivied around 5mV pulses.
> At 4mV the efficiency is at about 80% and at 3mV about 60%.
>
Is this with the 1 mF capcitor installed or not? This is a *crucial*
point since that is nominally the thing which really made people like
me think that electrolytics wouldn't work in the first place. With the
one that I looked at when I was in Italy, I could see no attenuation
to fast pulses... but maybe the one you are using here is creating
attenuation (dispersion)?
The general issue of SPE efficiency is an important one of course. I
suggest continuation of these types of studies. However, your 100%
effeciency at 5 mV seems inconsistent with the cutouts which you
describe in the table above. Can you explain?
Doug