Forwarded message:
> From kearns Thu Feb 6 18:57:25 1997
> Subject: Re: K's comments on the WFD memo
>
> really fast to implement. However, I have my reservations as to
> the extend that it will affect our ability to record monopole
> waveforms.
I agree- I wouldn't go ahead until we are sure. But I also have
reservations about uncabling the whole experiment to monkey with the
fanouts. I don't think cost is an issue.
> ~8 microsecs. However, the critical pulse height that is assumed
> here is maybe TOO OPTIMISTIC for a monopole at the 3-10microsec
> range and we already
We agree. Chris will replot at monopole levels. Just to help other
readers understand, this is the one case where low signals hurt rather
than help. So we need to get away from what we called the critical
voltage.
> 4| 15mV, 5usec |T=0 /T=2.5 & T=5.0/T=7.3 |T=2.5/T=5.0 | YES (50%) |
> 5| 8mV, 5usec |T=0 /T=1.7 & T=5.0/T=6.5 |T=1.7/T=5.0 | YES (66%) |
> 6| 4mV, 5usec |T=0 /T=1.0 & T=5.0/T=5.7 |T=1.0/T=5.0 | YES (80%) |
> 7| 8mV, 8usec |T=0 /T=1.7 & T=8.0/T=9.7 |T=1.7/T=8.0 | YES (79%) |
> Given that our expected slow monopole signatures will be at best the
> ones of test pulses #4,5,6,7 of the above table, we see that we will
> lose more than 50% of the monopole waveform for that width-pulse
> regime. Needless to say,
OK- I'm sure your measurements are right for square pulses. But we
have to keep in mind that there is no way to get a long 4 mV signal;
it has to be s.p.e. For much of this "dangerous" range, the spiky
photostatistics should completely restore the waveform. I'm anxious to
see Chris' SPICE simulations; I think they will be very instructive
since we have such a hard time generating long, high, fast, quiet
pulses with either LEDs or a pulse generator.
> Effect on Slow Massive Nuclearites
> ==================================
Let me state my personal opinion. We have to make the detector work
for monopoles and dyons. I would want to cover at least a factor of
100% for dyon dL/dX. After that, whatever nuclearite data we
get... fine. But I don't think we should take risks to get much more
coverage. Looking back on this, we have already sacrificed a lot in
time and complexity to try to do too many things. Long windows for
catalysis, the non-linear front-end come to mind; even though we made
them work, *in retrospect* it was not worth the extra time and effort
compared to say: twice as many channels with high/low gain. This
problem would have been a non-issue then, for example. We are in hot
water because we are trying to discriminate on 4 mV 15 ns single
photoelectrons at the same time as 10V 10usec pulses.
> a bit perplexed. The region marked as "LOST DATA" appears to be a bit
> after the "middle" (in the time axis) of the input pulse but *clearly*
Yes- this plot needs fixing. For example, the discriminator can't precede
the input pulse. It's an artifact of the scope data. I agree that we will
lose data from "some point" until the end of the input pulse. And exactly
how we catch the end remains to be proven (but we will see some indication.
I think the point here is that there is certainly some pathlength for which
we get in trouble at a given pulse height. Pathlengths of 12m are possible in
MACRO; just very unlikely. But even for the longest path, we will have some
signature to rely on.
In any case, we should try to minimize the lost data region for some 90-95%
of all pathlengths. We think 50cm is a good number.
> It is concluded in the memo that with the current WFD configuration
> we start becoming insensitive to monopoles at about beta=3x10^-3 and
> above; this might be too optimistic. I recall Doug estimating this
> to be at about beta~5x10^-4 and above, mostly coming from a
> measurement of his, showing that it cuts off at about 1Vx1usec
You are right, we've corrected the memo.
> BTW, in the summary figures 4 and 6, I had problems justifying the
> details of the bottom and top horizontal axes. For example in fig. 4
> a ~5usec pulse width is associated to ~10^-4 beta, something that I
Also fixed.
> Alternative Idea(s)??
> =====================
> On Feb. 3, 1997, Ioannis sent to all of you an alternative fix that
> involves the replacement of the capacitors in the two RC circuits
> that the signal goes through before the discrimination: one in the
> BU fanout and another one in the WFD daughtercard. It actually
> points to the reverse direction; it suggests increasing both of the
> capacitor values. The proper choice of capacitor values will
> guarantee that any positive overshoot or negative undershoot
> accompanying the PMT pulse will remain bounded in the
> [-2.5mV,+2.5mV] region, thus never resulting to any data loss. It
> will actually improve the quality of the digitized signal as it will
> get rid off any exponential decay currently present in long PMT
> pulses.
I agree that the absolute fix is to have such a large capacitance that
the problem goes away. The problems are practical. Uncabling the FMT,
TOHM, PHRASE, WFD, ERP to modify all the fanouts must be planned very
carefully and could potentially cause more of a disruption at all beta
(being down, having to fix things) than our acceptance loss is worth.
That being said: we are still considering those option! But we have some
comments to send back.
> used in a fix implementation. Capacitors of the right (physical) size and
> value (~1000uF and ~100uF) are available (found in product catalogs) BUT
> THEY WERE NOT available to us at the time of test. We invite you to
> test it on your work-bench. Cost and implementation are within reach
> (cost of <$2,000, implementation ~1week with 4 persons).
1) You don't really want to suggest of 100 uF electrolytics for the
d/cards. Electrolytics have limited lifetime (1000 hours for the
surface mount ones I found) and generally "ghastly" characteristics
(H&H page 22). I don't know about frequency response but that's
probably bad too.
The best we can find in a catalog that would fit on our pads (B or C
size can fit... also called "3528" and "6032" is a 68uF
tantalum. Chris will try to order some. When I was ordering these last
year, I found it very difficult to get quantities in stock. Lead times
were sometimes 6 months. But a year has passed, so we'll see...
2) We haven't found a good candidate non-electrolytic 1000 uF for the
fanout. When the fanouts were built I think 2.2uF was the max; looking
quickly through some catalogs we see up to 5.6uF, not a big change. Can
you tell us more about the ones you've found that you think will fit?
> Bottom Line
> ===========
> I believe we all agree that the proposed fix might come with data
> loss under some circumstances (long pulses-low ampls.). This might actually
> be a feature of *any* fix that anyone can come up with. The question is
> which has the least effect on our physics analyses. Although simplicity
> and straightforwardness of the fix is a major consideration, it should
> not be the only factor.
I do agree, it should not be the only factor. We should certainly add
the big-cap option to the memo, and anything else you want. And again,
thanks for the corrections. It sounds like we need some more time to
debate this and consider the choices before we make a final decision.
Ed