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Hi Tim,</div>
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I just looked up Goodfellow Metals to find the thickness variations of Tungsten foil. For 10 micron foils, it is about 25%.</div>
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Attached is the spec sheet.</div>
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Al</div>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> Hps <hps-bounces@jlab.org> on behalf of Nelson, Timothy Knight <tknelson@slac.stanford.edu><br>
<b>Sent:</b> Sunday, August 18, 2019 1:38 PM<br>
<b>To:</b> Valery Kubarovsky <vpk@jlab.org><br>
<b>Cc:</b> HPS <hps@jlab.org><br>
<b>Subject:</b> Re: [Hps] Daily RC Meeting</font>
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<div class="x_PlainText">Thanks Valery,<br>
<br>
For what it’s worth, here are some representative plots from 2016:<br>
<br>
<a href="https://logbooks.jlab.org/entry/3399754">https://logbooks.jlab.org/entry/3399754</a><br>
<br>
… where in these plots Sho had added a “low-charge hit" cut that eliminates most of the x-rays and low-charge hits during stacked triggers, AKA monster events. (You can also see the large low-charge peak has been cut away in the Cluster Charge plots.) You
can see that the MIP occupancy peaks at about 1.4%.<br>
<br>
The fact that we don’t have equivalent cuts in our current monitoring plots makes comparison somewhat difficult, but subtracting off the 0.2%-0.5% (depends on location and noise in a given chip) of flat (across the device) x-ray occupancy, I’m going to say
we are currently at about 1.7% peak occupancy for 100nA*20um. Although the lower multiplicity with the current sensors means the difference in particle occupancy is a bit larger, I think this is probably still in an acceptable range, partly because I think
our “isolation” cut is smarter than what we had when we assessed this before, and because I think we hope it can become smarter still. However… we won’t really know the answer until we do the analysis. It would be nice to hear from the real experts here
- Matt and Matt - but I will also try to dig up the quick study Sho did of this back in 2015, that convinced us we should run not double the current because of diminishing returns - doing more Si damage without any obvious advantage.<br>
<br>
As far as why these occupancies are higher than expected based on previous good agreement, I am not sure. One factor can be that the detector is simply a bit more closed than we planned to be, where even 75 microns is a 10% change in our solid angle coverage.
Takashi would predict a ~20% increase from such a change. There is some evidence of this possibility from early vertexing, although I don’t think we fully trust our alignment yet. Second, I think we can also be seeing the effect of the tails on the beamspot
that need much smaller than 15mrad scatters to get into the silicon. For example, from target-out running, we know that we don’t have tails hitting the active portions of the sensors. However, even moving the beam 50 microns either direction can make 1%
occupancy from tails in either half of the sensor. Such tails only need a 0.05mm/50mm = 1mrad scatter in the target to hit the active region. Such scatters are several sigma, but the tails can be rapidly increasing as one moves towards the center of the
spot, so without measuring them directly at the target, it’s difficult to say that this cannot contribute to our ~30% discrepancy.<br>
<br>
Hopefully we can have a good discussion of what to do tomorrow. I do think it’s imperative that we collect at least 2 “nominal weeks” of data to ensure results. I think we were on a path to this with the old target if running was smooth, and we should be
aiming higher now to make up for some lost time last week.<br>
<br>
Cheers,<br>
Tim<br>
<br>
P.S. See below a forwarded message Takashi sent about occupancies with these thicker targets - I don’t know that it has been widely distributed and discussed, but now is the time.<br>
<br>
> Begin forwarded message:<br>
> <br>
> From: "Maruyama, Takashi" <tvm@slac.stanford.edu><br>
> Subject: Running with thicker target<br>
> Date: August 1, 2019 at 6:18:34 PM PDT<br>
> To: 'Vitaliy Fadeyev' <fadeyev@ucsc.edu>, Timothy Nelson <tknightnelson@mac.com><br>
> Cc: "Holtrop, Maurik" <Maurik.Holtrop@unh.edu>, Stepan Stepanyan <stepanya@jlab.org>, "Graham, Mathew Thomas" <mgraham@slac.stanford.edu>, "Jaros, John A." <john@slac.stanford.edu>, "Bravo, Cameron B." <bravo@slac.stanford.edu>, "Moreno, Omar" <omoreno@slac.stanford.edu>,
Nathan Baltzell <baltzell@jlab.org><br>
> <br>
> Attached is the occupancy at 4.4 GeV. To compare with my previous calculation, I chose 4.4 GeV. One criterion we used to find the beam current was 1% occupancy at 1.5 mm at Z=10 cm. This current yielded roughly 1 MHz max. single rate in ECal crystal.
Based on this criterion, the beam current for 8 um target was 300 nA. Assuming the linear thickness dependence, the beam current for 16 um target is 150 nA, and for 20 um target is 120 nA. As seen in the plot, the occupancy for 20 um@120nA is slightly higher
than 1%., but it could be acceptable and 100 nA may be safer. <br>
> <br>
> Page 2 shows the distribution at L1. As you see, there are positrons from the bremsstrahlung conversion inside the target. The positron rate for 20 um target is (20/8)**2 = 6.25 times for 8 um. But these positrons from bremsstrahlung conversion inside the
target should have a negligible contribution to the positron trigger rate.<br>
> <br>
> Takashi <br>
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<div class="x_PlainText">> <br>
<br>
<br>
<br>
<br>
> On Aug 18, 2019, at 12:49 PM, Valery Kubarovsky <vpk@jlab.org> wrote:<br>
> <br>
> I made a comparison of new 8um and 20 um targets. The rates are almost proportional<br>
> to the tagger length. However, the old 8um target and new 8um target rates are different.<br>
> <br>
> We have 8% discrepancy between 2C21A and FC reading now. We did not have such a difference in the past.<br>
> FC/2C21A=1.08<br>
> There were several log entries about FC offset recently.<br>
> Valery<br>
> <br>
> <br>
> On 8/18/19, 3:09 PM, "Hps on behalf of Nelson, Timothy Knight" <hps-bounces@jlab.org on behalf of tknelson@slac.stanford.edu> wrote:<br>
> <br>
> Hi Raphael,<br>
> <br>
> I meant to ask today about the differences between the new 8um target and the old 8um target, where the difference is quite large considering that the two foils came from the same sheet. Do we have any understanding of this? Are we certain the beam current
measurement (mentioned today to be different between the FCup and 2C21) has not shifted in some way?<br>
> <br>
> The reason I ask is that the occupancies with 80nA*20um are higher than 200nA*8um(old) but lower than 200nA*8um(new), so it seems foolish to think we can decide on an operating point (high current*thin target vs. low current*thick target) when we don’t
understand this discrepancy. <br>
> <br>
> Best,<br>
> Tim<br>
> <br>
>> On Aug 17, 2019, at 6:34 PM, Raphael Dupre <dupre@ipno.in2p3.fr> wrote:<br>
>> <br>
>> Hello everyone,<br>
>> <br>
>> We are finally back in production since 1PM. Beam is good and steady and we have taken test data sets at various luminosities (see logbook for details). I propose that we meet rapidly tomorrow with those available to discuss our production luminosity and
the trigger adjustments to be made in consequence as live time can reduce significantly at the highest settings.<br>
>> <br>
>> The arrangements remain as usual: in the counting house 2nd floor meeting room and bluejeans for remote participants:<br>
>> <br>
>> Bluejeans: URL: <a href="https://urldefense.proofpoint.com/v2/url?u=https-3A__bluejeans.com_794825703&d=DwIFaQ&c=CJqEzB1piLOyyvZjb8YUQw&r=ErLkbYGWd8oSAZ70ywHxhA&m=YO8txpxO_ZRED7MUQZsqo1YPlQ_vZESUQ6jaR-_M54g&s=MmHeEB8bYIGfq1Hd2a-zFklj3B6l6X6irtpxd2ZZgMM&e=">
https://urldefense.proofpoint.com/v2/url?u=https-3A__bluejeans.com_794825703&d=DwIFaQ&c=CJqEzB1piLOyyvZjb8YUQw&r=ErLkbYGWd8oSAZ70ywHxhA&m=YO8txpxO_ZRED7MUQZsqo1YPlQ_vZESUQ6jaR-_M54g&s=MmHeEB8bYIGfq1Hd2a-zFklj3B6l6X6irtpxd2ZZgMM&e=</a>
<br>
>> Meeting ID: 794825703<br>
>> <br>
>> Best,<br>
>> <br>
>> ---<br>
>> Raphaël Dupré<br>
>> Institut de Physique Nucléaire d'Orsay<br>
>> CNRS - IN2P3<br>
>> Université Paris-Saclay<br>
>> <br>
>> _______________________________________________<br>
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>> Hps@jlab.org<br>
>> <a href="https://mailman.jlab.org/mailman/listinfo/hps">https://mailman.jlab.org/mailman/listinfo/hps</a><br>
> <br>
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