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Beni,<br>
<br>
This morning Elton and I were discussing an idea for providing a bit
more margin for the all-10-stage solution that is on the table.<br>
<br>
From the exchange between you and Elton, we seem to have a lifetime
estimate (again for the all-10-stage option) whose central range is
significantly less than two years. That is not very comfortable. We
have talked about using amplifiers, but building a custom PMT base
would add another design and build component to the plan. The idea
we discussed was to use commercial amplifiers, as a back-up in the
event that currents and/or PMT decay rates turn out to be as high or
higher than we expect.<br>
<br>
We could initially run the tubes without amplifiers and as you point
out we could raise the voltage if we see the gain start to sag
significantly. Finally, we could add a commercial 10X amplifier
downstream (electronically) of the PMT for any tubes that don't
look like they will make it to the nominal two years. These have
been available for years (actually decades). For this last step we
could maintain acceptable voltage steps in the first few dynodes
with some modest lowering of the voltage, but because the last
dynodes would be degraded, the overall gain would not be too large.
This gives a better shot at maintaining acceptable time resolution
(though likely degraded).<br>
<br>
The amplifiers would only have to be deployed on those tubes that
need it, likely only a few of the innermost PMT's. Hopefully, they
would not be needed at all.<br>
<br>
Note that we are talking here about a low-rate (10^7 photons/s)
solution, something that will hold us for the first two years. If we
needed to come up running at 10^8, this is clearly not a great
solution.<br>
<br>
-- Mark<br>
<br>
On 12/14/2011 05:15 PM, Beni Zihlmann wrote:
<blockquote cite="mid:4EE91FF2.6000808@jlab.org" type="cite">
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Hi All,<br>
I would like to address Elton's main concern the lifetime of the
PMTs.<br>
<br>
After 1000 hours of running at 0.1mA the gain reduces by about a
factor of 2.<br>
According to my calculations below the highest current PMT is
0.062mA so <br>
after about 1400 hours of running the gain would be reduced by
about a factor<br>
of 2. The HV is about 1600V for a pristine 10-stage PMT to get a
gain of 10^6.<br>
The maximum HV is 2000V for such a PMT with a gain of about
6*10^6. So we<br>
have some headroom to adjust the PMT high voltage to retain a gain
of 10^6<br>
for some time, but at least twice such that we can run for about
3000 hours<br>
with sufficient gain. That is not perfect but would mean we can
run for at least<br>
one year. At the end of one year running (120days full running)
one would have<br>
to assess the situation and may consider to replace the 8 PMTs of
the innermost<br>
paddles left and right of the beam hole of the two planes. At that
point we would<br>
also have enough experience with the TOF to take educated
decisions on what<br>
we need as amplifications and operating parameters for the 8-stage
PMTs in<br>
the high luminosity running.<br>
I hope this makes sense to you too.<br>
<br>
cheers,<br>
Beni<br>
<br>
<br>
<br>
<br>
<blockquote cite="mid:4EE7B2DA.4010108@jlab.org" type="cite">
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Hi all,<br>
<br>
See my comments attached. Cheers, Elton.<br>
<pre class="moz-signature" cols="72">Elton Smith
Jefferson Lab MS 12H5
12000 Jefferson Ave
Suite #16
Newport News, VA 23606
(757) 269-7625
(757) 269-6331 fax </pre>
<br>
On 12/13/11 11:54 AM, Mark M. Ito wrote:
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Folks,<br>
<br>
Yesterday Beni sent out an email summarizing thinking on the
choice of phototubes for the TOF (see below).<br>
<br>
Recall that there is a problem with PMT lifetime for the high
rate counters near the beamline, especially at 10^8 photons/s.
The choice of PMT's is central to how we deal with this
problem. <br>
<br>
Please have a look. He is asking for feedback so that we can
make a decision soon.<br>
<br>
-- Mark<br>
<br>
-------- Original Message --------
<table class="moz-email-headers-table" border="0"
cellpadding="0" cellspacing="0">
<tbody>
<tr>
<th align="RIGHT" nowrap="nowrap" valign="BASELINE">Subject:
</th>
<td>[Halld-pid] PMT choice for TOF</td>
</tr>
<tr>
<th align="RIGHT" nowrap="nowrap" valign="BASELINE">Date:
</th>
<td>Mon, 12 Dec 2011 12:01:22 -0500</td>
</tr>
<tr>
<th align="RIGHT" nowrap="nowrap" valign="BASELINE">From:
</th>
<td>Beni Zihlmann <a moz-do-not-send="true"
class="moz-txt-link-rfc2396E"
href="mailto:zihlmann@jlab.org"><zihlmann@jlab.org></a></td>
</tr>
<tr>
<th align="RIGHT" nowrap="nowrap" valign="BASELINE">To:
</th>
<td><a moz-do-not-send="true"
class="moz-txt-link-abbreviated"
href="mailto:halld-pid@jlab.org">halld-pid@jlab.org</a>,
Elton Smith <a moz-do-not-send="true"
class="moz-txt-link-rfc2396E"
href="mailto:elton@jlab.org"><elton@jlab.org></a>,
"Fernando J. Barbosa" <a moz-do-not-send="true"
class="moz-txt-link-rfc2396E"
href="mailto:barbosa@jlab.org"><barbosa@jlab.org></a></td>
</tr>
</tbody>
</table>
<br>
<br>
<pre>Dear colleagues,
In order to make a final decision on the choice and strategy with regard to
the PMTs for the TOF I put together a few facts and findings and ask for
your
opinion.
Please read the attached outline set in 4 main paragraphs:
Purpose, Facts, Calculations and Strategies
I am advocating strategy a) where we go with a two phase approach.
Let me know what you think and what makes the most sense. Depending
on your answers we will either take a decision right away or I will call
for a meeting at end of this week (beginning next) so that we can take a
decision on this matter before Christmas.
thanks everybody,
Beni
</pre>
<br>
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