[Halld-pid] Minutes, GlueX TOF Meeting, September 15, 2011

[Mark M. Ito]

Folks,

Find the minutes at 
http://www.jlab.org/Hall-D/software/wiki/index.php/GlueX_TOF_Meeting%2C_September_15%2C_2011#Minutes 
and below as text.

-- Mark
_____

GlueX TOF Meeting, September 15, 2011

Minutes

Present:
* FSU: Paul Eugenio, Sasha Ostrovidov
* JLab: Chuck Hutton, Mark Ito (chair), Simon Taylor, Tim Whitlatch,
Beni Zihlmann


Announcements

We talked about the meeting with Hamamatsu representatives scheduled
for tomorrow here at JLab, mainly to discuss SiPM's. Paul has been
talking to Andrew Allen of Hamamatsu who will be there and asked him
some of the questions on current limits that we have been discussing.
Beni and Mark will meet with the visitors over lunch to discuss these
issues as well.

Review of minutes from last meeting

We went over the [32]minutes from the September 1st meeting.
* Mark will circulate a new draft of the scintillator specifications
this afternoon.


Detector dimensions, light guide dimensions

We discussed many of the issues that have been raised over the past few
weeks. Chuck provided a [34]set of drawings of the current design to
guide discussion.
* Andrew Allen mentioned to Paul that we may be able to get the
μ-metal shield extended beyond the photocathode, given the quantity
we are ordering. Andrew is looking into it.
+ Tim said there was still a couple of Gauss at the photocathode
with the off-the-shelf dimensions.
* We decided to have a 1 cm straight section of the light guide where
it joins to the scintillator. This helps with the non-square
problem (see below) That way we can support both sides of the glue
joint.
+ If this new straight section is made too long, then the light
guide cannot be machined out of the currently used Lucite
cylinder.
* Tim reported that there are constraints on the geometry from the
FCAL and the FCAL support structure. If the light guides could be
made shorter, or be bent less, that would help.
+ Paul remarked the the in-board edge of the iron sleeve can be
trimmed (corner cut off) to help with the interference between
neighboring PMT's.
+ Tim reported that the iron shielding can be obtained with a
smaller diameter and still accept the PMT's.
* Non-square problem: when the thickness of wrapping is included the
array is no longer square and the ends of the scintillator would be
inside the outside edge of the scintillators of the other plane. We
will make the scintillator a bit longer (from 252 cm to 254 cm or
so) to avoid this as well as adding a straight section to the light
guides.
* Paul raised the possibility of needing extra Tedlar wrapping,
beyond the current design, to insure a good light shield around the
VM2000 wrapping should it be necessary to buy the VM2000 in small
sheets. If that is the case the VM2000 would be in sections butted
up against each other and each joint would constitute a thin spot
in the light-tight-ening. The extra layer of Tedlar changes the
total thickness of wrapping from 1.34 cm to 1.79 cm. We thought
that an external layer on the upstream and downstream faces might
suffice and avoid another Tedlar around each counter.
* We decided on a 3 mm gap between planes of the TOF to reduce stress
on the mechanical design process.
* There is 2 cm between the TOF and FCAL presently. Tim suggested we
open that up to 3 cm or so.

Tim and Chuck will work on a optimization of these parameters and of
the light guide shape and send around a proposal. Tentatively we will
have a special meeting, this Thursday at 9 am, with the mechanical
folks at FSU to discuss the proposal.

TOF schedule

Tim reminded us of where we are supposed to be, soon:
* PMT's procured in two weeks
* support structure, buy in the Spring
* by next May testing scintillators and light guides
* procure scintillator Nov. 2

This will be tough to make. We do not have a contract yet, so JLab may
have to buy the scintillator if we want to keep on the schedule.

PMT base design, rates, currents, amplification

Beni pointed us to his [37]wiki page where he studies the implications
of the rate on the TOF on PMT lifetime. If he assumes a maximum current
of 100 μA, then the inner three full-length counters on each side and
the half-length counters will have to be run at lower gain and an
amplifier used on the PMT output. In addition the voltage divider would
have to be modified so allow sufficient voltage on the first few
dynodes.
* Paul pointed out that the rates may have been calculated for 10^7
photon flux. If so we may be able to avoid the special arrangement
until we go to full luminosity.
* There was some controversy about the number used as the constraint
on the acceptable rate. We will ask the visitors from Hamamatsu
more on this tomorrow.


Strange shape of Delta(t) distribution in the short trigger counters

Sasha presented recent work trying to understand the longitudinal
position distribution in the trigger counters he is using in the cosmic
test set up. See [39]his page for details. He experimented with various
discriminator thresholds and with back termination of the signal
cables.

Retrieved from
"http://www.jlab.org/Hall-D/software/wiki/index.php/GlueX_TOF_Meeting%2C_September_15%2C_2011"

References

32. 
http://www.jlab.org/Hall-D/software/wiki/index.php/GlueX_TOF_Meeting%2C_September_1%2C_2011#Minutes
34. https://halldweb1.jlab.org/talks/2011-3Q/TOF%209_15_11.pptx
37. 
http://www.jlab.org/Hall-D/software/wiki/index.php/Detector_Rates_and_Lifetime
39. 
https://halldweb1.jlab.org/talks/2011-3Q/Shape%20of%20Delta(t)%20distribution.html