Minutes 3-14-2007
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FDC Weekly Meeting
Date: March 14, 2007
Participants: Daniel, Tim, Kim, Simon, Brian, Chuck, Roger
Next Meeting: Wednesday, March 21, 2007 @ 10:30 a.m.
Contents
Hall D Drift Chamber Review
- The Hall D Drift Chamber Review took place on Tuesday March 6.
The close-out and report was presented by the committee on
Thursday March 8. The recommendations of the committee for the
FDC system were as follows:
i). Work to reduce the material in the active area of the chambers.
Presently our design spec was 5% of a radiation length. Our
nominal design is presently about 4.5%. The committee was
concerned about conversions in the material that could result
in leakage effects in the partial wave analysis. This material
was of such a concern that privately they said that we might
even consider moving away from cathode chambers.
ii). Magnetic field affects on the cathode chamber resolution were
discussed, but the committee felt that from what they knew,
the difficulties in calibration and performance could be
dealt with and that our resolution spec was probably possible.
They stated (only in our private meetings) that we should move
to perform our magnetic field tests soon so that the choice of
cathode chambers could be shown to meet the design spec.
iii). In discussions in private, there was some concern about the
amount of material in the inactive portion of the chamber and
we should work on the design to minimize this (i.e. the frames
and spacers). Again, the issue is centered on conversions in
this material that could affect the reconstruction.
- Of course each of these areas were already known to us and we have
already been at work on each in our planning and our R&D work, but
at some point, we will have to address each of these areas to
satisfy ourselves, the Hall D group, and the lab management that
our design will not impact the physics and that we can meet our
resolution specs.
Material in the Active Area
- We have been working on two designs for the cathode sandwiches. One
design incorporates a foam backing for support and to define the
flatness of the cathodes. A second design is to tension the cathodes
and to remove the backing. Both designs are being considered in
the R&D phase. The design with the foam backing is currently the
nominal design choice due to the rigid support that it can provide.
I have been trying to "kill off" the design without backing as I
am worried about lateral distortions of the cathode board introduced
by the tensioning process. In light of the review and our own internal
questions about material, we should not be so hasty in eliminating
this design. In fact, we need to come up with a technique to make
this solution workable. Note that the foam accounts for roughly 1/3
of the material thickness in the active area.
- Work to be done: Simon and Chuck need to complete calculations of
the surface profile of a tensioned cathode plane due to the
electrostatic forces. Simon needs to work to understand how the
inward force on each cathode affects the chamber performance. As
the cell distorts, the capacitance changes, and hence the gain
changes. The good news here is that the inward distortion is most
likely quite gradual. We only require local surface flatness on
the cathodes to measure the charge distribution which is spread out
over 3 to 5 strips. Slow gain variations are not important as we
are performing a relative charge measurement to find the centroid of
the charge distribution. The impacts on the drift time calibration
do need to be considered.
- Work to be done: Construction of a cathode without backing is made
more difficult by the fact that our cathode planes are currently
designed from 3 separate pieces. The issue of how to align the
3 separate pieces, connect them, tension them, and attach them to
the support frame needs to be thoroughly investigated. Our lives
would be MUCH easier here if we could make the cathode from a
single circuit board. Roger has been asked to investigate if
this can be done.
- Note that if we do require the foam, there are several options
available to reduce the material thickness. These include:
> Cutting out a hole around the photon beam line (the 7-cm diameter
hole). This is an easy thing to do and will be done if we end
up using the foam. Daniel will talk to David Lawrence about
studying these options via our Monte Carlo.
> We could reduce the foam backing thickness below 5 mm.
> We could reduce the number of packages or the number of measurement
planes in each package. Our current studies were done with the
fast Monte Carlo program. They indicate that with 4 packages,
each with 6 planes, we are multiple scattering dominated. If
we reduce the number of layers per package to 5, the resolution is
unchanged. If we reduce the number to 4, the resolution gets worse
by nearly a factor of 2. The decision to go with 6 active layers
per package was not to maximize resolution, but instead to provide
sufficient numbers of measurements to aid in the pattern recognition.
This needs to be studied in Monte Carlo. Daniel will talk to
David about this issue.
- One suggestion by the committee was to investigate replacing the
copper on the cathodes with aluminum. The copper accounts for more
than half the thickness in the active area. However there are a
number of issues with aluminum. First PCB manufactures cannot handle
aluminum, secondly, soldering to aluminum is very difficult, next,
surface conductivity is an issue. Aluminum has a significantly
higher attenuation length. In this regard, gold is very similar in
density to copper (and surface adhesion to the kapton is an issue).
As for aluminum, we need to investigate the possibilities of going
outside the PCB industry to lithography sources. Brian will work
to make some contacts. Simon should talk to Fernando to seek his
advice. Note that Brian is worried about the sputtering techniques
used in this case. The accuracy of the strip gap is a possible issue.
Also our nominal design calls for 1/7 oz of copper for the cathode
strips. We have seen no performance degradation between 1/2 oz copper
and 1/7 oz copper. Can we investigate using still thinner copper
traces. Simon should talk to Fernando about this. It seems (from what
we know) that we can request any thickness that we wish, down to a few
hundred angstroms.
Magnetic Field Issues
- Work to be done: Simon has been asked to prepare a list of all gas mixtures in use in cathode chambers that should be considered. He will look at mixtures in use in ATLAS, PHENIX, CMS, and LASS. He will repeat the GARFIELD studies that Daniel did for the review and investigate 4, 5, and 6 mm half gaps. We need to converge on some candidate gas mixtures. - Work to be done: We need to identify a place where we can perform the magnetic field studies with the small-scale prototype. Brian will work to make contacts at BNL, FNAL, and SLAC. We need a dipole magnetic field a vertical field with a field strength of at least 1 T and a gap of about a foot wide (so we can fit in the chamber and the trigger scintillators). We need to devise a test plan so we know exactly what we would like to study and how long it will take. Daniel estimated that we would probably need to have about 1 week to complete the studies. - Work to be done: Just before the review, Daniel contacted Andrey Korytov about magnetic field issues in the CMS cathode chambers. He responded saying that we should set up a conference call to talk things over. Simon should set this meeting up and we need to prepare for it. - The magnetic field affects can be reduced by reducing the chamber half-gap. This will limit the spreading out of the charge distribution on the cathode planes. This needs to be investigated so that we have some understanding of the size of the affect as a function of half gap. Presently our half gap is defined by the size of the connectors, and this caused us to migrate up to 6 mm. We need to eliminate this constraint. We talked about a plan for putting the preamp daughter board connectors for the cathodes in the middle of the cathode sandwich. The spacing between active layers in a package is not a critical issue. As for the wire plane daughter boards, we began to discuss a plan to either mount them in the cathode sandwich or on the upstream and downstream faces of each FDC package. The connections to the daughter boards would then have to be made using jumpers. Simon will talk to Fernando to discuss this issue.
Material in the Inactive Chamber Region
- Simon is presently investigating alternative material choices for the frames and spacers. One thing that we believe now is that we have to construct the wire frames from G10 to minimize the deflections due to the wire load. However, each chamber still has the spacers and frames. We could replace the spacers with foam and the cathode frames with a foam/stainless steel or foam/aluminum sandwich. Simon will continue to investigate this. Chuck should work to perform compression calculations so that we can understand the impact on the half-gap definition as we compress the stack to compress the o-rings. - Simon will do some calculations of the material thickness for different options so that we can understand what the impacts are. He needs to communicate these options to Eugene to rerun his Monte Carlo for photon conversions.
FDC Package Design
- We are working to investigate ways to reduce the number of o-rings required in each package. This could make the assembly and the gas sealing much less of a concern (or a hassle).
Cathode Sandwich R&D
- We expect our dummy cathode boards with the copper strips next week. Brian will work to construct a second sandwich with the foam backing as well as to construct a full sandwich without the foam backing. - The dummy boards should be measured before the tensioning and after the tensioning to quantify the lateral distortion (possibly as a function of applied tension). - Chuck will think about incorporating strain gauges on the cathode that will be tensioned so that we can get some additional quantitative information on the technique. - Simon is responsible for measuring the strip capacitance as a function of strip length (from the shortest to the longest strips). This information should be communicated to Fernando and to Gerard for feedback. - Brian needs to contact the JLab Survey Group about a non-contact flatness measuring scheme. This should be available on the time scale of a month so that we can have a precision measurement of the surface flatness of the cathodes in our nominal design with the foam backing.
Small-Scale Prototype Plans
- Simon needs to provide resolution numbers for the chamber now that the grounding has been improved (by Gerard during his January visit). - The cathodes in the 90 deg orientation are ready for installation. Presently we cannot perform resolution studies with both 90 deg cathode installed as our cosmic ray chambers are only calibrated to the 1-mm level. Perhaps we can study the resolution with one 90 deg cathode and one 45 deg cathode? Simon should look into this. - We should work with Roger and Kim to layout a design with the cathode oriented at +/-75 deg and get this order out the door. We will also need to have support frames made for these cathodes. Again, Roger and Kim should prepare these designs. Brian brought up the fact that we need to make sure that there are no conflicts with gas distribution. - Summer student: Simon should put together a request for a summer student to work with him on the FDC prototype. This should be a grad student if at all possible. The project would be to calibrate the cosmic ray chambers and the scintillator hodoscope.
FDC Budget
- Daniel will update the FDC budget to incorporate the feedback that he has gotten. The items to update include: wire winding costs, preamp costs, signal cable costs, and costs for construction jigs for wire winding. The new budget is due by the end of March to Elton. Note that the budget will be based on the current nominal design.
FNAL MOU
- We have talked to Mike Syptak about what we need to do to setup an MOU with FNAL for the wire plane winding. He provided us with an example/template. Daniel will prepare a first draft and circulate for feedback. - We do have a budget estimate for the work from Karen Kephart.
Mechanical Drawings
- Chuck has been working on a modified set of drawings to use narrower copper strips on the cathode plane in a 10-cm strip about the beam hole to dead with the high rates on the central strips. When we were discussing the design, it became clear that we did not have a full understanding of the rate studies that David Lawrence has completed. We also began discussions of several possible designs that would use strips that did not go all the way through the beam line. This needs more discussion. In the mean time, Chuck should hold off on doing any more work until we have clarified things in our planning.
Wire Plane Design
- We need to begin to address the question of what is the optimal wire plane configuration/orientation in a package. Our nominal design is U,V,W,U,V,W (with a 60 deg rotation from wire plane to wire plane). We have begun discussions of a design of the layout U,U',V,V',W,W' where pairs of wire layers have the same orientation but are offset by half a cell spacing. The Us, Vs, and Ws would have nominally a 60 deg rotation between them. We need to perform some Monte Carlo studies to give us an indication of which design is optimal (or more appropriately, what is the optimal design). The goal is to be able to fit tracks locally within each package, which involves being able to resolve the L/R ambiguities in the wire planes via local fits. Daniel will work with Simon on developing a Monte Carlo for these studies. This is probably not something that David can handle in the short term, although Daniel will certainly discuss this with him.
Minutes prepared by Daniel. Send any comments or corrections along.
