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.

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
 - 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


 - 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.