BLTWG Meeting 03/12/2009

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  • Time: 9:30 EST
  • Place: EVO and ESNET (with telephone bridge)
  • Connecting: instructions are here
  • Present: Richard J., Alex S., Fernando B., Jim S., Eugene C., Tim W.


  1. Update from UConn - Richard
  2. Tagger dipole field uniformity specification - Jim
  3. Feedback on updated interface drawings - Tim
  4. Manpower for the spring 2009 CHESS run - Richard
  5. Plans for beam tests - Richard
  6. Talk at Diamond 2009 - Yang

Notes by R. Jones

Update from UConn

  • The type III diamond monocrystal on loan from BNL has arrived at UConn. Richard is discussing how best to mount it with experimental colleagues in the UConn department who have experience in mounting samples.
  • All electronic components for the prototype digital readout board for the tagger microscope SiPM electronics have finally arrived, sufficient to completely populate three boards. The prototype boards and components have been shipped to the firm Screaming Circuits for fabrication. We have purchased the Xilinx programmer for programming to the main FPGA on the prototype board. Students will begin bench tests of the prototypes as soon as they are received back from the vendor, near the end of March.
  • Work progresses on the prototype fiber array. Plans are to have the device ready for a beam test by the end of 2009. Schedule changes in Hall B mean that we may want to speed up that schedule.

Tagger dipole field uniformity specification

The tagger contract is now being formulated under two alternate schemes, to be chosen by the vendor.
  1. The vendor accepts the design provided by Jefferson Lab and builds the magnet to the specs that we provide, down to the grade of steel and tolerances of every part.
  2. The vendor wants to make modifications to accommodate their own equipment or reduce their cost of construction. They take responsibility for redoing the design and guarantee that our mechanical and field uniformity specs can be met.
The second option is the "design and build" contract that we have discussed before. Whether to build or design-and-build will be up to the contractor. We discussed the way in which the field uniformity spec will be expressed for the design-and-build option, and agreed with Jim's current plan. It was pointed out that if we award a design-and-build contract then we need to be prepared to be able to promptly turn around and do magnetic field studies to evaluate whatever changes the vendor decides to propose.
We reminded ourselves that we want to get together again with the accelerator group and discuss a plan for integrating the active collimator into the electron beam control system in the tagger hall. Jim will request a follow-up meeting with members of accelerator and CASA to address this.
There was discussion of the energy calibration scheme for the tagger. Richard reiterated the scheme that was presented at the tagger review, to measure the photon beam end-point in the pair spectrometer and use the accurate knowledge of field shape in the pair spectrometer and the relative positions of the pair spectrometer counters to transfer this calibration to all of the 144 energy channels in the PS. Then through coincidence with the tagger, this calibration can be transferred to the tagger. Eugene suggested a second method using symmetric Moller pairs. He suggested that if we can identify symmetric Moller pairs in the focal plane then the absolute energy scale of the tagger can be fixed by constraining their energy sum to the electron beam energy. A quick estimate showed that the polar angle for these pairs is quite large, of order 1/2 degree, but pairs close enough to the horizontal plane might make it into the tagger vacuum box and get tagged. Correction of their trajectories for the scattering angle at the vertex would yield two solutions for each event, of which one of them would be correct and lead to the correct sum. It was suggested that the acceptance for such pairs might be enhanced by running with a special field in the tagger quadrupole. Eugene will follow up with a rate estimate.

Feedback on updated interface drawings

Meetings between the Hall D and accelerator groups at Jefferson Lab are taking place, with the aim of closing the loop on the electron beam design in the tagger hall and adopting a common picture. Jim referred to this common design as "the song sheet". Let's hope that some of us can sing, or the song sheet might not help very much!

Manpower for the spring 2009 CHESS run

It was discussed whether any of the new collaborating institutions making use of photons in Hall D might want to get involved in the test run at CHESS. It was not deemed likely because GlueX is the only experiment right now with an interest in polarized photons. Between Richard and Franz, it is likely that they can find enough manpower to complete the May run at CHESS. There was discussion of the diamond radiator procurement pipeline. All of the stages of this pipeline are uncertain right now, but if the CVD diamond on loan from BNL proves to have sufficiently low mosaic spread to meet our requirements, it appears that the raw material supplier problem is pretty much solved. Thinning and quality assurance procedures are still very much under development.

Plans for beam tests

Based on earlier versions of the 2009-2010 run plan in Hall B, we had thought that the next time photon beams will be available at Jefferson Lab will be toward the beginning of 2009. Now with the latest "firm" schedule, it looks like the switchover to photons took place last month and photons will continue in Hall B until the end of 2009. This has not been confirmed, see correction below from Franz Klein. It is important because all of the beam tests that we want to do can be done parasitically but require prior planning and consultation with the CLAS collaboration and the agreement of the Hall B leadership.

Talk at Diamond 2009

Yang has indicated his plan to attend the Diamond 2009 conference that takes place 6-10 September 2009 in Athens, Greece. He also indicated that he submitted an abstract for a talk. The abstract deadline was March 6. Richard said that he would like to attend this conference as well, if at all possible. There was broad agreement that this would be a good idea.

Responses from F. Klein

  1. Hall-B experiment schedule: there is NO photon beam this years (despite the notation 'eg1-dvcs,eg6' are not photon runs but polarized target runs with NH3 target). We will have photon runs from March 2010 to April 2011 in Hall B (FROST-transversely polarized; HDice (or FROST with LiD if HDice has problems); PrimEx). It would be best if we schedule test runs during 2010/11 (if we want an earlier time slot, we cannot do anything parasitically. Besides, I'll be (co-)responsible for FROST and HDice runs.
  2. I heard about the Photonis decision from John Domingo but I'd like to switch to Hamamatsu anyway: Photonis tubes for high rates and reasonable gains are >1" (rather large for typical scintillator sizes of ~5-20mm). I'd like to test some Hamamatsu tubes with fast pulsers and in the Hall-B bremsstrahlung beam (but I still wait for my ordered tubes to arrive).
  3. Beam energy calibration using Moellers: that was a great success at Mainz but for our case I hardly can imagine how to get Moeller pairs into the hodoscope.