Meeting 21 February 2020

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  • Update from CPS beamline meeting
  • Update from KLong and GlueX collaboration meetings


  • Remind Radcon about sending numbers
  • CPS radiation simulations: additional tests to finalize CPS optimized geometry (see emails)
  • Hall D - prepare a sketch model of magnet
  • TCS: for Q2=5 GeV2 want reliable extraction of CFF and show that one can have a good measurement in Hall A/C



  • discussion of a number of different simulations carried out to optimize cost, material, dimensions - done in two ways: 1) geometry, material, layering, and 2) pure material
  • Comparing all W powder vs all Pb shows W much more effective to reduce prompt radiation. Conclusion: need W in the central area.

  • Replacing tungsten by lead in region upstream of z=0 makes the prompt radiation worse laterally outside the CPS, maybe a factor of ~3 - If one goes to z = 10, 20, 30, or 40 one sees slowly more prompt radiation “leaking out” and activation levels in the forward hemisphere growing. Conclusion: z = 0 is good compromise.
  • Adding a Pb layer on the outside helps to further reduce prompt radiation and greatly reduces the activation levels. Conclusion: always add a 5 cm layer of lead outside the poly. Having a lead "frame" outside (2.5 cm top, 5cm bottom) helps to reduce radiation levels even further
  • The W "football" has much less prompt radiation laterally out than the Russian doll (10, 40). This implies we likely need W laterally out to the poly layer at least in the region for z > 0. The activation levels for the W football are much higher than for the Russian doll (10,40). That reinforces we need some Pb layer (the 5 cm Pb skin).
  • The Russian doll (10,20) version allows more of the source radiation that we try to capture with W to "leak out". One can see that from elevated prompt radiation levels between z = 50 and 200 laterally out. The (10,40) seems safer, so extending the W up to 40 cm beyond the magnet may suffice, up to z ~ 70.


  • Meeting with Hall C hall leader, scientists, engineers and J. Benesch from CASA
  • No major problems foreseen from accelerator side
  • moving polarized target upstream is ok since none of the standard spectrometers is used (using BigBite for larger acceptance for WACS)
  • no vertical chicane needed
  • Discussions about development of the CPS engineering model including support structures and installation needs
  • estimated timeline: ~3 years. First step will be an improved conceptual design to see how/where things fit. The next phase is a more detailed design.
  • main challenge is fit with below-beamline level infrastructure
  • also discussed was assembly and details of installation, e.g. heavy items
  • Action item: send exact angles and geometry of detectors to engineering team (like the one for NPS)


  • Inquire about availability of ~200 tons of lead from SLAC
  • Inquire from NZ company if possible to assemble the entire magnet there including the Japanese radiation hard coil


  • Working on needed numbers, e.g. raster, for making initial design and simulations for Hall D CPS adaption
  • might need no secondary raster - estimate of beam size is >1mm
  • Action item: sketch a model for the magnet - initial model from CPS collaboration meeting is 4x longer
  • could use this model to check radiation levels if power two time larger
  • Note that 4x longer magnet does not mean that the magnet will be 4x heavier as the length goes as the square root and also the field will be smaller. Also note that the weight should be specified in terms of weight/unit area


  • Presentation of unpolarized TCS and discussion
  • an experiment would use the NPS, but not use CPS as one can get more photons with just high current on a 10% Cu radiator, as the unpolarized WACS experiment
  • if at some higher scale Q^2, it is really required to have a higher-precision data set to ensure a reliable extraction of the CFF and clearly learn about universality of twist-2 CFFs from TCS as compared to DVCS, maybe that is a good case
  • also see this publication: arXiv:1912.09853
  • need to show that there is a convincing statistically significant need-case for the reliable extraction (likely needs very good statistics in the various angular distributions)
  • Next step: show if a reliable extraction of the leading-twist CFF would be possible with measurable TCS data in the Hall C setup. So what can realistically be obtained in a possible Hall C experiment, and what would that mean for

a reliable CFF extraction (and, is it really better than SoLID).

NEXT MEETING: 27 March at 11AM (ET)