Hall-B Collimators and Flux

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Hi,

as I mentioned before (and presented on the Coll.Meeting in November - cf. http://www.jlab.org/~fklein/Hall-B%20beam%20characteristics.ppt), the spot size in the alcove is always of the order of >=1.5cm for existing collimators. In order to get a tagging range of ~137-653 MeV (mentioned in Rafael's mail), the electron beam energy has to be ~685MeV (Hall-B tagging range: 21.1% to 95.2% at this E0). Incoherent brems. has a comparatively wide spread at this energy causing a reduction of the photon flux after collimation to only:

  • 6.4% for Eg=145MeV to 7.0% for Eg=650MeV for 2.6mm colli
  • 3.0% for Eg=145MeV to 3.3% for Eg=650MeV for 2.0mm colli
  • 0.8% for Eg=145MeV to 0.9% for Eg=650MeV for 1.0mm colli

The beam sizes after collimation are as follows:

  • colli.(0.0m) PRIMEX (3.6m) CLAS center (15.9m) alcove (33.0m)
  • 1.0mm diam. 0.7mm 3.2mm 6.7mm
  • 2.0mm diam. 1.4mm 6.5mm 13.5mm
  • 2.6mm diam. 2.4mm 9.0mm 18.7mm

If you want a beam spot smaller than 1cm (at decent rates), you might consider to put the BCAL module at the PRIMEX position upstream of CLAS. An object of size 400x12x23cm^3 fits there quite good (even enough space for rotating the module).

Greetings

Franz

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