Last Revision February 04, 2003

Notes from a meeting at SLAC March, 24, 2001

Present: Peter Bosted, Eduardo do Couto e Silva, Hartmut Sadrozinski and Tune Kamae

We discussed  Peter's Presentation at EPAC Nov 8-9,2000.

Peter's calculation include:

• Beam emmittance
• Multiple Scattering
• Mosaic spread ( how well lattices are aligned with respect to the positron beam)

In Peter's presentation there are two effects not yet included in the calculations and could further reduce the flux below 1 GeV: Landau-Pomeranchuk-Migdal (LPM) effect and dielectric suppression. A nice description of suppression effects by Spencer-Klein is found at hep-ph/9802442

For the coherent beam the signal is proportional to 1/k² while the background goes with  1/k, k is the energy of the beam. Trying to use a photon tagger will be a big hit in intensity for Peter is worried about intensity and we do not have to worry (too much) about it since we mostly care about having 1 electron. Need to investigate tagger further and also verify the feasibility of having its information available in the main data stream. In the past test beams we treated as a black box for the analysis.

Low Energies ( ~ 20 MeV maybe with 10-15% dominated by beam divergence)

• Maybe 1 GeV positron beam will suffice (require degaussing magnets and switching quadrupoles to smaller power supplies). The smallest beam achieved at SLAC was 400 MeV. Achieving low energy may not be as hard as controlling beam dispersion.
• At the lowest beam current one may start at 10⁷ to 10⁶ electrons in the LINAC and get down to 10⁴ electrons after hitting the crystal, but the intensity will get even lower since one will have to go through the collimator. 
• Limiting factor may be emmitance of the beam
• To reduce multiple scattering use 80 mm diamond ( thinner is harder)
• Distance is the order of 90 m, so for 2 mm, one gets an angle of the order of mrad.
• Need collimator ~ 2 mm since smaller holes are harder to machine but have been done down to 1 mm (and a couple of sweeping magnets). If beam misses a hole by few mm one still gets a monochromatic beam but loses intensity.
• Need beam hardener (try to minimize its use since it is also a source of background)
• May need a CsI crystal to monitor intensity of the beam (maybe a converter with a spectrometer ?)

High energies ( 10-15 GeV ?)

• 50 GeV may not be an option since it is very hard to get reasonable intensity above 1/2 x beam energy and one would need to activate  SLED (energy doubler)
• The signal to noise ratio is better for lower energies
• Keep energy high as possible away from the desired photon energy
• Rule of thumb is to stay at 10% of end point energy. So for 20 GeV electrons one would get 2 GeV photons.
• Remember power bill goes with E²

^{Questions: (Al Odian & Gary Godfrey) - How do we monitor the spatial stability of the beam at 1 e/pulse , since this will affect the position of the monochromatic peak and we will not know which energy we are probing ?}

Peter agreed to investigate the possibility of running the following photon energies for GLAST

50 MeV, 100 MeV, 1 GeV, 10 GeV, 50 GeV (or as high as possible)