1. Constraints on Photon Beam Line
1. Energy resolution of photons on target: _E < 0.5%
2. Photon flux capability up to 10⁸ /s within the interval 75-95% E₀.
3. Linear polarization as large as possible, requires coherent bremsstrahlung from a crystal radiator and collimation of photon beam to 100µr or better.
4. Long collimation space between radiator and experimental target to allow angle-displacement dispersion to develop: 60-100m from radiator to defining collimator.
5. Virtual focus of electron beam at the entrance to defining collimator, r.m.s. of virtual beam spot 100µm.
6. Fine steering of electron beam just before bremsstrahlung target, stabilized by feedback from sensors built into the collimator, to an accuracy of 2µr.
7. Beam spot size at radiator as large as possible to extend crystal lifetime, alternative is to scan the crystal across the beam spot during running.
8. Sufficient beam current, dump capability to give necessary flux with collimation, 10µA is desirable.
9. Tagger magnet capable of bending 50% E₀ electrons into instrumented focal plane and bend beam part way into beam dump.
10. Special focal plane segment with high segmentation over a small bite for use with collimated beam.
11. Tagger magnet designed to be usable at 24GeV, in case of future upgrade.

2. Design Choices
1. Beam dumped in horizontal plane.
2. Beam transport, shielding, dump designed for a maximum of 10µA at 24GeV.
3. Two extra dipoles after tagger magnet bend the beam to 15^o into the beam dump.
4. Spearate buildings for tagger, experimental hall.
5. Facility housed in two buildings (tagger + experimental hall) above ground, connected by concrete tunnel into which one could mount a beam pipe or helium bag to transport the photon beam, signal cables, etc., dry but not air-tight.
6. Last 20' of tunnel before experimental hall is a rectangular cavity with concrete walls thick enough to shield the experiment from collimation debris. This is space for the collimation and sweeping magnet systems
7. Tagging counters can be small (1" high) with sigle phototubes on each counter. Instrumenting the full tagger focal plane 50-95% E₀ with E=0.5% counters = 90 channels. This segmentation is sufficient to handle the rates up to 10⁸/s within 25% of E₀ when running without collimation.
8. Focal plane must be reconfigurable to a more compact configuration for running with collimation and coherent bremsstrahlung..
9. Tagger magnet runs initially at 1T field so that at 24GeV it can still bend the beam into the same path and tag the same bite running at 2T.

Drawings and Specifications

• view of tagger magnet from above (PS)
• view of beam dump from above (PS)
• draft tagger magnet specification (PS)

Notes on Coherent Bremsstrahlung

Richard Jones

References

``Inclusive cross sections for pion and proton production by photons using collimated coherent bremsstrahlung'' by W. Kaune, G. Miller, W. Oliver, R.W. Williams, and K.K. Young (University of Washington) Phys. Rev. D11 no.3 pp.478-494,1975

``Coherent bremsstrahlung in crystals as a tool for producing high energy photon beams to be used in photoproducion experiments at CERN SPS'' by H. Bilokon, G. Bologna, F. Celani, B. D'Ettorre Piazzoli, R. Falcioni, G. Mannocchi, and P. Picchi Nucl. Instr. Meth. 204 pp.299-310,1983