1. Start and Vertex Detector W. Boeglin, A.Klein Current Design: 3300 scintillating fibers 1mm diameter 3 double layers (1 axial, 2 stereo) cylindrical geometry spherical cap for forward coverage estimated position resolution < 1mm

2. Photon beam Helical stereo layers Target chamber Scintillator array fast timing using scintillators/PMT (0.3cm thick) fibers for position only First design idea: fiber coverage of cone section virtually impossible increased multiple scattering no position information in conical section Pro: Con:

3. Current design Helical double layers Support structure Target volume Forward detector, with hole for  -beam fibers provide position and timing information forward detector coverage multiple scattering can be minimized forward detector feasible (at least so far)

4. Vertex Detector Dimensions

5. Views of detector I Double layer in front detector Down stream iso view

6. Rate Studies: Geant Simulation (thanks to Richard Jones) Target: 30 cm liquid Hydrogen diameter 3cm target cell : Al 0.5 mm vacuum vessel: Be 0.5 mm Beam: 3  A electron beam coherent bremsstrahlung Detectors: cylindrical detector surrounding the target cylinder flat detector with a 1.4 cm hole for the photon beam

7. hits on fibers e + /e - rate 3.3 MHz forward detector e + /e - rate 18 Mhz p >1 MeV rate per fiber: 10 kHz

8. Forward detector rates p>1 MeV all bin width: 1mm 700kHz 200 kHz

9. cylinder section has no problem with rate per fiber forward section:  large rate  needs large segmentation  possible radiation damage (to be studied)  creates readout problem (signal lines, to be studied) position resolution < 1mm (‘FWHM’,to be studied) timing resolution : extrapolated:  = 0.6ns conceptual design of vertex detector exists using proven technology

10. What do we need and how to we get the answer? What is the function of the START counter ?  Start signal for what : TOF, beam pulse identification ?  Vertex reconstruction: connect tracks to FDC’s, resolution (0.5 mm) ?  Part of the hardware trigger?  Part of the “software” trigger?  All of the above?  Readout/frontend electronics ADC/TDC logic Justification of requirements: physics driven minimal position resolution requirements as a function z and direction minimal timing resolution requirement efficiency, redundancy, uniqueness multiple scattering tolerance alignment requirements, fiber location phase space considerations, shadow regions (minimize impact of shadow regions) kinematic reconstruction, over determination of tracks, redundancy Trigger Integration any signals used in trigger if yes at what level, what information