1. The Gas Čerenkov Detector for the Crystal- Barrel Experiment at ELSA D. Kaiser Hemholtz-Institut für Strahlen- und Kernphysik der Uni Bonn supported by SFB/TR 16

2. Bosen 2007D. Kaiser 2 Nucleon Resonances photoproduction reactions with nucleon resonances at the CB experiment:  N  N* ... problem: overlapping resonances  partial wave analysis  polarisation observables Breit-Wigner parametrisation

3. Bosen 2007D. Kaiser 3 Double Polarisation polarised target polarised photonbeam - linear polarised photon beam by use of unpolarised electron beam and diamond radiator  50% @ 3.2 GeV - circular polarised photon beam by use of longitudinally polarised electron beam and amorphous radiator  55% @ 2.4 GeV Bonn Frozen Spin Target - longitudinally polarised protons in a butanol target @ 30-50 mK in frozen-spin mode - polarisation 70 – 90% - repolarisation every ~ 60 h - total relaxation time ~ 360 h time [hours] polarisation

4. Bosen 2007D. Kaiser 4 Electron Accelerator ELSA

5. Bosen 2007D. Kaiser 5 Crystal-Barrel Experiment Crystal-Barrel detector tagger MiniTAPS polarised target GIM  camera e - beam dump goniometer Gas Čerenkov detector e - beam

6. Bosen 2007D. Kaiser 6 Electromagnetic Background for butanol target under forward angles: ~100.000 e ±  1 hadr. due to: pair production Compton scattering for hydrogen target in full angular acceptance: ~1.000 e ±  1 hadr.  need for supression already on trigger level

7. Bosen 2007D. Kaiser 7 Čerenkov Effect discovered 1934 by P.A. Čerenkov (and S.I. Vavilov) Nobelprice 1958, together with I.E. Tamm and I.M. Frank charged particles with high velocity generate electromagnetic radiation in dielectric media

8. Bosen 2007D. Kaiser 8 Čerenkov Effect observable radiation for a particular angle  c : threshold for Čerenkov radiation: with

9. Bosen 2007D. Kaiser 9 Čerenkov Effect threshold energy “adjustable“ by use of different Č media useful for particle discrimination and determination simple set-up with Č medium and photon detector  Čerenkov threshold detector other set-ups possible (e.g. RICH)

10. Bosen 2007D. Kaiser 10 Čerenkov Effect  approx.1700 photons/cm in water approx. 2 photons/cm in CO 2 range of normal PM Used photomultiplier: Hamamatsu R1584-03SEL

11. Bosen 2007D. Kaiser 11 Gas Čerenkov Detector - drawing and picture

12. Bosen 2007D. Kaiser 12 Gas Čerenkov Detector forward crystals Gas Čerenkov detector MiniTAPS - positioning and angular space coverage

13. Bosen 2007D. Kaiser 13 Gas Čerenkov Detector periodic repolarisation of the target  easy in- and output of the detector is required

14. Bosen 2007D. Kaiser 14 Modifications gas system partial pressure sensor scale

15. Bosen 2007D. Kaiser 15 Simulation of the Efficiency verification of the efficiency of the detector  maximum efficiency of 99.97 %

16. Bosen 2007D. Kaiser 16 Determination of the Efficiency done at the old CB area e ± from pair production / Compton scattering on a carbon target  efficiency of 99.72 % ± 0.45 %

17. Bosen 2007D. Kaiser 17 Installation in the CB Set-up more adaptions needed: electronic trigger logic DAQ analysis software ExPlORA

18. Bosen 2007D. Kaiser 18 Use as Veto Detector Start of readout if two tagged hits in the Crystal-Barrel-, Forward- or MiniTAPS detector were found Start of readout if two tagged hits in the Crystal-Barrel-, Forward- or MiniTAPS detector were found, with applied Čerenkov veto

19. Bosen 2007D. Kaiser 19 Use as Veto Detector  number of suppressed events in the subdetectors cut on events correlated in time

20. Bosen 2007D. Kaiser 20 Čerenkov Detector as Time Reference

21. Bosen 2007D. Kaiser 21 Čerenkov Detector as Time Reference

22. Bosen 2007D. Kaiser 22 Summary Gas Čerenkov detector modified for use in the Crystal-Barrel experiment tests and determination of the efficiency detector integrated and in use –as veto detector –as time reference data taking has already started