1. Study of Background Noise in the DVCS Experiment Hall A - JLab Florian ITARD Monastir – December 15th 2008

2. Stakes Background noise Continuous signalInconvenience the measures Darkening Distort the measures New calibration Need to reduce the background noise Simulation

3. GEANT 3 Search of hot points Difficulties to identify clairly the origin of particles Idea: record deposited energy in all the calorimeter, Change geometry of experiment and notice the influence on the measure Target length, angle influence, shapes, tickness and nature of shieldings,… No evident identifications of parameters: « premature » conclusion that the most part of Background noise come directly from the target Difficulties to code, « heavy » commands with many parameters, and then code less and less supported by informatic farms

4. GEANT 4 News bases of research: Priority to the local aspect of background noise Restriction at one unique constraint: reduce the background noise without change the calorimeter position Benefits of GEANT 4: Facility of geometry visualisation Written in a modern language Simplified access to the informations Inconvenient: Still too few informations on commands

5. Experiment geometry

6. Results for the reference geometry Beam In KeV Electromagnetic processes Calorimeter with 2.5 cm aluminium front shielding Normal downstream beam pipe tube First depth Second depth Third depthFourth depth Mean deposited energy in each piece of block by electron sent into the target

7. Pavel Ambrozewicz Study http:/www.jlab.org/~pavel/dvcs/Calorimeter

8. Hadronics Processes Reference With hadronics processes Reference Bloc 8-1 Dose function on deposited energy With hadronics processes KeV Beam First depth

9. 6 degrees cone with insertion at 7 degrees with cutted iron beam side shielding Reference KeV Beam First depth 6 degrees cone

10. Rectangular downtream beam pipe Reference6 degrees cone First depth Rectangular KeV Beam

11. 7 cm tungsten block in front of the first column Reference 7 degrees cone KeV First depth 7 cm tungsten block Beam

12. Tungsten shielding at the intersection between scattering chamber and downstream beam pipe Reference Tungsten block KeV First depth Tungsten shielding Beam

13. Tungsten shielding with a 1 cm Tungsten plate along the downstream beam pipe tube ReferenceTungsten shielding Extended tungsten shielding First depth Beam KeV

14. Extended tungsten shielding and 8 cm polyethylen Front shielding ReferenceExtended tungsten shielding Tungsten and polyethylen First depth Beam KeV

15. Extended tungsten shielding and 35 cm LiH front shielding Reference Tungsten and polyethylen Beam KeV First depth Tungsten and LiH LiH Tungsten

16. Deposited Mean Dose Bloc 8-7 Bloc 8-1 On 100 MeV of deposited energy, 50% come from particles under 24 MeV Reference Bloc 8-13 Energy of Pocatello Beam: 20 MeV

17. Deposited Dose (Gy) into each block for E00-110 and E03-106 integrated luminosity Problem: study of pocatello on curing show that only 7 KGy could reduce the transmission by 20% Idea: compare the anode current simulation to the experiment values 300 000 Gy

18. Integrated Energy during 10 ns function on time (1500 p.e. / 10 ns) * (4.10 4 ) * (1,6.10 -19 C/e) = 960 uA Experiment anode current = 10 uAFactor 100 Approximation GEANT 3: 1000 photon / GeV Tungsten and LiH Block 8-2 Block 8-13 1500 MeV Gain PM: 4.10 4

19. Energy frequency Idea: low energy particles don’t give as much cerenkov photons than high energy particles On 100 particles reaching the block 8-1, 50% are inferior at 4.6 MeV

20. Cerenkov photons: one block study Mean number of cerenkov photons producted in the PbF2 crystal by a 1.280 GeV incident photon = 80 000

21. Cerenkov photons: one block study (suite) 850 photo-electrons by GeV Detected photons Photons reaching air Sum Zoom

22. Low efficiency reasons in the detection of cerenkov photons

23. Low efficiency reasons in the detection of cerenkov photons (suite)

24. Cluster of nine blocks: influence of shieldings Without shielding 2.5 cm aluminium front shielding 30 cm LiH and 3.39 cm polyethylen front shielding Deposited Energy in 9 blocks

25. Cerenkov photons: 208 blocks Calorimeter 0.25 MeV by ARS chanel Reference pulse

26. Cerenkov photons: 208 blocks Calorimeter (suite) 300 channels ~ 200 mV Block 8-2 Block 8-13 1.6 ARS channels / mV 200 mV / 50  4 mA Pre-amplification gain = 8 4 mA / 8 = 500 uA Factor 50

27. Curing? KeV Depth Block 8-1 Reference

28. Conclusion Background noise reduction: factor 8 dans la première colonne factor 3 dans la deuxième colonne reduction of one quarter in the other part of calorimeter Recommended geometry: addition of tungsten block with extended plate change of front shielding by a mixture of LiH and polyethyelen shielding according to the place and respect to the length radiation Avantages of the new geometry: no heavy modifications of the scatering chamber and down beam pipe tube

29. Conclusion (suite) Anode current: factor 50 with the most irradiated block (after reduction of background noise, otherwise factor 150 with the reference geometry) Curing: idiot at 300 000 Gy! law of « all or nothing » at 2000 Gy

30. Hall-A akbar