1. Status update on the Central Neutron Detector J. Bettane, J.L. Cercus, B. Garillon, T. Nguen Trung, S. Niccolai, D. Sokhan Detector goals and design Tests on three-layer prototype New PMTs Magnetic field tests News on mechanics Ongoing work and plans CLAS12 Workshop, 6/13/2012

2. Requirements and chosen design for the CND The light comes out only from one side of the CND, goes through bent light guides (1.5m) arriving to PMTs, placed in the low- field region CND design: scintillator barrel 3 radial layers, 48 bars per layer coupled two-by-two by “u-turn” lightguide Completed tests to choose: shape of light guide (semicircular) wrapping (Al foil) scintillator (ELJEN) PMT (R9779 or R10533)? magnetic shielding for the PMTs? Main physics goal: detect the recoling neutron in nDVCS → The CND must ensure: good neutron/photon separation for 0.2<p n <1 GeV/c → ~150 ps time resolution momentum resolution below 10% no stringent requirements for angular resolutions

3. Measuring time resolution with cosmic rays « Top-Bottom » method: Using two small scintillators (with 2 PMTs) for trigger and to fix hit position Pro: x « known » → t 1 and t 2 independently Con: small trigger scintillator size needed → low count rate Triple coincidence method: Pro: high count rate Con: only average resolution of the six PMTs is accessible

4. Three-layers prototype Thanks to J. Bettane & the technicians of IPN Completed at the end of December 2011 « Top-bottom » tests done before the February review reproduced performance of one-layer prototype  t ~140ps for « direct » PMT too low count rate to perform many tests → triple coincidence measurements

5. Three-layers prototype (with R9779)

6. Three-layers coincidence measurement to study dependence of time resolution on: - type of CFD - TDC slope Position coincidence cut of 3cm to select perpendicular hits only and compare with top-bottom tests. No major variations changing type of CFD or TDC slope Thanks to J.L. Cercus, B. Garillon, Daria Sokhan

7. Ongoing tests on new PMTs The tests with the three layer prototype showed the necessity to add an amplifier (x5) after the PMT, as the gain of R9779 is low (~5∙10 5 ) At the FebruaryJLab review, it was suggested to try a PMT with higher gain to avoid the need for amplifiers Discussion with the Hamamatsu headquarters in Orsay: a new PMT (R10533) is on the market, with same characteristics of R9779 (also price-wise) but with two extra stages (gain ~5 ∙10 6 ) Ordered 8 R10533’s to replace 8 R9779’s we had recently purchased – received a few days ago and being calibrated Ongoing: comparative tests R9779/R10533 of timing performances with single-bar « Top-bottom » setup – results coming soon Order to purchase all PMTs must be submitted by the end of June for IN2P3 budget rules. 70kE official offer by Hamamatsu Thanks to Thi Nguen Trung

8. Position of the PMTs of CND ~215 G B    ~ -71° Angle B/PMT ~ 110° ~ 215 G Magnetic field tests Solenoid field map (L. Quettier, spring 2011) Question: any foreseen changes on field map? Tests done in spring-summer 2011: µ-metal+5mm mild steel shielding JLab review (2/2012): more accurate field mapping needed

9. Magnetic field tests – May 2012 Same magnet as 2011 – solenoid at LAL Orsay Stray field used, as bore is too narrow (24 cm) to allow to vary the angle of the PMT Volume swept with 3D probe Thanks to J. Bettane, B. Garillon D. Sokhan

10. Magnetic field tests – May 2012 Field map: stray field is mostly parallel to the axis of the solenoid Magnitude dies fast moving away from magnet and from the bore’s center Tests had to be done in the center and near the « exit », varying PMT angle y x Thanks to J. Bettane, B. Garillon D. Sokhan Projection on the x-y plane

11. Magnetic field tests – May 2012 Dimensions (mm) of PMT and shielding µ metal mild steel (2.5 or 5mm) Optical fiber – connected to laser source Looked for variations of signal amplitude as a function of the angle of the PMT axis with respect to the axis of the magnet. From 90° to 45°, no variation PMT: R10533 – HV: 1900 V 5mm shielding Thanks to J. Bettane, B. Garillon D. Sokhan

12. Magnetic field tests – May 2012 Comparative tests between 2.5 and 5mm shielding done without PMT, directly measuring field with and without shielding Confirmed original choice of 5mm mild steel shielding Magnetic field values without shielding Thanks to J. Bettane, B. Garillon D. Sokhan Magnetic probe Position of photocathode Mild steel µ-metal Probe Shielding French/English legenda: Acier doux = mild steel Sonde = probe 45° 60°

13. Clearance requirements fullfilled Mechanical prototype built Installation plan ready Glue tests for strength done News on mechanics February review: pointed out that the azimuthal clearance between « S-blocks » (4mm) is too much. This was due to the size of scintillators already purchased (1/4 of the total) before new clearance requirements were imposed New « hybrid » design of detector, with 6 « S-blocks » with smaller bars, 18 with thicker ones (clearance now 2mm) Thanks to J. Bettane Julien’s new idea to improve magnetic shielding Steel plate µ-metal and mild steel shielding Recovered (from LAC) HV power supplies and HV cables provided by INFN Thanks to Miktat Imre and the INFN technicians!

14. What are the dimension of the support plate to be fixed on the solenoid? We need to know it to design and prototyping our support structure What are the dimensions of the solenoid and tolerances? We need to know it to dimenions our plastic shims and perform assembly tests What is the temperature of the flexible heaters? We need to know it to evaluate its impact on plastic shims and scintillators Questions (from Julien)

15. Plan for 2012 CHOICE PMTs - ORDER BY END OF JUNE (70 k€) FINAL DESIGN SCINTILLATORS ORDER BY SUMMER (30 k€) TESTS ON ELECTRONICS: CFD, FLASH-ADC, SPLITTERS, (AMPLIFIERS?) TESTS WITH MECH. PROTOTYPE (PROTO 144), SHIMS RADIATION TESTS MECHANICAL TESTS ON GLUE B-FIELD TESTS ON PMTs STUDIES ON SUPPORT STRUCTURE/PROTOTYPE/TOOLS – 10K€ TEST SUPPORT STRUCTURE PLANNING OF MACHINING OF LIGHT GUIDES TO BE DONE IN 2012 ONGOING DONE TO BE DONE Budget for 2012: 120 k€ 60% to spend by end of June Rest by end of December