Central Neutron Detector March ’11 Update

1. Central Neutron Detector March ’11 Update Daria Sokhan IPN Orsay CLAS 12 GeV Workshop Paris, France – 9th March 2011

2. e’ Four GPDs accessible in DVCS at large Q2: t  e L* and (Q2) x+ξ which are functions of and . x-ξ p p’ ~ ~ H, H, E, E (x,ξ,t) ~ Neutron DVCS (see Silvia Niccolai’s talk) Generalised Parton Distributions (GPDs) provide correlation between longitudinal momentum and transverse position of partons inside a nucleon and can give access to the contribution of orbital momentum of quarks to nucleon spin. Effective way of studying GPDs is through Deep Virtual Compton Scattering (DVCS): • Neutron DVCS: • needed for flavour separation • gives access to GPD E through the amplitude of the beam-spin asymmetry. GPD E is currently least well known. Important as it features in Ji’s Sum Rule, which relates E and H to the total angular momentum carried by each quark. longitudinal momentum transfer

3. The recoil neutrons • Proposed experimental programme on neutron DVCS is complementary to proton-target experiments at JLab aimed at accessing • For exclusive reconstruction of the DVCS process • require detection and measurement of all three final state particles. • Scattered electron and photon are typically produced at low forward angles (into forward detector of CLAS 12). ← Simulation at Ee= 11 GeV (Baptiste Guegan, Orsay). Over 80% of neutrons recoil at θlab > 40° with peak momentum at ~ 0.4 GeV/c. Requires central neutron detector sensitive to 0.2 < pn < 1.2 GeV/c.

4. Neutron Detector in CLAS 12 • Available: • 10 cm of radial space • in a high magnetic field (~ 5 T) • between the CTOF and the solenoid magnet. • Detector proposal approved @ PAC 37: • Barrel geometry • Plastic scintillator bars • Trapezoid cross-section • Long light-guides • PMT read-out upstream

5. y x Central Tracker CTOF z CND The Central Neutron Detector • 3 layers • 48 paddles (azimuthal segments) • Inner radius 28.5 cm, outer 38.1 cm • Length ~ 70 cm

6. The U-Turn Geometry Each couple of scintillator paddles connected at the downstream end with a semi-circular light-guide At the upstream end, curved light-guides take the signal from each paddle to PMTs out of the magnetic field Three-layer assembly, all PMTs on upstream end

7. Tests in the Lab

8. Tests @ Orsay (Giulia Hull) Measurements with cosmic rays, two short scintillator segments, above and below the test paddle, used for the trigger.

9. Reminder… Last year: Different “U-turn” light-guide geometries: Semicircular provides ~ 10% better time resolution! • Triangular • Semicircular Different wrapping materials: • Mylar • Aluminium foil • VM 2000 Al foil chosen based on charge-collection and timing tests, cost and ease of mechanical wrapping.

10. Current test set-up Scintillator BC408 (70 cm long) coupled to two R2083PMTs by means of 150 cm long light guides, wrapping in Al foil, semicircular light guide at the “U turn”

11. PMT-N PMT-D NEW Which PMTs? Previous tests with PMT R2083. A new PMT made by Hamamatsu, R9779, has recently become available at ~ 1/3 of the cost of R2083. Timing resolution of new R9779 ~ 10% worse than old R2083. Acceptable!

12. Simulations

13. PMT-N PMT-D Simulations Performance of CND simulated using GEMC. • Energy deposited by the neutron at each step propagated to the two PMTs, smeared, integrated and converted into ADC and TDC channels. • Thresholds applied to mimic ADC / TDC response. • Reconstruct hits using TDC signals from coupled pairs of scintillator paddles. • Require: • Reconstructed position within paddle length • Minimum reconstructed energy of hit 2 MeV • Maximum time threshold8 ns Contamination from mis-reconstructed events, after all time and energy cuts have been applied, is 1 - 3 %.

14. β β β Neutron / Photon Separation Error bars on the β - axis represent 3 σ θ = 60° in all plots Neutrons up to ~ 0.9 GeV/c can be well separated from photons on the basis of the measured β

15. Efficiency Photon efficiency is around 9 – 12% photons, θ = 60° neutrons Neutron efficiency is mostly in the range 8 – 9.5 %, depending on momentum

16. Momentum Resolution θ = 60° Momentum resolution in the range 4 – 10 %

17. θ Resolution Pn = 0.4 GeV/c θ-resolution in the range 2° – 3.5° φ-resolution determined by the azimuthal segmentation into 48 paddles, so 3.75°

18. Mechanics

19. CND – position within solenoid Julien Bettane, Orsay

20. Problem of Space Solenoid CND CTOF In the initial design, CND overlapped with CTOF! Solution: modify the magnet, extend CTOF.

21. Solenoid modifications – to be confirmed!

22. Solenoid modifications – to be confirmed! • Modifications: • Upstream magnet opening angle from 30° to 41°. • Reduce length of straight section (and therefore CND paddles) by a few cm. • Move cryogenic supply pipe to the top of the magnet. Pending agreement from magnet construction team…

23. Back-up plan If the magnet cryogenic pipe cannot be moved, one section of the CND can be removed (1/24th of the total)

24. CND and CTOF CTOF in blue CND in grey CTOF paddles need to be longer to accommodate CND light-guides. Pending agreement…

25. Construction Plan • Imminent: decision of the magnet construction team – within a number of weeks. By the Summer: • Design of the mechanical support structure • Study of the magnetic field shielding around the PMTs • Development of bases with amplifiers for the PMTs By early Autumn: • Completed detector segment of 3 layers of a coupled pair of bars each, with mechanical support, light-guides, PMTs and electronics – to be used in cosmic ray measurements: • Define electronic configuration for real experimental conditions • Compare with cosmic ray tests made with CTOF prototype

26. Thank you!