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

2. 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. Four GPDs accessible in DVCS at large Q 2 : and which are functions of and. 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. Effective way of studying GPDs is through Deep Virtual Compton Scattering (DVCS): longitudinal momentum transfer Important as it features in Ji’s Sum Rule, which relates E and H to the total angular momentum carried by each quark. t (Q 2 ) e L*L* x+ξ x-ξ H, H, E, E (x,ξ,t) ~ ~  p p’ e’ ~

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

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

10. Current test set-up Scintillator BC408 (70 cm long) coupled to two R2083 PMTs by means of 150 cm long light guides, wrapping in Al foil, semicircular light guide at the “U turn”

11. 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! PMT-N PMT-D NEW

12. Simulations

13. Simulations PMT-N PMT-D 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 threshold 8 ns Contamination from mis-reconstructed events, after all time and energy cuts have been applied, is 1 - 3 %.

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

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

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

17. θ Resolution θ-resolution in the range 2° – 3.5° P n = 0.4 GeV/c 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 In the initial design, CND overlapped with CTOF! Solution: modify the magnet, extend CTOF. Solenoid CTOF CND

21. Solenoid modifications – to be confirmed!

22. 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 (1/24 th 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