1. The BoNuS Detector: A Radial Time Projection Chamber for tracking Spectator Protons Howard Fenker, Jefferson Lab This work was partially supported by DOE Contract No. DE-AC05-84ER40150 under which the Southeastern Universities Research Association (SURA) operates the Thomas Jefferson National Accelerator Facility (Jefferson Lab).

2. Motivation Purpose –Provide almost-free neutron target to improve our understanding of neutron structure.

3. Method Measure slow protons –Identify spectator protons to tag e - d events in which the neutron was struck. n

4. Spectator Proton Characteristics Angular distribution is isotropic. Backwards proton almost certain to be a spectator. Momentum distribution favors low values. Tracks are 20x - 50x minimum ionizing.

5. Big Picture Track secondary e - in CLAS. Locate e - interaction point in target. Link p spectator with electron vertex (need  z ~8mm).

6. RTPC Concept 20 cm He Deuterium Gas Target (7 atm.) Thin (50 µm) Kapton Wall

7. RTPC Schematic View along beam line. Perspective view of 1/2 detector.

8. RTPC Schematic Acceptance

9. RTPC Schematic On-board electronics/cables

10. Curved GEM 60mm radius Negligible change in E-field Curving the GEMs should not be an issue. LOCAL FIELD DEFORMATION NEAR CURVED GEM IS NEGLIGIBLE

11. Radial Drift- Electric Field and Potential –Noticeable variation in E and therefore v drift –Lorentz angle spreads charge even for radial track. –These issues can be dealt with in analysis. target He Gas Drift Cathode GEMs Pad Electrodes

12. Simulation Results Momentum range Particle ID Tracks geometry Electron Drift - Lorentz angle effects

13. SIMULATION: RTPC Penetration 60 MeV/c 70 MeV/c 80 MeV/c 90 MeV/c 100 MeV/c 110 MeV/c 120 MeV/c 5 mm D 2 gas 50 mm Kapton He... Field Plane RTPC Gas GEMs, readout, etc. Energy Deposited in each layer of material as initial spectator momentum is varied. D k He Cu k Cu

14. SIMULATION: dE/dx and  Bdl => Proton ID 100 Mev/c Pion100 Mev/c Proton

15. SIMULATION: Tracks in RTPC Readout pad plane is shown as if unrolled.

16. SIMULATION: Lorentz Angle / Track Curvature

17. Prototype Construction Curved Prototype Designed to use existing 10x10cm 2 GEMs with same curvature needed in final detector.

18. Prototype Construction Curved Prototype Test Fit Drift Region Cathode Field Cage Electrodes GEM HV Connections (GEMs and Readout Board are not shown) ULTEM® Frame Parts

19. Readout Electronics Needs –Signals are typical of any TPC. –Channel Count: ~4000 4 mm x 5 mm pads. –Space only for simple preamp on-board, to drive ~3m cable to crate. –Drift time ~2 microseconds (2cm). –100ns time bins gives 20 samples/track.

20. Studies w/flat prototype Uses standard 10cm x 10cm GEMs. Drift region similar to planned final detector. Uses 3x 3M GEMs to allow tracking cosmics (min-I). At present, tests are performed using 80/20 Ar/CO 2.

21. Cosmic tracks easily recognized. Position resolution would be better with charge sharing over ≥3 pads. Gain ~ 30 3 Studies w/flat prototype

22. Future Better readout electronics for tests. –At present only 8 channels Low-Energy proton beam test. Curved prototype construction/test –Materials selection –Assembly methods –Performance Final design, readout selection/procurement, etc.

23. Experience w/3M GEMs Purchased 25 GEMs: 10x10 cm 2 Prototype uses first three - randomly chosen. –Stable gain up to ~30x each layer Inspection of remaining GEMs: –All show varying surface blemishes (next slide) –Of 24, based on applying voltage in air: 15 are probably good 6 might be good 3 are definitely bad

24. Experience w/3M GEMs Installed in Flat Prototype