1. BoNuS: Radial-Drift TPC using Curved GEMs
A Review
For the DarkLight Collaboration Meeting
17 March, 2011
A Time Projection Chamber having Radial Drift Direction, based on GEMs which have been Curved to form cylinders.
Howard Fenker, 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. Tracking a low energy, heavily-ionizing particle requires a low-mass detector
Time Projection Chamber (TPC)
Just a box of gas
Readout elements only on the surfaces.
Windows can be made thin.
Cathode
Anode
Readout
Information density is high, but each channel of readout may need to record an entire waveform for several s -- like having an oscilloscope per channel!

5. Radial TPC (RTPC) Alternative
Typical TPC
Radial TPC (RTPC) Alternative
TPCs are typically cylindrical - to fit around a beamline or interaction point. Ionization trails drift to one end where readout is located.
Drift distance is shorter if R/L is small.
Reduces material in forward (electron) direction.
Allows more readout channels -- less confusion.
But, drift path is curved if ExB ≠0.

6. BoNuS RTPC: added features
~100µm
To make mechanical structure even lighter, use no wires --> no wire tension to support.
Eliminating wires also eliminates local artifacts in the data.
Use GEMs instead of wires:
Gas Electron Multiplier
A sheet of material providing gas gain.
Sauli, ~1998
50µm
70µm

7. GEM Readout

8. Curved GEM for Prototype
Proton Track in Curved Prototype

9. BoNuS is just a curled-up TPC.
For convenience, the gas-gain elements are GEMs.

10. BonuS RTPC Schematic Beam’s view of target
And space within DVCS solenoid (~220mm diameter).
It would be nice to fill the empty space with detector…

11. BonuS RTPC Schematic
… but we will need some space for electronics and supports…
So place the drift anode at the largest radius we can

12. BonuS RTPC Schematic
Very near the beamline, the Moller and background rates will be too high.
Leave a dead zone surrounding the target. Make its mass small – fill with helium.

13. BonuS RTPC Schematic The cathode must be at several kV.
Protect the H2 target from HV by inserting a ground window.
Make BOTH of them as thin as possible!
( in.  6 m)

14. BonuS RTPC Schematic
To ensure that RTPC can be used for Minimum-Ionizing particles if desired, put in 3 stages of gain.
Readout board (pad electrodes).

15. BonuS RTPC Schematic
Add the electronics on the outside.

16. BonuS RTPC Schematic Finally, put in realistic framework and supports.
Perspective view of 1/2 detector frames.

17. BoNuS RTPC: Exploded View

18. Assembly

19. Curved GEM for BoNuS RTPC
GEMs mf’d by: Tech-Etch, Inc. Plymouth, MA
Duplication = sincere flattery

20. Readout Electrodes (Pads)

21. Readout Electronics Needs Signals are typical of any TPC.
Channel Count: mm x 5mm pads.
Space only for simple preamp on-board, to drive ~6m cable to crate.
Drift time ~5 microseconds (3cm).
100ns time bins gives 50 samples/track.

22. ALTRO Chip
Luciano Musa

23. BoNuS Readout Crate

24. A Way to Mount Everything

25. SIMULATION: Lorentz Angle / Track Curvature

26. Time-of-arrival information tells us where electrons causing each readout pulse originated in space.

27. Remember: we are working in 3D. That helps!

28. TPC sees all that happens in ~5 microsec window.
Background!
Early Track

29. Late track
Good track overlapping
another track

30. Side-view of the same event.
Late track
Good track overlapping
another track

31. Measured dE/dx vs. P Curves: Bethe-Bloch Formula:
proton / deuteron / 3He / 4He

32. Particle ID via dE/dx
After determining track momentum p, histogram the ratio:
under the assumption that the particle was a proton.

33. BoNuS: Next 6 GeV/c Experiment
DATA-TAKING COMPLETED: EG6 RUN IN LATE 2009

34. BoNuS: 12 GeV/c Experiment
APPROVED AS E
Will need a new/improved BONUS RTPC

35. Thank You!