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

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

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

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!

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.

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 http://gdd.web.cern.ch/GDD

GEM Readout http://gdd.web.cern.ch/GDD

Curved GEM for Prototype Proton Track in Curved Prototype

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

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…

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

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.

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! (0.00025 in.  6 m)

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).

BonuS RTPC Schematic Add the electronics on the outside.

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

BoNuS RTPC: Exploded View

Assembly

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

Readout Electrodes (Pads)

Readout Electronics Needs Signals are typical of any TPC. Channel Count: 3200 4mm 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.

ALTRO Chip http://ep-ed-alice-tpc.web.cern.ch Luciano Musa

BoNuS Readout Crate

A Way to Mount Everything

SIMULATION: Lorentz Angle / Track Curvature

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

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

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

Late track Good track overlapping another track

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

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

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

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

BoNuS: 12 GeV/c Experiment APPROVED AS E12-10-102 Will need a new/improved BONUS RTPC

Thank You!