BoNuS: Radial-Drift TPC using Curved GEMs

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BoNuS Radial-Drift TPC using Curved GEMs
A Time Projection Chamber having Radial Drift
Direction, based on GEMs which have been
Curved to form cylinders.
Howard Fenkera, Jefferson Lab
Hall-C Collaborators indicated by bold type
N. Baillieb, P. Bradshawc, S. Bueltmannc, V.
Burkherta, M. Christyd, G. Dodgec, D. Duttae, R.
Enta, J. Evansb, R. Ferschb, K. Giovanettif, K.
Griffioenb, M. Ispiryang, C. Jayalathd, N.
Kalantariansg, C. Keppeld, S. Kuhnc, G.
Niculescuf, I. Niculescuf, S. Tkachenkoc, V.
Tvaskisd, J. Zhangc
a Thomas Jefferson National Accelerator Facility
(Jlab) b College of William and Mary c Old
Dominion University d Hampton University e
Tri-Universities Nuclear Lab (TUNL) f James
Madison University g University of Houston
Graduate Student Undergraduate
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).
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Motivation
PR-97-107
LENT
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Motivation

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

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BoNuS Experiment - Structure Functions

• Purpose
• Neutron Structure-function measurements
  unencumbered by Final-State Interaction Effects

Ratio F2n/F2p vs.x. The small data points
indicate the expected results of the BONUS
experiment for several different bins in Q2 with
statistical error bars. Estimated systematic
errors due to experimental and theoretical
uncertainties are indicated by the band at the
bottom (total systematic error / point-to-point
error after normalization at low x). Arrows
indicate the different possible approaches to the
limit x-gt1. The shaded area indicates the range
of uncertainty from existing data due to
different treatment of nuclear effects.
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Method

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

n
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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.

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dE/dx is HIGH for slow protons
Tells us two things
1. Protons easy to identify
2. Detector must be thin
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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!

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

• http//gdd.web.cern.ch/GDD

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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/CO2.

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Cosmic Studies w/flat prototype

• Cosmic tracks easily recognized.
• Position resolution would be better with charge
  sharing over 3 pads.
• Gain 303

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Proton Studies w/flat prototype

• Heavily Ionizing Protons from TUNLs Tandem

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Curved GEM for Prototype
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Curved prototype RTPC
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Cosmic event from Curved GEM TPC
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BoNuS RTPC Exploded View
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Inserting Pre-amp cards
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BoNuS Readout Crate
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What it really looks like
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A Way to Mount Everything
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Tracks are Pretty Obvious
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Remember we are working in 3D. That helps!
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Results are Making Sense
Vertex position agreement between CLAS and BoNuS
Angle measurements agree, too.
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Measured dE/dx vs. P
proton / deuteron / 3He / 4He
Curves Bethe-Bloch Formula
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Particle ID via dE/dx
After determining track momentum p, histogram the
ratio
under the assumption that the particle was a
proton.
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NIM article to be submitted soon
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BoNuS Physics Analysis
Recoil mass with and w/o using measured ps
momentum.
Ebeam 4.223 GeV
N. Baillie
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Status of BoNuS Analysis
model for sn/sD by P. Bosted
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BoNuS Next 6 GeV/c Experiment
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BoNuS 12 GeV/c Experiment
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Conclusion

• Specialized spectator proton detector developed
• First use of GEMs at JLab
• First application of Curved GEMs
• Experiment Run
• More to come