BoNuS: Radial-Drift TPC using Curved GEMsA Time Projection Chamber having

Radial Drift Direction, based on

GEMs which have been Curved to form cylinders.

Howard Fenkera*, Jefferson Lab

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

Graduate Student Undergraduate a Thomas Jefferson National Accelerator Facility (Jlab)b College of William and Mary

c Old Dominion Universityd Hampton Universitye Tri-Universities Nuclear Lab (TUNL)f James Madison Universityg University of Houston

* 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).

Hall-C Collaborators indicated by bold type

MotivationPR-97-107

“LENT”

Motivation

• Purpose– Provide almost-free neutron

target to improve our understanding of neutron structure.

• Purpose– Neutron Structure-function

measurements unencumbered by Final-State Interaction Effects

BoNuS Experiment - Structure Functions

Ratio F2n/F2

p 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->1. The shaded area indicates the range of uncertainty from existing data due to different treatment of nuclear effects.

Method

• Measure slow protons– Identify spectator

protons to tag e-d events in which the neutron was struck.

np

e

before

e

p

?

after

n

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.

dE/dx is HIGH for slow protons

Tells us two things:

1. Protons easy to identify

2. Detector must be thin

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.– 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!

Cathode Anode Readout

BoNuS is just a curled-up TPC.

For convenience, the gas-gain elements are GEMs.

GEM Readout

• http://gdd.web.cern.ch/GDD

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.

Drift Plane (Cathode board + foil)

SegmentedCathode

GEM Foils

5/16 in 1.90 cm.

0.46 cm.

• Cosmic tracks easily recognized.

• Position resolution would be better with charge sharing over ≥3 pads.

• Gain ~ 303

drift

xpad #

xpad #

drift

20mm x 19mm 20mm x 19mm

20mm x 19mm 20mm x 19mm

box size represents Q(x,t)

Cosmic Studies w/flat prototype

• Heavily Ionizing Protons from TUNL’s Tandem

Proton Studies w/flat prototype

Curved GEM for Prototype

Curved prototype RTPC

Cosmic event from Curved GEM TPC

BoNuS RTPC: Exploded View

Inserting Pre-amp cards

BoNuS Readout Crate

What it really looks like

A Way to Mount Everything

Tracks are Pretty Obvious

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

Results are Making Sense

Vertex position agreement between CLAS and BoNuS:

Angle measurementsagree, too.

Measured dE/dx vs. P

proton / deuteron / 3He / 4HeCurves: Bethe-Bloch Formula:

Particle ID via dE/dx

After determining track momentum p, histogram the ratio:

dEdx measured

dEdx p

Bethe Bloch

under the assumption that the particle was a proton.

NIM article to be submitted soon:

0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

W (GeV/c2)

100

200

300

400

500

600

700

800

900

BoNuS Physics AnalysisBoNuS Physics Analysis

Ebeam = 4.223 GeV N. Baillie

Recoil mass with and w/o using measured ps momentum.

Status of BoNuS Analysis

n

np

CLAS

RTPC

N

N

)(

model for σn/σD by P. Bosted

BoNuS: Next 6 GeV/c Experiment

BoNuS: 12 GeV/c Experiment

Conclusion

• Specialized spectator proton detector developed

• First use of GEMs at JLab

• First application of Curved GEMs

• Experiment Run

• More to come