Measuring the Position Resolution of a COMPASS Drift Chamber
Prototype
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2
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Rojae Wright , Ihnjea Choi , Caroline Riedl , Matthias Grosse Perdekamp
(1) Alabama A&M University, (2) University of Illinois in Urbana Champaign
Introduction
Drift Chambers
Results
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The Drell-Yan process is the
creation of a lepton-antilepton pair from
the annihilation of a quark-antiquark
pair through a virtual photon.
COMPASS is a fixed target experiment at CERN
in Geneva, Switzerland which investigates the
quark and gluon structure of proton. The
experiment will study the transverse spin- and
momentum dependent quark structure for the
proton through pion-induced Drell-Yan
scattering off transversely polarized proton
targets. The observed Sivers asymmetries are
indicative of quark orbital angular momentum
inside the proton.
The drift chamber prototype B in the UIUC clean room.
UIUC is designing and building a drift chamber
to replace aging straw chamber stations in the
COMPASS spectrometer. UIUC has built two
drift chamber prototypes, A and B. Prototype A
consists of a single anode plane with 8 wires,
prototype B has two anode planes with 16
sense wires each. Electron beams with
momentum of 4-5 GeV at DESY, Hamburg and
cosmic rays are used to measure the position
resolution of the drift chamber prototypes A
and B. This poster describes the details of the
experimental method and steps that will lead to
the measurement of the position resolution
with cosmic rays.
Drift chambers are wire chambers that are built to
detect the passage of charged particles by means
of ionization of gas atoms and molecules. Planar
drift chambers generally feature a row of
alternating sense and field wires between two
cathode planes. The anode wires are at positive
electrical potential compared to the cathode
planes and field wires. The chamber is filled with
a mixture of 45% Ar, 50% Ethane and 5% CF4.
When a charged particle passes through a drift
chamber it creates electron-ion pairs. The applied
high voltage creates an electric field and prevents
electrons from recombining with the ionized
molecules or atoms. The field causes the freed
electrons to drift towards the anode sense wires
and the positively charged ions to drift towards
the field wires and the cathode planes. On
average electrons drift at a constant velocity.
Knowing the difference between initial and arrival
time, Δt, the relation Δx= v × Δt yields the
location of the passage of the charged particle.
The drift velocity in the gas can be accurately
computed and the use of high-resolution timing
electronics (TDC) allows the track position to be
determined from the drift time of the electrons.
Beam Test at DESY
20~30kHz
Noise rates
DC-A Channel #
Distance (R) -Time (T) correlation
from beam illuminated channel (ch3)
Rate [lHz]
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The diagram shows the cross
section of prototype A. The pitch
between sense wires is 8 mm.
Rate [lHz]
Prototype A, Beam Test at DESY
Electron beam
illuminates Ch3
DC-A Channel #
Close to sense wire
Close to field wire
Position resolution vs. drift time
Prototype B, Cosmic Ray Test at UIUC
Signal from Prototype B
Cosmic ray signal
Cosmic ray trigger
[ns]
Cosmic Ray Signal with
Trigger
Time of cosmic ray signal – Time of trigger
Conclusion and Future Work
4 - 5 GeV electron beam
5 x 5 mm incident beam size
Electron beam
Prototype A
Beam Telescope
Beam telescope, 6 layers of
silicon detectors, reconstruct
electron tracks.
- position resolution ~ 6 um
Prototype A placed between
silicon beam telescopes for
measuring the position
resolution & chamber
efficiency
The position resolution for prototype A is found
to be better than 200 micro meter. The frontend
electronics needs to be shielded carefully to
reduce noise levels. Prototype B will be used to
evaluate the performance of the second
generation FEE using cosmic rays.
Acknowledgements
This research was supported by NSF Grant