Electron Microscope and Spectral Analysis of Sense
Wires used in COMPASS Drift Chambers
Vivek Britto, Soobin Lim, IhnJea Choi, Pedro Montuenga – University of Illinois at Urbana-Champaign
COMPASS II Experiment
ESEM Imaging
Wire Spectrum Measurements (cont.)
Used, Clean Wire
<
The Drell-Yan process is the
creation of a lepton-antilepton pair
from the decay of a photon produced
in quark-antiquark annihilation.
The COMPASS II experiment will study the Drell-Yan process.
The COMPASS group at UIUC is responsible for the design and
construction of a large area tracking detector, the drift
chamber DC5. Multiple prototype drift chambers have been
built at UIUC and have undergone sustained testing with
cosmic rays.
Physics of Drift Chambers
< An Environmental
Scanning Electron
Microscope (ESEM).
For further examination of the used sense wires from the UIUC
prototype, an Environmental Scanning Electron Microscope (ESEM) was
used. This type of electron microscope allows for the imaging of dry,
uncoated samples and a detailed spectral analysis of the same. The
main concerns were to ensure that the diameter of the sense wires
remained unchanged, as well as to check for any possible organic
deposits on the wires.
^
The location on the wire where the spectrum
analysis was carried out is marked by the small red
circle.
^
Atomic spectrum from the examined
surface area. The most prominent peaks on the
graph represent various gold lines.
Used, Contaminated Wire
Wire Diameter Measurements
^ Cross section of the drift chamber. The
distance between sense and field wires is 4
mm and the gap between the cathode planes
is 8 mm.
^
The location on the wire where the spectrum
analysis was carried out is marked by the small red
circle.
> Equipotential lines in the drift chamber. The
sense wire is located at the origin, while the
field wires are at x =±4 mm, y = 0.
Drift chambers are charged-particle detectors that place rows of
alternating sense and field wires between two cathode planes.
The sense wires are set to a positive high voltage relative to the
field wires and the cathode planes. When a charged particle
passes through the drift chamber, gas molecules are ionized and
the freed electrons drift towards the sense wires.
Carbon Coating of Sense Wires
<
The picture shows the thick
carbon coating on the sense wire after
spending a significant amount of time in
the CEA Saclay drift chamber.
^
ESEM photograph of a sample of the gold-plated tungsten sense wire, along with a ruler to
determine a precise diameter. The horizontal shift of the ruler is a byproduct of the photographic process
occurring after the measurement of the diameter.
The diameter specification of the new sense wires was 20 µm.
Upon measurement under the microscope, it was found that all
the tested samples of sense wire had diameters between 19.5 µm
and 20.5 µm, with a typical deviation of 0.2 µm. Thus, if there was
a deposit on the wires, it would be much thinner than the one
found by CEA Saclay.
Wire Spectrum Measurements
New Wire
On a similar drift chamber built by CEA Saclay in France it was
previously found that the gold-plated tungsten sense wires had
accumulated significant carbon deposits on their surfaces after
spending long amounts of time in the chamber while it was
running. The carbon originated from the graphite-coated cathode
planes. If this were to occur for a sustained period of time, it would
force an increase in the operating voltage of the chamber to avoid
severely limiting its efficiency. This would result in higher dark
currents and faster ageing of the drift chamber.
Therefore several sense wires were removed from the UIUC
prototype after an extended period of operation and a detailed
microscopic and spectral analysis was performed in search for
possible deposits on the wires.
^
Atomic spectrum from the examined
surface area. The most prominent peaks on the
graph represent various gold lines. The smaller
peaks represent organic contaminants, specifically
carbon, oxygen, and calcium.
Almost all of the various wire samples were found to have a spectrum
practically matching that of the new wire straight off the spool. This
essentially eliminates any possibility of a uniform, pervasive organic
coating building up on the sense wires after repetitive use.
In some rare cases, some contaminated spots were found on the
sense wire, but due to their rarity and non-uniformity, it is most likely
that these are due to human mishandling and contamination. In
particular, the presence of calcium in those samples is a strong
indication that the contamination is from human error.
Future Work
It will be prudent and necessary to examine the wires from
future drift-chamber prototypes after use to check if these
deposits reappear, in order to avoid needing to increase the high
voltage which would result in higher dark currents and faster
detector ageing.
Acknowledgments
^
The location on the wire where the spectrum
analysis was carried out is marked by the small red
circle.
^
Atomic spectrum from the examined
surface area. The most prominent peaks on the
graph represent various gold lines.
We would like to thank Matthias Grosse Perdekamp, Caroline Riedl,
Ran Bi, John Blackburn, and Eric Thorsland for their work and
support in the design and construction of the prototype. This
research was funded by NSF grant PHY-12-05-671. In addition, I
would like to thank the CEU Program and the Office of
Undergraduate Research at the University of Illinois for travel
support.