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 <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 andconstruction 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. ^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 allthe 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 onefound by CEA Saclay. It will be prudent and necessary to examine the wires from futuredrift-chamber prototypes after use to check if these depositsreappear, in order to avoid needing to increase the high voltagewhich would result in higher dark currents and faster detectorageing. 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 In addition, I would like to thank the CEU Program and the Office ofUndergraduate Research at the University of Illinois for travelsupport. ^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. ESEM ImagingCOMPASS II Experiment Future Work Wire Diameter Measurements Wire Spectrum Measurements Physics of Drift Chambers Wire Spectrum Measurements (cont.) Acknowledgments Carbon Coating of Sense Wires ^ 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. 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. 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. 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. <The picture shows the thick carbon coating on the sense wire after spending a significant amount of time in the CEA Saclay drift chamber. < An Environmental Scanning Electron Microscope (ESEM). New Wire ^Atomic spectrum from the examined surface area. The most prominent peaks on the graph represent various gold lines. ^The location on the wire where the spectrum analysis was carried out is marked by the small red circle. Used, Clean Wire ^The location on the wire where the spectrum analysis was carried out is marked by the small red circle. Used, Contaminated Wire ^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. ^Atomic spectrum from the examined surface area. The most prominent peaks on the graph represent various gold lines.