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 detectors, the drift chamber DC5. Multiple prototype drift chambers have been built at UIUC, and have undergone sustained testing with cosmic rays. ^The pictures above show the 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 and20.5 µm, and most were even closer to 20 µm than that. As such,it is clear that if there was a deposit on the wires, it was muchthinner than the one found by CEA Saclay. While no carbon deposits were found on the sense wire samples,it is still unsure why these deposits appeared in the CEA Saclayprototype. As such, it will be prudent and necessary to examinethe wires from future prototypes after use to check if thesedeposits reappear. We would like to thank Professor 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. In addition, we would like to thank the CEU Program and the Officeof Undergraduate Research at the University of Illinois for theirgenerous support for this project. ^This picture shows the location on the wire where the spectrum analysis, marked by the small red circle. >The figure shows the contour lines for V in a 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 feature a row of alternating sense and field wires between two cathode planes. The sense wires are set at 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 resulting electrons drift towards the sense wires. ESEM Imaging COMPASS II Experiment Future Work Wire Diameter Measurements Wire Spectrum Measurements Physics of Drift Chambers Wire Spectrum Measurements (cont.) Acknowledgments Carbon Coating of Sense Wires ^The diagram above shows the layout of the drift chamber. The distance between sense and field wires is 4 mm and the gap between the cathode planes is 8 mm. As seen in the figures above, upon performing the spectral analysis on various samples of the sense wires, almost all of them were found to have a spectrum practically matching that of the new wires 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 nonuniformity, 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, it was decided to use an Environmental Scanning Electron Microscope (ESEM), a type of electron microscope which allows for the imaging of dry, uncoated specimens and a detailed spectral analysis of the same. The main areas of concern were ensuring that the diameter of the sense wires remained relatively unchanged, as well as checking for any possible organic contaminants on the wires. In earlier experiments performed on a similar drift chamber prototype built by CEA Saclay in France, it was 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. If this were to occur for a sustained period of time, it would force an increase in the operating voltage of the chamber and might severely limit its efficiency. As a result, it was decided to remove sense wires from the UIUC prototype after an extended period of operation, in order to perform detailed microscopic and spectral analysis in a search for possible deposits on the 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 prototype. <The picture shows an Environmental Scanning Electron Microscope (ESEM). for the COMPASS collaboration New Wire ^This figure shows the elemental spectrum of the examined area. The major spikes on the graph are the various gold lines. ^This picture shows the location on the wire where the spectrum analysis, marked by the small red circle. Used, Clean Wire ^This figure shows the elemental spectrum of the examined area. The major spikes on the graph are the various gold lines. ^This picture shows the location on the wire where the spectrum analysis, marked by the small red circle. Used, Dirty Wire ^This figure shows the elemental spectrum of the examined area. The major spikes on the graph are the various gold lines, while the smaller spikes are organic contaminants, specifically carbon, oxygen, and calcium.