| Summary: | A residual gas fluorescence beam profile monitor at the Relativistic Heavy Ion Collider (RHIC) has successfully recorded beam images of various species of relativistic heavy ions during FY2012 operations. These fully striped ions include gold, copper, and uranium at 100, 99.9, and 96.4 GeV/n, respectively. Their beam profiles give an independent measurement of the RHIC beam size and emittance. We estimated their corresponding fluorescence cross sections to be 2.1×10^{-16}, 1.8×10^{-17}, and 2.6×10^{-16} cm^{2}, and obtained their rms transverse beam sizes of 0.36, 0.37, 0.24 mm for gold, copper, and uranium ions, respectively. They are the smallest ion beam width, thus lowest beam emittance, ever produced at RHIC or any other high-energy heavy ion colliders. These extremely small beam sizes may have reached a fundamental limit to residual gas fluorescence based beam profile monitor. Nevertheless, this beam diagnostic technique, utilizing the beam-induced fluorescence from residual gas where hydrogen is still the dominant constituent in nearly all vacuum systems, represents a passive, robust, truly noninvasive, monitor for high-energy ion beams.
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