The crystal collimation system of the Relativistic Heavy Ion Collider

Crystal Channeling occurs when an ion enters a crystal with a small angle with respect to the crystal planes. The electrostatic interaction between the incoming ion and the lattice causes the ion to follow the crystal planes. By mechanically bending a crystal, it is possible to use a crystal to deflect ions. One novel use of a bent crystal is to use it to channel beam halo particles into a downstream collimator. By deflecting the halo particles into a collimator with a crystal it may be possible to improve the collimation efficiency as compared to a conventional two stage collimation system.; A bent crystal was installed in the counterclockwise ring of the Relativistic Heavy Ion Collider (RHIC) prior to the FY2001 run to be used as the first stage of a two stage collimation system. We present a model and simulations to the predict crystal channeling efficiency. The simulations and model predict a channeling efficiency between 59% and 74% depending on the choice of optics.; Attempts to reduce backgrounds in RHIC experiments using the crystal were unsuccessful because of the low channeling efficiency. Channeling results show a channeling efficiency 75% lower than originally predicted. Errors in the horizontal Twiss parameters at the crystal and inaccurate assumptions of the beam halo distribution are the cause of the large discrepancy. The strong scattering from the crystal made it impossible to intercept most of the scattered ions from the crystal using a single downstream absorber. Background at the STAR detector downstream of the crystal collimator was increased by as much as a factor of two and was rarely reduced below the uncollimated background rate. As a result, the crystal collimator was removed and the collimation system upgraded to a standard two stage system for the FY2004 gold-gold run.; We also discuss the uses and effectiveness of the copper scrapers used during all RHIC runs. Measurements of diffusion rates and frequency analysis of beam losses were performed. These measurements aid in the understanding of beam growth rates. We performed beam profile measurements using the scrapers.; The design and use of the upgraded collimation system is discussed. Future plans for collimation at RHIC are examined.