Design of an X-Ray Imaging System for the Low-Energy Ring of PEP-II

An x-ray beam-size monitor for positrons in the lowenergy ring (LER) of the PEP-II B Factory at SLAC is being designed to accommodate the present 2-A, 3.1-GeV beam and anticipated currents of up to 4.7 A. The final photon stop of an arc will be rebuilt to pass dipole radiation through cooled apertures to optics 17 m from the source. The initial installation will use pinhole optics with a calculated resolution of 35 µm. We then plan an upgrade using zone-plate imaging to obtain a resolution of 6 µm. Two multilayer x-ray mirrors precede the zone plate, limiting the bandwidth to 1%, in order to avoid chromatic blurring and protect the zone plate. Despite the narrow bandwidth, the zone plate’s larger diameter compared to a pinhole camera allows for a comparable photon flux. We will image all of the available 1700 LER bunches and also measure them individually, searching for variations along the train due to electron-cloud and beam-beam effects, using a scanning detector conceptually derived from a wire scanner. A mask with three narrow slots at different orientations will scan the image to obtain three projections. In one passage, signals from a fast scintillator and photomultiplier will be rapidly digitized and sorted to profile each bunch.