Design of a new xBSM Coded Aperture for 1.8 Gev Dan Peterson, 20130111 This design evaluation is based on the x-ray energy spectrum derived from December 2012 data. A new CA must be ordered on the time scale of 1 week, for installation for the April run.

The current Coded Aperture works well at 2.085 GeV beam energy. Here, I show a figure of merit for the xBSM measurement. It is a χ2 : ∑pixel [ h(σ+Δσ) – h(σ) ]2 / h(σ) where h(σ) is the signal height of a pixel for the given beam size, σ . It is plotted for constant Δσ /σ , has units of signal height, and measures the ability to resolve the beam size for constant current. The threshold for resolving the beam size for a typical current is estimated to be 0.6; we know that the PH is good down to about σ=10μm, and the CA is good up to about 30 μm. At 1.8 GeV, we do not have a usable optic.

The first attempts to design a new CA had mixed results. I tried a design with 2 symmetric sets of slits. John used his “pseudo random” pattern with 20μm minimum feature size. The grating with 62μm features is the first to show real improvement at 20μm beam size. But, none of these are above threshold at 10μm beam size.

Jim A. have the idea to start with a pseudo Fresnel Zone Plate. The “FZP” has only one significant feature. By departing from the regular FZP characteristic of decreasing feature size going away from the center, the performance is improved. Beam size can be resolved between 10 and 30 μm. CA feature sizes are between 10 and 70 μm.