Beam Background

With the elimination of the separation-bend magnets near IP, the synchrotron radiation backgrounds are not so severe as they were for TRISTAN, where the critical energy was higher. The apertures of the beam pipe and masks (SR-Mask) near IP were designed so that synchrotron radiation from QCS and QC1 go through without hitting them. In these synchrotron radiation calculations, we protect against beam tails out to 10 $\sigma_x$ and 30 $\sigma_y$. The beam-beam simulations indicate no long tails in the interaction region. Mask-A shields the beryllium beam pipe from back-scattered photons. Although some photons from QC2 and QC3 can hit SR-Mask, they have a critical energy less than 2 keV and are easily absorbed in the material of the mask. The masks are gold-plated for this purpose. Photons from upstream magnets far from IP are intercepted by movable masks installed just upstream of QC3.
Particle backgrounds were expected to be more critical at KEKB. A DECAY TURTLE calculation indicates that the rate of spent particles from both beams directly hitting the beam pipe between the two cryostats is 130 kHz for 10$^{-9}$ torr vacuum. Particle-Masks were installed outside the beam pipe to reduce the particle background. Movable masks installed in the arcs and the non-IR straight sections cut off the beam tails far from IR. These masks could reduce radiation levels at the detector during injection.