Recent experience with backgrounds at the SLC

During the 1990 SLC (Stanford linear Collider)/Mark II runs, machine backgrounds visible in the new vertex detectors were studied. From the start, synchrotron background in the main Mark II tracking chamber was significantly reduced from the prior year's run. Occupancies, which had been 15% to 20% during the previous year, were now below 5%. The improvement is attributed to the additional collimators at the end of the LINAC, which made it possible to do both primary and secondary collimation in a region with zero nominal dispersion. The additional masking material, including the material of the vertex detectors, also contributed. Backgrounds in the new vertex detectors were larger than expected. These detectors had active elements at radii from 3 to 17 cm and were subjected to backgrounds unique to linear colliders. The authors describe recent progress in identifying sources and developing control techniques. >