IARPE - SLAC

Stanford Linear Accelerator Center

News from SLAC

The Asymmetric B Factory is a combination of a major upgrade to the old PEP storage-ring facility, appropriately named PEP-II, and a new detector called BaBar. The PEP-II facility is made up of two independent storage rings, one located atop the other. The high-energy ring stores a 9-GeV electron beam and the low-energy ring that resides above it stores 3.1-GeV positrons. The term "asymmetric" refers to the fact that the e- and e+ energies are not equal, which results in a collision center of mass that is moving in the laboratory. This motion of the center of mass is crucial for the study of the matter vs. anti-matter question: "Why do we live in a matter-dominated Universe?" There is a nice discussion on the current belief for the cause, namely CP-violation, in SLAC's magazine, the "Beam Line" (Summer/Fall 1999, Vol.29, No.2). You can try to access it via Beam Line Online or you can write to SLAC's Publication Office for a free copy.

The construction of PEP-II is a collaboration of SLAC, LBL and LLNL; it was completed "on budget" and turned on several months ahead of schedule. The BaBar detector, built to study the millions of B mesons produced by the PEP-II storage ring, is a collaboration involving about 600 physicists and engineers from 85 institutions in 9 countries. It has been running very successfully ever since it started taking data at the end of May. The SLAC Radiation Physics Department actively participated in the design of all aspects of the radiation protection design for the B-Factory---the Personal Protection System (PPS), the Beam Containment System (BCS) and, of course, the design of the shielding. A paper has recently been published in Health Physics Journal that describes this work (see J. C. Liu et al, Health Physics 77, #5 (Nov. 1999) 588). The SLAC computer codes, EGS4 and SHIELD11, were extensively used in the shielding calculations, and radiation measurements were carried out during all phases of the B-Factory commissioning that successfully verified that all aspects of the shield design were correct.

The Radiation Physics Department has also contributed, in a variety of other ways, to the success of the B-Factory as a whole, including:

Design of the PEP-II Luminosity Monitor, which is currently in full use in the B-Factory program (see W. R. Nelson et al, First Int. Workshop on EGS4 (26-29 Aug 1997), KEK Proceedings 97-16).
Design of a CdZnTe telescope that was used during the PEP-II commissioning to check the adequacy of the synchrotron radiation mask (shield) for BaBar (also in the above KEK Proceedings 97-16).
EGS4 calculations to recognize areas within the PEP-II tunnel where high-dose levels would lead to failure of accelerator components, together with designs that could help prevent this from happening (see Liu et al, Nucl. Instr. Meth. A376 (1996) 9). Measurements were made during the PEP-II commissioning phase that verified the calculations and shielding methods were correct, and this dosimetry work continues as an essential part of the B-Factory program.

More recently we have performed EGS4 calculations in an effort to understand why so much damage (in the form of leakage current) occurred to silicon detectors that were exposed in a "beam dump" experiment near Interaction Region 8 of PEP-II. The motivation for this experiment was concern, by the experimenters, for the lifetime of the Silicon Vertex Detector (SVT)---the prime component of BaBar for recognizing the short-lived B-meson decays. Although dosimetry measurements made during the exposure demonstrated that the EGS4 calculations were, indeed, correct, the dose that was delivered was two orders of magnitude below damage threshold for the silicon devices. As a result, additional calculations were performed using FLUKA in order to account for damage caused by the production of hadrons. But this also was unable to explain the discrepancy, so the confusion still remains. Needless to say, the B-Factory people have taken all kinds of measures to prevent inadvertent beam loss in the vicinity of the SVT.