J. Stoner

Performance, radiation damage, and future plans of the BABAR silicon vertex tracker

The BABAR silicon vertex tracker (SVT) has been in operation for four years at the PEP-II electron-positron storage ring. During this time the SVT modules have accumulated a radiation dose up to 2 Mrad. We study the degradation in the performance of the SVT due to this accumulated dose which is highly non uniform across the device and also within the individual silicon detectors. To extrapolate the performance of the device to the future we study separately the effect of the irradiation on silicon detectors, front end integrated circuits and on a complete detector module under controlled radiation conditions, using a /sup 60/Co source and a 0.9 GeV e/sup -/ beam. We compare the results to the data from the SVT. In particular we show the dependence of the charge collection efficiency on the radiation dose even when a small stripe of the module is irradiated up to space charge sign inversion. Since the modules that are located in the plane of the beams will suffer significant radiation damage, we will describe our plans for their replacement in 2005 and for the operation of the SVT through the lifetime of the BABAR experiment.

Status and future plans of the BABAR silicon vertex tracker

Abstract A brief summary of the design goals, description, and performance of the BABAR Silicon Vertex Tracker is given. Results from radiation hardness tests are discussed, which indicate satisfactory operation up to 5 Mrad of accumulated radiation. The local alignment procedure has made significant improvements recently, and four readout sections were recovered during the BABAR shutdown in 2002.

Lessons learned from BaBar silicon vertex tracker, limits, and future perspectives of the detector

The silicon vertex tracker (SVT) of the BaBar experiment at PEP-II is described. This is the crucial device for the measurement of the B meson decay vertices to extract charge-conjugation parity (CP) asymmetries. It consists of five layers of double-sided ac-coupled silicon strip detectors, read out by a full-custom integrated circuit, capable of simultaneous acquisition, digitization, and transmission of data. It represents the core of the BaBar tracking system, providing position measurements with a precision of 10 /spl mu/m (inner layers) and 30 /spl mu/m (outer layers). The relevant performances of the SVT are presented, and the experience acquired during the construction, installation, and the first five years of data-taking is described. Innovative solutions are highlighted, like the sophisticated alignment procedure, imposed by the design of the silicon tracker, integrated in the beamline elements and mechanically separated from the other parts of BaBar. The harshness of the background conditions in the interaction region required several studies on the radiation damage of the sensors and the front-end chips, whose results are presented. Over the next five years the luminosity is predicted to increase by a factor three, leading to radiation and occupancy levels significantly exceeding the detector design. Extrapolation of future radiation doses and occupancies is shown together with the expected detector performance and lifetime. Upgrade scenarios to deal with the increased luminosity and backgrounds are discussed.

New effects observed in the BaBar silicon vertex tracker: interpretation and estimate of their impact on the future performance of the detector

The silicon vertex tracker (SVT) of the BaBar experiment at PEP II is briefly described. It consists of five layers of double-sided AC-coupled silicon strip detectors, constituting the core of the BaBar tracking system. After six years of operation, some unexpected effects have appeared. In particular, a shift in the pedestal for the channels of the AToM readout chips that are most exposed to radiation has been observed. The behavior has been understood and reproduced in AToM chip irradiations with 1-GeV electrons at Elettra (Trieste) and the results of the studies are presented here. A second unexpected behavior has been detected, consisting of an anomalous increase in the bias leakage current for the modules in the outer layers. The effect is beam-related but not directly linked to radiation damage, as suggested by the fact that it is not present in the inner layers. The cause has been understood and the conclusions are presented here. The effect has been reproduced in a qualitative way in the laboratory. Over the next three years the luminosity is predicted to increase by a factor of three, leading to radiation and occupancy levels significantly exceeding the detector design. Estimates of future radiation doses and occupancies are shown together with the extrapolated detector performance and lifetime, in light of the new observations. Upgrade scenarios to deal with the increased luminosity and backgrounds are discussed.