A Compact Beam Profile Probe Using Carbon Fibres’ C.Field, N.Hower’, and B.Scott

A beam profile monitor based on the use of carbon fibres is described. It has been designed to be compatible with the operation of the Mark II particle detector at the SLC, I including high resolution secondary vertex detectors mounted 25 mm radially from the beam. f Thin wire or fibre probes are in use at many accelerators to allow the measurement of beam profiles and positions [l]. In particular, at the Stanford Linear Collider (SLC) it is-. frequently necessary to measure the beam spot dimensions-in the range of microns-at the interaction point of the colliding electron and positron beams. Since this must be done at the centre of a large and densely packed experiment, the design of the fibre probe presents some challenges. The difficulties have been compounded by the recent development of high spatial resolution solid state or drift chamber detectors placed close to the beam line. These are designed-to detect and measure the decay vertices of short lived species, such as particles bearing charm or beauty quantum number. In this paper we report on a mechanism that inserts a fibre probe on to the SLC beam line at the interaction point, and is compatible with the small radius vertex apparatus in the Mark II detector [2]. The spatial constraints imposed by Mark II are typical of large solid angle detectors. At azimuthal angles of 15 to 25 mrad from the beam line are detectors for electron-positron elastic (Bhabha) scattering. A minimum of material is allowed in front of these. This is true also between 50 and 120 mrad., where the solid angle is devoted to the principal luminosity monitor. To the extent possible, only thin sections of the beam pipe intervene between the interaction point and these detectors. Above 25' also, the resolution of the secondary vertex detectors and main drift chamber must be preserved by keeping the multiple scattering to a minimum. The central thin section of the beam pipe, illustrated in fig. 1, extends +4.8 cm about the collision point, and its radius is 25 mm. The materials are 0.41 mm of aluminum with an internal deposit of 0.025 mm copper to suppress scattered synchrotron radiation, for a total of 0.65% of a radiation length. Immediately outside it is mounted a silicon strip vertex detector. For the next section, 6.2 cm long, the beam pipe is slightly thicker, 0.84 mm, but of the same radius. …