SPS Primary Targets

 

The primary proton beam of the SPS may be extracted from the long straight section (LSS2) to the North Experimental Area. The extracted beam, transported by bending magnets and focusing elements over few hundreds of meters is branched into three beam channels each terminating in a target whre the secondary particle beams are created. Switching magnets and beam-splitting systems distribute the protons among the targets.

There are four targets in the North Experimental hall, all having even numbers to distinguish them from those used to be in the West Area. All targets are intendend for use with 450 GeV/c primary protons. They lie in purpose-built target areas, near the surface but deep enough to be shielded by the landscape around them. Targets T2 and T4 produce beams for the EHN1 (Experimental Hall North 1), T6 is the source of the muon beam for EHN2 and the COMPASS experiment, while T10 serves the ECN3 (Experimental Cavern 3) for the NA48 experiment.

The maximum momentum of all primary beam channels and splitters in the North Area is 450 GeV/c. Most of the magnets in these channels are pulsed to conserve power, but only branch 21 can transport beams at different momenta during the same cycle. Branch 25 which feeds the muon target T6, may be set to different momentum from the branches leading to T2 and T4.

Two independent two-way splitting systems with steel septum magnets provide complete flexibility in sharing the beam simultaneously among the targets. In the mode of operation in which T6 receives a different momentum from the other targets, the first splitter simply bends the whole beam into branch 25.

The protons which do not interact in T4 in branch 24 are recuperated by the beam P0 and transmitted to the target T10.

All target stations of the SPS are similar in design. In each station, several beryllium or lead targets of different lengths and cross-sections are mounted on a common support, which may be moved by the SPS control room to select the target and adjust its position in the beam.

In order that targets may be easily removed, target supports are mounted in air, and thin vacuum windows seal off the primary and secondary beam pipes on either side. The assembly is completely surrounded by a massive iron shield, in which holes have been pierced for the passage of primary and secondary beams. A beam monitor station upstream of the target measures the incident proton flux, and horizontal and vertical split-foil detectors serve to centre the beam on the target. A mini-scanner placed before the target can measure the cross-sectional profile of the beam incident on the target. A second beam monitor downstream from the target stations (except T10) detects the emerging particle flux. The ratio of this flux divided by that of the incident beam gives the target multiplicity, a measure of secondary particle production efficiency.