Subatomic Physics – Beamline Polarimeters


The polarised ion source at iThemba LABS is of the atomic beam variety and can provide polarised proton beams with intensities of up to 100nA (SSC output) with the average degree of beam polarisation around 75%. The degree of polarisation normal to the scattering plane can be determined by measuring elastic scattered protons from targets with known analysing power values. Such measurements are performed either with the low energy polarimeter (between the injector cyclotron and the SSC) or with the high energy polarimeter in the P-line, which is the high energy beam-line leading from the SSC to the various experimental halls.

During a typical experiment the P-line polarimeter is periodically used to measure the beam polarisation (typically every two hours). The K- line polarimeter is used to optimise the degree of polarisation prior to the experiment. Deterioration of beam quality due of the presence of the target of the P-line polarimeter precludes continuous monitoring of beam polarisation during experiments.

The K-line polarimeter consists of two 150 μm thick Si-detectors and a target gas cell inside a vacuum chamber. The gas cell contains high purity He gas with Havar entrance and exit windows. The two Si-detectors are fixed at Θ{lab}=110°
and are each collimated with two Ta collimators.
The 4He(p,p) elastic scattering reaction is used to determine the beam polarization.
The P-line polarimeter consits of two NaI(Tl) detectors (12.7cm long crystals and 5.1cm in diameter) separated from a scattering chamber by Kapton exit windows. A target which consist of a 75 μm thick CH foil is mounted on a target ladder inside the scattering chamber. The 12C(p,p) elastic scattering reaction is used to determine the beam polarization.

For more information on these instruments please refer to the polarimeter section of the k600 document database.

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