A Compact Pair Polarimeter and Spectrometer

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The measurement of polarisation ovservables in hadron physics can provide detailed information about the excited stated of baryons and mesons. A programme of such measurements has been running at the world's major photon beam facilities for several years and has already produced execellent, and well publicised, results. These experiments require highly polarised beams and targets with precisely known properties. In particular, for linearly polarised photon beams, the degree of polarisation needs to be known with as much precision as possible. Recent improvements in luminosity and data taking rates have meant that is often the limiting systematic error in an experiment. This proposal aims to solve the problem by developing a new polarimiter which will be capable of measuring the degree of linear polarisation to better that 2% systematic error, by measuring the asymmetry in the orientation of e+e- pairs produced as the photon passes through a thin converter foil. The design is a compact, modular system based on Halbach dipole cylinders (permanent magnets) and TimePix3 pixel detectors. It is low cost, with minimal readout infrastructure and can be configured for any of the current photon beam facilities (MAMI in Mainz, ELSA in Bonn and CLAS and GlueX in Jefferson Lab, Virginia).