A Compact Pair Polarimeter and Spectrometer

强子物理中偏振观测的测量可以提供有关重子和介子激发态的详细信息。这样的测量项目已经在世界上主要的光子束设施中运行了好几年，并且已经产生了出色的、广为宣传的结果。这些实验需要高度极化的光束和具有精确已知特性的目标。特别是，对于线偏振的光子光束，偏振度需要尽可能精确地知道。最近在亮度和数据采集速率方面的改进意味着在实验中通常是限制系统误差的。本提案旨在通过开发一种新的偏振器来解决这个问题，该偏振器将能够通过测量光子通过薄转换器箔时产生的e+e对方向的不对称性来测量线性偏振度，从而使系统误差优于2%。该设计是一个紧凑的模块化系统，基于Halbach偶极子圆柱体（永久磁铁）和TimePix3像素探测器。它成本低，具有最小的读出基础设施，可以配置为任何当前的光子束设施（美因茨的MAMI，波恩的ELSA和弗吉尼亚州杰斐逊实验室的CLAS和GlueX）。

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).