Cherenkov Detector Prototype for ILC Polarimetry

Precise knowledge of all beam parameters is crucial to fully exploit the physics potential of the International Linear Collider (ILC). A sufficiently accurate measurement of the beam polarisation can only be achieved using dedicated high energy Compton polarimeters combined with well-designed Cherenkov detectors. The requirements have been evaluated and a suitable Cherenkov detector prototype has been designed, simulated and constructed accordingly. This prototype allows nearly all aspects of the final detector to be studied and has been operated successfully in a testbeam of which first results are presented as well. 1. High energy polarimetry at the ILC The physics programme of the ILC will rely heavily on how accurately the relevant beam parameters can be controled [1, 2]. For electroweak processes, the absolute normalisation of events rates depends on luminosity and polarisation. The luminosity will be measured to a precision of 10 to 10, while for the polarisation average an accuracy of 10 seems achievable. Contrary to luminosity and beam energy measurements for which permille level precisions have already been achieved, polarimetry needs to be improved by at least a factor of two. Measurements of two dedicated polarimeters located upand downstream of the ee interaction point will be combined with data from the ee annihilations themselves to determine the polarisation average. While the annihilation data will yield an absolute scale, the polarimeters provide fast measurements allowing to track variations over time (machine feedback) and to detect possible correlations with the luminosity or the polarisation of the other beam. Therefore, each polarimeter has to reach a systematic accuracy of at least δP/P = 0.25%, further reducing systematic uncertainties and adding redundancy to the system. Two polarimeters per beam are required in order to measure the polarisation of the beams in collisions. Both polarimeters have been designed for operation at beam energies between 45 GeV and 500 GeV. A detailed description of the polarimeters can be found in [3]. Compton polarimetry ensures a non-destructive measurement of the longitudinal beam polarisation. Circularly polarised laser light is shot under a small angle onto the individual bunches causing typically O(10) electrons per bunch to undergo Compton scattering. The energy spectrum of the scattered particles depends on the product of laser and beam polarisations, so that the measured rate asymmetry w.r.t. the laser helicity is directly proportional to the beam polarisation. The differential Compton cross section vs. scattered electron energy exhibits a large polarisation asymmetry near the Compton edge energy, which hardly depends on the chosen beam energy [3]. Since the scattering angle in the laboratory frame is below 10 μrad, a magnetic chicane transforms the energy spectrum into a spatial distribution which is then measured by an array of Cherenkov detectors chosen for several reasons: (i) It allows to measure the energy spectrum of many electrons arriving simultaneously. (ii) Cherenkov radiation is independent of the electron energy (β≈1). The number of Cherenkov photons is directly proportional to the number of Compton electrons per channel. (iii) Typical Cherenkov media are sufficiently rad-hard to withstand a high electron flux The final detector will consist of staggered U-shaped aluminium channels lining the tapered exit window of the beam pipe as illustrated in figure 1(a). All channels are filled with a Cherenkov gas and read out by photodetectors. Compton-scattered electrons traversing the U-base emit Cherenkov photons reflected upward to the photodetectors.