Christoph Bartels

Design and construction of a Cherenkov detector for Compton polarimetry at the ILC

This paper describes the design and construction of a Cherenkov detector conceived with regard to high energy Compton polarimeters for the International Linear Collider, where beam diagnostic systems of unprecedented precision must complement the interaction region detectors to pursue an ambitious physics programme. Besides the design of a prototype Cherenkov detector, detailed simulation studies are presented. Results of a first testbeam campaign with the main objective of validating the simulation in terms of the light distribution inside the channels and the channel response are presented. Furthermore, a new method for aligning the detector without the need of dedicated data taking has been developed.

Precision Polarimetry at the ILC: Concepts, Simulations and experiments

The precision physics program of the ILC requires precise knowledge of the state of beam polarisation. In fact the Compton polarimeters intended for the ILC will have to measure the polarisation with error a factor of 2 smaller than the previous best measurement at the SLAC SLD experiment. In order to further reduce measurement error, spin tracking simulations in the ILC Beam Delivery System subject to ground motion induced misalignment have been performed and the expected variation in polarisation has been quantified. A prototype of a high precision spectrometer to record Compton scattered electrons from the interaction of a longitudinal laser and the charged beams has been developed. The Compton electrons interact with a gas in the polarimeter channels to produce Cherenkov radiation measured by photodetectors. The calibration of the photodetectors is crucial and exhaustive bench tests of the photodetector linearity have been performed. The polarimeter prototype itself will be tested at the ELSA testbeam in Bonn in Spring 2009.

Photodetector studies and prototype simulation of a Cherenkov detector for an ILC polarimeter

At the International Linear Collider (ILC) the polarisation of the electron and positron beams needs to be measured with as yet unequaled precision of ΔP/P ≈ 0.25%. The key element of the polarimeter will be the precise detection of Cherenkov light from Compton scattered electrons. The poster deals with the choice of a suitable photodetector (PD). In a recently assembled test facility various types of PDs have been checked. Results are presented with a special focus on the linearity of the device, since this is expected to be the limiting factor on the precision of the polarisation measurement at the ILC. To study the performance of the entire Cherenkov detector design, a three channel testbox is constructed during summer 2008 and will be tested first in the laboratory (using LED a/o laser light), then in the DESY-II testbeam and finally in spring 2009 at ELSA in Bonn (higher rates and multi-electron events). The accompanying optical simulation of the testbox is done using Geant4. The photodetector response is simulated separately. First results are shown. The simulation is expected to help understand and interpret the testbeam data.

Towards precision polarimetry at the ILC: Concepts, simulation and testbeam results

At the International Linear Collider (ILC) the polarisation of the electron and positron beams at the e+e− interaction point needs to be determined with a precision of δP/P = 0.25% or even better in case of electroweak precision measurements. This can only be achieved by combining long term polarisation measurements based on e+e− collisions with fast measurements of Compton polarimeters placed up- and downstream of the e+e− IP. In order to control spin transport effects and depolarisation during the collisions, the Compton polarimeters are aiming for yet unrivaled instrumental precision at the permille level. This can be achieved by dedicated magnet configurations, which need to be designed carefully to take into account the very different beam and background conditions up- and downstream of the IP and to match in an optimal way the Cherenkov detectors forseen for the Compton scattered beam electrons. While the design of the magnetic chicane can be studied currently in simulations only, testbeam studies with the former SLD polarimeter Cherenkov detector have been performed to better understand the sources of limitations of this excellent device.

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.