[Paper Review] Precision Polarimetry at the International Linear Collider

C. Helebrant, D. Käfer+1 more|ArXiv.org|Sep 26, 2008

Particle Detector Development and Performance2 references3 citations

TL;DR

This paper presents a high-precision polarimetry system for the International Linear Collider (ILC), using Compton scattering with laser photons and Cherenkov detectors to measure electron and positron beam polarisation with a target precision of ΔP/P = 0.25% (improving to 0.1% with annihilation calibration). The method relies on measuring asymmetries in recoil electron energy spectra after switching laser polarisation, with photodetector linearity—particularly of SiPMs and PMTs—identified as the key limiting factor, addressed through rigorous non-linearity testing using pulse-length and mask-based methods.

ABSTRACT

The International Linear Collider (ILC) will collide polarised electrons and positrons at beam energies of 45.6 GeV to 250 GeV and optionally up to 500 GeV. To fully exploit the physics potential of this machine, not only the luminosity and beam energy have to be known precisely, but also the polarisation of the particles has to be measured with an unprecedented precision of dP/P ~ 0.25% for both beams. An overall concept of high precision polarisation measurements at high beam energies will be presented. The focus will be on the polarimeters (up- and downstream of the e+e- interaction point) embedded in the ILC beam delivery system. Some challenges concerning the design of the Compton spectrometers and the appropriate Cherenkov detectors for each polarimeter are discussed. Detailed studies of photodetectors and their readout electronics are presented focusing specifically 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.

Motivation & Objective

• To achieve unprecedented beam polarisation measurement precision of ΔP/P = 0.25% at the ILC, with potential improvement to 0.1% using annihilation data for absolute calibration.
• To address the dominant systematic uncertainty in polarimetry—non-linearity of photodetectors and readout electronics—by characterizing and minimizing it to the permille level.
• To evaluate and compare the performance of different photodetector technologies, including conventional PMTs, multianode PMTs (MAPM), and silicon photomultipliers (SiPMs), for use in ILC Cherenkov detectors.
• To develop and validate experimental methods for measuring differential and integral non-linearity (DNL, INL) of photodetectors and electronics with sub-0.1% precision.

Proposed method

• Utilizes two Compton polarimeters per beam (upstream and downstream of the interaction point) to enable redundancy and intercalibration.
• Employs a magnetic chicane to transform energy spectra of Compton-recoil electrons into spatial distributions for measurement.
• Uses a Cherenkov detector with a high-threshold medium (C4F10) or quartz fibers and SiPMs to detect Cherenkov light from relativistic electrons.
• Measures polarisation via asymmetry in electron spectra by switching the circular polarisation of the incident laser beam between +1 and -1.
• Employs a test facility with a blue LED (470 nm), function generator, and high-resolution 12-bit VME charge-to-digital converter (QDC) to characterize photodetector linearity.
• Applies multiple non-linearity measurement techniques: pulse-length variation, four-holed mask for differential non-linearity, and calibrated optical filters for attenuation.

Experimental results

Research questions

• RQ1 What level of photodetector and electronics non-linearity limits the precision of beam polarisation measurements at the ILC?
• RQ2 Can the pulse-length method detect non-linearities at the permille level (0.1%) in photodetectors used in ILC Cherenkov detectors?
• RQ3 How do different photodetector technologies—PMTs, MAPMs, and SiPMs—compare in linearity, magnetic field resilience, and suitability for ILC polarimetry?
• RQ4 To what extent can the differential and integral non-linearity (DNL, INL) of the QDC and photodetector system be measured and corrected to meet the 0.25% polarisation precision goal?
• RQ5 Can the combination of multiple measurement techniques (pulse-length, mask, filters) provide complementary and redundant validation of photodetector linearity?

Key findings

• The QDC used in the test setup exhibits good linearity within manufacturer specifications, with residual non-linearity expected to be correctable.
• The pulse-length method demonstrated sensitivity to non-linearities at the 0.05% level, confirming its suitability for detecting permille-level deviations.
• The differential non-linearity (DNL) and integral non-linearity (INL) of the QDC were measured over one billion samples, showing uniform code bin widths and low deviations.
• The pulse-length method was successfully applied to a 2×2 MAPM, yielding consistent results with reduced χ² values indicating good fit quality.
• Initial measurements on a 3×3 mm² SiPM show a well-resolved single photoelectron spectrum, with the most probable value (MPV) of photoelectron yield determined via a gaussian-smeared Landau fit.
• The use of optical filters for controlled light attenuation is being calibrated to the required precision for future non-linearity studies.