# Instrumentation for Colliding Beam Physics (INSTR-17)

from 27 February 2017 to 3 March 2017
Budker Institute of Nuclear Physics
Asia/Novosibirsk timezone
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# Contribution Poster

Budker Institute of Nuclear Physics
Colliders and detector integration

# Laser polarimeter at VEPP-4M

## Speakers

• Dr. Ivan NIKOLAEV

## Content

A new high precision measurement of the masses of $Upsilon$ mesons with KEDR detector at VEPP-4M collider requires beam energy calibration. The most accurate method of beam energy calibration is resonant depolarization technique. It is based on measurement of spin precession frequency which is connected to Lorentz factor of the beam and well known normal and anomalous part of magnetic moment of electron due to Thomas precession. Spin precession frequency is determined from frequency of resonant destruction of beam polarization. Intra-beam (Touschek) scattering is used for polarization measurement at the energies below $E=2.0$GeV. At higher energies $E=5$GeV around $Upsilon$ production Touschek intensity is rather small. This report describes a new polarization measurement system developing at VEPP-4M. It is based on up-down asymmetry of Compton backscattering of circular polarized photons on vertical polarized electrons. A pulse 527nm laser with 0.3W average power is used. Polarization is controlled by Pockels cell or by rotated $lambda/4$ phase plate. Scattered photons are registered by two-coordinate GEM based detector based with 12mm lead photon converter.

## Summary

First polarization measurements with the laser polarimeter were done in 2016 at $E=4.1~GeV$. Scattered photon count rate is about 600 Hz/mA. Measured polarization time due to Sokolov-Ternov radiative polarization is 36 minutes and polarization degree is $41pm4$%. The system requires a mirror quality and focusing improvement. A new GEM detector is designed and will be created. We expect the count rate to be about 16kHz for the 10mA beam current. It allows us to measure beam energy with an accuracy of $10^{-6}$.