An application of a Si/CdTe Compton camera for the polarization measurement of hard x-rays from highly charged heavy ions

TL;DR

This paper introduces a novel Compton camera-based polarimeter, EBIT-CC, capable of highly accurate polarization measurements of x-rays from highly charged heavy ions, enabling advanced tests of quantum electrodynamics effects.

Contribution

The development and experimental validation of the EBIT-CC, a new pixelated silicon and CdTe Compton camera for precise x-ray polarization measurement from heavy ions.

Findings

Achieved polarization degree measurement with an absolute uncertainty of 0.02.

Calibrated the Compton camera for 75 keV x-rays.

Demonstrated capability to distinguish Breit interaction effects.

Abstract

The methods to measure the polarization of the x-rays from highly charged heavy ions with a significantly higher accuracy than the existing technology is needed to explore relativistic and quantum electrodynamics (QED) effects including the Breit interaction. We developed the Electron Beam Ion Trap Compton Camera (EBIT-CC), a new Compton polarimeter with pixelated multi-layer silicon and cadmium telluride counters. The EBIT-CC detects the three-dimensional position of Compton scattering and photoelectric absorption, and thus the degree of polarization of incoming x-rays can be evaluated. We attached the EBIT-CC on the Tokyo Electron Beam Ion Trap (Tokyo-EBIT) in the University of Electro-Communications. An experiment was performed to evaluate its polarimetric capability through an observation of radiative recombination x-rays emitted from highly charged krypton ions, which were generated by the Tokyo-EBIT. The Compton camera of the EBIT-CC was calibrated for the 75 keV x-rays. We developed event reconstruction and selection procedures and applied them to every registered event. As a result, we successfully obtained the polarization degree with an absolute uncertainty of 0.02. This uncertainty is small enough to probe the difference between the zero-frequency approximation and full-frequency-dependent calculation for the Breit interaction, which is expected for dielectronic recombination x-rays of highly charged heavy ions.