Results on the Coherent Interaction of High Energy Electrons and Photons in Oriented Single Crystals

TL;DR

This study investigates high-energy electron and photon interactions with oriented single crystals, demonstrating new polarimetry techniques and observing birefringence effects in photon beams at CERN.

Contribution

It introduces a novel crystal polarimetry method based on coherent pair production and reports the first observation of birefringence in high-energy photon beams using oriented crystals.

Findings

Successful generation of multi-GeV linearly polarised photon beams.

Implementation of a new crystal polarimetry technique for photon polarisation measurement.

Observation of birefringence, converting linear to circular polarisation in high-energy photon beams.

Abstract

The CERN-NA-59 experiment examined a wide range of electromagnetic processes for multi-GeV electrons and photons interacting with oriented single crystals. The various types of crystals and their orientations were used for producing photon beams and for converting and measuring their polarisation. The radiation emitted by 178 GeV unpolarised electrons incident on a 1.5 cm thick Si crystal oriented in the Coherent Bremsstrahlung (CB) and the String-of-Strings (SOS) modes was used to obtain multi-GeV linearly polarised photon beams. A new crystal polarimetry technique was established for measuring the linear polarisation of the photon beam. The polarimeter is based on the dependence of the Coherent Pair Production (CPP) cross section in oriented single crystals on the direction of the photon polarisation with respect to the crystal plane. Both a 1 mm thick single crystal of Germanium and a 4 mm thick multi-tile set of synthetic Diamond crystals were used as analyzers of the linear polarisation. A birefringence phenomenon, the conversion of the linear polarisation of the photon beam into circular polarisation, was observed. This was achieved by letting the linearly polarised photon beam pass through a 10 cm thick Silicon single crystal that acted as a "quarter wave plate" (QWP) as suggested by N. Cabibbo et al.