Measurements of the transverse polarisation of K-shell photoelectrons produced by unpolarised 334 and 439 keV photons in thorium

Abstract

An experimental investigation of the transverse polarisation normal to the production plane of K-shell photoelectrons from a thin thorium (Z=90) converter produced by unpolarised 334 and 439 keV gamma rays has been carried out. Neutron capture gamma rays from an in-pile target enriched in 149Sm provided an intense source of photons. The photoelectrons were analysed in momentum with a high resolution, iron beta spectrometer which viewed the combined target plus converter situated near the core of the high flux reactor at the Institut Laue-Langevin. The electrons were detected after being scattered by a thin Bi foil in four silicon surface barrier detectors. The polarisation was deduced from the left-right asymmetry in Mott scattering from the Bi foil at backward angles. The left-right asymmetry in the two forward detectors allowed correction for spurious and instrumental asymmetries. The measurements were carried out at a mean angle of 45 degrees between the electron and photon momenta with the results: Egamma =334 keV, C02=0.074+or-0.018; Egamma =439 keV, C02=0.070+or-0.046.