Study of Doppler broadening of positrons annihilation radiation in (Smsub(0.8)Gdsub(0.2))S across valence transition

Doppler broadening of positron annihilation radiation was measured as a function of temperature in (Smsub(0.8)Gdsub(0.2))S. A hyperpure Ge spectrometer system having a resolution, FWHM = 0.97 keV for the 514 keV gamma ray of 85Sr has been used for the measurements. The assembly of the 22Na source and sample was mounted on the cold-finger of a variable temperature cryostat. Measurements were made at various temperatures from 71 K to 300 K. The spectra were analyzed to extract a line-shape parameter S, which is defined as the ratio of the total counts under a small segment of the photo-peak to the sum of counts under a segment on either wing of the spectrum. (author)