Influence of surface conditions on thermal positron reemission spectra from W(100)
Our measurements trace the temporal dependence of positron reemission spectral features, namely, the reemission intensity, energy and angular distributions with post-cleaning time and oxygen exposure. The unwanted inelastic component in the reemission spectra can be kept at less than 6 % of the total reemitted positrons during long and continuous operation by simply ensuring the cleanliness of the sample with better vacuum level. Simultaneously the other optimized spectral features of the beam can be maintained except for the reemission spectral intensity which decreased with increasing time after cleaning. Even though oxygen at high temperature is used normally in the cleaning process of the tungsten moderators, exposure of the sample to oxygen at room temperature caused an exponential reduction in the reemission yield with exposure time.