Positron Annihilation lifetime studies of inorganic materials using a new positron beamline

Abstract

In recent years, effects caused by the nanoscale structure of a material have become increasingly important for technological progress. With improvements in fabrication techniques, nanoscale structures can now be produced with growing control. As a result, techniques to characterise these properties have been developed and are also becoming increasingly important. One such collection of non-destructive techniques, which can provide unique information about a target sample, is Positron Annihilation Spectroscopy (PAS). The initial aim of the research presented here was the development and characterisation of a positron beamline for materials studies. This beamline, now operational, has been developed to measure the positron lifetime in a target material. This technique, referred to as Positron Annihilation Lifetime Spectroscopy (PALS), allows the size of open volume type defects (or voids) up to ~10 nm to be determined. In addition, as the technique is implemented using a variable energy positron beam, thin film samples can be characterised and measurements can be performed in a depth sensitive manner. The beamline uses a Surko trap to isolate and cool positrons from a moderated radioactive source. Positrons are released from the trap using a timed potential ramp, though hardware restrictions limit the effectiveness of this technique. This arrangement results in a pulsed positron beam of up to two thousand pulses per second with a temporal Full Width at Half Maximum (FWHM) of 800 ps. Typica l spectra, which contain ~1 000 000 counts, can be recorded in two hours; although to resolve shorter lifetime components (<600 ps) longer measurement times are required. Due to consequences from the unusual operating regime of the Surko trap, a new analysis program was developed for the analysis of the resulting PALS spectra. This software, dubbed CamsFit, is specifically designed to account for the temporal distribution of positron pulses in beam based systems. This is achieved by analysing the PALS spectrum of the target material relative to the spectrum of a well characterised sample which contains only very short positron lifetimes. This analysis technique allows lifetimes greater than ~300 ps to be resolved when using pulses of 800 ps. With these tools, two studies of inorganic materials have been performed. The first study focused on investigating nanometre sized voids in natural zircon crystals. Such voids are the result of self-irradiation from radioactive substitutional impurities such as uranium. Samples which had suffered a range of radiation doses were investigated and such voids were observed, though not universally. The second study focused on characterising zeolite sheets prepared using a papermaking technique. Several sheets were prepared and the concentrations of the starting reagents were varied. While only a preliminary study, it has been found that it may be possible to tailor the properties of the sheets to specific applications.