Design and Preparation Of Positron Source For Non-Destructiive Positron Annihilation Lifetime Spectroscopy

Positron annihilation lifetime (PAL) spectroscopy uses positrons to characterize void content in materials at the sub-nm scale. The time a positron survives in a defect corresponds to the free volume of the defect and its associated charge. Radioactive sodium (²²Na) salt prepared from aqua solution is mostly used as a positron source in laboratory PAL systems, which is sandwiched between two identical glass samples. Encapsulating the source between two layers of Kapton film allows successive use of the same source for nondestructive PAL measurements of different samples, reducing measurement uncertainties, cost of experimentation and raising reproducibility of the results. Quality of the obtained data and precision of further analysis depends significantly on the condition of the encapsulated positron source, which in turn, depends on the uniformity of ²²Na salt layer deposited on Kapton films. Our work reports our first results on the design of an apparatus for positron source preparation and encapsulation. Uniformity of the source layer is achieved by adjusting water evaporation rate and solution supply rate. In this way a positron sources of ~5 mm in diameter are obtained.