Production of miniaturized 72Se/72As positron generators for applications in materials science

Position lifetime measurements using β+-γ-coincidence is a promising tool for non-destructive testing of materials [1]. Mobile equipment for the application of this technique requires miniaturised collimated positron sources emitting positrons of high energy, such as those emitted by 72As (2.5 and 3.3 MeV) in order to probe bulk properties of the material. The present paper describes the production of miniaturised 72Se/72As positron generators fulfilling these requirements. 72Se was produced by the reaction 70Ge(α,2n)72Se by irradiating a Ge disc with 38 MeV α-particles. The positron generator was then produced by a vapour deposition process. By milting the irradiated Ge disc in a closed graphite crucible with a fine nozzle in the top cover a Se vapour jet was produced that allowed the deposition of Se vapour inside a cylindrical bore of 0.6 mm diameter in a gold cylinder of 2 mm outer diameter. With preliminary process parameters 72Se/72As positron generators of 0.6 mm diameter and about 0.5 to 1 MBq positron activity have been produced in 1 hour of vapour deposition. About 20% of the 72Se activity produced in the Ge was recovered. The process could be further optimised if required. (author)