Recoil thin layer activation technique

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Wallace, G. 2000 Recoil thin layer activation technique. Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 2000/03 20 p.

Abstract: Thin layer activation (TLA) is a technique for labelling a surface with small amounts of radioactivity. Provided that the depth profile of this radioactivity is known, and that the decay is accompanied by gamma ray emission, measurement of the remaining gamma activity enables any losses of the surface to be detected. These losses can be by wear, erosion or corrosion. There have been many industrial applications of this technique as it provides a direct, on-line determination of surface loss and this can be measured through several centimeters of steel. The implanted TLA activity is created by nuclear reaction with substrate atoms that result from directing a sub-atomic particle beam into a surface. There are difficulties in doing this with plastic or composite surfaces because these substrates usually lack suitable target atoms. Furthermore, the surfaces are liable to damage due to heating and charging effect from the ion beam. In the work reported here, an alternative technique has been investigated. This uses a sacrificial foil in front of the surface that is to be activated. The nuclear reactions that generate the radioactive product occur in the foil by bombarding it with an energetic heavy ion beam. The foil is thin enough that these recoil products can exit through the foil, and be implanted into the backing substrate. As the reaction products recoil at some angle to the incident beam direction, the incident ions can be blocked from reaching the substrate. The recoil TLA technique has been investigated both theoretically and experimentally in this report. The reaction H(7Li, n)7Be which has a 53 day half-life. Good agreement was achieved between theory and experiment. However, the levels of activity that could be implanted are much lower than for direct TLA, with the consequence that measurement of implanted activity would take longer, and need more efficient detectors. While longer activation times can compensate, it was found that due to damage sustained by the sacrificial foil, it would be difficult to maintain constant implantation conditions. Finally, the contamination caused by generation of the 7Li ion beam impacted on some other accelerator applications. An alternative to the recoil TLA technique is proposed, based on direct implantation of a radioactive ion. (auth)