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Date of Award

3-1996

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Engineering Physics

Supervisor

Professor D.A. Thompson

Abstract

Trapping of 30 keV ion-implanted helium by the radiation-produced damage in hot isostatic pressed beryllium foils of 99.5 wt% purity has been investigated by a series of ion implantation/thermal desorption experiments. Different experimental regimes were designed to obtain some fundamental insight into the behaviour of helium in beryllium. The helium release was related to the surface morphology changes observed on the desorbed surfaces by scanning eIectron microscopy.

The nature of helium trapping in beryllium has been found to strongly depend on the implantation parameters as well as on the thermal treatment of the implanted samples. Desorption peaks have been analyzed in terms of the dissociation of simple helium-vacancy trapping centres and/or helium release from microbubbles that nucleate and grow during the annealing of the implanted samples.

Linear-ramp annealing following room-temperature 30 keV He⁺ implantation in beryllium to a total fluence in the range of 10²⁰ to 10²¹ /m² has produced two desorption stages above 890 K and below 830 K, respectively. The high temperature desorption peak was analyzed in terms of a first-order dissociation mechanism with an activation energy that depends on the relative occupation of the trapping site. The low temperature peak corresponds to a higher-order helium-vacancy cluster that begins to fill once the deeper trapping site approaches saturation. A third trapping site, with higher dissociation energy, has been inferred from the fact that a sizeable fraction of the implanted helium has not been released after heating up to 75% of the melting temperature.

Some samples have been heated by a stepped anneal regime, for two hours at 573 or 773 K, that allows reconfiguration of the trapped helium before being desorbed. The release spectra and the desorbed surface morphology show that the nature of helium trapping has changed to a more stable form of helium bubbles. A model is proposed to account for the helium bubble nucleation and growth by a migration and coalescence mechanism. Linearly ramped thermal desorption after high temperature implantations at 600 and 773 K reveals the formation of more stable trapping sites. The helium release has been related to the formation of holes on the surface and the desorption was inferred to result from microbubble growth by thermal vacancy assisted mechanism.

Some of the samples have been deliberately corroded to investigate the effect of the surface contamination on the helium release. The desorption curves show the formation of an additional broad desorption peak at temperatures higher than 950 K. This is associated with a relative drop in the population of the primary and secondary desorption peaks and implies that the corroded surface contains additional trapping sites that delay the release of the helium detrapped from the bulk.

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