Volatile Evolution from Polymer Materials Induced by Irradiation with Accelerated He ++ Ions
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Volatile Evolution from Polymer Materials Induced by Irradiation with Accelerated He++ Ions Julian.J. Murphy† and Christopher.J. Wetteland † AWE, Aldermaston, Reading, Berkshire, RG7 4PR, UK. Ion Beam Materials Laboratory, Los Alamos National Laboratory, NM 87545 U.S.A. ABSTRACT Experimentally investigating ageing caused by irradiation with energetic particles is very difficult. Radioactive sources can be employed but these are difficult to handle and contaminate the material being irradiated precluding subsequent chemical and physical characterisation. The penetration of energetic particles also tends to be small so any change is localised in the near surface region so only a small amount of material is irradiated. Analysing changes in such thin layers causes a number of problems. To simulate ageing induced by particle radiation polymer samples have been exposed to fast He++ ions in an accelerated ion beam. The ions pass through a 10µm thick window of Havar foil before impacting upon the sample. Volatile species evolved from the materials upon bombardment are contained within the irradiation chamber by the foil window. Analysis of such species is shown to be a highly sensitive probe for investigating chemical changes in the exposed materials. A number of important chemical changes induced in polymer materials have been identified. Trends in the relative rates of volatile evolution have been identified which correlate with chemical changes identified in other radiation experiments. As these experiments are performed at far slower irradiation rates the large acceleration factors used in ion beam irradiation are discussed along with the implications for using ion beams to simulate alpha particle irradiation. INTRODUCTION Performing investigations into long term ageing effects caused by exposure to high doses of alpha radiation is very difficult from an experimental perspective. Irradiating a material with large radiation doses is extremely difficult in a short time scale. Furthermore, alpha radiation effects are localised near the surface, so only a small amount of material is actually exposed. The exposed portion of material in any sample will therefore only constitute a small percentage of the whole. The unexposed material dominates measurements performed upon such samples making identification of changes brought about as a consequence of exposure to alpha radiation extremely difficult. Accelerated ion beams allow a stream of He++ ions to be implanted into the surface of a material. The energy of the He++ ions can be controlled allowing the major components of α radiation from chosen sources to be simulated. The vastly increased dose rates available in such a procedure allows exposure to high doses to be performed in a limited time. The energy of the He++ particles can also be increased so that they have enough to pass through a metal foil window before impacting on the sample. An extremely strong foil can act as an interface between a chamber housing the irradiated sample and the vacuum chamber of the ion
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