With the increase in nuclear power, it may be necessary to contain for 100–200 years the radioactive 85 krypton released during fuel reprocessing. The ideal method of immobilizing radioactive materials is in the form of stable, monolithic solids which are resistant to the effects of the environment or accidents, and which can retain the radioactivity under all envisaged conditions. Since krypton is a rare gas, not forming thermally stable compounds, conventional methods of storage are not possible. A process is being developed to immobilize the krypton in the form of small gas bubbles in a metal matrix by implanting the gas into a metal layer from a glow discharge and then burying the implanted layer by sputter deposition. By repeating the process, a thick layer of deposit is built up with the krypton dispersed throughout the matrix as bubbles of diameter less than 20 Å. This process offers an ideal form of storage since gas in bubbles is not thermally released until the temperature of the matrix is close to the melting point, and also leakage of gas by corrosion or mechanical damage will be small. A pilot plant is being built in order to demonstrate the process on a scale comparable with that required for a reprocessing plant. The efficiency of the process is dependent upon the amount of gas which can be implanted at low energy into a thin layer and its subsequent retention. More information is required on the processes occuring when krypton ions are implanted close to the surface, in particular, the retention and re-emission of the gas, and the formation of clusters and bubbles.
A comparison has been made of the lifetimes under irradiation of carbon stripper foils prepared by cracking hydrocarbon vapour in a glow discharge with those of foils made by the carbon arc process. Greatly enhanced lifetimes have been observed.
The behaviour of carbon stripper foils under heavy ion bombardment has become the subject of extensive study. It is well known that for tandem electrostatic accelerators the yield of high energy heavy ions is greater for solid than gas strippers. The disadvantage of stripper foils is that their lifetime can be short.
The knowledge that the lifetime of carbon stripper foils may be limited by radiation induced shrinkage has been exploited to produce carbon foils with a significantly increased lifetime against rupture.
Irradiation lifetimes of carbon stripper foils made by cracking ethylene in a DC glow discharge are reported. A beam of 1.2 MeV Ar+ ions was used and lifetimes were compared with those for foils made by the carbon arc process. The thicker ethylene foils (15–30 μg cm−2) lasted more than 20 times longer than the carbon arc foils while the thinner ones (5–10 μg cm−2) lasted 9 times longer. A further factor of 1.8 improvement was obtained for thicker foils by mechanically slackening them before irradiation. Possible reasons for the differences in lifetime are discussed.
Carbon stripper foils have been prepared by a variety of methods and tested in a special clean irradiation facility with 4.8 MeV Ar ions. The physical properties of the foils have been studied. The effects of irradiation induced shrinkage have been partially overcome by the use of a special mounting technique which has resulted in a lifetime ten times greater than that obtained with conventional carbon arc foils. Foils prepared by the cracking of hydrocarbon vapour in a glow discharge have greatly reduced shrinkage rate and preliminary measurements have indicated that the lifetime may be extended by a factor of at least twenty five. Finally an attempt is made to understand the mechanism of foil shrinkage and eventual rupture under heavy ion bombardment.
Preliminary measurements have shown that the lifetime of carbon stripper foils prepared by the cracking of hydrocarbon vapour in a glow discharge is greater than for conventional carbon arc evaporated carbon foils by a factor of at least 25.
The knowledge that the lifetime of carbon stripper foils may be limited by radiation induced shrinkage has been exploited to produce carbon foils with a significantly increased lifetime against rupture.
A new technique of measuring the range of helium ions implanted at low concentrations into nickel is described. The residual amount of helium in a foil is measured by a combination of gas release and selective vibro-polishing. The method, illustrated by the results of some preliminary experiments, is sufficiently sensitive to enable the range, to be measured with peak implanted gas concentrations of the order of 10−7 atoms per nickel atom (i.e. a sensitivity limit of 10−8 atom/atom) where bubble formation is unlikely. The mean projected range of 100 keV helium ions in nickel was found to be 2800 Å, together with a second peak at 5250 Å which is thought to be due to channelling. A significant fraction of the helium diffused interstitially to depths greater than 1 μm.
The lifetime of carbon stripper foils when bombarded whith 4.8 MeV A ++ ions in a hydrocarbon-free vacuum environment has been investigated. The lifetime of the foils is limited not by thickening but by shrinkage and rupture, and can be improved by a factor of 3 by irradiating at a temperature of 600 °C. Thinner foils (nominally half thickness) produced on a cellulose backing had a shorter life, but since the transmitted beam is greater, they show an improvement of 30 %. Further improvements in foil lifetime can only be achieved by reduction of the shrinkage rate, or by improvements to the resistance of the foils to shrinkage.
Hard surface layers with a Vickers hardness of greater than 2000 have been deposited on titanium alloys, Nimonic, high tensile steel and stainless steels by cracking ethylene in a glow discharge. Thin deposits were extremely tenacious, but films thicker than 3–5 μm tended to blister and peel away from the substrate.
A JEM 200A electron microscope has been linked to a 120 kV heavy-ion accelerator in order to observe the dynamic effects of heavy-ion bombardment of materials. The ions, produced in a sputtering ion source, are accelerated, analysed, directed through a flight line and then deflected to strike the specimen in the electron microscope. The effects of the ion bombardment on the specimen are recorded on videotape as they occur. Details of the system and examples of its use are presented.
Abstract The release of helium from nickel, 316 and 18/8 stainless steels and nimonic PE16 foils uniformly filled with helium to concentrations of the order of 10−5 atom/atom has been measured during linear rise anneals and compared with the release from nickel foils implanted with low energy helium ions. Annealing peaks have been observed and attributed to radiation damage annealing, helium diffusion to form gas bubble nuclei, and gas release by bubble migration. The fraction of helium released has been found to be concentration dependent and to be much smaller from stainless steels than from nickel. This is attributed to increased trapping. The activation energy for diffusion of helium in nickel was found to be ∼ 2.50 eV. In nickel annealed at 600°C all of the helium is incorporated in bubbles, which are predicted to be approximately 20 A, diameter. It is concluded that voids formed by ion bombardment to high doses at elevated temperatures in helium filled foils are nucleated on these bubbles.
The deposition of thin films of iridium on molybdenum has been studied by field-ion microscopy. When the substrate was held at low temperatures during and after deposition, a localised thin deposit coherently bound to the substrate was formed. At higher substrate temperatures, diffusion of the iridum took place, and thicker films were grown epitaxially on the substrate with (111) Ir ∥ (110) Mo and [101̄] Ir ∥ [11̄1] Mo. Twin crystals caused by multiple positioning, and stacking faults, were found. There was no evidence of alloying by surface diffusion.
