Detailed three-dimensional size and shape characterisation of UHMWPE wear debris

Atomic force microscopy (AFM) is used to characterise UHMWPE wear debris from a knee prosthesis actuated under constant load. The size and shape of debris particles is quantified in all three spatial dimensions. Artificially limiting the analysis to the two-dimensional projections of the particles onto the substrate plane, it has been found that equivalent shape ratio (ESR) plotted as a function of equivalent circle diameter (ECD) follows a trend observed before. Inclusion of the third, vertical spatial dimension of particle height shows that such two-dimensional analysis, as it is often based on SEM images, can greatly misrepresent the actual particle shape. The three-dimensional AFM information indicates that for the prosthesis and the conditions studied here debris particles tend to be deformed independent of their volumetric size. Fractionation of debris particles according to size was achieved with a new filtration protocol. It is demonstrated that with this protocol debris particles settle uniformly across the filter and that particle abundance as function of size can thus reliable be established

26 Al measurements with 10 Be counting statistics

The potential of the 26Al isotope for exposure and burial dating applications has not been fully realized to date due to the low efficiency for producing the Al- ion in negative ion sources. Here, we show that it is possible to use the AlO- ion instead, which is produced an order of magnitude more efficiently, provided that a gas-filled magnet is employed to reduce the intense flux of 26Mg ions at the detector. It is then possible to achieve comparable counting statistics for 26Al as for the much more widely used 10Be

Exploration of the size, shape and abundance of UHMWPE wear particles using atomic force microscopy

Atomic force microscopy is proposed and explored as a technique for the characterisation of ultra-high molecular weight polyethylene (UHMWPE) wear particles from knee prostheses. This new approach may be an alternative to other techniques such as filtration and scanning electron microscopy. Atomic force microscopy can measure the size and geometry of polyethylene wear particles extracted from the joint lubricant with a precision on the nanometre-scale. This can further the understanding of wear processes at this length-scale. However, information about the absolute and the fractionated abundances of the particles in the lubricant is lost, since the particles precipitate non-uniformly on substrates suitable for atomic force microscopy. It is demonstrated that conventional filtration of polyethylene particles can cause similar non-uniform precipitation on a filter medium. This result gives support to the view that uniformity across a substrate or filter medium needs to be established in detail and quantitatively using an appropriate experimental protocol so that abundance distributions may be obtained reliably

Nucleogenic 36 Cl, 236 U and 239 Pu in uranium ores

The nucleogenic isotopes 36Cl, 236U and 239Pu are produced naturally in subsurface environments via neutron capture of thermal and epithermal neutrons. Concentrations are, however, very low and accelerator mass spectrometry (AMS) is required for quantitative measurements. A particular challenge is presented by the measurement of 236U/238U ratios down to the level of 10-13 that is expected from rocks with low uranium concentration. Here, we present the AMS methodology that has been developed at the ANU for measuring 236U/238U ratios at this level. The more established methodologies for 36Cl and 239Pu measurements are also summarised. These capabilities are then used to characterize the 36Cl, 236U and 239Pu concentrations in a range of uranium ores. A simple model of the neutron production and capture processes in subsurface environments has been developed and is presented. It is shown that nucleogenic 36Cl, 236U and 239Pu can be used to determine both thermal and epithermal neutron fluxes in subsurface environments. Potential applications include uranium exploration and monitoring of the environmental impact of uranium mining

Demonstration of a new technique using radioisotope tracers to measure the backside wear rate on tibial inserts

Local backside wear measurements on ultrahigh molecular weight polyethylene (UHMWPE) tibial inserts in LCS mobile bearing knee prostheses have been performed using a new radioisotope tracing technique. The radioisotope tracers97Ru and101mRh were synthesized via a fusion evaporation reaction and recoilimplanted into cylindrical plugs of UHMWPE. The labelled plugs were carefully fitted into tibial inserts at two relevant locations. With bovine serum acting as a lubricant, the tibial inserts were then worn in vitro for 500,000 and 780,000 cycles, respectively, in a pneumatic knee motion simulator. Results reflect the non-linear change of wear during the wear-in phase and its evolution to a long-term steady-state rate. This new technique shows potential for extracting localized wear rates across the backside of a tibial insert in order to develop a comprehensive backside wear model

Exploration of activity measurements and equilibrium checks for sediment dating using thick-window Germanium detectors

Activity measurements on sediment samples for trapped-charge geological dating using gamma-ray spectroscopy are an important verification of the field-site dose rate determination. Furthermore gamma-ray spectroscopy can check if the natural decay series are in secular equilibrium which is a crucial assumption in such dating. Typically the activities of leading members of the Thorium and Uranium decay series are measured, which requires Germanium detectors with thin windows and good energy resolution in order to effectively detect the associated low energy gamma-rays. Such equipment is not always readily available. The potential of conventional Germanium detectors with thick entrance window has been explored towards routine gamma-ray spectroscopy of sediment samples using higher energy gamma-rays. Alternative isotopes, such as Ac-228 and Pb-212 for the Thorium series, and Pa-234m, Ra-226 and Bi-214 for the Uranium series, have been measured in order to determine the mass-specific activity for the respective series and possibly provide a check of secular equilibrium. In addition to measurements of the K-40 activity, with the alternative approach, the activities of both decay series can be accurately determined. The secular equilibrium condition may be tested for the Thorium series. Measurement accuracy for Pa-234m is, however, not sufficient to permit also a reliable check of equilibrium for the Uranium series