Biomedical subjects

J P McCaffrey

Publications and source records attributed to J P McCaffrey.

8 recordsLinked to original sources

PubMed2005

Pre-irradiation effects on ionization chambers used in radiation therapy.

Dosimetry protocols recommend that ionization chambers used in radiation therapy be pre-irradiated until they 'settle', i.e., until a stable reading is obtained. Previous reports have claimed that a lack of pre-irradiation could result in errors up to several per cent. Recently, data collected for a large number of commonly used ion chambers at the Institute for National Measurement Standards, NRC, Canada, have been collated and analysed, with additional data contributed by the National Physical Laboratory, UK. With this data set, it was possible to relate patterns of ion chamber behaviour to design parameters. While several mechanisms seem to contribute to this behaviour, the most obvious correlations implicate the type of insulator surrounding the central collector electrode, the extent of collector electrode shielding and possibly the area of the insulator exposed at the base of the active air volume. The results show that ion chambers with electrode connections guarded up to the active air volume settle quickly (approximately 9 min) and the change in response is small (less than approximately 0.2%). For ion chambers where the guard connection surrounding the central collector does not extend up to the active air volume, settling times of 15-20 min and an associated change in response of up to 1% are typical. For some models of ion chambers, the irradiation rate may also play a role in settling behaviour. Settling times for the ion chambers studied here were found to be independent of beam quality.

Artifacts↗

PubMed2004

Evidence for using Monte Carlo calculated wall attenuation and scatter correction factors for three styles of graphite-walled ion chamber.

The basic equation for establishing a 60Co air-kerma standard based on a cavity ionization chamber includes a wall correction term that corrects for the attenuation and scatter of photons in the chamber wall. For over a decade, the validity of the wall correction terms determined by extrapolation methods (K(w)K(cep)) has been strongly challenged by Monte Carlo (MC) calculation methods (K(wall)). Using the linear extrapolation method with experimental data, K(w)K(cep) was determined in this study for three different styles of primary-standard-grade graphite ionization chamber: cylindrical, spherical and plane-parallel. For measurements taken with the same 60Co source, the air-kerma rates for these three chambers, determined using extrapolated K(w)K(cep) values, differed by up to 2%. The MC code 'EGSnrc' was used to calculate the values of K(wall) for these three chambers. Use of the calculated K(wall) values gave air-kerma rates that agreed within 0.3%. The accuracy of this code was affirmed by its reliability in modelling the complex structure of the response curve obtained by rotation of the non-rotationally symmetric plane-parallel chamber. These results demonstrate that the linear extrapolation technique leads to errors in the determination of air-kerma.

Algorithms↗

PubMed2001

Role of ripening and defects in the formation of mesotaxial cobalt-disilicide layers.

Previous studies of the formation of buried silicide layers through mesotaxy, i.e., the introduction of the metal component by ion implantation with subsequent annealing of the structure, have shown that high-quality films can be achieved, but at processing temperatures too high to be useful for integration with silicon device technology. Recently, we proposed a modified ripening model to describe the early stages of the formation of a buried film which intentionally excluded the role of defects. In the current study, the predictions of that model are tested by varying sample preparation conditions. We show that the shift of the silicide precipitate layer is correlated with the narrowing of the cobalt profile, thus requiring both processes to be described by the same mechanism. We also show in a TEM study that specific defect morphologies are correlated with specific properties of the forming buried film, requiring an extension of the previous model.

Journal Article↗

PubMed2001

Surface damage formation during ion-beam thinning of samples for transmission electron microscopy.

All techniques employed in the preparation of samples for transmission electron microscopy (TEM) introduce or include artifacts that can degrade the images of the materials being studied. One significant cause of this image degradation is surface amorphization. The damaged top and bottom surface layers of TEM samples can obscure subtle detail, particularly at high magnification. Of the techniques typically used for TEM sample preparation of semiconducting materials, cleaving produces samples with the least surface amorphization, followed by low-angle ion milling, conventional ion milling, and focused ion beam (FIB) preparation. In this work, we present direct measurements of surface damage on silicon produced during TEM sample preparation utilizing these techniques. The thinnest damaged layer formed on a silicon surface was measured as 1.5 nm thick, while an optimized FIB sample preparation process results in the formation of a 22 nm thick damaged layer. Lattice images are obtainable from all samples.

Journal Article↗

PubMed1997

Preparation of cross-sectional TEM samples for low-angle ion milling.

The ideal ion-milled transmission electron microscope (TEM) sample would contain large, thin areas in selected regions, minimal top and bottom surface amorphization, and minimal preferential etching of adjacent materials. This desire has led to studies of these effects and improvements in designs and techniques for ion mills. Minimizing the incident angle of the ion beam to the surface of the sample has been found to be one of the most effective techniques for fulfilling these requirements. A new generation of ion mills has recently been designed to allow low-angle ion milling for the production of very high quality cross-sectional TEM samples. To take advantage of the low-angle ion milling capabilities and high-powered ion sources in these mills, new sample preparation techniques are required. In this paper, a simple technique of mounting material in a specialized titanium grid for cross-sectional TEM sample preparation is presented. This technique is designed to take full advantage of the powerful capabilities of these new low-angle ion mills.

Microscopy, Electron↗

PubMed1997

Application of the small-angle cleavage technique to thickness measurement of TEM samples.

In modelling transmission electron microscopy (TEM) images with the dynamical theory of electron diffraction, the sample thickness at the region of interest must be accurately known. A technique of sample preparation for cross-sectional single-crystal samples has been developed to provide this information. This technique, based on the small-angle cleavage technique, can allow a direct measurement of the sample thickness at the region where cross-sectional TEM analysis has been undertaken. Several cross-sectional samples of GaAs-based multiple quantum wells were prepared with parallel cleaved faces and thicknesses in the range of 50 to 200 nm. The samples were initially mounted so that cross-sectional images were obtained, then on suitable samples the mount was adjusted to obtain a plan view image of the same area. With suitable care, this technique provided an accurate thickness measurement of a cross-sectional region of a sample, which allowed subsequent modelling and analysis.

Microscopy, Electron↗

PubMed1995

A transmission electron microscope (TEM) calibration standard sample for all magnification, camera constant, and image/diffraction pattern rotation calibrations.

A calibration sample for transmission electron microscopy (TEM) has been developed that performs the three major instrument calibrations for a transmission electron microscope: the image magnification calibration for measurements of images, the camera constant calibration for indexing diffraction patterns, and the image/diffraction pattern rotation calibration for relating crystal directions to features in the image. This offers an improvement over commercially available calibration standards, where up to five different samples are required to perform these three calibrations. The new calibration sample consists of an electron-transparent cross-sectional TEM sample made from a molecular beam epitaxy (MBE)-grown, single-crystal semiconductor wafer. When the calibration structure is viewed in a TEM, it appears as a series of light and dark layers where the layer thicknesses are very accurately known. The calibrated thickness measurements of these light (silicon) and dark (SiGe alloy) layers are based on careful TEM measurements of the [111] lattice spacing of silicon which is visible on the calibration sample itself, and are supported by X-ray diffraction measurements. Furthermore, the layer thickness variation across the entire silicon wafer has been verified to be less than 1%, allowing all samples prepared from the same wafer to have errors in the given layer thickness values of less than 1%. As the sample is a single crystal of silicon, the calibrations requiring electron diffraction information such as the camera constant calibration and the image/diffraction pattern rotation calibration can also be performed easily and unambiguously. One single calibration sample can therefore be used to provide all three of the major TEM instrument calibrations at all magnifications and all camera lengths.

Calibration↗

PubMed1993

Improved TEM samples of semiconductors prepared by a small-angle cleavage technique.

A small-angle cleavage technique has been developed that produces superior transmission electron microscope (TEM) samples of semiconductors and related materials. The technique involves back-thinning the sample to approximately 100 microns, then scribing a groove on this back face at a specified small angle to a standard cleavage plane. The sample is cleaved along this scribe line followed by cleaving along the standard cleavage plane to produce a thin wedged sample. Samples prepared by this method are characterized and compared with conventional and low-angle ion milled samples. The technique is illustrated, and the characteristic geometry of the cleaved sample is explained in terms of a simplified cleavage model.

Microscopy, Electron↗