E.J. Morton

An X-ray micro-tomography system optimised for the low-dose study of living organisms

An X-ray micro-tomography system has been designed that is dedicated to the low-dose imaging of radiation sensitive living organisms and has been used to image the early development of the first few days of plant development immediately after germination. The system is based on third-generation X-ray micro-tomography system and consists of an X-ray tube, two-dimensional X-ray detector and a mechanical sample manipulation stage. The X-ray source is a 50kVp X-ray tube with a silver target with a filter to centre the X-ray spectrum on 22keV.A 100mm diameter X-ray image intensifier (XRII) is used to collect the two-dimensional projection images. The rotation tomography table incorporates a linear translation mechanism to eliminate ring artefact that is commonly associated with third-generation tomography systems. Developing maize seeds (Triticum aestivum) have been imaged using the system with a cubic voxel linear dimension of 100 microm, over a diameter of 25mm and the root lengths and volumes measured. The X-ray dose to the plants was also assessed and found to have no effect on the plant root development.

Infra-red microscopy of Cd(Zn)Te radiation detectors revealing their internal electric field structure under bias

This paper describes the use of the Pockels electro-optic effect to both visualise and measure the internal electric field present within CdZnTe detectors under normal bias conditions. Use was made of a silicon charge coupled device (CCD), as an image sensor, in conjunction with a 940 nm LED to illuminate a 3/spl times/3/spl times/5 mm detector crystal with polarised infra-red light. The experiment was arranged so that a /spl times/40 image of the crystal showing a contour plot of electric field intensity could be generated. Further, more precise E-field measurements, were obtained using a 1.2 /spl mu/m laser diode (LD) and a germanium photodiode detector. Together these experiments indicate substantially non-uniform electric fields within the detector crystals used.

Development of a high speed X-ray tomography system for multiphase flow imaging

This paper describes a new system for quantitative measurement of multiphase flow. It is based on the use of high speed (50 frames per second) X-ray tomographic imaging. This requires a new X-ray tube concept to be implemented, together with an annular detector array and data acquisition system. A pressurised krypton microgap detector is being developed to give 90% quantum efficiency for detection of K/sub /spl alpha// radiation from the silver anode X-ray tube. Results from simulation work are presented to justify the overall design strategy selected.

Energy dispersive X-ray scatter for measurement of oil/water ratios

Abstract A technique for measuring oil and water concentrations in a bulk liquid using energy-dispersive X-ray scatter will be discussed. It is a requirement of the oil industry to measure accurately on-line the oil/water ratio of fluids extracted from oil fields across a wide range of gas–liquid flow regimes. To this end, a low cost, robust system is being developed using the energy dependence of coherent scatter as a probe to measure oil/water ratios. The scatter profiles are inherently broad and thus the relaxed energy resolution requirements allow the use of CdZnTe detectors suitable for field deployment. Energy-dispersive diffraction spectra of a range of oil–water emulsions have been measured. The ratio of the scattering into two energy windows, where the diffraction spectrum from oil is maximised and minimised with respect to that of water, is shown to be dependent upon the oil/water mixture ratios. The measurement is made independent of gas volume flow rate by normalisation of the scattered data to the transmission data. A relative error of 0.6% in the oil/water ratio measurement at 80% water fraction, and independent of gas fraction, is obtained.

Dose mapping of inhomogeneities positioned in radiosensitive polymer gels

Abstract A gel matrix (gelatin), infused with acrylamide monomer and a crosslinking agent has been used as a three-dimensional dosimeter of X- and γ-rays are used in radiotherapy. Magnetic resonance imaging is used to create a parameter map of transverse relaxation time (T2) from which a map of absorbed dose is calculated. We are using the technique to investigate the interfacial radiation patterns resulting from the presence of implanted inhomogeneities in the gel phantom being irradiated, a concept which is as yet underdeveloped. The inhomogeneities, which may be either low or high density with respect to the surrounding medium, are designed to simulate the effects of air cavities or bone in the body and their effects on the absorbed dose during radiotherapy. This paper explores ways of introducing inhomogeneities into the gel and presents results of dose maps obtained post irradiation. An inherent problem of the dosimeter is the inhibition of polymerisation due to dissolved oxygen. Free radicals produced by the incident radiation are key to the polymerisation and crosslinking effects but are consumed by the dissolved oxygen. This paper describes the observed effects of oxygen contamination within a gel and suggests steps to be taken to minimise the problem.

X-ray tomographic imaging in industrial process control

Abstract Optimisation is essential in modern industrial and chemical process industries to increase efficiency and decrease downtime and maintenance costs. X-ray tomography is being developed to address these issues both on the microscopic level to characterise and quantify unit cell parameters and the macroscopic level for real time measurement of multiphase flow. The behaviour of macroscopic flow can be predicted using finite element simulations. Three-dimensional X-ray micro-tomography with 100 micron resolution has been developed and could be used to determine, for example volume fractions, contact surface area and particle size distribution and used as basic data for modelling of macroscopic systems. Additionally, a high speed X-ray tomography instrument is being developed to measure on-line multiphase flow in fast moving systems. This system does not require moving parts and is expected to operate at up to 50 frames per second. In addition to a practical implementation of this system in an industrial environment it can also be used as a cross-validation of the macroscopic models. Details of each system will be described and the suitability of the applications discussed.

Measuring the bulk resistivity of CdZnTe single crystal detectors using a contactless alternating electric field method

Abstract It is of some importance to know the bulk resistivity of semiconductor materials. Conventionally, measurements of bulk resistivity require the use of electrical contact directly to the sample itself. This raises questions related to the type of contact so formed. To address this issue, a contactless method of measurement has been devised, based on the dielectric heating produced by inducing an alternating current in the dielectric of a capacitor. This effect has been demonstrated to be both feasible and accurate, resulting in the measurement of a bulk resistivity of 4.5 × 10 10 Ω cm for small samples of CdZnTe.

Photo-electronic investigation of CdZnTe spectral detectors

Using a focused and pulsed beam of 850-nm laser radiation, it is possible to assess a number of bulk properties of CdZnTe. The subbandgap pulsed radiation (pulse width 80 ns) excites a detectable signal from defects near the band edges in CdZnTe. This is detected using a conventional charge sensitive preamplifier. Further information is obtained by observing the transmitted optical beam simultaneously with the detected photo-induced current from the CdZnTe detector. The intensity of the laser beam is adjusted such that an average of 152 keV is deposited in the CdZnTe detector per optical pulse. Using optical polarizers, the electric field distribution has been measured without the space-charge-induced distortion of the electric field normally experienced when using higher intensity dc light sources. These results have been correlated with charge collection efficiency measurements. By varying the pulse repetition frequency (f/sub p/), it may be possible to obtain information about the lifetime of shallow defects at room temperature.

Analysis of the pulse shapes obtained from single crystal Cd0.9Zn0.1Te radiation detectors

Abstract Over the past thirty years there has been considerable research into the properties of CdTe semiconductor radiation detectors. Recently, the quality of CdTe detectors has improved (notably by the substitution of around 10% of the cadmium by zinc) to the point where high quality room temperature gamma-ray spectroscopy is possible using devices of volume 1 cm 3 or greater. We are currently investigating the variation in pulse shape resulting from γ-ray interactions in CdZnTe detectors supplied by eV Products. The impact of these variations is assessed in terms of its effect on the resolution in the pulse height spectrum. Parameters studied for individual pulses include the shaping amplifier amplitude and slew rate of the preamplifier output pulse. These signals are digitised using an oscilloscope controlled by a PC using IEEE-488 data transfers. Three-dimensional plots of this data are used to observe the distribution of detector outputs. Results are shown for 3 mm × 3 mm × 2 mm thick Cd 0.9 Zn 0.1 . Te crystals which indicate that significant improvement in peak to background ratio can be obtained by selective histogramming of the main amplifier output pulses.

Design considerations for transmission x-ray tubes operated at diagnostic energies

A low-energy expansion of the EGS4 Monte Carlo code system was used in studies that aimed to understand photon production in transmission target x-ray tubes. In each case, foil targets were modelled with incident electrons of a few tens of keV. The production of bremsstrahlung and fluorescence radiation at different depths was studied and the angular distribution of radiation analysed. The characteristics of x-ray spectra measured along the beam path, for targets of different materials, thicknesses and several beam energies have been determined and the relation between efficiency and the fluorescence-to-bremsstrahlung ratio is shown.