W.B. Gilboy

Tissue-equivalent gel for non-invasive spatial radiation dose measurements

Abstract In almost every single application of ionising radiation, the accurate determination of absorbed radiation energy is of great importance. Radiation sensitive gels were first proposed for non-destructive and non-invasive measurements of the spatial distribution of radiation absorbed dose in 1984 [1] . Two different systems were developed but neither of these systems is widely used in practice due to some technical drawbacks. To overcome the problems associated with previous systems a gelatin gel of different composition has been devised and characterised for radiation dosimetry. This system is based on the changes in the optical properties of a transparent gel medium when it is exposed to ionising radiation. The detection material was carefully investigated and the role of each constituent of this system has been studied in order to determine the optimum composition. Gel with this formula has a linear dose response in the range 0.1–30 Gy, this range can be altered to suit other applications by changing the concentration of the active chemicals. The gel sensitivity, defined as the change in optical density per unit radiation dose, was found to be 70×10−3 cm−1 Gy−1 and the system stability was studied for both pre- and post-irradiation effects. This new formulation allows a convenient, rapid and economic readout method using an optical tomography system operating at visible wavelengths.

Neutron activation studies of trace elements in tree rings

The levels of twelve elements most of which are either considered essential to plant growth or have been detected in air filter samples in an air pollution survey, have been monitored in three transverse sections of trees, two elms (ring porous trees) and one cedar (a conifer). Two dimensional distributions of these elements around the tree rings of the section and radially from ring to ring, have been obtained to see if tree rings can be analysed for use as a record of historical pollution.

An optical method for three-dimensional dosimetry

Accurate determination of the spatial distribution of the absorbed dose of ionising radiation plays an important role in radiotherapy, industrial radiation processing and many other applications. Computer calculations have frequently been used to estimate three-dimensional (3D) dose distributions in complex geometries and it becomes important to validate these by accurate 3D measurements. For this purpose we have been investigating the use of gelatin gels loaded with a modified Fricke solution which are pale orange in appearance and which, upon irradiation, become increasingly purple when viewed in normal light. This ferrous sulphate xylenol orange in gelatin gel (FXG) system displays very good properties, such as sensitivity, linearity and dynamic range, that make it suitable for 3D dosimetry applications. A high-speed optical tomography readout technique has been developed enabling two-dimensional projections of optical absorption data to be recorded rapidly. From these data the 3D absorbed dose distribution can quickly be derived with minimal degradation due to ion diffusion.

TIME VARIATIONS IN ENVIRONMENTAL POLLUTION

Two programmes of work are described involving the measurement of environmental pollution over periods from two hours to many years. The long period measurements are deduced from neutron activation analysis of whole wood samples taken from individual tree rings for trees grown in various locations, and some results for cadmium, antimony and germanium levels are reported. In a second series of experiments the concentration of airborne particulate matter is being measured using a moving filter (streak sampler) technique to give a continuous record with two hour resolution. The filter samples are being analysed by proton induced X-ray emission (PIXE) and preliminary data is presented.

Si(Li) Detector efficiency below 10 keV

The full-energy peak efficiency of a Si(Li) detector has been experimentally determined over the photon energy range 3–26 keV for use in accurate ion-induced X-ray cross-section measurements. The efficiency uncertainties are ± 2% to ± 4% for 4.5–9.9 keV photons — the energy region of primary interest in the present work. The techniques utilised are described in detail since it was found that the use of absolute theoretical efficiencies can lead to errors in excess of 30% over the whole efficiency curve with even greater errors appearing below about 3 keV. Recent electron-capture data are used to calculate the yield of 3.1 keV Ag L X-ray from the decay of a calibrated 109Cd source in order to extend the efficiency curve down to 3 keV. The use of fluorescence sources as a novel way of accurately measuring efficiencies in the photon energy region 1–4 keV is outlined.

A comparison of thin and thick target methods of measuring proton-induced K-shell ionization cross-sections

Abstract Absolute proton-induced K-shell ionization cross-sections, in the energy range 1–3 MeV, have been measured for Ti and Cu using the thin and thick target methods. The thin target method is shown to be slightly more accurate. The potential of using thick targets of chemical compounds to measure these cross-sections is examined. For the highly reactive element Ca, preliminary results from different compounds are inconsistent thereby indicating the importance of the correct choice of compound and the need for photon attenuation and stopping cross-section data of higher accuracy.

The analysis of air particulate deposits using 2 MeV protons

Particle-induced X-ray emission (PIXE) analysis of the lighter elements in time-resolved air particulate deposits has been carried out. Minimum detection limits have been determined for 1.0, 2.0 and 3.5 MeVprotons. Quantitative PIXE analysis results, obtained with 2 MeV protons, are given for temporal variations in the elemental concentrations of Na, Al, Si, S, Cl, K, Ca and Fe. Rutherford backscattering (RBS) spectra were taken simultaneously with the PIXE spectra to provide information on lead concentrations and deposit thicknesses. The experimental problems associated with the quantitative analysis of light elements on cellulose acetate filters are described. The relationship between these results and meteorological data is discussed.

Study of Compton scattering signals in single-sided imaging applications using Monte Carlo methods

Compton scatter imaging (CSI) is well establish one-sided imaging approach, where the image is formed using photons that are singly scattered in a region of the object. However, the measured data includes also photons that are multiply scattered and is also affected by the fact that incoming and outgoing beams are attenuated. In this paper, the EGS4 code system is used to gain some insight into the physics that constrains CSI. The yields of singly and multiply scattered photons, obtained with monochromatic and X-ray tube inspecting beams, were assessed for a variety of media and depths.