A. Notea

The Ge(Li) spectrometer as a point detector

The behaviour of a Ge(Li) spectrometer as a point detector was considered. The intrinsic peak efficiency of the point detector is suggested as a characteristic parameter. A procedure for relative calibration of photon sources, and for comparison between various shape spectrometers, based on accurate corrections of solid angle as a function of photon energy is presented. Examples of the procedure are given.

Near infrared spectrometry of milk in its heterogeneous state.

Composition analysis by NIR spectrometry usually uses models that assume the sample to be homogeneous. However, agricultural fluid products are naturally heterogeneous, and in some cases of on-line sensing, it is very difficult to fulfill the homogeneity requirement, and some precautions are needed. The flow itself might create a non-uniform distribution of the measured ingredient concentrations across the cross-section of the flow. A modified discrete model was developed, which incorporates the concentration distribution of the ingredients. The effects of model parameters (number of layers and of iterations, valid ranges of absorbency and specular reflectance factors) were studied and evaluated. Samples of fresh raw milk were tested, in order to create a known distribution of the fat content in the sample, and were scanned by a NIR spectrometer. In the case of monotonic variation, along the sample, of the concentration of the measured ingredient (fat), the effect on the measured spectrum was similar to that of changes in the average concentration of a homogeneous sample. Comparison of the errors predicted by the model with the result of NIR measurements in heterogeneous conditions of fresh raw cows milk gave close correlation in their direction and their relative size. This created the basis for a technique to evaluate other fluids, in which the distribution of the concentration of the measured ingredient could be evaluated and incorporated in the model simulations.

Point spread functions due to neutron scattering in thermal neutron radiography of aluminum, iron, zircaloy and polyethelene objects

Abstract Recently, image restoration methods have penetrated to the nondestructive testing field. Statistical and averaging methods are not adequate for the extraction of thicknesses and dimensions. The present work deals with the calculation of the point spread function (PSF) in thermal neutron radiography. The contribution of the scattering of neutrons in the radiographed object is calculated using Monte Carlo computational methods. Calculations were performed for polyethylene, iron, zircaloy and aluminum. The results are based on the physical phenomena, therefore, a restoration operator based on it has to be preferred over a general statistical operator. However, one has to bear in mind that scattering is not the only contributor to the image blurring. A complete operator has to take into account the other phenomena as well.

Recent developments in secondary emission gaseous X-ray imaging detectors

Abstract The conversion of X-ray photons in a thin solid film and the detection of the subsequent secondary electrons in a low-pressure avalanche wire chamber is the basis for a new type of fast, high resolution parallax-free X-ray imaging detectors. The combination of the high emission yield of low energy secondary electrons from alkali halides with the particular properties of low-pressure avalanche electron multipliers results in a fast detection process, free of space charge effects even at high radiation flux. We summarize recent experimental and theoretical results connected with these secondary emission gaseous detectors which have a broad field of potential applications. Simulations of a real-size ultrafast transition radiation detector based on this principle indicate its outstanding particle identification properties, of prime importance for the very demanding environment of future colliders.

Inversion of Abel's integral equation and its application to NDT by X-ray radiography

Abstract In X-ray radiography, inversion of Abels integral equation is an essential step in the reconstruction of the radial optical density profile of a cylindrically symmetric object from its radiograph. The original inversion formulae developed by Abel a century and a half ago are not well suited for use on measured data, since they require differentiation which greatly amplifies inherent random measurement errors. Recent developments in the field are presented, including a derivative-free analytic inversion formula and a spline-based numerical inversion method. These methods were applied to the interpretation of radiographs of industrial products. Several practical problems were encountered such as the Gibbs phenomenon near sharp density changes in the object. Countermeasures developed to eliminate these problems are discussed and their use is demonstrated in the reconstruction of the density profile of a cylindrical ceramic fuse housing from a single radiograph.

Low energy photons from 152gEu for Ge(Li) calibration

Abstract Intensities of low energy gamma and X-rays in the decay of 12.4 y 152 Eu were measured. A branching ratio of 1.84±0.17 between the decay by β − emission to 152 Sm and via EC and β + to 125 Gd, was found from the relative X-rays and 344 keV intensities. The measured energies in keV and the absolute intensities in photons per 100 disintegrations are: 39.5±0.2(45. 3 ±5.8), 45.2±0.2 (11. 2 ±1.4), 121.9±0.1 (27. 7 ±3.1), 244.75±0.08(7.5 2 ±0.85), 295.8±0.1 (4.5 3 ±0.61), 344.44±0.09 (24. 5 ±2.7), 367.8±0.2 (0.72 1 ±0.086), 411.3±0.2 (1.9 4 ±0.24), 444.2±0.2 (2.9 6 ±0.33) The nucleus is recommended as a calibration standard for Ge(Li) detectors.

Abel reconstruction of piecewise constant radial density profiles from x-ray radiographs

We present a method for reconstructing the radial density profile of a cylindrically symmetric object from a single x-ray projection, when the profile consists of a number of different constant sections. A forward Abel transform based algorithm is employed whereby the profile is recovered recursively, onion peelinglike, starting from the outside diameter of the object and moving in. Distortions originating in the Gibbs phenomenon, unavoidable in most available Abel inversion methods, are completely eliminated. The method is simple enough to be carried out on a handheld calculator or a spreadsheet program on a personal computer, and no elaborate computer fits or application programming are required. The method is demonstrated by inverting a simulated three-section noisy set of data and is shown to yield results of a quality equal to that of a recent powerful Abel inversion method, based on full nonlinear least-squares computer fits.

Resolving power of dynamic radiation gauges

The characteristic functions of dynamic gauges, based on nuclear or atomic radiation, were developed. These gauges are applied to the examination of material whose properties may vary continuously with time. The approach presented takes into consideration contributions to the uncertainty and blurring from various effects, such as radiation scattering, gauge geometry, and the systems time constant. The analysis is based on the concept of the line spread function obtained from the derivation of the response to a step change in the inspected property. The response and relative resolving functions were demonstrated for a rectangular change with a gamma-through transmission gauge. The procedure provides a systematic method of obtaining the optimal values for the design parameters of the radio gauge, such as radiation energy, source emission rate, detection efficiency, detector-sample distance, and measurement time. The time constant, for example, reveals a pronounced minimal value for large relative velocity. Due to the radiation scattering in the examined material, there is an advantage to large detector-material distance. The design values may differ considerably more for the dynamic gauge than for a static gauge, i.e., a gauge applied to samples whose properties do not vary during the measurement period.

Gamma transmission gauge for assay of integral water content in soil

Abstract A photon transmission gauge applied for integral water content measurement in a soil layer was analyzed. The gauge may be used as a control unit for automatic irrigation in a field, or as a scanner employed for establishing an irrigation policy. The characteristic functions of the gauge; response and relative resolving power were developed. The functions provide parameter study at the design stage and interpretational ability at the operational stage. The model led to a design which eliminates sensitivity to water distribution in the examined soil. It is shown that a resolving power of 2% was obtained for a 2.3 mCi 137 Cs source at 53 cm below surface, in measuring water content of 0.2 g water/cm 3 soil during 1000 s.

Secondary electron emission gaseous detectors for fast X-ray imaging

Abstract A new class of X-ray imaging gaseous detectors was developed, based on photon conversion in a thin film and the detection of the emitted secondary electrons with a low-pressure avalanche wire chamber. Their main features are parallax-free imaging with a submillimeter spatial resolution over a broad range of X-ray energies, a nanosecond response and a counting rate capability above 1 MHz/mm 2 . The imaging characteristics of a large area secondary electron emission (SEE) detector equipped with CsI, Ag and Ta photoconvertors, in the photon energy range of 6–60 keV are summarized. In localization capability this detector can successfully compete with commercial image intensifier systems and medical film-screen combinations.