Patent Number: 052415694
Section: summary

The present invention relates in general to imaging radionuclide analysis apparatus and method and more particularly to imaging neutron activation analysis apparatus and method. BACKGROUND OF THE INVENTION Normal neutron activation analysis measures the average concentration of one or more analytes in a single analysis volume. Neutron activation analysis is an extremely powerful method for measuring major, minor, and trace element concentrations in a wide variety of samples. Analyte elements absorb a neutron to form a radionuclide which usually decays by emitting a .beta.-particle and a .gamma.-ray. The .gamma.-ray energies are characteristic of the analyte element and they are normally measured with a germanium detector. Modern germanium crystal .gamma.-ray detectors have excellent energy resolution which provides for simultaneous in situ determination of many elements. This procedure, performed without chemical separations, is called instrumental neutron activation analysis (INAA). Although the INAA takes place on elements located in situ within unaltered samples, information on the three-dimensional locations of the elements is never acquired. Beta-electrons provide a method for gathering lateral position information for individual radionuclide decompositions in thin samples or particles. Neutron activated nuclides usually decompose by .beta.-decay, effectively producing a nucleus in which a neutron has been converted to a proton. The nucleus emits a neutrino, and usually a .gamma.-ray in addition to the .beta.-electron. The emitted electrons have substantial energies which are largely expended in the production of secondary electrons. Secondary electrons with energies of a few electron volts can be imaged if they pass out of the sample. BRIEF SUMMARY OF THE INVENTION Broadly stated, the present invention, to be described in greater detail below is directed to radionuclide imaging method and apparatus wherein the time when and the energy of .gamma.-rays emitted from the sample are detected and the presence of certain elements in the sample established from the detected ray energies. Secondary electrons emitted from the sample are detected and imaged showing the location on the sample from which the secondary electrons were emitted. Coincidence between detection of .gamma.-rays and secondary electrons is determined to establish the location of certain elements on the sample. In accordance with a principle aspect of the present invention, the location of the certain elements on the sample is established by producing a distribution image of the certain elements of the sample from the determined coincidence of the detected rays and the detected secondary electrons and the established ray energies and the image of the location on the sample from which the secondary electrons are emitted. Thus, when .gamma.-rays and .beta.-particle induced secondary electrons are detected in coincidence, the .gamma.-ray energy answers the question of "what" and the secondary electron position answers the question "where" for individual radionuclide disintegrations. In accordance with another aspect of the present invention, the secondary electrons are detected and imaged using an image intensifier and a resistive anode encoder. The features and advantages of the present invention will be appreciated by a perusal of the following specification taken in conjunction with the accompanying drawings wherein similar characters of reference refer to similar elements in each of the several views. DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view, partially in broken away elevational sectional form and partially in block diagram form. FIG. 2 is a schematic elevational sectional view of the charged particle optics for electron extraction and imaging of secondary electrons arising where energetic .beta.-particles pass out of the sample. FIG. 3 is an enlarged sectional view of the charged particle optics for electron extraction shown in FIG. 2. FIG. 4 is a graph of electron counts plotted against energy level of detected .gamma.-rays in an operative example of use of the present invention. FIGS. 5A and 5B illustrate the distributions of gold and nickel, respectively, on a portion of a particle sample.