Abstract:

Starting in 1969, the U.S. Geological Survey (USGS) developed neutron techniques for borehole measurement of the elemental composition of ores, and it successfully made a borehole ultimate analysis of coal in 1977. Borehole measurements permit real-time evaluation of ore quality without the expense of coring or the delays and expense associated with laboratory analyses. Two technological innovations have made such measurements possible: the availability of small californium-252 fission neutron sources from the Savannah River Operations Office of the Department of Energy, and the development, by USGS and Princeton Gamma-Tech, of the melting-cryogen-cooled high-purity germanium borehole gamma-ray detector. A technique of relating mass fractions to measured gamma-ray intensities, which eliminates the need for detailed knowledge of the geometry of the neutron distribution, was used to calculate elemental compositions without using test pits or computer borehole modeling. Most of the common elements in the earth’s crust can be detected by neutron techniques. In coal all of the major constituents except oxygen (C, H, N, S, Si, Al, Fe, Ti) can be determined quantitatively by thermal neutron capture gamma-ray spectroscopy. The latest innovation in this field is the replacement of the 252Cf neutron source with a neutron generator, a type of ion accelerator. These generators, used for many years by the petroleum logging industry, produce neutrons having an energy of 14 MeV. The neutron generator is a safer tool than californium, because no radiation is emitted by the device until it is turned on in the borehole. Coupling a neutron generator with a high-resolution detector to form a borehole measuring system was pioneered by workers at Sandia National Laboratories. USGS has built and put into service one neutron generator based on the Sandia design, and now is building a second. This new device enables the experimenter to use higher energy (n,n′), (n,p), (n,2n), and (n,α) reactions as well as the (n,γ) thermal neutron capture reaction. Both the (n,n′) and the (n,p) reactions on 16O permit quantitative measurement of oxygen, and the inelastic scattering excitation of carbon in coal provides increased sensitivity over that of the (n,γ) reaction. Reactions caused by 14 MeV-neutron irradiation that are used in exploration for strategic metals such as Cr, Ni, Mn, V, Co, Ti, and W are tabulated.

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