Nuclear detectors find natural resources For two days in Houston, scientists heard the latest chapter in the fastdeveloping story of nuclear techniques for finding increasingly scarce U.S. natural resources. Papers at a symposium on nuclear methods of field exploration for such use clustered around neutron and x-ray fluorescence methods for spotting substances ranging from oil, copper, and iron to gold, gadolinium, and a long list of minor elements. Although some nuclear mineral detectors have been available for several years, others—such as instruments using neutron-emitting actinide elements—are just now being developed. The actinides have been under scrutiny for some time (C&EN, Oct. 14, 1968, page 54) and now some companies, Du Pont among them, are mulling possible commercial formulations and price levels. One company in the thick of the development is Schlumberger through its Schlumberger-Doll Research Cen-
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C & E N M A R C H 9, 1970
ter in Ridgefield, Conn. Dr. J. Tittman, physicist at the center, described Schlumberger's probes for determining subsurface chemical compositions in cased oil-well bores. Bursts. The company's present tools—3:V,s and l n / i 6 inches in diameter—employ miniature accelerators. These are 6 inches long and 1 inch in diameter and accelerate deuterons on tritium to generate 14-m.e.v. neutrons in short bursts (about 200 per second). The neutrons slow down to thermal energies immediately after emission and pass through the steel casing into the surrounding mud. The neutrons are gradually absorbed at different rates by hydrocarbons, salt water, and other substances. The absorption rates are determined by a delayed measurement of the resulting gamma-ray emissions. These rates and supporting information, including the rock matrix composition, yield chlorine (salt water) content and hydrocarbon saturation. In noncased wells, hydrocarbonbearing formations are usually identified by well logs using the electrical conductivity difference between oil or gas and salt water. This can't be done in a bore with steel casing because of the short-circuiting effect of the metal sheath. The niceties involved in developing the probes make an interesting story of physics. Neutron bursts must be spaced efficiently and yet allow enough time for diffusion. Absorption measurements must be timed precisely and corrected for differing diffusion rates of the neutrons in various bore-mud environments. It took years of research and development to arrive at the present parameters, in the simplest system, for 250 microseconds for the bursts, 15Vi» inches of source-detector spacing, and a continuously variable time between bursts related to the time constant of absorption (the reciprocal of the relative rate of decay of the thermal neutron population). More sophisticated systems have variable burst times and repetition rates. Potent. Dr. Tittman states that Schlumberger is also looking at califomium-252 as a neutron source for future probes. This isotope would operate by a different technology, either through neutron activation analysis or neutron-induced prompt gamma-ray spectroscopy. The actinide, a favorite candidate among the heavy elements for commercial development, is a potent neutron emitter (2.34 X 10 12 neutrons per second per gram). Its half-life is 2.65 years. According to Dr. J. L. Crandall of
Du Pont's Savannah River laboratory, Aiken, S.C., californium-252 is now available only in research quantities. However, Dr. Crandall notes, production programs at Oak Ridge and Savannah River are expected to be yielding more than a gram a year within the next two years. Dr. Crandall foresees production rates by the 1980's of hundreds or even thousands of grams a year from advanced production reactors irradiating by-product americium and curium from spent power reactor fuels. Californium prices may drop to about $1.00 per microgram in the next 10 years. At these prices, Dr. Crandall states, californium neutron sources could win major roles in mineral exploration and many other fields.
High-voltage supply (left) and neutron accelerator are miniaturized to go down narrow borehole
