Well logging with californium-252

Abstract

Californium 252 is an isotopic neutron source that has only recently become available for experimental well logging. One curie of 252Cf emits 4.4 x 109 neutrons per second by spontaneous fission, 300 times the emission rate of any other one curie radioisotopic source. California 252 has several other advantages as a high yield source for well logging: very small physical size, low gamma and heat emission, and expected low cost relative to other sources. A 50-millicurie 252Cf source fabricated at the Savannah River Laboratory was made available by the U. S. Atomic Energy Commission to the U. S. Geological Survey for a feasibility study on well logging. The nuclear and physical characteristics of this source and some of the health physics aspects of its use in the field are discussed. The source was used to make epithermal neutron logs, which are compared with logs made with plutonium beryllium and americium beryllium sources in the same well. The high neutron flux available from 252Cf permitted the use of longer than usual spacing while maintaining a high count rate and excellent sensitivity. In addition, continuous activation logs were made utilizing a spacing of 5. 5 feet from the source to detector. Aluminum 28 was identified as the chief radioisotope contributing to the log response. This new technique may provide a log more closely related to clay content than the natural gamma log. Stationary irradiation experiments were also carried out in boreholes, and sodium 24 and manganese 56 were readily produced and identified. Suggestions for additional research on logging applications and problems resulted from this feasibility study. Potential well logging applications not investigated include the activation of temporary depth markers and the use of stable tracers that can be activated at the site or in the well. The high neutron yield of californium 252 will facilitate in situ activation analysis for many elements as an aid to exploration for oil, water, and other minerals.