Issue 5, p. 157 (2015)
Historically, the application of nuclear science to borehole logging began with the detection of the natural radioactivity emitted by rocks and soils. This simple and effective method found many applications, particularly in sedimentary, coal and uranium deposits. Whilst this technique is still widely used today, the industry quickly moved to more sophisticated logging techniques that activate rocks with neutrons and measure the induced radiation to infer characteristics of the material surrounding the hole. Neutrons, as primary particles, were found to be an excellent radiation source, which opened large opportunities for in-situ material analyses. Neutron based commercial instruments were introduced1 for the first time in the 1940’s. Early versions of neutron logging tools used isotopic neutron sources that primarily responded to the amount of hydrogen in the formation. These were adopted by the oil industry to identify zones of porosity. Common isotopic neutron sources, such as 252Cf or 241Am-Be, are environmentally problematic if, for any reason, the tool cannot be retrieved from the borehole. Tools using continuously-on isotopic sources also suffer from limited capacity to distinguish between water and oil. Pulsed sources were found to overcome this obstacle since, by exploiting differences in time response, they made it possible to distinguish water from oil beds. Thus, the need for switchable neutron generators was driven by the oil industry, which in turn prompted the development of compact industrial grade equipment suitable for the requirements of their logging tools. There is a variety of active logging techniques based on the use of pulsed neutrons. These can be classified according to the implementation of the neutron source and the type of induced particles that are detected. Among the latter, gamma photons are of considerable interest as they enable elemental analysis of rocks. Pulsed Fast and Thermal Neutron Activation (PFTNA) is one of the most commonly used techniques combining a neutron generator with a gamma scintillation detector. Application of PFTNA for borehole logging is not limited to the oil industry. Nevertheless, most commercial tools have been developed to withstand the severe pressure and temperature conditions inherent at great depths in oil wells and thus tend to be oversized and too costly for mining applications. Without the need to withstand high temperatures and pressures, the technology can be optimized to make it more cost effective. This article presents a new PFTNA tool developed for the mining industry.