Apparatus for Elemental Analyses of Gold by Activation Using a Neutron Source, 20-9064

Principal Investigators
James R. Weldy
English C. Pearcy
James D Prikryl
David A. Pickett

Inclusive Dates: 01/01/98 - Current

Background - Commercial exploration for mineral resources is increasingly competitive. At present, there are no means to analyze rocks or soil in the field for gold (or other precious metals) at the low levels required for effective exploration. The current practice of evaluating gold content is to collect the samples in the field, take them to a commercial laboratory for laboratory analysis, and wait a number of weeks for the result. With the results in hand, the individual then returns to a promising field and collects additional samples as indicated by the laboratory results. Under some circumstances, the delay in obtaining the analytical results may result in the loss of a potentially valuable property. The capability to perform trace-level elemental analyses in the field would provide significant competitive advantages to mineral exploration companies.

Approach - The purpose of this project is to design and construct a field-portable prototype neutron source-moderator assembly capable of measuring the gold content of geologic samples through neutron activation analysis. Once proven, development and deployment of this analytical technology should generate substantial interest from gold exploration companies. Using a ²⁵²Cf neutron source available at the Institute, a potentially field-portable source and moderator assembly was constructed. Using high-density polyethylene as the hydrogenated media for thermalizing neutrons emitted from the source, this assembly irradiated samples to transmutate gold (100 percent ¹⁹⁷Au natural abundance) contained in the sample to a radioactive isotope (¹⁹⁸Au; half life of 2.7 days with a characteristic gamma ray at 412 keV).

Accomplishments - Two types of samples have been investigated in this project: 1) silicon dioxide samples doped with a known amount of gold chloride solution, and 2) several United States Geological Survey (USGS) rock standards. Initially, the technique was able to measure accurately gold concentrations in geologic samples down to approximately 10 parts per billion in pure silicon dioxide samples, a level well below the 10 parts per million threshold of economic importance. Despite these promising results, measurement of the gold concentration of the actual USGS samples was compromised due to interference from compton events in the detector from activated sodium and magnesium, also contained in the geologic samples. This interference led to fairly lengthy measurement turnaround times for the samples with very low gold concentrations. Using the sample most representative of gold ore (U.S. Geological Survey standard DGPM-1; Nevada Disseminated Gold Ore), the turnaround time was approximately two days. Soils with lower gold concentrations took significantly longer. The initial step for decreasing the measurement time was the purchase of a more powerful neutron source. Initial testing with the new neutron source has led to a substantial decrease in the time required to determine the concentration of gold in a sample.

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