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