Gamma ray fluorescence for in situ evaluation of ore in Witwatersrand gold mines
Date
2015-08-18
Authors
Rolle, Rainer
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Abstract
A system tor quantitative in situ evaluation of ore in
Witwatersrand gold mines was researched and subsequently
developed.
The principle of measurement is based on the excitation
of gold K x-rays in rock face samples by the 88 keV gamma
radiation from a Cadmium-109 radioisotope source. The X-rays
and scattered radiation from the rock matrix are detected by
a hyperpure germanium detector cooled by liquid nitrogen in
a portable probe. In the fluorescence spectrum the intensity
ratio of the gold Kb peaks to their immediate scattered
background is evaluated and quantitatively converted in the
portable analyser to area concentration units.
All aspects of the physical and instrumental measurement
had to be investigated to arrive at a system capable ot
quantitative evaluation of trace concentrations in stope
face ore samples. The parameters of efficiency of excitation
of the gold K X-rays, and the energy distribution after
scattering from the rock matrix at different angles were
investigated from basic principles to determine an optimum
source - sample - detector g e o m e t r y which would allow
quantitative evaluation of homogeneous ore concentrations,
for edged-on measurement of rough-surfaced thin layer
deposits a method or controlling the measurement geometry
through ratemeter feedback was developed to allow conversion
of mass concentration values to units of area concentration.
The parameters of spectrum evaluation were investigated from
fundamental principles to allow quantitative assessment of
different methods of peak evaluation for optimization of the
method as a whole. The basic concepts of random signal
processing times were developed together with new concepts
of pileup parameters to allow a quantitative description of
the data acquisition rate of a complete analog pulse
processing system.
With this foundation a practical measuring geometry and
optimum values for signal processing time parameters, for
detector size and for discriminator positions for spectrum
evaluation could be determined.
Parallel with the derivation of optimum measurement
parameters went the development of instruments, their field
testing and appraisal of the method. The underground results
obtained with prototype versions of the gamma ray
fluorescence analyser were in all instances found to have a
highly significant correlation with those obtained from the
same locations by conventional chip or bulk sampling and
fire assay.
The development of the gamma ray fluorescence method has
shown the potential of the method to serve as an ore
valuation tool and to assist in the geological identificaion
of strata in Witwatersrand gold mines.
Description
A Thesis Submitted to the Faculty of Science
University of the Witwatersrand, Johannesburg
for the Degree of Doctor of Philosophy
Johannesburg 1979