Ore analysis

A method for the rapid and sensitive analysis of heavy metal ores, especially those of gold and uranium, uses high-engery X-ray fluorescence spectroscopy. The invention is of particular interest for the measurement of samples from gold or bodies which typically have concentrations up to 10 ppm by mass. Preferred features include the use of an X-ray tube as a source, the counting of emitted fluorescence photons in energy bands selected to correlate with the characteristic x-ray fluorescence emissions of elements of interest, the excitement of the ore sample by irradiation with high energy bremsstrahlung radiation filtered through tin, the exploitation of polarization in analysis for uranium, the interposition of a platinum-group metal filter between the sample and the detector, and the use of high-purity germanium detectors. Techniques are described for the detection and elimination of inaccuracies due to the presence of certain interfering metals and correction for variations in sample density. Apparatus for use in the method of the invention is also disclosed and claimed.

This invention relates to a method and apparatus for the analysis of 
samples of materials in which a finely divided heavy metal is dispersed in 
a non-metallic matrix, especially mineral ores containing gold, uranium, 
lead or platinum, using the technique of X-ray fluorescence (XRF). 
The expression "heavy metal" used herein means tungsten and metals of 
higher atomic number. 
A "non-metallic matrix" means a matrix consisting predominantly of 
non-metallic elements of low atomic number and/or their compounds, such as 
silica, but may also contain metals which are not "heavy metals" as 
defined above, either in elemental or in combined form, such as iron or 
barium. 
The phenomenon of energy-dispersive X-ray fluorescence (XRF) is well-known. 
A sample, for example, of a mineral ore, is bombarded by high-energy 
X-rays and the fluorescence spectrum is analysed by counting the rate of 
emission of photons over a range of photon energies. 
Assay laboratories of gold mines have a daily chore of analysing large 
numbers of samples of gold ore; these include both exploration samples 
taken to locate the gold-bearing ore, and random samples of ore actually 
mined, for accounting purposes. The total number of samples to be analysed 
per day typically ranges from a few hundred to several thousand depending 
on the size of the mine. A high throughput of samples is therefore highly 
desirable, in the order of say one sample per hundred seconds, and the 
only practicable assay method which can achieve this is the traditional 
fire assay method. This method, however, is expensive in time, labour, 
capital and running costs, and less accurate than desirable. It is 
essentially a manual process requiring large quantities of electricity and 
other consumables and in a typical laboratory dealing with 1,000-2,000 
samples per day it is difficult to keep track of individual samples. 
The use of X-ray fluorescence techniques to analyse ores for heavy metals 
is known, and is described for example in U.S. Pat. No. 3,404,275 
(Martinelli) UK Patent Patent Specification 1080346 (VEB Vacutronic), 
German Offenlegungschriften 2046606 and 2140794 (Siemens AG), U.S. Pat. 
No. 4,224,517 (Lubecki) and European Patent Application 0166914 
(Kernforschungzentrum Karlsruhe GmbH). Most of these disclosures 
contemplate the use of the characteristic K-emission peaks of the elements 
of interest. Such fluorescent emission may be stimulated by various 
sources, principally X-ray tubes (as in UK Patent Specifications 1070337 
(Laurila) and 1017595 (UKAEA), and German Offenlegungsschriften 2054464) 
and radionucleides (as in U.S. Pat. No. 3,404,275). None of these 
disclosures however uses high energy X-radiation within the meaning of the 
present disclosure. 
None of the above prior art discloses the particular technique of counting 
the emitted fluorescence photons in energy bands selected in relation to 
the K-emission bands of the elements under analysis according to the 
present invention. 
The use of metallic filters to shape the radiation incident upon the sample 
under analysis is known, for example, from U.S. Pat. No. 3,404,275 where a 
cadmium filter is used, but not for the particular purpose relevant to the 
present invention; this U.S. Patent, for example, uses for exciting the 
sample not high-energy X-rays, but .delta.-rays. Again, UK Patent 1070337 
discloses the use of nickel or cobalt filters but in the context of 
analysis for lighter metals than those contemplated by the present 
invention. 
It is especially significant that none of the prior art discloses the 
preferred method according to the invention of producing the stimulating 
radiation--that is the exploitation of the broad bremsstrahlung peaks, 
filtered through a suitable metallic filter to excite the characteristic 
K-emission peaks of the metals under analysis. The use of germanium 
detectors, both of high purity (European Patent Application 0166 914 and 
UK Patent 4224571) and silicon or lithium-drifted (Offenlegungsschriften 
2046606 and 2054464) is known but not in the context of the particular 
energy profile of the fluorescence photons analysed according to the 
invention. 
It will be noted that none of the extensive prior art mentioned above is 
specifically directed to the analysis of ores for gold. 
X-ray fluorescence techniques have been employed to analyze gold ores but 
these generally use the gold L band which does not give the sensitivity 
required for measurement of the one part per million level of gold which 
is critical. Small samples effectively of a few grams are the maximum that 
can be used in these known machines. The sample must be ground to an 
extremely fine powder and a binding resin or glaze added. 
On the other hand some of the prior disclosures mentioned above (e.g. U.S. 
Pat. No. 4,224,517) are specifically concerned with analysis for uranium, 
but do not disclose the particular use of filtered high-energy 
bremsstrahlung radiation from a high-energy X-ray source according to the 
invention. 
Metallic ores when bombarded by high energy X-radiation (which for the 
purposes of this description means radiation of by energy greater than 80 
keV) emit fluorescence photons which are grouped into characteristic 
peaks. 
The K X-ray fluorescence emission spectrum of gold is characterised by two 
peaks known respectively as the gold K.alpha..sub.1 and the gold 
K.alpha..sub.2 peaks respectively at approximately 68.8 keV and 67.0 keV. 
According to the invention we have discovered a method of rapidly analyzing 
the ores of gold and other heavy metals which gives significant advantages 
over the known fire assay and L X-ray fluorescence techniques mentioned 
above. We are able, using preferred embodiments of the invention, to 
detect a concentration of 1 ppm by mass of gold at a confidence level of 
95% in a counting time of 100 seconds. 
The invention is applicable not only to gold but also to other heavy 
elements such as uranium, platinum and lead. 
According to the invention we provide a method for analyzing a sample of 
ore for at least one heavy metal comprising exciting the ore with high 
energy X-rays to produce a fluorescence emission spectrum and measuring 
the intensity of the K-emission bands of the said metal or metals in the 
spectrum, characterised in that: 
(a) The X-rays are produced by an X-ray tube. 
(b) That most of the high energy bremsstrahlung peak is eliminated by a 
metallic filter interposed between the source and the sample to give high 
energy bremsstrahlung radiation of 100 to 130 kev incident upon the sample 
and 
(c) The number of fluorescence photons emitted in each of a plurality of 
energy bands is counted and compared, the width and energy of the bands 
being chosen in relation to the K-emission peaks of the metal or metals in 
the sample. 
The anode material generally preferred for use in the method according to 
the invention is tungsten but the use of other anode materials is within 
the scope of the invention. 
The method of the invention is especially relevant to the analysis of 
powdered gold ores containing below 10,000 ppm of gold and of ores 
containing both gold and uranium. Typically, in gold mining applications, 
90% of the ore samples will be of concentration up to 10 ppm gold by mass. 
(All ppm data in this description are by mass). 
The metallic filter interposed between the source and the sample is 
preferably tin metal at least 4 mm thick. 
As will be described in more detail below, it is a preferred feature of the 
invention to pass the fluorescence photons emitted by the sample through a 
heavy metal filter to reduce the bremsstrahlung energy peak and thereby to 
enhance the relative number of counts in the K-bands of the metals under 
analysis. This filter is preferably of iridium or platinum when the ore 
contains gold, and of osmium when the ore contains platinum. 
A preferred feature of the method according to the invention is that the 
photons emitted by the sample are counted by at least one detector of high 
purity germanium, preferably in the form of a disc of active thickness 2-4 
mm. 
In a preferred method according to the invention photons are counted in 
each of two background energy bands lying either side of the 
K.alpha..sub.1 peak and in each of two signal energy bands lying between 
the background bands either side of the peak maximum and the total counts 
of photons in the background bands and in the signal bands compared, the 
two background bands being substantially equal in energy width and the two 
signal bands also being substantially equal in energy width. 
This "symmetry" of the energy bands counted is an important preferred 
feature of the invention. The bands are chosen so as to show symmetry 
about the K.alpha..sub.1 peak maximum, the object being to reduce errors 
due to shifts in the incident energy peak due to external causes such as 
supply voltage variations and temperature sensitivity of components. It is 
advantageous to compress the width of the signal bands and increase that 
of the background bands thus achieving a better signal:background count 
ratio. This "compression" of the peak is achieved by better detector 
resolution which in turn results from improvement in the physical 
configuration of the detector and its following electronics, reduction of 
noise and the like. 
A further preferred method according to the invention uses not four but six 
energy bands. Using this method, as will be more particularly described 
hereinafter, it is possible to detect and eliminate interference from 
certain metals present in the heavy metal ore, for example thorium, 
mercury and tungsten. 
In one such preferred method, applicable to the analysis for gold of ore 
samples containing mercury and/or tungsten, photons are counted in each of 
six adjacent energy bands embracing respectively: 
(1) the gold K.alpha..sub.2 peak on both sides of the maximum 
(2) the trough between the gold K.alpha..sub.2 and gold K.alpha..sub.1 
peaks 
(3) the slope of the gold K.alpha..sub.1 peak below its maximum 
(4) the slope of the gold K.alpha..sub.1 peak above its maximum 
(5) the trough between the gold K.alpha..sub.1 peak and the mercury 
K.alpha..sub.1 peak 
(6) the mercury K.alpha..sub.1 peak on both sides of the maximum 
In another such preferred method, applicable to the analysis for uranium 
and gold of ore samples containing thorium, photons are counted in each of 
six adjacent energy bands embracing respectively: 
(1) the region immediately below band (2) described below 
(2) the thorium K.beta..sub.2 peak 
(3) the uranium K.beta..sub.1 peak 
(4) the uranium K.beta..sub.3 peak 
(5) the trough between the uranium K.beta..sub.2 and K.beta..sub.1+3 peaks 
(6) the uranium K.beta..sub.2 peak 
An important preferred method according to the invention for the analysis 
of gold in an ore is characterised in that: 
(a) The X-rays are produced by an X-ray tube with a plutonium or uranium 
anode or secondary target. 
(b) The ore sample is excited with the characteristic K X-rays of the 
material of the anode or secondary target and 
(c) The fluorescence photons emitted by the sample are passed through an 
iridium filter. 
According to the invention, a particular method of analysing a sample of 
ore, especially a gold ore or an ore containing gold and uranium, 
comprises: (1) Exciting the sample with high-energy bremsstrahlung X-rays 
having their maximum energy at about 115 keV produced by an X-ray tube 
with a tungsten anode and filtration through a metallic tin filter 
(2) Passing the X-ray fluorescence spectrum emitted by the sample at right 
angles to the exciting rays through a metallic iridium or platinum filter 
(3) Detecting the fluorescence photons by a germanium detector 
(4) Measuring the intensity of the K.alpha..sub.1 emission bands of the 
gold content of the sample. 
The invention further provides an apparatus for analysing the heavy metal 
content of an ore comprising: 
(1) A source of high-energy X-rays 
(2) Means to hold a sample of ore in the path of the X-rays 
(3) Detector means to count the fluorescence photons emitted by the sample 
and characterised by 
(4) Means to compare the counts of emitted photons in selected energy 
bands. 
Characterised in that the X-ray source is a tube and a metallic filter is 
interposed between the source and the sample which eliminates part of the 
high-energy bremsstrahlung peak whereby high energy bremsstrahlung 
radiation of 100-130 keV is incident upon the sample. 
In the apparatus according to the invention, the anode is preferably of 
tungsten and the filter of tin metal; the filter is perferably of tin 4-5 
mm thick. Preferably, the apparatus also comprises a heavy metal filter 
interposed between the sample and the detecting means, preferably of 
osmium, iridium or platinum. Preferably the detector means is at least one 
body of high purity germanium, especially a plurality of high purity 
germanium discs each 2-4 mm thick. Also in the apparatus according to the 
invention, the fluorescence spectrum is preferably viewed at a scattering 
angle of from 80-100 degrees to the exciting radiation. 
In an important preferred emodiment of the apparatus according to the 
invention, the X-ray source is a tube with a plutonium or uranium anode or 
secondary target and an iridium filter is interposed between the sample 
and the detecting means. 
A preferred form of the apparatus according to the invention, associated 
with the preferred method set forth above, comprises means to count the 
emitted fluorescence photons in each of two background energy bands lying 
either side of the K.alpha..sub.1 peak of the heavy metal under analysis 
and in each of two signal energy bands lying between the background bands 
and either side of the said K.alpha..sub.1 peak maximum and to compare the 
total counts of photons in the background bands with the total counts of 
photons in the signal bands, the two background bands being substantially 
equal in energy width and the two signal bands also being substantially 
equal in signal width. 
A particular aspect of the apparatus according to the invention is an 
apparatus for analyzing the gold and/or uranium content of a sample of ore 
by X-ray fluorescence, comprising an X-ray tube with a tungsten anode, a 
metallic tin filter, means to hold and retain the sample in the path of 
the X-rays emitted from the source and passed through the filter, an X-ray 
detector and means to detect the emission of photons of various energies 
from the sample, characterised by the interposition of a metallic platinum 
or iridium filter between the sample and the detectors and the use of 
germanium detectors. An iridium filter is preferred. 
It is an important feature of the invention that the fluorescence radiation 
emitted by the sample is viewed at a scattering angle of about 90.degree. 
to the exciting radiation. However, the scattering angle in any 
application is a matter of compromise between conflicting requirements as 
described below and the invention is not limited to any particular 
scattering angle. Generally it is found that an angle within the range 
80.degree.-100.degree. is suitable. For purely mechanical considerations, 
an angle of exactly 90.degree. is easiest to work with. However, the angle 
must also be chosen to minimise scattered non-parallel rays which give 
rise to background counts and from this point of view is preferably set at 
the Compton scattering minimum angle. Further, there is a correlation 
between peak energies and scattering angle--generally a particular peak 
shifts to a lower energy with an increase in scattering angle. A 
compromise between these three factors must be reached and it is generally 
found for the applications described herein that best results are obtained 
at a scattering angle of about 100.degree.. 
The X-ray tube may be any known type that produces high energy X-rays (i.e. 
photons) of the appropriate energy (as to which, see below) and is 
preferably water-cooled in a closed circuit cooling system with an 
integral radiator or chiller. In this description the expression "high 
energy" is used to indicate an exciting energy of 80 keV or more; the 
energy region 80-160 keV being of principal interest. 
However, a preferred method of producing high energy X-rays for use in the 
method according to the invention uses an X-ray tube with a tungsten 
anode. Such tubes emit, in addition to the tungsten characteristic peaks, 
the highest of which lies at 69 keV and is too low to be of practical 
value in analysis for gold (the K-edge of which lies at 80.7 keV), a 
bremsstrahlung peak at 65 keV the upper edge of which lies at about 130 
keV. The bremsstrahlung peak is generated by the slowing down of electrons 
in the tube anode. In this preferred method the tungsten characteristic 
peaks and most of the bremsstrahlung peak are eliminated by a metallic 
filter interposed between the source and the sample to give high energy 
bremsstrahlung radiation of 100-130 keV, preferably with a maximum at 
about 115 keV, incident upon the sample. This is in the form of a 
semi-Gaussian peak. 
The metallic filter is preferably of tin metal and at least 4 mm, generally 
from 4-5 mm, thick. 
The excitation voltage for the X-ray tube is optimised at about 130 kV when 
a 4 mm tin filter is used. The optimal voltage decreases as the tin 
thickness is increased, for example 125 kV with 5 mm tin. 
The effective incident photon energy can be varied by altering the 
excitation voltage on the X-ray tube and the thickness of tin filter in 
front of it. Thicker filtration will both harden the energy (i.e. increase 
the average energy) and narrow the bremsstrahlung peak, but of course 
reduces the number of photons striking the sample. As the average energy 
moves downwards towards the K-edge of gold at 80.7 keV, the probability of 
exciting the gold will increase. However, in practice the resulting 
increase in signal will be partially counteracted from a signal to noise 
point of view by an increase in background and attenuation effects. Most 
of the peak is in the energy range 100-130 keV but the low energy tail 
does extend lower than this and, after single or multiple scattering by 
the sample contributes significantly to the background counts in the gold 
energy measurement region. 
There are a number of practical limitations which restrict the ranges of 
excitation voltages and tin filter thicknesses one can use. 
(a) With falling excitation voltage and increasing filter thickness the 
photon flux falls. We can compensate up to a point by reducing the 
distance between the X-ray tube anode and the sample, and between the 
sample and the detectors, but this introduces a number of other problems. 
(b) It is advisable to restrict the effective photon energy to above 100 
keV to minimise particle size effects. 
(c) Simultaneous measurement of gold and uranium content requires a 
significant number of incident photons to be of higher energy than 115.6 
keV, the K-edge of uranium. 
The invention is not however limited to the use of highly filtered 
bremsstrahlung radiation as described above. It is also possible to 
achieve greater sensitivity by the use of an X-ray tube with uranium or 
(preferably) plutonium anode or secondary target, to generate uranium or 
plutonium characteristic K X-rays as well as bremsstrahlung. In this 
instance the characteristic radiation of the anode or secondary target 
material used to excite the sample and the bremsstrahlung radiation has 
its maximum at 150 keV, well above the energy region of interest. Some 
filtration of the bremsstrahlung radiation is still required, however, for 
example by the use of a tin filter as described above, to attenuate 
substantially the bremsstrahlung "tail" below 80 keV relative to the 
characteristic K-bands. 
The secondary target may be internal or external; it will be appreciated 
that the use of a secondary external target has advantages when tubes with 
anodes of these metals are unavailable. 
The lowest energy plutonium K X-ray, the K.alpha..sub.2, is at 99.2 keV. 
Being characteristic radiation, this is monoenergetic, and there is no low 
energy tail, as with bremsstrahlung radiation. Similarly, the lowest 
energy uranium K X-ray is 94.6 keV as compared with the gold K absorbtion 
edge at 80.7 keV. 
The relative attenuation of the low energy tail significantly reduces the 
background counts in the gold measurement and allows detection of lower 
gold levels. Another advantage is that the scattered characteristic 
radiation would be attenuated better by an iridium filter than the present 
bremsstrahlung, which would result in a higher sensitivity. 
Plutonium K X-rays will excite both gold and uranium, whereas uranium K 
X-rays will excite only gold. This is one reason for preferring plutonium. 
A second reason is that the plutonium K X-rays are several keV higher in 
energy and, when scattered from the sample, will not contribute so much to 
the background counts in the gold energy measurement region. 
The operating voltage of the uranium or plutonium anode X-ray tube is 
ideally higher than the tungsten anode tube, possibly as high as 300 kV or 
thereabouts. The higher the voltage, the higher the efficiency of 
production of characteristic radiation from the anode material. Also, the 
higher the energy of the bremsstrahlung, which is produced at the same 
time. As stated above some tin filtration is still needed to reduce the 
low energy tail of the bremsstrahlung but the latter should be a much 
lower fraction of the total exciting intensity. 
An alternative to an X-ray tube using electrons for production of 
characteristic K X-rays of plutonium or uranium, is to bombard the target 
material with protons or alpha-particles. These produce much less 
bremsstrahlung when they are slowed down in the target material. 
The ore samples must be finely ground but the average grain size, which 
should preferably be less than 100 microns, is not critical, although it 
has been found that with larger gold grain sizes in the ore better results 
are obtained by fine grinding. With fine gold grain sizes the sample may 
be ground to sub-millimetric size only. The sample is preferably contained 
in a cylindrical thin-walled container of plastic material such as acetal 
plastic, certain features of which are, as more particularly described 
below, preferred according to the invention. 
A preferred feature of the invention is the use of a heavy metal filter 
interposed between the fluorescent sample and the detectors. For gold 
analysis using X-rays from a tungsten anode tube the heavy metal is 
preferably platinum or iridium; for platinum analysis an osmium filter is 
preferred. As will be more particularly described later with reference to 
the drawings, the use of a filter of one of these metals reduces the 
scattered bremsstrahlung peak at about 100 keV and enables the 
characteristic bands to be more readily detected by allowing the detection 
of higher count rates in the energy region of interest. 
Apart from considerations of cost, a platinum filter should be used where 
the content of lead in the sample is to be measured, as the iridium 
K.beta. band overlaps the lead K.alpha. band. 
The invention is not, however, restricted to the use of a filter of iridium 
or platinum, or indeed of any metal. As described herein, these particular 
metals are especially useful in analysis for gold and certain other 
elements using particular sources of exciting radiation. For example, 
platinum may be detected and analysed using an osmium filter. However, it 
is possible using other sources (for example radioisotopes) to analyse for 
low concentrations within the range contemplated by the invention and this 
is of particular relevance for the detection and analysis of lighter 
elements such as silver and copper as well as for that of gold, using the 
same detectors as are contemplated herein; such methods are, however, not 
within the scope of the present invention. A heavy metal filter cannot be 
used for the simultaneous measurement of light and heavy elements in 
similar dilutions. Nevertheless, whatever the excitation source, the use 
of an appropriate filter such as those described herein will generally 
make possible analysis at lower concentrations than is possible without 
such filter. 
The above principles may be summarised by the statement that the filter 
medium is chosen to reduce the bremsstrahlung peak so as to enhance the 
count rate in the energy region of interest. 
The invention is not limited to the use of filters consisting of a foil or 
sheet of the metal in question. For example, the filter may be made from 
finely divided metal dispersed in an epoxy or other resin matrix. Similar 
considerations also apply to the tin or other metal filter which, as 
mentioned above, is to be interposed between the radiation source and the 
sample. 
The fluorescence photons passing through the iridium or platinum filter are 
preferably counted by at least one detector consisting substantially of 
metallic germanium. It is found that circular germanium detectors arranged 
in a regular array and orientated axially to the incident radiation are 
preferable. The combined use of germanium detectors and an iridium filter 
in preferred embodiments of the invention is an important feature which 
gives the capacity to detect gold in ore samples down to a concentration 
of 1 ppm at a confidence level of 95% in a counting time of 100 seconds. 
The detector unit is preferably cooled to cryogenic temperatures by liquid 
nitrogen from a small cryostat automatically topped up from a large 
cryostatic reservoir. 
The bombardment of the detectors by the high energy fluorescence photons 
emitted from the sample passing through an iridium filter generates a 
signal the intensity of which is amplified by pre-amplifiers (generally, 
one for each detector) and processed to give only the readings for the 
metals to be analysed. 
The invention includes the analysis of sample containing other heavy 
elements such as uranium, platinum and lead. Indeed these elements are 
commonly found in gold ores. It is possible using the invention to measure 
two or more of these metals simultaneously, for example, especially 
uranium and gold. In principle it is possible to adapt the apparatus and 
method according to the invention to detect and analyze for elements in 
the periodic table down to, say, tungsten. 
A method and apparatus corresponding to those of the invention may be used 
for measuring heavy metals such as gold in materials other than mineral 
ores; of especial importance is the measurement of gold in carbon from 
carbon-in-pulp plants. 
The drawings illustrate the underlying physic and the layout of an 
apparatus according to preferred embodiments of the invention.

In FIGS. 1a and 1b the vertical axis represents the count of photons and 
the horizontal axis the energy of the exciting X-rays; this illustrates 
the spectrum of the X-rays bombarding the sample in the method and 
apparatus according to the invention. FIG. 1a shows, at 1, the broad 
bremsstrahlung peak emitted by an X-ray tube with a tungsten anode 
operated at 130 kV. It will be seen that there is a broad maximum at 
photon energy about 65 keV. According to the invention this broad maximum 
is reduced by filtration through metallic tin to give a reduced maximum 
for irradiation of the sample. This reduced maximum is shown on a larger 
scale in FIG. 1b, from which it will be seen that there is a peak at 115 
keV with a Gaussian fall-off in number of counts on either side. A typical 
ore containing gold, lead and uranium, after bombardment with incident 
X-rays having the spectrum shown in FIG. 1b, emits at 90.degree. 
fluorescence radiation having the spectrum illustrated in FIG. 2 in which 
the axes represent the same parameters. This Figure illustrates the 
characteristic peaks of gold, lead and uranium, those of particular 
significance for the invention being the gold K.alpha..sub.1 peak at 68.8 
keV and the gold K.alpha..sub.2 peak at 67.0 keV. The maximum of the 
overall bremsstrahlung peak is shifted from about 115 to about 100 keV. 
At the concentrations with which the invention is primarily concerned, 
(i.e. up to 10 ppm of ore) the gold peaks in the spectrum shown in FIG. 2 
cannot be measured accurately with a time span convenient for industrial 
application by even the most sensitive means presently known. The maximum 
count-rate which each detector and associated electronics channel can 
handle is an inherently limiting factor. With known pulse-shaping 
techniques, as are envisaged for use in the invention, there is a 
trade-off between count-rate and detector resolution. At the detector 
resolution required for the analysis of the ores and other materials in 
which we are interested, the maximum input counting-rate is about 150,000 
counts per second. 
In the context of analysis for gold and uranium, however, only the fairly 
narrow energy bands around the gold K.alpha. (and uranium K.beta.) peaks 
are of interest. An ideal detector would respond only to these. 
Unfortunately the detector response (illustrated diagrammatically in FIG. 
5, see below) can only be partially optimized by careful selection of the 
detector thickness (in the range of 2-4 mm) and the photons in the large 
bremsstrahlung peak, which are of no interest, use up a large proportion 
of the detector live time. 
This difficulty is overcome according to the invention by the use of an 
iridium or platinum filter, the characteristic absorbtion spectra of which 
are illustrated in FIG. 3 in which the vertical axis represents absorbtion 
and the horizontal axis the energy of the incident radiation. When X-rays 
emitted by fluorescence from a sample in the apparatus according to the 
invention are passed through such a filter before detection the absorbtion 
spectrum of the filter is effectively superimposed upon the peak shown in 
FIG. 2 with result that the gold K.alpha..sub.1 and K.alpha..sub.2 peaks 
are much more readily detectable for the same total detector count rate, 
because there are more counts in the energy region of interest and 
statistical errors are reduced. 
In other words, the iridium and platinum filters used according to the 
invention preferentially attenuate the higher energy radiations in which 
we are not interested. For example, a 0.125 mm thick iridium filter will 
transmit about 40% of the photons in the regions of interest (i.e. 
channels 0-5 in FIG. 6), but will transmit only about 20% of photons in 
the range 80-120 keV (the bremsstrahlung peak). As this latter peak 
comprises most of the photons, when we use the iridium filter we require 
about 5 times more power from the X-ray source to get the count-rate back 
to the maximum which the detectors can handle. However, the use of an 
iridium (for example) filter converts the spectrum shown in FIG. 2 to that 
shown in FIG. 4, and the proportion of photons in the region of interest, 
relative to the total count-rate, has increased by a factor of 0.4.times.5 
i.e. doubled. Thus the use of the iridium filter has the same effect as 
doubling the number of detectors, and the statistical error in the result 
is reduced by about 2. A thicker filter would give more improvement. 
This selectivity is enhanced according to the invention by the use of 
germanium detectors, the efficiency curve of which is illustrated in FIG. 
5 in which detection efficiency (%) assuming photoelectric cross-section 
only is plotted against excitation energy (keV) for various detector 
active thicknesses. The efficiency falls assymptotically away from 100% 
with increasing exciting energy, being about 90% for a 4 mm thick detector 
at the crucial energy of 68.8 keV (the energy of the gold K.alpha..sub.1 
band). With increasing energy the efficiency falls off more rapidly to 
about 50% at 110 keV. It will be seen that a thickness of 2-4 mm gives a 
curve corresponding most closely to that of FIG. 4, and a detector 
thickness within this range is therefore preferred according to the 
invention for use in gold analysis. 
The spectrum resulting from the combined use of an iridium filter and 
germanium detectors in the apparatus according to the invention gives a 
spectrum as shown in FIG. 6 which corresponds generally to FIG. 4 but 
shows the gold K.alpha..sub.1 and K.alpha..sub.2 bands on a greatly 
enlarged scale. The associated electronic instrumentation is programmed in 
known manner to measure the number of impacts in selected band widths (or 
channels) such as those shown in FIG. 6. The instrumentation is programmed 
to compare the total signal plus background from the bands numbered 2 and 
3 with the total background signal from the bands 1 and 4. The measurement 
of the gold content of the ore is therefore based not on an absolute 
measurement but on comparative measurements, thus eliminating 
uncertainties and inaccuracies inherent in the measurement of absolute 
values (e.g. the variations in excitation voltage or current in the X-ray 
source). 
For gold measurement, bands 1-4 are used. Each band has a nominal width of 
600 eV, but in practice these widths may be altered to suit detector 
resolution, for example bands 1 and 4 may be 500 eV wide, and bands 2 and 
3 may be 700 eV wide, or vice versa. Bands 0 and 5 are used for diagnostic 
purposes. 
The symmetry of the curve around the K.alpha..sub.1 peak is important. In a 
diagnostic routine, a solid gold check source is used to determine the 
proportion of gold counts falling into each of channels 1-4. The spectrum 
from a solid gold source does not contain much scattered radiation (which 
originates predominantly from low atomic number material) and is shown in 
FIG. 8. The count in channel 2 is compared with the count in channel 3, 
and if these are unequal by more than a predetermined amount, the 
mid-point position between channels 2 and 3 is altered electronically, 
until symmetry is restored. Thus the fraction X of counts in channels 
(2+3) over channels (1+2+3+4) is measured and stored, and this indicates 
what fraction of the signal representing the gold K.alpha..sub.1 band, 
which is nominally in channels 2 and 3, is spilling over into channels 1 
and 4. 
As will be mentioned in more detail below, the use of a check source of the 
metal being analysed is an important technique in practising the 
invention. When a series of samples are being analysed, as will normally 
happen, the check source is interposed between samples at intervals of, 
say, 4 or 5 samples, to determine and correct for any instrument drift due 
for example to temperature changes. 
This process for gold is given by the algorithm: 
##EQU1## 
where G=gold concentration in suitable units (e.g. g/tonne) S=a normalized 
sensitivity factor (corrected for sample density and predetermined with 
calibration samples as described in more detail below) 
K=a background factor (predetermined and corrected for sample density with 
calibration samples) 
T.sub.1 =counts in channels (2+3) 
T.sub.2 =counts in channels (1+4) 
X=fraction of total signal counts in channels (2+3) (predetermined) 
Y=fraction of total signal counts in channels (1+4) 
(The total number of signal counts being the numbers in channels (1+2+3+4), 
so that Y=(1-X)) 
Ideally, with no spillover of gold signal into channels 1 and 4, X=1 and 
Y=0, and 
##EQU2## 
The preferred normalizing technique according to the invention for the 
factor S is as follows. The counts from the sample depend both on the 
sample density and on the number of photons exciting the sample. 
Variations due to the latter (e.g. due to changes in the X-ray generator 
current and high voltage) can be minimised by reference to the counts (H) 
obtained recently from a standard scatterer, which can be a solid gold 
check source or an aluminium background standard. The assumption is made 
that the instrument has remained stable since the last reading of the 
standard, which is reasonable. Thus, the variation in counts from the 
sample can be normalised to a standard excitation intensity, and the 
remaining variations are due to sample density alone, which is corrected 
for as described below. 
It is within the scope of the invention to use a separate detector or 
detectors to check dynamically the excitation intensity and so remove the 
remaining uncertainty due to the time delay. 
It has been found that, as sample density increases, the counts B in the 
background channels (corrected for overlap of signal) also increase, due 
both to more primary scattering and to more multiple scattering. The 
increase is partially offset by more attenuation and absorption. At the 
same time the signal counts per ppm also increase, due to the increased 
number of interactions, but not so fast as B. It has been determined, with 
a high correlation coefficient, that this process can be described by the 
equation: 
##EQU3## 
where M and N are constants determined by regression analysis from a set 
of values of S and B obtained from known high value ore samples, having a 
range of densities. Even single samples can be prepared with a range of 
densities, by a combination of compression and grinding to different grain 
sizes. B should ideally be corrected for system deadtime, but in most 
practical cases the latter is largely compensated for in the actual 
measurement of M and N for similar photon energy spectrums. Variable lead 
peaks can cause a slight error, as they vary system deadtime. 
The ratio T1/T2 in the gold equation given above is independent of system 
deadtime. 
Other techniques for refining the density correction will readily be 
apparent, for example by consideration of higher energy sections of the 
photon energy spectrum which are less affected by multiple scattering. 
However, a particular advantage of basing the density correction on 
background channels on both sides of the signal channels is that primary 
attenuation and absorption effects are matched for both signal and 
background. 
The values of M and N determined also compensate for counts due to the 
sample container itself, and other scattered radiation not originating 
from the sample. 
The software can be made slightly easier by redefining the signal counts 
simply as those appearing in channels (2+3), in which case X=1. Then the 
equation becomes: 
##EQU4## 
This is not different, but the same process in slightly different format. 
FIG. 9 illustrates schematically the geometry of excitation and detection 
according to the invention. As mentioned above, an important feature of 
the invention is that it provides a geometry for the apparatus which 
exploits the polarization of X-rays emitted from a source with a thick 
target. In FIG. 9, the X-ray source is indicated generally at 91 and 
comprises an X-ray tube 92, with tungsten cathode at 93 and tungsten anode 
at 94. The exciting radiation 90 from the anode 94 emerges through a lead 
collimator 95 and a tin filter 96 to strike a sample 97. Scattered and 
fluorescent radiation 102 from the sample emitted at about 90.degree. to 
the incident exciting radiation passes through an aperture in a tin 
collimator 98 and an iridium or platinum filter 99 to the germanium 
detector array 100. This array may consist for example of two vertical 
rows of 8 detectors each. These are preferably circular and may for 
example be about 8 mm in diameter and about 2-4 mm thick. The tolerance in 
thickness is about 15% owing to limitations in the reproducibility of the 
lithium-diffused contact. Square cross-sectioned detectors would be 
preferable but currently available detectors are not suitable at 
high-count rates due to insufficient field strength in the rear corners 
resulting in low-energy tailing. A second filter and detector array 
(similar to that already described) may be provided at 101. Radiation 103 
scattered from the tin filter 96 is stopped by the collimator 98. 
The preferred use according to the invention of a scattering angle of about 
90.degree. is of especial significance for the measurement of uranium 
because the X-rays emitted from a thick target are often partially 
polarized. It is therefore important that, as shown in FIG. 9, the sample 
should be positioned so as to intercept X-rays emerging frOm the tube at 
90.degree. to the electron beam passing from cathode 93 to anode 94 in the 
tube. 
This polarization phenomenon can, according to a preferred feature of the 
invention, be applied advantageously to the simultaneous analysis of gold 
and uranium. This is illustrated in FIG. 7(a), in which, as in previous 
figures, number of photon impacts is plotted vertically and photon energy 
in keV horizontally. The curve illustrated is that containing the uranium 
K bands on the high-energy side of the energy peak in FIG. 4. Broken curve 
(a) in FIG. 7(a) illustrates the curve obtained without the advantage of 
the reduction of background radiation obtained using the polarization 
technique described above. Curve (b) is obtained using the polarization 
technique, and illustrates the reduction in background counts. The 
channels 0-5 illustrated in FIG. 7 are counted and compared in a manner 
generally similar to that described above with reference to FIG. 6. In 
FIG. 7(a), the signal channels are channels 1, 2 and 4 and the background 
channels 0, 3 and 5. The edge of the uranium K-band is at 115.6 keV, thus 
if we are operating at 125-130 keV, reasonable excitation and polarization 
effects are obtained. 
This technique for measuring uranium could suffer from interference by a 
thorium peak which occurs in channel 0 as shown in FIG. 7(a). In FIG. 7(b) 
is illustrated a method according to the invention of reducing or 
eliminating this interference. In FIG. 7(b) the channel numbers and 
energies have been shifted to lower energies and part of channel 0 
re-allocated to provide an additional channel for thorium. The principal 
uranium K.beta..sub.1 and K.beta..sub.3 peaks then appearin channels (2+3) 
and the background measurement is determined from channels (0+4). The 
thorium K.beta..sub.2 peak appears in channel 1. This not only reduces or 
eliminates thorium interference where uranium is being determined but also 
provides a method of determining thorium concentration. 
A further application of the selection and allocation of channels according 
to preferred embodiments of the invention to reduce or eliminate 
interference from unwanted elements or even to determine the concentration 
of the same elements is illustrated in FIGS. 7(c) and 7(d). A preferred 
feature of the invention is the use of an analysis board with six 
channels, which not only facilitates the determination of certain elements 
as described below, and is the number required for uranium determination 
but also simplifies electronic design. FIGS. 7(c) and 7(d), which have 
been separated for clarity, illustrate the simultaneous detection of 
interference from mercury and tungsten in analysis for gold. 
As shown in FIG. 7(c) the mercury K.alpha..sub.2 peak overlaps the gold 
K.alpha..sub.1 peak. The presence of mercury is detected via its 
K.alpha..sub.1 peak in channel 5. This enables the gold result to be 
questioned or corrected. 
In most gold mining areas, the ratio of mercury to gold is very low, and in 
practice mercury in ore samples is not expected to present a problem. In 
processed material, however, the mercury may be concentrated relative to 
the gold. The K.alpha..sub.2 peak of thallium overlaps the mercury 
K.alpha..sub.1 peak and can give a false mercury indication. In practice, 
the occurrence of thallium is rare, and the indication of possible 
interference is fail-safe. 
FIG. 7(d) illustrates the application of this technique to tungsten, the 
K.beta. peaks of which occur at virtually the same energy as the gold 
peaks but are differently proportioned as regards amplitude. The tungsten 
K.beta..sub.2 peak occurs at a slightly higher energy than the gold 
K.alpha..sub.1 peak, and most of it falls in the signal channels (2+3) 
with a small amount in channel 4. Part of the tungsten K.beta..sub.1 peaks 
fall in channel 1. The tungsten signal appearing in channel 0 is much 
greater than the signal appearing in channels (2+3), which contrasts with 
the gold signal, which is higher in channels (2+3) than in channel 0. Thus 
the ratio of the total signal in channels (2+3) to the total signal in 
channel 0 provides an indication of whether or not tungsten is present. 
The platinum K.alpha..sub.1 peak and the tantalum K.beta..sub.2 peaks also 
appear in channel 0, and can give false indications of tungsten. However, 
in practice, the occurrence of these elements in ore bodies when gold is 
the major mineral is rare. The indication of possible interference is 
fail-safe. 
FIG. 10 is a schematic block diagram showing the method and apparatus 
according to the invention in use. The apparatus consists essentially of a 
source of high energy X-rays (photons) 201 arranged to bombard a sample in 
a cylindrical container 202 as described above and below through a 
collimator 203. Interposed between the X-ray source and the sample 202 is 
a metallic tin filter 204. The fluorescence photons emitted at right 
angles to the bombarding rays pass through an iridium filter 205 before 
collimation to a detector 206 comprising a regular array of germanium 
detector elements with axes parallel to the incident radiation, each with 
its own pre-amplification and signal conditioning circuitry. 
In FIG. 10, 209 represents a check source, consisting for example of a 
piece of gold foil in a sample container which is interposed between 
every, say 4 or 5 ore samples during readings. 
The sample container 202 is preferably a cylindrical thin walled container 
of plastic material such as acetal plastic. A thin container is necessary 
so that counts due to the sample container are very much less than the 
counts due to the sample. We have found that generally a diameter in the 
range of 10-30 mm gives satisfactory results depending on the 
considerations set forth below, while a wall thickness of 0.35 to 0.5 mm 
typically about 0.4 mm provides sufficient rigidity, can be made 
reproducibly, and complies with the above requirement. 
The diameter of the sample container can be varied to suit the application 
and ore bodies. For inhomogenous ore bodies, when sampling error is high, 
it is desirable to maximise the mass of sample irradiated, and an internal 
diameter of typically about 18.7 mm is a good compromise between the 
conflicting requirements of sensitivity and sampling error. 
However, the smaller the diameter of the sample container, the higher the 
sensitivity will be, as there will be less attenuation or absorption of 
the signal, and less multiple scattering. However, if the sample container 
is made too small, the effect of counts from its wall will become more 
noticeable, and also the power of the X-ray generator will have to be 
greatly increased to provide the high count rate necessary for 
minimisation of errors due to counting statistics. The sample size will be 
smaller, and may cause sampling errors. However, for the homogeneous ore 
bodies, as occur in several parts of the world, the sampling error is low, 
and a good compromise is a tube having an internal diameter of about 12.7 
mm. 
The invention is not, however, limited to the use of cylindrical containers 
with the above dimensions. It is also possible to use differently shaped 
sample containers with, for example, rectangular cross sections in which 
the long axis of the rectangular cross section is angled at about 
45.degree. to both the exciting beam and the detected beam, so that the 
desired scattering angle, as described above, is still in the order of 
90.degree.. 
A further precaution by which inaccuracies due to non-uniform particle size 
or packing in the sample can be reduced is to shift the sample container, 
for example along its longitudinal axis, and to measure the scattered 
radiation in two or more positions of the containers; alternatively the 
zone of the sample scanned (with the preferred containers of the 
dimensions mentioned above this is generally about 7 cm long) may be moved 
along the sample. 
The apparatus shown schematically is FIG. 10 employs an automatic sample 
changer to achieve a continuous throughput of, for example, one sample per 
100 seconds. Such rapid throughput is as described above essential in 
mining applications where continuous analysis of a succession of samples 
must continue at all times. 
It is within the scope of the invention to irradiate the sample with a 
second beam of exciting X-radiation from a source diametrically opposite 
to the first source. This technique reduces the gradient of the exciting 
photons throughout the thickness of the irradiated sample and may also be 
applied to the use of other types of irradiation source such as special 
X-ray tubes and radioisotopes; the use of radiootopes source is not 
however within the scope of the present invention.