Patent Number: 050200844
Section: description

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) PA1 T.sub.1 =counts in channels (2+3) PA1 T.sub.2 =counts in channels (1+4) PA1 X=fraction of total signal counts in channels (2+3) (predetermined) PA1 Y=fraction of total signal counts in channels (1+4) PA1 (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.