Patent Number: 043404430
Section: description

Referring to the drawings, auriferous rock 1 is fed to a hopper 2 which supplies it to a rock crusher 3 in which it is crushed into lumps 4 corresponding to a mesh size of some 5 cm. The stream of crushed rock leaving the crusher 3 is divided into a number of streams 5 only one of which is shown, which pass through a neutron irradiation assembly 6, to be described more fully later. Having been irradiated by neutrons generated within the assembly 6, each of the streams of lumps 4 of rock is caused to pass a .gamma.-ray detector assembly 7 which is arranged to detect any .gamma.-rays having an energy of 279 keV arising from the nuclear reaction .sup.197 Au (nn') .sup.197.sbsp.m Au, occurring in any gold contained in the lumps 4 of rock. Each lump 4 of rock is interrogated individually to establish whether its gold content lies above or below some predetermined concentration. For example, the critical concentration might be 5 ppm. In general it might be in the range 1 to 10 ppm. Downstream of the .gamma.-ray detector assembly 7 is a sorter 8 of a type which is well known in the art of material sorting, and which will not be described further. The sorter 8 is arranged to respond to signals from the .gamma.-ray detector assembly 7 to accept or reject for further processing each lump 4 of rock passing through it. Referring to FIG. 2, the neutron irradiation assembly 6 consists of a cylindrical body 21 made of lead which is surrounded by a biological radiation shield 22 which is made to be impervious to neutrons and to .gamma.-rays. In the body 21 there is a central bore 23 around the periphery of which there are positioned six tubes 24 made of boron. The tubes 24 extend throughout the length of the body 21. Each of the tubes 24 has a bore which is such that only a single stream of lumps 4 of rock can pass through the relevant tube. In the central region of the bore 23 in the body 21 there is a target 25 made of a material which will produce neutrons in response to bombardment by a beam of deuterons from a source which is not shown. Suitably, the target 25 can be made of a material which contains deuterons, or beryllium. The important thing is that the neutron source should be made of a material which does not produce neutrons which are energetic enough to excite fast neutron reactions in the constituent elements of the rock in which the gold is contained, i.e. aluminum, silicon, calcium, iron and oxygen. The target 25 will emit neutrons over a solid angle of 4II, but as the lumps 4 of rock pass through the maximum neutron field at 90.degree. to the direction of the neutrons, it can be arranged that while the neutron source energy equates to the maximum energy of the reaction EQU .sup.197 Au(nn').sup.197.sbsp.m Au, the neutron energy is below the threshold energies of (n,p) reactions in such as those previously mentioned which are likely to be present in the rock in high concentrations. In particular care should be taken to ensure that the neutron energy is below the threshold of the fluorine reaction EQU .sup.19 F(n,.alpha.).sup.16.sbsp.N.sbsb..beta. .fwdarw..sup.16 O(T.sub.1/2 =7.3 secs) This reaction generates .gamma.-rays having energies of 6.1 and 7.2 Mev. Although these are considerably greater than the 0.279 Mev from the .sup.197.sbsp.m Au, the half-life is the same, and the reaction could be the source of low-energy collided .gamma.-rays which would have the same decay pattern as those to be detected, and so interfere with the estimation of the gold content of the lumps 4 of rock, particularly if the fluorine is present in concentrations which are relatively high when compared to that of the gold. The .gamma.-ray detector assembly 7, which is not illustrated in detail, has six linear arrays of .gamma.-ray detectors, one for each stream of lumps of rock. The signals which operate the sorter 8, which again has six input channels, are derived from the combined output signals from each of the individual .gamma.-ray detectors appropriate to each channel. A neutron output of some 10.sup.11 n/s from the neutron source enables a lump of rock having a gold concentration of 1 ppm to be differentiated from one having a gold concentration of 2 ppm in one of three measurements, and a lump having a gold concentration of 2 ppm to be differentiated from one having a gold concentration of 5 ppm in 99 in 100 measurements.