GB2151837A

GB2151837A - Lithium target

Info

Publication number: GB2151837A
Application number: GB08429284A
Authority: GB
Inventors: Colin Geoffrey Clayton
Original assignee: UNITED KINGDOM ATOMIC EMERGY A
Current assignee: UNITED KINGDOM ATOMIC EMERGY A
Priority date: 1983-11-30
Filing date: 1984-11-20
Publication date: 1985-07-24
Also published as: SE8406080L; SE458241B; ZA849308B; FI844689L; FI844689A0; GB8429284D0; FR2555853B1; GB2151837B; GB8331913D0; DE3443795A1; AU561875B2; FR2555853A1; AU3593984A; JPS60198100A; CA1226683A; BR8406075A; SE8406080D0

Abstract

A target for generating fast neutrons by the reaction 3<7> Li (p, n) 4<7> Be consists of a thin lithium foil 30 supported within an evacuated beam tube 10 by a water-cooled support structure 12. Gaps 24 in the support structure 12 are bridged by the foil 30, and in use a proton beam is scanned over these bridging portions of the foil. <IMAGE>

Description

SPECIFICATION Lithium Target This invention relates to a lithium target for bombardment by protons to generate neutrons by the reaction 3 Li (p, n) 47 Be, and to apparatus for detecting the presence of a selected substance in ores by neutron activation analogous, for example the gold content of gold-bearing ores.

A practical gold ore sorting plant needs to be able to process several tens of tonnes of ore an hour, and hence must use a rapid analytical technique. A suitable technique is neutron activation analysis using the reaction 197 Au (n, nty) '97"Au to activate gold present in a lump of ore, the 197mAu nuclides so produced decaying with a half-life of about 7.8 seconds, with the emission of y-rays of energy 297 keV.British Patent Specifications Nos 2 055 465B and 2 101 304A (US Patent No. 4 340 443, and US Serial No. 383 686 filed 27 May 1982, respectively) which are incorporated by reference herein, describe apparatus for sorting gold bearing ores in which lumps of ore are activated by the above reaction, the y-rays emitted subsequently being detected and analysed to assess the gold content of the ores.

Such an ore-sorting plant requires an intense source of fast neutrons to bring about the activation, and one possible source is a target consisting of a lithium layer, coated onto a silver backing plate, and bombarded by protons. However the effect of the bombardment is to generate heat and hydrogen in the target, which can be detrimental to the target structure. In the above target, for example, hydrogen bubbles tend to form at the interface between the lithium and the silver.

According to the present invention there is provided a lithium target comprising a relatively thin foil of lithium supported by a support structure whereby the foil may be supported within an evacuated beam tube to be bombarded by a proton beam, the support structure including a pipe for the passage of a coolant fluid and having at least one gap therethrough such that a portion of the foil bridges the gap and is exposed to vacuum on both sides thereof.

In use of the target the proton beam is preferably incident, at least most of the time, on the portion of the foil which bridges the gap.

In a preferred embodiment, the lithium foil is supported near to an end wall of the beam tube, the end wall being of a material having a low cross-section for neutrons from the foil and through which hydrogen diffuses sufficiently rapidly to prevent a build-up of hydrogen within the beam tube. The end wall may be of palladium, and may be heated to ensure rapid diffusion of the hydrogen.

In a preferred embodiment the support structure comprises a plate defining at least one substantially annular gap therethrough, and the foil bridges the gap or gaps, being supported at each side of the gap.

The invention thus provides a target, to act as a source of high energy neutrons, which can be expected to suffer less damage than known targets, and which generates neutrons of a relatively welldefined energy range since the thickness of the lithium is well-defined.

The invention also provides an irradiator for irradiating lumps of ore for detecting the presence of a selected substance in the lumps, the irradiator including as a neutron source the lithium target defined above.

The invention will now be further described by way of example only and with reference to the accompanying drawings, in which: Figure 1 is a flow diagram of a gold ore sorting apparatus including a lithium target according to the invention; Figure 2 is a sectional view of the lithium target of Figure 1; and Figure 3 is a view in the direction of arrow A of Figure 2.

Referring to Figure 1, a gold ore sorting apparatus comprises a rock crusher and classifier 2 to which mined ore is supplied, in which the ore is crushed into lumps, and from which emerges a stream of lumps corresponding to mesh size of about 75 mm, while lumps smaller than mesh size about 35 mm are rejected. The stream of lumps is passed through an iradiation chamber 4 adjacent to a lithium target 5 to be described in more detail later, and then all the lumps are caused to pass a y-ray detector assembly 6 arranged to detect trays having an energy of 279 keV arising from the decay of 97,7Au nuclides and so signifying the presence of gold in the lumps of ore. Each lump of ore is interrogated individually by the detector assembly 6 to establish whether its gold content lies above or below some predetermined concentration.The critical concentration is typically in the range 0.5 to 5 parts per million (ppm), and might for example be set at 1 ppm. Each lump of ore is then passed into a sorter 8 arranged by means of a cable 7 to respond to signals from the detector assembly 6, and to sort each lump of ore into one of two outlet streams depending on whether the gold concentration in the lump lies above or below the predetermined concentration.

The crusher and classifier 2 and the sorter 8 may be of types well known in the art, while the detector assembly 6 may be as described more fully in the aforementioned specifications to which reference may be made, the crusher and classifier 2, the sorter 8 and the detector assembly 6 not being the subject of the invention.

Referring to Figures 2 and 3, the lithium target 5 is located at one end of an evacuated beam tube 10 along which a 1 mA beam of 4.5 MeV protons is passed during operation of the apparatus of Figure 1. The target 5 includes an aluminium circular support plate 12, consisting of an inner hub 14, three concentric rings 16, and an outer ring 18, linked by three equally spaced radial struts 20 (only one of which extends to the hub 14) so as to define four concentric gaps 24 of substantially annular shape, crossed by the struts 20. The four gaps 24 are respectively bridged by four annular lithium foils 30, each of width 30 mm and of thickness 0.3 mm, attached along each edge to the support plate 12.A small hole 32 through the centre of the hub 14 ensures there is no pressure difference between the two sides of the support plate 12, and the support plate 12 is attached around its perimeter to the wall of the beam tube 10. Five pipes 34, 35, 36, 37, 38 are attached along concentric circles to the hub 14 the three rings 16 and the outer ring 18 respectively, and their ends extend radially outwards to pass through the wall of the beam tube 10. The radial portions of the pipes 34, 35 and 36 are aligned with the struts 20.

As shown in Figure 2, across the end of the beam tube 10, about 20 mm away from the lithium foils 30, is a palladium plate 40 sufficiently thick to withstand a pressure difference of one atmosphere, and with a circumferential rim 42. A copper end plate 44 is attached around the rim 42, and has two diametrically opposite ports 46 connected to ducts 48.

In operation of the apparatus of Figure 1, the proton beam is accelerated down the beam tube 10 onto the target 5, being scanned around over each of the lithium foils 30 in succession. At the same time a coolant liquid such as water is passed through the tubes 34, 38 to ensure that the temperature of the lithium does not reach its melting point, 186-C, and air is passed through the ducts 48 so as to flow over one side of the palladium plate 40 and so to extract excess heat from it.

As a result of the nuclear reaction 3 Li (p, n) 7 Be, an intense flux of fast neutrons of energy between about 0.6 MeV and 2.8 MeV emerges from the target 5, to irradiate the lumps of ore passing through the adjacent irradiation chamber 4 (see Figure 1).

The thickness of the lithium foil 30 and the energy of the incident protons determine the range of energy of the emitted neutrons. The cross-section for activating gold nuclei,19?Au, is a maximum at about 2.5 MeV, and neutrons of energy between 0.6 MeV and 2.8 MeV can bring about this activation but have insufficient energy to bring about activation by (n, p) reactions of other elements which are likely to be present in the ore, such as aluminium and silicon. Neutrons of energy below about 0.6 MeV are unlikely to activate the gold nuclei but may bring about activation by (n, y) reactions of for example aluminium, and so are undesirable.

The thickness of the foils 30, which is 0.3mm in this case, is governed by these considerations. It will be appreciated that since there is a vacuum on both sides of the foils 30, the portions of the foils 30 which bridge the gaps 24 need only support their own weight.

Those protons which do not undergo the above reaction with lithium atoms emerge from the lithium foil 30 with an energy of about 3.3 MeV and are then incident onto the palladium plate 40, in which their energy is dissipated to heat by coilisions with the palladium lattice. The plate 40 is thus heated, its temperature being controlled by the air flow through the ducts 48, and hydrogen atoms (i.e. protons which have been slowed down) diffuse rapidly through the hot plate 40 and are removed in the air flow.

It will be appreciated that since the lithium foils 30 are separated by a gap of about 20 mm from the plate 40, the plate 40 may be heated to a temperature well above the melting point of lithium, heat transfer across the gap only being by heat radiation. Consequently the rate of diffusion of hydrogen through the plate 40 is greater than for a plate 40 held at a low temperature. Furthermore, although the plate 40 has been described as being of palladium, another metal such as niobium may be used through which hydrogen diffuses at a sufficiently rapid rate.

Claims

1. A lithium target comprising a relatively thin foil of lithium supported by a support structure whereby the foil may be supported within an evacuated beam tube to be bombarded by a proton beam, the support structure including a pipe for the passage of a coolant fluid and having at least one gap therethrough such that a portion of the foil bridges the gap and is exposed to vacuum on both sides thereof.

2. A lithium target as claimed in Claim 1 wherein the support structure comprises a plate defining at least one substantially annular gap therethrough, and the foil bridges the gap or gaps, being supported at each side of the gap.

3. A lithium target as claimed in Claim 1 or Claim 2 further comprising an end wall for the beam tube, the end wall being spaced apart from the foil and being of a material having a low crosssection for neutrons from the foil and through which hydrogen diffuses sufficiently rapidly to prevent a build-up of hydrogen within the beam tube.

4. A lithium target as claimed in Claim 3 wherein the end wall is of palladium.

5. A lithium target as claimed in Claim 3 or Claim 4 also comprising means for controlling the temperature of the end wall to ensure rapid diffusion of the hydrogen.

6. A lithium target substantially as hereinbefore described with reference to, and as shown in, Figures 2 and 3 of the accompanying drawings.

7. An irradiator for irradiating lumps of ore for detecting the presence of a selected substance in the lumps, the irradiator including as a neutron source a lithium target as claimed in any one of the preceding Claims.