FR2555853A1 - LITHIUM TARGET FOR BOMBARDMENT BY PROTONS TO PRODUCE NEUTRONS - Google Patents

Abstract

THE OBJECT OF THE INVENTION IS A TARGET FOR PRODUCING FAST NEUTRONS BY REACTION 73 LI (P, N) 74 BE. IT INCLUDES A THIN CLINQUANT OF ALUMINUM 30 SUPPORTED IN A TUBE 10 WITH DIRECTED BEAMS AND IN WHICH A VACUUM HAS BEEN ESTABLISHED BY A WATER-COOLED SUPPORT STRUCTURE 12; THE BREACHS 24 IN THE SUPPORT STRUCTURE 12 ARE COVERED BY THE CLINQUANT 30; IN SERVICE, A PROTON BEAM EXPLORES THESE CLINQUANT COVERAGE PARTS. SORTING A GOLD ORE TO DETECT THE POSSIBLE PRESENCE OF A CERTAIN CONCENTRATION OF GOLD.

Description

The present invention relates to a lithium target for bombardment by

  protons to produce 7 L (, n) 7 Btu neutrons by reaction 7 Li (p, n) Be and a

3 4

  apparatus for detecting the presence of a selected substance in ores by neutron activation, for example to detect the gold content of a

gold ore.

  A practical gold ore sorting workshop should be able to process several tens of tons of ore per hour and therefore must use a fast analytical technique. A suitable technique is neutron activation analysis using the reaction 97Au 19 7mA (n, N'Y) 97mAu to activate the gold present in a piece of ore, the 197mAu nuclides as well

  products being disintegrated after a period of

  activity of about 7.8 seconds, with emission of gamma rays having an energy of 279 keV. British Patent Nos. 2,055,465 B and 2,101,304 A (US Patent 4,340,443 and US Patent Application Serial No. 383,686 filed May 27, 1982, respectively) describe

  an apparatus for sorting gold-bearing ores in

  which pieces of ore are active by the

  reaction above, gamma rays emitted later-

  being detected and analyzed to determine with

  accuracy the gold content of the ores.

  An ore sorting workshop requires an intense source of fast neutrons to trigger activation, and a possible source is a target consisting of a layer of lithium applied as a coating on a silver support plate and bombarded with protons. However, the effect of the bombardment is to release-heat and hydrogen into the target, which can be detrimental to the target structure. In this target, for example, hydrogen bubbles tend to form

  c the interface between lithium and silver.

  According to the present invention there is provided a target for producing neutrons by proton bombardment, which comprises at least one lithium foil and a foil-supporting structure within a directed beam tube which has been evacuated, where it is bombarded by a beam of protons, the structure having at least one gap crossing it so that a part of the foil covers the gap and is exposed to vacuum on its

two sides.

  In use of the target, the proton beam is preferably incidental, at least most

  of time, to the part of the foil that covers the breach.

  In a preferred embodiment, the foil

  of lithium is supported near a

  of the directed beam tube, this wall terminates

  It is a material having a small cross section for the neutrons from the foil and through which the hydrogen diffuses with sufficient rapidity to prevent the accumulation of hydrogen inside the tube. The end wall may be of palladium and may be heated to provide a

rapid diffusion of hydrogen.

  In a preferred embodiment, the support structure comprises a tray defining at least one substantially annular gap therethrough, and the foil covers the gap (s) and is supported on each side of the gap or each gap. The invention thus provides a target to act as a source of high energy neutrons, which is believed to be subject to less damage than known targets, and which produces neutrons in a relatively well-defined range of energy since the thickness

  lithium can be well defined.

  The invention also provides an apparatus for

  diode for irradiating the ore pieces to detect the presence of a selected substance in the pieces, the irradiation apparatus comprising as a neutron source the defined lithium target

upper.

  Other goals, features and benefits

  of the invention will emerge from the detailed description

  which will follow with regard to the annexed drawings, on

  which: - Figure 1 is a general diagram of a gold ore sorting apparatus comprising a lithium target according to the invention; FIG. 2 is a section of the lithium target shown in FIG. 1; and FIG. 3 is a view of the target in

  lithium in the direction of arrow A in Figure 2.

  Referring to Figure 1, a gold ore sorting apparatus includes a rock crusher and sorter 2 which is supplied with ore mined and in which the ore is crushed into pieces and where the ore is discharged in the form of ore. a stream of pieces whose sizes are about 75 mm (mesh size) while pieces smaller than 35-mm (mesh size) are rejected. The current of

  pieces passes through an irradiation chamber 4 adja-

  5 to a lithium target 5 which will be described in more detail later, and then all the pieces are forced to pass through a detector assembly 6 of gamma rays in which are detected the gamma rays having an energy of 279 keV from the disintegration of nuclides of 197m Au and indicating

  thus the presence of gold in the pieces of ore.

  Each piece of ore is interrogated individually-

  by the detector assembly 6 to establish whether its

25558 $ 53

  gold neur is above or below a predetermined concentration. The critical concentration is normally between 0.5 and 5 ppm and can be set for example at 1 ppm. Each piece of ore is then passed through a sorter 8 which is arranged to respond by means of the cables 7 to the signals coming from the detector assembly 6 and which sorts each piece of ore into one of the two output streams according to that the gold concentration in the piece is above or below the predetermined concentration. The crusher and sorter 2 and the sorter 8 may be of the type well known to those skilled in the art whereas the detector assembly 6 may be of the type

  described in more detail in the aforementioned patents

  what can be referred to, the crusher and sorter 2, the sorter 8 and the detector assembly 6 not doing

the subject of the present invention.

  Referring to FIGS. 2 and 3, the lithium target 5 is disposed at one end of a directed beam tube and in which the vacuum has been established, in which a beam of 1 mA protons of 4.5 Mev passes. during the operation of the device

  of Figure 1. Target 5 includes a circular tray

  aluminum support bracket 12 consisting of an inner hub 14, three concentric rings 16 and an outer ring 18, connected by three regularly spaced radial spacers 20 (of which only one ends

  hub 14) in order to define four concen-

  24 of a substantially annular shape, cut by the spacers 20. The four gaps 24 are

  respectively covered by four foils annulled

  each of which is 30 mm wide and 0.3 mm thick, which are attached along each edge to the support plate 12. A small hole 32 through the center of the hub 14 ensures the absence of a difference pressure between the two sides of the support plate 12 and this support plate is fixed on the

  around its perimeter at the wall of the tube 10.

  Five ducts 34, 35, 36, 37, 38 are fixed in concentric circles to the hub 14, to the three rings 16 and to the outer ring 18, respectively, and their ends are directed radially towards

  the outside to cross the wall of the tube 10.

  The radial portions of the pipes 34, 35, 36 are

aligned with the spacers 20.

  As seen in FIG. 2, across the end of the tube 10, at a distance of about 20 nm from the lithium foils 30, is mounted a palladium tray 40 of sufficient thickness to resist a difference. of pressure

  of 1 atmosphere, and having a circumferential

  feral 42. A copper end plate 44

  is fixed around the border 42 and has two ori-

  diametrically opposed fonts 46 connected to conduits 48.

  In operation of the apparatus shown in Figure 1, the proton beam is accelerated down the tube 10 to reach the target, with exploration around each of the successive foils of lithium 30. At the same time, a coolant as the water circulates in the tubes 34 to 38 to ensure that the temperature of the lithium does not reach its melting point (1860C) and air is passed through the ducts 48 so that this air passes on one side of the palladium plate 40 and

  allow to extract the excess heat.

  As a result of the nuclear reaction 7 Li (p, n) 7 Be an intense fast neutron flux having an energy of between about 0.6 and 2.8 MeV emerges from the target to irradiate the ore pieces passing through the adjacent irradiation chamber 4 (see FIG. 1). The thickness of the lithium foil 30 and the energy of the incident protons determine the energy level of the emitted neutrons. The cross-section to activate the gold nuclei, 197Au, is at most about 2.5 MeV and the neutrons with an energy of 0.6 to 2.8 MeV can cause this activation

  but have insufficient energy to bring the activity

  by (n, p) reactions of other elements that are probably present in the ore, such as aluminum and silicon. Neutrons having a

  energy below about 0.6 MeV are unlikely to

  not activate the gold nuclei but may provoke

  an activation by (n, T) reactions of the aluminum

  minium, for example, and so they are undesirable.

  The thickness of the foils 30, which in this case is 0.3 mm, is governed by these considerations. It is conceivable that since a vacuum exists on both sides of the foils 30, the parts of the foils 30 which cover the gaps 24 need only

bear their own weight.

  Protons that do not undergo the reaction

  above with the lithium atoms emerging from the clin-

  quantum of lithium with an energy of about 3.3 MeV and are then incident on the palladium plate 40 in which their energy is dissipated in heat by collisions with the palladium network. The plate 40 is thus heated, its temperature being regulated by the stream of air through the conduits 48, and the hydrogen atoms (that is to say the protons which have been slowed down) are rapidly diffused through the hot plate 40 and are discharged into the air stream. It will be appreciated that since the lithium foils 30 are separated from the plate 40 by a gap of about 20 mm, the plate 40 can be heated to a temperature well above the melting point of the lithium, the transmission of heat through the gap

  only by thermal radiation.

  As a result, the rate of diffusion of hydrogen through the plate 40 is greater than in the case

  a plate 40 maintained at a low temperature.

  In addition, although the plate 40 has been described as being in palladium, another metal such as niobium could be used through which the diffusion

  of hydrogen is done with sufficient speed.

  It goes without saying that various modifications can be made to the embodiments that were

  described, without departing from the scope of the invention.

Claims (7)

  1 - Target for producing neutrons by bombardment with protons, comprising at least one foil (5) made of lithium, characterized in that it comprises a structure (12) which supports the foil (5) in a tube (10) to directed beams in which the void has been established and wherein the target is bombarded by a proton beam, --the structure (12) having at least one gap (24) therethrough such that a portion of the foil (5) covers the gap (24) and is exposed to vacuum on both sides.   2 - Target according to claim 1, characterized   in that the lithium foil 5 is supported near an end wall (40) of the tube (10), said end wall (40) being of a low cross section material for the neutrons from the foil and through which the diffusion of hydrogen is fast enough to prevent the accumulation of hydrogen in the directed beam tube.   3 - Target according to claim 2, characterized in that the end wall (40) is palladium. 4 - Target according to claim 2 or 3, characterized in that it comprises means (48, 44) for heating the end wall (40) to ensure   rapid diffusion of hydrogen.   Target according to any one of claims 1 to 4, characterized in that the support structure (12) comprises a plate defining at least one substantially annular gap (24) therethrough and the foil (5) overlies the or breaches by being   supported on each side of the or each breach.   6 - Target according to any one of the   1 to 5, characterized in that the support structure (12) comprises means (34 to 38) for   the passage of a cooling fluid.   7 - Target for producing neutrons by proton bombardment, characterized in that it comprises a support structure (12) mounted inside said directed beam tube in which the vacuum has been established to a terminal wall (40) thereof, the structure (12) having at least one substantially annular gap (24) and means   (34 to 38) for the passage of a cooling fluid   at least one foil (5) made of lithium which is   supported by the structure and covering the gap.   8 - Target according to claim 7, characterized in that said end wall (40) is in palladium or niobium.   9 - Target according to claim 7 or 8, characterized in that means (48, 44) are provided to heat the end wall.   - Irradiation apparatus for irradiating pieces of ore to detect the presence of a substance selected in the pieces, said irradiation apparatus being characterized in that it comprises as a neutron source, the lithium target   according to any one of claims 1 to 9.