EP3557955A1 - Procédé de préparation d'une cible pour la génération d'isotope radioactif - Google Patents

Procédé de préparation d'une cible pour la génération d'isotope radioactif Download PDF

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EP3557955A1
EP3557955A1 EP19166708.8A EP19166708A EP3557955A1 EP 3557955 A1 EP3557955 A1 EP 3557955A1 EP 19166708 A EP19166708 A EP 19166708A EP 3557955 A1 EP3557955 A1 EP 3557955A1
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Prior art keywords
layer
target
chemical element
support layer
isotope
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German (de)
English (en)
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EP3557955C0 (fr
EP3557955B1 (fr
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GIRARDINI Luca
ZADRA Mario
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Spm Pressofusione Srl
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K4SINT Srl
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    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05H—PLASMA TECHNIQUE; PRODUCTION OF ACCELERATED ELECTRICALLY-CHARGED PARTICLES OR OF NEUTRONS; PRODUCTION OR ACCELERATION OF NEUTRAL MOLECULAR OR ATOMIC BEAMS
    • H05H6/00—Targets for producing nuclear reactions
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F3/00—Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces
    • B22F3/10—Sintering only
    • B22F3/105—Sintering only by using electric current other than for infrared radiant energy, laser radiation or plasma ; by ultrasonic bonding
    • G—PHYSICS
    • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
    • G21G—CONVERSION OF CHEMICAL ELEMENTS; RADIOACTIVE SOURCES
    • G21G1/00—Arrangements for converting chemical elements by electromagnetic radiation, corpuscular radiation or particle bombardment, e.g. producing radioactive isotopes
    • G21G1/04—Arrangements for converting chemical elements by electromagnetic radiation, corpuscular radiation or particle bombardment, e.g. producing radioactive isotopes outside nuclear reactors or particle accelerators
    • G21G1/10—Arrangements for converting chemical elements by electromagnetic radiation, corpuscular radiation or particle bombardment, e.g. producing radioactive isotopes outside nuclear reactors or particle accelerators by bombardment with electrically charged particles

Definitions

  • the present invention relates to a process for the manufacture of targets suitable for the production of radioactive isotopes, in particular 100 Mo targets for the production of the radioactive isotope 99m Tc by means of a cyclotron.
  • Radioactive isotopes are widely used in medicine for diagnostic procedures, for example in the field of oncology, cardiology, endocrinology, etc.
  • a radionuclide is used as a tracer in order to have an emission of radiations from the inside of the human body and be able to evaluate the functional aspect of an organ or a drug, which are thus visualised by means of the PET (Positron Emission Tomography) or SPECT (Single Photon Emission Computed Tomography) techniques.
  • the most used radionuclide is 99m Tc, with a half-life of 6 h: every day about 60,000 patients around the world undergo diagnostic procedures with this isotope, which represents about 80% of the total images obtained with techniques of nuclear medicine.
  • 99 Mo is an unstable isotope with a half-life of 66 h: it is mostly produced in several nuclear plants and subsequently quickly sent to nuclear medicine units where the radioactive decay is exploited for the formation of 99m Tc.
  • 100 Mo is a rather stable isotope and can be easily obtained with an enrichment greater than 99% starting from natural isotopes of molybdenum: it is usually supplied in the material form of an angular powder with a particle size of around 100 ⁇ m.
  • the target composed of 100 Mo has been irradiated and thus contains a minimum amount of 99m Tc it is quickly dissolved by chemical attack and the different isotopes/elements can be separated and used. Only a minimal part of the isotope 100 Mo is converted into 99m Tc, but it is sufficient to make the process economically and environmentally sustainable. It is therefore possible to produce 99m Tc directly from 100 Mo in every hospital unit equipped with a cyclotron suited to the purpose.
  • radioactive isotopes used for diagnostic purposes are: 57 Co obtained from 58 Ni, 44 Ti obtained from 45 Sc, 88 Y obtained from 88 Sr and still others are continually discovered and used.
  • radioactive isotopes require the preparation of a target, which is definable as the physical object against which the appropriately accelerated physical particles (protons, electrons, alpha particles, beta particles, light ions), also called an accelerated beam, collide.
  • a target which is definable as the physical object against which the appropriately accelerated physical particles (protons, electrons, alpha particles, beta particles, light ions), also called an accelerated beam, collide.
  • the target is usually composed of two disks or layers placed one on top of the other and joined together: a first disk (diameter D 1 and thickness t 1 ) composed of the chemical element from which the radioactive isotope is obtained by bombardment/irradiation of accelerated particles - for example 100 Mo bombarded by protons, from which 99m Tc is obtained - and a second disk (diameter D 2 and thickness t 2 ) that acts as a mechanical support, whose function is to dissipate the thermal power generated by the particle beam incident upon the first disk.
  • a first disk diameter D 1 and thickness t 1
  • a second disk that acts as a mechanical support, whose function is to dissipate the thermal power generated by the particle beam incident upon the first disk.
  • FIG. 1 An example of a target is illustrated in Figure 1 , in which the number 1 indicates the first layer consisting of the chemical element from which the radioactive isotope is generated by bombardment of particles and the number 2 indicates the second layer that acts as a mechanical support, whose function is to dissipate the thermal power generated by the particle beam incident upon the first disk.
  • the production of the target must take account of the fact that the starting material from which the first layer (the active part of the object where the nuclear reaction takes place) will be formed is usually, albeit not exclusively, a powder.
  • the starting material can be a metal foil with a controlled composition.
  • the methods which make use of plasma spray, sputtering or electrodeposition have a low coefficient of use: a large part of the enriched isotope is lost in the solution (electrodeposition) or is not deposited solely on the target (plasma spray, sputtering).
  • brazing could be the best method in that it has a rate of use of the element/isotope that is close to 100% and the junctions are mechanically very resistant: unfortunately, however, brazing alloys are composed of elements with a high atomic number (Z) (silver, platinum, palladium, tin, zinc, cadmium, lead), which, following the irradiation phase, can transmute into highly radioactive isotopes that are difficult to control.
  • Z atomic number
  • the present invention relates to a process for preparing a two-layer or three-layer target for the generation of radioactive isotopes (radionuclides) by bombardment with accelerated particles, in particular protons generated by a cyclotron.
  • the radioactive isotopes obtained starting from the target prepared with the process according to the invention are used in medicine for diagnostic investigations.
  • the process of the invention envisages the preparation of a layer (called "first layer”), characterised by a thickness/diameter ratio ⁇ 0.25, comprising a chemical element or an isotope of a starting chemical element, i.e. a chemical element or an isotope of a chemical element capable of generating a known radionuclide when subjected to bombardment with accelerated particles.
  • the starting chemical element or isotope of a chemical element can also be in the form of a chemical compound, such as, for example, a carbide, oxide, nitride, boride, or silicide.
  • a chemical compound such as, for example, a carbide, oxide, nitride, boride, or silicide.
  • an intermediate layer made of metal material whose function is to prevent contact of the support layer with the chemical reagents used for the dissolution and recovery of the radionuclide from the first layer after irradiation with the accelerated particles.
  • the coupling between the first layer, the support layer and an intermediate layer, if present, takes place through the use of the Spark Plasma Sintering (SPS) technique, without the use of a containing mould.
  • SPS Spark Plasma Sintering
  • This technique can be defined as a technique of sintering in the presence of electric current and pressure.
  • the invention also relates to a target for the generation of radionuclides by bombardment with protons which comprises a first layer comprising a chemical element or isotope, for example 100 Mo, in an amount ⁇ 99% by weight, and a support layer of copper, preferably containing from 0.1% to 0.2% by weight of Al 2 O 3 .
  • the isotope 100 Mo can also be in the form of a chemical compound such as a carbide 100 MoC/ 100 Mo 2 C, oxide 100 MoO 2 , boride 100 MoB, nitride 100 Mo 2 N or silicide 100 MoSi 2 . In such a case the compound itself will be present in an amount greater than 98%.
  • the support layer is a layer made of diamond or a composite material comprising from 60% to 80% by volume of synthetic diamond powder and 20% to 40% by volume of silver alloy powder. Said alloy is preferably a silver alloy containing 3% by weight of silicon.
  • the target also comprises an intermediate layer, which can be made of nickel, gold, tantalum, niobium, silver, zirconium, titanium, chrome, yttrium, vanadium, tungsten, manganese, cobalt, platinum, zinc, aluminium, tin, said intermediate layer being positioned between the first layer and the support layer.
  • an intermediate layer which can be made of nickel, gold, tantalum, niobium, silver, zirconium, titanium, chrome, yttrium, vanadium, tungsten, manganese, cobalt, platinum, zinc, aluminium, tin, said intermediate layer being positioned between the first layer and the support layer.
  • First layer means a layer comprising a starting chemical element or isotope of a chemical element from which the radionuclide is obtained by bombardment with accelerated particles.
  • the starting chemical element or isotope of a chemical element can also be in the form of a chemical compound, such as, for example, a carbide, oxide, nitride, boride or silicide.
  • Green layer means a first layer that is not sintered according to the sintering step a1).
  • Tin green layer means a first layer subjected to sintering according to step a1).
  • First sintered layer means the first layer of the two- or three-layer target after step d) of the process of the invention.
  • “Support layer” means a layer on which the first layer is applied.
  • “Intermediate layer” means a layer that is positioned between the first layer and the support layer.
  • Angular powder means a non-spherical powder or, in other words, a powder of an irregular shape.
  • “Target” means the physical object against which the accelerated particles collide and which for the purposes of the invention can be a two-layer or a three-layer one.
  • “Intermetal phase” means a chemically and crystallographically established phase defined by a metallic bond and an ordered lattice, characterised by low tenacity.
  • Protective atmosphere means a non-oxidising atmosphere, for example a nitrogen, argon or hydrogen gas. For the purposes of the present invention, the current density and pressure measurements always refer to the dimensions of the first layer.
  • the present invention relates to a process for preparing a two-layer or three-layer target for the generation of radioactive isotopes (radionuclides) by bombardment with accelerated particles, comprising the steps of:
  • the first layer is a green layer prepared from a powder of a chemical element or of an isotope of a chemical element or of a compound of a chemical element or of a compound of an isotope of a chemical element.
  • the powder contains an amount of the element/isotope/chemical compound ⁇ 98% by weight, preferably ⁇ 99% by weight.
  • the powder is preferably a powder isotopically enriched with an element/isotope/chemical compound, for example it is a 100 Mo or 100 MoC/ 100 MoC 2 powder enriched to values greater than 99% by weight.
  • the powder is introduced inside a metal mould and subjected to a pressure comprised between 100 and 2000 MPa, preferably between 500 and 900 MPa.
  • the metal mould can have cavities of different shape, for example it can be rectangular, cylinder-shaped or of an ellipsoidal shape so as to provide a corresponding green layer of the desired shape.
  • An ellipsoidal shape of the first layer is particularly useful in the event of use of the target in high-power cyclotrons.
  • the first ellipse-shaped layer is coupled with a support layer of a rectangular shape for maximum performance and inclined by a certain angle relative to the direction of the incident ray of bombardment so as to increase the area of incidence.
  • the green layer has a relative density comprised between 60% and 90%, according to the compressibility of the powder; this means that the porosity of the green layer is comprised between 10% and 40%.
  • a greater or lesser porosity of the green layer determines a greater or lesser reactivity of the layer during the chemical attack phase following the bombardment with accelerated particles, which can thus be carried out with variable times.
  • the mechanical strength of the green layer is determined by the mechanical gripping due to the rearrangement and plastic deformation of the powder particles, in particular the surface roughness. Therefore, the mechanical strength of the green layer depends on the morphology of the initial powder, which is preferably an angular powder. Spherical powders are less preferred, since they provide green layers with poor mechanical strength and which are thus difficult to handle. The amount of powder to be introduced into the mould depends on the diameter of the mould itself and the thickness of the green layer it is desired to obtain.
  • the percentage of use of the powder of the element/isotope/chemical compound is ⁇ 95% by weight, preferably ⁇ 98% by weight.
  • This result represents an undoubted advantage compared to the known processes in which the percentage of use of the powder is about 50%.
  • the process of the invention presents itself as an absolutely advantageous process, compared to the known ones, also from an economic viewpoint.
  • the chemical element or the isotope of a chemical element which is used, in powder form, to form the green layer can be selected from: 100 Mo, 58 Ni, 45 Sc, 88 Sr, 54 Cu, 57 Cu, 51 Cr, 59 Fe, 89 Y, 68 Zn, 112 Cd, 64 Ni , 48 Ti, 55 Mn, 50 Cr, 52 Cr, 44 Ca, 54 Fe, 56 Fe, 61 Ni, 59 Co, 63 Cu, 60 Ni, 66 Zn, 65 Cu, 94 Mo or from the natural isotopes or chemical compounds thereof, such as, for example, carbides, oxides, nitrides, borides, silicides.
  • the isotope is preferably 100 Mo, which gives rise to the radionuclide 99m Tc when subjected to proton bombardment.
  • Other radionuclides obtained by proton bombardment are: 57 Co obtained from 58 Ni, 44 Ti obtained from 45 Sc and 88 Y obtained from 88 Sr.
  • a first layer is illustrated in figure 1 with the number 1.
  • the first layer can be a preformed foil of an element/isotope/chemical compound, in which case no pressing step is necessary.
  • the support layer of step b) can have a rectangular or disk shape and dimensions greater than those of the first layer, as schematically illustrated with the number 2 in figure 1 .
  • the support layer must be characterised by a good ability to dissipate heat and thus by a thermal conductivity that is as high as possible.
  • the support layer is made of a metal material selected from: copper, preferably containing from 0.1% to 0.2% by weight of Al 2 O 3 , aluminium, gold, silver and alloys thereof.
  • the support layer can be made of synthetic diamond which has a K of up to 2000 W/(m•K) and is highly resistant to chemical agents, even if fragile and costly, or of electrically conductive diamond obtained by chemical vapor deposition (CVD) using boron as the doping agent. It is possible to make the support layer from a composite material obtained by mixing 65-80% by volume of synthetic diamond powder and 20-35% by volume of silver alloy powder. Said alloy is preferably a silver alloy containing 3% by weight of silicon.
  • the support layer in this case is produced by means of the Spark Plasma Sintering (SPS) technique or with other techniques, for example infiltration, classic pressing & sintering, or metal injection moulding.
  • SPS Spark Plasma Sintering
  • the support layer can be made of a conductive or semi-conductive ceramic material, for example TiB 2 or SiC, or electrically insulating ceramic materials, for example Al 2 O 3 , rendered electrically conductive by using carbon nanotubes or graphene.
  • the support layer can also be made of isotropic graphite, C/SiC (carbon fibres infiltrated with Si), C/C (carbon reinforced with carbon fibres), SiSiC (silicon carbide infiltrated with silicon) or graphite/SiC, commercially known as CarbocellTM, composed of spherical graphite particles bound together by SiC.
  • the support layer can be made of metals with a modest thermal conductivity, such as niobium, tantalum, tungsten, titanium, zirconium or vanadium, in order to meet other needs, in particular resistance to specific thermochemical treatments for dissolving the first layer.
  • Copper material containing from 0.1% to 0.5% by weight, preferably from 0.2% to 0.4% by weight, of Al 2 O 3 known, for example, by the name Glidcop AL-15®, is particularly preferred. This material, besides having excellent thermal conductivity, shows good resistance to deformation at high temperatures and also a high capacity to maintain good mechanical strength at high temperatures.
  • the support layer is obtained by machining a bar or sheet of one of the materials listed above or else, if it is made of a composite diamond/silver alloy material, by using one of the techniques listed above.
  • the first layer is placed on top of and aligned with the support layer and then the two-layer target thus obtained is inserted into a vacuum chamber.
  • Preferred combinations of the first layer and support layer are: first layer of 100 Mo and support layer of copper or copper alloy, for example copper containing from 0.1% to 0.5% by weight, preferably from 0.2% to 0.4% by weight, of Al 2 O 3 ; first layer of 100 Mo and support layer of CarbocellTM; first layer of 100 Mo and support layer of synthetic diamond or a composite material obtained by mixing 65-80% by volume of synthetic diamond powder and 20-35% by volume of silver alloy powder.
  • Said alloy is preferably a silver alloy containing 3% by weight of silicon.
  • step d) after the insertion of the two-layer target into a vacuum chamber and setting of a vacuum or a protective atmosphere, an electric current density comprised between 0.5 and 25 A/mm 2 , preferably between 3 and 16 A/mm 2 , is applied, while the two-layer target is simultaneously subjected to a pressure comprised between 0.1 and 100 MPa, preferably between 2 and 30 MPa.
  • the electric current applied can be pulsed, alternating or direct.
  • This step is schematically illustrated in figure 2 , in which the number (1) indicates the first layer, the number (2) the support layer and the number (3) the vacuum chamber.
  • the graphite blocks (also called "spacers") are indicated with the letters A, B, C and D. The sizes of said graphite blocks depend on the available equipment, the sizes of the target and the type of graphite they are made from.
  • both the first layer (1) and the support layer (2) are electrically conductive, the circuit is closed and it is possible to apply a pulsed, alternating or direct electric current. Since the two layers are electrically conductive, it is not necessary to apply a conductive or semi-conductive lateral containing mould, for example made of graphite, to close the electrical circuit, since the electric current passes totally through the first layer and the support layer, thus determining an excellent junction between the layers and a consolidation/sintering of the first layer.
  • the graphite of the spacers being a semiconductor, has a much higher resistivity than the materials of the target: therefore, a large part of the heating power generated as a result of the Joule effect is produced in the spacers and not in the target, which in fact remains at a lower temperature. Therefore, by making geometrically different graphite blocks it is possible to generate and control temperature gradients during the whole of step d). It is for this reason that the spacers C and D in figure 2 have been made with different dimensions: the spacer C, having a smaller cross section, has a higher resistance than the spacer D and therefore, given that the current in transit is the same, heats up more quickly and reaches a higher temperature.
  • the choice of the dimensions of the graphite blocks means that the spacer C at a higher temperature is in direct contact with the first layer to be sintered, whilst the spacer D is in contact with the support layer: the first layer is thus positively subjected to a temperature that is higher than that of the support layer.
  • This fact is advantageous because the material of the first layer requires high temperatures in order to be sintered, much higher than the melting temperature of the support layer. Since no lateral containing mould is present, there arises the problem of measuring the temperature during step d). In order to remedy this problem, the temperature of the spacer C can be measured immediately near the first layer and also that of the spacer D near the support layer by means of a pyrometer or thermocouple.
  • This measurement represents a fairly accurate estimate of the temperature of the first layer and of the support layer.
  • the mechanical pressure applied throughout the cycle (2-30 MPa) tends to be low in order to avoid the deformation and compression of the layers, but sufficient to ensure electrical contact.
  • the application of the current density it can take place in two ways: by applying a certain fixed value, for example 5 A/mm 2 , and allowing heating to take place freely up to a certain temperature, or setting a thermal cycle in terms of heating rate, maximum temperature and time of stasis.
  • a certain fixed value for example 5 A/mm 2
  • the temperature measured at the graphite blocks during step d) will be comprised between 700 and 1000°C.
  • step d) the target is cooled and shows excellent solidity: it is necessary to apply a considerable shear stress between the first layer and the support layer in order to be able to detach them. Not even the use of a scalpel or cutter enables the two layers to be detached.
  • the analysis of the separated layers has made it possible to verify that the first layer has a slightly smaller mass than the initial one (thus a rate of use ⁇ 98%) and the same density as at the start, but the mechanical strength is higher compared to the initial green layer.
  • the first layer for example of molybdenum
  • the support layer for example made of Glidcop AL-15®
  • Figure 3 shows an enlargement of the 100 Mo/Glidcop AL-15® interface after the application of step d).
  • the treatment with an electric current and application of pressure in step d) takes place in a short time, for example comprised between 10 and 600 seconds, preferably between 60 and 300 seconds. The duration of the entire process can be longer because of the time necessary for reaching a vacuum or introducing a protective atmosphere.
  • the first layer (or green layer) is subjected to sintering after step a) and before the subsequent steps c) and d) so as to increase its mechanical strength.
  • step a) the powder of an element/isotope/chemical compound is inserted into a metal mould and subjected to a pressure comprised between 100 and 2000 MPa, preferably between 500 and 900 MPa, so as to form the green layer. This operation imparts to the layer a certain mechanical strength and manageability which, however, can be insufficient during phases of irradiation with high-energy accelerated particles.
  • step a1 a sintering step which precedes the subsequent steps c) and d) of coupling to the support layer.
  • step d the same method as described for step d), and schematically illustrated in figure 4 .
  • step d the same method as provided for step d) of the process, i.e. the total passage of current inside the layer to be sintered. This ensures a high productivity and short processing times.
  • the graphite blocks are of the same size, because no temperature gradient is necessary, as only one layer is to be sintered.
  • the applied electric current density has values comprised between 1 and 25 A/mm 2 , preferably between 7 and 16 A/mm 2 .
  • the applied pressure is comprised between 5 and 100 MPa.
  • the density of the green layer (pre- and post-sintering) remains practically unchanged: the sintered green layer remains considerably porous and thus maintains the benefits with respect to chemical reactivity and adhesion with the subsequent support layer.
  • the mechanical strength is considerably greater: it is no longer possible to break the sintered green layer with one's hands.
  • an intermediate layer which has the purpose of preventing contact between the support layer and the solution used for the chemical attack of the first sintered layer and the recovery of the radionuclides generated by the irradiation with particles.
  • the solutions commonly used for the chemical attack of the first layer are acidic or basic. They normally also enter into contact with the support layer linked to the first layer.
  • the support layer may also not be sufficiently resistant to such reactions and thus partially degrade, thereby contaminating the phase of recovery of the radionuclides.
  • the intermediate layer can be a metal layer of nickel, gold, tantalum, niobium, silver, zirconium, titanium, chrome, yttrium, vanadium, tungsten, manganese, cobalt, platinum, zinc, aluminium, tin, lead, cadmium or iron.
  • the lateral dimensions of the intermediate layer are greater than those of the first layer and equal to or less than those of the support layer, as schematically illustrated in figure 5 .
  • step d) of the process two times: a first cycle to form the junction between the intermediate layer and the support layer and a second cycle to form the junction between the green layer, or the green layer sintered according to step a1), and the intermediate layer anchored to the support layer.
  • the beneficial effects of the passage of current between the various interfaces enable solid, resistant junctions to be obtained.
  • the intermediate layer is as thin as possible so as to influence the thermal conductivity as little as possible, but at the same ensure the separation between the first layer and the support layer.
  • the diameter of the intermediate layer depends on the system of chemical dissolution.
  • the preferred combinations for the three-layer target are: support layer of copper or copper alloy, for example copper containing from 0.1% to 0.5% by weight, preferably from 0.2% to 0.4% by weight, of Al 2 O 3 , intermediate layer of nickel and first layer comprising 100 Mo; support layer of copper or copper alloy, for example copper containing from 0.1% to 0.5% by weight, preferably from 0.2% to 0.4% by weight, of Al 2 O 3 , intermediate layer of gold and first layer comprising 100 Mo.
  • An alternative to this production method is a process wherein the intermediate layer is applied and anchored to the support layer by means of a conventional method selected from: welding, electrochemical deposition, physical vapor deposition (PVD), chemical vapor deposition (CVD) and the like; the first layer can subsequently be positioned and anchored to the two-layer target comprising the support layer and the intermediate layer according to step d) of the process of the invention.
  • a conventional method selected from: welding, electrochemical deposition, physical vapor deposition (PVD), chemical vapor deposition (CVD) and the like
  • a further alternative to the two processes illustrated above is a process wherein the three-layer target is obtained with a single cycle of step d) of the invention by suitably increasing the pressure during step d).
  • the green layer or the green layer sintered according to step a1) is positioned on the support layer together with the intermediate layer according to the configuration in figure 5 .
  • the assembled target is inserted into a vacuum chamber according to step c) and the vacuum is applied or a protective atmosphere is introduced.
  • the configuration of the graphite blocks A, B, C and D is equal to the one illustrated, for example, in figure 2 , the spacer C being smaller than the spacer D.
  • an electric current is made to circulate at the values previously indicated for step d); a constant current is preferably applied.
  • an initial pressure comprised between 5 and 20 MPa is applied.
  • a sintering temperature comprised between 900°C and 1100°C (measured on the graphite block C as represented in figure 2 ) is reached, the pressure is increased to a value comprised between 20 and 80 MPa. The increase in pressure causes the intermediate layer and the first layer to penetrate inside the support layer until the graphite block C is in contact with the support layer. See, by way of example, the configuration in figure 6 .
  • the support layer will have dimensions larger than the initial ones because of the interpenetration between the first layer and the intermediate layer.
  • the adhesion of the first layer is good and poses no problems of detachment from the other two layers.
  • the thickness of every target is very limited, it is possible to stack various targets, as illustrated, for example, in figure 7 and then proceed to a single sintering cycle so as to obtain a high productivity and low costs.
  • the targets to be sintered are stacked in such a way that the graphite blocks C are facing towards the first layer and the graphite blocks D are facing towards the support layer of each target. Therefore, each target is stacked in a manner opposite to that of the next target, as illustrated in figure 7 .
  • This embodiment of the process of the invention can also be applied to the three-layer targets that comprise an intermediate layer between the first layer and the support layer, both in the case where the intermediate layer has already been anchored to the support layer with other techniques, and in the case where the 3 layers undergo simultaneous sintering according to the configuration in figure 6 .
  • the process of the invention can be definable as a process in which the known sintering technique aided by mechanical pressure and electric current, called Spark Plasma Sintering (SPS), suitably adapted, is applied without the use of a mould for containing the powder to be sintered.
  • SPS Spark Plasma Sintering
  • the SPS technique exploits the heating resulting from the Joule effect thanks to the passage of current through the layers and thus enables many materials to be easily sintered in a short time and at lower temperatures compared to the usual sintering times and temperatures.
  • the SPS technique is also called FAST (Field Assisted Sintering Technique), PECS (Pulsed Electric Current Sintering), EFAS (Electric Field Assisted Sintering), DHP (Direct Hot Pressing) or DCS (Direct Current Sintering).
  • FAST Field Assisted Sintering Technique
  • PECS Pulsed Electric Current Sintering
  • EFAS Electro Field Assisted Sintering
  • DHP Direct Hot Pressing
  • DCS Direct Current Sintering
  • ESF Electro Sinter Forging
  • Another variant is the technique that uses electrical induction to generate an electromagnetic field and hence an electric current inside the sample to be sintered.
  • the process of the invention is an SPS process for preparing a target for the generation of radioactive isotopes by bombardment with accelerated particles, comprising the steps of:
  • the two-layer or three-layer target sintered with the process according to the invention is used as a source of radionuclides when subjected to irradiation with accelerated particles which strike the first layer comprising a chemical element or an isotope of a chemical element or a compound of a chemical element or a compound of an isotope of a chemical element.
  • the non-transmuted part i.e.
  • the part not converted into the radionuclide can be recovered after the chemical attack, for example with hydrogen peroxide solutions or basic solutions, converted into the powder of the starting element/isotope/chemical compound (for example 100 Mo) and again used for the production of further targets.
  • the starting element/isotope/chemical compound for example 100 Mo
  • the first layer is anchored to the support layer by sintering according to the process of the invention.
  • the anchorage between the first layer and the support layer is a gripping of a mechanical type, wherein the support layer has penetrated into the pores of the first layer.
  • the anchorage/junction will mainly be due to a mechanical action, whereas in the case of materials/elements that exhibit solubility the anchorage/junction will be mainly due to diffusion/interdiffusion.
  • the presence of intermetal phases between the two layers in contact is preferably nearly or completely nil.
  • the chemical element or the isotope of a chemical element that is used, in powder form, to form the green layer can be selected from: 100 Mo, 58 Ni, 45 Sc, 88 Sr, 54 Cu, 57 Cu, 51 Cr, 59 Fe, 89 Y, 68 Zn, 112 Cd, 64 Ni, 48 Ti, 55 Mn, 50 Cr, 52 Cr, 44 Ca, 54 Fe, 56 Fe, 61 Ni, 59 Co, 63 Cu, 60 Ni, 66 Zn, 65 Cu, 94 Mo or from the natural isotopes or chemical compounds thereof, such as, for example, carbides, oxides, nitrides, borides, silicides.
  • the isotope is preferably 100 Mo, which gives rise to the radionuclide 99m Tc when subjected to proton bombardment.
  • the support layer is made of a material selected from: copper, preferably containing from 0.1% to 0.2% by weight of Al 2 O 3 (for example Glidcop AL-15®), aluminium, gold, silver and alloys thereof, diamond, a composite material obtained by mixing 65-80% by volume of synthetic diamond powder and 20-35% by volume of silver alloy powder (preferably a silver alloy containing 3% by weight of silicon), TiB 2 , SiC, Al 2 O 3 rendered conductive by using carbon nanotubes or graphene, isotropic graphite, C/SiC (carbon fibres infiltrated with Si), C/C (carbon reinforced with carbon fibres), SiSiC (silicon carbide infiltrated with silicon) or graphite/SiC, commercially known as CarbocellTM, composed of spherical graphite particles bound together by SiC.
  • CarbocellTM composed of sp
  • the two-layer target of the invention preferably comprises the first layer of 100 Mo and the support layer made of copper containing from 0.1% to 0.2% by weight of Al 2 O 3 (for example Glidcop AL-15®) or of diamond or a composite obtained by mixing 65-80% by volume of synthetic diamond powder and 20-35% by volume of silver alloy.
  • Said alloy is preferably a silver alloy containing 3% by weight of silicon.
  • the three layers are anchored to one another by sintering according to the process of the invention.
  • the anchorage between the first layer and the intermediate layer is a gripping of a mechanical type wherein the intermediate metal layer has penetrated into the pores of the first layer, whereas between the intermediate layer and the support there is an interdiffusion between the elements such as to form a metallurgical junction.
  • the anchorage/junction will mainly be due to a mechanical action, whereas in the case of materials/elements that exhibit solubility the anchorage/junction will mainly be due to diffusion/interdiffusion.
  • the presence of intermetal phases between the three layers in contact is preferably nearly or completely nil.
  • the first layer and the support layer are as defined above.
  • the intermediate metal layer is a layer of nickel or gold, tantalum, niobium, silver, zirconium, titanium, chrome, yttrium, vanadium, tungsten, manganese, cobalt, platinum, zinc, aluminium, tin.
  • the three-layer target comprises the first layer of 100 Mo, the intermediate layer of gold and the support layer of copper containing from 0.1% to 0.2% by weight of Al 2 O 3 (for example Glidcop AL-15®) or of synthetic diamond or a composite obtained by mixing 65-80% by volume of synthetic diamond powder and 20-35% by volume of silver alloy.
  • Said alloy is preferably a silver alloy containing 3% by weight of silicon.
  • the three-layer target comprises the first layer of 100 Mo, the intermediate layer of nickel and the support layer of copper containing from 0.1% to 0.2% by weight of Al 2 O 3 (for example Glidcop AL-15®) or of diamond or a composite obtained by mixing 65-80% by volume of synthetic diamond powder and 20-35% by volume of silver alloy.
  • Said alloy is preferably a silver alloy containing 3% by weight of silicon.
  • the first layer has a relative density comprised between 60% and 90% depending on the compressibility of the starting powder; this means that the porosity of the first layer is comprised between 10% and 40%.
  • the first layer can be a green layer or a sintered green layer.
  • the invention also relates to the use of the two-layer or three-layer target according to the invention for the production of radionuclides by bombardment with protons produced by a cyclotron.
  • the radionuclides are used in diagnostic techniques such as PET (Positron Emission Tomography) or SPECT (Single Photon Emission Computed Tomography).
  • the green layer is solid and resistant and weighs 0.495 g: the rate of use of this first step is thus around 99%.
  • a disk with a diameter of 32 mm and thickness of 1.5 mm was obtained from a bar of Oxygen-Free High Conductivity (OFHC) copper; a small foil with a diameter of 25 mm was punched out from a pure gold foil with a thickness of 0.1 mm.
  • the copper disk and small gold foil were positioned in an SPS apparatus with a spacer C with a diameter of 30 mm and height of 30 mm and a spacer D with a diameter of 45 mm and height of 35 mm.
  • a second SPS cycle was then carried out using the green 100 Mo disk and the Cu/Au support just mentioned: the same graphite block configuration as in the first SPS cycle was used.
  • the chamber was again evacuated and a constant load of 2 kN (corresponding to a pressure of 10.6 MPa) and a current of 1400 A (corresponding to a current density of 7.4 A/mm 2 ) were applied until a temperature of 850°C was reached in the spacer C: the time necessary to reach that temperature was 140 s.
  • the target thus obtained was used in a cyclotron and bombarded with protons with a specific power of up to 1 kW/cm 2 for 6 hours: no detachment of the disk (1) from the small gold foil was noted.
  • the chemical dissolution test also gave excellent results.
  • a 58 Ni target for the production of 57 Co was produced.
  • the 58 Ni isotope was supplied in the form of a 13 x 13 x 0.1 mm small foil.
  • CarbocellTM Toyo Tanso, Osaka, Japan: it is composed of spherical graphite particles bound together by SiC.
  • the CarbocellTM disks used had a diameter of 25.4 mm and thickness of 2 mm: the object obtained easily passed the proton bombardment and chemical resistance test during the dissolution step.
  • the configuration of the process is the one shown in figure 2 , but with the assembly of the target as per figure 5 : the disk (2) made of Glidcop AI-15, a nickel foil (4), and the disk (1) (green disk or sintered green disk, according to circumstances) are positioned in the SPS chamber with the small-sized spacer C and the large-sized spacer D.
  • a constant current for example 1700 A, corresponding to a current density of 9 A/mm 2
  • a low initial load (3 KN, corresponding to an applied pressure of 15.9 MPa) are imposed until a maximum sintering temperature (1050°C) is reached: at this point, the load is increased to 11 kN (corresponding to an applied pressure of 58 MPa).
  • the increase in the load causes the nickel foil (4) and the disk (1) to penetrate into the disk (2) of Glidcop AI-15 until the spacer C is in contact with the disk (2) ( figure 6 ).
  • the only drawback is that the disk (2) of Glidcop AI-15 will have larger dimensions than initially due to the interpenetration of the foil and the disk (1).
  • the adhesion of the disk (1) is good and poses no problems of detachment.
  • the SPS cycle provides for a constant load of 3 kN (corresponding to an applied pressure of 15.9 MPa) and the application of a constant current of 1800 A (corresponding to a current density of 9.5 A/mm 2 ): after about 60 s the pyrometer read a temperature of 1050°C in the spacer C and the SPS cycle was interrupted. It should be noted that this is not the temperature value either of the disk (1) or the disk (2): it is not possible to read the temperature of the two disks because they are too thin. What can be measured is the temperature of the spacer D, which was equal to 700°C, a good 350°C less than the spacer C. It may be presumed that the local temperature of contact between the particles of 100 Mo is much greater. After cooling, the target has excellent solidity: it is necessary to apply a considerable shear stress between the disks (1) and (2) in order to be able to detach them. Not even the use of a scalpel or cutter enables the two disks to be detached.

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RU2736310C1 (ru) * 2020-03-04 2020-11-13 Российская Федерация, от имени которой выступает Государственная корпорация по атомной энергии "Росатом" Способ изготовления изделий из электропроводных порошков, содержащих радионуклиды
CN113770467A (zh) * 2021-09-07 2021-12-10 合肥工业大学 一种tzm合金和石墨的sps无压钎焊方法
CN114531768A (zh) * 2022-03-07 2022-05-24 中国原子能科学研究院 一种医用核素生产的高功率固体靶
US20220220586A1 (en) * 2019-06-11 2022-07-14 Wisconsin Alumni Research Foundation Intermetallic compounds of cobalt as targets for the production of theranostic radionuclides

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Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20220220586A1 (en) * 2019-06-11 2022-07-14 Wisconsin Alumni Research Foundation Intermetallic compounds of cobalt as targets for the production of theranostic radionuclides
RU2736310C1 (ru) * 2020-03-04 2020-11-13 Российская Федерация, от имени которой выступает Государственная корпорация по атомной энергии "Росатом" Способ изготовления изделий из электропроводных порошков, содержащих радионуклиды
CN113770467A (zh) * 2021-09-07 2021-12-10 合肥工业大学 一种tzm合金和石墨的sps无压钎焊方法
CN113770467B (zh) * 2021-09-07 2023-06-20 安徽尚欣晶工新材料科技有限公司 一种tzm合金和石墨的sps无压钎焊方法
CN114531768A (zh) * 2022-03-07 2022-05-24 中国原子能科学研究院 一种医用核素生产的高功率固体靶
CN114531768B (zh) * 2022-03-07 2023-03-10 中国原子能科学研究院 一种医用核素生产的高功率固体靶

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