Container, method for obtaining same and target assembly for the production of radioisotopes using such a container

Abstract

The invention relates to a container (100, 901, 902, 903, 904, 905, 906, 907, 908, 909, 910) for the production of radioisotopes by irradiation of a precursor material formed by a one-piece metal casing, the wall of said casing including one thin portion (130) having a thickness of between 5 and 100 m, the remainder having a thickness greater than 100 m. The invention also relates to a method for obtaining the container and to a target assembly using same.

Claims

1. A container for producing radioisotopes by irradiation of a precursor material, the container comprising: a metal jacket of integral construction, the metal jacket having a symmetry of revolution about an axis, and having from top to bottom (i) an upper portion being open and having a conical shape, the opening of the cone being oriented upwards, (ii) a first cylindrical portion connected to the upper portion, (iii) a thin wall cylindrical section connected to the first cylindrical portion, (iv) a second cylindrical portion connected to the thin wall cylindrical section, and (v) a dome connected to the second cylindrical portion, the thin wall cylindrical section having a first thickness between 5 m and 100 m, the first and second cylindrical section and the dome having a second thickness larger than 100 m.

2. The container as claimed in claim 1, wherein thin wall cylindrical section has an outside diameter between 4 mm and 100 mm.

3. The container as claimed in claim 1, wherein the container is at least partially made from at least one of nickel, titanium, niobium, tantalum, iron, chromium, cobalt or a stainless steel.

4. A method for obtaining a container as claimed in claim 1, the method comprising: providing a matrix; electrodepositing on the matrix a thickness of a metallic material, until a first thickness between 5 m and 100 m is obtained; masking a fraction of a surface of the matrix; electrodepositing on an unmasked section until a thickness larger than 100 m is obtained; removing the matrix; and obtaining the container as claimed in claim 1.

5. The method as claimed in claim 4, wherein the matrix is removed by dissolution.

6. A target assembly for producing radioisotopes, including: a container as claimed in claim 1; a holding tube including at one end a threaded portion; and a ring including a suitable interior thread, the holding tube and the ring being configured to encase the container.

7. The target assembly as claimed in claim 6, wherein the container has an end having a conical shape, a base of the cone being oriented toward an exterior of the container, the holding tube has a conical end congruent with the end of the container, and the ring has a conical end congruent with the end of the container.

8. The target assembly as claimed in claim 6, wherein the holding tube and the container are mounted so as to be able to rotate about an axis, and the target assembly includes a motor arranged to make the holding tube and the container rotate.

9. The target assembly as claimed claim 6, further including a cooling tube placed inside the container and arranged to allow a cooling liquid to flow.

10. The target assembly as claimed in claim 9, wherein the cooling tube includes a cooling head that has a recess on a portion of a periphery of the cooling head, the recess to give incident beam a longer path in a precursor liquid.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a cross-sectional view of a prior-art container, namely that of WO2013049809.

(2) FIG. 2 is a semi-isometric perspective view of a container according to the invention.

(3) FIG. 3 is an exploded semi-isometric perspective view of the lower portion of a target assembly according to the invention.

(4) FIG. 4 is a cross-sectional view of the lower portion of a target assembly according to the invention.

(5) FIG. 5 is a perspective view of an axial cross section through the upper portion of a target assembly according to the invention, in an embodiment allowing the container to be rotated.

(6) FIGS. 6a, 6b and 6c are a cross-sectional and semi-isometric perspective view, a cross-sectional view and a detailed view, respectively, of a cyclotron in which a target assembly according to the invention, with possibility of rotation, is arranged as an internal target.

(7) FIG. 7a is an isometric perspective view of the lower end of a cooling tube of a pocket according to one particular embodiment of the invention. FIG. 7b is a top view of a cross section perpendicular to the axis of this tube in position in a container.

(8) FIG. 8 shows cross-sectional views of a plurality of embodiments of containers according to the invention and a semi-isometric perspective view of one thereof.

DETAILED DESCRIPTION OF THE INVENTION

(9) FIG. 1 is a cross-sectional view of a prior-art container, namely that of WO2013049809, and was described above.

(10) FIG. 2 is a semi-isometric perspective view of a container 100 according to the invention. This container 100 takes the form of a thimble, having a symmetry of revolution about an axis. The upper portion 110 is open and may have a conical shape, the opening of the cone being oriented upward. As explained below, this arrangement is of benefit as regards the assemblage of the container 100 into a target assembly. The top of a first cylindrical portion 120 is connected to the upper portion 110 and its bottom is connected to a thin wall section 130. This thin wall section 130 is connected to a second cylindrical portion 140, that itself is connected to a dome 150 closing the container 100 at the bottom. The thickness of the thin fraction is smaller than or equal to 100 m and for example 80, 60, 40, 20, 10 or even 5 m. A smaller thickness gives a better transparency to the beam and therefore a better production yield, but is more fragile. The applicant has determined experimentally that the value of 20 m is a good compromise between these contradictory requirements. The non-thinned portions, namely the open upper portion 110, the first 120 and second 140 cylindrical portion and the dome 150 are produced with a thickness larger than the thickness of the thin wall fraction 130. For example, when the thin fraction has a thickness of 20 m, the non-thinned portions may have a thickness larger than or equal to 100 m, 200 m or more for example. The various portions of the container 100 connect to one another without sharp angles, such that a better mechanical resistance, especially to pressure, is obtained. The inside diameter may be about 10 mm and the total height 11 mm and the angle of the cone may be 30. The container 100 shown has a cylindrical shape. However, it is possible, without departing from the scope of the present invention, to produce a container 100 having a more complex shape, with a curvature toward the interior, such as a one-sheet hyperboloid, or a bulging shape, such as a barrel. The container 100 has been shown with an upward-facing opening and a closed bottom side. However, it is possible to imagine, without departing from the scope of the invention, a container 100 having two openings such as shown. A container 100 that may be supplied with target material from above or below and through which a coolant fluid or fluid precursor may be made to flow from top to bottom is then obtained.

(11) The obtainment of a container 100 according to the invention, in particular when the thin fraction 130 is very thin, presents many difficulties. The applicant has developed a manufacturing method by virtue of which the shape shown, or other shapes, may be produced easily. This method is based on electroforming: A matrix having the shape of the interior of the container 100 is produced. This matrix may for example be made of aluminum; A metal layer is deposited by electrodeposition on all the exterior surface of the matrix, until the thickness desired for the thin portion has been obtained; A fraction of the height of the matrix is masked by applying an insulating layer, a lacquer or a plastic tape for example; the electrodeposition is continued until the thickness desired for the non-thinned portions has been obtained; the matrix is removed, for example in a caustic solution.
The thickness of the deposit is determined by the magnitude of the current and the duration of application thereof. The following metals may be used: nickel, titanium, niobium and tantalum, and alloys may also be obtained such as stainless steel, Havar (cobalt-based alloy), Invar or Kovar. In the case of a rotating target, the point of penetration of the beam into the container is a hotspot that is in continuous motion. This spot is a source of thermal expansion/contraction that may lead to fatigue of the metal. The choice of a material with a low thermal expansion coefficient, such as Invar and Kovar, may then be advantageous. It is also possible to deposit different alloys or metals in successive electrodeposition steps so as to obtain a first layer in one material, and one or more other layers in other materials. It is thus possible to choose the constituent material of the thin fraction for its resistance to the beam, or to make the layer making contact with the precursor material from a material having a chemical compatibility with the precursor material. Niobium may advantageously be used for the first layer forming the internal wall of the container i.e. the wall making contact with the precursor material. Specifically, it is known that the use of niobium does not lead to contamination of the produced radioisotope by undesired radioisotopes.

(12) The choice of the thickness of the thin portion 130 is an important element of the invention. In the table below, the residual energy that a beam of protons having an energy of 7, 10, 15, 20 and 30 MeV, respectively, has after passage through a nickel sheet of various thicknesses has been indicated. It may be seen that when the sheet has a thickness of 5 m, the energy loss of the protons is negligible i.e. less than 3% at 7 MeV and less than 0.2% at 30 MeV. In contrast, at 100 m and low energy, the loss in the sheet is substantial. It is then necessary to make recourse to a higher energy and therefore a more expensive accelerator. It is known that the production yield of .sup.18F from H.sub.2.sup.18O by (p,n) reaction is practically zero when the protons have an energy below 3 MeV. To obtain a yield higher than 60 mCi/A, it is necessary to use protons of 6 MeV at least. The thickness values indicated in bold in the table below are therefore maximum preferred thicknesses, depending on the energy of the available beam. If a yield even higher than 60 mCi/A is desired, it is necessary to further decrease the thickness of the thin fraction.

(13) TABLE-US-00001 NICKEL Sheet Incident E <MeV> thickness 7 10 15 20 30 <m> Transmitted E <MeV> 5 6.84 9.87 14.91 19.92 29.94 10 6.67 9.74 14.81 19.85 29.89 20 6.32 9.48 14.62 19.70 29.78 40 5.59 8.95 14.24 19.39 29.55 60 4.77 8.38 13.85 19.07 29.33 80 3.86 7.80 13.43 18.76 29.10 100 2.75 7.16 13.01 18.44 28.86 200 Stopped 3.00 10.79 16.75 27.72
The choice of a thinner wall, for example of thickness smaller than or equal to 100 m, allows the production of heat as the beam passes through to be limited. The above table may be used to guide the choice of the thickness when the chosen material is nickel. Other metals, such as niobium, titanium or Havar, have a slightly higher transparency and will give better results.

(14) FIG. 3 is an exploded semi-isometric perspective view of the lower portion of a target assembly according to the invention and shows how the container 100 is arranged in a holding tube 200. The tube has a male threaded portion 220. A ring 300 has a corresponding female threaded portion 310. The ring covers the upper portion 110 of the container 100 and presses it against the lower portion of the holding tube 200. At least the thin wall fraction 130 of the container 100 then emerges from the assembly thus formed. The holding tube 200 and the ring 300 may include flats 210, 320 that then allow an operator to assemble and disassemble the assembly very rapidly by means of two open-ended wrenches. The holding tube 200 and the ring 300 may for example be produced from stainless steel. Other mechanical assembling means may also be used without departing from the scope of the invention, such as quick-release hose clamps. In one preferred embodiment of the invention, the lower portion of the holding tube 200 includes a conical end 230 that is congruent with the conical portion 110 of the container 100, said conical portion itself being congruent with a conical end 330 of the ring 300. In this embodiment, an excellent seal tightness may be obtained without having to make recourse to a seal: the seal tightness is ensured by the metal-to-metal contact.

(15) FIG. 4 is a cross-sectional view of the lower portion of a target assembly according to the invention. Apart from the elements described above with reference to FIG. 3, the pocket assembly 400 is also shown, this pocket assembly playing the dual role of ensuring the cooling of the precursor material contained in the container and that cools in its turn the container, and of allowing the precursor material to be loaded into or unloaded from the container. A cooling tube 410 that is closed at its lower end may be inserted into the holding tube 200 and end in the container 100. In one exemplary embodiment, the container 100 has an inside diameter of 10 mm and a height of 10 mm and the cooling tube 410 an outside diameter of 8 mm, the irradiation chamber 440 having a useful volume of approximately 350 mm.sup.3. An intermediate tube 420, which is open at its lower end 425, and of diameter smaller than that of the cooling tube, is inserted into the latter. It is thus possible to make a cooling liquid such as water flow through the space comprised between this cooling tube 410 and this interior tube 420. The arrows A represent the entrance of the cooling liquid and the arrows B the exit of the cooling liquid. The directions of flow A and B may be inverted. Since the heat transfer area is large and uniformly distributed, this arrangement allows excellent cooling to be obtained. In the case where the target assembly allows the assembly made up of the container 100, the holding tube 200 and the ring 300 to be rotated, the pocket assembly 400 remains stationary. The relative movement of these 2 assemblies produces a stirring effect that further improves the cooling by inducing a forced convection. A capillary tube 430 placed axially inside the intermediate tube 420 and sealably passing through the lower end of the cooling tube 410 in order to end in the space comprised between the container 100 and the cooling tube 410 allows the precursor material to be loaded and unloaded as indicated by the two-headed arrow C. The enlarged view shows how the conical portion 110 of the container is clamped between the conical end of the ring 330 and the conical end of the holding tube 230, thus ensuring the seal tightness without using a seal.

(16) Independently of whether the target of the invention is used as an internal or external target, it is advantageous to be able to make it rotate. It is possible to either successively give thereto various orientations, for example to rotate it by 10 each time it is used, or preferably, to continuously rotate the container 100 during the irradiation. It is thus possible to ensure that all the periphery of the thin wall fraction is passed through by the beam, thereby ensuring a better distribution of the production of heat over a larger area. Furthermore, in the case of a liquid target, the rotation induces stirring of the precursor material, thereby improving the cooling by convection. FIG. 5 is a perspective view of an axial cross section through the upper portion 500 of a target assembly according to the invention, in one embodiment allowing the container 100 to be made to rotate. The container 100 (not shown in the figure) and the holding tube 200 are arranged in the rotor 570 of an electric motor. The stator 560 is secured to a housing 510 that is fixed. Maintenance and seal-tightness are ensured by a seal-bearing having a fixed portion 540 and a rotating portion 542. This seal-bearing may include ball bearings 550 and 550. This seal may for example be a magnetic fluid seal such as those sold by Rigaku. The distributing head of the pocket 400 emerges from the upper portion of the target assembly and gives access to the orifices 452, 454 through which the cooling fluid respectively enters and exits. and to 430 through which the precursor material is filled/emptied. There may be two separate entrance and exit tubes.

(17) FIGS. 6a and 6b show a cyclotron 700 in which a target assembly according to the invention is placed. The upper portion 500 emerges from the upper face of the cyclotron 700. The holding tube 200 has a length such that the container 701 is located in the median plane of the cyclotron, the thin fraction thereof being exposed to the beam, as shown in the detailed view 6c. When the target assembly of the invention is used as an external target, it may be placed at the end of the beamline and receive the beam radially. It is also possible to produce a container the thin portion of which is located on the base, such as in the containers 907 and 909 shown in FIG. 9, and to orient the beam toward this base, parallelly to the axis of symmetry of the container.

(18) Certain radioisotope precursors, such as H.sub.2.sup.18O, are precious and expensive. Moreover, it is sometimes advantageous to be able to synthesize radiochemicals from a concentrated product. It is therefore advantageous to minimize the amount used. To this end, a preferred embodiment of the invention has been designed, in which embodiment (shown in FIGS. 7a and 7b) the volume of the chamber is even smaller. FIG. 7a is a semi-isometric perspective view of the lower end of a cooling head 800 of a pocket of this preferred embodiment. This tube has a face 801 having an optimized profile as discussed below. The entrance/exit orifices 802 of the cooling liquid allow the cooling liquid to be made to flow through the interior of the cooling head 800. In this example, there are two parallel entrance and exit tubes, but there could be only a single one thereof as in the example in FIG. 4. The entrance/exit orifices 803 of the precursor liquid open below the lower end of the cooling head 800 and allow the space comprised between the container and the cooling head 800 to be accessed. Notches or grooves 804 may be provided for the placement of temperature probes, thermocouples for example. FIG. 7b is a top view of a cross section perpendicular to the axis of this cooling head 800 in position in a container 860. As may be seen from this cross section, the cooling head 800 has, on a portion of its periphery, a recess 851, which gives to the incident beam, represented by the arrows F, a longer path 852 in the precursor liquid, although the space between the cooling head 800 and the container 160 is smaller in the places where there is no incident beam. The length of this path is defined so that the beam can deposit all its useful energy in the precursor material. This arrangement has the following advantages: decrease of the necessary volume of precursor; maximization of cooling, due to a minimum thickness of liquid; use of all the useful energy (for example the energy higher than 4 MeV for protons in H.sub.2.sup.18O) of the particles of the beam in the precursor. The thermocouples 805 allow the temperature of the target to be controlled in real time. In the embodiment in which the target is rotated, the container 860 rotates whereas the cooling head 800 remain stationary, thereby promoting the stirring of the precursor liquid and the exchange of heat. In this example, the inside diameter of the container 860 is 10 mm, the outside diameter of the cooling head is 9.5 mm and the useful volume of the chamber is 100 mm.sup.3.

(19) FIG. 9 shows cross-sectional views of a plurality of embodiments of containers according to the invention. The arrow X represents the direction of the incident beam. The arrow X also indicates the position of the thin wall. The cross sections are limited to the facial segment of the solid bodies so as to facilitate the representation of the thin walls.

(20) The container 901, which has symmetry of revolution, is cylindrical and has an upper end of conical shape, is one of the preferred embodiments of the invention. The container 902, which has a symmetry of revolution, has two open ends, both of which are of conical shape. The containers 903 and 904 are similar to the container 901, except that they have an open end with a flat edge and an open end with a cylindrical edge, respectively. The container 905 is similar to the container 901, except that it has a barrel shape.

(21) The container 906 is similar to the container 901, except that it has a one-sheet-hyperboloid shape.

(22) The container 907 is similar to the container 901, except that it has a thin wall in the closed end. It thus allows an axial penetration of the beam.

(23) The container 908, in contrast to the other containers shown, does not have symmetry of revolution, but a square or rectangular cross section, the thin wall possibly extending over a portion of two or three faces. This container is also shown in semi-isometric perspective. The container 910 is similar to the container 901, except that it has a larger diameter (for example 50 mm) and a flat bottom.

(24) The container 909 is similar to the container 910, except that the thin portion is arranged in a ring on the flat bottom and allows an axial penetration of the beam. This container may advantageously be used in an external target, in which the incident beam is parallel to the axis of rotation, as shown by the arrow X.

(25) In case of use as an external target, the targets 901 to 907 may be placed such that the beam penetrates into the target radially.

Advantages of the Invention

(26) The container 100 according to the invention has the advantage of being of integral construction, i.e. of not requiring assembling means or working, mounting or demounting means. The thin fraction 130 of the container 100 forms as it were a window integrated into the container 100. The target and the container 100 according to the invention may be easily demounted and remounted. The operator may act rapidly and may therefore limit his exposure to radiation. The container of the invention requires little material. It is therefore inexpensive and creates little waste when it must be scrapped. The target assembly according to the invention may if needs be serve as a beam stop, for example during the setup of an accelerator.

(27) The present invention has been described with reference to specific embodiments, which have been given purely by way a of illustration and which must not be considered to be limiting. Generally, it will appear obvious to those skilled in the art that the present invention is not limited to the examples illustrated and/or described above. The presence of reference numbers in the drawings must not be considered to be limiting, including when these numbers are indicated in the claims. The use of the verbs comprise, contain, include, or any other variant, and their conjugations, in no way excludes the presence of elements other than those mentioned. The use of the indefinite article a, an or the definite article the to introduce an element does not exclude the presence of a plurality of these elements. The use of the words top/bottom lower/upper is to be understood as being relative to the orientation of the components shown in the drawings. Although the examples described relate to the production of .sup.18F by irradiation by a beam of protons of a target material containing .sup.18O-enriched water, the invention may be applied to other liquid precursors, such as ordinary water H.sub.2.sup.16O, which produces .sup.13N during irradiation with protons, or gaseous precursors, such as .sup.14N.sub.2 to obtain .sup.11C. It is also possible to apply the invention to pulverulent precursor materials or to powders in suspension in a liquid and forming slurries. Lastly, the invention is also applicable to the case of a precursor material such as .sup.11B.sub.2O.sub.3, which produces .sup.11C by (p,n) reaction and forms .sup.11CO.sub.2 that may be collected. Other particles such as deuterons and alpha particles may be used. Likewise, the target according to the invention may be used with the chamber of the container at atmospheric pressure, or with the chamber placed under pressure.