WO2002007183A1

WO2002007183A1 - Process for manufacturing assemblies comprising a base and a device integrating the functions of cathode and getter for miniaturized x-ray emitters and assemblies so produced

Info

Publication number: WO2002007183A1
Application number: PCT/IT2001/000376
Authority: WO
Inventors: Paolo Battilana; Alessandro Gallitognotta; Alessio Corazza
Original assignee: Saes Getters S.P.A.
Priority date: 2000-07-19
Filing date: 2001-07-17
Publication date: 2002-01-24
Also published as: IT1318202B1; ITMI20001645A0; ITMI20001645A1

Abstract

A process is described for manufacturing with exact and reproducible geometry devices integrating the functions of cathode and getter for miniaturized X-ray emitters (10), which have an application in the medical field; some variants of the basic method as well as the resulting devices (40; 60; 80; 100) are also described.

Description

"PROCESS FOR MANUFACTURING ASSEMBLIES COMPRISING A BASE

AND A DEVICE INTEGRATING THE FUNCTIONS OF CATHODE AND GETTER FOR MINIATURIZED X-RAY EMITTERS AND ASSEMBLIES SO

PRODUCED"

The present invention relates to a process for manufacturing assemblies comprising a device having the double function of cathode and getter joined to a metal base for use in miniaturized X-ray emitters, particularly for use in the medical field; in particular, the invention provides such assemblies with a very high degree of geometrical precision, that is important for the proper functioning of the X-ray emitters. The invention also relates to the assemblies so manufactured. For the sake of simplicity, the devices according to the invention will be also be referred to in the following as getter cathodes.

It is known that a treatment with X-rays can cause the necrotization of tissues of the human body. This effect can be employed for treating some forms of tumor or restenosis, that is, the thickening of the artery's walls following to an operation of mechanical removal of an obstruction of blood vessels.

Obviously, in these treatment methods it is important to prevent as much as possible healthy parts too of the body from being subjected to the treatment. Consequently, systems wherein a miniaturized X-ray emitter is mounted on the top of a thin rigid tube or of a catheter are under advanced study. The tube or the catheter are then introduced into the patient's body, bringing the X-ray source as near as possible to the area which has to be irradiated. In this way it is possible to maximize the efficiency of the treatment, preventing at the same time the neighboring healthy tissues from being involved.

Miniaturized X-ray emitters mounted on the top of rigid tubes, in particular for the treatment of cerebral tumors, are disclosed for instance in patents US 5,153,900, 5,369,679, 5,422,926, 5,566,221, 5,621,780 and 5,748,699, all assigned to US company Photoelectron Corporation. Patents US 5,729,583, 5,816,999 and 6,069,938, European patent application EP-A-860181 and the international patent applications WO-A-97/07740, WO-A-99/36938, WO-A- 99/44687, WO-A-99/45562 and WO-A-99/45563, disclose systems wherein the miniaturized X-ray source is mounted on a flexible catheter, which are therefore suitable for irradiating zones which can be reached through the blood vessels or other lumens of the human body. The miniaturized X-ray emitters of these patents have different structures in the details, but they always comprise a cathodic portion for the emission of electrons and an anodic portion (generally made with a heavy metal) which emits X-rays when it is hit by the electronic beam accelerated by the potential difference between the two electrodes. The electrodes are contained in an evacuated housing, in order to avoid that the electrons emitted by the cathode are absorbed by gas molecules. This requirement involves in general the need of using a getter material inserted into the housing which can sorb the traces of gas released by the same components of the X-ray miniemitter, such as the walls of the housing or the anode, which during the functioning overheats because of the electron bombardment.

Getter devices inserted into miniaturized emitters of X-rays are described in some documents assigned to XRT Coiporation, St. Paul, Minnesota, USA.

European patent application EP-A-860180 describes getter devices which can have the shape of a small ring arranged around the cathode; alternatively, the cathodic portion itself can be foimed of a getter material on which a thin layer of diamond, which is the active element for electron emission, is deposited. The formation of the diamond layer can be carried out by Chemical Vapor Deposition (also commonly referred to with its acronym CVD), that is, a technique of deposition of layers of materials from precursors in the gas phase; or by "laser ablation", which consists in producing by laser heating vapors of the material whose deposit is desired, and then condensing these vapors on a suitable substrate. However, at least the case in which the getter has the shape of a small ring around the cathode, is very difficult to implement because of the very reduced space which is available. Therefore, the getter devices which integrate also the cathode function are preferable. Patent US 5,854,822 describes getter devices which also perform the cathode function. These getter cathodes are obtained by sinterization of powders of a getter material and therefore have a granular surface: as it is known, the resulting microtips, which correspond to the morphology of the granules of the getter material, are preferential points for electron emission. According to this patent, in order to improve the features of cathodic emission, it is also possible to mix diamond powder with the getter material powder during cathode manufacture.

Finally, international patent application WO-A-09580 discloses various cathodes having getter functionality, with a smooth or granular surface, wherein the efficiency of electron emission can be increased either by depositing a thin diamond layer on a well defined area of the getter material surface, or by mixing powders of diamond and of getter material before the sinterization of the latter.

However, all the cathode getters described in the patents above suffer from serious construction problems, due to the fact that the geometrical precision and invariability of the electrodes position have to be guaranteed with very low tolerances in the miniaturized X-ray emitters for medical purposes. As a matter of fact, X-rays emission is generated by applying differences of tens of thousands of volts between anode and cathode, which are arranged at distances not higher than 1.5 millimeters, and preferably lower than one millimeter; under these conditions very high electric fields are present, and deviations from the theoretical geometry of the reciprocal arrangement of cathode and anode (both in distance, and in the lateral deviation) even of a tenth of millimeter or lower can lead to notable variations of the quantity of emitted X-rays, with the risk that the radiation is too weak for a effective treatment or too strong and therefore capable of reaching beyond the region to be treated and destroying healthy neighboring tissues. Therefore, it is absolutely necessary that cathode and anode are at a precise distance and aligned with the axis of the miniemitter.

The manufacturing methods according to the mentioned patents cannot guarantee the required absolute geometrical precision. According to the known art, the getter devices are prepared by introducing a suitable quantity of powder into an upperly open- die and by subjecting the powder to a sintering thermal , treatment. During this treatment a reduction of the total volume of the powders occurs, which involves a reduction, although a minimal one, of the height of the powder level and a rearrangement of the free upper surface thereof. This free surface is the one intended to be used, in a subsequent step, to comiect the getter cathode to a base of the finished device, that preferably forms one of the internal walls of the housing of the miniaturized X-ray emitter. Irregularities of said free surface of the sintered powders are reflected in imprecisions in the distance from the cathode getter tip from said base, and in the perpendicularity with respect to the base surface; these imprecisions, in their turn, respectively result in variations of the distance between cathode and anode and of the orientation of the cathode, which can be non-coaxial with the emitter, leading to the above mentioned drawbacks.

Object of the present invention is to provide a process for manufacturing assemblies for use in miniaturized X-ray emitters, comprising a device integrating the functions of cathode and getter joined to a metal base, which avoid the problems and the difficulties of the known processes, as well as to provide the resulting assemblies.

These objects are achieved according to the present invention by means of a process which comprises the steps of: a) preparing a first die of an inert material, with at least one recess in a flat upper surface thereof, said recess having an axis perpendicular to said upper surface; b) scrape filling said recess with at least one mixture comprising powders of at least one sintering material and at least one material selected between a getter material and diamond; c) placing over said first die a second die of an inert material having at least a through hole, so that said through hole is coaxial with said recess in the first die; d) introducing into said through hole a metal piece having previously provided for a layer of a material favoring, in a subsequent thermal treatment, the adhesion between said at least one mixture of powders and said metal piece; e) introducing the assembly formed of said dies and of powders and pieces therein contained into an oven and subjecting it to a thermal treatment for sintering said at least one mixture of powders; f) at the end of the sintering thermal treatment, extracting from the dies the resulting assembly formed of the device integrating cathode and getter functions and the metal piece.

With the definition "scrape filling" it is meant a rather common method used in the field of powders handling for filling to the top a cavity in a die; the method consists in introducing the powder in a cavity until this is completely filled and cleaning the upper surface of the die with a scraping blade in order to remove the portion of exceeding powder which comes out of the cavity.

The invention will be now described with reference to the drawings wherein:

- Figure 1 shows schematically in enlarged scale a cross-section of a miniaturized X-ray emitter;

- Figure 2 shows a metal piece of the assembly of the invention;

- Figure 3 shows the result of the main steps of a first process according to the invention;

- Figure 4 shows an assembly manufactured according to the process shown in figure 3;

- Figure 5 shows the result of two steps of a second possible process according to the invention;

- Figure 6 shows an assembly manufactured according to the process of figure 5; - Figure 7 shows the result of two steps of another process according to the invention;

- Figure 8 shows an assembly manufactured according to the process of figure 7;

- Figure 9 shows the result of four steps of a further process according to the invention; and

- Figure 10 shows an assembly manufactured according to the process of figure 9.

Figure 1 shows schematically the cross-section of a miniaturized X-ray emitter 10. The emitter 10 is formed of a lateral wall 11, generally cylindrical, closed at one end with a piece 12, generally made of metal, which forms the base carrying cathode 13, and at the other end with a piece 14 made with a heavy metal, which forms the anode. Wall 11 is generally welded by brazing to piece 12 and to anode 14 (or to apiece carrying the anode) respectively in the zones 15 and 16, thus defining a sealed space 17, which has to be maintained evacuated in order to guarantee that the electrons emitted by the cathode 13 may reach the anode 14. Piece 12 has a surface 18 which faces space 17 and constitutes a wall for this space. The drawing does not show the getter device, which can be integrated inside cathode 13, or be present as a separate member. Emitter 10 has an external diameter generally lower than 3 mm, and preferably lower than 1.5 mm, and a length lower than 5 mm and preferably lower than 3 mm. The mounting and electric connection of emitter 10 on a catheter or on the top of a thin tube (not shown in the drawing) are known from the above mentioned patents. Wall 11 is generally made of materials containing atoms having a low atomic number, such as diamond or boron nitride, in order to guarantee the maximum outward transmission of the X-rays; the base 12 of the cathode is generally made of metals, such as molybdenum or tungsten, whereas the anode is made of a heavy metal, such as tungsten or platinum.

Figure 2 shows the metal piece 12, turned upside down with respect to its orientation in emitter 10 as shown in figure 1; the drawing shows surface 18, and its portion 19 where the getter cathode is to be comiected. Figure 3 shows in cross-section the result of the main steps which compose the process according to the invention in its simplest variant.

In step a it is prepared a first die 30 of inert material having at least a recess, 31. Recess 31 has an axis which is perpendicular to the flat upper surface 32 of the die. Die 30 is made of a material which can resist, without alterations and without interacting with the materials therein contained, during the thermal treatments of the process of the invention, which are carried out generally at temperatures between about 750 and 1100 °C under vacuum. Materials which are suitable for the purpose are for instance graphite, molybdenum or ceramic materials such as boron nitride. hi step b recess 31 is scrape filled with a mixture 33 comprising powders of at least one getter material and of at least one sintering material. The getter material is generally an alloy comprising zirconium or titanium with one or more elements selected among transition elements and aluminum, such as for example the Zr-Al alloys disclosed in patent US 3,203,901 and in particular the alloy having weight percent composition Zr 84% - Al 16%, produced and sold by the applicant under the name St 101; the Zr-V-Fe alloys disclosed in patent US 4,312,669 and in particular the alloy having weight percent composition Zr 70% - V 24.6% - Fe 5.4%, produced and sold by the applicant with the name St 707; and the Ti-V-Mn alloys disclosed in patent US 4,457,891. The sintering material is generally zirconium, titanium, hydrides thereof, or a mixture thereof. The preferred mixtures for the purposes of the invention are the mixture comprising, by weight, 70% of Ti and 30% of alloy St 101; the mixture comprising 70% of Ti and 30% of alloy 707; the mixture comprising 40% of Zr and 60% of alloy St 707; and the mixture comprising 60% of Ti and 40% of alloy St 707. Preferably, at least 10% of titanium or zirconium is substituted by their hydrides, which during the thermal treatment release hydrogen, thus contributing to de-oxidize the particle surface and favoring sinterization. The powders which form mixture 33 preferably have particle size lower than 50 μm: higher particle sizes would give rise to getter cathodes having a very non-homogeneous surface (with resulting difficulties in the control of electron emission) and excessively porous (with problems of poor electronic emissivity).

After the first filling the powder of mixture 33 may be slightly compressed in recess 31 with a suitable punch, and it is then possible to carry out a new scrape filling, in order to improve compactness of the powder inside the recess. Compactness can also be helped by vibrating the die: this causes the settlement of the powders and the lowering of the level thereof inside recess 31. Finally, it is possible to resort to both methods, by first causing compaction of the powder by vibrating the die and then slightly compressing this in the recess.

In step c a second die 34, made with a material inert in the conditions of the thennal treatments of the process of the invention, is placed over die 30; said material is generally (but not necessarily) the same of die 30. Die 34 has at least one through hole 35, having a diameter corresponding to piece 12, and is arranged on die 30 in such a position that hole 35 is coaxial with recess 31. The exact reciprocal positioning of dies 30 and 34 is preferably guaranteed in the different productive steps with the well-known "pinning" method, according to which the dies are provided with at least two (better three or four) thin through holes in corcesponding positions in the different dies, wherein pins whose height is at least equal to the sum of the height of all the superimposed dies are introduced.

In step d piece 12 is introduced into hole 35 with surface 18 facing die 30, having previously provided for a layer, shown in the drawing as element 36, of a material capable of favoring the adhesion between mixture 33 and metal piece 12. The adhesion- favoring layer 36 can be either in the form of a film coated with any suitable technique onto piece 12 or in the fom of a layer of powders, as will be described in detail in the following. h step e (not shown in figure 3), the so formed assembly is introduced into an oven and subjected to a thermal treatment for sintering the powders of mixture 33. The thermal treatment is preferably earned out under vacuum (pressures lower than 10^"6 mbar) and generally requires from 1 minute to 2 hours. The treatment temperature is generally comprised between 750 and 1100 °C. The exact value is determined as a function of the specific mixture which is used. During the treatment, a process of solid-state interdiffusion of the materials of the mixture and the material used to form layer 36 takes place, with the result that piece 12 and the part resulting from sintering of mixture 33 result joined at the end of the thennal treatment.

Finally, the last step f of the process (not shown in figure 3), consists in taking out from the oven the assembly of the dies and in taking out from these the final product. This product is assembly 40 shown (in a side view) in figure 4, which comprises piece 12 connected to getter cathode 41 through layer 36. Because of the way hi which it is produced, assembly 40 guarantees the exactness and reproducibility of the distance between the tip of getter cathode 41 (indicated with 42 in the drawing) and surface 18 of piece 12, and therefore the distance between cathode and anode in emitter 10; assembly 40 also guarantees perpendicularity between surface 18 and the axis of getter cathode 41, and therefore the coaxial condition between this latter and emitter 10. This high degree of geometrical exactness and reproducibility allows exact control and proper functioning of emitter 10.

In a first considered embodiment of the invention, layer 36 is in form of a film of a material 37 coated onto piece 12. This film can be formed by the above mentioned CVD technique, by electroplating, or by Physical Vapor Deposition (PVD, also commonly referred in the field as sputtering). Layer 36 can be formed over entire surface 18 (possibility not shown in figure 3) but it is preferably obtained only on portion 19 of said surface by masking techniques, as it is well known in the field of films deposition (embodiment shown in figure 3; the thickness of the film of material 37 is greatly exaggerated for the sake of clarity). The prefened technique for forming layer 36 is PVD, that is relatively more easy to implement than the other cited techniques. It has been found that a suitable material 37 for forming the adhesion-favoring film onto piece 12 can be selected among the metals Ti, Ni, Cu, Co; the binary alloys Ni-Cr, Co-Cr or Fe-Cr; the ternary alloys Ni-Cr-M or Co-Cr-Mi; and the quaternary alloys Ni-Cr-Mi-M₂ or Co-Cr-M₁-M₂, wherein Mi and M₂ are metals of the first transition row, or of LTI or IV group of the periodic table of elements.

According to a second embodiment of the process of the invention, layer 36 can be in the form of a layer of powders of a brazing material. In this case the process depicted in figure 3 is modified by the addition of the two extra steps represented in figure 5. After having carried out step b of the process of fig. 3, in this embodiment it is earned out step g, by which a depression 50 with respect to surface 32 is fonned, by slightly pressing mixture 33 into recess 31 with a suitable punch (not shown in the figure), by vibrating die 30, or both, as described above. Depression 50 is then scrape filled in step h with powder of a brazing material 51. This step too may be aided by vibrating die 30. Material 51 must have a melting temperature similar to that subsequently used in the process for sintering mixture 33. hi particular, said melting temperature must be not lower than the sintering temperature of mixture 33, h order to avoid that, while melting, material 51 incorporates the powders of the mixture, thus reducing or canceling their gettering effect towards gases; on the other hand, the melting temperature of material 51 is preferably not more than 10-15 °C higher than said sintering temperature, in order to enable the brazing thereof and adhesion to mixture 33 and to metal piece 12 in step e of the process. Material 51 preferably has a particle size lower than about 100 μm: higher particle sizes would make difficult the contact between the body formed by sinterization of mixture 33 and metal piece 12. The prefened brazing materials are silver, silver-copper alloys, and silver-copper-gold alloys, among silver-copper alloys, preferred are those having a silver content higher than about 60% and particularly the eutectic composition containing 72% of silver. Pure silver and the eutectic composition have the advantage of a well- defined melting temperature, thus allowing the process parameters to be set at best, whereas the other alloys are chosen when the sinterization must be carried out at temperatures different from the melting ones of silver and of the eutectic Ag-Cu composition; in particular, the Ag-Cu-Au compositions are employed when it is necessary to resort to high sintering temperatures.

Upon completion of step h, the process is continued with steps c-f as described above, by superimposing die 34 to die 30 filled with mixture 33 and brazing material 51, and subjecting this assembly to the described sintering thermal treatment. The result of this second embodiment of the process of the invention product is assembly 60 shown in a side view in figure 6. Assembly 60 comprises metal piece 12 comiected to getter cathode 61 by means of layer 36, formed of the brazing material 51.

In a modification of the processes described with reference to figures 3 and 5, it is possible to use, in place of piece 12, a metal piece 12' having a part of surface 18 (corresponding, hi section, to portion 19) raised with respect to this latter; this modification is represented in figure 7, making reference by way of example to the embodiment of figure 3. Upon completion of step b, a step i is carried out by which a depression 70 with respect to surface 32 is obtained by slightly pressing mixture 33 into recess 31 (or by vibrating die 30). Then step c is performed, superimposing die 34 to die 30. A piece 12', having a raised part 71 (con-esponding to portion 19), is prepared aside, depositing a layer 36 of material 37 only on the circular surface of part 71. Step d is then carried out, inserting the so obtained piece 12' as shown into through hole 35 of die 34, with part 71 extending into depression 70 and layer 36 contacting mixture 33. By applying a compression (dynamic, for example by means of a punch, or static, by placing a weight over it) to piece 12', a better contact between this and mixture 33 during sinterization is ensured, and therefore a better mechanical strength of the finished assembly. The process is then completed by carrying out steps e and f as described before. The result of this modification of the base process is assembly 80 shown in figure 8, composed of a getter cathode 81, joined through layer 36 to piece 12'. This modification employing step i and piece 12' in step d is also possible based on the embodiment of the process described with reference to figure 5, by obtaining a further depression in the upper surface of the powders of brazing material 51 after step h, and inserting a piece of kind 12' (on which no layer of material 37 is obtained) into die 34; this latter process modification and the assembly thereby obtained between piece 12' and the getter cathode are not represented in the drawings.

In order to improve the electronic emission properties (cathodic functionality) of the getter cathodes, in any embodiments or modifications of the process of the invention so far described a thin diamond film can be deposited on the tip of the getter cathode (42) by means of one of the above mentioned "CVD" or "laser ablation" methods. Since the thin films obtained by these techniques have thickness values of the order of a thousandth of millimeter or less, this operation does not modify the geometrical relationships between the thus obtained getter cathode and anode 14 in emitter 10. Alternatively, it is possible to use in place of mixture 33 a mixture 33', admixing . diamond powder to the former. Preferably, the diamond powder has a particle size between about 1 and 50 μm, and even more preferably of about 10 μm.

In a further modification of the processes described in the foregoing, it is possible to add diamond powder only in the tip portion of the getter cathodes, as described in the following with reference to figure 9 that represents in cross- section the result of the main steps of this last modification of the process of the invention. According to this modified process, in a step j it is prepared a die 90 of an inert material, provided with a recess 91 having an axis perpendicular to the upper surface 92 of the die, and a depth which is only a fraction of the length of the getter cathode to be produced.

Recess 91 is scrape filled (step k) with a mixture, 93, fonned of diamond powder having particle size as previously described and of powder of an easily sinterable material, preferably titanium hydride having a particle size lower than 50 μm and preferably comprised between about 5 and 10 μm. Mixture 93 could also be formed of diamond powder and of one of the above described mixtures 33.

In the following step, 1, a die 94 of an inert material which is provided with a through hole 95 having the same diameter of the upper portion of recess 91 is superimposed to die 90; dies 90 and 94, taken together, conespond to die 30, with upper surface 96 of die 94 coiresponding to surface 32 of die 30. Finally, in step m hole 95 is scrape filled with powder of the same mixture

33 previously described.

Upon completion of step m an assembly is obtained, comprising dies 90 and 94 filled with mixtures 93 and 33, conesponding to the result of step b in the process of figure 3; this assembly can then be subjected to steps c-f of the process described with reference to figure 3, to step g (and following steps) of the process described with reference to figure 5, or to step i (and following steps) of the process described with reference to figure 7.

Figure 10 shows in lateral view the assembly obtained according to this last modification of the process of the invention; the assembly 100 here represented comprises a metal piece 12, that is joined to the getter cathode 101 by means of a layer 36 of brazing material 51; in its turn, getter cathode 101 is fonned of a main body 102 resulting from sinterization of the mixture 33, and a tip portion 103 made of mixture 93 described above.

The process has been described up to now (in its different embodiments and modifications) with reference to the manufacture of a single assembly between a metal piece and a getter cathode, but because of the small size of these assemblies, in order to increase productivity, they are generally produced more than one at a time by using dies with a multiplicity of recesses and coreesponding holes.

Some methods above described, while referring to particular embodiments or modifications of the process of the invention, can be used in any of such embodiments or modifications: for instance, a piece of kind 12' can always be employed in place of piece 12, as well as a process employing a film of material 37 can always be changed by a process where a brazing material is used as adhesion-favoring layer 36; all dies will be preferably made of graphite, molybdenum or boron nitride; the exact geometric relationships between all dies will be preferably ensured with the pinning method; and any step of filling with powders a recess or hole in a die may be assisted by vibrating such die.

Claims

1. A process for the production of assemblies (40; 60; 80; 100) comprismg a metal piece (12; 12') and a device (41; 61; 81; 101) integrating the functions of cathode and getter for miniaturized X-ray emitters (10), which comprises the steps of: a) preparing a first die (30) of an inert material, with at least one recess (31) in a fiat upper surface (32) thereof, said recess having an axis perpendicular to said upper surface; b) scrape filling said recess with at least one mixture (33; 33') comprising powders of at least one sintering material and at least one material selected between a getter material and diamond; c) placing over said first die a second die (34) of an inert material having at least one through hole (35), so that said through hole is coaxial with said recess in the first die; d) introducing into said through hole a metal piece (12; 12'), having previously provided for a layer (36) of a material (37; 51) favormg, in a subsequent thermal treatment, the adhesion between said at least one mixture of powders and said metal piece; e) introducing the assembly formed of said dies and of powders and pieces therein contained into an oven and subjecting it to a thermal treatment for sintering said at least one mixture of powders (33; 33'); f) at the end of the sintering thermal treatment, extracting from the dies the resulting assembly formed of the device integrating cathode and getter functions and the metal piece.

2. A process according to claim 1, wherein said layer (36) of material favoring adhesion between said at least one mixture of powders and said metal piece is selected between:

- a film of a material (37) coated onto a surface (18) or a portion thereof (19) of said metal piece; or

- a layer of powder of a brazing material (51).

3. A process according to claim 2, wherein the material favormg adhesion is a brazing material, and: - after step b, a step g is carried out, by which a depression (50) with respect to said upper surface of the first die is formed by slightly pressing said mixture of powders (33; 33') into said recess, by vibrating said first die, or by first vibrating said first die and than pressing the mixture;

- after step g, a step h is carried out wherein said depression is scrape filled with powder of the brazing material (51);

- after step h, steps c-f are carried out.

4. A process accord ig to claim 2, wherein said layer of material favoring adliesion is a film of a material (37) coated onto a surface (18) or a portion thereof (19) of said metal piece, and:

- between steps b and c, a step i is carried out, by which a depression (70) with respect to said upper surface of the first die is formed by slightly pressing said mixture of powders (33; 33') into said recess, by vibrating said first die, or by first vibrating said die and than pressing the mixture;

- in step d, a metal piece (12') having a central part (71) of one surface (18) raised with respect to the rest of said surface, said part having a section corresponding to that of said recess, is inserted into said hole (35) of said second die (34), with said raised part (71) extending into said depression (70).

5. A process accordhig to claim 3, wherein:

- between steps li and c,. a step i is carried out, by which a depression (70) with respect to said upper surface of the first die is formed by slightly pressing said brazing material (51) into said recess, by vibrating said first die, or by first vibrating said first die and than pressing the brazing material;

- i step d, a metal piece (12') having a central part (71) of one surface (18) raised with respect to the rest of said surface, said part having a section corresponding to that of said recess, is inserted into said hole (35) of said second die (34), with said raised part (71) extending into said depression (70).

6. A process accord g to any of the previous claims, wherein step a is composed in its turn of the following steps: j) preparing a third die (90) of an inert material provided with at least one recess (91) h a flat upper surface (92), said recess having an axis perpendicular to said upper surface and a depth which is only a fraction of the length of the getter cathode to be produced; k) scrape filling said recess (91) with a mixture (93) comprismg at least powder of diamond and powder of an easily sinter able material; 1) superimposing to said third die (90) a fourth die (94) of an inert material having a flat upper surface (96) and which is provided with a second through hole (95) whose axis is perpendicular to said surface having the same diameter of the upper portion of said recess (91); m) scrape filling said second through hole (95) with powder of a mixture comprising a getter material and a sintering material (33).

7. A process according to any of the previous claims, wherein the getter material is an alloy comprising zirconium or titanium with one or more elements selected among transition elements and aluminum.

8. A process according to claim 7, wherein said alloy is selected among the Zr-Al alloys, the Zr-V-Fe alloys, and the Ti-V-Mn alloys.

9. A process accordmg to claim 8, wherein said alloy has weight percent composition Zr 84% - Al 16%.

10. A process according to claim 8, wherein said alloy has weight percent composition Zr 70% - V 24.6% - Fe 5.4%.

11. A process according to claims 1 to 6, wherein the sintering material is selected among zirconium, titanium, hydrides thereof, or a mixture thereof.

12. A process accordmg to claims 1 to 6, wherein said mixture (33; 33') comprises, by weight, 70% of Ti and 30% of the alloy of weight percent composition Zr 84%

- Al 16%.

13. A process according to claims 1 to 6, wherein said mixture (33; 33') comprises, by weight, 70% of Ti and 30% of the alloy of weight percent composition Zr 10%

- V 24.6% - Fe 5.4%.

14. A process according to claims 1 to 6, wherem said mixture (33; 33') comprises, by weight, 40% of Zr and 60% of the alloy of weight percent composition Zr 70%

- V 24.6% - Fe 5.4%.

15. A process accordmg to claims 1 to 6, wherein said mixture (33; 33') comprises, by weight, 60% of Ti and 40% of the alloy of weight percent composition Zr 70% - V 24.6% - Fe 5.4%.

16. A process accordhig to any of claims 12 to 15, wherein the powders of said mixture (33; 33') have particle size lower than 50 μm.

17. A process according to claim 2, wherein said layer (36) of material favoring adhesion between said at least one mixture of powders and said metal piece is a film of a material (37) selected among the metals Ti, Ni, Cu, Co; the binary alloys Ni-Cr, Co-Cr or Fe-Cr; the ternary alloys Ni-Cr-Mϊ or Co-Cr-M}; and the quaternary alloys Ni-Cr-Mι-M₂ or Co-Cr-MrM₂, wherein Mi and M₂ are metals of the first transition row, or of III or IV group of the periodic table of elements.

18. A process according to claim 17, wherein said film is obtained by a technique selected among Chemical Vapor Deposition, electroplating, or Physical Vapor Deposition.

19. A process according to claim 2, wherein said layer (36) of material favoring adhesion between said at least one mixture of powders and said metal piece is a layer of powder of a brazing material (51).

20. A process accordmg to claim 19, wherem said brazing material has a melting temperature not lower than the temperature of the sintering treatment of said mixture (33; 33').

21. A process accordhig to claim 20, wherein said brazing material has a melting temperature not more than 15 °C higher than the temperature of the sintering treatment of said mixture (33; 33').

22. A process according to claim 19, wherein said brazing material (51) is selected among silver, silver-copper alloys, and silver-copper-gold alloys.

23. A process according to claim 22, wherein said brazing material is a silver-copper alloy having a silver content higher than about 60% by weight.

24. A process according to claim 23, wherein said brazing material is the silver- copper alloy containing 72% by weight of silver.

25. A process according to claim 19, wherein said brazing material (51) has a particle size lower than about 100 μm.

26. A process according to any of claims 1 to 5, wherein at the end of the process a thin diamond film is deposited on the tip (42) of the device integrating the fimction of getter and cathode, by means of a technique selected between Chemical Vapor Deposition and laser ablation.

27. A process accordmg to claim 6, wherem said diamond powder has a particle size between about 1 and 50 μm.

28. A process according to claim 27, wherein said diamond powder has a particle size of about 10 μm.

29. A process accord g to claim 6, wherein said easily sinterable material is titanium hydride.

30. A process accordmg to claim 29, wherein said titanium hydride has a particle size lower than 50 μm.

31. A process accordmg to claim 30, wherem said titanium hydride has a particle size comprised between about 5 and 10 μm.

32. A process according to any of claims 1 to 6, wherein said inert material of the dies is selected among graphite, molybdenum or ceramic materials.

33. A process according to claim 32 wherein such ceramic material is boron nitride.

34. A process accordmg to claim 1, wherein said sintering thermal treatment is carried out at a temperature between about 750 and 1100 °C and at a pressure lower than 10^"6 mbar.

35. A process accordmg to any of claims 1 to 6, wherem the exact reciprocal positioning of any assembly of dies is obtained by providing these with at least two thin through holes in corresponding positions in the different dies, and inserting into said holes phis whose height is at least equal to the sum of the height of all the superimposed dies.

36. An assembly comprising a metal piece (12; 12') having the function of base for a device (41; 61; 81; 101) having the double function of cathode and getter for a miniaturized X-ray emitter (10), said device being formed of at least one sintered mixture (33; 33'; 92) comprising powders of at least one sintering material and at least one material selected between a getter material and diamond, and said device being joined to a surface (18) or a portion thereof (19) of said metal piece by means of a layer (36) of a material selected among: - a film of a material (37) coated onto said surface or portion thereof of said metal piece; or

- a layer of powder of a brazing material (51).

37. An assembly (40; 60; 80) accordhig to claim 36, wherein said device (41; 61; 81) is formed of a sintered mixture (33) consisting of at least one getter material and at least one sintering material.

38. An assembly according to claim 37, wherein a thin diamond film is deposited on the tip (42) of said device.

39. An assembly (100) according to claim 36, wherein said device (101) comprises a main body (102) formed of a sintered mixture (33) comprising powders of at least one getter material and at least one sintering material, and a tip portion (103) formed of a sintered mixture (92) comprising powders of at least a sintering material and diamond.