Abstract:
A method of manufacturing a target for the generation of radiation of photons, protons or electrons by means of a laser, including: forming a support including first and second surfaces connected by openings, and forming in an enclosure a layer of material on the first surface by protecting the first surface with a protection element, injecting into the enclosure a gas of filling material, adjusting the pressure in the enclosure and the temperature of the support to form plugs of material in the openings of the support, and maintaining the temperature of the support and the pressure in the enclosure at values to maintain the plugs, followed by withdrawing the protection clement from the first surface, and forming a layer of metallic material on the first surface of the support and on the plugs. The pressure and support temperature are then modified to remove the plugs.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to the emission of X rays, or of particles, particularly protons and electrons, and more particularly to the manufacturing of targets used in such an emission. 
       BACKGROUND 
       [0002]    A known principle of X-ray generation comprises focusing a laser beam on a solid metal target, preferably a metal having a high atomic number, for example, gold, copper, or any other dense metal. The interaction of the laser beam and of the metal thus generates a plasma and a mono-energetic X-ray emission, which are particularly used in mammography or angiography. The target is generally placed in a vacuum enclosure, where the laser/matter interaction takes place, and is mounted on a mobile support, for example, a rotating support, so that each laser firing impacts a portion of the target which has not been impacted by a previous tiring. 
         [0003]    When the metal target has a large thickness, particularly greater than 500 nanometer, the X rays are emitted in the half-space where the laser beam propagates. It is then spoken of a “retro-emission”. However, when the metal target has a small thickness, particularly a thickness smaller than 500 nanometers, the rays are emitted in the direction of the laser beam and are accordingly emitted by the surface of the metal target opposite to that impacted by the laser beam. It is then spoken of a “transmission” emission. 
         [0004]    Usually, thin metal targets are obtained by depositing a metal layer on a copper or plastic support, after which the substrate is removed by means of a chemical bath and/or of a plasma etching. The layer thus exposed is then washed, for example, in water or alcohol, to remove a maximum amount of impurities, after which the target is mounted on the mobile support, and the support is mounted in the enclosure. 
         [0005]    Such a manufacturing is long and the substrate removal step is besides very delicate. It is further necessary to introduce the target and its support into the enclosure, which assumes breaking the vacuum existing therein, and then reforming the vacuum, which also takes a long time. In practice, the time of use of an X-ray emission system is very limited, and its down time is long. 
         [0006]    Similar problems are posed for the generation of particles, for example, electrons and protons, by means of a laser firing on a thin target. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0007]    The present invention aims at providing a method of manufacturing a target for the generation of photon or particle radiation, particularly the generation of X rays, electrons, or protons, which does not require removing a physical support of the metal target, and which can be directly used in a vacuum enclosure, and particularly the vacuum enclosure used for the radiation or particle emission. 
         [0008]    For this purpose, the invention aims at a method of manufacturing a target for the generation of photon or particle radiations by means of a laser, comprising:
       forming a support comprising a first and a second surfaces and crossed by openings; and   forming in a tight enclosure a layer of material on the first surface of the support.       
 
         [0011]    According to the invention, the forming of the layer of material on the support comprises:
       protecting the first surface of the support with a protection element;   injecting into the enclosure a gas of filling material;   adjusting the pressure in the enclosure and the temperature of the support to form solid plugs of filling material in the openings of the support;   stopping the injection of the gas of filling material into the enclosure;   maintaining the temperature of the support and the pressure in the enclosure at values maintaining the plugs solid in the openings of the support, and jointly to said maintaining:
           withdrawing the protection element from the first surface of the support to expose said surface; and   forming the layer of material on he first surface of the support and on the solid plugs filling the openings;   
           modifying the conditions of pressure in the enclosure and of support temperature to clear the openings of the support of the solid plugs.       
 
         [0020]    In other words, a solid support is formed by filling preexisting openings of the support, the layer, for example, metallic, forming the target is formed on the solid support thus created, after which the material for filling the openings is removed. The portions of the metal layer covering the openings then form the areas of the target which are subsequently impacted by the laser beam to produce rays (X) or particles (protons or electrons). The support is thus not removed and may for example directly be the mobile support used to expose to the laser beam areas of the target which are free or any impact. Further, the filling material in solid form may be easily removed once the layer has been deposited, for example, by modifying the pressure in the enclosure and/or the support temperature to sublimate the filling material. 
         [0021]    Further, the filling material may be introduced in gaseous form into the vacuum enclosure used for the emission of rays or particles, and the layer of material may be directly formed in this enclosure by means, for example, of a physical vapor deposition. 
         [0022]    According to an embodiment, the first surface of the support is planar, and the protection thereof comprises placing the first surface of the support against a planar solid surface. 
         [0023]    According to an embodiment, the placing of the support in contact with the gas comprises placing the support in a tight enclosure have a predetermined low pressure and cooling the support to a temperature lower than the temperature of the triple point of the material forming the gas. 
         [0024]    According to an embodiment, the layer of material is deposited on the first surface of the support by means of a physical vapor deposition. The vapor phase of the metal is for example produced by the ohmic heating of a support having a solid mass of the metal resting thereon or by the electronic bombarding of a target made of the metal. 
         [0025]    Particularly, the tight enclosure comprises means capable of injecting the gas into the enclosure, means capable of adjusting the internal pressure thereof, means capable of adjusting the temperature of the support to temperatures lower than the triple point of the material forming the gas, and means capable of vaporizing a solid metal element placed in contact therewith. Further, the placing into contact of the support with the gas, the forming of the plugs filling the support openings, and the physical vapor deposition are performed in the enclosure by maintaining a low pressure, particularly lower than 10 −3  mbar. 
         [0026]    According to an embodiment, the withdrawal of the protection element from the first surface of the support comprises heating said support to separate the element from the solid plugs filling the openings of the support and drawing the protection element away from the first surface of the support. 
         [0027]    According to an embodiment, the clearing of the support openings comprises adjusting the support temperature and/or the pressure in the enclosure. Particularly, the support is re-heated above the sublimation point of the filling material and the enclosure is maintained at a pressure lower than the saturation pressure corresponding to the support temperature. 
         [0028]    According to an embodiment, the filling material is argon, nitrogen, krypton, or xenon. 
         [0029]    According to an embodiment, the layer of material is metallic. More specifically, the thickness of the layer of metallic material is smaller than or equal to 500 nanometers, and preferably smaller than or equal to 50 nanometers. 
         [0030]    As a variation, a layer of material comprises a metal layer and a dihydrogen or deuterium layer. More specifically, the metal layer has a thickness in the range from 20 nanometers to 100 nanometers, and the dihydrogen or deuterium layer has a thickness in the range from 20 nanometers to 100 nanometers. 
         [0031]    According to an embodiment, the openings of the support are truncated cones widening from the first surface of the support to the second surface of the support. 
         [0032]    According to an embodiment, openings of the support are arranged in a circle and angularly spaced apart in regular fashion. 
         [0033]    The invention also aims at a system for implementing a method of manufacturing a target for the generation of radiation, particularly X rays or particles, particularly of protons or electrons by means of a laser, of the previously-mentioned type, comprising:
       a tight enclosure;   a support and means for positioning the support in the enclosure;   a protection element and means for positioning the protection element in the enclosure between a withdrawal position and a position where the protection element is placed against the surface of the support;   means for pumping the internal volume of the enclosure;   means for controlling the support temperature;   means for injecting gas into the enclosure;   means for heating the protection element; and   means for vaporizing a metallic element in the enclosure.       
 
         [0042]    According to an embodiment, the means for positioning the support in the enclosure comprise means for rotating said support. 
         [0043]    According to an embodiment, the enclosure comprises a window transparent to the laser beam arranged in front of the support surface. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0044]    The present invention will be better understood on reading of the following description provided as an example only in relation with the accompanying drawings, where the same reference numerals designate the same or similar elements, among which: 
           [0045]      FIG. 1  is a simplified view of a system according to the invention for producing a target for X rays and for producing X rays; 
           [0046]      FIGS. 2 and 3  respectively are simplified top and cross-section views of a support for a target according to the invention; 
           [0047]      FIGS. 4 to 9  are simplified views illustrating different phases of the operation of the system of  FIG. 1  to implement a method according to the invention; and 
           [0048]      FIG. 10  shows saturation curves of different gases of filling material for temperatures lower than the triple point of these gases. 
       
    
    
     DETAILED DESCRIPTION 
       [0049]    In relation with  FIG. 1 , a system  10  according to the invention enabling to form a metal target for the generation of X rays and to generate X rays from a single vacuum enclosure will now be described. 
         [0050]    System  10  particularly comprises:
       a tight enclosure  12  comprising a first wall portion forming a first window  14  transparent to a laser beam, and a second wall portion forming a second window  16  transparent to X rays and arranged in front of first window  14 . Tight enclosure  12  further comprises a heat shield enabling to limit heat inputs by radiation from the surrounding environment and from the elements internal to the enclosure having their temperature maintained high, for example, close to 300 K, for their operation.   a pumping circuit  18  of enclosure  12 , capable of creating therein a low pressure, particularly lower than 10 −3  mbar, and more particularly a pressure lower than or equal to 10 −4  mbar;   an injection circuit  20  capable of injecting gas into enclosure  12 ;   a circuit  22  for controlling the temperature of enclosure  12  capable of establishing in enclosure  12  a low temperature, particularly a temperature lower than 100 K. Advantageously, circuit  22  also comprises a cold source, preferably having a temperature lower than the temperature of the triple point of a filling material, capable of discharging the heat flows generated in enclosure  12 , particularly the flows generated by a process of deposition of a metal layer formed in enclosure  12 , as will be described in further detail hereafter; and   a laser  24  capable of performing laser firings through first window  14  along a beam axis  26 .       
 
         [0056]    System  10  also comprises, in enclosure  12 :
       a planar support  28  thoroughly crossed by openings  30 ;   a first circuit for displacing support  32  capable of displacing said support to position each of openings  30  on laser beam axis  26 , as will be explained in further detail hereafter;   a solid plate  34 , for example of same dimensions as support  28 ;   a second displacement and heating circuit  36 , capable of displacing plate  34  between a storage position, where plate  34  does not disturb the subsequent deposition of a metal layer on support  28 , and a protection position, where plate  34  is placed against support  28  to close openings  30  and prevent the condensation on the surface of support  28  having plate  34  placed against it. Circuit  36  is further capable of heating plate  34 . Circuit  36  is for example capable of delivering a heat pulse to plate  34  and/or plate  34  is metallic and circuit  36  is capable of injecting a current therein to cause a heating by Joule effect;   a support  38  capable of receiving a metal layer to be evaporated; and   a circuit  40  for heating support  38  to take the support to a temperature greater than the evaporation temperature of the metal deposited on support  38 .       
 
         [0063]    Temperature control circuit  22  also comprises a circuit for controlling the temperature of support  28 , and particularly means for cooling support  28 , for example, ducts in contact therewith, and/or inside thereof, and having gaseous helium flowing therethrough, and/or a cryorefrigerator comprising a connection by metal braids or by thermosyphon, and means for heating support  28 , for example a regulated electric heating. 
         [0064]    Finally, system  10  comprises a unit  42  controlling the operation of the elements which have just been described according to a plurality of phases described hereafter. 
         [0065]    Referring to  FIGS. 2 and 3 , support  28  for example takes the shape of a disk where openings  30  are positioned in a circle and angularly spaced apart in regular fashion. Advantageously, openings  30  have millimetric or submillimetric dimensions and have a truncated cone cross-section which widens from surface  44  of support  28  facing window  14  of enclosure  12  to surface  46  facing window  16  of enclosure  12 , to conform to the angle of divergence of the X-ray beam produced on impact of laser firings. 
         [0066]    Advantageously, support  28  is made of silicon or germanium, such materials being very good heat conductors, which enables to easily control the support temperature. Further, a silicon or germanium support may be significantly polished. More specifically, surface  44  of support  28  is polished to have very few roughnesses, and particularly to have a so-called “optical quality” polish, which provides a subsequent metal deposition on surface  44  of substantially constant thickness. 
         [0067]    Circuit  32  for displacing support  28  for example comprises a step-by-step electric motor to rotate support  28  around an axis  48  perpendicular to the main plane of support  28 , and running through the center of the circle having openings  30  positioned thereon. Circuit  32  thus enables to position each of openings  30  on axis  26  of the laser beam emitted by laser  24 . 
         [0068]    In relation with  FIGS. 4 to 9 , a method according to the invention of forming a metal target for the generation of X rays and generating the X rays implemented by system  10  will now be described. 
         [0069]    In a first step of the method, a metal layer  60  is positioned on support  38 , the metal of layer  60  being that forming the target for the subsequently performed X-ray generation ( FIG. 4 ). The metal of layer  60  is for example tantalum, tungsten, gold, or copper. Control unit  42  further controls displacement and heating circuit  36  to place plate  34  against support  28 . 
         [0070]    Consecutively or simultaneously, unit  42  controls pumping circuit  18  and temperature control circuit  22  to create and maintain a pressure and a temperature in enclosure  12  such that:
       support  28  and plate  34  have a temperature lower than the temperature of the triple point of a material for filling openings  30  subsequently introduced into enclosure  12  in gaseous form; and   the pressure in enclosure  12  is maintained at a pressure lower than the saturation vapor pressure of the filling material corresponding to the temperature of support  28  to avoid any parasitic condensation.       
 
         [0073]    Once the pressure and the temperature of enclosure  12  have been adjusted, unit  42  then controls injection circuit  20  so that the latter injects into enclosure  12  filling material  62  in gaseous form. Filling material  62  particularly is argon, nitrogen, krypton, xenon, or a mixture thereof. Due to the pressure and temperature conditions in enclosure  12  which are adjusted according to injected gas  62 , the latter condenses in solid form in openings  30 . 
         [0074]    Once openings  30  have been filled with solid material  62 , unit  42  controls the stopping of the gas injection into enclosure  12 . Advantageously, unit  42  controls pumping circuit  18  to lower the pressure in enclosure  12  and temperature control circuit  22  to lower the temperature of support  28  by a few Kelvin to guarantee the maintaining of the plugs in the solid state during a subsequent heat inflow from plate  34  and a metal deposition on support  28  and avoid the self-evaporation of the plugs. 
         [0075]    Unit  42  then controls displacement and heating circuit  36  so that it heats plate  34 . Plate  34  then separates from material  62  filling openings  30  of support  28  ( FIG. 5 ). Unit  42  then controls displacement and heating circuit  36  to position plate  34  in its storage posit ( FIG. 6 ). 
         [0076]    In a next step, unit  42  controls heating circuit  40  so that the temperature of support  38  is higher than the evaporation temperature of the metal of layer  60  laid on support  38 . A physical vapor deposition is then implemented. More specifically, vaporized metal  64  deposits on surface  44  of support  28  and on plugs  62  of solid material filling openings  30  ( FIG. 7 ), thus forming a metal layer  66 . 
         [0077]    Once the thickness desired for metal layer  66  has been obtained, particularly a thickness smaller than or equal than 500 nanometers, and preferably a thickness smaller than or equal to 50 nanometers, to implement an X-ray emission by transmission, unit  42  controls the stopping of the heating of support  38  and accordingly the stopping of the metal deposition on support  28 . Unit  42  then controls pumping circuit  18  and temperature control circuit  22  to remove material  62  tilling openings  30  of support  28 , preferably by sublimation. More specifically, unit  42  controls circuit  22  to adjust the temperature of support  28  to a temperature greater than the saturation vapor temperature of the filling material corresponding to the pressure in enclosure  12  maintained constant. Thus, the temperature of the solid plugs crosses the saturation vapor pressure curve of the filling material as illustrated in  FIG. 10 , so that the solid plugs sublimate. Sublimated material  68  is then pumped by pumping circuit  18  and discharged from enclosure  12  ( FIG. 8 ). 
         [0078]    Finally, once openings  30  have been cleared of tilling material  62 , unit  42  controls laser  24  and displacement circuit  32  to cause laser firings  70  through window  14  on the portions of metal layer  66  arranged on openings  30  of support  28 , particularly on portions which have received no impact. For each laser firing, a beam of monochromatic X-rays  72  is then generated along axis  26  of the laser beam, said beam being emitted towards the outside of enclosure  12  through window  16 . 
         [0079]    For example, for a filling material formed of argon, the temperature of support  28  and of plate  34  is selected from range [30 K, 40 K] and the pressure of the enclosure is substantially selected to be equal to 10 −4  mbar. 
         [0080]    Advantageously, the selection of the filling material and the selection of the adjustments of the pressure in the enclosure and of the support temperature are performed as follows. First, the filling material is selected according to support  28  to obtain a homogeneous metal deposition. Particularly, if the support comprises a crystal lattice, the filling material is selected so that its solid phase is also crystalline so that the metal target deposition is performed on a homogeneous surface. 
         [0081]    Once the filling material has been selected, the support temperature is selected for a pressure in the enclosure lower than 10 −3  mbar, for example, 25 K for nitrogen, 35 K for argon, 52 K for xenon, or 75 K for krypton, and the gas of filling material is introduced into the enclosure. The pressure in the enclosure is thus increased, particularly to be in the range from 10 −3  mbar to 5.10 −3  mbar to solidify the gas and till the support openings. Once the openings have been filled, the gas injection is stopped and the pressure is lowered back to a value lower than 10 −3  mbar. 
         [0082]    During the metal deposition of the target on the support, the temperature is lowered by a few Kelvin and the pressure in the enclosure is lowered by some 10 −4  mbar to limit phenomena of evaporation by heat inflow and self-evaporation. When the deposition of the target on the support is finished, the support temperature is then raised by a few Kelvin to cause the sublimation of the filling material present in the openings. 
         [0083]    In a first variation, when target  66  is worn out, enclosure  12  is opened and support  28  is removed along with worn-out target  66 . A new support  28  is then introduced into enclosure  12  for a new target manufacturing and X-ray production cycle, as described previously. 
         [0084]    In a second variation, circuit  22  for controlling the temperature of support  28  is capable of raising the temperature thereof to cause the total evaporation of worn-out target  66 . The evaporated metal is then pumped by pumping circuit  18  and a new target manufacturing cycle can then be implemented without having to break the vacuum in enclosure  12 . 
         [0085]    A specific embodiment where the target support comprises openings according to a specific arrangement has been described. Of course, the invention applies to any type of support and of opening arrangement. 
         [0086]    A plate which is placed against the support during the filling of the openings to, particularly, protect the surface of the support having the condensation target subsequently deposited thereon, has been described. Other protection elements may however be envisaged. Particularly, a second mobile protection element, for example, a second solid plate, is provided to be placed against the surface of support  28  opposite to the surface against which protection element  34  is capable of being placed. The second protection element is placed against support  28  on deposition of the metal layer to protect the rear surface from a parasitic deposition on the plugs filling openings  30 . The second protection element is advantageously connected to a displacement and heating circuit for displacing and heating said circuit. 
         [0087]    Similarly, an embodiment where the forming of the solid plugs in the openings of the target support, the withdrawal of the protection plate, and the deposition of the target on the support are carried out at a constant pressure and temperature has been described. During these phases, the pressure and the temperature may vary within a whole range enabling to keep the plugs tilling the support openings in their solid form. Similarly, different temperatures and pressures may be selected during the X-ray generation. 
         [0088]    The deposition of a single metallic material on support  28  has been described. As a variation, a plurality of metal layers are successively deposited on support  28 . For this purpose, the system according to the invention for example comprises several supports  38  having their temperature controlled independently from one another by a heating circuit  40  provided for and connected to each support  38  or by a single heating circuit  40  connected to each support  38 . Different metallic materials may thus be deposited on supports  38 , and unit  42  controls heating circuit  40  to deposit successive metallic layers on support  28  in a predetermined order. Once the deposition of the different metal layers has been performed, the filling material present in openings  30  of support  28  is then removed, as described previously, and the laser firings can take place. 
         [0089]    As a variation or additionally, once the metal layer has been deposited, for example, first layer, the second protection element is placed against support  28 , for example, after the plugs filling openings  30  of support  28  have been removed, before the deposition of the next metal layer(s), which particularly avoids the deposition of metal in openings  30 . Thereby, the deposition conditions are less restrictive since it is no longer necessary to adjust the pressure and the temperature in the enclosure to keep the plugs solid. 
         [0090]    For example, the next metal layer(s) may be deposited by condensation in solid form of a gas injected into the enclosure via injection circuit  20 , the pressure and the temperature of the enclosure being controlled to obtain such a condensation. 
         [0091]    The forming of a metal target for the generation of X rays has been described. The invention also applies to the manufacturing of non-metal or partially metallic ultra-thin targets, such as for example targets used in particle accelerators. For example, the targets comprise a solid deuterium or dihydrogen layer in addition to or instead of a metal layer. Laser firings on such a target thus generate protons and/or electrons. 
         [0092]    Particularly, a metal layer, having a thickness between 20 nanometers and 100 nanometers, is deposited on support  28 , after which a second solid deuterium or dihydrogen layer, having a thickness between 20 nanometers and 100 nanometers, is deposited on the metal layer. The deposition of multiple materials on support  28  is for example performed by depositing the dihydrogen or deuterium layer on the metal layer once the plugs filling openings  30  have been removed and the second protection element has been placed against support  28 .