EP0425718B1 - X-ray generator - Google Patents

Description

The invention relates to an X-ray generator according to the preamble of patent claim 1.

US-A-2,665,390 describes an X-ray tube in which the electrons emitted by a hot cathode are focused on a metal that is liquid during operation with the aid of electrostatic lenses. In order to prevent evaporation of the metal serving as the anode, it is circulated in a closed circuit. In the area of the electron focus, the metal flows in a preferably tubular container, the wall of which is made of beryllium.

An X-ray tube with an anode consisting of a circulating metallic liquid, for example mercury, is known from DE-C-890 246. The mercury is located in a pot-like anode body and is set in rotation by means of a rotating field generated by a stator. During operation, the circulating mercury forms a paraboloid of revolution, on the surface of which the electrons emitted by a hot cathode are focused.

The X-ray tube known from US-A-3,646,380 has a rotating anode, the axis of rotation of which includes an angle of 90 ° with the electron beam direction. In order to shield the rotating anode from the high voltage applied to the hot cathode, the X-ray tube is equipped with a perforated diaphragm located directly above the rotating anode.

X-ray tubes for medical diagnostics are described in US-A-4,357,555, EP-A-0 136 762 and in Philips Tech. Rev. 41, 1983/84 No. 4, pages 126 to 134.

X-ray tubes for fine structure examinations are known from J. Urlaub, X-ray analysis vol. 1, X-rays and detectors (Siemens, Karlsruhe 1974) pages 71 to 75.

• Verringerung der Größe des Elektronenfokus auf der Anode (Erhöhung der Leistungsdichte des Elektronenstrahls)
• Verwendung einer Drehanode
  (Verteilung der thermischen Belastung auf die Mantelfläche einer schnell rotierenden Anode)
• Verringerung der effektiven Röntgenemissionsfläche durch flachen Strahlabgriff
  (s. beispielsweise J. Urlaub, Röntgenanalyse Bd. 1, S. 96 bis 98).

To perform highly sensitive X-ray analysis methods (total reflection X-ray fluorescence analysis, reflectometry, interferometry, diffractometry, etc.), X-ray sources with a high spectral brilliance are required. Since synchrotrons as high-intensity X-ray sources are not yet available as laboratory sources, attempts are being made to increase the brilliance of conventional X-ray tubes by using the following techniques:

• Reduction in the size of the electron focus on the anode (increase in the power density of the electron beam)
• Use of a rotating anode
  (Distribution of the thermal load on the outer surface of a rapidly rotating anode)
• Reduction of the effective x-ray emission area through flat beam tapping
  (See, for example, J. Urlaub, Röntgenanalyse Vol. 1, pp. 96 to 98).

With immobile anodes as well as with rotating anodes, the increase in brilliance that can be achieved with these techniques has already been exhausted up to the material limit values. The use of rotating anodes also presents considerable technical difficulties, since the rotary unions required to drive the anode and to replace the coolant still have to reliably seal the coolant circuit and the evacuated tube housing even at speeds of up to 6000 rpm. Despite complex constructions, leaks always lead to failures. In addition, the electron beam causes a strong local heating of the Anode, which is subject to extreme mechanical stresses and therefore ages very quickly. Cracks form with increasing operating time. This causes a reduction in the brilliance due to the stronger self-absorption. Cracks can also get coolant inside the tube. The strong local heating of the anode can also cause the anode material to evaporate and lead to flashovers in the case of the high electric field strengths.

The aim of the invention is to provide an X-ray generator of simple construction and having a high level of brilliance. This object is achieved by an X-ray generator according to claim 1.

The advantage that can be achieved with the invention is, on the one hand, that the brilliance of an X-ray tube can generally be increased, since liquid anodes tolerate a higher electron beam power density (no crack formation, better heat dissipation by mixing). On the other hand, the brilliance can also be increased selectively in terms of energy or wavelength by means of X-ray optical effects with a flat beam tap. The prerequisite for this, a smooth anode surface, is ideally met by liquid anodes.

While dependent claims 2 to 11 relate to configurations of the x-ray generator according to claim 1, claims 12 to 14 are directed to a method for operating an x-ray generator.

Fig. 1

ein erstes Ausführungsbeispiel eines erfindungsgemäßen Röntgenstrahlerzeugers,

Fig. 2

die relative Brillanz eines Röntgenstrahlerzeugers in Abhängigkeit vom Abgriffswinkel α₂ und der Photonenenergie E_ν ,

Fig. 3

die geometrischen Verhältnisse beim Austritt der Röntgenstrahlung in das Röhrenvakuum,

Fig. 4 bis 7

weitere Ausführungsbeispiele erfindungsgemäßer Röntgenstrahlerzeuger.

The invention is explained below with reference to the drawings. Here shows:

Fig. 1

a first embodiment of an X-ray generator according to the invention,

Fig. 2

the relative brilliance of an X-ray generator as a function of the tap angle α₂ and the photon energy E _ν ,

Fig. 3

the geometrical relationships when the X-rays emit into the tube vacuum,

4 to 7

further exemplary embodiments of the X-ray generator according to the invention.

The X-ray generator shown schematically in FIG. 1 essentially consists of a housing formed by the metal wall 1, the beam exit windows 2, the anode support 3/4 and the glass high-voltage bushing 5, a filament 7 arranged as a cathode in the high vacuum of the housing and connected to voltage supply lines 6 a Wehnelt electrode 8 for focusing the electrons emitted by the incandescent filament 7 onto the anode 9 which is liquid during operation. To cool the anode liquid 9 which completely wets the top of the anode support 3 (flow is prevented by the surface tension), water 16 or another coolant is added via the im Fastening flange 10 existing channel 11 brought up to the anode support base 4 and derived via the channel 12. The coolant circuit between the anode support base 4 and the mounting flange 10 is sealed by an O-ring 13. When the X-ray generator is operated at low power, there is no significant shrinkage fear of anode liquid 9 by evaporation, provided that it is sufficiently cooled. The evaporation rate rises considerably at high tube outputs, so that the loss of material can no longer be neglected. Strong cooling of the anode support 3/4 and simultaneous heating of the remaining tube housing, in particular the anode liquid supply 18 with the aid of the heating conductor 46, can ensure, however, that the vaporized anode liquid condenses again on the top 3 of the anode support. There is a dynamic equilibrium between the evaporation rate and the condensation rate, since the cooling effect increases with decreasing thickness of the anode liquid 9. The heating and cooling capacity must be set so that the housing pressure does not exceed 10 (-9) bar during operation.

Metals with a low melting point FP and a high boiling point KP as well as low vapor pressure and high thermal conductivity are particularly suitable as anode materials, in particular gallium Ga, indium In, tin Sn and their alloys. The melting and boiling points FP and KP of the metals Ga, In and Sn are given in Table 1. In this case, the heating by the electron beam is generally so high that no additional heating devices are required to liquefy the anode material. Table 1 Ga: FP 29.5 ° C KP 2064 ° C In: FP 156.2 ° C KP 2050 ° C Sn: FP 231.8 ° C KP 2700 ° C

die die Anzahl N_ν der pro Zeitintervall dt, Raumwinkelelement dΩ₂ und Energieintervall dE_ν emittierten Photonen bezogen auf die effektive Größe dA₂ der Röntgenstrahlquelle angibt.Liquids have a low surface roughness and, if vibrations are avoided, also a low ripple. Since the average roughness of liquids (thermally excited capillary waves) at temperatures that are not too high T << T _{KP is} typically less than 1 nm, it is possible that of the Anode 9 emitted X-rays 14 at extremely flat angles α₂ <1 °. This is particularly important for increasing the spectral brilliance of the X-ray generator. The size is referred to as spectral brilliance B _E

${\text{B}}_{\text{E}}{\text{= d⁴N}}_{\text{ν}}{\text{/ (dt · dA₂ · dΩ₂ · dE}}_{\text{ν}}\text{) = Photons / (s · mm² · mrad² · eV) (1)}$

which indicates the number N _{ν of} the photons emitted per time interval dt, solid angle element dΩ₂ and energy interval dE _ν based on the effective size dA₂ of the X-ray source.

erfüllen. Der dem Grenzwinkel der Totalreflexion entsprechende, von der Photonenenergie E_ν sowie dem verwendeten Anodenmaterial abhängige Austrittsgrenzwinkel α_2C errechnet sich hierbei aus dem Dispersionsanteil des Brechungsindex

$\text{n = 1 - δ - i β (3)}$

gemäß der Formel

${\text{α}}_{\text{2C}}{\text{= (2δ(E}}_{\text{ν}}{\text{))}}^{\text{1/2}}\text{. (4)}$

Dabei hängt β über

${\text{µ = 4π E}}_{\text{ν}}\text{β /(hc) (5)}$

mit dem Absorptionskoeffizenten µ zusammen. Bei hohen Photonenenergien E_ν > E_AK (E_AK: Energie der K-Schalen-Absorbtionskante) ist δ näherungsweise durch

mit

r_o

: klassischer Elektronenradius

N_A

: Avogadro-Konstante

h

: Planck-Konstante

c

: Vakuumlichtgeschwindigkeit

e

: Elementarladung

ζ

: Dichte des Anodenmaterials

Z

: Kernladungszahl des Anodenmaterials

A_r

: relative Atommasse des Anodenmaterials

E_ν

: Photonenenergie

gegeben.To increase the spectral brilliance B _{E of} the X-ray generator, the one in Jap. Journ. of Appl. Phys. Vol. 24, No. 6, 1985, p. L 387 - L 390 described solid angle concentration of the X-ray radiation 14 generated in near-surface layers of the anode and emerging in a vacuum. Since this effect caused by refraction is only effective in the area of the exit limit angle α _2C , the tapping angle α₂ predetermined by the aperture 15 should relate to the relationship

${\text{α}}_{\text{2C}}{\text{<α₂ <3α}}_{\text{2C}}\text{(2)}$

fulfill. The exit angle α _2C , which corresponds to the critical angle of total reflection and is dependent on the photon energy E _ν and the anode material used, is calculated here from the dispersion component of the refractive index

$\text{n = 1 - δ - i β (3)}$

according to the formula

${\text{α}}_{\text{2C}}{\text{= (2δ (E}}_{\text{ν}}{\text{))}}^{\text{1/2}}\text{. (4)}$

Here β overhangs

${\text{µ = 4π E}}_{\text{ν}}\text{β / (hc) (5)}$

together with the absorption coefficient µ. At high photon energies E _ν > E _AK (E _AK : energy of the K-shell absorption edge) δ is approximately through

With

r _o

: classic electron radius

N _A

: Avogadro constant

H

: Planck constant

c

: Vacuum speed of light

e

: Elementary charge

ζ

: Density of the anode material

Z.

: Atomic number of the anode material

A _r

: relative atomic mass of the anode material

E _ν

: Photon energy

given.

Typical values for the exit limit angle α _2C are 0.5 °. Since in the present invention the X-ray optical properties of the anode surface in the area of extremely small tap angles (see Eq. (2)) are used to increase the brilliance, the flatness of the anode surface has to meet the highest requirements.

genügt (λ: Wellenläge λ = hc/E_ν). Diese Bedingung läßt sich aus den Arbeiten von B. Vidal und P. Vincent, Applied Optics, 23 No 11 (1984) S. 1794 - 1801 und S. K. Sinha, E.B. Sirota, S. Garoff und H.B. Stanley, Phys. Rev. B38 No 4 (1988) S. 2297 - 2311 herleiten. Für Ga-Kα-Strahlung aus einer flüssigen Ga-Anode bedeutet dies für ${\text{α₂ = 1,5α}}_{\text{2C}}\text{= 1,5 0,28°:σ ≾ 2nm}$

. Eine solche Anforderung ist mit Flüssiganoden zu erfüllen.In order to ensure a defined tap angle α _2C in this angular range and to prevent beam expansion, the ripple must not exceed 0.1α _2C . Too much ripple would cause the radiation on the differently inclined facets of the anode surface to refract into different ones Experiences exit directions. This would have a lubrication of the emerging X-ray intensity via the exit angle α₂ and thus a reduction in the increase in brilliance that can be achieved by flat beam tapping. In addition to the ripple that covers the long-wave, gently oscillating part of the anode unevenness, the roughness that describes the short-wave oscillations is also important. This roughness causes interference with both the transmitted and the reflected X-rays. This reduces the intensity of the radiation reflected back into the anode and increases the intensity of the transmitted radiation to the same extent. However, the transmitted intensity is increased in part in the form of diffuse radiation, which does nothing to increase the brilliance. Overall, the influence of the roughness on the transmitted intensity is slight due to the moderate reflectivity when the average roughness σ of the anode surface of the condition

$\text{σ ≾ λ / (4π √}\overline{\text{sinα₁ · sinα₂}}\text{) (7)}$

is sufficient (λ: wavelength λ = hc / E _ν ). This condition can be derived from the work of B. Vidal and P. Vincent, Applied Optics, 23 No 11 (1984) pp. 1794-1801 and SK Sinha, EB Sirota, S. Garoff and HB Stanley, Phys. Rev. B38 No 4 (1988) pp. 2297 - 2311. For Ga-Kα radiation from a liquid Ga anode, this means for ${\text{α₂ = 1.5α}}_{\text{2C}}\text{= 1.5 0.28 °: σ ≾ 2nm}$

. Such a requirement must be met with liquid anodes.

für eine konventionelle Cu-Anode in Abhängigkeit von der Photonenenergie E_ν und dem Abgriffswinkel α₂ für eine Elektronenenergie E_e = 30 keV), läßt sich beispielsweise die Brillanz der Cu-Kα-Linie um einen Faktor 3 steigern, wenn man die Strahlung, nicht wie bisher üblich, bei einem Winkel α₂ = 6°, sondern bei einem Winkel α₂ = 0,8° (Austrittsgrenzwinkel für Cu-K_α-Strahlung in Cu:α_2C = 0,4°) abgreift. Im Bereich der hochenergetischen Grenze des Bremsstrahlungskontinuums fällt der Gewinn an Brillanz für einen extrem flachen Abgriffswinkel von α₂ = 0,2° noch deutlich höher aus (Faktor 30 gegenüber dem Abgriff bei α₂ = 6°). Außerdem ist zu erkennen, daß man den Photonenfluß durch geeignete Wahl des Winkels α₂ spektralselektiv verstärken oder schwächen kann. Dies ist ein entscheidender Vorteil gegenüber konventionellen Röntgenröhren, in denen der Photonenfluß durch Verwendung der das Signal- Untergrundverhältnis verbessernden primär- oder sekundärseitigen Monochromatoren, Filter und Blenden winkel- oder spektralselektiv geschwächt, jedoch nie erhöht wird.The gain in brilliance B _E with a flat beam tap is based on a geometric effect (projective reduction of the emitting anode area) and an X-ray optical effect that makes the main contribution (solid angle concentration due to refraction at the anode-vacuum interface). As FIG. 2 shows (the relative brilliance is shown ${\text{B}}_{\text{E}}{\text{(E}}_{\text{ν}}{\text{, α₂) / B}}_{\text{E}}{\text{(E}}_{\text{ν}}\text{, α₂ = 90 °)}$

for a conventional Cu anode depending on the photon energy E _ν and the Tap angle α₂ for an electron energy E _e = 30 keV), for example, the brilliance of the Cu-Kα line can be increased by a factor of 3 if the radiation is not at an angle α₂ = 6 °, but as usual, but at one Angle α₂ = 0.8 ° (exit limit angle for Cu-K _α radiation in Cu: α _2C = 0.4 °). In the area of the high-energy limit of the brake radiation continuum, the gain in brilliance is even higher for an extremely flat tap angle of α₂ = 0.2 ° (factor 30 compared to the tap at α₂ = 6 °). In addition, it can be seen that you can spectrally selectively amplify or weaken the photon flux by suitable choice of the angle. This is a decisive advantage over conventional X-ray tubes, in which the photon flow is weakened by angle or spectrally selectively, but never increased, by using the primary or secondary monochromators, filters and diaphragms which improve the signal-background ratio.

• (I)
  
  einfallende Elektronenstromdichte,

  N_e:

  Anzahl der Elektronen,

  dt:

  Zeitintervall,

  A₁:

  Strahlquerschnitt des Elektronenstrahls.

  j_e sin γ Anzahl der Elektronen pro Zeiteinheit und pro Flächeneinheit der Anodenoberfläche A_o.
• (II)
  
  beschreibt die Verkleinerung der Quellfläche in der Projektion des Strahlabgriffs gemäß: A₂ = A_o sinα₂.
• (III)
  Φ(z, E_ν) Photonenproduktion als Funktion der Entstehungstiefe z und der Photonenenergie E_ν. Sie gibt die Anzahl der Photonen der Energie E_ν an, die pro einfallendem Elektron der Energie E_e in der Tiefe z pro.Tiefenintervall dz erzeugt werden. Als Parameter gehen noch die Kernladungszahl Z und die Dichte ζ der Anode sowie der Elektroneneinschlußwinkel γ ein (J.I. Goldstein, Scanning Electron Microscopy and X-Ray Microanalysis; Plenum Press, New York, 1981 S. 355 ff.)
• (IV)
  exp(-z|Im k_1z|) Schwächungsfaktor der Strahlungsflußdichte der austretenden Photonen innerhalb der Anode.
  
• (V)
  
  Transmissionsgrad der Photonen durch die Anodenoberfläche.

  n₁:

  Brechungsindex des Anodenmaterials,
  
  $\text{n₁ = 1 - δ - iβ ,}$

  

  
  

  n₂:

  Brechungsindex des Vakuums, n₂ = 1,

  T₁₂:

  Transmissionskoeffizient

  
  
  ${\text{|T₁₂|² = 0,5 |<figure-callout id="12" label="T" filenames="imgf0001.png,imgf0004.png" state="{{state}}">T</figure-callout>}}_{\text{12⊥}}{\text{|² + 0,5|<figure-callout id="12" label="T" filenames="imgf0001.png,imgf0004.png" state="{{state}}">T</figure-callout>}}_{\text{12∥}}\text{|² (10)}$
  
  
  
  ${\text{<figure-callout id="12" label="T" filenames="imgf0001.png,imgf0004.png" state="{{state}}">T</figure-callout>}}_{\text{12⊥}}\text{= 2n₁ sinα₁/(n₁ sinα₁ + n₂ sinα₂) (11)}$
  
  
  
  ${\text{<figure-callout id="12" label="T" filenames="imgf0001.png,imgf0004.png" state="{{state}}">T</figure-callout>}}_{\text{12∥}}\text{= 2n₁ sinα₁/(n₁ sinα₂ + n₂ sinα₁) (12)}$
  
  
  
• (VI)
  
  Raumwinkelkonzentration;
  Verhältnis des Raumwinkelelements im Anodenmaterial dΩ₁= dα₁dτ zum Raumwinkelelement im Vakuum $\text{dΩ₂ = dα₂dτ . dτ}$
  
  beschreibt die Ausdehnung des Strahlenbündels senkrecht zu dα₁ bzw. dα₂.

The spectral brilliance B _E (E _ν , α₂) for the X-rays emerging from an anode when excited with a monoenergetic electron beam results from the following relationship:

• (I)
  
  incident electron current density,

  N _e :

  Number of electrons,

  dt:

  Time interval,

  A₁:

  Beam cross section of the electron beam.

  j _e sin γ number of electrons per unit time and per unit area of the anode surface A _o .
• (II)
  
  describes the reduction of the source area in the projection of the beam tap according to: A₂ = A _o sinα₂.
• (III)
  Φ (z, E _ν ) photon production as a function of the depth of origin z and the photon energy E _ν . It specifies the number of photons of energy E _ν that are generated per incident electron of energy E _e at depth z per depth interval dz. The nuclear charge number Z and the density ζ of the anode and the electron inclusion angle γ are also taken into account as parameters (JI Goldstein, Scanning Electron Microscopy and X-Ray Microanalysis; Plenum Press, New York, 1981 pp. 355 ff.)
• (IV)
  exp (-z | Im k _1z |) attenuation _{factor of} the radiation flux density of the emerging photons within the anode.
  
• (V)
  
  Transmittance of the photons through the anode surface.

  n₁:

  Refractive index of the anode material,
  
  $\text{n₁ = 1 - δ - iβ,}$

  

  
  

  n₂:

  Refractive index of the vacuum, n₂ = 1,

  T₁₂:

  Transmission coefficient

  
  
  ${\text{| T₁₂ | ² = 0.5 | <figure-callout id="12" label="T" filenames="imgf0001.png,imgf0004.png" state="{{state}}">T</figure-callout>}}_{\text{12⊥}}{\text{| ² + 0.5 | <figure-callout id="12" label="T" filenames="imgf0001.png,imgf0004.png" state="{{state}}">T</figure-callout>}}_{\text{12∥}}\text{| ² (10)}$
  
  
  
  ${\text{<figure-callout id="12" label="T" filenames="imgf0001.png,imgf0004.png" state="{{state}}">T</figure-callout>}}_{\text{12⊥}}\text{= 2n₁ sinα₁ / (n₁ sinα₁ + n₂ sinα₂) (11)}$
  
  
  
  ${\text{<figure-callout id="12" label="T" filenames="imgf0001.png,imgf0004.png" state="{{state}}">T</figure-callout>}}_{\text{12∥}}\text{= 2n₁ sinα₁ / (n₁ sinα₂ + n₂ sinα₁) (12)}$
  
  
  
• (VI)
  
  Solid angle concentration;
  Ratio of the solid angle element in the anode material dΩ₁ = dα₁dτ to the solid angle element in a vacuum $\text{dΩ₂ = dα₂dτ. dτ}$
  
  describes the expansion of the beam perpendicular to dα₁ or dα₂.

3 shows the beam geometry and the associated variables.

FIG. 4 shows an X-ray generator in which the electrons pass through a funnel-shaped constriction 17 between the filament 7 and the anti-cathode 9, which is liquid during operation. This taper 17, which acts as a hollow anode, also has the task of coating the top side 3 of the carrier again after the tube has been transported with the anti-cathode liquid 18 which collects on the tube bottom. In order to achieve this, the tube is briefly turned over and straightened up again, so that the liquid 18 strikes the carrier top 3 arranged below the hollow anode and completely wets it. The use of a border arranged on the upper side 3 of the support and projecting in the direction of the cathode 7 is out of the question since this would hinder the desired flat steel tap.

The exemplary embodiment according to FIG. 5 shows an X-ray generator in which the electrons emitted by the cathode 7 and accelerated in the direction of the hollow anode 17 pass through a window 20 which closes the housing 19 in a vacuum-tight manner, in order to dispose in the anti-cathode liquid arranged outside the housing 19 on the water-cooled upper side 3 of the carrier 9 to generate braking radiation and characteristic X-ray radiation 14. Due to the flat beam tap, the height d of the spacer 21 screwed to the housing 19, the anti-cathode support 3 and the fastening flange 10 can be selected to be very small (d ≾ 1 mm), so that no appreciable electron absorption takes place in the atmosphere. The absorption in Electron exit window 20 also remains very small when using 0.5 μm thick quantum as window material (to be obtained from Kevex Cooperation, Foster City CA). Since a low vapor pressure does not have to be required for the materials which can be used as the anti-cathode, sodium and mercury can also be used as anti-cathode materials in addition to gallium, indium and tin. The advantage of the beam generator described here is in particular that the low-energy spectral components can also be used experimentally.

The embodiment of FIG. 6 shows an X-ray generator, the anode of which is formed by an electrically conductive liquid 9 with a low vapor pressure. To circulate this anode liquid 9, which is guided in an insulating body 22, a Faraday pump 23 is provided, the horseshoe magnet 24 of which generates a magnetic field oriented perpendicular to the desired flow direction 25. An electrical current flowing between the electrodes 26 perpendicular to the magnetic field and flow direction 25 ensures the Lorentz force accelerating the anode liquid 9. In the backflow area, the heated anode liquid 9 is cooled in a heat exchanger 27. The cooling water enters through the opening 28 in the heat exchanger 27 in order to flow off again at the outlet 29. The nozzle 30 (Laval nozzle) provided in the channel of the anode liquid 9 is used to adapt the magnetic circulation pressure to the gas pressure p <10 (-9) bar present in the housing, in order thereby to provide a smooth interface between the nozzle 30 and the point of impact 31 of the electron beam to ensure flowing anode liquid 9. As mentioned at the beginning, this is an essential prerequisite for tapping the X-rays at the critical angle of total reflection.

The arrangement consisting of the ceramic insulating body 22, the cathode 7 and the focusing unit 8 (Wehnelt electrode, focusing trough or Pierce electrode) is located in an evacuated housing (not shown), the vacuum-tight voltage and cooling water feedthroughs as well Window for the exit of the X-rays 14 tapped at an angle α₂.

In the exemplary embodiment shown, the liquid 9 heated by the electron beam is exchanged very quickly and supplied to the cooling unit 27. The comparatively low thermal conductivity of the anode materials used, gallium, indium and tin, does not have a disadvantageous effect, since the anode liquid 9 stores the heat and, as a result of the mixing in the backflow region, releases it very quickly. The electron beam thus constantly strikes the liquid flowing in with cooling, as a result of which the permissible power density of the electron beam can be significantly increased compared to a liquid anode which has not been circulated.

In the embodiment shown in FIG. 7, the anode liquid 9 is circulated with the aid of a rotating drum 32. An electric motor 36, which is rigidly connected to the evacuated housing 34/35 via the carrier 33, is used as the drive unit, a coupling 38 consisting of two opposite magnets 37 each transmitting the rotary movement of the outer cylinder 39 to the drum 32. According to the principle of the centrifugal pump, the rotating drum 32 with the paddle wheels 40 exerts a pressure on the anode liquid 9 flowing off at the open end faces, so that it starts to move in the pipeline 41. It flows through the heat exchanger 42 and the central tube 41 in order to exit again via the diffuser 43. Here the anode liquid 9 is gripped by the rotating drum 32 and pressed against the inner wall by the centrifugal force. It then flows off again via the paddle wheels 40, so that the pressure required for the recirculation builds up again. The electrons emitted by the cathode 7 are accelerated by a high voltage supplied via the connections 44 and focused on the anode liquid 9 with the aid of a Pierce or Wehnelt electrode 8. Here they generate brake radiation and characteristic X-rays 14, which can be picked up at a flat angle α₂ and through which Window 2 decouples. A vacuum pump (not shown in FIG. 7), in particular a turbomolecular pump, is used to evacuate the housing, which consists of two parts 34/35 and is rigidly connected to the motor mount by a screw connection, and which sucks off the residual gas via the connection piece 45. A vacuum seal, in particular a gold wire seal, is provided between the two housing parts 34 and 35.

The invention is of course not limited to the exemplary embodiments described. it is thus easily possible to replace the drum described above with a rotating disk, the anode liquid emerging from a hollow axis carrying the disk and wetting the disk surface.

Claims (14)

1. X-ray generator having a cathode (7) disposed in an evacuated housing (1 to 5, 19), an anticathode (9) comprising an electrically conductive fluid and a device (8) for focusing the particles emitted by the cathode (7) onto the anticathode (9), characterized by a shielding system (15) to define a tab angle α₂ for the X-radiation (14) generated at a surface of the anticathode (9) where α₂ satisfies the condition
   
   ${\text{α}}_{\text{2c}}{\text{< α₂ < 3α}}_{\text{2c}}$

   

   
   
   and α_2c designates the critical angle of total reflection of the X-radiation (14) at the surface of the anticathode (9).
2. X-ray generator according to Claim 1, characterized by an anticathode (9) comprising a metallic melt.
3. X-ray generator according to Claim 1 or 2, characterized in that the housing (19) exhibits a window (20) which is transmissive for the particles emitted by the cathode (7), and in that the anticathode (9) is disposed outside the housing (19).
4. X-ray generator according to one of Claims 1 to 3, characterized by an anode (17) disposed inside the housing (1 to 5, 19) between cathode (7) and anticathode (9).
5. X-ray generator according to Claim 4, characterized in that the anode (17) is designed as a hollow anode.
6. X-ray generator according to one of Claims 1 to 5, characterized by a device (23, 32, 36, 37) to circulate the electrically conductive fluid.
7. X-ray generator according to Claim 6, characterized by a Faraday pump as device to circulate the electrically conductive fluid.
8. X-ray generator according to Claim 6 or 7, characterized in that the electrically conductive fluid is guided in a closed circuit.
9. X-ray generator according to one of Claims 1 to 5, characterized in that the electrically conductive fluid is disposed on a cooled carrier device (3, 4).
10. X-ray generator according to one of Claims 1 to 5, characterized by a drum (32), which is rotatable about an axis and the internal wall of which is wetted with the electrically conductive fluid.
11. X-ray generator according to Claim 10, characterized in that a motor (36) and a magnetic coupling (38) are provided to rotate the drum (32) about the axis.
12. Method for operating an X-ray generator which exhibits a cathode (7) disposed in an evacuated housing (1 to 5, 19), an anticathode (9) comprising an electrically conductive fluid and a device (8) for focusing the particles emitted by the cathode (7) onto the anticathode (9), characterized in that the X-radiation (14) generated at a surface of the anticathode (9) is tapped at an angle α₂ satisfying the condition
    
    ${\text{α}}_{\text{2c}}{\text{< α₂ < 3α}}_{\text{2c}}$

    

    
    
    where α_2c designates the critical angle of the total reflection at the surface of the anticathode (9).
13. Method according to Claim 12, characterized in that the electrically conductive fluid is brought to a temperature which is lower than the temperature of the housing (1 to 5, 19).
14. Method according to Claim 12 or 13, characterized in that the electrically conductive fluid is circulated.

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