EP0815582B1 - Microfocus x-ray device - Google Patents

Abstract

PCT No. PCT/EP96/01145 Sec. 371 Date Jan. 8, 1998 Sec. 102(e) Date Jan. 8, 1998 PCT Filed Mar. 16, 1996 PCT Pub. No. WO96/29723 PCT Pub. Date Sep. 26, 1996In microfocus X-ray equipment for enlarging radiographic short-time recordings, a focussed electron beam for the production of X-radiation (16) impinges on the retarding material of a target (23). In this case, the retarding material in the focal spot (22) passes over into the liquid aggregate state due to the high thermal loading. For this reason, the equipment is operated in pulsed operation, wherein the position of the focal spot (22) on the target (23) is, when each loading occurs, displaced relative to the previous position. The retarding material is arranged in a retarding layer (32) on a carrier layer (33) and the electron beam (16) impinges on the retarding layer (32) oriented perpendicularly to the electron beam (16). A control interrupts the irradiation at the latest when the carrier layer (33) starts to melt.

Description

The invention relates to a device according to the Preamble of claim 1. Such a device is known from U.S. Patent 4,344,013 (Ledley).

The usability of so-called direct and magnification radiographic facilities, in particular in the fields of materials testing and medicine in the article "Development and Perspectives of medical enlargement radiography "by G.Reuther, H.-L. Kronholz and K.B. Hüttenbrink in RADIOLOGE Vol.31 (1991) 403-406, described in more detail. The function of such Facilities is based on the geometric geometry Lawfulness, according to which a radiation source only then high-contrast silhouettes of high spatial resolution leads if the effective radiation area is very small compared to the irradiated area of the subject Object. Otherwise, every point of the object at different angles, namely from different Producing the radiation source that would be irradiated each object point would result in the projection into the Shadows offset from each other in the image plane, and overall the result would be a washed out contour of the Object, according to its distance from the Image plane is shown enlarged.

Despite the achievable improvement in resolution have microfocus X-ray devices in practice, especially medical diagnostics, not really can enforce. Above all, that seems to be attributed to being limited X-ray power can work. Because the very narrow focus of the electron beam on the Brake target gives a very small focal spot (focus) Diameter with a correspondingly very high energy density. This large specific load quickly leads to the fact that usually in a direction of 10 ° to 45 ° irradiated target one - for converting the incident Electron beam energy to be emitted in X-ray energy - disadvantageous change of its topography with sooner First experienced brake layer failure. On the other hand, the exposure time per x-ray is extended, when working with lower power x-rays would what the requirement for short exposure times in the range of tenths to hundredths of a second contradict to an unnecessarily high radiation exposure and Avoid blurring due to object movement. Each however, the thermal focal spot on the target anode is smaller is, the lower the electrical Power absorbed by the small target area before it starts to melt. This behavior contradicts the demand for higher density of the target impacting electron beams for higher X-ray power output.

From the aforementioned US Pat. No. 4,344,013 (Ledley) known a microfocus X-ray device, which already with a melted target works. At this The electron beam strikes the device inclined target so that the generated X-rays also at an angle from the target is emitted. However, this facility is not been taken into account that even before the complete Blowing the target is a rapidly progressing one Crater formation causes the optical axis of the radiated useful x-rays a shade of the swelling crater rim that experiences the X-rays largely absorbed. It results in a diffuse x-ray light that is not considered by one point source can be viewed starting. Therefore, such a device with a incident electron beam oblique position of the target not proven.

DE-OS, A, 33 07 019 (Scanray) is a microfocus X-ray device known where the electron beam Draw perpendicular to the target. As useful radiation the outgoing at an angle of 0 to 10 degrees X-ray related. However, it comes with a solid Target worked. A meltdown transmission target is not intended and not addressed.

The invention is therefore based on the object Capture the point in time at which the vertical incidence Electron beam has melted through the target and becomes one other target point must be directed.

This object is achieved according to the invention in that the generic microfocus x-ray device also after the characterizing part of claim 1 is designed.

In the subclaim is a development of the invention claimed.

Fig. 1

einen schematischen Längsschnitt durch eine Mikrofocus-Röntgeneinrichtung,

Fig. 2

einen Schnitt durch das Target in vergrößertem Maßstab,

Fig. 3

das Target nach Figur 2 mit einer Messung des Targetstroms,

Fig. 3A

den Verlauf des Targetstroms in Abhängigkeit von der Bestrahlungsdauer,

Fig. 4

ein Target mit einem eingezeichneten Bremsvolumen und

Fig. 4A

eine Trägerschicht mit Trägermaterial-Dotierungen.

In the drawings, an embodiment of the invention is shown. Show it :

Fig. 1

2 shows a schematic longitudinal section through a microfocus x-ray device,

Fig. 2

a section through the target on an enlarged scale,

Fig. 3

2 with a measurement of the target current,

Figure 3A

the course of the target current as a function of the irradiation time,

Fig. 4

a target with a marked braking volume and

Figure 4A

a carrier layer with carrier material doping.

The microfocus x-ray device 1 consists of a evacuated housing 11, 12 made of glass or non-ferromagnetic metal. The tube 12 has one any, usually round cross-section. By a rear end face 11 of the tube 12 protrude electrically Feed wires 13 for a hairpin-shaped cathode 14 ins Inside of tube 12. The heated cathode 14 acts as an electron source, from whose radiation by means of a cap-shaped grating 15 a narrow divergent Electron beam 16 is hidden. The beam 16 occurs through the central opening of a perforated disk anode 17 through and experiences a bundling into one virtual focal spot 18. The widening afterwards Beam 16 passes through the cross-sectional zone outside the tube 12 arranged deflection coil 19 and is in magnetic gap 20 a subsequent Focusing coil 21 bundled. The focusing coil 21 forms as an electromagnetic lens, a reduced image of the virtual focal spot 18 as focal spot 22 on a Transmission target 23 from which is in the outlet opening 24 of the tube 12 is located. The focusing coil 21 is generated an extremely small-area focal spot 22 in the Typical order of magnitude of 0.5 ... 100 µm. The target 23 consists of a thin brake layer 32 made of a metal high atomic number in the periodic system of elements, such as Tungsten, gold, copper or molybdenum, and one weak X-ray absorbing but good heat conductor Carrier layer 33, preferably made of aluminum or Beryllium. As a result of the braking effect of the target material solve the beam 16's incident electrons X-ray radiation 25 off. Part of the X-ray radiation 25 penetrates the target 23 with the beam direction 28 which coincides with the beam axis 10 of the electron beam 16 and leaves tube 12 toward sample 26 as divergent x-ray beam 25. Due to the geometric The structure of the sample 26, insofar as they are more or less for the X-rays 25 is impermeable, enlarged accordingly as a silhouette parallel to a larger distance behind the sample 26 to the transmission target 23 and thus perpendicular to the Beam direction 28 arranged film in the image plane 29th projected.

A suction system 37 for maintaining the vacuum in the tube 12 and for withdrawing vaporous Traces of material from the burning cathode 14 causes at the same time keeping the interior of the tube 12 clean melted material particles from the focal spot hole 31 in target 23.

The particularly high yield of X-rays 25 results from the extremely small-area stimulated braking volume 40 (Figure 4) in the transmission target 23. The high Power density, so the high area-specific physical stress with the microfocused Electron beam 16, leads to the burning in of a Focal spot hole 31 in the target 23, so that in Departure direction 28 of the X-rays 25 the remaining Target material and thus its radiation-weakening Self-absorption continuously reduced. The brake layer 32 is targeted by the incident electron beam 16 melted, which is a regarding their physical state dynamically changing x-ray source represents.

If the brake material as a thin layer 32, approximately from Tungsten, on a thick backing layer 33 made of good heat-conducting material, such as beryllium or aluminum, is stored, then it is hardly avoidable but also not critical that at the bottom of the hole 31 in the brake layer 32 finally from the microfused electron beam 16 also the backing layer behind in the beam direction 28 33 is melted. Then, however, the radiation must of the target 23 are ended at this point, that is in the Application of this X-ray device 1 ends the recording his; because the application of the carrier layer 33 with Electron beams 16 only lead to a very soft one X-ray radiation 25 and thus hardly in the image plane 29 usable diffuse silhouettes of the translucent sample 26.

For the next x-ray silhouette to be taken again the very short-term radiation of the Transmission targets 23 with a microfocused Electron beam 16, for which in turn the cathode 14 only operated for a short time and / or the beam 16 via a pivotable aperture, not shown in the drawing only released for a short time or the beam 16 via a corresponding control of the deflection coil 19 briefly from a non-functional waiting direction in the device and Active axis 10 of the beam direction 28 is pivoted. However, the transmission target 23 must not be used again a spot is irradiated where a hole 31 has been branded because otherwise soon or even immediately the carrier layer 33 instead of the brake line 32 would be melted from brake material. Therefore an offset control 34 is provided which is controlled by the The beam deflection described above by means of the deflection coil 19 out of the device axis 10 and / or by displacement of the target 23 relative to the device axis 10 ensures that just along a meandering or spiral arch successive focal spots 22 are caused. This ensures that only unused areas of the target 23 in succession are claimed and thus a destruction of the carrier layer 33 with triggers only a little more useful because it is too low in energy X-rays are avoided. The target 23 is thus by the vertical exposure to electrons in the Transmitted light operation so loaded until an aggregate conversion in the molten phase.

To illustrate the displacement of the target 23 relative to the tube 12 or its axis 10 is in the Drawing a positioning motor 35 placed in the tube, shown in the drawing. Instead, the target 23 together with positioning motor 35 in principle also on the front the outlet opening 24 of the tube 12 held vacuum-tight his; or from an external arrangement of the positioning motor A rod attacks through the wall a rotating or sliding bracket 36 for the target in Inside the tube 12.

As stated above, the relocation must of the target 23 always take place when the electron beam 16 the micro-hole 31 as deep in the brake layer 32 has burned in that it reaches the carrier layer 33.

A simple procedure for determining this point in time is after one in terms of performance assessable or easier to determine empirically short exposure time on the order of milli- or microseconds the focal spot generation on the target 23 for what the electron beam, as above already described, switched off, dimmed or off the target area can be pivoted out. This However, the process takes no account of the individual Condition of the micro hole 31. It may well be that the carrier layer 33 has already been irradiated in this method or that on the other hand, the micro-hole 31 is not yet Boundary between brake layer 32 and carrier layer 33 has reached.

A much more precise method for determining the point in time t _a at which the brake layer 32 has melted and the electrons strike the carrier layer 33 is the measurement of the target current I shown in FIG. 3. As shown in FIG. 3, the target current I becomes measured as a function of the irradiation time t, then this has the course shown in Figure 3A. At time t _{a there} is a sudden increase in the target current. The point in time t _a is the point in time at which the electron beam has penetrated the braking layer 32 and the micro-hole 31 extends to the carrier layer 33. By measuring the target current I, a command for the deflection of the electron beam 16 can thus very easily be obtained by the control. All local special features of brake layer 32 and carrier layer 33 are automatically taken into account here.

If an electron accelerated in a high-voltage field penetrates the surface of matter, it experiences in interaction with the matter a series of elastic collisions, each of which loses some of its kinetic energy, which is converted into radiation. Part of this radiation consists of X-rays. During the sequence of elastic impacts, the electron passes through a braking volume 40 (FIG. 4) within the target material, the expansion of which is primarily determined by the atomic number Z of the target material, the energy E _{o of} the electrons and the electron beam diameter d.

The X-ray radiation arises within the braking volume 40 described. The extent of the radiation source is thus determined by the size of the braking volume 40. Even if an electron beam diameter d going towards "zero" is assumed, a finite braking volume 40 remains due to the spreading of the electrons. Thus, a minimum radiation source size, which is essentially determined by E _o and Z, cannot be fallen below in principle.

Now, a further reduction in the radiation source must be achieved in the carrier material Target material doping 41 (Figure 4A) are introduced whose volumes are significantly smaller than that prescribed braking volume 40 of the electrons in one contiguous target material.

The usable X-rays are only generated in the target material high atomic number. That from the target material doping 41 in the base material low atomic number penetrated electrons do not contribute to the usable X-ray radiation, as well as that in addition to the doping 41 electrons penetrating directly into the carrier material do not contribute significantly to the usable radiation.

Since in the small doping volumes according to FIG. 4A same electron beam density less X-ray photons are generated per time than in the larger one Brake volumes 40 in a brake layer 32 (Figure 2), must the electron beam density (current) can be increased. The leads to faster melting of the target material dopings 41 and their substrate environment, however can also arise during the melting process X-rays can be used. For the next X-ray image of the electron beam 16 is known Directed to a still unused doping point 41, etc. The doping 41 can for example in one defined grid can be arranged.

Reference list

1

Microfocus X-ray device

10th

Device and beam axis

11

Face

12th

tube

13

Edging wires

14

cathode

15

Grid

16

Electron beam

17th

Perforated disc

18th

virtual focal spot

19th

Deflection coil

20th

magnetic gap

21

Focusing coil

22

Focal spot

23

Transmission target

24th

Outlet opening

25th

X-rays

26

sample

28

X-ray beam direction

29

Image plane

31

Micro hole

32

Brake layer

33

Carrier layer

34

Offset control

35

Positioning motor

36

Swivel or sliding bracket

37

Extraction system

40

Braking volume

41

Endowments

Claims (2)

1. Microfocus X-ray device, wherein a focussed electron beam for generation of the X-ray radiation is incident perpendicularly on a retarding material of a target (23), the retarding material is converted at the focal point (22) at least into the liquid state of aggregation by the high thermal loading and the position of focal point (22) on the target (23) is displaced with each loading relative to the previous position, wherein the retarding material is arranged in a retarding layer (32) on a carrier layer (33), the retarding layer (32) is arranged at the side of the carrier layer (33) oriented towards the electron beam (16), and a control (34) is provided which interrupts the electron beam (16) at the latest or melting of the carrier layer (33), characterised in that the control (34) is a target current measuring apparatus, which ascertains the instant (t_a), at which the electron beam (16) begins to melt the carrier layer (33), by measurement of the target current (I).
2. X-ray device according to claim 1, characterised in that the retarding material is arranged in the form of dopings (41) in the carrier layer (33).

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