Patent Number: 052221130
Section: summary

BACKGROUND OF THE INVENTION Various kinds of X-ray microscopes are known which differ more or less with respect to the following: the optical configuration with respect to the beam source used, the optics for focussing the X-ray beam on the specimen to be investigated and the optics for imaging the specimen on the X-ray detector used to provide the image. X-ray microscopes are described, for example, wherein mirror optics are used for imaging the specimen on the detector such as a Wolter optic which images the specimen with a grazing incidence of the X-radiation. The quality of the microscopic image generated with such microscopes is however not especially good since considerable imaging errors are associated with the mirror optics. In mirror optics operating with grazing incidence, the image error associated therewith is the so-called angle-tangent error. These image errors limit in principle the possible resolution which can be obtained with the microscope and is pregiven by the aperture of the optics. X-ray microscopes are known wherein so-called zone plates are utilized for focussing the X-radiation on the specimen as well as for imaging the specimen on the detector. These zone plates make it possible (similar to very thin lenses) to provide an imaging of the object or specimen which is free of image faults and therefore of high resolution. However, the zone plates have a significantly less efficiency than mirror optics. The efficiency in practice lies between 5% and 15%, that is, a maximum of only 15% of the X-radiation impinging on the zone plate is utilized for imaging. An overview of the various X-ray microscopes is provided in the text of D. Rudolph et al entitled "X-Ray Microscopy", Volume 43 (1984) and published by Springer. Starting on page 192 of this text, an X-ray microscope is described wherein the condenser as well as the objective are configured as zone plates. The zone plate used as the condenser not only focusses the X-radiation on the object but also functions as a monochromator and separates the monochromatic radiation required for the high resolution imaging from the more or less expanded wavelength range supplied by the X-ray source. This takes place simply by a suitable pin-hole diaphragm on the optical axis which effects the condition that only one of the monochromatic images passes through the diaphragm with the image arising on the optical axis as a consequence of the wavelength dependency of the focal width of the zone plate. The X-ray microscope described above is relatively light attenuating with the above-mentioned low efficiency because of the use of zone plates so that long exposure times result which can lead to motional blurring during exposure when taking recordings of living cells. For this reason, one is dependent upon the most intensive X-radiation sources. For the reasons given above, synchroton radiation from electron storage rings is used almost exclusively. However, this brings with it the disadvantage that the X-ray microscope is not self-contained, that is, the user is tied to the few electron storage rings with respect to the measuring time which is available. So-called plasma focus sources are also known as X-radiation sources. Such X-ray sources are described for example in U.S. Pat. No. 4,596,030 and do not however continuously supply X-radiation; instead, they supply short X-ray pulses which are followed by a relatively long dead time during which the capacitors of the X-radiation sources must be recharged. The X-radiation contained in one pulse is in many cases inadequate. SUMMARY OF THE INVENTION From the foregoing, it is seen that a light-intense X-ray microscope which is self-contained while at the same time having high resolution does not exist up to now. However, for biological applications, it is precisely this type of microscope which is required for the investigation of living cells because of the required short exposure times. The X-ray microscope according to the invention includes the following features: a pulsed X-ray source which supplies an intensive line radiation; a reflecting condenser which focusses the radiation of the radiation source on the specimen to be investigated; and, an X-ray optic configured as a zone plate which images the object with high resolution on the X-ray detector. By combining the pulsed X-ray source, which supplies intensive line radiation, and a reflecting condenser, the energy which is available is optimally utilized. The use of the mirror optics on the illumination side is not disadvantageous since, on the one hand, the image errors of the reflecting condenser are significantly less critical than on the imaging end of the microscope. In contrast, a 20 to 30 multiple in savings of light is obtained in comparison to a zone plate on the illumination side. Although the reflecting condenser cannot be used as a monochromator, this is not, however, necessary since X-ray sources such as the plasma focus already supply an adequately intense monochromatic radiation. In view of the light savings achieved, the zone plate can be retained on the imaging side with its excellent imaging characteristics. With the combination described above, adequate X-radiation is available for the first time in order to image biological specimens adequately; that is, the X-radiation contained in one X-ray pulse is optimally utilized and is adequate for recording an X-ray image of biological specimens. For example, the reflecting condenser can be a segment of an ellipsoid which focusses the X-radiation with a grazing incidence on the specimen. It is advantageous to provide the reflecting condenser with a multilayer for increasing the reflective capacity. In this way, the efficiency of the microscope can be again improved. The zone plate utilized for imaging the specimen on the detector is preferably a phase zone plate which has a higher efficiency than an amplitude zone plate. It is also advantageous when the condenser images the X-radiation source directly on the specimen in the manner of a so-called critical illumination. In contrast to microscopes which are conventional and utilize a so-called "Kohler illumination", this affords the advantage that a single condenser optic is adequate; that is, the efficiency on the illuminating side is optimized. It is advantageous when the reflecting condenser is protected by one or more foils through which the radiation beam passes. With this foil, the sensitive mirror surfaces are protected against dust and dirt from the ambient and against vapor from the plasma focus source which would otherwise condense on the optical faces of the condenser and reduce its efficiency. A photoplate or an X-ray sensitive CCD-camera can be used as a detector. An image memory is preferably connected downstream of the camera into which the images of the specimen to be investigated are read in and further processed with the known methods of image processing. The images stored in this manner are generated with each X-ray pulse.