Patent Number: 051329948
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT Prior to the description of the embodiments according to the present invention, filter means used in the present invention will be explained in detail below. When a substance layer with a thickness of d is provided in an optical path of light of high energy such as X rays, a spectral transmittance t (E) of the substance layer, with an absorption coefficient of the substance taken as .mu. [F. Biggs, "Analytical Approximations for X-ray Cross Sections II", Sandia Lab. Research Report SC-PRT-710507 (1971)], is given by EQU t(E)=exp(-d.multidot..mu.) (1) The absorption coefficient .mu. is the amount depending on the kind of substance and the energy (namely, the wavelength) of incident light and has a general trend to diminish as the energy of radiation increases. Accordingly, the substance layer of this kind has the function of a high-pass filter and can behave as the high-pass filter (X-ray filter) with a desired spectral characteristic by selecting the material and thickness of the substance layer. FIG. 6 shows the spectral transmittance characteristic of an Fe filter of d=0.5 .mu.m calculated according to Equation (1). As is apparent from this figure, the X-ray filter suppresses the transmittance of radiation on the low energy side to a small value and therefore fulfils the function of the high-pass filter with respect to photon energy. Further, by varying the material and thickness of the filter, cutoff energy can be selected. Next, when a ray of light is incident at a particular grazing angle on a plane mirror, its reflectance is given by EQU R={(.theta.-a).sup.2 +b.sup.2 }/{(.theta.+a).sup.2 +b.sup.2 }(2) where ##EQU1## Here, the complex index of refraction of the substance constituting the mirror surface can be expressed as n.sub.c =1-.delta.-i.beta.. Further, EQU .delta.=(N.sub.a .multidot.r.sub.e .multidot..lambda..sup.2 .multidot.f.sub.1)/2.pi. EQU .beta.=(N.sub.a .multidot.r.sub.e .multidot..lambda..sup.2 .multidot.f.sub.2)/2.pi. where N.sub.a is the number of atoms per unit volume, r.sub.e the classical electron radius, .lambda. the wavelength of light, and f.sub.1 and f.sub.2 the scattering and absorption factors in the table of Henke [ATOMIC DATA AND NUCLEAR DATA TABLES, Vol. 27, No. 1, p. 1-144 (1982)]. Also, .theta. denotes the grazing angle of light. The grazing incidence mirror, as shown in FIG. 7 [the dependence of the wavelength .lambda. of the reflectance on a Pt reflecting surface at the grazing angle .theta. which is calculated from Equation (2)], has the effect that when radiation with various wavelengths is incident at particular grazing angles (2.degree., 3.degree., 5.degree. and 7.degree.), the reflectance of the radiation on the short wavelength side is suppressed to a small value. That is, it fulfills the function of the low-pass filter suppressing the radiation of high energy. Further, by changing the material of the mirror surface and the grazing angle, the cutoff energy can be selected. As such, if the X-ray filter is used in combination with the grazing incidence mirror, a band-pass filter can be constructed. In particular, a proper selection of characteristics of the filter makes it possible to secure the filter transmitting selectively the radiation of the region of wavelengths of 10-100 .ANG. called "water window" in which the following absorption edges of substances governing a living phenomenon exist. ______________________________________ Substance Absorption edge Wavelength (.ANG.) ______________________________________ P L2, 3 94 S L3** 75.1 Na K 11.569 C K 43.68 N K 30.99 Ca L2 35.13 L3 35.49 ______________________________________ [From: L. Henke, Atomic Data and Nuclear Data Tables 27, p. 1-144 (1982) Also, as the filter for this wavelength region, its thickness is moderate to range from nearly 5 to several .mu.m (although it depends on substances as a matter of course). The filter of larger thickness will cut even the soft X rays and, with smaller thickness, the long wavelength light such as vacuum ultraviolet rays cannot be blocked. Furthermore, the filter of smaller thickness has difficulties in respect of the latest manufacturing technology and the strength. In accordance with the embodiments shown, the present invention will be described in detail below. However, the substances constituting the filter means used in the present invention is not necessarily limited to those shown in individual embodiments. FIRST EMBODIMENT FIG. 8 is a schematic view showing the construction of a scanning X-ray microscope equipped with the Walter optical system. In this figure, the Walter optical system, though shown in regard to only the one side of the optical axis, has the arrangement in which an annular ellipsoidal mirror 1 and a hyperboloidal mirror 2 are coaxially connected with each other. Further, an X-ray source O is disposed at a focal point F.sub.1 of the ellipsoidal mirror 1 and radiation emitted from the X-ray source O is reflected from the order of the ellipsoidal mirror 1 and the hyperboloidal mirror 2 and converged at a focal point F.sub.2 of the hyperboloidal mirror 2. At this position is provided a stage on which a specimen is placed. The radiation transmitted through the specimen is conducted to a detector 5 through an X-ray filter 4. The stage 3 is such that a two-dimensional movement, which is possible in a plane normal to the optical axis, enables the specimen to be scanned by a radiation spot. Here, the laser plasma source having the characteristic such as is shown in FIG. 1B is used as the X-ray source O, the Fe filter of the characteristic shown in FIG. 6 as the X-ray filter 4, and the MCP shown in FIG. 3 as the detector. Also, the entire system is contained in a vacuum vessel, although not shown. For a scanning technique, there is a method of providing a movable grazing incidence mirror on the optical axis, instead of changing the position of the stage, to move the radiation spot by turning the grazing incidence mirror. In this embodiment, a detecting efficiency G(.lambda.) of the radiation with the particular wavelength .lambda. emitted from the radiation source is given by ##EQU2## where I(.lambda.) is the spectrum of the radiation emitted from the plasma radiation source, I max the maximum of I(.lambda.), .alpha.(.lambda.) the convergent efficiency =.intg.R.sub.1 R.sub.2 d.omega. (the integration covers the range of an effective solid angle at which the radiation can be incident on the optical system) of the Walter optical system, R.sub.1 the reflectance at the ellipsoidal mirror 1, R.sub.2 the reflectance at the hyperboloidal mirror 2, t(.lambda.) the spectral transmittance of the X-ray filter 4, and QE(.lambda.) the quantum detecting efficiency of the detector 5. FIG. 9 shows the Walter optical system comprising a Pt reflecting mirror which is favorable for the embodiment and FIG. 10 depicts the wavelength dependence of the convergent efficiency .alpha.(.lambda.) thereof. FIG. 11 diagrams the detecting efficiency G(.lambda.) calculated from Equation (3) with respect to the X-ray microscope constructed by the combination in which the Walter optical system such as is shown in FIG. 9 is adopted as a converging optical system, the laser plasma source radiating radiation with the spectrum shown in FIG. 1B as the X-ray source O, the filter having the spectral transmittance shown in FIG. 6, namely, the Fe filter with a thickness of 0.5 .mu.m, as the X-ray filter 4, and the MCP of the characteristic shown in FIG. 4A as the detector 5. FIG. 12, on the other hand, shows the detecting efficiency G(.lambda.) of an arrangement in which the X-ray filter 4 is removed from the preceding X-ray microscope. In FIG. 11, energy where the detecting efficiency becomes about 10% of the maximum on the low energy side is approximately 230 eV, which is equivalent to 55 .ANG. more or less in terms of the wavelength. Accordingly, the radiation of longer wavelength is cut by the X-ray filter. In FIG. 12, although the diagram may be rather hard to read because the peak of the detecting efficiency G(.lambda.) is cut, the energy where the detecting efficiency G(.lambda.) becomes about 10% of the peak is 100 eV more or less. It will thus be seen that the radiation of longer wavelengths is cut by the X-ray filter 4. SECOND EMBODIMENT FIG. 13 is a view showing an outline of the arrangement of a Walter type soft X-ray scanning microscope which is designed so that in the optical system of FIG. 8, a grazing incidence mirror 6 is disposed on the emergence side of the specimen and the radiation transmitted through the specimen, after being reflected from the grazing incidence mirror 6, is incident on the detector 5 though the X-ray filter 4. The detecting efficiency G(.lambda.) relative to the light of the wavelength .lambda. of this embodiment is given by ##EQU3## where R(.lambda.) is the spectral reflectance of the grazing incidence mirror, which is as shown in FIG. 7. FIG. 14 shows the detecting efficiency G(.lambda.) calculated according to Equation (4) by adding a Pt grazing incidence mirror with a grazing angle of 5.degree. to the example of FIG. 11. As is evident from this diagram, the photon energy where the value of the detecting efficiency G(.lambda.) becomes about 10% of the peak is less than 700 eV and consequently the short wavelength region is cut to the extent of 18 .ANG.. It will thus be seen that the use of the grazing incidence mirror makes it possible to cut the radiation of the short wavelength region compared with the example in FIG. 11. THIRD EMBODIMENT FIG. 15 shows the detecting efficiency G(.lambda.) calculated from Equation (3) in regard to the X-ray microscope constructed by the combination in which the Walter optical system such as is shown in FIG. 9 is adopted as the converging optical system, the synchrotron radiation source emitting the radiation with the spectrum shown in FIG. 1A as the X-ray source O, the filter having the spectral transmittance shown in FIG. 16, namely, an Ni filter with a thickness of 0.4 .mu.m, as the X-ray filter 4, and the MCP of the characteristic shown in FIG. 4A as the detector 5. FIG. 17 depicts the detecting efficiency of the X-ray microscope devoid of the X-ray filter. As is evident from these diagrams, it is noted that the photon energy such that the S/N ratio of the detecting efficiency is held to nearly 10% (that is, such that the detecting efficiency becomes nearly 10% of the peak) comes to more than 300 eV and thus the long wavelength radiation is cut to the extent of 41 .ANG.. FOURTH EMBODIMENT FIG. 18 is a view showing an outline of the arrangement of a soft X-ray scanning microscope for microscopy of biological specimens. The radiation emitted from the X-ray source O and converged through an optical system 7 traverses a window member 9 of a vacuum chamber 8 to be incident on and transmitted through the specimen located in the atmosphere and after passing through a window member 11 of a vacuum chamber 10, is detected by the detector 5. At this time, the detecting efficiency G(.lambda.) of the radiation with the wavelength .lambda. is ##EQU4## where t.sub.1 (.lambda.) is the X-ray transmittance of the window member 9, t.sub.2 (.lambda.) the X-ray transmittance of the window member 11, and AIR(.lambda.) the X-ray transmittance of an atmospheric layer in which the specimen and the stage 3 are located. In this way, where a living body is observed in vivo, it is required that the specimen and the stage 3 are disposed in the atmosphere and, as illustrated in FIG. 19, a microscope body and a detecting section positioned in the vacuum chambers 8 and 10, respectively, are separated somehow from each other by windows. If the X-ray filters are used as the windows, the window members 9 and 11 separating the vacuum from the atmosphere will be secured and unnecessary radiation with low energy can be cut. Moreover, the atmosphere between the microscope body and the detecting section serves as a high-pass filter such that the radiation with low energy is attenuated by the atmosphere per se, as seen from, for example, the spectral transmittance [of N.sub.2 (d=650 .mu.m) constituting principally the atmosphere which is calculated from Equation (1)] shown in FIG. 20. Hence, even if the atmospheric layer exists, the high-pass filter with good performance can be designed. FIG. 21 shows the detecting efficiency G(.lambda.) calculated from Equation (5) in relation to the X-ray microscope constructed by the combination in which the Walter optical system such as is shown in FIG. 9 is adopted as the converging optical system 7, the synchrotron radiation source emitting the radiation with the spectrum shown in FIG. 1A as the X-ray source O, Be filters each having a thickness of 0.3 .mu.m (the spectral transmittance of a 0.6-.mu.m-thick Be filter is as shown in FIG. 22) as the window members (X-ray filters) 9 and 11, a layer with a thickness of 650 .mu.m (whose spectral transmittance is as shown in FIG. 20) as the atmospheric layer, and the MCP of the characteristic shown in FIG. 4A as the detector 5. As is evident from this diagram, it is seen that the region of wavelengths detected with the S/N ratio of more than 10% is reduced to less than nearly 60 .ANG.. FIFTH EMBODIMENT FIG. 23 shows the detecting efficiency G(.lambda.) of the radiation with the wavelength .lambda. in the case where the Pt grazing incidence mirror with a grazing angle of 2.degree. is disposed on the emergence side of the specimen in the X-ray microscope of FIG. 18. As is apparent from this diagram, it is seen that the radiation of the short wavelength region is cut to the extent of 15 .ANG. compared with FIG. 21. SIXTH EMBODIMENT FIG. 24 shows the detecting efficiency G(.lambda.) calculated from Equation (5) with respect to the X-ray microscope in which in FIG. 18, the Walter optical system such as is shown in FIG. 9 is adopted as the converging optical system 7, the synchrotron radiation source emitting the radiation of the spectrum shown in FIG. 1A as the X-ray source O, a 0.3-.mu.m-thick Ni filter (whose spectral transmittance is as shown in FIG. 25) and a 0.3-.mu.m-thick Al filter (whose spectral transmittance is as shown in FIG. 26) as the windows members (X-ray filters) 9 and 11, respectively, a layer with a thickness of 50 .mu.m (whose spectral transmittance is as shown in FIG. 27) as the atmospheric layer, and the MCP of the characteristic shown in FIG. 4A as the detector 5. As is apparent from this diagram, by combining substances different from each other as in the foregoing to construct the window members, the substance transmitting the X-rays to some extent in the low energy region (namely, on the long wavelength side) can also be utilized as the window member if the Al filter with such a thickness is used alone. That is, it will be noted from FIG. 24 that the wavelength region detected with the S/N ratio of more than 10% is reduced to the extent of less than 41 .ANG.. SEVENTH EMBODIMENT FIG. 28 is a schematic view showing the arrangement of a Schwarzschild type soft X-ray scanning microscope. In this case, the radiation radiating from the X-ray source O and converged by a Schwarzschild optical system 12 is incident on and transmitted through the specimen placed on the stage 3 and after passing through the X-ray filter 4, is detected by the detector 5. The Schwarzschild optical system, as depicted in FIG. 29, possesses per se remarkable properties of wavelength dispersion in a soft X-ray region due to the effect of multilayer films applied to the mirror surfaces of individual reflecting mirrors [FIG. 29 indicates the property of wavelength dispersion of the multilayer film alternately laminated with 201 Ni-Si layers which is optimally designed under the conditions of a wavelength of 39.8 .ANG. and an incident angle of 6.degree.]. For the radiation of the long wavelength beyond the vacuum ultraviolet rays, however, the reflectance increases again, so that the X-ray filter 4 is effective to cut such radiation. FIG. 30 depicts the Schwarzschild optical system with a numerical aperture of 0.25 on the specimen side and a magnification of 100.times. which is favorable for this embodiment, and a concave mirror 12.sub.1 and a convex mirror 12.sub.2 constituting the optical system are coated with the multilayer films of the following specification: ______________________________________ 201 Ni--Si layers ______________________________________ Film thickness Concave mirror 12.sub.1 Ni = 9.1 .ANG. Si = 11.1 .ANG. Convex mirror 12.sub.2 Ni = 9.2 .ANG. Si = 11.3 .ANG. ______________________________________ FIG. 31 shows the wavelength (that is, energy) dependence of the convergent efficiency .alpha.(.lambda.) in the Schwarzschild optical system. FIG. 32 shows the detecting efficiency G(.lambda.) calculated from Equation (3) in regard to the X-ray microscope constructed by the combination in which the multilayer film Schwarzschild optical system such as is shown in FIG. 30 is adopted as the converging optical system, the synchrotron radiation source emitting the radiation with the spectrum shown in FIG. 1A as the X-ray source, the filter having the spectral transmittance shown in FIG. 6, namely, a 0.5-.mu.m-thick Fe filter, as the X-ray filter 4, and the MCP of the characteristic shown in FIG. 4A as the detector 5. As is obvious from the diagram, it is noted that the long wavelength radiation such as vacuum ultraviolet rays is cut in comparison with the characteristic of the example (comparison example) making no use of the X-ray filter 4. EIGHTH EMBODIMENT FIG. 34 is a view showing an outline of the arrangement of a zone plate type soft X-ray scanning microscope. In such an instance, the radiation emitted from the X-ray source O and converged by a zone plate 13 (refer to FIG. 2C) traverses a pinhole 14 to be incident on and transmitted through the specimen on the stage 3 and after passing through the X-ray filter 4, is detected by the detector 5. Also in this embodiment, since the long wavelength radiation diffracted by the pinhole 14 and the short wavelength radiation transmitted therethrough adversely affect image formation, the X-ray filter 4 is available. NINTH EMBODIMENT FIG. 35 is a schematic view showing the arrangement of an imaging mode X-ray microscope. The imaging mode, unlike the scanning mode, is such that by forming an image of an object of predetermined size, the image of certain size can be observed without moving the object. This embodiment is designed so that the specimen on the stage 3 is irradiated with the radiation emitted from the X-ray source O and the radiation transmitted through the specimen is imaged by an imaging optical system 15, thereby causing the image of the specimen to be formed through the X-ray filter 4 at the position of the detector 5. A condenser lens may be disposed between the X-ray source O and the specimen, as necessary. TENTH EMBODIMENT FIG. 36 shows a schematic arrangement of the imaging mode X-ray microscope constructed so that in the optical system of FIG. 35, the grazing incidence mirror 6 is disposed at the imaging position of the specimen secured by the imaging optical system and the radiation transmitted through the specimen is reflected from the grazing incidence mirror 6 to enter the detector 5 through the X-ray filter 4. ELEVENTH EMBODIMENT FIG. 37 shows a schematic arrangement of an imaging mode X-ray microscope for microscopy of biological specimens which comprises the optical system of FIG. 35 or 36 incorporated in the vacuum chambers 8 and 10, except for the stage 3.