X - R A Y M I C R O S C O P Y

A N D M I C R O R A D I O G R A P H Y

PROCEEDINGS OF A SYMPOSIUM HELD AT THE

CAVENDISH LABORATORY, CAMBRIDGE, 1956

Edited by

V. E. COSSLETT Cavendish Laboratory, University of Cambridge, England

ARNE ENGSTROM Department of Medical Physics, Karolinska Institutet, Stockholm, Sweden

H. H. PATTEE, JR. Department of Physics, Stanford University, California

1957

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Possibilities of the Scanning X-Ray Microscope

HOWARD H. PATTEE, JR. Department of Physics, Stanford University, Stanford, California

The original idea of a scanning X-ray microscope (1) came about during the time that the author was engaged primarily in the design and construction of reflection X-ray optical systems, which are probably less akin to the scanning system than either microradiography or point projection. At that time, however, these other methods were known and their limitations understood to a certain extent. In order to describe the imaging properties of a scanning X-ray microscope, it will be interesting to discuss, by way of comparison, these other known methods of image formation.

Historically, it is not unreasonable to assume that refraction X-ray microscopes were the first to be considered, since Roentgen himself men­tions the impracticability of X-ray lenses; and although lenses are perhaps not so impractical as he may have believed, there are still no refraction X-ray microscopes in existence to my knowledge.

It is natural, therefore, that contact radiography was for so long the only useful method of making enlarged X-ray images, since until quite recently the term "microscope" was usually thought of as an instrument using lenses or perhaps mirrors. An inherent advantage of any contact method is its relative freedom from diffraction, which nevertheless re­mains the fundamental limit of all microscopes. Using visible light, it is quite possible to produce contact images with a diffraction resolution well below 1 p, whereas the point projection technique is quite im­practical for light microscopy because of the diffraction. The idea of using point projection for X-ray enlargement may have been delayed partly by the near-instinctive disregard that most physicists give any "microscope" which has no "optics."

The diffraction limit of a point projection X-ray microscope may be taken as the minimum width, r, of the first Fresnel fringe divided by the image magnification. In most cases this can be expressed as (2)

r/M = (pX)1'2

where M is the image magnification, p is the source-to-object distance,

• This work was supported by grants from Research Corporation and the American Cancer Society.

367

368 HOWARD H. PATTEE, JR.

and A is the X-ray wavelength. By making p very small, the diffraction can be correspondingly reduced. For a wavelength of 1 A and a source-to-object distance 0.1 mm, the diffraction limit, by this criterion, is 1000 A. If a wavelength of 50 A is used, then the specimen must be moved 50 times closer to maintain the same resolution. This results in a smaller field of view, since the width of field of a projection microscope cannot be much wider than the source-to-object distance without exces­sive increase in p at the edges, and a correspondingly large obliquity of the incident rays. If we set the width of the field equal to p, then the number of resolvable elements, N, in a shadow picture will be N = (pM/r)2 = p/X. For the above resolution of 1000 A using a wavelength of 50 A, the projection microscope would give only 400 resolvable picture elements. For 100-A resolution this would be reduced to only 4 resolvable picture elements. In order to produce a good picture by television standards at least 105 resolvable picture elements are needed, and a good photographic standard requires well over 106 elements. It therefore appears that high resolution is incompatible with great width of field in projection microscopes. For a resolution of several hundred angstroms, using a wavelength of about 10 A, a compound reflection system such as that proposed by Wolter (3) would have over ten times the useful width of field as the point projection system.

If one is to have the highest possible resolution, it is natural to con­sider placing the specimen as close to the source as possible, which in the case of an X-ray tube with a Lenard window would be directly on the outside of the window. The source and specimen would then be separated by the thickness of the specimen itself. However, if the speci­men and source remain relatively fixed, the useful field would be little more than the minimum resolving distance. One is therefore led to con­sider moving the source across the specimen so that a useful field is covered. The image of the specimen is thereby broken into elements each of which is no larger than the minimum resolving distance, which in turn is approximately equal to the size of the X-ray source, since diffraction is essentially eliminated. The reassembly of these picture elements into a representation of the object is accomplished by using a kinescope with a scan which is geometrically similar to the X-ray source scan and whose light output is modulated by the instantaneous X-ray intensity passing through an element of the specimen.

By such a scanning system, we retain the advantage of a contact imaging process where diffraction effects are reduced to a minimum for whatever wavelength is used, and at the same time allow the width of field to be increased without affecting the resolution limit of the X-rays. The electron beam which produces the scanning X-ray source is, of

THE SCANNING X-RAY MICROSCOPE 369

course, subject to the aberrations and intensity limits of any electron optical system.

Since the picture information is carried by a time-varying signal in a scanning microscope, it is possible to subject this information to electronic control more easily than if the picture arrives simultaneously over a two-dimensional surface. The contrast, intensity, and magnifica­tion of the final picture are all controllable over wide ranges by simple electronic adjustments. Furthermore, wavelength discrimination is pos­sible, and has been described by Cosslett and Duncumb (4), which makes the scanning system most attractive for microanalysis.

Unfortunately, the speed of the scanning microscope cannot be made independent of the resolution. The minimum resolving distance in a scanning system of this type is very nearly equal to the effective diameter of the X-ray source, and the maximum current which can be delivered to a given spot diameter is limited by the properties of the electron optics. The functional dependence of the maximum target current on the spot radius, r, is different for the two types of electron sources which have been used in microfocus X-ray tubes. For a conventional thermionic emitter, which is greatly demagnified by magnetic electron lenses, Cosslett and Haine (5) give maximum deliverable current as proportional to rs/3, whereas for the field emission electron source they give maximum target current as proportional to r- / !. The target spot radius below which the field emitter is superior to the thermionic emitter is somewhat uncertain, owing to a lack of experimental data, but prob­ably lies near 1 p (6,7) . In any case, the scanning microscope fares no better or worse than the point projection type as far as the X-ray in­tensity at the detector is concerned. In the method of detection, however, the scanning system has some advantages.

Because the X-ray signal is a one-dimensional time-dependent function in a scanning system, it is possible to utilize the most sensitive types of X-ray detectors. Both scintillation and proportional counters may be used to detect individual X-ray photons within a wide range of energies and at the same time to discriminate between photons of different energies, as we have mentioned. The fundamental sensitivity limit of the scanning X-ray microscope is therefore the statistical fluctuations in the photons which pass through a given picture element. No type of micro­scope can overcome this limit.

We may easily estimate the order of magnitude of the target current which would be necessary to produce a scanning image with a given quality. If we choose to have 500-line resolution, which is a minimum value for a good-quality picture, and a minimum contrast of 0.1 de­tectable above the photon noise, then each frame will require 2.5 X 10s

370 HOWARD H. PATTEE, JR.

photons (8). If we further specify a frame-time of 0.1 sec, and assume an electron-to-photon conversion efficiency of 10", we find that we require a minimum target current of close to 4 ^a. Notice that this figure is independent of the minimum resolving distance which we desire to obtain in this picture. In order to obtain this quality of picture in the time specified, we must deliver the 4-pa current to a spot diameter which is about equal to the desired resolving distance.

According to Cosslett and Haine (5), it would be very difficult to focus a beam current of the order of microamperes onto a target spot of several hundred angstroms radius using a thermionic emitter as a source, whereas with a field emitter as a source this would just be pos­sible. The author's field-emission experiments (9) indicate that such cur­rents are quite feasible even down to 100-A spot radius.

Experimentally the development of the scanning X-ray microscope at Stanford has followed two lines of endeavor: the first is involved prin­cipally with field emission electron sources which is described fully in another paper (7); the second has been with detectors and electronic techniques, which we will describe here. As we mentioned previously, the minimum target current which is necessary to produce a picture with a certain number of resolvable elements and contrast detail in a given time is determined without regard to the resolution. This means that we may study most of the aspects of the scanning, detection, and display of the image with relatively low resolution and expect this information to apply, unchanged, to a high-resolution instrument, assuming only that the target current remains constant.

Such an instrument has been in operation for some time, and much useful data has been gathered which will be applied to a higher resolu­tion instrument which is now under construction using a field-emission X-ray tube. The X-ray tube is a Metropolitan-Vickers microfocus dif­fraction tube which has had the regular target replaced by a chamber containing two pairs of electrostatic deflection plates and a target disk on which can be mounted several target-specimen combinations (see Fig. 1). The targets can be positioned in the vacuum. The tube was designed to produce a line focus at the target about 10 p wide, but the gun structure was changed to produce a circular focus of about 8 p diameter. The detector consists of an anthracene crystal mounted with Canada balsam to the face of the RCA 6199 photomultiplier. The photo-multiplier voltage is supplied by batteries. Several video amplifiers were used giving amplifications of from 103 to 104 and a bandwidth of about 2 Mc/sec. The oscilloscope provided the scanning voltages. A photograph of this apparatus is shown in Fig. 2.

The most significant data which this instrument provided were con-

THE SCANNING X-RAY MICROSCOPE 371

cerned with sensitivity. It was judged by several observers that the kine­scope image with about 104 elements was barely discernible above the noise at a target current of 10 8 amp and a frame time of 0.5 sec. (The specimen was a 500-mesh/in. copper grid.) This would indicate that an average of about 10 photons were incident on each picture element

X-ray Filament and

Bias Supply

X-ray High Voltage Supply

.Electron Gun

Anode o o

Lens Current Supply

Electron Lens-Scanning Plates-

Target" Crystal

Oscilloscope

Z-axis

Scanning Voltage

Attenuator

Photomultiplier High Voltage n

Video Amplifier

Photomultiplier FIG. 1. Block diagram of scanning X-ray microscope. The target structure of an

experimental Metropolitan-Vickers microfocus X-ray tube was replaced with trans­mission-type targets and electrostatic deflection plates. The targets can be changed in the vacuum by a simple rotation of a disk which can hold 20 target foils. A 0.00025-in. Mylar window separates the scintillation crystal from the high vacuum. The oscilloscope chassis provides both x and y sweep voltages. H

during each frame, and that the noise was therefore the random fluctua­tion of the individual photons. This target current under these conditions is consistent with the previously estimated 4-^a target current which was necessary for the 500-line picture.

Our next step in the development of a high-resolution scanning X-ray microscope is to attempt to build a field-emission scanning X-ray tube. Our progress along these lines is reported in a separate paper (7). By

372 HOWARD H. PATTEE, JR.

this means we hope to increase the resolution without undue loss in speed; but until such a tube has been demonstrated to be practical, the microfocus tubes now used in point projection microscopes should be applicable to scanning systems to some degree as the excellent work

FIG. 2. Photograph of the equipment shown in the block diagram of Fig. 1. The X-ray tube with its vacuum pumps is on the left. The scintillation counter and its power supply is directly below the tube. The equipment rack holds the high and low voltage supplies and the display oscilloscope.

described in the next paper will show. Since any scanning X-ray micro­scope is convertible to a point projection type with only a few changes, it is probably worth carefully designing such an instrument so that the advantages of both methods of X-ray imaging may be used.

THE SCANNING X-RAY MICROSCOPE 373

With our present knowledge of the properties of X-ray images, the behavior of field-emission electron sources, and the sensitivity of X-ray detectors, it would appear not only possible but reasonable to work toward the development of a scanning X-ray microscope which produces in a few seconds a picture with at least 105 elements at a resolution of well below 0.1 p. We can furthermore reasonably hope that such a scanning system would be useful over a wide range of useful wave­lengths from 1 A to 50 A without significant differences in the picture quality. Several proposed reflection X-ray designs offer similar picture quality, but without the speed or wavelength range of a scanning micro­scope (3,10). The point projection method may match the speed of the scanning system if suitable image intensifiers are developed; and it can equal the scanning method in either resolution or width of field but not both, since the diffraction can be reduced only at the expense of the number of resolvable picture elements.

References

1. H. H. Pattee, Jr., The scanning X-ray microscope. /. Opt. Soc. Amer. 43, 61 (1953).

2. W. C. Nixon, High-resolution X-ray projection microscopy. Proc. Roy Soc. A232, 475 (1955).

3. H. Wolter, Spiegelsysteme streifenden Einfalls als abbildende Optiken fur Ront-genstrahlen. Ann. Physik [6] 10, 94 (1952).

4. V. E. Cosslett and P. Duncumb. Microanalysis by a flying spot X-ray method. Nature 177, 1172 (1956).

5. V. E. Cosslett and M. E. Haine, The tungsten point cathode as an electron source. Proc. Intern. Conf. Electron Microscopy London 1954, p. 639 (1956).

6. P. Kirkpatrick and H. H. Pattee, Jr., X-ray microscopy. In "Handbuch der Physik" (S. Fliigge, ed.), Vol. 30. Springer, Berlin, 1957.

7. H. H. Pattee, Jr., this volume, 278. 8. T. Tol, W. J. Oosterkamp, and J. Proper, Limits of detail perceptibility in

radiology particularly when using the image intensifier. Philips Research Repts. 10, 141 (1955).

9. H. H. Pattee, Jr., A metal vacuum joint suitable for field emitters. Phys. Rev. 98, 283 (1955).

10. H. H. Pattee, Jr., The compound reflection X-ray microscope. Ph. D. Disserta­tion, Stanford University (1953).

Discussion

DR. I. MACARTHUR (Dept. of Biomolecular Structure, University of Leeds)! Is the MetroVick tube the Haine-Witty model whose character­istics were reported in 1950 (Brit. J. Appl. Phys.) or has it a still finer focus?

AUTHOR'S REPLY: Yes, it is basically the same tube, but with both the gun and target modified as mentioned. The original gun produced a line focus about 10 p wide. It now produces a circular focus 8 p in diameter.