76R
ANALYTICAL CHEMISTRY, VOL. 50, NO. 5, APRIL 1978
(113) Niernan, T. A., Holler, F. J., Enke, C. G., Anal. Chem., 48, 899 (1976). Kordl, E. V., Hadjiioannou, T. P., Anal. (114) Nlkolelis, D. P., Karayannis, M. I., Chlm. Acta, 90, 209 (1977). (115) Bartels, P. C., Roljers, A. F. M.. Clin. Chim. Acta, 81, 135 (1975). (116) Simonescu. T.. Rusu. V.. Kiss. L.. Rev. Chim.. 28. 75 (19751: Anal. Abstr.. 30, 364 (1976) (117) Simonescu, T , Rusu, V I Rev Chim , 2 7 , 164 (1976), Anal Abstr , 31, 357 (1976) ~ . . . .,. (118) Wilson, R. L., Ingle, J. D., Jr., Anal. Chim. Acta, 92, 417 (1977). (119) Tamarchenko, L. M., Zh. Anal. Khlm., 30, 127 (1975); Anal. Abstr., 30, 113 (1976). (120) Connors, K. A., Anal. Chem., 48, 87 (1976). (121) Connors, K. A., Anal. Chem., 49, 1650 (1977). (122) CsIzBr, E., Gorog, S.,Anal. Chlm. Acta, 88, 217 (1976). (123) Koprlvc, L., Polla, E., Hranllovlc, J., Acta Pharm. Suec., 13, 421 (1976); Anal. Abstr., 33, 60 (1977). (124) Mentastl, E., Pellzzetti, E., Salnl, G., Anal. Chim. Acta, 88, 303 (1976). (125) Kopanica, M., Stara, V., Collect. Czech. Chem. Commun., 41, 3175 (1976); Anal. Abstr., 32, 488 (1977). (126) Wolff, C. M., Schwlng, J. P., Bull. Soc. Chlm. f r . , I. (5-6); 679 (1976); Anal. Abstr., 31, 438 (1976). (127) Collin, J. P., Lagrange, P., Bull. SOC. Chlm. Fr., I. (9-lo), 1309 (1976); Anal. Abstr., 32, 300 (1977). (128) Haraguchi, K., Ito, S., Jpn. Anal., 24, 405 (1975); Anal. Abstr., 30, 210 (1976). (129) Pantel, S., Weisz, H., Anal. Chlm. Acta, 89, 47 (1977). (130) Weisz, H., Rothrnaier, K., Anal. Chim. Acta, 82, 155 (1976).
(131) Wilson, R. L., Engie, J. D., Jr., Anal. Chim. Acta, 83, 203 (1976). (132) Porterfield, R. I., Olson, C. L., Anal. Chem., 48, 556 (1976). (133) Mertens, J., den Winkel, P. V., Massart, D. L., Anal. Chem., 48, 272 (1976). (134) Snyder, L., Levlne, J., Soy, R., Conetta, A., Anal. Chem., 48, 942A (1976). (135) Ridder, G. M., Margerum, D. W., Anal. Chem., 49, 2090 (1977). (136) Rldder, G. M., Margerum, D. W., Anal. Chem., 49, 2098 (1977). (137) Mieling, G. E., Taylor, R. W., Hargis, L. G., English, J., Pardue, H. L., Anal. Chem., 48, 1686 (1976). (136) Patton, C. J., Crouch, S. R., Anal. Chem., 49, 464 (1977). (139) Nlernan, T. A., Enke, C. G., Anal. Chem., 48, 619 (1976). (140) Patel, R. C., Chem. Instrum., 7, 83 (1978). (141) Relch, R. M., Sutter, J. R., Anal. Chem., 49, 1081 (1977). (142) Cllne Love, L. J., Shaver, L. A,, Anal. Chem.. 48, 364A (1976). (143) Mousa, J. J., Winefordner, J. D., Anal. Chem., 48, 1195 (1974). (144) Chen, R. F., Anal. Lett., 10, 767 (1977). (145) Johnson, D. W., Callis, J. G., Chrlstlan, G. D., Anal. Chem., 49, 747A (1977). (146) Krug, F. J., F o l k , B. H., Zagath, E. A. G., Jbgensen, S . S.,Analyst(Lorxbn), 102, 503 (1977). (147) Hadjiioannou, T. P., Hadjiioannou, S.F., Avery, J., Malrnstadt, H. V., Anal. Chim. Acta, 89, 231 (1977). (148) McCreery, R. L., Anal. Chem., 49, 206 (1977). (149) Perrln, D: D., Talanta, 24, 339 (1977). (150) Gochrnan, N., Bowie, L. J., Anal. Chem., 49, 1183A (1977). 11511 Dessv. R. E.. Anal. Chem.. 49. llOOA (19771. (152) Yang: R T Steinberg. M I Anal Chem , 49, 998 (1977)
Electron Microscopy John M. Cowley Department of Physics, Arizona State University, Tempe, Arizona
T h e past two years have seen developments in the instrumentation and techniques of electron microscopy which, while not fundamentally new, provide important advances in the power of the instruments as research tools in many areas of science. Since previous reviews in this series have tended to emphasize advances relevant to biological applications, this review will be written more from the viewpoint of users in the materials sciences and will include some reference to achievements previous to this two-year period. The overlap with t h e interests of biologists is still considerable.
INSTRUMENTATION CTEM Instruments. The new generation of commercial 100-ke\' high resolution transmission electron microscopes offers several important improvements in performance. \lost notable, for many purposes, is the improved vacuum. \Vhereas pre\iously the yacuum in the specimen region was rarely much betrer than 10 Torr, it is now possible to achieve better than IO-.Torr with the aid of additional, clean pumping systems. l h e incentive for this development was, in part, the need for good vacuum in the microscope column which would not degrade the very much better vacuum in the gun chamber needed for the operation of field emission guns. However. the lower gas pressure in the specimen area should reduce greatly the contamination of the specimen which occuri under electron irradiation. For the imaging of thin specimens with atomic-level resolution, this will eliminate much of the unwanted background contrast (noise) of the images. It will allow more convenient use of the microdiffraction methods employing very fine electron beams [see below). It will allow the surface structure of crystals to be imaged with greatly improved clarity and convenience. Improvements in stability of the high voltage supplies and lens currents. together with the development of special high resolution pole-pieces for the lenses (with the spherical aberration constant as low as C, = 0.: mm in m e case) have allowed better resolution, as demonstrated by several groups in Japan. Hashimoto et al. (18) have publiihed pictures ( i t ' thin gold crystals in [lo01 orientation showing clearly the intensity maxima corresponding to the projected ro\vs of gold
8528 7
atoms 2 A apart and also showing intensity variations on a scale of about 0.5 8, within these maxima. Although these details are not directly interpretable in terms of specimen structure they demonstrate a major advance in instrument performance. Izui e t al. (29) have obtained images showing projections of thin silicon single crystals in the [110] orientation with well-resolved spots corresponding to silicon atom positions 1.36 A apart. It must not be assumed t h a t these images represent a resolution, in any meaningful sense of the term, of 0.5 A or 1.36 A. Such images can be obtained only because the specimens are periodic in two dimensions and have relatively small unit cell periodicities. With the same microscopes, the resolution, defined as the least distance between recognizable structural units of the specimen in nonperiodic array which can be clearly distinguished, is more like 2.6 8, or probably 3.0 to 3.5 8, with a goniometer specimen stage. Since for most electron microscopy of crystalline materials a goniometer stage is essential, it is this latter figure which represents t h e current microscope performance level for most applications in materials science. High Voltage Electron Microscopes. Resolution beyond these limits has been achieved only with microscopes using high voltages. Theoretical estimates have suggested that if the necessary levels of electrical and mechanical stability can be maintained and if the spherical aberration constant of the objective lens can be kept to within a few millimeters, the resolution should be improved by a factor of 2 or 3 if the voltage is increased from 100 keV to 1 MeV. The contrast of individual small objects and the thickness of specimens which can conveniently be used should also be improved by much the same factor ( 5 ) . Resolution approaching 2 A has already been demonstrated with the dedicated high resolution, high voltage microscopes in Japan. Apart from the Japanese efforts, special high resolution high voltage microscopes are being built in England and Germany (the latter with superconducting lenses). A proposal has been made that such an instrument should be imported for a projected high resolution microscopy research center in Berkeley, Calif. For other than high resolution applications, a n increasing number of operational high voltage microscopes is providing
ANALYTICAL CHEMISTRY, VOL. 50,
John M. Cawley is Galvin Professor of Physics ^E at Arizona State University He was born in Australia and received his B Sc degree at the University of Adelaide in 1942 and his M SC and D.Sc degrees at the same University in 1945 and 1957 He attended M I 1 from 1947 to 1949 and obtained his Ph D degree there From 1945 to 1962 he worked in the Chemical Physics Division of the Commonwealth Scientific and Industrial Research Organization in Melbourne. then became Protessor of Physics at the Unrvsrsity of Melbourne in 1963 In 1970 he moved to this country to take his present poshon He has been a Director of the Electron Microscopy Society of America and a member of the Executive Committee of the International Union of Crystallography. I
a wealth of significant results on the studies of defects in thick crystals, on the production and examination of radiation damage in simulation of reactor irradiation effects, and for the three-dimensional study of thick biological specimens by use of stereojcopic methods. The reports of the International High Voltage Electron Microscope Symposia a t Toulouse (33) a n d Kyoto (27) summarize the progress in these areas. T h e progression of microscopes produced with higher and higher accelerating voltages have stopped with the 3-MeV machines a t Toulouse and Osaka, except for a German program to build a 5-MeV microscope with a new type of field-emission gun and a superconducting linear accelerator as electron source and possibly superconducting lenses (13). STEM Instruments. The performance and versatility of scanning transmission electron microscopy (STEM) instruments continues to improve with two commercial manufacturers (VG Microscopes in England and Siemens in Germany) now active. The VG microscope has produced crystal lattice fringes with 1.4-A spacing. The resolution of these instruments is now comparable with that of the 100-keV conventional-type (CTEM) transmission instruments. While t h e first S T E M instruments, following the initial designs of Crewe and colleagues (12) were designed and used for biophysical applications, the possibility of applications in materials science have now been realized, some of the relevant theory has been developed (6,22) and appropriate instrument modifications have been planned ( 7 , 46). Currently most materials science applications of S T E M are made using attachments to CTEM instruments which may be more limited in their high-resolution capabilities but which combine medium-resolution performance with the convenience and versatility for diffraction observation and specimen treatment which result from the many years of commercial development of C T E M technology. The use of the S T E M mode a t high voltages offers a number of potential advantages. For very thick specimens, t h e “penetration” (the resolution or contrast for a given thickness) is several times better than for CTEM instruments of the same voltage. This has been confirmed experimentally by Strojnik (40) using the S T E M machine which he built a t Arizona State University a t 500 keV (lo-A resolution) and by various observers using S T E M attachments to high voltage CTEM machines. Efforts to build 1-MeV STEM instruments for ultra-high resolution are proceeding slowly in Chicago and Toulouse.
ANALYTICAL ELECTRON MICROSCOPY Increasing emphasis is being placed on the combination within one instrument, whether CTEM or STEM, of a variety of imaging, diffraction, and analytical modes. These last include x-ray microprobe analysis using energy-dispersive spectroscopy and electron energy loss spectroscopy applied to the transmitted electron beams. The value and convenience of correlating the chemical information from these analytical techniques with t h e more usual morphological and crystallographic information on very small specimen regions has led to the introduction of the concept of Analytical Electron Microscopy (AEM). New microscopes have been designed as AEM instruments and an increasing number of AEM symposia and workshops are being held and planned (30). Microprobe Analysis. The electron probe used for STEM imaging in either dedicated S T E M instruments or CTEM
NO. 5, APRIL 1978
77R
attachments will generate characteristic x-rays which can be detected and analyzed to identify the elements of the sample, as in the pure microprobe instruments. Display of the detected signal as the incident beam scans the specimen can give the distribution of particular elements in the sample. Because the signals are weak, it is necessary to use relatively large beam diameters with high beam currents and detectors of high efficiency, namely t h e energy dispersive, semiconductor detector elements. For thin transmission samples, the lateral spread of the electron beam is much less than for bulk specimens and analysis can be made of regions having diameters of hundreds of Angstroms rather than micrometers (31).
Electron Energy Loss Analysis. The provision of a simple magnetic sector energy analyzer, applied t o the transmitted beam in the STEM mode, or a more complicated two-dimensionally focusing analyzer for CTEM images, offers the possibility of forming images with electrons which have produced the characteristic inner-shell electron excitations of specimen atoms; or spot analyses of chosen specimen regions may be performed. This analytical technique is complementary to the x-ray microprobe analysis in that it has greater sensitivity for the lighter elements. Current estimates suggest (38) that as little as 2 X g of carbon, for example, may be detected in this way. The detection of the -3000 iron atoms within a single molecule of the iron-storage protein ferritin has become a common test of sensitivity of the method. As with x-ray microprobe analysis, considerable refinement of the technique is necessary to improve the reliability and sensitivity. Work by Jouffrey et al. (32) suggests that the use of high voltages of the incident electrons offers considerable advantages, allowing improved signal-to-noise ratios, improved sDatial resolution and so a further advance toward the ultimate goal of “single-atom chemistry“. Microdiffraction. The introduction of STEM techniaues has provided a major revolution in the use of electron diffraction as a complementary tool for imaging of crystals and their defects. In the conventional selected area diffraction technique of CTEM, the minimum areas giving diffraction patterns have diameters of 0.5 Fm or more. If t h e beam in a S T E M instrument is held stationary, a diffraction pattern will be produced from a n area of diameter comparable to the microscope resolution Le., 5-10 A in principle. In practice microdiffraction from areas with diameters of about 20 A has been reported ( 4 ) . By use of incident beam scanning techniques, microdiffraction from regions of similar diameter can be achieved in CTEM instruments also (14). The diffraction patterns produced are convergent-beam diffraction patterns, such as have been explored for years by Goodman and coworkers and used for the absolute symmetry determination of crystal structures ( 1 5 ) ,and the highly accurate determination of potential fields in crystals (If?), as well as for t h e simple identification of phases and the detection of faults and disorder (17). The use of the microdiffraction pattern intensities as a means for extending the resolution of S T E M imaging and for overcoming the limitations of radiation damage has been suggested by Cowley (8). Applications of AEM. Investigations which depend for their success on the correlation of results obtained from the various analytical, imaging, and diffraction modes possible with a single AEM microscope are becoming increasingly common in metallurgy ( 3 ) ,mineralogy (35),and other areas of materials science. One application of particular topical interest is t h a t of distinguishing chrysotile and amphibole asbestos minerals. This may best be done by microanalysis on the individual single-crystal fibers which have dimensions of 0.1 to 1 fim. The distinction has legal connotations in relation to anti-pollution legislation because of the alleged carcinogenic properties of the amphiboles (37). HIGH RESOLUTION IMAGING OF CRYSTALS Lattice Fringe Images. Observations of fringes indicating periodicities of crystal lattices of less than 1 A have Seen used as a test of performance (not resolution) of electron microscopes. The realization t h a t these fringe images cannot be interpreted directly in terms of atom positions has tended to discourage electron microscopists from making use of the information which may legitimately be derived from them. However, under properly controlled conditions of specimen thickness and orientation, taking account of the perturbations
78R
ANALYTICAL CHEMISTRY, VOL. 50,
NO.5,
APRIL 1978
of fringe spacing and contrast which may arise from dynamical scattering effects, fringe observations can be used to gain important information on variations and perturbations of crystal structure over distances of 5-10 A or more. Interesting applications have been made to the ordering and spinodal decomposition phenomena in alloys (41) and related phenomena in minerals (43). Crystal Structure Images. Over the past seven years, the techniques have been developed, first by Iijima at Arizona State University and more recently also a t a number of laboratories around the world, for the direct imaging of atom positions in thin crystals. Under well defined conditions of crystal orientation and thickness and objective lens defocus, the images give direct representations of projections of the crystal structure and the form of defects with a resolution which has been about 3.5 A but is now improved (9, 10). Among the many applications which have been made of this technique are the direct determination of the crystal structures of unknown phases, the detection and characterization of linear and planar defects in complex oxide and sulfide phases in terms of metal atom positions, inrestigations of the nature of short-range order and ordering prccesses in oxides (23),the investigations of the structures and structural perturbations of minerals (24, 44) and the study of superlattices in alloys (20). The present level of resolution with 100-keV electron microscopes limits the technique to materials in which heavy atoms, or characteristic clumps of atoms, are separated by 3.5 8, or more in projection. For the many metals, semiconductors, and inorganic materials which have typical atom separations of 1.5 to 2 A, the information obtainable is strictly limited. The resolution necessary to extend the atom resolution techniques to these materials will in principle be given by the high voltage, high resolution microscopes now under construction or being planned. Already resolution approaching 2 8, has been achieved with the dedicated high resolution 500-keV microscope a t the University of Kyoto (42)and with the 1-MeV microscopes a t the National Institute for Research in Inorganic Materials, Saka-mura (21) and a t the Tohoku University, Sendai (20). With the first of these microscopes, clear images have been obtained showing the details of the atom arrangements in a grain boundary in a thin crystal of germanium (34). Computer Simulation of Crystal Images. The scattering of electrons in crystals is complicated by dynamical scattering effects involving the coherent interactions of a large number of diffracted beams. Although an image which seems to be directly interpretable in terms of atom positions can be obtained in some cases, image interpretations can be made with confidence only if the image calculated on the basis of the proposed atom configuration gives good agreement with the observations. The calculations can be made using a moderate size digital computer with programs based on either the matrix formulation or the multi-slice formulation of the many beam dynamical theory (11). The latter formulation is more convenient for structures with large unit cells for which the coherent interactions of 5 W O O beams must be considered and for the calculation of images of nonperiodic objects, such as crystal defects, for which up to 12000 beams have been used. The available programs allow the images predicted for any combination of specimen parameters and instrumental conditions (objective lens aberrations and defocus) to be plotted in displays convenient for comparison with observed images. For known structures, the agreement is excellent (39).
SINGLE ATOMS A N D SURFACES It has been established for a number of years that individual heavy atoms, suitably isolated from other strongly scattering material, can be imaged as distinct black or white dots by use of either STEM or CTEM. More recently attempts have been made to use this ability in studies on the configurations of heavy atoms attached to biologically significant macromolecules (47),the aggregation of heavy atoms to form microcrystals (19),and the interactions of heavy atoms with the surfaces of crystals or other substrates. Isaacson et al. (28) have used extended time-lapse series of images of uranium atoms moving on carbon supports to detect preferred associations of the atoms with each other and with surface steps. Iijima (25)has imaged clearly the association of single tungsten
atoms with single-atom high steps on thin graphite single crystal surfaces. He has shown that holes in the surface layers of graphite formed by the removal of 4 to 10 carbon atoms can be detected (26). Monolayers or less of absorbed molecules on single crystal surfaces have been studied with CTEM imaging by Moodie and Warble (36) and by Venables et al. (46) using transmission electron diffraction techniques at low temperature. Atl these results refer to specially favorable cases in which it has been possible to overcome the difficulties of obtaining extremely clean surface conditions with a minimum of interfering sources of image contrast. However, current improvements in instrument resolution and vacuum will greatly enhance the potential of transmission electron microscopy for research on surface structure and surface reactions. This should be of extreme value as the only technique by which diffraction information can be combined with atom-level resolution of surface detail.
ELECTRON DIFFRACTION The first observations of the diffraction of electrons from solids were made in 1927 by Davisson and Germer (Bell Laboratories) using low energy electrons and by Thomson and Reid (Aberdeen) using high energy electrons in transmission. During 1977, a series of symposia and conferences have been held to mark the 50th anniversary of the discovery and to survey the present status of the subject. The reader is referred to the collections of review and contributed papers in the published proceedings of the symposia held a t the meetings of the American Crystallographic Association in February (2) and the Electron Microscopy Society of America in August (1)and in the forthcoming publication of the proceedings of the conference “50 Years of Electron Diffraction” sponsored by the Institute of Physics in London in September. ELECTRON MICROSCOPY NOMENCLATURE ACRONYMS - - -_ AEM Analytical electron microscopy: the combination of CTEM and/or S T E M with EDS, EELS, and microdiffraction, preferably in one instrument. AES Auger electron spectroscopy: the use of Auger electrons, emitted when an electron from an inner electron shell of a specimen atom is excited by incident radiation, for the identification of atomic species. Bright-field electron microscopy in which the BFEM forward scattered portion of the incident beam contributes to the images so that, with no specimen, the image is uniformly bright. Convergent beam electron diffraction, in which the CBED incident beam is focused on or near the specimen with a finite angle of convergence so that each spot of the diffraction pattern from a crystal is spread into a disk. CTEM Conventional transmission electron microscopy: used to distinguish the more established fixed-beam form of T E M from STEM. Contrast transfer function; the function repreCTF senting the modification of the Fourier coefficients of an image intensity distribution as a result of the aberrations and angular aperture of a lens. DFEM Dark field electron microscopy in which the forward scattered, transmitted part of the incident beam is prevented from contributing to the image so that for no specimen the image is dark. Electron channelling pattern: the pattern of lines ECP and bands obtained in SEM when the scanning system is used to vary the angle of incidence of the electron beam on a sinele-crvstal mecimen. ED Electron diffraction: thgscattering of electrons into preferred directions, as a result of the electron interactions with nonrandom atom arrangements in the specimen. Energy dispersive spectroscopy: analysis of a small EDS region of a specimen by detection of the characteristic x-rays from the specimen elements, making use of semiconductor or other detector elements which respond differently to different photon energies. Electron energy-lossspectroscopy: analysis of small EELS regions of thin specimens by recording of the
ANALYTICAL CHEMISTRY, VOL. 50, NO.
variation of the numbers of transmitted electrons which have lost particular amounts of energy in the neighborhood of t h e energies required to excite inner-shell electrons of the specimen atoms. FIM Field ion microscopy: microscopy of t h e atom configurations on the surface of a sharply pointed metal or other tip when the image is formed by ions accelerated by a strong electric field near the tip surface. HEED High energy electron diffraction: diffraction of electrons by transmission or reflection from specimens of electrons with energies in the range of 20 to 100 keV or more. H R E M High resolution electron microscopy. HVEM High voltage electron microscopy, usually with accelerating voltages of greater than about 250 kV. ILEED Inelastic low energy electron diffraction; LEED with energy analysis of the diffracted electrons to allow t h e study of inelastic scattering processes. LEED Low energy electron diffraction; diffraction from bulk solid surfaces of electrons with energies in the range 10 t o 200 eV with near-normal incidence: used for the study of surface structure. M E E D Medium energy electron diffraction: diffraction from bulk solid surfaces of electrons with energies in the range of approximately 500 t o 10000 eV. PCDA Projected charge density approximation, which states that for thin specimens the bright field image contrast is proportional to the amount of defocus and t o t h e projected value of the charge density. POA Phase object approximation, in which the effect of a very thin specimen on an incident electron wave is that of a two-dimensional phase object, changing t h e phase b u t not t h e amplitude of the wave. R H E E D Reflection high energy electron diffraction: HEED by reflection from a flat solid surface with a glancing angle of incidence. SAED Selected area electron diffraction: E D of a region of a T E M specimen selected by use of an aperture in the image plane of t h e objective lens. SEM Scanning electron microscopy, referring usually to microscopy in which the incident electron beam of 1 to 30 keV energy is scanned over a solid surface and t h e image signal is derived from either low energy secondary electrons or back-scattered electrons. STEM Scanning transmission electron microscopy in which an SEM system is used with a thin specimen and transmitted electrons are collected to produce t h e image signal, with applications as for CTEM. TEM Transmission electron microscopy; usually refers to CTEM but may be used collectively for CTEM and STEM. TF Transfer function: the function which multiplies the Fourier transform of an object transmission function t o represent the effects on the image of lens aberrations and aperture limitations. U'POA Weak phase object approximation; the POA for weakly scattering objects in which it is assumed that the phase change is very much less than unity. ZAP Zone axis pattern; a symmetrical pattern of extinction contours seen in the T E M image of a thin uniformly bent single crystal around the point where the incident beam is parallel to a zone axis.
OTHER TERMS Absorption: the loss of electrons from the particular signal being considered or recorded, e.g. the loss from sharp Bragg diffraction peaks due t o diffuse scattering. Bloch Wave: a wave function, having the periodicity of the lattice, which is a solution of the wave equation for an electron wave in an infinite periodic crystal. Chromatic Aberration: the variation of the focal length of a lens with the energy (and hence wavelength) of the incident electrons: measured by a chromatic aberration constant C,. Contrast: t h e relative variation of intensity in a n image: measured quantitatively in terms of the maximum intensity I,, and minimum intensity I,, for an image feature, often
5, APRIL 1978
79 R
in terms of the visibility, (I,,, - 1,in)/.(Imax + Imin). Excitation Error: the distance in reciprocal space of a reciprocal lattice point from the Ewald Sphere. Extinction Length: the periodicity in t h e direction of the incident beam of the diffracted beam intensities in a crystal for those special cases in which such periodicity occurs. Kinematical Theory: an approximation to ED theory, useful in the limit of weakly scattering objects, in which only single scattering processes are included (equivalent of the First Born Approximation). Microdiffraction: E D from specimen regions much smaller than is customary; currently refers to E D from regions less than about 0.1 fim in diameter. R -Beam Dynamical Theory: t h e theory of electron scattering by crystals which includes the coherent interaction of a large number of diffracted waves (beams): as distinct from the simple 2-beam approximation commonly used. Resolution: In normal usage, a n abbreviation for "least resolvable distance". I t may refer to the smallest distance between two intensity maxima or minima in an image which may be observed reproducibly ("instrumental resolution") or it may refer to the smallest distance between two small objects within t h e specimen which may be recognized as distinct ("interpretable resolution"). Spherical Aberration: the variation of focal length of a lens with the angle between a beam and the lens axis. Measured by a spherical aberration constant C,.
UNITS A: Angstrom unit, m: commonly used, convenient unit: not an SI unit but officially endorsed by the International Union of Crystallography. ym: micrometer, m: SI unit, to be used in place of micron. nm: nanometer, IO-' m: SI unit, increasingly used in electron microscopy. LITERATURE CITED (1) Bailey, G. W., Ed., Proceedings of 35th Annual Meeting, Electron Microscopy Society of America, Claitor's Publ. Div., Baton Rouge. La., 1977. (2) Brockway, L. O.,Ed., "Transactions of the American Crystallographic Association", Vol. 13, American Crystallographic Association, New York, N.Y., 1977. (3) Carpenter, R. W., Bentley, J., and Kenik, E. A,, in "Scanning Electron Microscopy/l977", Vol. 1, Om Johari, Ed., IIT Res. Inst., Chicago, IN., 1977, p 411. (4) Chevalier, J-P. A. A., and Craven, A. J., Phil. Mag., 36, 67 (1977). (5) Cowley, J. M., Ref. 33, p 129 (1975). (6) Cowley, J. M., Ultramicroscopy, 2, 3 (1976). (7) Cowley, J. M., in Ref. 27 (1978). ( 8 ) Cowley, J. M., Ultramicroscopy, 1, 255 (1976). (9) Cowley, J. M., and Iijima, Sumio, Phys. Today, March 1977, 30, 32 (1977). (10) Cowley, J. M., Annu. Rev. Mater. Sci., 6, 53 (1976). (11) Cowley, J. M., "Diffraction Physics", North Holland Publ. Co., Amsterdam, 1975. (12) Crewe, A. V., and Wall, J., J . Mol. B i d , 46, 375 (1970). (13) Dietrich, I., Herrmann, K.-H., and Passow, C., Optik, 42, 439 (1975). (14) Geiss, R., in "Scanning Electron Microscopy/l976", Vol. l . , Om Johari, Ed., IIT Res. Inst., Chicago, Ill,, 1976, p 337. (151 Goodman, P., Acta Crystallogr., Sect. A , , 31, 804 (1975). (16) Goodman, P., and Lehmpfuhl, G., Acta Crystallogr.. Sect. A , , 24, 339 (1968). (17) Goodman, P.. Acta Crystallogr., Sect. A , , 32, 793 (1976). (18) Hashimoto, H., Endoh, H., Tanji, T., Ono, A., Watanabe. E., J . Phys. SOC. Jpn., 42, 1073 (1977). (19) Hashimoto, H., Kumao, A., Hino, K., Endoh, H., Yotsumoto, H., and Ono, A . , J , Nectron Microsc., 22, 123 (1973). (20) Hiraga, K., Hirabayashi, M., and Shindo, D., in Ref. 17, (1978). (21) Horiuchi, S., Matsui, Y., and Bando, Y. Jpn J . Appl. Phys., 15, 2483 (1976); see also paper in Ref. 27. (22) Humphreys, C. J., and Drummond, R. A., in "Electron Microscopy 1976", Vol. 1, D. G. Brandon, Ed., TAL Internat. Publ. Co., 1976, p 176. (23) Iijima, Sumio, and Cowley, J. M., J . Phys. (Paris), in press, 1978. (24) Iijima, S., and Buseck, P. R., A m . Mineral., 60, 758 (1975). (25) Iijima, W.. Optik, 47, 437 (1977). (26) Iijima, S., Optik, 48, 193 (1977). (27) Imura, T., Hashimoto, H., Eds., "High Voltage Electron Microscopy 1977". Japanese Society of Electron Microscopy, Kyoto, 1978. (28) Isaacson, M. S.. Langmore, J., Parker, W. W.. Kopf, D., and Utlaut, M., Ultramicroscopy, 1, 359 (1976). (29) Izui, K., Furuno, S.. and Otso, H., J . Nectron Microsc., 26, in press. (30) Johari, Om, Ed., "Scanning Electron Microscopy/l978", in press. (31) Joy, D. C., and Maher, D. M.. in "Scanning Electron Microscopy/l977", Vol. l . , Om Johari, Ed., IIT Res. Inst.. Chicago, Ill., 1977, p 325. (32) Jouffrey, B., Kihn, Y. PBrez, J. Ph., Sevely, J., and Zanchi, G. in Ref. 27, in press, 1978. (33) Jouffrey, B., and Favard, P., Ed., "Microscopie ElectroniqueB Haute Tension 1975", Societe Francaise de Microscopie Eiectronique, Paris, 1976.
80R
ANALYTICAL CHEMISTRY, VOL. 50, NO. 5, APRIL 1978
(34) Krivanek, 0. L., Isoda, S., and Kobayashi, K., Phil. Mag., 36, 931 (1977). (35) Lorimer, G. W., and Cliff, G., in "Electron Microscopy in Mineralogy", H.-R. Wenk, Ed., Springer-Verlag, Heidelberg, 1976, p 506. (36) Moodie, A. F., and Warble, C. E., in "Electron Microscopy 1974", J. V. Sanders and D. G. Goodchild, Ed., Australian Acad. Sci., Canberra 1974, p 230. (37) Pooley. F. D., Philos. Trans. R . S . London, Ser. A , . 286, 625 (1977). (38) Silcox, J., in "Scanning Electron Microscopy/l977", Vol 1. Om Johari, Ed., ITT Res. Inst., Chicago, Ill., 1977, p 393. (39) Skarnulis, A. J., Iijima, S., and Cowley, J. M., Acta Crystallogr., Sect. A , , 32, 799 (1976). (40) Strojnik, A,, in Ref. 33, p 27 (1976). (41) Thomas, G., Sinciair, R., and Gronsky, R., in "Electron Microscopy 1976", Vol. l . , D. G. Brandon, Ed., TAL Internat. Publ. Co., 1976, p 114.
(42) Uyeda, N., Ishizuka, K., Saito, Y . , Murata, Y . , Kobayashi, K., and Ohara, M. In "Electron Microscopy 1974", J. V. Sanders and D. G.Goodchild, Ed., Australian Acad. Sci., Canberra, 1974, p 266. (43) Van Landuyt, J., and Amelinckx, S., Am. Mineral., 60, 351 (1975). (44) Veblen, D. R., Buseck, P. R., and Burnham, C. W., Science, 198, 359 (1977). (45) Venables, J. A.. Janssen, A. P., and Harland, C. J., in Proceedingsof "5Mh Anniversary of the Discovery of Electron Diffraction" to be published by the Institute of Physics, London, 1978. (46) Venables, J. A., Kramer, H. M., and Price, G. L., Surf Sci., 55, 373 (1976). (47) Wiggins, J. W., Beer, M., Rose, S. D., Cole, M., Waldrop, A. A,, Zubin, J., Phtner, J. W., Marzilll, L., Chang, C. H., and Kapili, L., in "Scanning Electron Microscopy/l976", Pari I, Om Johari, Ed., IIT Res. Inst., Chicago, Ill., 1976, p 295.
Nucleonics W. S. Lyon" and
H. H. Ross
Analytical Chemistry Division, Oak Ridge National Laboratory,
' Oak Ridge,
Singular events mark the two-year period (November 1975-November 1977) covered by this report. ERDA, brought into existence in early 1975 when the old AEC fissioned into ERDA and NRC, exhibited a half-life so short that no trace of it remained scarcely two years after conception, but arising in its place as a Phoenix-like fusion product incorporating a number of agencies appeared the DOE. During ERDA's short life-time, we witnessed t h e death of Nuclear Science Abstracts, singularly useful and now sorely missed. A number of alphabetical nuclear projects (LMFBR, MSR, CRBR) died by the axe, and several others (HTGR, LWR) appear to be mortally wounded. Doubly singular has been the demonstration of one atom detection by two separate groups; the work qualifies as nuclear since proportional counting was used in one ( I ) and the other involves measurement of radioactive species (2). T h e gargantuan effort t o detect solar neutrinos by the capture reaction 37Cl + u = 37Ar + e- has given essentially negative results ( 3 ) ,and a new proposal has been made to use mass spectrometric assay of 1.6 X lo7 year "'Pb produced by another reaction ( 4 ) . A singular event that apparently occured about 20 years ago in the southern Urals of t h e USSR was just recently made generally known to the scientific community; we refer, of course, to the disasterous nuclear explosion t h a t seems to have involved only nuclear waste ( 5 ) . Contaminated birds from this area have been used to study migration patterns (6) and the entire episode has afforded an opportunity for considerable scientific sleuthing. Amongst all these bureau-busting, project-killing, atomfinding, particle-seeking and earth-shaking occurrences, several quieter events stand out: some genuine innovations and promising techniques using large machines (mentioned below), and the great increase in conference papers, patents, and journal articles from Japan. Unfortunately, many of these are in their incomprehensible language, but those that appear in English are indicative of quite excellent work. Indeed, a new radiochemical sun may be rising, and remembering how easily nuclear power has been accepted in Japan as compared to our country, perhaps we may be permitted to change one word of the opening lines of Richard I11 and say:
"Now is the winter of our discontent Made glorious summer by this sun of Nippon". Few Japanese papers are referenced here because they do not meet our criteria of easy availability and common language. B u t the abstracts and the few English translations we have seen whet our appetite. Perhaps the next two years will see more Japanese work appearing in American journals, or a t least in widely distributed English language Japanese journals. Oak Ridge National Laboratory is operated by Union Carbide Corporation under contract with the US.Department of Energy. This paper not subject to U S Copyright
Tennessee 37830
And speaking of journals, we should forget neither the encyclopedic Nuclear Data and the Nuclear Data Sheets which bring decay scheme data up to date nor the biennial Applications Reviews issue of Analytical Chemistry which references many applications of nuclear techniques. Throughout this report are listed tables of new books, reviews, bibliographies, and conference proceedings (Tables I-IV). The latter are not as complete as we would wish because of the often-experienced long delay time between oral presentation and proceedings publication. Table I11 indicates those proceedings t h a t are already available as well as those to watch for. By referring to these tables, one can find information on almost any facet of nucleonics. Our brief writeup below, therefore, emphasizes only the new, the interesting, or occasionally, the peculiar. Detectors. Developments in radiation detectors continue to be one of t h e more innovative aspects of this review. A particularly intensive development effort is being directed toward the use of solid state detector configurations, and, although many are called, few are chosen; several devices that have been studied appear to be only marginally suitable for practical application. Most of the success emerges in the detection of x-rays. A high resolution detector has been fabricated from a 100-g single crystal of HgIz (7). Vapor phase growth in a vertical furnace appears to be the key to the large crystal size. The 1.5-KeV A1 x-ray was resolved a t room temperature with the prototype crystal. A semiconductor that is widely used as a video detector (CCD; charge coupled device) has also been tested for use with soft x-rays (8). Here the objective is spatial resolution in two dimensions. Another position sensitive x-ray detector has been described (9) that uses LC (inductance, capacitance) rather than RC (resistance, capacitance) encoding. The particular feature of the detector is that high radiation levels can be tolerated without damage to the device. A new imaging optical detector called a "photicon" has been noted (10). Single photon events produce a digital word that defines the position of the event. Spatial resolution of 20 pm has already been achieved in a 25-mm diameter device. Another paper describes a new photomultiplier tube (11) that employs a n electron-multiplier channel-plate. The tube features high gain, low transit time and jitter, and a very low sensitivity to magnetic fields. Practical counting systems frequently require some fancy electronic footwork to discriminate against unwanted radiation. A relatively straightforward system has been described (12) for Compton suppression in a low-level y-counting unit. An attractive feature is t h a t all sub-systems of the counter are commercially available. We also note a method for high-energy 0 counting that discriminates against y radiation (13). Another way to avoid y response in p assays is to use a Cerenkov counter. A new Cerenkov counting vial design has been developed that permits the use of a wave-shifter in
Published 1978 by the American Chemical Society
