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Contrast of electron micrographs
130
Classified Abstracts 239--246
distortion at higher temperatures. Recent work has shown that iodine vapour under specific conditions of concentration and temperature combines with evaporated tungsten to form tungsten iodide which, at the temperature of the filament, decomposes into tungsten and iodine vapour. This therefore provides a method by which the operating temperature of the filament may be increased without decreasing its life, and lamps of this type are under development. T.L. E. F. Edwards, New Scientist, 11 (253), 21 Sept. 1961, 730-732.
Electrode contamination and arc formation. 32.
31 : 18 See Abstr. No. 173.
Nucleonics
32 239. A 60 ° sector type electromagnetic isotope separator. Italy. The paper describes a 60 ° sector type electromagnetic isotope separator. The separator produces ion current of the order of some milliamperes, in the range of light and intermediate masses. Some results of a few experimental separations, carried out by using zinc and magnesium, are also reported. (Authors) F. Bisi, A. Cattoni, B. de Mechelis and P. Piechele, Nuovo Cimento Suppl., 21 (1), 1961, 39-47.
33.
G e n e r a l P h y s i c s and E l e c t r o n i c s
33 240. Numerical investigation of a range of unipotential electron lenses. Great Britain. An electronic digital computer has been applied to the computation of focal lengths and spherical aberration constants of unipotential lenses, utilizing field data obtained from a resistance network analogue. Results obtained are presented in graphical form and shown to be in good agreement with published experimental results, but not, in the ease of spherical aberration constants, with results based on analytical approximation to the axial potential distributions. The sources of error in the computations are discussed, and the accuracy of the results estimated. (Author) J. Vine, Brit. J. Appl. Phys., 11 (9), Sept. 1960, 408-411. 33 241. Emission of negative ions of oxygen during the activation of oxide-coated cathodes. Great Britain. Oxide-coated cathodes have been used as ion sources in a simple mass spectrometer and have been found to emit atomic negative ions of oxygen during their activation at high temperatures (1150-1275°K). A retarding potential at the collector was used to separate the ions emitted by the cathode from the ions formed in the residual gas. Most of the oxygen ions from the cathode arrived at the collector with more energy than they could have obtained from the potential difference across the electron gun, and their number increased as the cathode became more active. The evidence suggests that the ions were formed by dissociation of the oxide coating, then diffused to the surface and were removed by positive ion bombardment. Such a process would leave oxygen vacancies in the oxide which would act as electron donors and increase its electron emission. (Author) N. A. Surplice, Brit. J. Appl. Phys., 11 (9), Sept. 1960, 430-433. 33 242. Contrast of electron micrographs. Great Britain. The variations of contrast in electron micrographs of amorphous films, with film thickness, electron accelerating voltage and objective aperture angle, are discussed ; elastic and inelastic electron scattering and the possibility of plural scattering are taken into account. Experimental results, obtained with vacuum-operated iron films, confirm a formula of the type
first proposed by Hall. However, the contrast is about four times smaller than predicted, using scattering cross-sections based on Lenz's data. Results for other materials, obtained by previous workers, indicate similar discrepancies. At present, the likeliest explanation appears to be a theoretical underestimation of the scattering into angles smaller than 10 -a tad. The limitations of contrast measurements for assessing specimen thickness or constitution are discussed, and a new criterion for replica and shadowing materials suggested. (Authors) J. S. Halliday and T. F. J. Quinn, Brit. J. Appl. Phys., 11 (11), Nov. 1960, 486-491. 33 243. Selected-area diffraction in the electron microscope: Great Britain. A direct demonstration is given of the error in the area contributing to the recorded diffraction pattern, which arises from use of the incorrect objective focal length in microdiffraction experiments in the electron microscope. The effect of errors in the final intermediate lens setting is also discussed, and reproducibility of the camera constant AL in selected area diffraction is investigated. Agar has emphasized that a certain operational procedure has to be closely followed, if a selected area diffraction pattern in a three-stage electron microscope is to come solely from the area defined by the selector aperture under magnified image conditions. It is the purpose of this paper to demonstrate directly the contributions which may occur from an annulus outside the image of the aperture, due to incorrect excitation of the objective lens, and to comment on another important aspect of selected area diffraction--the reproducibility of AL. (Author) R. Phillips, Brit. J. Appl. Phys., 11 (i1), Nov. 1960, 504-506. 33 244. Fifth order spherical aberration of magnetic lenses. Great Britain. Knowledge of the fifth order spherical aberration of magnetic lenses is required in order to show how much error is left when third order aberration is corrected. This is found theoretically for lenses of the bell-shaped field type. It is found that, for lenses of moderate strength, third and fifth order coefficients are numerically very similar. (Author) G. D. Archard, Brit. J. Appl. Phys., 11 (11), Nov. 1960, 521-522. 33 245. Heating of metallic foils in an electron microscope. Great Britain. The temperature of a thin metal foil illuminated by the high intensity electron beam of the modern microscope may at the centre of illumination reach a temperature above that of the melting point of iron. An attempt has been made to measure and calculate the temperature distribution for any given metal under various illumination conditions and to determine also the stress fields produced in the foil by such temperature distribution. (Authors) B. Gale and K. F. Hale, Brit. J. Appl. Phys., 12 (3), March 1961, 115-117. 33 : 57 246. The investigation of surface deformation. Great Britain. The methods for studying surface deformation used in the National Engineering Laboratory are described. Deformed metal surfaces show an enhanced photoelectric emission which is studied with the aid of open-ended GeigerMtiller counters or electron multipliers. Oxide films stripped from deformed metal surfaces are studied by electron-microscopy and changes in chemical reactivity by radioactive tracers. Surface topography is an important factor in lubrication and wear research and is being studied with the aid of interferometry, phase contrast and electron microscopy. Replica techniques are often applied with advantage. (Authors) L. Grunberg, D. Scott and K. H. R. Wright, Brit. J. Appl. Phys., 12 (4), April 1961, 134-140.
Classified Abstracts 239--246
distortion at higher temperatures. Recent work has shown that iodine vapour under specific conditions of concentration and temperature combines with evaporated tungsten to form tungsten iodide which, at the temperature of the filament, decomposes into tungsten and iodine vapour. This therefore provides a method by which the operating temperature of the filament may be increased without decreasing its life, and lamps of this type are under development. T.L. E. F. Edwards, New Scientist, 11 (253), 21 Sept. 1961, 730-732.
Electrode contamination and arc formation. 32.
31 : 18 See Abstr. No. 173.
Nucleonics
32 239. A 60 ° sector type electromagnetic isotope separator. Italy. The paper describes a 60 ° sector type electromagnetic isotope separator. The separator produces ion current of the order of some milliamperes, in the range of light and intermediate masses. Some results of a few experimental separations, carried out by using zinc and magnesium, are also reported. (Authors) F. Bisi, A. Cattoni, B. de Mechelis and P. Piechele, Nuovo Cimento Suppl., 21 (1), 1961, 39-47.
33.
G e n e r a l P h y s i c s and E l e c t r o n i c s
33 240. Numerical investigation of a range of unipotential electron lenses. Great Britain. An electronic digital computer has been applied to the computation of focal lengths and spherical aberration constants of unipotential lenses, utilizing field data obtained from a resistance network analogue. Results obtained are presented in graphical form and shown to be in good agreement with published experimental results, but not, in the ease of spherical aberration constants, with results based on analytical approximation to the axial potential distributions. The sources of error in the computations are discussed, and the accuracy of the results estimated. (Author) J. Vine, Brit. J. Appl. Phys., 11 (9), Sept. 1960, 408-411. 33 241. Emission of negative ions of oxygen during the activation of oxide-coated cathodes. Great Britain. Oxide-coated cathodes have been used as ion sources in a simple mass spectrometer and have been found to emit atomic negative ions of oxygen during their activation at high temperatures (1150-1275°K). A retarding potential at the collector was used to separate the ions emitted by the cathode from the ions formed in the residual gas. Most of the oxygen ions from the cathode arrived at the collector with more energy than they could have obtained from the potential difference across the electron gun, and their number increased as the cathode became more active. The evidence suggests that the ions were formed by dissociation of the oxide coating, then diffused to the surface and were removed by positive ion bombardment. Such a process would leave oxygen vacancies in the oxide which would act as electron donors and increase its electron emission. (Author) N. A. Surplice, Brit. J. Appl. Phys., 11 (9), Sept. 1960, 430-433. 33 242. Contrast of electron micrographs. Great Britain. The variations of contrast in electron micrographs of amorphous films, with film thickness, electron accelerating voltage and objective aperture angle, are discussed ; elastic and inelastic electron scattering and the possibility of plural scattering are taken into account. Experimental results, obtained with vacuum-operated iron films, confirm a formula of the type
first proposed by Hall. However, the contrast is about four times smaller than predicted, using scattering cross-sections based on Lenz's data. Results for other materials, obtained by previous workers, indicate similar discrepancies. At present, the likeliest explanation appears to be a theoretical underestimation of the scattering into angles smaller than 10 -a tad. The limitations of contrast measurements for assessing specimen thickness or constitution are discussed, and a new criterion for replica and shadowing materials suggested. (Authors) J. S. Halliday and T. F. J. Quinn, Brit. J. Appl. Phys., 11 (11), Nov. 1960, 486-491. 33 243. Selected-area diffraction in the electron microscope: Great Britain. A direct demonstration is given of the error in the area contributing to the recorded diffraction pattern, which arises from use of the incorrect objective focal length in microdiffraction experiments in the electron microscope. The effect of errors in the final intermediate lens setting is also discussed, and reproducibility of the camera constant AL in selected area diffraction is investigated. Agar has emphasized that a certain operational procedure has to be closely followed, if a selected area diffraction pattern in a three-stage electron microscope is to come solely from the area defined by the selector aperture under magnified image conditions. It is the purpose of this paper to demonstrate directly the contributions which may occur from an annulus outside the image of the aperture, due to incorrect excitation of the objective lens, and to comment on another important aspect of selected area diffraction--the reproducibility of AL. (Author) R. Phillips, Brit. J. Appl. Phys., 11 (i1), Nov. 1960, 504-506. 33 244. Fifth order spherical aberration of magnetic lenses. Great Britain. Knowledge of the fifth order spherical aberration of magnetic lenses is required in order to show how much error is left when third order aberration is corrected. This is found theoretically for lenses of the bell-shaped field type. It is found that, for lenses of moderate strength, third and fifth order coefficients are numerically very similar. (Author) G. D. Archard, Brit. J. Appl. Phys., 11 (11), Nov. 1960, 521-522. 33 245. Heating of metallic foils in an electron microscope. Great Britain. The temperature of a thin metal foil illuminated by the high intensity electron beam of the modern microscope may at the centre of illumination reach a temperature above that of the melting point of iron. An attempt has been made to measure and calculate the temperature distribution for any given metal under various illumination conditions and to determine also the stress fields produced in the foil by such temperature distribution. (Authors) B. Gale and K. F. Hale, Brit. J. Appl. Phys., 12 (3), March 1961, 115-117. 33 : 57 246. The investigation of surface deformation. Great Britain. The methods for studying surface deformation used in the National Engineering Laboratory are described. Deformed metal surfaces show an enhanced photoelectric emission which is studied with the aid of open-ended GeigerMtiller counters or electron multipliers. Oxide films stripped from deformed metal surfaces are studied by electron-microscopy and changes in chemical reactivity by radioactive tracers. Surface topography is an important factor in lubrication and wear research and is being studied with the aid of interferometry, phase contrast and electron microscopy. Replica techniques are often applied with advantage. (Authors) L. Grunberg, D. Scott and K. H. R. Wright, Brit. J. Appl. Phys., 12 (4), April 1961, 134-140.