274
8.
MICROSCOPY
area diffraction.* A cross-over beam (but with very low, unbiased beam
current) is passed through any sort of material which will diffract electrons. The beam is focused so that it appears as a tiny bright spot. The
neighboring areas are illuminated diffusely by a cloud of diffracted
electrons. The rim of the aperture is silhouetted against this fairly sharply
[Figs. 31(a)-(c)] having an apparent diameter of ^ to J in. Naturally, when
a new aperture is first introduced, its disk may hide the bright beam.
One first seeks the area of diffuse illumination, and then, by manipulating the controls, an off-center aperture [Fig. 31(c)] can be exactly
aligned [Fig. 31(b)], so that the beam spot is in the center of the illuminated circle.
This method of centering is accomplished easily if anticipatory precautions are followed. Before beginning aperture replacement a suitably
diffracting object should be set up. A thick section will do, or perhaps a
fold in a section or a place where there is contaminating dirt. If necessary,
one can prepare special screens with carbon or magnesium oxide smoke
deposits. Obtain the focused image that will be used later for centering
the new aperture and note the beam current at saturation and also the
condenser setting at cross-over. Next, spread out the beam with the condenser control and decrease the beam current until a faint diffuse illumination is all that is visible. Note this beam current, and return to this
when the search is started for the new aperture. This low order diffuse
bombardment should not harm the new aperture.
When the new aperture is first put in, the microscope is returned to
conditions which are known to be safe and also capable of providing the
images necessary for alignment. One first finds the illuminated image of
the aperture rim. Then one begins to condense the beam toward the
cross-over position, reducing the beam current as this is done to keep the
aperture barely visible. As the cross section of the beam is reduced the
aperture is moved correspondingly to keep it in view. After several
successive adjustments a cross-over beam will be achieved within the
boundaries of the aperture, and final alignment is close [Figs. 31(b) and
(c)]. Only after alignment can the beam current safely be increased to
the bias position.
* The RCA EMU-2 microscopes are not equipped for selected area diffraction except
for the final model (2E). But the Canal Industrial Corporation markets a "Centering
Guide and Diffraction Device" as an accessory for these microscopes which creates the
desired optical conditions for aperture alignment.
8.
MICROSCOPY
area diffraction.* A cross-over beam (but with very low, unbiased beam
current) is passed through any sort of material which will diffract electrons. The beam is focused so that it appears as a tiny bright spot. The
neighboring areas are illuminated diffusely by a cloud of diffracted
electrons. The rim of the aperture is silhouetted against this fairly sharply
[Figs. 31(a)-(c)] having an apparent diameter of ^ to J in. Naturally, when
a new aperture is first introduced, its disk may hide the bright beam.
One first seeks the area of diffuse illumination, and then, by manipulating the controls, an off-center aperture [Fig. 31(c)] can be exactly
aligned [Fig. 31(b)], so that the beam spot is in the center of the illuminated circle.
This method of centering is accomplished easily if anticipatory precautions are followed. Before beginning aperture replacement a suitably
diffracting object should be set up. A thick section will do, or perhaps a
fold in a section or a place where there is contaminating dirt. If necessary,
one can prepare special screens with carbon or magnesium oxide smoke
deposits. Obtain the focused image that will be used later for centering
the new aperture and note the beam current at saturation and also the
condenser setting at cross-over. Next, spread out the beam with the condenser control and decrease the beam current until a faint diffuse illumination is all that is visible. Note this beam current, and return to this
when the search is started for the new aperture. This low order diffuse
bombardment should not harm the new aperture.
When the new aperture is first put in, the microscope is returned to
conditions which are known to be safe and also capable of providing the
images necessary for alignment. One first finds the illuminated image of
the aperture rim. Then one begins to condense the beam toward the
cross-over position, reducing the beam current as this is done to keep the
aperture barely visible. As the cross section of the beam is reduced the
aperture is moved correspondingly to keep it in view. After several
successive adjustments a cross-over beam will be achieved within the
boundaries of the aperture, and final alignment is close [Figs. 31(b) and
(c)]. Only after alignment can the beam current safely be increased to
the bias position.
* The RCA EMU-2 microscopes are not equipped for selected area diffraction except
for the final model (2E). But the Canal Industrial Corporation markets a "Centering
Guide and Diffraction Device" as an accessory for these microscopes which creates the
desired optical conditions for aperture alignment.