The resolution of photographic emulsions for electrons in the energy range 7-60 keV

Abstract

The quality of images recorded photographically with electrons with energies from 7-60 keV is examined by measurements on the granularity of large areas exposed to a uniform optical density and the density profiles found in the images of sharp opaque edges. The work has been mainly carried out with Ilford Special Lantern Contrasty (SLC), Ilford Nuclear Research G5 and Kodak C-type DC3 emulsions. The granularity of SLC and G5 at 60 and 7 keV respectively has been examined over a range of densitometer scanning aperture diameters from 65 to 63 μm and these measurements are related to the corresponding correlation curves. Grain correlation is found for SLC at 60 kV extending over image regions about 15 μm in diameter; no correlation was found for G5 at 7 kV, or at 60 kV for which case a few measurements were also made. The changes in image granularity with electron energy were examined for SLC, G5 and two other nuclear emulsions using a scanning aperture 45 μm in diameter. Similar measurements on DC3 were limited to the energy range 7-20 keV. The detective quantum efficiencies were evaluated from the granularity measurements for all these emulsions. As examples of the results, over the energy range from 7-60 keV the values of detective quantum efficiency for G5 and SLC fell by factors of about 2 and 30 respectively. A comparison between observed and calculated values of the granularity shows reasonable agreement allowing for the assumptions made. The edge density profiles for SLC, G5 and DC3 were measured at 60 and 7 kV and, after the profiles are fitted to a function of exponential form, the corresponding modulation transfer functions (MTF) are considered. Inter-relating the measurements on the image granularity and the MTF allows the determination of the minimum detectable density difference corresponding to a Fourier image component of a given wavelength, and results are given for SLC at 60 kV and G5 at 7 kV. A comparison of these results shows that electron images of good quality may be recorded on G5 at low voltages and at readily realized image magnifications.