X-RAY PROJECTOR

The advantages associated with the short wavelength compel a persistent search for possibilities of using X-rays for microscopy. As is well known, the difficulty of carrying out X-ray microscopy is due to the impossibility of effectively using lenses. One possibility for overcoming this difficulty is to realize a lensless shadow microscope-projector with a point source, long since used in electron microscopy. The resolving power of such a projector depends entirely on the size of the point source of radiation, and the magnification obtained with its aid depends on the distance between the source and the object.

In the device described in the reviewed note[^1], the dimensions of the X-ray source were about 1 micron. This was achieved by focusing the exciting electron beam (3–12 keV) with the aid of electron optics. The target was a Lenard window separating the vacuum system from the atmosphere. A beam of X-rays emerging through the window to the outside was used.

The object under consideration was placed (in air), as close as possible, near the window. At a distance of 2–5 cm from it there was a photographic plate (or a fluorescent screen), which received the shadow image of the object.

The figure shows one of the photographs obtained in this way of a silver mesh (1000 openings per inch, wire thickness 5 microns),

superimposed on a coarser copper mesh (400 openings per inch). Projection was carried out by means of X-rays with wavelength \(\lambda = 1.5\) Å (the energy of the exciting electron beam was 10 kV). The exposure time was 20 s. The initial magnification was 95 times; the final magnification, 285 times. The resolving power was 1 micron.

Since X-rays of this wavelength have high penetrating power, the projector can be used almost exclusively in metallurgical studies; biological objects prove to be practically transparent. Reducing the penetrating power by increasing the radiation wavelength would require placing the object under study inside the vacuum system, which would deprive the entire apparatus of a significant part of its advantages.

R. G.

References

1. V. E. Cosslett and W. C. Nixon, Nature, 168, 24 (1951).