CONTACT MICRORADIOGRAPHY 



recording the ultraviolet spectrum at wave materials which permit enlargement by 

 lengths shorter than 3200 A. A similar electron optical systems, 

 material, pararosaniline leuconitrile (30), has Although several promising techniques 

 also proved useful for high resolution contact have been studied none of them, at this 

 x-ray images (31). This dyestuff may be stage, may be considered as more then ex- 

 dissolved in several materials including perimental methods. Much development 

 gelatin and nylon, and since it is insensitive work must be done before any reliable 

 to light the problems of specimen mounting quantitative results may be expected. We 

 are greatly simplified. Furthermore, the shall only mention the various approaches 

 x-ray image may be viewed as it is produced to this problem to illustrate the difficulties 

 during the exposure, provided that a suitable and possibilities, 

 optical system is available. An x-ray sensitive material which gives a 



This dye shows no granularity under the good electron image must have certain 



highest possible optical magnification. The peculiar characteristics. It must be thin 



resolution at the samples tested so far is enough to form an electron image of rea- 



limited by the thickness of the sensitive sonable resolution and contrast, and at the 



layer which greatly exceeds the depth of same time it must be thick enough to ab- 



focus of the enlarging microscope. sorb the x-rays in the initial recording 



A fourth type of contact x-ray recording, process. Yor all but the ultrasoft x-rays these 

 recently reported by Auld and McNeil (32), two requirements are incompatible in the 

 makes use of the xerographic process, in same material. The Ladd technique (27) 

 which the x-ray image is produced by elec- overcomes this difficulty by forming the 

 trostatic variations on a semiconducting x-ray image on a thick sheet and then re- 

 surface. In this case the charge "picture" is plicating the topographic x-ray image wdth 

 made visible by depositing fine opaque a thin material which is view^ed in the elec- 

 particles on the exposed surface. The tron microscope. This method may be used 

 charged regions of the surface collect the with any topographic method of image 

 largest number of particles thereby produc- formation, however for an electron image 

 ing visible contrast. Although this method contrast which corresponds to x-ray absorp- 

 also suffers from grain structure, it requires tion the same recording material should be 

 considerably less exposure time for a given used for both x-ray exposure and electron 

 resolution than for any of the photographic imaging. For x-ray wave lengths from 4 to 8 

 materials. A it is possible to use films of micron thick- 

 ness, although speed is lost from incomplete 

 Materials Useful for Electron Optical x-ray absorption and resolution will suffer 



Enlargement from too much electron scattering. For wave 



lengths of 12 to 24 A the situation is much 



All the materials previously described improved and recording fihns of 1000 A or 



have resolutions of a micron or better, and less are practical. 



therefore they approach the resolution The mechanism of image formation which 



limit of the optical microscope by which they has shown the most promise from the work 



are enlarged. A few of the materials have of Asunmaa (33) is photoreactive staining 



sufficiently small background structure that which is produced in a uniform, low atomic 



no graininess is visible even at the highest weight substrate by the combined action 



optical resolution. Therefore to carry the of the x-rays and a heavy atom stain. The 



contact method to its potential limit of stain is more chemically reactive under 



resolution requires finding x-ray recording irradiation so that electron density is de- 



565. 



