COiNTACT MICRORADIOGRAPHY 



This is especially true in the ultrasoft x-ray 

 region where suitable target materials are 

 scarce and where intensities are discourag- 

 ingly low. Three general approaches to 

 monochromatization may be used, generally 

 in combination. First, the target material 

 may be chosen to yield emission lines as near 

 the desired wavelength as possible. Second, 

 filters may be interposed in the beam to 

 selectively suppress the remaining unwanted 

 portions of the spectrum. A filter may be of 

 the direct absorption type which sharply 

 attenuates the unwanted wave length near 

 the short wave length side of the character- 

 istic absorption edge of the filter element 

 (12, 13) ; it may consist of a grazing incidence 

 reflector which eliminates all the x-rays 

 shorter than a critical wave length, but with 

 a cut-off which is not sharp (14, 15); or it 

 may be a crystal reflector which achieves 

 practically complete monochromatization, 

 but only with such a considerable loss of 

 intensity that it is not likely to be useful 

 for normal image production. 



Finally the response of the detector may 

 be used to eliminate unwanted wave lengths 

 either by failing to absorb the harder com- 

 ponents (16, 17) or by discrmiinating be- 

 tween different x-ray energies as in the case 

 of proportional counters (18, 19). These 

 methods of monochromatization are covered 

 in detail in the references given. Their 

 proper use will depend on the specific ap- 

 plication and the type of information re- 

 quired. 



Recording and Detecting Materials 



Since both penumbral blurring and dif- 

 fraction effects may be reduced to practically 

 negligible amounts by proper contact image 

 geometry (20), the resolution of the method 

 is limited by the grain or structure of the 

 recording material or by the optical system 

 which is used to view the recorded image. 

 For enlargement with the light microscope 

 there are several recording materials with 

 background structure which is visible only 



under the highest magnification, and some 

 which show no background structure at all. 

 For electron optical enlargement, on the 

 other hand, the resolution is limited by the 

 structure of the recording material. We shall 

 describe briefly some of the existing materials 

 useful for contact x-ray image recording and 

 indicate their range of usefulness. 



Photographic Materials. Most standard 

 photographic films are designed for normal 

 enlargement, which is seldom greater than 

 10 or 20 times in linear magnification. Con- 

 sequently they have a resolving power in 

 the range of 50 to 150 lines/mm. For photo- 

 micrographic magnifications of 200 or 500 

 times an order of magnitude higher resolu- 

 tion is essential. The films most commonly 

 used for these high magnifications are 

 Gevaert "Lippmann", Agfa "Mikrat", and 

 Eastman Kodak "Type 649" and "Maxi- 

 mum Resolution Plate". These materials 

 have a resolution of approximately 1000 

 fines/mm, or better, depending on condi- 

 tions of use and of measurement. In spite 

 of the very small individual grain diameters 

 of these emulsions (0.05/i) they nevertheless 

 exhibit visible granularity after development 

 which often limits their usefulness for 

 optical magnifications over 500 X. They are 

 also too thick for the optimum optical en- 

 largement, being about ten times the depth 

 of field of a microscope with a N.A. of 1.0. 

 This thickness may be necessary to ef- 

 ficiently absorb x-rays of wave length shorter 

 than 2 A, but it is a disadvantage when the 

 recording of only the ultrasoft x-ray com- 

 ponents from a continuous spectrum is de- 

 sired. Several experimental emulsions have 

 been made (21, 22) to try to improve the 

 qualities of these emulsions for contact 

 microradiography. The adaptation of ultra- 

 fine grained photographic emulsions for 

 electron optical enlargement has also been 

 considered by Recourt (23). 



Fluorescent Screens. Instead of forming 

 a permanent optical image directly from 

 the x-ray exposure, it is possible to convert 



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