X-RAY MICROSCOPY 



the x-rays to visible light by means of a or add to the original measurements should 

 fluorescent screen. The image may then be any doubts or questions arise. There are, of 

 viewed directly with a microscope or photo- course, possibilities of photometric errors, 

 graphed for permanent recording. The but although they are different for each 

 production of a visible microfluoroscopic method the effort necessary to correct them 

 image requires a fine-grained fluorescent is about the same in both cases. 

 screen, an intense microfocus source of There is one great advantage of the 

 x-rays, and a well-designed, high -aperture fluorescent recording which should be men- 

 optical viewing system. An instrument com- tioned, although it falls under the topic of 

 prising these features is illustrated in a later monochromatization rather than image re- 

 section. The fluorescent screens themselves cording. Because of the method of fluorescent 

 may be produced by vacuum evaporation screen production it is possible to control 

 of a suitable phosphor, such as Zn2Si04 , the screen thickness and uniformity with 

 onto a thin, optically polished substrate, great accuracy down to sub-micron thick- 

 Methods for producing such screens are nesses. This makes it practical to design the 

 described by Feldman and O'Hara (24) and screen to filter out effectively all but the 

 Pattee (16). Also commercial screens have ultrasoft components of the x-rays by not 

 been obtained on special order (25). absorbing them. Photographic emulsions 



A general comparison between direct may also be made uniformly thick, but only 



photographic recording and the x-ray con- with difficulty at submicron dimensions, 

 version with the fluorescent screen is not 



simple to make. Certainly in problems where ^*^«^ Recording Methods 



motion or change in the specimen occurs in There are many possibilities for x-ray 



short intervals of time the fluoroscopic recording materials besides the conventional 



method of viewing is superior. On the other silver halide emulsions. It is well known, 



hand when a permanent record of an ex- for example, that some ionic crystals take 



tensive area of a specimen is desired, the on a characteristic color as a result of ir- 



direct photographic recording is the best radiation (26) . Another well known effect of 



choice. For observing a small region of a irradiation on plastics is a change in solu- 



specimen at the highest resolution it is not bility produced by cross-linking or change of 



easy to make the choice without trying both polymer length. This effect has been used 



methods. The fluorescent screen has less by the Ladds (27) to produce microradio- 



granularity than some batches of type 649 graphic images for electron optical enlarge- 



plate, but at the same time the intensity ment, and by Warnes (28) for ordinary light 



and contrast of the direct fluorescent image optical magnification. Common plastics 



may not be as good as that obtained in view- such as vinylidene chloride (saran) will give 



ing a direct contact photographic image. visible x-ray contact images without de- 



For quantitative absorption measure- velopment of any kind, although a clearer 



ments, again there is no clear-cut choice, image is produced by washing the exposed 



The intensity of the fluorescent image may plastic in a solvent. 



be measured directly with a photometer so A third type of x-ray record may be pro- 



that no errors of photographic recording or duced by photoreactive dye derivatives 



development enter. Measurements may also which become dyes on exposure to suffi- 



be made more rapidly on the fluorescent ciently energetic radiation. Many of these 



image. Nevertheless, a permanent record of materials are used in light photography for 



the image is a great strategic advantage in low speed copying. One material of this type 



any quantitative work since one may check has been described by Chalkley (29) for 



564 



