RAY TRACKS AND RADIOAUTOGRAPHS 217 



9. HAY TRACKS AND RADIOAUTOGRAPHS 



The tracks in a developed photographic emulsion after exposure to 

 cosmic or other radiation consist of fine lines of silver grains. Since 

 there is usually considerable contrast between the dark silver grains 

 and the rest of the emulsion, less increase of contrast might be expected 

 from phase microscopy. The short side paths of proton tracks and other 

 regions of slight silver deposit do show better with a 0.14A— 0.25X 

 or a IB— 0.25X diffraction plate. Some investigators prefer to reverse 

 the contrast of the silver particles from dark to bright with an A+ phase 

 objective, because the eye is more sensitive to small bright regions on a 

 dark background than to dark regions on a bright background. When 

 yeast, bacteria, or other organisms are included in the emulsion they 

 show better with phase than with a l)rightfield microscope. 



A radioautograph, or autoradiograph, is formed when a section of 

 tissue containing radioactive materials is placed in contact with a 

 photographic emulsion in the dark for a period of time and subsequently 

 developed. The ray tracks show in the emulsion, and the problem is to 

 locate their origin with reference to the tissue elements. The develop- 

 ment process in strong alkaline solution tends to lessen the visibility of 

 the tissue, and the decreased visibility of the tracks in the emulsion 

 under a stained tissue is undesirable. Tissues mounted in balsam are 

 too transparent for analysis, and in glycerin jelly and other aqueous 

 media the contrast is so great that the tracks are not seen with the phase 

 microscope. The dilemma is resolved by means of a transparentizing 

 medium of the proper refractive index to reveal the tissue pattern and 

 yet not obscure the ray tracks. One solution is Groat's organic phos- 

 phate mixtures described in Section 3.2 of Chapter IV; for example, a 

 mixture of about 1.528 refractive index was satisfactory for kidney 

 tissue. 



The 8-mm objective is preferable as it has enough resolving power to 

 show the organization of the tissue and enough depth of field to show 

 both the tissue and track levels. The medium dark A— contrast has 

 been found useful (Fig. V.9). With 15 X to 20X oculars, or by subse- 

 quent enlargement of a photomicrograph, adequate magnification may 

 be obtained. With objectives of greater aperture the depth of field 

 may not be adequate to show the tissue and the tracks simultaneously. 

 Photomicrographs can be made also by divided exposure. Expose at 

 the tissue layer for one-half to two-thirds of the total exposure required, 

 tiu'n the fine adjustment the amount previously determined to focus at 

 the track level, and expose for the remainder of the required time. Since 

 the path of the rays in the emulsion is often not parallel with the surface, 

 stereophotomicrographs with bright or dark contrast phase may be 



