X-RAY MICROSCOPY 



Two important facts are immediately cvi- resolution limit of both contact and projec- 

 dent from this result: (1) In order to gain tion microscopy are essentially the same and 

 maximimi speed and because of the depend- equal to that of light microscopy, viz. about 

 ence upon 8*, it is very essential that the 0.2 ju- The present practical limit of resolu- 

 recording material and the source size be tion of both methods is dependent somewhat 

 "matched" to the particular resolution prob- upon the resolving power of present day con- 

 lem as required by Eq. (6). (2) The micro- centrated Lippmann emulsions and other re- 

 scope efficiency, which is equal to SA, is cording materials for contact microradiog- 

 very much greater for the contact method raphy and upon the intensity problem and 

 than for the projection method as illustrated consequent instability problem for high-res- 

 in Fig. 4. olution projection microscopy. For dense, 



For high-resolution projection microscopy heavy element samples, and with shorter 

 (x ^ 1) the camera speed, S, is equal to wavelength x-rays, resolutions of about 0.1 ju 

 Kiod^/d^ and the efficiency, SA, is a constant might be achieved with the projection 

 equal to Kio8*Q. For contact microradiog- method as demonstrated by Nixon using a 

 raphy, the camera speed is essentially con- silver grid (3). (2) The contact method can 

 stant for a given sample thickness, d, and yield shorter exposure times (higher speed) 

 equal to KioS^/d' and the efficiency is equal and shorter exposure-times-per-unit-sample- 

 Kio8*Q/x^. (In the expressions for S, Q has area (higher efficiency) than are possible 

 been equated to A/d^.) It should be noted with the projection method — this fact is of 

 that the maximum speed, S, for contact utmost importance in the ultrasoft x-ray 

 microradiography is higher than that for analysis. (3) Inasmuch as intense, sharply 

 projection microscopy if the same source or focused sources are required for projection 

 source brightness, io , is used since the sam- microscopy, and are not needed for contact 

 ple-to-film distance, d, for contact micro- microscopy, the instrumentation for the con- 

 radiography is usually less than the sample tact method is much simpler and is easier to 

 to source distance, c, in projection micros- operate for high resolution work, 

 copy. It should be emphasized here, neverthe- 



The specific intensity, io , is limited by the less, that the projection microscope becomes 



rate of heat dissipation in the conventional of considerable advantage for certain special 



large focal spot tubes. For the microfocus problems (see paper of Ong Sing Poen) and 



source, however, the efficiency of heat trans- also that the excellent electron-optical sys- 



fer is much higher for geometrical reasons terns which have been developed for projec- 



and io is not set by the target loading lunit tion microscopy are of very great and unique 



but rather it is set by the maximum beam value as tools in other kinds of microanalysis, 



current as allowed for a given resolution re- ultrasoft X-Ray Absorption Measure- 



quired for the electron optical system. In 

 practice to may be as much as fifty times 

 higher for the microfocus tubes. This fact is 

 often utilized in order to reduce exposure 

 time in contact microradiograph}^, at the 

 sacrifice of sample field, by placing the sam- 

 ple and film very close to the microfocus 

 source. 



ment 



Most of the ultrasoft x-ray interaction is 

 by photoelectric absorption, and, in general, 

 the relatively small amount of the incident 

 energy that is scattered is coherent and in 

 the forward direction (low-angle scattering 

 and diffraction) (4). The energy which is 

 photoelectrically absorbed is re-emitted first 



Contact Method or Projection Method? as a photoelectron and subsequently, as the 

 From the foregoing discussion, certain con- atom returns to its normal state, as fluores- 



clusions may now be summarized: (1) The cent radiation and Auger electrons. Because 



680 



