ULTKASOFT X-RAY MICROSCOPY 



of the extremely low fluorescent yield for 

 ultrasoft x-ray interactions the fraction of 

 this energy which reaches the recording ma- 

 terial in either projection or contact micros- 

 copy is negligible. Because of the extremely 

 short range of the emitted electrons within 

 the sample and its supporting membrane, 

 this component of the transmitted energy is 

 also negligible. 



It is thus evident that projection micros- 

 copy measures the amount of energy which 

 is photoelectrically absorbed and low-angle 

 scattered out of the direct beam. It is im- 

 portant to note that contact microscopy, 

 with ultrasoft radiations, measures only the 

 amount of energy which is photoelectrically 

 absorbed since low-angle scattered energy re- 

 mains effectively in the direct beam within 

 the short distance to the recording material. 

 Because the amount of energy that is not 

 scattered at the very low angles is so small, 

 the contrast, or absorption signal, in contact 

 microradiograms may be considered as re- 

 sulting purely from photoelectric absorption. 



The measured sample transmission by Eq. 

 (1) depends only upon the mass photoelectric 

 absorption coefficient, ju, and the mass-per- 

 unit-area-thickness, m, of the sample. The 

 photoelectric absorption cross-section is a 

 function only of the x-ray wavelength and 

 the atomic nmnber of the absorbing element ; 

 it is not sensitive to the molecular combina- 

 tion of the elements, and, unlike the scatter- 

 ing cross section, it is not a function of the 

 physical structure of the sample. This simple 

 and direct connection between the mass-and- 

 elementary -chemistry of the sample and the 

 x-ray transmission as easily measured from the 

 microradiogram is probably the greatest and 

 most distinctive advantage of this form of mi- 

 croscopy. 



It is for this reason the writer has felt that 

 it is important that work be carried out on 

 the measurement and tabulation of photo- 

 electric absorption coefficients for the ultra- 

 soft x-ray region and on the development of 

 efficient ultrasoft x-ray sources of monochro- 



matic radiation. A tentative tabulation (see 

 Table 1) of light element mass absorption 

 coefficients has been established (5) based 

 upon a semiempirical method of interpola- 

 tion from the best available absorption data 

 and listed here for the several monochro- 

 matic ultrasoft wavelengths which have been 

 found to be useful in this laboratory for 

 quantitative microradiographic analysis. 



Example of an Ultrasoft X-Ray Source 

 and Camera for Contact Microra- 

 diography 



There are three basic types of x-ray 

 sources for contact microradiography: The 

 first is a microf ocus source used for maximum 

 speed contact microradiography, as men- 

 tioned above, in which the sample and film 

 are placed very near the source. The high 

 speed is at the expense of sample field which 

 is of the order of size equal to the source-to- 

 sample distance. The second type is a source 

 of conventional focal spot size, a few mm or 

 less, and it permits sample fields of the order 

 of one to two cm in diameter at the minimum 

 working distance for high resolution. There 

 are many such sources described in the litera- 

 ture (1) and these have been applied pri- 

 marily for polychromatic radiation. The 

 third type, which has been under develop- 

 ment in this laboratory, utihzes a large focal 

 spot area and consequently requires rela- 

 tively large anode power; but it is designed 

 for quantitative, microradiographic analysis, 

 presenting monochromatic radiation and a 

 relatively large, uniform field. It is described 

 below. 



Monochromatic ultrasoft radiation is 

 gained by utilizing the K or L characteristic 

 radiation from an appropriate target mate- 

 rial. This radiation is isolated from the rela- 

 tively low intensity background radiation by 

 a proper choice of filter and excitation volt- 

 age. In order to obtain such radiation it is 

 extremely important that the target be kept 

 free from tungsten and carbonaceous con- 

 tamination otherwise the low-voltage elec- 



681 



