PRODUCTION OF CONTINUOUS AiND CHARACTERISTIC X- RADIATION 



fill approach is to use the x-ray fluorescent 

 emission from the specimen when the pri- 

 mary x-ray beam has sufficient energy to ex- 

 cite the characteristic hnes sought. This is 

 the same method that is widely practiced on 

 the macro scale for x-ray analysis of large 

 amounts of material. The projection x-ray 

 tube has a very high specific loading of the 

 x-ray focal spot and this high brightness 

 makes it specially suitable for micro-diffrac- 

 tion, as mentioned above, and for micro- 

 fluorescent analysis. In this case the pri- 

 mary x-ray beam is collimated to a narrow 

 beam and this strikes a very small area of 

 the specimen. The emitted radiation is de- 

 tected by a counter placed to one side of the 

 main beam and for analysis a spectrometer 

 is placed between the specimen and the 

 counter. 



A similar but perhaps more powerful vari- 

 ant of x-ray microanalysis is the use of the 

 x-ray emission from the target itself. In this 

 case the specimen replaces the normal x-ray 

 target, is struck by the electron beam, and 

 the emitted x-rays from an area equal to the 

 size of the beam are analyzed by a spectrome- 

 ter and counter. Point to point detection 

 was achieved by mechanical scanning of the 

 specimen in early models of this method with 

 the position of the electron probe determined 

 by means of an additional optical micro- 

 scope focused on the same area. A recent 

 more elegant approach is where the specimen 

 is still moved for general viewing of the sur- 

 face but the electron beam is scanned as well, 

 as in a television raster, and the x-ray output 

 modulates a cathode-ray tube that is scanned 

 with the same speed. The resultant picture 

 on the face of the tube represents the exact 

 element analysis for each particular setting 

 of the spectrometer detector. This method of 

 scanning x-ray microanalysis seems to be the 

 true strength of the scanning x-ray micro- 

 scope and efforts are being made to extend 

 the analysis to lower atomic numbers, lower 

 concentrations and to determine the limit of 

 detection. (See "Encyclopedia of Spectros- 

 copy," pp. 745, 768.) 



These analytical tools use the same beam 

 forming sj^stems as the static x-ray projec- 

 tion microscope and the combination of x-ray 

 microscopy and x-ray analysis although not 

 at the resolution of the electron microscope, 

 should become a well established technicjue 

 of microscopical investigaion. 



REFERENCES 



Nixon, W. C, Research, 8, 473 (1955) (reprinted 

 in "Modern Methods of Microscopj'", p. 92, 

 Butterworths, London, 1956 and Interscience, 

 N. Y.) 



Proceedings of the Cambridge Symposium "X-ray 

 Microscopy and Microradiography", August 

 1956, Cambridge, England; editors, V. E. 

 Cosslett, A. Engstrom and H. H. Pattee, Aca- 

 demic Press, N. Y., 1957. 



Proceedings of the Stockholm Symposium "X-ray 

 Microscopy and X-ray Microanalysis", June 

 1959, Stockholm, Sweden; editors, A. Eng- 

 strom, H. H. Pattee and V. E. Cosslett, El- 

 sevier Press, Amsterdam, 1960. 



Ong Sing Poen. "Microprojection with X-rays", 

 Hoogland and Waltman, Delft, Netherlands, 

 1959. 



Cosslett, V. E. and Nixon, W. C, "X-ray ^Micros- 

 copy", Cambridge Universitj- Press, 1960. 



W. C. Nixon 



PRODUCTION OF CONTINUOUS AND 

 CHARACTERISTIC X-RADIATION FOR 

 CONTACT AND PROJECTION 

 MICRORADIOGRAPHY 



When discussing the production of charac- 

 teristic or continuous x-radiation, by bom- 

 barding a target with electrons, it is con- 

 venient to separate the elementary processes 

 of bremsstrahlung and inner-shell ionization 

 from the modifying effects of electron scat- 

 tering and energy loss. The so-called 'ideally 

 thin' target is one in which only a small frac- 

 tion of the electrons undergo any encounter 

 (radiative or otherwise) within the target so 

 that the electrons are undeviated and are of 

 full energy. This ensures that any encounter 

 resulting in the production of x-radiation 

 takes place when the bombarding electron 

 has a known energy and direction. Targets 



653 



