218 Subsurface Geologic Methods 



The Geiger-Mueller apparatus is constructed on the principle of the 

 focusing powder camera (See a of fig. 94), although gross deviations 

 from the principle have been permitted in the actual construction of the 

 apparatus (See b of fig. 94). In the focusing camera the diffracted lines 

 are focused so that they appear as sharp lines on the film, which lies on the 

 circumference of the circle that passes through the slit. Inspection shows 

 that even though gross deviations from the principles were made in the 

 construction of the apparatus, it nevertheless has relatively good focusing 

 at the countertube slit. On the other hand, even small irregularities in 

 focusing will readily show up when line intensities are based on the num- 

 ber of counts at the peak, and disagreement between the data of the two 

 methods will result. The 29 angle, from which the diffraction effects are 

 calculated by means of the Bragg equation, must be obtained by calcula- 

 tion, using the geometric relations of the camera for patterns obtained with 

 diffraction cameras employing films. For the Geiger-Mueller-counter appa- 

 ratus the positions of the diffraction lines are obtained directly as the 26 

 angle (in degrees) from a graduated arc on the countertube track. 



The film methods of recording the patterns were developed shortly 

 after the interaction between matter and X-rays was discovered and have 

 been improved gradually so that today their reliability is no longer ques- 

 tioned. Furthermore, reliable methods of measuring and evaluating the 

 data have been developed simultaneously, although they are not so short 

 and simple as those used for Geiger-Mueller-counter data. Actually, film 

 methods record and preserve a full, detailed pattern, giving such addi- 

 tional information as orientation effects and particle sizes of every indi- 

 vidual sample analyzed. All this information is lost in Geiger-Mueller- 

 counter data. Furthermore, these effects, such as orientation, may cause 

 appreciable error and distortions in the data without forewarning the 

 operator. 



At present the reliability of some types of Geiger-Mueller-counter 

 data is not well established, but further instrumental improvements and 

 development and refinement of operational techniques may soon increase 

 the reliability of the method. Variations in line intensity of twenty to 

 thirty percent have been observed by workers ^^ studying platy minerals 

 with Geiger-Mueller-counter apparatus. The effect of orientation on the 

 intensity of the strongest line in the pattern has also been studied by the 

 writer. Relatively soft, lathlike organic material was ground for 1| 

 hours and the powder carefully packed into a special sample holder with 

 a thin-bladed spatula. The sample was then made flush with the surface 

 of the holder with a single pass of the spatula across the holder. The 

 sample was approximately 2 cm. in diameter and 2^ mm. thick. The 

 holder was so constructed that the sample could be rotated in steps about 

 an axis perpendicular to the front surface of the sample. These steps 



" Beatty, Van IX, X-ray Spectrometer Study of Mica Powders: Am. Mineralogist, vol. 34, pp. 74 ff., 

 1949. 



