RADIOACTIVITY METHODS 997 



It is common practice to incorporate a guard ring in the insulator of 

 ionization chambers (see Figure 609). The guard ring is usually connected 

 to ground and has two functions. First, it provides an electrostatic shield 

 which keeps the collecting electrode from seeing any external A.C. "an- 

 tenna," such as a light bulb or an electric cord. Second, it collects all 

 current that may leak across or through the high- voltage (outer) insulator. 

 The collecting electrode is kept at a low potential with respect to the guard 

 ring, so that the leakage across the inner insulator is negligible. 



The Geiger-MUller Counter 



In the literaturef and on the market are many varieties of counter 

 tubes. Their essential parts, however, have remained unchanged since Hans 

 Geiger and Walther Miiller announced their Zdhlrohr more than 20 years 

 ago.l 



All the tubes have a thin center wire (anode) surrounded by a usually 

 cylindrical cathode. The volume of the counter is filled by a gas at pressures 

 ranging from a few centimeters to atmospheric. A wide variety of gases 

 can be used for the purpose, but the most common mixture consists of argon 

 with a small amount of polyatomic vapor (alcohol, amyl acetate, etc.) 

 added to quench the discharge. 



Gamma counters may have walls of glass or metal tubing, which absorbs 

 all /3's and a's. The efficiency of y counters is very low. About one out of 

 every 100 y's which enter the counter actually produces a pulse-giving 

 discharge. 



Thin-wall (beta-gamma) counters (Figure 610) are sometimes made of 

 glass 0.005 to 0.007 inch thick in the sensitive region, and will admit the 

 more energetic betas. All-metal counters are available with an aluminum 

 wall 0.004 inch thick (about 25 milligrams/cm^), and will count /3's of 

 medium energy very well. Counters are also frequently provided with 

 mica windows that can be made sufficiently thin to admit even some a 

 particles. The thinnest windows conveniently used in the laboratory weigh 

 about 1.5 mg/cm^. 



For a counting, however, it is usually preferable to introduce the 

 sample into the counting volume without any intervening wall. Such coun- 

 ters are generally operated in the proportional region below the Geiger- 

 Miiller threshold (see Figure 611). The counter efficiency for /3's and a's 

 is very high. Almost all charged particles that penetrate into the counting 

 volume produce a pulse. 



Portable Field Instruments 



Attempts have been made to use ionization chambers of various types, 

 but the inherent instability of their D.C. amplifying circuits and their 



t D. R. Corson and R. R. Wilson, "Particle and Quantum Counters," Rev. Sci. Inst. 19, 207-33 

 (1948). 



S. C. Brown, "Theory and Operation of Geiger-Muller Counters," Nucleonics, 2, No. 6, 10-22 

 (June, 1948); ibid. 3, No. 2, 50-64 (August, 1948); ibid. 3, No. 4, 46-61 (October, 1948). 



t H. Geiger and W. Miiller, "Das Elektronenzahlrohr," Phys. Zeit., 29. 839-41 (1928) ; ibid. 30, 

 489-93 (1929). 



