Sec. 1-5] 



MK( II A \l( 'A L IXJ'CT 77,',! A".S7;/ VERS 



115 



Mass.) a vacuum-tube electrometer is used to detect the current by 

 measuring the voltage drop across the resistance R. 



The transfer characteristic (meter reading versus pressure) is 

 linear. The error, i.e., the practical deviation from the characteristic, 

 is '2 per cent. The slope of the characteristics is different for different 

 gases, as shown in Fig. (1-5)13. This has the advantage that the gauge 

 can be used as a leak detector. The 

 gauge will show an indication if a 

 gas with a relative response differ- 

 ent from air diffuses through a 

 leak into the gauge (e.g. , H 2 or He) . 

 On the other hand, the different 

 calibration characteristics for dif- 

 ferent gases can cause uncertain- 

 ties and errors when impurities of 

 an unknown concentration or 

 composition are present. 



The system can be used over a 

 range from 1 3 to 1 0~ 3 mm Hg ( 1 -3 

 mm Hg is 10 per cent of lowest 

 full-scale reading). At lower gas 

 pressures the mean free path of 

 the a particles increases beyond the 

 dimensions of the vessel, so that the probability of a collision is 

 reduced and the ionization current is very small. X-ray effects as 

 described under Thermionic Ion Gauges, l-57a, ultimately limit the 

 useful range of the gauge. 



An advantage of the Alphatron over the thermionic ionization 

 gauge is the absence of the heated filament. The instrument cannot 

 be damaged by exposure to air of atmospheric pressure and requires 

 no degassing. The indication is continuous and follows the variation 

 of the pressure without inertia. The activity of the a source (radium- 

 gold alloy, rhodium -plated to prevent loss of the short-lived gaseous 

 daughter products of radium, radon) will rise slightly as the source 

 comes to equilibrium with its daughter products over the course of 

 about fifty years. For practical purposes the radiation from the 

 source will be constant within a few per cent for a period of five years. 



For references see J. R. Downing and G. Mellen, Rev. Sci. Instr., 17, 218 

 (1946); also Electronics, 19, 142 (April, 1946); G. Mellen, Ind. Eng. Chem., 

 40, 787 (1948); A. Boburieth, Le Vide, 1, 61 (1946); J. H. Beynon and G. R. 

 Nicholson, J. Sci. Instr., 33, 376 (1956). A similar instrument in which ioniza- 

 tion by beta rays takes place is described by C. Deal et al., Anal. Chem., 28, 

 1958 (1956). 



2 4 6 



Pressure, x 0001-100 mm Hg 

 depending upon goin of meter 



Fig. (1-5)13. Characteristics of a radio- 

 active ionization gauge (alphatron, 

 model 511, National Research Corp., 

 Cambridge, Mass.; by permission). 



