114 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



to log i£ — log i~ , eliminates the need for accurately stabilizing 

 the electron current and permits the reading of the pressure in the 

 range from 10~ 3 to 10 -7 mm Hg on a single logarithmic scale. The 

 maximum deviation from a true logarithmic response is 3 per cent 

 of the full-scale value. 



The influence of the filament, grid, and plate geometry upon the 

 sensitivity of conventional ionization gauges has been investigated 

 by Kinsella. 1 A relative optimum has been found for a grid structure 

 of 1 cm diameter with a pitch of 4 mm, 0.2 to 0.3 mm wire diameter; 

 the sensitivity of the gauge increases with the plate diameter. 



Application of excessively high grid or plate voltages to the gauge 

 may lead to instability (Barkhausen-Kurz oscillations). 



Difficulties are likely to arise if gases within the gauge react with 

 the cathode or with other elements of the gauge (e.g., water vapor, 



carbon monoxide, carbon dioxide, 

 halogen gases, and hydrocarbon 

 vapors). Oxidation of the filament 



c- 



R 



E or gas absorption may occur if 



Tl'- the gauge is operated at a pressure 



. . ,. higher than 10~ 3 mm Hg. Under 



Fig. (1-5)12. Radioactive ionization & & 



gauge. A, a source; B, housing; c, ion normal conditions a filament life 



collector; E, voltage source {alphatron, f 10 3 to 10 4 hr can be expected. 

 model 511, National Research Corp., r\ ± • c ±\ • „„„ 



~ , ., ' , , . . , * Outgassing of the gauge, m par- 



Cambrtdge, Mass.; by permission). & & & & ' r m 



ticular of the grid by passing a high 

 current through it, is required when the system has been subjected 

 to a pressure exceeding 10 -2 mm Hg. Some gauges are constructed 

 with a grid and a plate in the form of helical filaments which can 

 be heated and outgassed by passing a current through each helix. 

 Extended overloading during outgassing may cause evaporation of 

 metal on the glass wall and may lead to leakage paths. 



For bibliography, see H. Schwarz, Arch. tech. Messen, V 1341-5, May, 1952; 

 also ibid., V 1341-2, September, 1951, and V 1341-4, March, 1952. 



b. Radioactive Ionization Gauge. The instrument is shown sche- 

 matically in Fig. (1-5)12. It contains a radioactive source A that 

 emits a particles which ionize the gas in the gauge. The number of 

 ions formed in the gas is directl}' proportional to the gas pressure as 

 long as the range of the a particles is longer than the dimensions 

 of the chamber. The ions are collected at an electrode C and form a 

 current of the order of 10~ 9 to 10 -13 amp. In the commercial model 

 (Alphatron, Model 511, National Research Corporation, Cambridge, 



1 J. J. Kinsella, Trans. Vacuum Symposium, 1954, p. 65. 



