Guarded-Field Thimble Ionization Chambers

Abstract

IN a previous communication¹ a new guarded-field principle as applied to ionization chambers was outlined, together with its attendant advantages. A thimble chamber employing the new system comprises a cylindrical conducting wall, subdivided into a number of annular sections of equal width ω situated between two end-sections of width ½ω, as shown in Fig. 1,a. These wall-sections are all insulated from one another, either by narrow surface grooves in the case of a wall of insulating material having a conductive coating, or by thin insulating washers in the case of a wall of conducting material. The two half-sections at either end are in electrical continuity with the adjacent (plane) end-surfaces of the cylindrical chamber cavity. If equal potential differences are maintained across each of the insulating gaps in the cylindrical wall, the associated electrostatic field distribution is somewhat as shown in Fig. l,a. (An alternative mode of subdivision of the wall into sections of equal width throughout is also possible, but involves the use of unequal voltage steps.)