Sec. 4-1] ELECTRICAL TRANSDUCERS 1\1 



enters the cage through a small opening. Reflected primary charges 

 and secondary electrons traverse the inside of the cage at different 

 angles, but their chances to leave the cage are very small, so that 

 practically all incident charges are measured. A diaphragm D in 

 front of the opening prevents incident charges from reaching the out- 

 side of the cage where they could produce secondary electrons. The 

 diaphragm has either the form of a disk in front of the opening side of 

 the cage or it surrounds the cage and 

 serves as an electrostatic shield. 



The few electrons leaving the cage ^ D ^c 



have in general a velocity of the order 

 of 100 eV and can be driven back into --- 

 the cage by the application of a poten- 

 tial between the cage and the shield or 



by a magnetic field. For high-energy FicfeL(4-1)3. Faraday cage, 

 particles, the bottom of the cage should 



be made from a material with low secondary emission, such as 

 beryllium or carbon. Most of the secondary electrons emitted from 

 such surfaces have an energy of only a few hundred electron-volts, 

 even if the primary energy is about 400 keV. The use of such 

 materials in the cup tends to reduce the number of double second- 

 aries from the walls of the cup. 1 



Errors are likely to arise from the collection of electrons and ions 

 in the vicinity of the cup. Enclosing the cup in an evacuated 

 envelope reduces this effect. Further errors may arise if high-energy 

 particles penetrate the cup; the influence may be reduced by using a 

 thick layer of carbon and lead at the bottom of the cup. A Faraday- 

 cup monitor for electron beams of an energy ranging from 4 to 300 

 MeV with an error of less than 0.5 per cent is described by Brown and 

 Tautfest. 2 



The potential to which the Faraday cage is charged is proportional 

 to the charge and the number of the incident particles and is inde- 

 pendent of their kinetic energy. For large incident beam currents, 

 the current from the cage to ground (or to any other reference elec- 

 trode) can be measured with a galvanometer or by measuring the 

 voltage drop across a large resistor. If the beam current is small, it is 

 more advantageous to accumulate a charge on the cage during the 

 time t and measure the cage potential E. This potential should be 

 small to reduce leakage currents. If E is small compared with the 



1 J. G. Trump and R. J. Van de Graaff, Phys. Rev., 75, 44 (1949). 



2 K. L. Brown and G. W. Tautfest, Rev. Sci. Instr., 27, 696 (1956). 



