Sec. 5-1] HADIATIOS T h'A XS I )(_■(' E h'S 243 



The response time of the vacuum photoelectric cell is limited only 

 by the transit time of the electrons and is of the order of 10 -9 sec. 

 However, the response time of a circuit connected to the photo- 

 electric cell may be increased by the capacitance of the tube (of the 

 order of 1 to 5 fifiF) and the resistance of the circuit. 



The range of light intensities for which a photoelectric cell can be 

 used is limited at the lower end by noise consideration and dark 

 current (see the general considerations mentioned above) and at the 

 upper end by fatigue effects and stability considerations. The illumi- 

 nation should be such that the emission current for continuous opera- 

 tion does not exceed the order of 1 //A/cm 2 of the cathode surface. 

 The output impedance is of the order of several megohms. 



The maximum ambient temperatures which can be tolerated are 

 100°C for tubes with S-l, S-d, and £-8 surfaces and 75°C for £-4 and 

 $-5 surfaces. For constant sensitivity over long periods, operating 

 voltages of 20 volts or less are desirable, since at high voltages 

 positive-ion bombardment from the residual gases may alter the 

 cathode. 



b. Gas-filled Phototubes. The sensitivity of a phototube can be 

 increased by amplifying the number of electrons in a gas discharge. 

 An inert gas at a pressure of about 1 mm Hg is introduced and a 

 potential of about 1 00 volts is applied to the electrodes. The emitted 

 photoelectrons are accelerated in the direction of the field, taking 

 energy from the field and losing some of it by elastic collisions with 

 the gas molecules. Eventually the electron, after attaining an energy 

 of about 20 eV, will ionize an atom and produce a positive ion and a 

 new electron. The two electrons are then accelerated again to pro- 

 duce more electrons; the positive ions are directed to the cathode, 

 where a fraction of them will release further electrons. The result- 

 ant amplification of the photocurrent may provide a gain by a 

 factor of 5 to 10. The gain depends upon the applied voltage and, 

 to some extent, upon the tube geometry. The current enhancement 

 by gas filling may be greater, by 20 to 30 per cent, for infrared light 

 (900 m^) than for red light (660 m/t) 1 . 



The luminous sensitivity of gas-filled phototubes is between 40 and 

 150 //A/lumen, the radiant sensitivity between 0.01 and 0.15 /j,A[juW. 

 The permissible current density is between 10 and 30 //A/cm 2 of the 

 cathode surface for continuous operation and up to 600 //A/cm 2 for 

 short-time applications. 



Voltage-current characteristics of gas-filled phototubes are shown 

 in Fig. (5-1)10. Below the ionization potential of the gas (~15 volts) 



1 W. S. Huxford, Phys. Rev., 55, 754 (1939). 



