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RADIATION BIOLOGY 



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lated light beam. This capacitance siuiiit Hmits the useful range of 

 modulation frcciuencies to below 10, ()()() cycles/sec. 



Although all commercially available barrier-layer cells have peak sensi- 

 tivity in the \isible spectral region, they are available in quartz envelopes 

 which permit apprecial)le response to wave lengths as short as 270 m/i. 

 These cells are somewhat temperature sensitive, and may display an 

 initial "fatigue" for 15-20 min on exposure to radiation (Lange, 1938; 



Barbrow, 1940). For use in appro- 

 priate applications, matched pairs of 

 cells are available. 



Photoemissive Detectors. Because 

 of their high sensitivity, linearity and 

 speed of response, and convenience 

 of operation, photoemissive detectors 

 have become the most widely used 

 means for quantitative measurement 

 of ultraviolet radiation. The elec- 

 tric currents deri\^ed from these de- 

 vices are easily amplified and may 

 then be used to operate meters or 

 au}^ of various kinds of automatic 

 recording devices. With modern 

 techniques (Engstrom, 1947a; Som- 

 mer and Turk, 1950) it is possible to 

 reduce the extraneous sources of elec- 

 trical fluctuation, such as the thermal 

 emission of electrons from the photo- 

 cathode and the thermal motion of 

 electrons in the amplifier input cir- 

 cuit, to levels sufficiently low for the 

 principal limitation on the precision 

 of measurement of w'eak beams of radiation to arise from the (juantized 

 nature of the radiation itself and from the concomitant statistical fluctua- 

 tions in radiation intensity (Johnson and Llewellyn, 1934). 



The operation of photoemissive cells depends on the release of electrons 

 from a photosensitive surface on incidence of quanta of adequate energy. 

 Since the energy of a quantum is proportional to the frequency of the 

 radiation, there is for any surface a minimum value of frequency — or a 

 maximum value of wave length — below (or above) which the quanta will 

 not have sufficient energy to release electrons. This maximum wave 

 length is known as the threshold wave length for the photosurface in 

 ciuestion. 



For many metallic svafaces, the threshold wave length lies in the ultra- 

 violet. This circumstance has made possible the design of photocells 



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 ILLUMINATION, ft-C 



Fig. 4-;J. Influence of external circuit 

 resistance upon current output of 

 photovoltaic cell. Photocurrent char- 

 acteristics with several external resist- 

 ances; rectangular cell Model lOA; 

 active area 0.70 sq in. Figures on 

 curves, ohms. {Bradley Laboratories, 

 Inc.) 



