612 



ADOLF F. VOIGT 



trostatic chargers are available, a small power pack is much more re- 

 liable for providing thi» potential. 



The scale of the electi'oscope is not linear throughout its length so 

 that it should be calibrated, or only one part of it, preferably the 

 first third, should be used. In the absence of any radioactive sub- 

 stance, an electroscope fiber will still discharge at a slow background 

 rate. The magnitude of this background will depend on conditions 

 but is usually 0.05 to 0.25 scale divisions per minute. It is caused by 

 cosmic rays, minute amounts of radioactive substances near and in the 

 instruments, and by leakage across the insulator. After the produc- 

 tion of ion pairs by radiation, there is always a possibility that the 

 ions will recombine and not be measured. This possibility increases 



Eyepiece of 

 opticol system 



Adjustment knob 

 optical system Fiber 



\ 



Window 



Objective 



Ctiarging button Charging wire 



Fig. 3. Lauritsen electroscope. (Courtesy F. C. Henson Co.) 



with increasing density of ionization, so that as the intensity of radia- 

 tion increases the electroscope no longer collects as large a fraction of 

 the ions. There is then a limit above which the response of the elec- 

 troscope will not be proportional to the amount of radioactive sub- 

 stance. One should either staj' below this point or make a correction. 

 For a particular case the point at which an error greater than 1% was 

 introduced by this nonlinearity was found to be 60-70 scale divisions 

 per minute. 



Numerous modifications of the chamber of the Lauritsen electro- 

 scope have been made for different purposes (2, p. 119). The instru- 

 ments come w4th a cylindrical chamber of sheet aluminum or a similar 

 shell with a thin aluminum window, to permit the measurement of 

 low energy j8 rays. Other modifications available from the manufac- 

 turer include a brass chamber into which the sample can be put. 



A chamber, preferable to those on market, can readily be made in 

 the shop. Its advantages are rigidity, more favorable geometrical 



