Sec. 5-2] RADIATION TRANSDUCERS 295 



background counting rate, the probable error can be expressed by 



VrjS + 2B 1 



e = 



vs Vt 



V.'. •.•.•.•■ .^.^.^.'.'. l .'.^.^ 



^ Window 



/-Anode 



Cathode-- \, 



mm 



) + 



The error, at a given level of radiation, diminishes in proportion to 

 the root of the observation time t. The minimum radiation intensity 



5 that can be measured with a given error, or the time required to 

 obtain a measurement, diminishes, of course, with a reduction of 

 the background counting rate. An 

 increase of the counter efficiency 

 ■i] does not necessarily produce 

 the same result, since it may also 

 lead to an increase of the back- 

 ground counting rate. 1 The back- 



i ,. r Fig. (5-2)14. Cross section of a Geiger- 



ground counting rate oi commer- ; , , .., , 



6 o counter tube with window. 



cial counters is between 1 and 50 



counts/min; the efficiency, for instance for the ft radiation of C 14 , 



is between 2 and 10 per cent. 



Commercial Geiger counters are predominantly of two types, the 

 cylindrical type as illustrated in Fig. (5-2)9 and the end-on type as 

 shown in Fig. (5-2)14. 



The cylindrical type is primarily used for gamma radiation and, 

 if the walls are sufficiently thin, for the more energetic ft radiation. 

 The wall thickness of commercial counters expressed in absorbing 

 mass per unit wall surface area can be made as small as 30 mg/cm 2 , 

 but is in general above 300 mg/cm 2 . Counters vary in length from 

 2 to 40 in. and in diameter from 0. 125 to 2 in. The operating voltage 

 is between 400 and 1,500 volts; the length of the plateau is around 

 200 volts, the slope of the plateau of the order of 1 to 10 per cent/ 100 

 volts. The recovery time of the counters varies from less than 100 

 to 400 /^sec. 



End-on-type counters are used in general for the counting of a 

 and ft rays and soft X rays which can penetrate the thin windows 

 usually made from mica or Mylar (trade name, E. I. du Pont de 

 Nemours & Company). Counters of the type shown in Fig. (5-2)14 

 can also be used open (windowless counter). The radioactive speci- 

 men is placed in front of the opening within a closed housing, and the 

 filling gas (for Geiger counters, 99 per cent helium plus 1 per cent 

 isobutane; for proportional counters 90 per cent argon plus 10 per 



1 See Tracerlog, no. 84, p. 7, issued by the Tracerlab, Inc., Boston, April, 

 1957. 



