XVII. K A D 1 O A ( r I V E T R A C K U S (V25 



background moasiirement is B and the duration of this measurement 

 is t\ the background counting rate and its standard deviation will be 

 {B =t \/B)li' ■ The true counting rate of the sourro. /?, and its 

 standard deviation will be given hy the expression: 



C B C B^^' 



K = 7 



t t' 



•)■ 



or, in words, the resultant standard deviation will be equal to the 

 square root of the sum of the squares of the standard deviations of 

 the measured and background rates. 



It can be seen from the al)ove that the error may be larger if the 

 measured activity is similar to the background than if it is consider- 

 ably greater. For this reason and to reduce the time required for a 

 measurement it is desiraljle to deal with activities at least of the ordei- 

 of ten times background. For lower activities it may be well to 

 consider the length of time that should be alloted to the measurements 

 of the activity and of the background. If counter time is limited 

 these should be adjusted so that each measurement has a similar 

 error. In general, however, counter time is not limited to such an 

 extent since the background measurement may be taken overnight 

 when the counter is not in use for other purposes. Such a measure- 

 ment is applicable to correct the daytime measurements unless the 

 amount of active material near the counter varies from day to night. 



Another point to which statistical reasoning gives emphasis is 

 that the background should be made as low as possible, if samples of 

 low activity are to be measured. Since a large part of this back- 

 ground is due to cosmic rays and to radiation from samples near the 

 counter, shielding with lead or other dense material will reduce the 

 background of a GIM tube considerably, frequently to ahout half its 

 unshielded value. Lead shields, available commerically, include 

 mounts for the tube and samples and have only the disadvantage of 

 expense (84,85). Storage of samples and all other radioactive mate- 

 rials at a distance from the counter, preferably in another room, 

 eliminates one source of background radiation. Construction of the 

 counter and allied equipment with materials scrupulously free of 

 radioactivity is of utmost importance. Counting equipment should 

 only for very special reasons be used in a room in which radiochemical 

 operations are done, since this is a rather sure way of contaminating the 

 counter. 

 .The statistical approacli is not, in general, applied to measurements 



