184 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



expensive automatic equipment will be based on the number of samples 

 to be counted and the technician time available. It must also be remem- 

 bered that the more complicated the equipment, the greater the chances 

 of its getting out of order. The general references may be consulted for 

 detailed descriptions and explanations of principles of operation. 



Commercial electroscopes can be used for measurement of radioivsotopes 

 and are simple, reliable, and modestly priced. However, they are not 

 widely used, possibly because of the time required for each measurement, 

 and will not be discussed further. Conventional instrumentation consists 

 of the detecting device (e.g., counter tube, ionization chamber) and the 

 required electronic circuits. The electronic equipment is common to 

 most of the detecting devices and is discussed below, w^hereas the detec- 

 tors are described in later sections. 



The Scaling Unit. The scaling unit, or scaler, is the basic electronic 

 item for radioassay and supplies the voltage for operation of the counter 

 tube and the circuits for registering the counts. The most expensive 

 units usually have a built-in timer and register, and it is necessary only 

 to connect the counter tube. With the simpler units it may be necessary 

 to use an external timer and register. The scaler has an electronic cir- 

 cuit which totalizes the individual counts received and trips the mechan- 

 ical register after some given total. With a scale of 64, for example, each 

 unit recorded on the mechanical register indicates that 64 counts have 

 been received. The most popular circuits operate with scale multiples 

 of 2. However, many instruments on the market are decade scalers 

 (multiples of 10). The latter have more complicated circuits to be kept 

 in order, but the readings are somewhat easier to totalize, although in 

 practice this does not appear to offer much advantage. It is recom- 

 mended that the unit have at least a scale of 64 and, for more flexibility, 

 a scale of 256. The decade units usually have a scale of 100 or 1000. 



With a scale of 64 there will be interpolation lights labeled 1, 2, 4, 8, 

 16, 32. The number of pulses received by the circuit will be the sum of 

 the numbers of the lights that are glowing. For example, if 63 counts 

 have been received, all the lights will be on, and the next impulse will 

 extinguish them and actuate the mechanical register. With decade cir- 

 cuits the total is read directly from the lights and register without the 

 necessity of addition. Depending upon the instrument, the measure- 

 ments can be made for a predetermined count or a predetermined time. 

 In the former method, all samples are measured to the same number of 

 total counts, and therefore all values will have the same counting accu- 

 racy; also there will be no interpolation lights to totalize. Basic scaling 

 units, however, can usually be more easily adapted for predetermined 

 time measurements. In practice, there seems to be little choice between 

 the procedures. 



