GENERAL PROCEDURES FOR RADIOASSAY 185 



The general procedure for operation of the scaUng unit is as follows: 

 (a) connect the counter tube to the unit and, if necessary, also connect 

 the timer and register; (6) see that all switches are off and the high- 

 voltage adjustment set at zero, since damage may result if the high volt- 

 age is turned on before warmup of the circuit; (c) plug the unit into the 

 regular 110-volt receptacle; (d) turn on the master switch and allow the 

 circuit to warm up for at least 30 sec; (e) place a source of radioactive 

 material under the counter tube; (f) set the count-stop switch on count 

 and set the automatic timer, if there is one, to the count position; (g) turn 

 on the high voltage; (h) increase the high voltage slowly until the inter- 

 polation Ughts begin to register; (i) adjust the high voltage to about 50 

 volts above the starting voltage. Accurate measurements should not be 

 attempted until the instrument has been in operation for al)()ut 30 min, 

 at which time the operating voltage should again be determined as above. 

 In practice, the instrument should be left on at all times. 



Before the high voltage is adjusted, the approximate operating voltage 

 of the counter tube should be noted from the specifications, and if the 

 unit does not operate when this voltage is reached, all switches should be 

 checked to make sure that the counting circuit is on. This may prevent 

 the application of excess voltage to the counter which may cause damage. 

 A background count in the normal range is the first criterion of correct 

 operation of the assembly. The actual background will vary with the 

 size of the tube, location, etc. However, an ordinary end-window counter 

 will run about 20 counts/min shielded and about 60 unshielded. If the 

 background is extraordinarily high, one should look for contaminating 

 activity in the area, light sensitivity, or spurious counts due to loose cable 

 connections. During a background count it is possible to follow the 

 individual pulses through the interpolation lights and ascertain if the 

 scaling circuits are functioning properly. Very often a simple tube 

 replacement will correct any difficulty in this part of the circuit. If the 

 background is normal, then a standard source should be measured to 

 make sure that the counter sensitivity is in the expected range. By keep- 

 ing records of background and standard-source measurements from day 

 to day, it becomes easy to detect a counter tube that is going bad or any 

 other malfunction. 



In the procurement of scaling units it is wise to ensure that the ecjuip- 

 ment as obtained will accommodate the particular types of detection 

 devices which are to be employed. 



Instead of scaling units, count-rate meters may be employed. These 

 have the advantage of giving direct readings of the counting rate and are 

 useful for in vivo or other studies where quantitative results are not 

 recfuired. However, the scaling unit still appears to be the instrument of 

 choice for general radioassay. 



