168 ISOTOPIC TRACERS 



trie neutrality is re-established in the gas, and the tube is ready to ac- 

 cept another particle or quantum. The pulse of current crosses the capaci- 

 tor to be registered on a meter or on a scaling or counting circuit. The 

 whole operation is completed in about a microsecond or so. If a radio- 

 active sample is placed under the tube, some constant fraction of the 

 radioactivity will enter the tube to be counted, the exact fraction depend- 

 ing on the geometry of the system. Amounts of radioactivity measured 

 this way are usually expressed as counts per minute (c/m). 



The Scintillation Counter: This instrument is more easily adapted to 

 special measurements. The scintillator is a material which responds to 

 radioactivity by producing flashes of light. These individual flashes are 

 counted by a multiplier phototube (Chapter 13); the electrical output 

 of the phototube is counted by a scaling circuit. The scintillating phos- 

 phor may be dissolved in a liquid, which means that the counting cham- 

 ber can take almost any shape. Whole-body counters have been con- 

 structed as hollow cylinders in which dogs or other animals are placed. 

 The scintillation counter surrounds the body and detects any radioac- 

 tivity emitted from the animal's body. 



Scaling Circuits: The output from a G-M tube or a scintillation 

 counter is a series of electric pulses. The rate at which these pulses 

 are produced can be measured with an ammeter calibrated in amperes 

 or in c/m. Alternatively, each pulse can be counted singly with a digital 

 computing circuit known as a scaler. The electronic scaler can be ar- 

 ranged in a variety of ways to indicate a total number of counts in a 

 certain length of time or the length of time required to reach some pre- 

 determined number of counts. 



Counts of radioactive materials always are accompanied by determina- 

 tions of "background," that is, pulses produced from uncontrollable 

 sources such as cosmic rays or y-emitters in the general vicinity of the 

 counting tube. The background count, assumed to be constant during 

 the measurement, is subtracted from the total number of counts. Since 

 background varies from day to day, each measurement must be corrected 

 for background. Background can be reduced by shielding the counting 

 tube with lead or by rather intricate electronic correction, but it can never 

 be eliminated. 



Accessory Equipment: Samples of materials to be counted are placed 

 in small metal dishes and placed beneath the window of the G-M tube. 

 (The scintillation counter, of course, is more flexible, and the sample can 

 take any shape.) The samples are counted for a period of time adequate 

 to obtain a good measurement of average rate of radioactive disintegra- 



