110 



RADIOACTIVITY; BIOLOGICAL TRACERS 



Figure 5-4. Measurement of Radioactivity. Left: thin- 

 walled Geiger tube for alpha- and gamma-ray detection. 

 Right: a typical survey instrument with protected, detach- 

 able detector tube. Typical ranges: to 0.25 mr/hr; to 

 2.5 mr/hr. 



photocathode of a photomultiplier tube, causing the ejection of more elec- 

 trons. These are multiplied in number by an internal secondary-emission 

 system to produce a measurable current pulse for each scintillation. A typi- 

 cal arrangement is shown in Figure 5-5. Certain organic liquids also 

 fluoresce, and very sensitive liquid counters have recently been developed. 



Each of the counters discussed in paragraphs (2) and (3) has specific uses, 

 tor a radiation such as the l.Z-Mev gamma from Co 60 , for instance, the 

 scintillation counter can have efficiencies as high as 15 per cent as compared 

 to 1 per cent for a Geiger counter. Therefore, for medical tracer applica- 

 tions of gamma in which the intensity is low, a scintillation counter would 

 be preferred over a Geiger counter. However, if dosage is high, as it may 

 be in radiation therapy, the extra sensitivity is not important. Figure 5-6 



photons 





scintillation 

 phosphor 



photocathode 







JT 



\ 



*fl 



X 



optical coupling 

 to photocathode 



pre-amplifier 



T 



photomultiplier 

 assembly 



pulse 

 output 

 to 

 ammeter 



Figure 5-5. Schematic Drawing of Scintillation Counter. (See text.) 



