TYPES OF GEIGER TUBE 



/^-emitter like ^^K the counting rate for a given amount of the isotope is 

 about the same for a skirted liquid counter used in the manner just described 

 as it is for a conventional end-window counter. With weaker radiation the 

 efficiency falls because of absorption losses, but is adequate for many of the 

 isotopes of importance in biological work. In using liquid counters care 

 must be taken to avoid errors from adsorption of the isotope on the glass 

 surfaces, which in the author's experience may give trouble when working 

 with an anion such as ^^P04, but is not a serious problem with cations like 

 24Na and *^K. It is essential to wash out the tube thoroughly between 



To wash bottle 

 mounted above 

 castle 



Removable 

 plug to allow 

 observation of 

 fluid level 



Lead castle 



Standard ground 

 glass joint fitting 

 either receptacle 

 for washings or 

 tubes for re-collec 

 -tion of samples. 



Figure 31.8 An arrangement for washing out liquid counter tubes without 



removing them from the castle 



samples, either removing it from the lead castle for the purpose or else 

 fitting the castle with an arrangement for washing in situ. The system shown 

 in Figure 31.8, with a reservoir of wash-fluid mounted above the castle and 

 a fine glass tube inserted into the counter in order to empty it by suction, 

 proves very convenient to use. Two or three washes suffice to bring the 

 counting rate down to background, even after a sample giving several 

 thousand counts per minute. 



Inside counters 



For the weakest /5-emitters, even a thin mica end-window can cause 

 appreciable absorption losses, and some workers have therefore used 

 demountable tubes, similar in shape to an end-window tube, in which 

 solid samples can be counted inside rather than outside the gas space. With 

 such an arrangement there are still losses from self-absorption in the sample, 



29 437 



