TYPES OF GEIGER TUBE 



Absorption errors — Any radiation that is absorbed by the window or wall 

 of the Geiger tube, or within the sample itself, will not be recorded, thus 

 cutting down the counting rate. Self-absorption by the upper layer of 

 radiation emitted by the lower layer of a solid sample is a particularly 

 dangerous source of error when working with soft ^-emitters. Again, the 

 usual way of overcoming this source of error is not to correct for it, but to 

 ensure that there is the same degree of self-absorption in both unknown 

 samples and in standards. This means that in counting solid samples, 

 unknowns and standards must occupy the same area (if they do not, there 

 may also be positioning errors) and have the same total weight of material 

 per unit area, while in using liquid counters, densities must be kept the same 

 in unknowns and standards. When attempting to balance out errors in this 

 fashion it is, of course, desirable to keep all absorption losses as low as 

 possible, using the most appropriate type of counter for any given isotope. 



Scattering errors — To radiation travelling directly from the sample into 

 the sensitive volume of the Geiger tube is added a certain amount of radiation 

 scattered from the walls of the enclosure in which tube and sample are 

 contained, from the support on which the sample is mounted, and from 

 within the sample itself. It is possible to correct for such scattering effects, 

 but simpler once more to see that they are the same for unknowns and 

 standards. It should be noted that back-scattering from the sample mount 

 may make an appreciable contribution to the total count. 



Timing of counts 



For many purposes it is sufficient to time the counts of isotope samples 

 by hand, using a stop-watch. In handling large numbers of samples it is, 

 however, more convenient to use some form of automatic timer. Timers 

 incorporating a standard time source and scaling circuits have been devised 

 (and are available commercially) to give a choice of two methods of control- 

 ling a scaler: (1) to determine the time taken for a pre-set number of pulses 

 to be recorded, (2) to determine the number of pulses recorded during a 

 pre-set time interval. The advantage of method (1) is that it pre-determines 

 the standard error of each count. A timer of this sort is complex and 

 expensive, and the writer has always used a simple automatic timer constructed 

 from a cheap clock and a relay (see Cook and Keynes^^), which over a 

 number of years has proved entirely reliable. 



TYPES OF GEIGER TUBE 



End-window counters 



Probably the most commonly employed type of Geiger tube is the end- 

 window counter illustrated in Figure 31.4. End-window tubes are available 

 from several makers, e.g. Mullard (type MX123), G.E.C. (types GM4 and 

 EHM2S), 20th Century Electronics (type EW3H), and others. Most of 

 these tubes have very thin mica end-windows, making them suitable for 

 work with soft j5-emitters like ^^C and ^^S; but tubes with duralumin or 

 glass end-windows are also manufactured, these being more robust but also 

 somewhat thicker. Some are high-voltage tubes quenched with alcohol 



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