ASSAY OF RADIOACTIVITY 



and others have preferred to introduce the samples by adding them in 

 gaseous form to the fiUing gas. This technique has the double advantage of 

 completely avoiding absorption losses, and of achieving Att geometry, so 

 that it can be used for absolute as well as relative measurements of radio- 

 activity; however it entails a somewhat complicated filling system. In order 

 to exploit the undoubted advantages of counting isotopes hke ^H and ^*C in 

 the form of gases, it is probably better to employ a proportional counter 

 operating at atmospheric pressure, as discussed below. 



y-ray counters 



As was pointed out earlier, y-rays have only a low probability of producing 

 an ionization in the gas space of a Geiger tube, so that the counting efficiency 

 for y-radiation is apt to be very small. The secondary electrons produced by 

 photoelectric absorption and Compton scattering in the wall of the tube can, 

 however, be detected satisfactorily, and the overall y-counting efficiency of 

 a Geiger tube can therefore be somewhat increased by building it with 

 fairly thick walls and by using for these a material of high atomic number 

 (e.g. lead). Even with this refinement the maximum efficiency of Geiger 

 tubes for y-ray detection is not more than 1 or 2 per cent. For monitoring 

 y-radiation, Geiger counters have the important merit of simplicity, but for 

 quantitative measurements they have now been supplanted by the scintillation 

 counters described below, whose efficiency is very much higher. 



PROPORTIONAL COUNTERS 



In recent years proportional counters — that is to say gas chambers operating 

 in region (C) of Figure 31.1 — have come into increasing use for the assay of 

 radioisotopes. Their main advantages are that the dead time is very much 

 shorter than in a Geiger tube, enabling them to be used at much higher 

 counting rates, and that they are more stable in operation than Geiger tubes, 

 being less affected by such factors as changes in temperature. Their most 

 obvious disadvantage is that they produce relatively small pulses of current, 

 and their output therefore has to be fed to a high-gain linear pulse amplifier 

 in order to make the pulses large enough to trigger a scaler. They also 

 necessitate a rather well stabilized EHT power supply, and, for some 

 applications, elaborate discriminator circuits which select only pulses within 

 a given size range. However, for most purposes it is sufficient to use a 

 simple circuit which merely rejects all pulses below a certain size (i.e. to 

 provide a threshold control for the scaler), and provision of a suitable 

 amplifier is not difficult. Figure 31.9 shows a circuit with a good enough 

 frequency response to provide undistorted and linear amplification of the /usee 

 pulses produced by proportional counter tubes. 



It is possible to use a conventional end-window tube in the proportional 

 region of its characteristic, but most proportional counters are of the 'flow' 

 type, filled with continuously flowing gas at atmospheric pressure. Almost 

 any gas can be used, but methane is the usual choice since it has some 

 quenching action and allows a greater gas multiplication factor to be 

 employed. Samples may be placed beneath a thin side-window, or introduced 

 inside the counter with a sliding shelf arrangement. For the assay of tritium, 



438 



