SCINTILLATION COUNTERS 



has to be at a distance from the photomultiplier (e.g. where the phosphor 

 is used as a probe which can be inserted into living tissues containing 

 y-active isotopes) hght can be piped from the phosphor along an internally 

 polished metal tube, or along a light-guide consisting of a rod of trans- 

 parent plastic material. 



(4) A photomultiplier tube (see Chapter 28) whose spectral sensitivity 

 matches the wavelength of the light produced in the phosphor. 



(5) A well stabilized high-voltage supply (see Chapter 37) and a chain 

 of high-stability resistors to provide suitable voltages for the electrodes 

 of the photomultiplier tube. The overall gain of the photomultiplier varies 

 as a high power of the applied voltage, so that good stabilization of the 

 supply is essential. 



(6) A linear pulse amplifier to make the output pulses from the photo- 

 multiplier large enough to trigger a scaling circuit, or to be fed to a dis- 

 criminator circuit. 



(7) A scaling circuit (see Chapter 41) with adjustable threshold. 



(8) For some applications it is advantageous to use a single-channel 

 analyser (a 'kick-sorter') between the amplifier and scaler. This consists 

 of a double discriminator circuit, arranged so that only pulses whose height 

 falls between the two discriminator settings are allowed to reach the scaler, 

 smaller and larger pulses being rejected. Since, as in a proportional counter, 

 pulse height depends on the energy of the radiation emitted by the isotope, 

 such a device may permit discrimination against background radiation 

 or between radiation emitted by two diff'erent isotopes. An appropriate 

 circuit is described by Farley^^. 



(9) Another refinement for lowering the background in liquid scintillator 

 low-level /5-counting is to use two photomultipliers mounted on opposite 

 sides of the sample container, and after feeding their outputs to separate 

 amplifiers and single-channel analysers to use a coincidence circuit to 

 count only those pulses which appear simultaneously in both channels 

 (see Arnold^'). 



The pulse height given by a scintillation counter varies with: (a) the 

 voltage applied to the photomultiplier tube, and (b) the energy of the 

 exciting radiation. If a scaler with variable threshold is employed so that 

 all pulses below a certain size can be rejected, the counting rate for a given 

 isotope sample is affected both by the threshold setting and by the operating 

 voltage. The curve relating counting rate to voltage does not show a 

 plateau like the characteristic curve of a Geiger tube (Figure 31.7), but 

 since, in general, the background pulses differ in size from those due to 

 the sample, a setting of the controls can usually be found which represents 

 the optimum compromise between high sensitivity and high background. 

 By using a single-channel analyser it is possible to discriminate still better 

 against background pulses. Good discrimination is fairly easy to achieve 

 for isotopes giving strong ^- or y-radiation, but for weak radiation the 

 gain of the photomultiplier tube may have to be increased to the point 

 where pulses originating from the emission of thermal electrons at the 

 photocathode begin to be recorded. Unless special measures are taken 

 this sets a limit to the energy of the radiation which can usefully be detected. 

 However, by such stratagems as cooling the photomultiplier to — 20°C and 



441 



