ASSAY OF RADIOACTIVITY 



the molecules of a gas by interaction with ionizing radiation. The interaction 

 that occurs in a phosphor is rather different, since electrons are not neces- 

 sarily displaced altogether from atoms in the path of the radiation, but 

 instead the energy level of some of the atoms is temporarily raised. When 

 such excited atoms return to their ground state, the energy imparted to 

 them in the interaction is emitted as photons of electromagnetic radiation. 

 In fluorescent or phosphorescent substances the wavelength of this radiation 

 is in the ultraviolet or visible part of the spectrum. Any material, liquid or 

 solid, which can readily form excited atoms is a potential phosphor, but 

 obviously only those materials that are transparent to the photons produced 

 within them are of practical value for scintillation counting. Another 

 criterion that needs to be satisfied for a substance to be a good phosphor is 

 that the period of light emission should be very brief. In general, this is 

 easily achieved, and the resolution times of scintillation counters are 

 appreciably less than those of gas chamber counters, being well below 1 

 ^sec for most inorganic phosphors, and shorter still for organic phosphors. 

 In order to detect y-rays with maximum efficiency it is necessary for the 

 radiation to traverse a substantial thickness (of the order of several cm) of 

 the phosphor, so that each y-photon has a good chance of giving rise to at 

 least one flash of light somewhere within the phosphor. A further require- 

 ment which somewhat limits the choice of inorganic phosphors is therefore 

 that it should be possible to obtain them as large clear crystals. The most 

 commonly used inorganic phosphors are calcium tungstate, zinc sulphide 

 and sodium or potassium iodide. The first of these substances is an efficient 

 phosphor in its naturally occurring pure state (scheelite), but the phosphor- 

 escence yields of the others can be considerably increased by the presence of 

 certain impurities known as 'activators' ; thus zinc sulphide crystals can be 

 activated by traces of manganese, while sodium iodide is usually thallium- 

 activated. The function of the activators is to produce imperfections in the 

 crystal lattice which help to ensure that the excited atoms return to the 

 ground state by emitting radiation in the visible spectrum, rather than by 

 handing the energy on to neighbouring atoms in such a way that in the end 

 it merely appears as heat. Various organic compounds are also good 

 phosphors, notably (in solid form) naphthalene or anthracene. For 

 liquid scintillation counting of low-energy ^-emitters, solutions of terphenyl 

 in xylene or of diphenyloxazole in toluene have given the best results. 

 The components of a complete scintillation counting system are as foflows: 



(1) Container for the isotope sample. If the sample is in liquid form, 

 as it often is when working with y-active isotopes, it may either be placed 

 in a small cylindrical vessel fitting into an annular slab of phosphor or 

 placed in an annular vessel which surrounds a cylindrical slab of phosphor. 



(2) A thick, light-tight lead castle enclosing sample, phosphor, photo- 

 multiplier and pre-amplifier. For very low-level fi- or y-counting, traces of 

 radioactive impurities in the lead may give unacceptably high background 

 counts, which can be reduced by using instead a massive iron shield and 

 an annular tank of mercury surrounding the phosphor (see Johnston^^). 



(3) An efficient light connection between the phosphor and the window 

 of the photomultiplier in order to reduce internal reflections at the surface 

 of the phosphor to a minimum. For special applications where the phosphor 



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