SCINTILLATIONS. 



By E. MARSDEN. 

 Lecturer in Physics, University of Manchester. 



It is well known that there are three principal types 

 of radiation from radioactive substances designated 

 a, /3 and y. The first of these types, the a rays, 

 or a particles, are single atoms of helium carrying 

 a double elementary positive charge and moving 

 initially with a velocity of the order 2x10" centi- 

 metres per second or one fifteenth the velocitv of 

 light. Different radioactive products emit a particles 

 of different speeds but the speed is characteristic 

 of the product. The a particles are very easily 

 absorbed by material substances, 

 the swiftest known, those from 

 Thorium C, being completely 

 absorbed by 8-6 centimetres of 

 air and the slowest known, those 

 of Uranium I, only penetrating 

 2-5 centimetres of air at atmos- 

 pheric pressure. 



The second type of radiation, 

 the /3 rays, are known to consist 



of electrons or isolated elementary negative electric 

 charges. They travel with enormous velocities 

 varying for different products, and also for the same 

 product, up to as much as 0-998 of the velocity of 

 light. The /? rays are not so easily stopped as the 

 a rays, travelling on the average about one metre in 

 air before they are stopped. 



The third type of radiation, they rays, differ from 

 the two preceding types in that they appear to be 

 electrically uncharged. They are similar to very 

 penetrating Rontgen rays, but their exact nature 

 constitutes the battling ground of many rival 

 theorists, for the problem appears also to involve that 

 of the fundamental constitution of light waves. 

 The y rays seem to be closely connected with the 

 ft rays in much the same way as Rontgen rays are 

 connected with cathode rays. All radioactive 

 products which emit y rays also give /3 rays, although 

 certain products are known which emit $ rays with- 

 out any appreciable y radiation. 



One of the most remarkable of the properties of the 

 various radiations is that of producing luminescence 

 in certain substances on which they fall, and more 

 particularly remarkable is the scintillating property of 

 the a rays. This property was first discovered by Sir 

 William Crookes, and independently by Elster and 

 Geitel, in 1903, but five years elapsed before its full 

 significance was recognised ; for it is now known that 

 each scintillation is produced by a single a particle 

 or atom of Helium. Crookes made a screen by 

 dusting Sidot's blende (phosphorescent crystalline 

 zinc sulphide) on glass. On bringing up a source of 

 radium the screen lit up with a greenish phosphor- 

 escent light which, when examined under a 



Figure 64. 



magnifying glass, was found to consist of a number 

 of scintillating points of light. 



Subsequent work has shown that many other 

 substances show this scintillating property though 

 the scintillations are generally fainter than with 

 zinc sulphide; the best known substances being 

 willemite (a mineral containing zinc silicate) barium 

 platinocyanide and diamond. Many of these 

 materials appear to require the presence of some 

 impurity ; thus pure zinc sulphide does not show 

 scintillations while some of the 

 purest diamonds fail to respond 

 to any of the radium radiations. 

 In nearly all cases also the scin- 

 tillating substances appear to lose 

 their sensitiveness under pro- 

 longed action of the a rays, and 

 for this reason the zinc sulphide 

 screens of Crooke's spinthari- 

 scopes after a time need renewal. 

 Diamond and willemite are more stable than zinc 

 sulphide, for they retain their scintillating power 

 longer, while barium platinocyanide, on the other 

 hand, is very rapidly transformed under the action of 

 a rays. 



It was at first thought by Becquerel and others 

 that the scintillations produced in zinc sulphide, for 

 example, are the direct result of the mechanical 

 fracture or cleavage of the crystals by the a particles, 

 since it is well known that zinc sulphide is very 

 sensitive to mechanical shocks. Recent evidence, 

 however, appears to discredit this hypothesis and 

 points to the conclusion that the scintillations are in 

 some way the result of the enormous local ionisation 

 produced in the zinc sulphide by the a particles ; for 

 it is well known that an a particle produces about 

 two hundred thousand ions, before its energy is 

 absorbed. 



In 1908, Rutherford and Geiger succeeded in 

 detecting the emission of a single a particle by an 

 electrical method and were thus able to count the 

 a particles from a given quantity of radium. By 

 comparing this number with the number of scintil- 

 lations produced under proper conditions on a zinc 

 sulphide screen they were able to show that practi- 

 cally every u particle produces a scintillation. Thus 

 the scintillation method can be used for quantitative 

 measurements in radioactivity and this method has 

 since proved of very great value in such investiga- 

 tions ; for the electrical method of counting a par- 

 ticles is cumbersome and requires very special 

 apparatus. A piece of glass or other transparent 

 material is coated with small crystals of sine sulphide 

 and the observations are made in a dark room with a 



55 



