420 History of Luminescence 



terial by a current of air. A similar emanation from radium was 

 explained as a gas by E. Dorn (1900) , and F. Giesel (1903) obtained 

 it from actinium, another radioactive element, discovered in 1899 

 by A. Dieberne. Uranium does not produce an emanation. 



The three emanations are actually elements, isotopes, belonging 

 in the series of inert gases— helium, argon, etc. Radium emanation 

 has been given various names, exradio, radeon, radon, or niton, 

 from nitere, to shine. Some very striking luminous experiments 

 can be carried out with radon or niton, whose rays cause a transi- 

 tory luminescence of zinc sulphide when blown over a screen of 

 this material. If drawn through a tube containing willemite (Zn 

 silicate) , there results a brilliant luminescence of the mineral, 

 despite the infinitesimal concentration in which the gas is present. 

 Kept in any container, the emanation causes luminescence of the 

 walls, blue in quartz, lilac in sodium glass, and blue green in lead- 

 potassium glass. 



The fact that cathode rays will cause the development of color in 

 certain crystals, which then emit light on gentle heating, has been 

 mentioned previously. The same result can be obtained with ioniz- 

 ing radiation such as X-rays and gamma rays. In the twentieth cen- 

 tury it has been established that this effect is of two sorts. In some 

 material, the ionizing radiation may bring about changes which re- 

 sult in luminescence after heating or exposure to light and the 

 change is irreversible, i. e., a second heating or exposure to light 

 will not cause a second luminescence. This effect is called thermo- 

 stimulation or photostimulation. In other materials, new permanent 

 stable centers are formed by ionizing radiation, thereby creating a 

 material which will fluoresce when exposed to light, whereas the 

 unradiated material is not fluorescent. This effect, which is called 

 radiophotoluminescence is permanent, and has become important, 

 in detecting X-radiation and gamma radiation. 



Scintillation Counters and New Particles 



A trace of radium salt added to a zinc sulphide phosphor pro- 

 duces the luminous paint of watch dials. If this paint is examined 

 with a low power microscope, numerous little flashes of light or 

 scintillations may be seen, each one of which is due to an alpha 

 particle (a charged helium atom) hitting the zinc sulphide. The 

 effect,^* discovered independently by Wm. Crookes, by H. Becquerel, 

 and by E. Elster and H. Geitel, all in 1903, can be used to count 



^* H. Becquerel believed that the alpha particles actually make a small fracture in 

 the ciystal, resulting in a triboluminescence. 



