CHAPTER XII 



RADIOLUMINESCENCE 



Introduction 



THE WORD " RADIOLUMINESCENCE " has been used in a broad sense 

 to indicate emission of light by substances when exposed to radia- 

 tion of various kinds other than light, whether streams of particles, 

 or certain regions of the electromagnetic spectrum, X-rays, and 

 gamma rays. A radioluminescence from streams of particles arising 

 from cathode or anode in a vacuum tube was a middle nineteenth- 

 century discovery (1858). Depending on the kind of particle in- 

 volved, corresponding names have been applied to the resulting 

 light— cathodoluminescence and anodoluminescence. In modern 

 terminology electroluminescence and ionoluminescence have been 

 used to indicate the nature of the particles, electrons or ions. 



More specifically, radioluminescence applies to the luminescence 

 from impact of X-rays or the gamma rays given off by radioactive 

 elements. Knowledge of such specific radioluminescence effects 

 began with Wilhelm Konrad Roentgen's (1845-1923) demonstra- 

 tion of X-rays in 1895 and played an important part in the search 

 for penetrating radiation from naturally occurring materials. This 

 led to the discovery of uranium radioactivity and to the isolation of 

 radium itself and the demonstration of gamma rays. Never before 

 had observation of luminescence been followed by such momentous 

 results. 



Cathodoluminescence 



Experiments on electrical discharges through a vacuum soon 

 demonstrated that a luminescence (fluorescence) appeared when 

 rays which were obviously not light rays, since they could be bent 

 by a small magnet (J. Pliicker, 1858) ,^ struck various solid sub- 

 stances. Peculiar luminescent effects occurred in highly exhausted 

 tubes when something from the negative electrode or cathode struck 

 the glass of the tube. This phenomenon had been observed as early 

 as 1858-1859 by J. Pliicker (1801-1868), by E. Becquerel (1820- 

 1891) and by J. P. Gassiot (1817-1879). Becquerel noted that the 

 light at the cathode was of a different wave-length from that at the 



^ De la Rive (1849, 1858) had noted movement of luminous discharges in gas under 

 the influence of a magnet, and used the observation in explanation of the aurora 

 borealis, but made no mention of luminescence of the glass. 



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