92 



STUDIES IN LUMINESCENCE. 



temperature and allowed to decay to 7 = o.8 before heating. The points 

 are joined by straight lines to aid the eye in following the individual curves. 

 Two effects are noticeable at a glance. The maximum intensity increases 

 with excitation and is shifted to the right, i. e., comes at a later time. The 

 effect of saturation is shown. This is brought out more clearly in Fig. 89. 



Fig. 88. Curves similar to Fig. 87. Effect of varying the 

 length of excitation. The length of excitation and the 

 temperature of the furnace are as follows: Run 6, 320.1 

 sec, 309 C; run 5, 159.9 sec, 308 C; run 4, 80.2 sec. 

 308 C; run 3, 39.9 sec, 308 C; run 2, 20.0 sec, 309" 

 C; run i, 10.0 sec, 307 C. 



Fig. 89. Saturation effect. 7 M vs. length of excitation. 

 Curve A , temperature 308 C : curve B, temperature 267 

 C; curve C, temperature 207 C.; curve D, temperature 

 155 5 C; curve E, temperature 98 C 



Fig 88. Fig. 89. 



To get the maximum point in each run, smooth curves, not shown in 

 the figures, were drawn through the different points. These curves show 

 that increasing the excitation beyond a certain length does not increase 

 the energy manifested as thermo-luminescence. That saturation takes 

 place is further shown by the time that luminescence lasts. Figs. 87 and 88 

 show that the duration of luminescence has about reached a maximum. 

 The areas included between the curves and the coordinate axes also show 



Fig. 90. 



Effect of delay in heating. Time measured from the end of excitation. Excited 320 seconds at room 

 temperature. Temperature of furnace 303 C Curve A, decay at room temperature. The time 

 between the end of excitation and the beginning of heating is as follows: 



Curve B, 2.5 seconds; C, 21.4 seconds; D, 41.3 seconds; E. 81. 1 seconds; F. 162.0 seconds; G, 321.6 

 seconds; //, 631.8 seconds; /, 1280.0 seconds. 



that further increase with increased excitation is limited. The shifting 

 of maximum intensity resembles an inertia effect; the longer the excitation 

 the longer it takes the temperature to produce the maximum thermo-effect. 



