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impinge on a nncleus before it is slowed down and its energy reduced 

 beneath this particular value. The curve of Fig. 15 for lithium 

 should then become horizontal at abscissa 400. At lower voltages, 

 both curves concur in implying that the probability of disintegration 

 depends on the energy of the proton. I will revert to this topic in a 

 later article. 



500 550 600 650 700 750 



ENERGY IN ELECTRON-MLOVOLTS 



Fig. 18 — Intensity of the mixture of neutrons and gamma-rays resulting when lithium 

 is bombarded by deutons. (Crane & Lauritsen) 



There are also modes of disintegration of lithium by deutons and 



by protons, in which neutrons and gamma-rays are emitted. These 



have been observed in Pasadena by Crane, Lauritsen and Soltan. 



Deutons are the more efficient of the two, but protons are sufficiently 



potent to have enabled Crane and Lauritsen to trace the curves of 



Fig. 18, in which the significant quantity is the difference between the 



ordinates of the two.^" The ionization-chamber was walled inwardly 



'"' Dr. Lauritsen writes me that the readings from which the lower curve is drawn 

 were unchanged when the high voltage was removed; presumably therefore they 

 represent the "background" due to the natural leaks of the electroscope. 1 am 

 indebted to his letter for other as-yet-unpublishe,d statements. 



