THE ACTIOX OF XEITROXS ON THE RARE EARTH ELEMENTS 51 



A very intense aclivity was ol)lained by Sugden (3) on bombarding 

 europium with slow neutrons. It decayed with a period of 9.2 h. The 

 intensity of the europium radiation was found by us (2) to be 80% 

 of the dysprosium radiation emitted l)y the same amount of dysprosium, 

 both preparations being activated until saturation was reached. Care 

 was taken, too, that the neutron beam was weakened only to a small 

 extent by the activation process, i.e. very thin layers were activated. 

 Europium has two stable isotopes 151 and 153 and the activity is 

 possibly due to the formation of ^^^Eu. Th.e europium : dysprosmm 

 activity ratio is found to be smaller for thick layers than for thin 



The value 40 was found for the hydrogen-effect, a. The half-value 

 thickness (2) of the ^-rays emitted is 0.02 cm Al, and it was concluded 

 from absorption measurements that energies up to 2.0-106 eV occur.* 

 In addition, y-rays have been detected which are little absorbed by 4 

 mm. lead. 



Gadolinium 



Fermi and others (1) found gadolinium to decay with a period of 

 8 h. after neutron bombardment. McLennan and Rann (5) found a 

 half-life of 6.4 h. and twice the intensity found for neodymium. The 

 combination of the last mentioned figure with our intensity data leads 

 to an intensity value which is 250 times as small as that observed for 

 dysprosium. Marsh and Sugden (4) could not find any activity. 



Terbium 



The activity of terbium (3) decays with a period of 3.9 h. As this ele- 

 ment has only one stable isotope, ^^Tb, the activity observed is presum- 

 ably due to the formation and decay of TsTb. The intensity of the 

 radiation (2) observed is 2.5 per cent of that of dysprosium. 



Dysprosium 



The activity of dysprosium (2), (4) decays with a period of 2.5 h. 

 and is the strongest yet observed in the domain of artificial radioactivity. 

 We have therefore chosen it (2) as a standard of comparison for the 



* R. Naidxj and R. E. Siduy {Proc. Roy. Soc A 48, 332, 3(5) by using a cloud 

 chamber determined recently the energies of the ^-ray spectra and found that 

 the maximum energy lies at 1 .3 ■ 10« eV, while the upper limit of the spectrum 

 is 2.6 • 106 eV. 



4* 



