50 ADVEXTTEES IX RAmOISOTOPE RESEARCH 



As lanthanum has but one stable isotope, ^^^ha, the activity obtained 

 is presumably due to the formation of ^|!^La. Fermi's coefficient a, 

 indicating the increase in activity when the bombarding neutrons 

 are slowed down by a thick layer of paraffin or other hydrogen - 

 containing substances, instead of being allowed to impinge directly from 

 the beryllium source on to the substance to be activated, was found 1o 

 be 12. 



Cerium 



No activity was observed after Ijomljardment of cerium for several 

 days with a neutron source of few hundred millicurie. 



Praseodymium 



Amaldi, Fermi, and others (1) found the artificial radioactivity of 

 praseodymium to decay with a 19 h. period, the same value being found 

 later by other experimenters (4), (5). Although only one stable isotope 

 of praseodymium is known. ^^^Pr, the above-mentioned investigators 

 found a second period of decay (5 min) which in contrast to the first 

 period is not hydrogen-sensitive. * 



Neodymium 



Fermi and his collaborators (1) found that activated neodymium 

 decays with a period of 1 h. ; we find (2) that this activity is 2500 times 

 as small as that of dysprosium. Marsh and Sugden (4) found no activity, 

 while according to McLennan and Rann (5) the half-life is 35 min. 

 Neodymium has the stable isotopes 142, 143, 144, 145, 146 and 148, and 

 the activity observed is presumably due to the formation and decay of 



497 

 fiO-l 



"Wd 



Samarium 



The artificial radioactivity of samarium decays, as was found ))y 

 Fermi (1), and later by us, with a period of 40 min. We find (2) the 

 intensity of the activity to be 0.6% of that of dysprosium. Samarium has 

 furthermore, as was first noticed by Marsh and Sugden (4), a much 

 longer period as well. We determined the period of this isotope to be 

 2 d., as can be seen from Fig. 1 and found its intensity to be ~ of that 

 of dysprosium, i.e. 2.0 on our relative scale. Samarium has the stable iso- 

 topes 144, 147, 148, 149, 150, 152, and 154, and it is not possible to deter- 

 mine the mass number of the active samarium isotopes with certainty. 



