54 ADYEXTIEES IX KADIOISOTOPE KESEAECH 



orof ^^^Yl). The ytterbium radiation (2) is somewhat weaker than that 

 of erbium, and amounts to 0.3 per cent of that of dysprosium. The 

 half-value thickness of the ;5-rays emitted is 0.04 cm Al. 



Lutecium 



Lutecium (cassiopeium) exhibits an activity of fairly long life (2), 

 namely one decaying with a period of 6 — 7 d.. and having an intensity 

 of L4 per cent of that of dysprosium ; there is a second activity of 

 somewhat less intensity decaying with a period (2) of 4 h.; as lutecium 

 (cassiopeium) and ytterbium are very closel^y related elements, and 

 lutecium being usually contaminated with ytterbium, we considered it 

 possible that the 4 h. period observed might be due to the presence of 

 ytterbium in the sample investigated. A very pure lutecium (cassiopeium) 

 preparation, however, prepared l)y Auer and kindly lent us by Prof. 

 HoNiGSCHMiD. also showcd the 4 h. period. Furthermore the intensity 

 of this radiation was stronger than that emitted by a pure ytterbium 

 preparation activated by a neutron source of the same strength. So we 

 must conclude that both the periods observed are due to lutecium. 

 The long period of decay has not been observed by any experimenter 

 besides us, presumably because the times of exposure have been too 

 short. For the shorter period McLennan and Rann (5) give a value of 

 3.6 h. and Rona (6) 4—5 h. The decay of the lutecium preparation lent 

 us by Prof. Honigschmid is seen in Fig. 2. the time of exposure 

 being 2.8 days. In comparing the intensities of the long and the short 

 periods the former must be divided by 0.267 which value follows from 

 a consideration of the relation J^ = '\ (1— c'^'M- where J^ is the 

 saturation value of the activity. J^. the value obtained after t days, and 

 P. the decay constant (= half-life/ln 2). As can be seen from the Fig. 

 2b a third long period is present in the activated lutecium which is 

 possibly due to the presence of small amounts of thulium. 



ABSORPTION OF SLOW NEUTRONS BY RARE EARTH ELEMENTS 

 Determination cf the period of decay from absorption data 



When faced with the problem of determining the period of very slo^\ly 

 decaying radioactive isotopes having half-lives of several months or 

 years, decay measurements become very tedious. In such a case we can 

 obtain information about the decay constant required by comparing 

 the absorption of slow neutrons in the rare earth element in question 

 with that in another rare earth element of known period. A knowledge 

 of this ratio and of the activities obtained for Iwth elements after a known 



