52 ADVENTURES IN RADIOISOTOPE RESEARCH 



activities of the rare earth elements : we denote the intensity of dys- 

 prosium arbitrarily by 100. It is of interest to remark that the 2.3 min 

 activity of silver, which is considered a very strong activity, is 12 times 

 as weak as the activity of an equal amount of dysprosium. The hydrogen 

 effect (a) was found to be 100, the half-value thickness of the /?-rays 

 emitted was 0.025 cm Al ; and the upper limit of the continuous /5- 

 spectrum concluded from absorption measurements with aluminium 

 has an energy of 1.4 • 10^ eV (2).* Dysprosium is one of the commoner 

 rare earth elements of the yttria group and as it is very strongly active, 

 activated samples of rare earth elements denoted as "erbia", "holmia", 

 "yttria", etc. often decay with the period of dysprosium. 



Hoi 



nuum 



We found (2) the activity of holmium to decay with a period of 35 h., 

 while E. RoNA (6) recently found the value of 33 h. The half-life of 2.6 h. 

 measured by Marsh and Sugden (4) and later by McLennan and Rann 

 (5) is presumably due to the presence of dysprosium in their preparations ; 

 some of our impure preparations, too, showed an initial decay with the 

 period of dysprosium. The samples of holmia investigated were given 

 us by the late Baron Auer. Holmium has one stable isotope, 165 ; the 

 activity observed is therefore presumably due to the decay of ^evHo, 

 the intensity of the activity observed being 20 per cent of that ol 

 dysprosium. The hydrogen-effect (a) is much smaller (2) than that of 

 dysprosium; the half value thickness is 0.04 kgm/cm^Al ; and the upper 

 limit of the /^-ray spectrum has an energy of 1.6 • 10^ eV. 



Erbium 



Erbium has a very weak activity of similar intensity to the 40 min 

 samarium radiation, decaying with a 7 min period according to Marsh 

 and Sugden (4), and with a 4.5 min according to McLennan and Rann 

 (5). A second period (2) was found by us to be 12 h. ; the period of 2.5 h. 

 ascertained by Sugden (3) using a commercial preparation is presum- 

 ably due to the presence of dysprosium, and that found by Marsh and 

 Sugden (4), 1.6 d., to the presence of holmium in the sample investi- 

 gated. Recently Rona (6) has given the value of 13 h. for the longer 

 period. The intensity (2) of the longer period of erbium is 0.35 per cent 

 of that of dysprosium, and the half- value thickness of the ^-rays emitted 

 is 0.03 cm Al. 



* R. Naidu and R. E. Siday, loc. cit. found that the maximum energy lies 

 at 0.75 • 108 (.y. while l.S • ]0« eV is ihe upper limit. 



