56 



ADVEXTURES IX RADIOISOTOPE RESEARCH 



Table 1. — Absorption of Slow Neutrons in Rare Earth Elements 

 (Amount necessary to reduce the activity of the indicator by ten per cent) 



that the isotope formed by the capture process is not a stable one already 

 known but an active one hitherto unknown. It is a matter of experience 

 that a mass number cannot be occupied both by a stable and an active 

 isotope of the same element, so that should the mass number 166 be 

 occupied by a stable dysprosium isotope the high capturing power for 

 slow neutrons shown by ^HDy would not lead to an active but to a 

 stable dysprosium isotope. The appearance of a strong activity shows 

 that at least one isotope of this element captures neutrons strongly, 

 but high absorption does not necessarily imply strong activity because 

 nuclei yielding stable isotopes can also be very strong absorbers of neu- 

 trons. To obtain information about the existence of strongly capturing 

 rare-earth nuclei not leading to the formation of radioactive products, 

 we compared the activities of dysprosium, europium, and holmium with 

 their absorbing powers for the same neutron beam as was used to activate 

 them. The results of these measurements, in which the absorbing el- 

 ement itself was used as indicator, are seen in Table 2. 



Table 2. — Absorption of Slow Neutrons in Rare Earth Elements 

 ( Amount necessary to reduce the activity of the indicator by ten per cent) 



While the activity of europium is slightly smaller than that of dyspro- 

 sium its absorbing power is more than twice as big ; europium thus 

 absorbs slow neutrons to an appreciably larger extent than is to be 

 expected from the activity of the radioactive europium isotope formed. 

 To explain this discrepancy we have to assume that besides the 



