The Neutron Absorbing Isotopes in Gadolinium and Samarium

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Abstract
The large absorption for slow neutrons in samarium was found to be caused by the isotope at mass 149, and in gadolinium to the isotopes at masses 155 and 157, the cross section for 157 being about three times that for 155. Mass spectra were made with the double focusing mass spectrograph, using the spark with nickel tubes packed with oxide as electrodes. Mass spectra of normal samples were compared photometrically with those of samples in which this isotope abundance had been altered by long exposure to slow neutrons. In the altered gadolinium the abundance of mass 157 was less than 155 instead of greater, and 156 became greater than 160 rather than less. In samarium the abundance of mass 149 was reduced to be nearly as weak as 148, and mass 150 was correspondingly increased.

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