Europium Activation Studies with Monochromatic Neutrons

Resonances in the europium slow neutron cross section at -0.011, +0.327, 0.461, 1.055, 2.74, 3.35, and 7.36 ev have been assigned to ${\mathrm{Eu}}^{151}$. This was done by observing the intensity of the 9.3-hour activity of ${\mathrm{Eu}}^{152}$ produced as a function of neutron energy. The remainder of the europium resonances below 10 ev were assumed to be due to ${\mathrm{Eu}}^{153}$; i.e., 1.76, 2.47, 3.84, 6.25, and 8.98 ev. The fraction of the compound nuclei decaying to the isomeric state of ${\mathrm{Eu}}^{152}$ was found to vary by a factor of four for different resonances in ${\mathrm{Eu}}^{151}$. These fractions were obtained for six of the seven resonances in ${\mathrm{Eu}}^{151}$.