Radioactive Decay ofLa137andCe137

A long-lived radioactive isotope of lanthanum has been produced by neutron bombardment of natural cerium. It decays with emission of Ba x-rays and has been assigned as ${\mathrm{La}}^{137}$. The $K$-electron capture halflife is (6±2)×${10}^4$ yr calculated from the Ba $K$ x-ray intensity and the yield of mass 137 atoms in the bombardment. Approximate measurements of neutron cross sections of ${\mathrm{Ce}}^{136}$ gave 6.3 barns for activation of ${\mathrm{Ce}}^{137}$ and 0.6 barn for activation of ${\mathrm{Ce}}^{137m}$. A previously unobserved 10-kev $M1\mathrm{}$ transition has been discovered in the decay of ${\mathrm{Ce}}^{137}$. A decay scheme is proposed which has a $g_{\frac{7}{2}}$ ground-state spin assignment for ${\mathrm{La}}^{137}$. Electron capture to the $d_{\frac{3}{2}}$ ground state of ${\mathrm{Ba}}^{137}$ on the basis of this assignment is second-forbidden.