Nuclides formed as spallation and fission products during bombardment of tantalum metal with 340-Mev protons in the 184-inch Berkeley cyclotron were separated chemically, identified, and their formation cross sections were calculated. A very broad fission peak which extends from mass 20 to mass 132 is observed. The maximum fission yield occurs in the region of the nuclide ${\mathrm{Kr}}^{83}$ and analysis of a set of contour curves fitted to the data indicates that either ${\mathrm{Hf}}^{166}$ or ${\mathrm{Lu}}^{166}$ is "the most probable fissioning nucleus." The total cross section for fission is estimated to be 4.1 mb. Comparison of the fission data of tantalum with those of uranium and bismuth under the same bombardment conditions indicates that asymmetric fission is much more probable in tantalum than in either of the other elements. In the spallation region it is observed that neutron emission is the predominant spallation reaction. Integration under the spallation yield curve indicates that of those tantalum target nuclei which received at least enough excitation energy to reach the region of "the most probable fissioning nucleus" less than 1 percent undergo fission; the remainder undergo spallation reactions.

Activities which have been observed here for the first time include a 136-minute rhodium activity which has tentatively been assigned as an isomer of ${\mathrm{Rh}}^{107}$; 29.4-hour ${\mathrm{Er}}^{160}$; 5.0-hour ${\mathrm{Ho}}^{160}$; and 74-minute ${\mathrm{Yb}}^{167}$.

• Received 27 September 1954

DOI:https://doi.org/10.1103/PhysRev.97.1092