Neutron yields from thick targets bombarded by 32- and 18-Mev protons were measured for 59 elements and compounds. For 23 metallic elements, yields were also measured from targets thick enough to degrade the proton energy from 32 to 18 Mev. The neutrons were detected by counting the ${\mathrm{Mn}}^{56}$ activity produced by neutron capture in MnS${\mathrm{O}}_4$ solution contained in a 3-ft by 3-ft tank that surrounded the targets. The efficiency of the neutron-detection system was determined with a calibrated Ra-$\alpha$-Be source. For all three energy intervals, the neutron yields increased abruptly at $Z=20\mathrm{}$ and $Z=30\mathrm{}$, and Ni had a yield $\frac{1}{3}$ that of neighboring elements.

The yields were compared with calculations based on the statistical theory of nuclear reactions. The agreement was fair for $Z>\mathrm{}30\mathrm{}$, but the calculated yields did not reproduce the sudden jump at $Z=30\mathrm{}$.

• Received 8 November 1957

DOI:https://doi.org/10.1103/PhysRev.109.2086