Using 14-Mev deuterons from the M.I.T. cyclotron, proton spectra have been measured for many heavy and medium atomic weight elements. The range of the protons was measured in aluminum foils by the method of "peaking" with a triple proportional counter. From the $Q$-value of the ground-state proton group (measured relative to a $Q$-value of 5.50 Mev for the aluminum ground-state group), the neutron binding energy of the residual nucleus has been obtained. These values are compared to the maximum gamma-ray energy obtained from thermal neutron capture. A few neutron binding energies have been measured from the ($d,t$) reaction and are compared with values obtained from the ($\gamma ,n$) threshold. Several neutron binding energies have been computed from radioactive decay energies and measured neutron binding energies. All neutron binding energies have been compared with the values predicted from the semi-empirical mass formula. A sharp drop in neutron binding energy of about 2.2 Mev occurs at the completion of the closed shell of 126 neutrons. A drop, a little greater than 2 Mev, occurs at 50 neutrons. There is also a decrease in the region of 82 neutrons. There also seems to be a decrease of about 1 Mev in the region of 29 neutrons, but this is not very conclusive.

The differential cross section of the ground-state peaks from even $Z$, odd $A$ isotopes at forward angles are much smaller (about a factor of ten) than the differential cross sections of the ground-state peaks from the same $Z$ but even $A$ isotopes. The ground-state peaks from ${\mathrm{Sn}}^{117}$ and ${\mathrm{Pb}}^{207}$, however, are about 50 percent that of the even $A$ isotopes.

Close

PrevNext

View figure in article