Excitation Functions for Single-Nucleon Transfer Reactions Close to the Barrier

Thick targets of KI, RbCl, ZnS, and Al were bombarded with 27.5-MeV ${\mathrm{N}}^{14}$ ions and excitation functions were obtained for the following single-nucleon transfer reactions: ${\mathrm{K}}^{39}$(${\mathrm{N}}^{14}$,${\mathrm{O}}^{15}$)${\mathrm{Ar}}^{38}$; ${\mathrm{Cl}}^{35}$(${\mathrm{N}}^{14}$,${\mathrm{O}}^{15}$)${\mathrm{S}}^{34}$; ${\mathrm{Cl}}^{35}({\mathrm{N}}^{14}, {\mathrm{N}}^{15}){\mathrm{Cl}}^{34m}$; ${\mathrm{S}}^{32}$(${\mathrm{N}}^{14}$,${\mathrm{O}}^{15}$)${\mathrm{P}}^{31}$; and ${\mathrm{Al}}^{27}$(${\mathrm{N}}^{14}$,${\mathrm{O}}^{15}$)${\mathrm{Mg}}^{26}$. The proton transfer reactions were identified by the annihilation radiation due to the ${\mathrm{O}}^{15}$ (2.1 min) positrons; the neutron transfer reaction was identified by the characteristic 145-keV $\gamma$ ray of ${\mathrm{Cl}}^{34m}$ (32.4 min). The cross sections at 27.0 MeV (lab) were found to be: 0.35, 1.35, 0.73, 1.55, and 0.45 mb for the reactions in the order listed above. Single-nucleon transfer excitation functions at low bombarding energies, available from previous works, were accumulated and examined from the standpoint of the tunneling theory. It was found that reactions involving the targets ${\mathrm{B}}^{10}$ and ${\mathrm{N}}^{14}$ invariably behave anomalously. Explanations of this behavior are offered. The ratios of reduced widths for the transferring neutron in ${\mathrm{N}}^{14}$ and ${\mathrm{F}}^{19}$ were determined from four pairs of reactions. These four ratios, which should be identical, agree to within a factor of 3. The "universal curve" noted earlier for transfer reactions was found to be invalid when all the available data were considered.