Influence of additional neutrons on the fusion cross section beyond the $N=8$ shell

Fusion enhancement for neutron-rich isotopes of oxygen on carbon nuclei was probed. To measure the fusion cross section a ${}^{20}\mathrm{O}$ beam accelerated to $E_{\mathrm{lab}}/A=2.7$ MeV bombarded the active-target detector MuSIC@Indiana with a fill gas of ${\mathrm{CH}}_4$. Examination of the average fusion cross section over the interval $0.5\le (E_{\mathrm{c}.\mathrm{m}.}-V_B)/V_B\le 1.2$ for ${}^{16–20}\mathrm{O}+{}^{12}\mathrm{C}$ reveals that while even isotopes of oxygen exhibit essentially the same cross section, the cross section for odd isotopes can be either enhanced or suppressed relative to the even $A$ members of the isotopic chain. Theoretical models fail to explain the observed experimental results.

Figure 1

(a) Schematic of the experimental setup used to measure fusion of ${}^{20}\mathrm{O}+{}^{12}\mathrm{C}$. The tilted foil MCP detector, MuSIC@Indiana, the veto detector, and the downstream surface barrier silicon detector (SBD) are indicated. (b) Identification of the incident beam utilizing the energy deposit in the first two anodes of MuSIC@Indiana.

Figure 2

Representative traces from MuSIC@Indiana for different types of events. Indicated as the black line in all panels is the average trace for the ${}^{20}\mathrm{O}$ beam. Error bars indicate the standard deviation of the energy distribution for the beam on each anode. (a) Fusion in anode 5, (b) proton capture on ${}^1\mathrm{H}$ early in MuSIC@Indiana, and (c) a two-body event at anode 5.

Figure 3

Comparison of the excitation functions for ${}^{17–20}\mathrm{O}+{}^{12}\mathrm{C}$ with coupled channels and DC-TDHF calculations. (a) Data taken from [29, 30, 31, 32]; (b) data measured in the present experiment (solid symbols) along with prior measurements (open symbols) [13, 28, 30, 33, 34]; (c) data taken from [14]; (d) fusion excitation function for ${}^{20}\mathrm{O}+{}^{12}\mathrm{C}$.

Figure 4

Dependence of the average cross-section on neutron excess. Error bars reflect both the statistical and systematic uncertainties in the measurements. Data are taken from ${}^{16}\mathrm{O}$ [30, 33, 39, 40]; ${}^{17}\mathrm{O}$ [29, 30, 31, 32]; ${}^{18}\mathrm{O}$ [13, 28, 30, 33, 34]; and ${}^{19}\mathrm{O}$ [16].