Nonresonant p -wave direct capture and interference effect observed in the O16(n,γ)O17 reaction

Abstract

We measured the total capture cross section of the $^{16}\mathrm{O}(n,\ensuremath{\gamma})^{17}\mathrm{O}$ reaction, as well as partial cross sections leading to the ground (${J}^{\ensuremath{\pi}}=5/{2}^{+})$ and first-excited (${J}^{\ensuremath{\pi}}=1/{2}^{+})$ states in $^{17}\mathrm{O}$. The measurement was carried out at average neutron energies of 157, 349, 398, 427, 468, 498, and 556 keV by using pulsed neutrons produced via the $^{7}\mathrm{Li}(p,n)^{7}\mathrm{Be}$ reaction and a $\ensuremath{\gamma}$-ray detection system based on an anti-Compton NaI(Tl) spectrometer. We observed the interference effect between the $3/{2}^{\ensuremath{-}}$ resonance state at 4554 keV of $^{17}\mathrm{O}$, corresponding to a neutron energy of 411 keV in the center-of-mass system, and a nonresonant contribution in the capture process. The measured partial cross sections are in good agreement with theoretical calculations obtained taking into account the interference between the $3/{2}^{\ensuremath{-}}$ resonance and a nonresonant $p$-wave direct radiative capture contribution. Using the present results, together with our previous measurement, we derived the Maxwellian-averaged capture cross sections (MACS) for thermal energies between $kT=5$ and 100 keV, key quantities for $s$ process nucleosynthesis studies in massive stars.