We have measured the neutron energy spectrum of the ${}^{18}\mathrm{O}$($p,n$)${}^{18}\mathrm{F}$ reaction at a proton energy of 2582 keV, 8 keV above the reaction threshold. At this energy the resulting neutron spectrum resembles almost perfectly a Maxwellian distribution at a thermal energy of $\mathrm{kT}=5.1\pm 0.1\mathrm{keV}$. Since all neutrons are emitted in a forward cone of 140${}^{\circ }$ opening angle, this reaction can be used for neutron activation measurements similar to the ${}^7\mathrm{Li}$(p,n)${}^7\mathrm{Be}$ reaction, which is known for producing a thermal spectrum with $\mathrm{kT}=25\mathrm{keV}$. Measured neutron capture cross sections at $\mathrm{kT}=5.1\mathrm{keV}$ and $\mathrm{kT}=25\mathrm{keV}$ can be used to interpolate to $\mathrm{kT}=8\mathrm{keV}$, which characterizes the dominant neutron exposure during s-process nucleosynthesis in thermally pulsing low-mass AGB stars. In a first application of this new method the Maxwellian-averaged neutron capture cross section of ${}^{138}\mathrm{Ba}$ was measured to be $〈\sigma v〉/v_T=13.0\pm 0.5\mathrm{mb}$ at $\mathrm{kT}=5.1\mathrm{keV}$.

• Received 27 July 2004

DOI:https://doi.org/10.1103/PhysRevC.71.025803