Measurement of neutron capture on 50 Ti at thermonuclear energies.

Abstract

At the Karlsruhe and T\"ubingen 3.75 MV Van de Graaff accelerators the thermonuclear ${}^{50}\mathrm{Ti}(n,\ensuremath{\gamma}{)}^{51}\mathrm{Ti}(5.8\mathrm{min})$ cross section was measured by the fast cyclic activation technique via the 320.852 and 928.65 keV $\ensuremath{\gamma}$-ray lines of the ${}^{51}\mathrm{Ti}$ decay. Metallic Ti samples of natural isotopic composition and samples of ${\mathrm{TiO}}_{2}$ enriched in ${}^{50}\mathrm{Ti}$ by 67.53% were irradiated between two gold foils which served as capture standards. The capture cross section was measured at the neutron energies of 25, 30, 52, and 145 keV, respectively. The direct capture cross section was determined to be $0.387\ifmmode\pm\else\textpm\fi{}0.011\mathrm{mb}$ at 30 keV. We found evidence for a bound state s-wave resonance with an estimated radiative width of 0.34 eV which destructively interferes with direct capture. The strength of a suggested s-wave resonance at 146.8 keV was determined. The present data served to calculate, in addition to the directly measured Maxwellian averaged capture cross sections at 25 and 52 keV, an improved stellar ${}^{50}\mathrm{Ti}(n,\ensuremath{\gamma}{)}^{51}\mathrm{Ti}$ rate in the thermonuclear energy region from 1 to 250 keV. The new stellar rate leads at low temperatures to much higher values than the previously recommended rate; e.g., at $kT=8\mathrm{keV}$ the increase amounts to about 50%. The new reaction rate therefore reduces the abundance of ${}^{50}\mathrm{Ti}$ due to s processing in asymptotic giant branch stars.