Nonresonant direct p- and d-wave neutron capture by 12C

Discrete gamma rays from the neutron capture state of C-13 to its low-lying bound states have been measured using pulsed neutrons at E-n=550 keV. The partial capture cross. sections have been determined to be 1.7+/-0.5, 24.2+/-1.0, 2.0+/-0.4, and 1.0+/-0.4 mu b for the ground (J(pi)=1/2(-)) and first-(J(pi)=1/2(+)), second-(J(pi)=3/2(-)), and third- (J(pi)=5/2(+)) excited states, respectively. From a comparison with theoretical predictions based on the nonresonant direct radiative capture mechanism, we could determine the spectroscopic factor for the J(pi) = 1/2(+) state to be 0.80+/-0.04, free from neutron-nucleus interaction ambiguities in the continuum. In addition we have detected the contribution of the nonresonant d-wave capture component in the partial cross sections for transitions leading to the J(pi)=1/2(-) and J(pi)=3/2(-) states. While the s-wave capture dominates at E-n less than or equal to 100 keV, the d-wave component turns out to be very important at higher energies. From the present investigation the C-12(n,gamma)C-13 reaction rate is obtained for temperatures in the range 10(7)-10(10) K. [S0556-2813(98)05905-6].