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

Discrete γ rays from the neutron capture state of ¹³C to its low-lying bound states have been measured using pulsed neutrons at E_n=550keV. 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±0.4μb for the ground (J^π=1/2^-) and first- (J^π=1/2⁺), second- (J^π=3/2^-), and third- (J^π=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^π=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^π=1/2^- and J^π=3/2^- states. While the s-wave capture dominates at E_n<~100keV, the d-wave component turns out to be very important at higher energies. From the present investigation the ¹²C(n,γ)¹³C reaction rate is obtained for temperatures in the range 10⁷–10¹⁰K.