Interference effects in 12C(p, γ) 13N and direct capture to unbound states

Abstract

Excitation functions of the capture reaction 12C(p, γ 0) 13N have been obtained at θ γ = 0° and 90° and E p = 150–2500 keV. The results can be explained if a direct radiative capture process, E1(s and d → p), to the ground state in 13N is included in the analysis in addition to the two well-known resonances in this beam energy range [E p = 457( 1 2 +) and 1699 ( 3 2 −) keV] . The direct capture component is enhanced through interference effects with the two resonance amplitudes. From the observed direct capture cross section, a spectroscopic factor of C 2 S( l = 1) = 0.49 ± 0.15 has been deduced for the 1 2 − ground state in 13N. Excitation functions for the reaction 12C(p,γ 1p ∗) 12C have been obtained at θ γ = 0° and 90° and E p = 610–2700 keV. Away from the 1699 keV resonance the capture γ-ray yield is dominated by the direct capture process E1 (p → s) to the 2366 ( 1 2 +) keV unbound state. Above E p = 1 MeV, the observed excitation functions are well reproduced by the direct capture theory to unbound states (bremsstrahlung theory). Below E p = 1 MeV, i.e., E p → 457 keV, the theory diverges in contrast to observation. This discrepancy is well known in bremsstrahlung theory as the “infrared problem”. From the observed direct capture cross sections at E p ⪆ 1 MeV , a spectroscopic factor of C 2 S( l = 0) = 1.02 ± 0.15 has been found for the 2366 ( 1 2 +) keV unbound state. A search for direct capture transitions to the 3512 ( 3 2 −) and 3547 ( 5 2 +) keV unbound states resulted in upper limits of C 2 S( l = 1) ≦ 0.5 and C 2 S( l = 2) ⪅ 1.0, respectively. The results are compared with available stripping data as well as shell-model calculations. The astrophysical aspect of the 12C(p, γ 0) 13N reaction also is discussed.