The reduced-alpha-width of the lowest 1¯ state of ^(16)O

Abstract

Cross sections for the reaction 12 C(α,γ) 16 O have been measured for a range of center-of-mass alpha particle energies extending from 1.72 MeV to 2.94 MeV. Two 8x5 NaI (Tl) crystals were used to detect gamma rays; time-of-flight technique was employed to suppress cosmic ray background and background due to neutrons arising mainly from the 13 C(α,n) 16 O reaction. Angular distributions were measured at center-of-mass alpha energies of 2.18, 2.42, 2.56 and 2.83 MeV. Upper limits were placed on the amount of radiation cascading through the 6.92 or 7.12-MeV states in 16 O. By means of theoretical fits to the measured electric dipole component of the total cross section, in which interference between the 1¯ states in 16 O at 7.12 MeV and at 9.60 MeV is taken into account, it is possible to extract the dimensionless, reduced-alpha-width of the 7.12-MeV state in 16 O. A three-level R-matrix parameterization of the data yields the width Θ α,F 2 = 0.14 +0.10 -0.08 . A hybrid R-matrix-optical-model parameterization yields Θ α,F 2 = 0.11 +0.11 -0.07 . This quantity is of crucial importance in determining the abundances of 12 C and 16 O at the end of helium burning in stars.