The supernova-nucleosynthesis ⁴⁰Ca (α,γ) ⁴⁴Ti reaction

The ⁴⁴Ti (t_1/2 = 59 y) nuclide is considered an important signature of core-collapse supernova (SN) nucleosynthesis and has recently been observed as live radioactivity by γ-ray astronomy from the Cas A SN remnant. We investigated in the laboratory the major ⁴⁴Ti production reaction ⁴⁰Ca(α,γ)⁴⁴Ti (E _cm ∼ 0.6-1.2 MeV/u) by off-line counting of ⁴⁴Ti nuclei using accelerator mass spectrometry. The observed yield is significantly higher than inferred from previous prompt γ-spectroscopy experiments. The present data are interpreted in terms of the BRUSLIB statistical model which incorporates a microscopic model of nuclear level densities and of the γ-ray strength function, and a global α-nucleus optical-model potential and confirm the strong suppression in yield expected for (α,γ) reactions on self-conjugate (N =Z) nuclei. The derived astrophysical rate of the ⁴⁰Ca(α,γ)⁴⁴Ti reaction is a factor 5-10 higher than calculated in current models. We will present results of stellar calculations in spherical hydrodynamics using this reaction rate, showing an increase of the calculated SN ⁴⁴Ti yield by a factor ∼ 2 over current estimates. An increase by a factor of ∼ 2 in ⁴⁴Ti is found also in the calculated fall back material. The yields calculated by multi-dimensional SN explosion calculations proposed to explain the observed ⁴⁴Ti yield of Cas A, in which parts of deeper layers can be ejected while some of the outer layers fall back, are expected to be enhanced in ⁴⁴Ti as well.