The radioactive 44Ti nuclide has long been considered an important nucleosynthesis signature during the expansion phase of a core-collapse supernova. Live 44Ti has since been observed from the Cassiopaea A supernova remnant by γ-ray astronomy, one of the few cases where the total yield of a specific nuclide in a stellar event can be quantitatively measured. The relation of this quantity with astrophysical models requires knowledge of nuclear cross sections in the relevant path. We have studied the major reaction for production of 44Ti, namely 40Ca(α,γ)44Ti in the range of energies relevant to supernova nucleosynthesis. Preliminary results of our experiment are presented here, showing a significantly stronger yield than observed in previous measurements. The experimental method consisted of the bombardment of a He gas target by a 40Ca beam and implantation of recoil products in a forward-positioned Cu catcher. 44Ti atoms were chemically extracted together with a natTi carrier by etching the catcher and separated by ion-exchange methods. The 44Ti/Ti ratio (rTi) of the order of 10-12 was then measured by accelerator mass spectrometry and the yield of 44Ti nuclei produced in the activation is obtained from the relation Y44 = rTinTi where nTi denotes the number of atoms of natTi carrier used. The thickness of the He target was selected to integrate the reaction yield from Ecm= 4.2 MeV down to Ecm= 1.7 MeV. The derived 44Ti yield (5.1 x 107 atoms) corresponds to an overall resonance strength of 40 to 60 eV (depending on the energies of contributing resonances), much stronger than that measured by prompt-γ measurements (Σωγ= 10 eV). The figure compares the average cross section measured in this work with recent Hauser-Feschbach calculations. The stronger yield measured here for 44Ti production relative to that currently assumed in stellar calculations, has important consequences regarding the comparison with 44Ti activity of supernova remnants measured by γ-ray astronomy and the production of heavier nuclei up to the iron group.
See more of Poster Session II
See more of The 10th International Conference on Accelerator Mass Spectrometry (September 5-10, 2005)