TUNL Seminar: Mikołaj Ćwiok, University of Warsaw and Robin Smith, Sheffield Hallam University

Toward a Measurement of Stellar Oxygen Formation with the Warsaw Active Target TPC Operating in Gamma Beams

Mikołaj Ćwiok, University of Warsaw & Robin Smith, Sheffield Hallam University

Stellar formation of oxygen, the capture of alpha particles by 12C, denoted as the 12C(α,γ)16O reaction, is widely recognised as the most important nuclear input of stellar evolution theory. The cross section of this reaction at stellar energies, around 300 keV, determines the carbon-to-oxygen ratio (C/O) at the end of helium burning, a key parameter of this theory. Despite more than 50 years of measurements of angular distributions, current data, for the most part, fail to measure the interference of the l=1 (E1) and the l=2 (E2) partial waves, predicted by quantum mechanics. Previous measurements using the UConn-TUNL optical-readout TPC (O-TPC) in the HIγS gamma beam demonstrated the validity of measuring the direct 12C(α,γ0)16O reaction via the time-reversed photo-dissociation reaction, 16O(γ,α0)12C. This talk presents a significantly improved measurement using the Warsaw electronic-readout TPC, which operates at much higher rates and with over 1,000 readout channels, enabling 4π three-dimensional reconstruction of nuclear events. Background-free angular distributions measured at centre-of-mass energies 1.46-2.72 MeV are reported, with E1-E2 interference angles agreeing with quantum mechanical predictions over the entire measured energy range. These have motivated new measurements, currently on-going at HIγS (5-28 August 2026), probing the reaction down to centre-of-mass energies of 1.25 MeV, helping to constrain the cross section at stellar conditions. 

Local location: Grainger Hall 1112
Time Zone: ET
Zoom: https://duke.zoom.us/j/94471581751?pwd=yi0iSLM7JKgLehUQsWkmuayNEi2xjI.1
              Meeting ID: 944 7158 1751 , Passcode: TUNL