Elastic scattering of He3+He4 with the SONIK scattering chamber

Measurements of the elastic scattering cross section of He3 and He4 are important in order to improve constraints on theoretical models of He4(He3,γ)Be7, a key reaction in Big Bang nucleosynthesis and solar neutrino production. The astrophysical S factor for this reaction is a significant source of uncertainty in the standard-solar-model prediction of the Be7 and B8 solar neutrino fluxes. The elastic scattering measurements reported in the literature do not extend to low energies and lack proper uncertainty quantification. A new measurement of the He4(He3,He3)He4 reaction has been made at center-of-mass energies Ec.m.=0.38−3.13 MeV using the Scattering of Nuclei in Inverse Kinematics (SONIK) scattering chamber: a windowless, extended gas target surrounded by an array of 30 collimated silicon charged particle detectors situated at TRIUMF. This is the first elastic scattering measurement of He3+He4 made below 500 keV and it has greater angular range and better precision than previous measurements. The elastic scattering data were analyzed using both R-matrix and halo effective field theory frameworks, and values of the s-wave scattering length and effective range were extracted. The resulting improvement in knowledge of the s-wave effective-range function at low energies reduces the overall uncertainty in S34 at solar energies.