The total cross section and angular distributions for the reaction ${}^4\mathrm{He}(\gamma ,{\pi }^0)$ from a cryogenic liquid helium target have been measured within 25 MeV of threshold using tagged photons and a large acceptance ${\pi }^0$ spectrometer at the Saskatchewan Accelerator Laboratory. The reduced isovector amplitude $p_3^{(+)}$ has been determined from the total cross-section measurement using a distorted-wave impulse approximation analysis. Refinements from earlier analytical methods, specifically an improved background event rejection scheme and a corrective tagged photon energy calibration analysis, have also yielded an improved estimate on the $p_3^{(+)}$ amplitude for the ${}^{12}\mathrm{C}(\gamma ,{\pi }^0)$ reaction explored previously.

• Received 3 March 2008

DOI:https://doi.org/10.1103/PhysRevC.77.064601