³He+α resonances in ⁷Be

Resonances in 7Be which decay into the 3He+α exit channel have been measured with improved precision using preexisting data sets. The energy and width of the Jπ = 7/2− state have been extracted from an invariant-mass study of projectile-breakup products originating from interactions of an E/A = 10.7 − MeV10C beam on Be and C targets. The excitation energy of this state (from the pole of the S matrix) is determined to be E∗ = 4.545(6) MeV, a factor of 8 improvement in precision as compared to the ENSDF value. This improvement is enabled by fine tuning the detector calibrations using calibration resonances in 6Li, 7Li, 6Be, 9B, and 12C whose decay energies are known to high precision. The Jπ = 7/2− resonance in 7Be can now itself be used as a calibration resonance in invariant-mass experiments. This utility is particularly helpful for 3He energy calibrations of CsI(Tl) detectors which are often used in detector arrays employed for measurements with fast beams. This utility is demonstrated with a data set associated with E/A = 70-MeV 7Be beams which are inelastically excited to the 7/2− and 5/2− 1 resonances. Again using the pole of the S matrix as the definition of the resonance parameters, the fitted excitation energy of the 5/2− 1 resonance is 6.376(17) MeV, approximately 300 keV lower than the ENSDF value. Its width of 565(4) keV is roughly half of the ENSDF value.