3 He +𝛼 resonances in 7 Be

Abstract

Resonances in 7 Be which decay into the 3 He+α exit channel have been measured with improved precision using preexisting data sets. The energy and width of the J π =7/2 - state have been extracted from an invariant-mass study of projectile-breakup products originating from interactions of an E/A=10.7-MeV 10 C 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 6 Li, 7 Li, 6 Be, 9 B, and 12 C whose decay energies are known to high precision. The J π =7/2- resonance in 7 Be can now itself be used as a calibration resonance in invariant-mass experiments. This utility is particularly helpful for 3 He 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 7 Be 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. Finally, its width of 565(4) keV is roughly half of the ENSDF value.