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Missing mass spectroscopy of the unbound ^8C nucleus was performed by the one-neutron transfer ^9C (p, d) ^8C reaction at 55 MeV/nucleon. Besides the known ground state, two new resonant states were observed, the first at an excitation energy of 3. 40 (25) MeV with a width of 3. 0 (5) MeV, the second at 18. 6 (5) MeV with a width of 3. 9 (11) MeV. Spin and parity J^=2^+ were assigned to the first resonance from the distorted-wave Born approximation analysis of the experimental differential cross section. The excitation energy of the 2^+ resonance in ^8C supports the persistence of the subshell closure at the semimagic number Z = 6, as is the case for N = 6. The mirror energy difference relative to the 2^+ state in ^8He, Eₗ=-0. 14 (25) MeV, is compatible with zero. Both states represent resonances in the continuum, unbound by about 1. 4 and 6. 9 MeV, respectively, above the particle thresholds. A simple theoretical model emphasizes the difference in unboundedness to account for a symmetry in mirror energies. This unique system is expected to provide a salient test of theoretical models, which include the treatment of the continuum.
Koyama et al. (Wed,) studied this question.