Spectroscopy of ¹³Be through isobaric analog states in ¹³B

Background: Spectroscopy of the exotic, neutron unbound beryllium isotope ¹³Be is still a puzzle despite significant experimental efforts.

Purpose: We aim to observe 𝑇=5/2 states in ¹³B , establish spin-parities and spectroscopic factors, and use isospin symmetry to inform spectroscopy of ¹³Be .

Methods: Excitation functions for resonance elastic scattering of 12 Be on protons were measured in the center-of-mass energy range from 1 to 5 MeV.

Results: Two 𝑇=5/2 states in ¹³B at excitation energies 18.25 and 19.95 MeV were observed. Unambiguous 1/2⁺ and 5/2⁺ spin-parity assignments were made, and spectroscopic factors were established using the 𝑅-matrix analysis of the measured ¹²Be +𝑝 excitation functions.

Conclusions: We provide the first direct and unambiguous spin-parity assignments and spectroscopic factor measurements for the resonances in the 𝐴=13, 𝑇=5/2 isobaric multiplet. This puts us on a solid footing to identify the 2.3 MeV state in ¹³Be as 5/2⁺ and infer the existence of an 𝑠-wave (1/2⁺) resonance at around 0.6 MeV with relatively small spectroscopic factor.