Isospin Symmetry Breaking Disclosed in the Decay of Three-Proton Emitter <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mmultiscripts> <mml:mrow> <mml:mi>Al</mml:mi> </mml:mrow> <mml:mprescripts/> <mml:none/> <mml:mrow> <mml:mn>20</mml:mn> </mml:mrow> </mml:mmultiscripts> </mml:mrow> </mml:math>
X.-D. Xu, I. Mukha, J. G. Li, S. M. Wang, L. Acosta, M. Bajzek, E. Casarejos, D. Cortina‐Gil, J. M. Espino, A. Fomichev, H. Geissel, J. Gómez‐Camacho, Л. В. Григоренко, O. Kiselev, A.A. Korsheninnikov, D. Kostyleva, N. Kurz, Yu. A. Litvinov, I. Martel, C. Nociforo, M. Pfützner, C. Rodríguez–Tajes, C. Scheidenberger, M. Stănoiu, K. Sümmerer, H. Weick, P. J. Woods, M.V. Zhukov
Abstract
The previously unknown nucleus ^{20}Al has been observed for the first time by detecting its in-flight decays. Tracking trajectories of all decay products with silicon microstrip detectors allowed for a conclusion that ^{20}Al is unbound with respect to three-proton (3p) emission. The 3p-decay energy of the ^{20}Al ground state has been determined to be 1.93(_{-0.10}^{+0.12}) MeV through a detailed study of angular correlations of its decay products, ^{17}Ne+p+p+p. This value is significantly smaller than the predictions inferred from the isospin symmetry by using the known neutron separation energy of its mirror nucleus ^{20}N, which indicates a possible isospin symmetry breaking in the mirror nuclei ^{20}Al and ^{20}N. This observed isospin symmetry breaking is supported by the calculations of the continuum embedded theoretical frameworks, describing the observed ^{20}Al ground state as a 1p s-wave state with a spin-parity of 1^{-}, which differs from the spin-parity (2^{-}) of the ^{20}N ground state. The ^{20}Al ground state decays by sequential 1p-2p emission via the intermediate ground state of ^{19}Mg, which is the first observed case of "daughter" 2p radioactivity following 1p decay of the parent state.