Intruder Configurations in theA=33Isobars:Mg33andAl

Abstract

The beta decay of 33Mg (N=21) presented in this Letter reveals intruder configurations in both the parent and the daughter nucleus. The lowest excited states in the N=20 daughter nucleus, 33Al, are found to have nearly 2p-2h intruder configuration, thus extending the "island of inversion" beyond Mg. The allowed direct beta-decay branch to the 5/2{+} ground state of the daughter nucleus 33Al implies positive parity for the ground state of the parent 33Mg, contrary to an earlier suggestion of negative parity from a g-factor measurement. An admixture of 1p-1h and 3p-3h configurations is proposed for the ground state of 33Mg to explain all of the experimental observables.

Direct experimental evidence for a multiparticle-hole ground state configuration of deformedMg33

“…The present analysis of Coulomb breakup of 33 Mg has been performed in a similar manner to that of 17 C [22]. The ground-state spin of 33 Mg is 3/2 as suggested by two recent independent experiments [9,10]. But the parity and the single-particle configuration are under debate.…”

“…From the measured negative sign of the magnetic moment, a pure 3p-2h ground state configuration and 3/2 − ground state spin and parity were suggested [10]. In contrast, 3/2 + ground state spin and parity with a 4p-3h or 2p-1h configuration was assigned through β-decay measurements, i.e., 33 Mg to 33 Al [9] and 33 Na to 33 Mg [6]. A narrow longitudinal momentum distribution (150 ± 3 MeV/c) of 32 Mg was observed in the knockout reaction of this (S n = 2.22 MeV) nucleus [11].…”

“…The observation of the breakdown of traditional magic numbers [1] in exotic nuclei far from the β-stability line has been an important issue in nuclear structure physics and significant effort has been put into both experimental investigations [2][3][4][5][6][7][8][9][10][11][12] and theoretical studies [13][14][15][16]. The first observation of the disappearance of the so-called magic number was reported by Thibault et al [2], based on mass measurements of the neutron-rich 31,32 Na isotopes [2].…”

“…Many different experiments [3,6,[8][9][10][11][12] have been performed to unravel the structure of 33 Mg (N = 21) but until now neither the ground state parity nor the shell configuration has been confirmed. From the measured negative sign of the magnetic moment, a pure 3p-2h ground state configuration and 3/2 − ground state spin and parity were suggested [10].…”

High-precision quadrupole moment reveals significant intruder component inAl201333ground state

“…Experimental measurements to determine this intruder component have led to conflicting results. Whereas mass measurements [14] and the 33 Al → 33 Si β-decay study [15] place 33 Al outside the island of inversion, the g factor [16] and the (debated) 33 Mg → 33 Al β-decay scheme [17][18][19] suggest a sizable intruder component in the 33 Al ground state, while also one-neutron removal cross sections and longitudinal momentum distributions do not exclude the presence of intruder configurations [20]. The quadrupole moment is a particularly well-suited observable to shed light on this issue due to its sensitivity to E2 collectivity and 2p-2h excitations in the wave function.…”