g factors of 31,32,33Al: Indication for intruder configurations in the 33Al ground state

Abstract

The g factors of 31,32,33 Al have been measured using the β-nuclear magnetic resonance (β-NMR) technique on spin-polarized beams produced in the fragmentation of a 36 S (77.5 MeV/u) beam on a 9 Be target. Nearly pure beams of Al (Z = 13) isotopes were selected with the highresolution fragment separator LISE at GANIL. An asymmetry as high as 6% has been observed in the β-NMR curve for 32 Al implanted in a Si single crystal. The magnetic moment of the N = 20 nucleus 33 Al is obtained for the first time: μ( 33 Al, I π = 5/2 + ) = 4.088(5)μ N , while those of 31,32 Al are obtained with improved accuracy: μ( 31 Al, I π = 5/2 + ) = 3.830(5)μ N and μ( 32 Al, I π = 1 + ) = 1.9516(22)μ N . Comparison of the results to shell-model calculations in the sd and the sdpf shell-model spaces leads to the conclusion that 33 Al must contain some contribution from 2p-2h intruder configurations in its ground-state wave function. This indicates a gradual transition from the normal sd shell Si (Z = 14) isotopes to the intruder Mg (Z = 12) isotopes.

High-precision quadrupole moment reveals significant intruder component inAl201333ground state

“…This is inconsistent with a picture in which no particle-hole excitations occur: for a closed-shell isotope like 33 Al 20 the quadrupole moment should decrease with respect to the open-neutron shell isotopes 27,31 Al. That is illustrated in the shell model calculations using the USD shell model interaction [28] with a 16 O core and protons and neutrons restricted to the sd-shell model space, the triangles in Fig. 2.…”

“…2. The ANTOINE shell model code has been used with SDPF-U-MIX [29], the most recent effective interaction for this region, which builds upon a 16 O core and comprises the full sd and pf shell for neutrons. This allows excitations of neutrons across N = 20 and mixing between np-nh configurations.…”

“…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.…”

Ground-state configuration of neutron-rich Aluminum isotopes through Coulomb Breakup

“…Nuclei like 34,35 Al are lying at the boundary of this 'Island of Inversion'. Little experimental information about their ground state configurations are available in literature [7][8][9]. g-factor measurement of 34 Al by Himpe et al [7] showed the need of intruder components in 34 Al wave function to account for the observed large magnetic moment.…”

“…Little experimental information about their ground state configurations are available in literature [7][8][9]. g-factor measurement of 34 Al by Himpe et al [7] showed the need of intruder components in 34 Al wave function to account for the observed large magnetic moment. Recently, an experiment (S306) has been performed [5] at GSI, Darmstadt using LAND-FRS setup [10] to explore the neutron-rich exotic nuclei in and around N ∼ 20 'Island of Inversion' (Fig.…”