In-beam γ-ray spectroscopy at the proton dripline: 23Al

Abstract

We report on the first in-beam γ-ray spectroscopy of 23 Al using two different reactions at intermediate beam energies: inelastic scattering off 9 Be and heavy-ion induced one-proton pickup, 9 Be( 22 Mg, 23 Al+γ)X, at 75.1 MeV/nucleon. A γ-ray transition at 1616(8) keV -exceeding the proton separation energy by 1494 keVwas observed in both reactions. From shell model and proton decay calculations we argue that this γ-ray decay proceeds from the core-excited 7/2 + state to the 5/2 + ground state of 23 Al. The proposed nature of this state, [ 22 Mg(2 + 1 ) ⊗ πd 5/2 ] 7/2 + , is consistent with the presence of a γ-branch and with the population of this state in the two reactions.Since its discovery in 1969 [1], the neutron-deficient nucleus 23 Al has attracted much attention. 23 Al is four neutrons removed from stable 27 Al and is the last proton-bound, odd-mass aluminum isotope known to exist. The low proton separation energy of S p = 122(19) keV [2] made 23 Al a candidate for a proton halo system. From the measurement of an enhanced reaction cross section, 23 Al was indeed proposed to have a proton-halo structure with a J π = 1/2 +

Intermediate-energy inverse-kinematics one-proton pickup reactions on neutron-deficientfp-shell nuclei

“…These are comparable in magnitude to the proton pickup cross section to the sd-shell nucleus 23 Al, measured on a 9 Be target, where σ inc exp = 0.54(5) mb [8]. We note that 23 Al is also very weakly bound with S p ( 23 Al) = 142.11(43) keV [32].…”

“…In this case the spectroscopic strengths of the different 8 Li final states (for values, see [7]) are more equal and the ground state transition is less dominant. As previously, the one-proton pickup cross section to a particular 49 Mn final state is the sum of these singleparticle cross sections for the three 8 Li final states. The calculations confirm the selectivity of pickup to the high-ℓ final-state configuration.…”

“…To complement this arsenal of fast-beam, inverse kinematics nucleon removal reactions, one-nucleon pickup reactions onto fast projectile beams from thick carbon and beryllium targets have recently been studied as a spectroscopic probe sensitive to particle-like states/configurations [7][8][9]. The choice of reaction target is now motivated by generic linear and angular momentum matching considerations and that, at high collision energies, the pickup of well-bound target nucleons is favored [10,11].…”

“…Only 9 Be targets have been used to date for measured one-proton pickup cases [7,8]. The goals of the present work are two-fold.…”

Spectroscopic study of the exotic nucleusP25

“…Some states can be narrow because they are built on excited-core configurations. Examples of experimental evidence of such states can be found in 19 Na [1], 16 Ne [2], 15 F [2], 15 Ne [3], and 23 Al [4]. Theory also predicts excited-core-based narrow states in 17 Na [5][6][7] and 21 Al [8].…”