Halo NucleusBe11: A Spectroscopic Study via Neutron Transfer

Abstract

The best examples of halo nuclei, exotic systems with a diffuse nuclear cloud surrounding a tightlybound core, are found in the light, neutron-rich region, where the halo neutrons experience only weak binding and a weak, or no, potential barrier. Modern direct reaction measurement techniques provide powerful probes of the structure of exotic nuclei. Despite more than four decades of these studies on the benchmark one-neutron halo nucleus 11 Be, the spectroscopic factors for the two bound states remain poorly constrained. In the present work, the 10 Be(d,p) reaction has been used in inverse kinematics at four beam energies to study the structure of 11 Be. The spectroscopic factors extracted using the adiabatic model, were found to be consistent across the four measurements, and were largely insensitive to the optical potential used. The extracted spectroscopic factor for a neutron in a n j = 2s 1/2 state coupled to the ground state of 10 Be is 0.71(5). For the first excited state at 0.32 MeV, a spectroscopic factor of 0.62(4) is found for the halo neutron in a 1p 1/2 state. Nuclear halos are a phenomenon associated with certain weakly-bound nuclei, in which a tail of dilute nuclear matter is distributed around a tightly bound core [1][2][3]. This effect is only possible for bound states with no strong Coulomb or centrifugal barrier, and which lie close to a particle-emission threshold. Though excited-state halos exist, the number of well-studied halo states is predominantly limited to a handful of light, weakly-bound nuclei which exhibit the phenomenon in their ground state.The neutron-rich nucleus 11 Be is a brilliant example of this phenomenon, with halo structures in both of its bound states, and light enough to be modeled with an ab initio approach. It is well documented that the 1/2 + ground state and 1/2 − first excited state in 11 Be are inverted with respect to level ordering predicted from a naïve shell model. There has been considerable theoretical effort toward reproducing this level inversion in a systematic manner, while maintaining the standard ordering in the nearby nuclide 13 C, where the 1/2 + state lies over 3 MeV above the 1/2 − ground state. A Variational Shell Model approach [4] and models which vary the singleparticle energies via vibrational [5] and rotational [6] core couplings reproduce this level inversion in a systematic manner. Common to the success of these models is the inclusion of core excitation. Ab initio No-Core Shell Model calculations [7] have been unable to reproduce this level inversion though a significant drop in the energy of the 1/2 + state in 11 Be is reported with increasing model space. In all of these models, the wave functions for the 11 Be halo states show a considerable overlap with a valence neutron coupled to an excited 10 Be(2 + ) core, in addition to the naïve n⊗ 10 Be(0 + gs ) component. Despite decades of study, the extent of this mixing is not well understood, with both structure calculations and the interpretation of experimental results ranging from a few...

The ratio method: A new tool to study one-neutron halo nuclei

“…In the standard eikonal implementation, a subsequent adiabatic approximation is performed to solve Eq. (9). That approximation corresponds to neglecting the excitation energy of the projectile compared to the beam energy.…”

“…Narrow momentum distributions are also an indication of the large spatial extension of the wave functions [7,8]. While identifying a halo can be in itself a challenge, one would also like to have a better understanding of the structure of the valence orbital and its wave function, such as accomplished in (d, p) studies [9]. A new observable based on a ratio of cross sections, and therefore referred to as the ratio method [10], offers this possibility.…”

Experimental study of bound states in12Be through low-energy11Be(d,p)Johansen1, Bildstein2, Borge3 et al. 2013Phys. Rev. C283180View full textAdd to dashboardCite

“…This method has recently been employed also for the study of other exotic nuclei [12][13][14]. In neither of the neighboring isotopes 11 Li and 11 Be can the ground states be written as a simple single-particle configuration (see [15] and the references above). This seems to also be the case for 12 Be.…”

Reactions with a10Be beam to study the one-neutron halo nucleus11Be

“…The 10 Be(d,p) 11 Be reaction was performed in inverse kinematics at four different energies. The data from the elastic, inelastic, and transfer channels were analyzed in a distorted wave and adiabatic framework, as presented in [8,16] and spectroscopic factors were extracted. The values of S for the ground state of 11 Be are consistent with 0.69±0.06, for the analysis using the Koning and Deleroche optical potential [18].…”

“…The experiment discussed here [8,16] was performed with the goal of providing consistent 10 Be + d data at four different energies, including elastic and inelastic scattering as well as transfer to bound and resonant states. S was extracted in a consistent fashion, using the same optical potentials and reaction formalisms.…”