The Ratio Method: measuring haloes from angular distributions

by Dr Pierre Capel (Physique Nucléaire et Physique Quantique (CP 229) Université Libre de Bruxelles)

Thursday 12 Dec 2013, 14:30 → 15:30 Europe/Zurich

26-1-022 (CERN)

The development of radioactive-ion beams in the mid 80s has enabled us to explore

the nuclear landscape far from stability. This led to the discovery of exotic nuclear

structures such as halo nuclei. These neutron-rich nuclei exhibit a large matter radius

in comparison to their isobars. This peculiarity is due to the strongly clusterised structure

of halo nuclei: they can be seen as a dense core to which one or two valence neutrons are

loosely bound. By tunnel effect, these neutrons exhibit a large probability of presence at

a large distance from the other nucleons and hence form a sort of halo around the core.

Being far away from stability, halo nuclei cannot be studied through usual spectro-

scopic techniques and we must rely on indirect methods, such as reactions, to infer in-

formation about their structure. Breakup reaction is probably the most used tool to

study halo nuclei. In this reaction, the halo neutrons dissociate from the core through

the interaction with a target. In order to get valuable information from breakup observ-

ables, a good understanding of the reaction process is required and its sensitivity to the

properties of the projectile structure must be assessed.

In this presentation, I will review the basics of the theoretical description of breakup

reactions involving halo nuclei and present results obtained within the Dynamical Eikonal

Approximation (DEA) for the breakup of ¹¹Be, the archetypical one-neutron halo nucleus,

and explain what kind of information can be inferred from usual breakup observables.

Based on these results, I will present a new way to extract information about the

structure of halo nuclei through reactions. The basic idea of this new method is to perform

the ratio of angular distributions for breakup and scattering. These two mechanisms

exhibit very similar features that depend mostly on the projectile-target interaction.

Taking the ration of these two cross sections hence removes most of the dependence on

the reaction mechanism, leading to an observable much more sensitive to the projectile

internal structure than usual reaction cross sections. The ratio method should thus

help us better understand the structure of nuclear matter far from stability.