Coulomb breakup of halo nuclei by a time-dependent method

Description

Halo nuclei are among the strangest nuclear structures.
They are viewed as a core containing most of the nucleons
surrounded by one or two loosely bound nucleons.
These have a high probability of presence at a large distance
from the core.
Therefore, they constitute a sort of halo surrounding the other nucleons.
The core, remaining almost unperturbed by the presence
of the halo is seen as a usual nucleus.
<P>
The Coulomb breakup reaction is one of the most useful
tools to study these nuclei. It corresponds to the
dissociation of the halo from the core during a collision
with a heavy (high <I>Z</I>) target.
In order to correctly extract information about the structure of
these nuclei from experimental cross sections, an accurate
theoretical description of this mechanism is necessary.
<P>
In this work, we present a theoretical method
for studying the Coulomb breakup of one-nucleon halo nuclei.
This method is based on a semiclassical approximation
in which the projectile is assumed to follow a classical trajectory.
In this approximation, the projectile is seen as evolving
in a time-varying potential simulating its interaction with the target.
This leads to the resolution of a time-dependent Schrödinger
equation for the projectile wave function.
<P>
In our method, the halo nucleus is described
with a two-body structure: a pointlike nucleon linked to a
pointlike core.
In the present state of our model, the interaction between
the two clusters is modelled by a local potential.
<P>
The main idea of our method is to expand the projectile wave function
on a three-dimensional spherical mesh.
With this mesh, the representation of the time-dependent potential
is fully diagonal.
Furthermore, it leads to a simple
representation of the Hamiltonian modelling the halo nucleus.
This expansion is used to derive an accurate evolution algorithm.
<P>
With this method, we study the Coulomb breakup
of three nuclei: ¹¹Be, ¹⁵C and ⁸B.
¹¹Be is the best known one-neutron halo nucleus.
Its Coulomb breakup has been extensively studied both experimentally
and theoretically.
Nevertheless, some uncertainty remains about its structure.
The good agreement between our calculations and recent
experimental data suggests that it can be seen as a
<I>s1/2</I> neutron loosely bound to a ¹⁰Be core in its
0⁺ ground state.
However, the extraction of the corresponding spectroscopic factor
have to wait for the publication of these data.
<P>
¹⁵C is a candidate one-neutron halo nucleus
whose Coulomb breakup has just been studied experimentally.
The results of our model are in good agreement with
the preliminary experimental data. It seems therefore that
¹⁵C can be seen as a ¹⁴C core in its 0⁺
ground state surrounded by a <I>s1/2</I> neutron.
Our analysis suggests that the spectroscopic factor
corresponding to this configuration should be slightly lower
than unity.
<P>
We have also used our method to study the Coulomb breakup
of the candidate one-proton halo nucleus ⁸B.
Unfortunately, no quantitative agreement could be obtained
between our results and the experimental data.
This is mainly due to an inaccuracy in the treatment
of the results of our calculations.
Accordingly, no conclusion can be drawn about the pertinence
of the two-body model of ⁸B before an accurate reanalysis of these
results.
<P>
In the future, we plan to improve our method in two ways.
The first concerns the modelling of the halo nuclei.
It would be indeed of particular interest to test
other models of halo nuclei than the simple two-body structure
used up to now.
The second is the extension of this semiclassical model to
two-neutron halo nuclei.
However, this cannot be achieved
without improving significantly the time-evolution algorithm so as to
reach affordable computational times.

Links & Downloads

1. http://theses.ulb.ac.be/ETD-db/collection/available/ULBetd-12172003-095023/

Tags

dissociation reaction

three-dimensional mesh method

time-dependent Schrödinger equation

semiclassical approximation

exotic nuclei

Additional Fields

Identifer	oai:union.ndltd.org:BICfB/oai:ulb.ac.be:ETDULB:ULBetd-12172003-095023
Date	29 January 2004
Creators	Capel, Pierre
Contributors	Beauwens, Robert, Godefroid, Michel, Baye, Daniel, Leclercq-Willain, Christiane, Michel, Francis, Thompson, Ian
Publisher	Universite Libre de Bruxelles
Source Sets	Bibliothèque interuniversitaire de la Communauté française de Belgique
Language	English
Detected Language	English
Type	text
Format	application/pdf
Source	http://theses.ulb.ac.be/ETD-db/collection/available/ULBetd-12172003-095023/
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