Developing Techniques for High Fidelity Studies of Reactions with Light Weakly Bound Nuclei
Date
2016
Authors
Carter, Ian Paul
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Abstract
Research capabilities in nuclear physics have greatly expanded in
recent years with
the availability of radioactive ion beams and exotic nuclei near
the drip line. As a
result, new phenomena are being discovered in areas of nuclear
reactions and nuclear
structure. This thesis work is focused on studies of reaction
mechanisms of light
weakly bound nuclei at energies near the Coulomb barrier, where
nuclear structure
influences nuclear reactions outcomes. Two strands towards this
end were followed
concurrently; the first, to develop a radioactive beam capability
to enable reaction
studies with 6 He and 8 Li nuclei, and the second, to study the
systematics of breakup
mechanisms of the stable but weakly bound nucleus 9 Be in
interactions with targets
of mass A = 40-124.
The radioactive beam capability at the Australian National
University uses in-flight
transfer reactions to produce light unstable beams. The
radioactive ion species of
interest are then transported and focused onto a secondary target
using the magnetic
field generated by a superconducting solenoid. The relatively low
purities of the
unstable beam obtained using a single solenoid (typically 30%)
normally necessitates
the use of two solenoids in tandem to further purify the
radioactive ion beam as
done at the TwinSol (USA) and RIBRAS (BRAZIL) facilities. A
unique feature of
the Australian National University (ANU) radioactive beam
capability is a pair of
tracking detectors placed at the exit of the solenoid that allows
identification and
determination of the trajectories of the radioactive species, and
electronic tagging
event-by-event. These detectors were developed and successfully
implemented as
part of this thesis work. The reconstruction of ion trajectories
using these detectors
aids in rejection of contaminant species. Effective beam purities
of greater than 90%
have been achieved for 6 He and 8 Li, with most impurities being
tritons. The tracking
detectors have demonstrated rate handling capability of 3×10 6
particles per second. The trajectory reconstruction also provides
information on the point of interaction
and the angle of incidence of the ion on the secondary target,
allowing precise
reconstruction of reaction kinematics which is necessary for high
fidelity studies of
nuclear reactions. Details of the ion transport, tracking
detector performance and
secondary beam characteristics are described in this thesis,
along with the results of
the first experiment using a radioactive beam of 8 Li from the
ANU capability.
Parallel to developing the tracking detectors, experiments with 9
Be, identifying
and characterising all breakup mechanisms of 9 Be incident on
targets of mass A =
40-124 were carried out. These experiments were done at several
energies below
the fusion barrier to minimise absorption of breakup fragments by
the target. The
charged breakup fragments were detected in BALiN, a highly
pixelated double sided
silicon detector array. The dominance of n-transfer from 9 Be to
the target, forming
8
Be, is observed over the entire target mass-region studied in
this thesis. Following
transfer the 8 Be formed breaks up into two alpha particles. The
relative energies of
the two coincident alpha particles are used to separate breakup
following population
of the long-lived ground state of 8 Be from the shorter-lived
excited states. This
separation is significant since complete fusion cannot be
affected by breakup occurring
on a time-scale slower than fusion. Selecting the near-target
breakup events, and
presenting their probability as a function of the radial
separation of the projectile and
target, can be used in a classical trajectory model to predict
suppression of complete
fusion at above-barrier energies. The experimental results
obtained in this work,
combined with the previous studies of 9 Be on heavy targets, give
the systematics of
breakup in reactions with masses ranging from 40 to 209 u. Such
systematics should
aid in the developments of models of reactions with weakly bound
nuclei.
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Keywords
Radioactive Ion Beams, RIB, breakup, elastic scattering, PPACs, gas detectors, Li-8, Be-9, SOLEROO, BALiN
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