Download nuclear brochure 1.2 - Research School of Physics and Engineering

INTERACTIONS
OF NUCLEI
WITH
MATERIALS
The motions of protons and neutrons
within the nucleus give rise to
electric and magnetic fields, which
both affect and are affected by
nearby atomic electrons. These
hyperfine interactions change the
orientation of the nucleus, and can
perturb the spatial distributions of
emitted radiations.
Blue Lake in the Snowy Mountains, formed by glaciers
Direct measurement of the concentration of the undecayed isotopes is a much
superior method. The Accelerator Mass Spectrometry (AMS) group uses the
14UD accelerator to provide a mass-selected beam of the isotope from a
sample, having sufficient energy to enable the counting and unambiguous
identification of each ion, using techniques from nuclear physics. A sensitivity
down to 1 atom of isotope in 1015 normal atoms has been achieved, from
samples of only a few milligrams. This previously unattainable sensitivity has
opened up whole new areas of research in fields as diverse as global climate
change, bio-medicine and archaeology.
A large boulder deposited 20,600 years ago by a
glacier at Blue Lake in the Snowy Mountains of
Australia. This date was obtained by determining
how much 10Be isotope had built up in the rock
surface over time due to cosmic-ray
bombardment.
The group has a strong program of developing new measurement techniques
and apparatus, to extend the range of isotopes that can be studied. New
applications of AMS measurements are also being investigated, both for
natural and man-made isotopes. Vigorous collaborations with both Australian
and international scientists have been established for projects including the
dating of glacial advance and retreat as an indicator of past global climatic
changes, tracing the effect of land clearance on the salinity of the MurrayDarling river system, and dating the time of arrival of Aboriginal People in
Australia.
The detection of scattered beam
particles, and of recoiling target
nuclei, are the basis of the
extremely powerful materials
analysis
techniques
called
Rutherford Backscattering and
Elastic Recoil Detection Analysis.
Collaborators from the Australian
Defence Force Academy (ADFA)
and the Department of Electronic
Materials Engineering (EME), use
accelerated heavy-ion beams and a
unique large solid angle gasionization detector to obtain the
depth profiles of all elements in a
sample simultaneously.
Measurement of perturbed radiation
patterns allows the investigation of
a wide range of nuclear structure and
materials science problems. For
example, the nuclei of ions moving
swiftly within ferromagnetic hosts
experience high transient hyperfine
INTERNATIONAL
LINKS
magnetic fields (several thousand Tesla).
These allow the measurement of
magnetic moments of very short-lived
(10-12 seconds) nuclear states, critically
testing nuclear theories. Transient fields
are also studied as a unique probe of
ion-solid interactions.
In collaboration with EME, perturbed
angular correlation measurements are
used to study atomic-scale electric
fields due to dopant-defect interactions
in semiconductor materials, through
implantation of radioactive isotopes
using the 14UD accelerator, or a
dedicated ion implanter developed with
ADFA. The control and understanding
of dopant-defect interactions is crucial
in the design and fabrication of
semiconductor devices.
Department of Nuclear Physics
Heavy-ion Accelerator
Facility
25
Elastic Recoil Detection Analysis
Doped Gallium Nitride Film
20
Ga
15
(ch1 )
200
Si
150
10
'E(1)
The interaction of cosmic radiation with the atmosphere and surface layer of
the Earth results in the production of minute quantities of radioactive nuclei
such as 14C, 10Be and 36Cl. These isotopes decay sufficiently slowly to be useful
in dating man-made artefacts or natural features in the environment. One
approach is to measure the radiation emitted during their decay. In practice,
this is only practicable for 14C dating, and then only for large samples.
'E (MeV)
ACCELERATOR
MASS
SPECTROMETRY
H
100
Mg
50
H
5
O
C
0
N
0
100'E(2)200
300
0
0
20
40
60
80
Energy (MeV)
Vigorous international links have been developed with physicists from many countries.
These often take the form of short or long-term visits by individual scientists, or by
small groups, who may perform experiments in collaboration with local researchers.
Experiments are also performed under an agreement with the EPSRC in the UK,
whereby a large group of external users carry out their own experiments.
Local researchers also travel to
overseas facilities (e.g. in France, Italy,
USA) to make use of apparatus
complementary to that available in
Canberra.
Strong collaborations exist with
theoreticians in all the research fields.
CONTACTS
Department of Nuclear Physics, RSPhysSE, Australian National University, ACT 0200
+61 (0)2 6125 2083
wwwrsphysse.anu.edu.au/nuclear
wwwrsphysse.anu.edu.au/nuclear
385.7
Our nuclear structure research investigates the properties of,
and interactions between individual excited states of metastable
nuclei, whilst the complex interactions between colliding nuclei
are the subject of nuclear reaction dynamics studies. In our
applied research, the accelerated beams are used in AMS
measurements and in advanced materials characterization, to
investigate subjects as diverse as landform evolution and internal
electric fields in semiconductors.
delayed gammas
202Po
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The different strands of research within the
Group have the ultimate goal of developing
a unified picture of the dynamics of nuclear
break-up, fusion and fission.
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delayed electrons
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High resolution
delayed electron
and γ-ray spectra,
from which the
nuclear energy
levels of 202Po
were deduced
fluid or solid matter. Further, the energies
of single-particle excitation and
collective motion such as rotation and
vibration are very similar and essentially
in competition. The quantum states that
arise occur in a specific pattern, but they
are sparse and therefore observable as
individual states. If they do overlap they
can interfere, leading to modifications of
the decay probabilities which are
indicative of the coupling.
Identification and characterisation of
exotic metastable states is one focus of
current spectroscopic studies. This
exploits their relatively long-lived nature
(nanoseconds to milliseconds), and pulsed
beams from the accelerator, to achieve
very high sensitivity. Recent results have
given new insights into the coupling
between collective vibrations and
octupole-shaped proton and neutron
orbits near the surface of spherical nuclei,
and into the constituents of superfluid
motion in deformed nuclei.
Another focus of the research program is
the identification of nuclei that are very
far from stability. Some exhibit different
shapes at low energies, which is a
manifestation of the competition
between dual minima in the nuclear
potential. Their study, which often
involves the primary identification of
nuclei never before formed, is particularly
important for testing models of nuclear
stability.
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Fusion can also be inhibited by break-up of
the colliding nuclei before contact, a process
important in the interactions of the fragile
nuclei found at the boundary of stability
(becoming available from new radioactive
beam facilities). Our precision measurements
for stable, yet fragile, light nuclei have
thrown light on the interplay between
break-up and fusion.
The 14UD Van der Graaff accelerator, housed in a 40m tall tower,
can operate at over 15 million Volts, and delivers ion beams
with pulse widths from a nanosecond to seconds. Beam energies
can be doubled by a superconducting linear post-accelerator
(Linac). The varied interactions of beam nuclei with target nuclei
are studied using state-of-the-art detector arrays, developed
in-house, and located at the ends of the 10 beam lines.
These main areas of research are complementary, overlapping
in terms of shared techniques, and the understanding achieved
of interrelated aspects of nuclear behaviour.
After contact, the two nuclei can merge into
one (fusion) or separate after exchanging
mass (quasi-fission). The latter inhibits the
formation of new heavy elements. We have
shown that quasi-fission is more prevalent
than expected, and have developed an
experimental framework to investigate the
delicate balance of forces that determines
whether fusion or quasi-fission occurs.
Using Gammasphere in the USA
The Department of Nuclear Physics operates two accelerators,
producing charged atoms (ions) with up to 10% of the speed of
light. This is enough to overcome the electrostatic repulsion
between atomic nuclei, and initiate nuclear reactions.
The nucleus is a highly symmetric
mesoscopic system whose structure and
motions are coupled: mesoscopic, rather
than microscopic like a group of individual
nucleons, or macroscopic, like a chunk of
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To fuse, nuclei can tunnel through the
potential barrier created by the sum of the
long-range repulsive electrostatic and
short-range attractive nuclear forces. The
excitation of other nuclear degrees of
freedom (e.g. rotation, vibration) during the
collision results in a distribution of barrier
heights.
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These distributions can be extracted from
extremely precise fusion probability
measurements, through a simple and elegant
mathematical transformation. They give a
unique picture of the complex interactions
of the nuclear surfaces as they come into
contact.
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The Nuclear Reaction Dynamics group
carries out highly regarded research into the
fundamental processes of nuclear fusion,
where two nuclei merge into one, and
nuclear fission, where one nucleus splits into
two. This work is built on our broad expertise
in the design and development of unique
and efficient particle detection systems,
matched to the high-quality particle beams
from the accelerator.
Spectroscopic studies are aimed at
identifying and characterising individual
quantum states in nuclei, using
instrumentation for high-resolution
gamma-ray and electron detection. The
nuclei that are studied are not just the
specific combinations of protons and
neutrons which form the stable species,
but the very wide range which are
accessible by combining stable nuclei with
any one of the beams available from the
accelerator. The result is efficient
production of a nucleus, under conditions
which force it to emit the gamma-rays or
electrons connecting quantum levels, thus
revealing the states that characterise its
properties, motion and degrees of
freedom.
NUCLEAR
STRUCTURE
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Located on the ANU campus
15 million Volt electrostatic accelerator
Superconducting post-accelerator
Ph.D. research opportunities, working
closely with distinguished staff having
high international profiles
Ph.D. graduates obtain positions in top
research laboratories worldwide
ARC-funded postdoctoral positions
International collaborations
World-leading research in nuclear
properties and nuclear reactions
Outstanding applications in bio-medical
and environmental science
Atomic nuclei are completely invisible, being
less than 10-14 m across, and a collision of
two nuclei takes only 10-20 seconds. In such
seemingly infinitesimal and transient events,
a wide variety of phenomena occur.
Understanding them represents a
fascinating intellectual challenge, and also
impacts on other fields of science.
Velocity components of fission events
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NUCLEAR
REACTION
DYNAMICS
Some members of the SOLITAIRE team
AUSTRALIA’S TOP NUCLEAR PHYSICS
LABORATORY
The group’s CUBE fission detector array
THE
FACILITY
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830.9 12 142.7
10+ τ =141 ns 436.1
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τ =23 ns
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τ =168 ns
∆
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6+
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202Po
84 118
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Energy
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