N A T U R E A T T H E F E M T O - S C A L E 1
This advance is largely the result of technological
breakthroughs in developing equipment for nuclear
physics experiments. Until recently, nuclear scientists
had to be content with conducting experiments on
stable nuclei, of which there are only about 300. In
the past decade, however, we have learned how to
build high-energy facilities for producing short-lived,
radioactive nuclei. With these new beams of unstable
nuclei we can make and study many thousands of
exotic nuclear species – most of which have never
existed before, or are created fleetingly only in the hot
interiors of stars. These will tell us more about the
structure of matter and how it evolved in the Universe.
Radioactive beams also offer exciting
opportunities for new medical procedures, and for
applications in other areas of research and industry.
Europe has been at the forefront of this endeavour,
with pioneering technology developed at CERN
(ISOLDE), and the world’s first dedicated facility built
at CRC’s CYCLONE in Louvain-la-Neuve, Belgium. New,
second-generation facilities are now planned or being
built in a number of European laboratories. They will
enable European scientists to remain at the forefront
of these new developments for the next decade.
High-energy experiments are also being designed
to study not only the structure and behaviour of
exotic nuclei but also other, related short-lived
subatomic particles, some of which may have existed
in the very early Universe. Such studies throw light
on the fundamental forces that hold matter
together. Another complementary area of nuclear
study involves carrying out sensitive experiments on
nuclei and nuclear constituents at low energies,
which may point the way to a deeper understanding
of the physics of the cosmos.
All these nuclear experiments are underpinned by
advances in theory which, thanks to developments in
high-performance computing, continue apace.
Nuclear research is very much a science of the
future. The aim of this booklet is to illustrate its
scientific and technological potential in the 21st
century through highlights of work carried out in
laboratories and institutions across Europe.
FINUPHYFrontiers in Nuclear Physics
uclear science is entering a new era of discovery inunderstanding how Nature works at the most basic leveland in applying that knowledge in useful waysN
forewordG
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N A T U R E A T T H E F E M T O - S C A L E 3
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Foreword
Journey into the heart of matter
A world-leading programme of nuclear research
The complex nucleus
Nuclear physics in the Universe
Exploiting the nucleus
From nuclear liquid to quark soup
Nuclei and the universal forces
What holds nuclei together?
Producing exotic nuclei
Why become a nuclear physicist?
Contacts
About FINUPHY
INTRODUCTION
THE EUROPEAN DIMENSION
NUCLEAR STRUCTURE
NUCLEAR ASTROPHYSICS
APPLICATIONS
PHASES OF NUCLEAR MATTER
FUNDAMENTAL INTERACTIONS
QCD
RADIOACTIVE ION BEAMS
EDUCATION AND TRAININGco
nten
ts
4 N A T U R E A T T H E F E M T O - S C A L E
Nuclear science now and the future Physicists soon realised that the electrically charged
subatomic particles they had uncovered, such as
electrons and protons, could be accelerated to higher
energies in electric fields, and fired at target materials
so that they actually penetrated the nucleus and
broke it up. In this way they discovered a whole new
zoo of subatomic particles, such as muons, pions and
kaons, which were stable only for short amounts of
time. Some of them were first discovered in cosmic
rays coming from space – giving clues to the
existence of violent processes in the Universe.
Over the past 50 years, such high-energy physics
experiments have slowly unravelled the fundamental
building blocks of matter and their interactions,
while advances in quantum theory have produced a
cohesive description of them, as described on p.22.
We now know that protons and neutrons (as well as
some of the other particles discovered) are made of
elementary particles called quarks held together by
the strong force (p.24). Theorists also have some
powerful ideas as to how these building blocks came
into existence when the Universe was born, and how
they were built up into the elements in stars (p.12).
Today, nuclear physics laboratories across Europe
carry out vigorous programmes to investigate the
properties of particles at different energy scales, and
to test ideas about stellar nucleosynthetic processes.
The arrangements of protons and neutrons in
nuclei remain an intriguing, but still poorly
understood area of nuclear science. Nuclei exhibit
incredibly complex behaviour as a result of the forces
holding them together (p.8). They represent one of the
richest quantum systems in Nature, demonstrating
many aspects of structure and behaviour mirrored at
larger scales; nuclei can behave like atoms, clusters of
atoms, molecules, quantum liquids and even
superconductors. Indeed, they can be regarded as
miniature laboratories for testing broad ideas about
the way Nature organises itself.
A vast variety of nuclei is possible with widely
differing proportions of protons and neutrons. Most
of them are very unstable. However, research into
their structure has tremendous potential – not just in
understanding nuclear forces, astrophysical processes
The starting point was the truly visionary work
carried out in European laboratories on the structure
of the atom in the early years of the 20th century.
The discovery of the electron, and work on
radioactivity and transmutation (see box), provided
the first hints of a coherent organisation of
fundamental particles that explained the existence
of the elements. But the key discovery that unlocked
the door to modern science was made by Ernest
Rutherford at the University of Manchester in the UK,
when he showed that one form of radioactivity –
alpha particles – could be sharply deflected by a thin
gold sheet. He correctly deduced that these particles
were occasionally hitting the dense, positively
charged cores of the gold atoms and being bounced
back. The concept of the atomic nucleus was born.
With the idea of the atom as consisting of a
positively-charged nucleus surrounded by orbiting
negative electrons came the notion that electrons
must occupy states differing by fixed units of energy
called quanta, which could be emitted or absorbed
as electromagnetic radiation. This tremendous
conceptual leap led to the development of one of the
main cornerstones of modern scientific thinking –
quantum theory – now used to explain interactions
between the microscopic building blocks of matter –
whether subatomic particles, atoms or molecules.
It offers a profound, if perplexing, insight into the
nature of reality.
Rutherford and others later showed that nuclei
were made of smaller units, protons and neutrons,
which explained fully the relationship of elements in
the Periodic Table as differing in the numbers of
protons they possess, and the existence of isotopes
as variants of elements with differing numbers of
neutrons. This deeper perception of matter ultimately
underpins all modern chemistry and biology.
t is only during the past 100 years that we haveachieved any real understanding of matter andthe Universe at a fundamental level
I N T R O D U C T I O N ::
I
heart ofmatterJourney into the
The ISOLDE target
for generating
radioactive beams
Alpha particle emission
CE
RN
N A T U R E A T T H E F E M T O - S C A L E 5
and quantum concepts better, but also in exploring
future technological applications. Nuclei and
nuclear processes are already successfully applied
in analysis and in medical therapies, but the vast
energy trapped in nuclei has been exploited only
in the crudest of ways through uranium fission.
A better understanding of the nucleus through
innovative experiments could lead to new sources
of safe, clean energy and other environmentally
significant technologies (p.16).
Nanotechnology – the ability to manipulate
matter at a scale of one-billionth of a metre (atoms)
is expected to have an enormous impact on
human progress. It may be that ‘femtotechnology’
(a femtometre is one million billionth of a metre) –
the scale of the nucleus – will have even more impact
in the future.
Radioactivity – messagesfrom the nucleusNuclear physics really started with an observationby Henry Becquerel in 1896, that ‘emanations’from a uranium salt fogged a photographic plate.This radioactivity was investigated further byMarie and Pierre Curie who isolated two newradioactive elements, polonium and radium. Itwas Rutherford who showed that there werethree types of radioactivity – alpha, beta andgamma radiation. Together with Frederick Soddy,his studies of radioactivity in thorium revealed forthe first time that an element could transmuteinto another element by spitting out particles inthe form of radiation; elements were not soelementary after all. Alpha particles turned out tobe helium ions, while beta rays are electrons.Gamma-rays are very high energyelectromagnetic radiation. Their characteristicenergies measured in experiments today giveclues to the quantum states in nuclei. Rutherford’searly experiments firing alpha particles at variouselements led to the discovery of the proton andthus helped to elucidate the nuclear constituents.
Becquerel’s first evidence
of radioactivity
Preparing radioisotopes for medical use
Part of the COSY
accelerator at FZJ
Ernest Rutherford
PS
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6 N A T U R E A T T H E F E M T O - S C A L E
Today, European nuclear research is spread over a
network of large and medium-sized regional
facilities, and university departments. Because
experiments usually involve attaining very high
energies, the equipment – accelerators, detectors and
so on – is physically large, involves incredibly complex
engineering requiring multidisciplinary teams of
people, and so is expensive. As a result, even the
smaller laboratories are becoming increasingly
international in nature. One of the main tasks of the
European nuclear physics community is to develop a
coordinated research strategy that makes the best
use of current and proposed facilities. The large
international and the smaller, national laboratories
have complementary roles in providing a balanced
framework of experimental programmes for
supporting creative research.
The most famous European high-energy physics
laboratory, now regarded as a world facility, is CERN,
the European Organisation for Nuclear Research,
T H E E U R O P E A N D I M E N S I O N ::
A world-leading E urope has a noble heritage in nuclear
physics that started with the discoveries ofradioactivity and nuclear structure
Part of the AGOR accelerator
at KVI in the Netherlands
The accelerator
at JYFL in Finland
N A T U R E A T T H E F E M T O - S C A L E 7
based in Geneva. As well as building the world’s
highest-energy particle machine, the Large Hadron
Collider (LHC) to investigate a new realm of
fundamental physics, CERN carries out a number of
lower energy nuclear physics experiments. The
electron-proton collider HERA, investigating the
structure of the proton, has established the DESY
laboratory in Hamburg as an international laboratory.
Germany also has another major laboratory devoted
to nuclear physics, GSI in Darmstadt, which has
ambitious plans to upgrade and extend its
accelerator complex so as to open up new frontiers in
nuclear science. France and Italy also host large
international facilities carrying out many
experiments – respectively, GANIL in Caen and the
GRAN SASSO underground laboratory near Rome. All
these research centres have made major frontier
discoveries in nuclear physics, as the pages in this
booklet show.
The role of medium-sized facilities While certain experiments require the cutting-edge
facilities and infrastructure of a large laboratory, the
medium-sized, nationally-based research institutes*play a crucial role within the research framework.
These laboratories are able to focus on specialised
experiments perhaps requiring particular kinds of
particle beams and detectors. Because the
equipment is less costly to operate, very precise data
can be obtained by running the experiment for a
long time. It may also be advantageous to develop
and test new detectors and experimental techniques
first in university laboratories. The instrumentation
can then be shared by several facilities across Europe.
The national laboratories can readily offer students
the necessary broad education in both physics and
engineering. Finally, many of these laboratories have
close relationships with local industry, and have their
own spin-out companies offering, for example,
isotopes and cyclotrons for medical applications,
microfabrication facilities and radiation-hazard
assessment services.
Most nuclear physics research centres are involved
in technology transfer, for example, developing novel
semiconductor devices that may find use in medical
and industrial equipment. Many European facilities
now run particle-beam cancer therapy programmes
which may take up a substantial proportion of
accelerator time. The public is thus benefiting from
nuclear research in a very direct way.
Finally, the nuclear physics community is aware of
the importance of communicating to the public the
significance of its research findings. Many laboratories
organise regular public outreach and education
activities, including open days and school visits.
*JYFL Finland; KVI The Netherlands; TSL Sweden; FZJ Germany;CRC Belgium; IReS France; LNL, LNF and ECT Italy
programme of nuclear research
The accelerator
hall at FZJ in
Germany
The multidetector
ICARE used for
nuclear reaction
studies at IReS
8 N A T U R E A T T H E F E M T O - S C A L E
of the neutron dripline has so far been explored. There
is a vast territory of nuclei with high numbers of
neutrons which is still terra incognita. At the top end
of the nuclear map exist possible islands of superheavy
nuclei which are expected to be relatively stable.
Why is it important to make and study the broad
range of nuclei possible? First, nuclei with specific
combinations of protons and neutrons allow us to
test theories of nuclear structure (see box) and
fundamental interactions; secondly, their properties
may uncover the pathways by which the elements
are created astrophysically, and finally, many isotopes
have potentially significant applications – in analysis,
medical treatment and perhaps most excitingly,
though more speculatively, in safe energy production
and storage (p.16).
To create the nuclei we want to study requires a
wide range of experimental techniques. First,
particles such as protons, neutrons and various
nuclei, accelerated to moderately high energies, are
projected against a target. This produces new nuclei
in several ways – neutrons or protons may flow
between the target and the projectile nuclei, or the
nuclei may fuse into heavier species or break up into
lighter ones. Depending on the energy of the beam and
the target material, specific nuclei can be selected and
then guided by electromagnetic fields to a detector.
As physicists begin to explore out into the
unstable frontiers of the nuclear landscape, the first
important measurements to make are those of mass,
lifetime and mode of decay, which give information
on the stability and the binding energy of the
nucleus. Nuclei are often prepared in high-energy
states, perhaps set spinning by glancing collisions.
The energy spectra of the gamma-rays emitted as
the nucleus returns to its ground state, or particles
emitted as it decays, provide vital information on its
structure and shape.
The limits of nuclear stabilityDuring the past few years, the neutron dripline has
been mapped as far as fluorine-31 (9 protons and 22
neutrons, neon-34 (10 protons and 24 neutrons) and
sodium-37 (11 protons and 26 neutrons). What is
remarkable is that the dripline jumps from oxygen-24
Nuclei are well-defined, yet somewhat mysterious
systems containing up to a couple of hundred
protons and neutrons (collectively called nucleons)
held together by nuclear forces. Nuclei are capable of
immense variation and complexity due to the subtle
interplay of these forces.
The stable nuclei making up the elements we are
familiar with in everyday life represent just a small
fraction of those that can exist. Other nuclei with
vastly varying proportions of protons and neutrons
can be created in experiments, and play a vital role in
the synthesis of elements in stars (p.12): at least
6000 different proton-neutron combinations are
possible. These can be plotted across a ‘landscape’ of
protons and neutrons (see diagram). The chart
reveals a long ‘valley of stability’ rising diagonally,
which is inhabited by families of stable nuclei; north
and south of the valley, unstable nuclei with widening
ratios of proton and neutron numbers survive for a
time. Out on the wild frontiers of these regions,
nuclei live dangerously – the proton-neutron ratios
are so extreme that the nucleons leak out. These
‘driplines’ are a very active area of study. We know
where the proton dripline is but only the lower part
N U C L E A R S T R U C T U R E
82
82
126
50
50
28
2820
20
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8
The nuclear landscape
PROT
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NUM
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NEUTRON NUMBER N
proton dripline
neutron dripline
r-process path
rp-process path
unknown nuclei
known nuclei
stable nuclei
::
complexnucleusThe
ost of the Universe that we can see is made of nuclei – the dense coresof atoms only about one million millionth of a centimetre acrossM
N A T U R E A T T H E F E M T O - S C A L E 9
(8 protons, 16 neutrons) to fluorine-31 with six more
neutrons. Now, the eight protons of oxygen represent
a stable, ‘magic number’ (see box) so that adding
an extra proton requires a fair amount of energy;
nevertheless that one extra proton orbiting the
closed eight-proton shell binds six extra neutrons –
giving us important insights into the subtleties of
the interactions between the two sets of nucleons.
:: HALOS AND SKINSThe complexity of these interactions are revealed in
light nuclei close to the neutron dripline in which the
excess neutrons are loosely bound, forming a halo
around the central core. The best known are helium-6
and lithium-11, which have been studied at a number
of European laboratories. In these nuclei, two
neutrons sit far outside the respective helium-4 and
lithium-9 cores, each forming a mutually attractive
three-body system but which any separate pair of
the constituent bodies does not bind. The
system is called Borromean after the
heraldic sign of the Italian princes of
Borromeo of three interlocked rings
which fall apart when one ring is
removed. So far, several neutron halo systems have
been discovered, the largest being carbon-19.
Bound quartets of neutrons may even exist on
their own (tetraneutrons) – recent results at GANIL
have yet to be confirmed. However, hydrogen-5 and
hydrogen-7 containing one proton, and four and six
neutrons respectively have recently been made. Studies
of these extraordinary neutron-rich species may
throw light on the structure of neutron stars (p.12).
In heavier neutron-rich nuclei, a mantle of
neutrons can envelop the nuclear core to form a low-
density neutron ‘skin’. Such systems are expected to
show complex behaviour – an experimental
challenge for the future. The neutron dripline seems
to be much further out than originally thought and
embraces some fascinating phenomena. Protons
might also form halos in proton-rich nuclei and will
be investigated in the future. One important recent
discovery are nuclei teetering on the proton dripline
which fall apart by spitting out two protons at a
time, such as oxygen-12 and iron-45.
Nuclear modelsThe nucleus is a self-organised, many-body quantumsystem which interacts through the strong, weak andelectromagnetic forces (p.22), so that developing asingle theoretical description is extremely challenging.At the moment, there are several approaches whichwork well for different types of nuclear species. Oneimportant characteristic, first noticed in the 1940s,was that certain proton-neutron combinations areparticularly stable. These ‘magic numbers’ (2, 8, 20, 28,50, 82...) can be explained by the so-called shell model,in which each nucleon moves in an orbit held by acentral force calculated from the average effects of allthe nucleons. The nucleons build up in shells accordingto quantum mechanical principles, as for electrons inatoms; the magic numbers represent fully occupied, orclosed stable shells of protons or neutrons. Anynucleons orbiting beyond the outermost closed shellbehave as ‘valence’ nucleons (single particles), whichcan be excited to higher quantum states. The shellmodel works well for most light nuclei and those withnumbers of neutrons and protons near a closed shell.
Another model, successful for many nuclei,describes the nucleons as pairing up into nuclearbuilding blocks called ‘bosons’, which are characterisedby particular quantum properties also found in thepairs of electrons responsible for superconductivity.Changes in structure can then be characterised interms of the pairs being excited or breaking up.
For heavy nuclei, and those with nucleons not neara magic number, the interactions are best described byanother class of models in which the nucleons aretreated collectively as a liquid drop, held together bysurface tension. When excited, the nucleons all movetogether causing the nucleus to vibrate or rotate, oreven change shape. Many nuclear species showcomplex behaviour involving interplay betweencollective and single-particle or boson excitations.
Investigating the intricate changes in structuretriggered by adding or removing a nucleon, changing aproton for a neutron (a parameter called isospin), orexciting the nucleus to high energies can be used totest these models.
>>
Equipment used in
experiments to study
heavy nuclei at GSI
The protons
(green) and
neutrons (pink)
in a nucleus
Shell model
Liquid drop
model
10 N A T U R E A T T H E F E M T O - S C A L E
at GSI and GANIL have coaxed into existence two
unusual doubly-magic nuclei, tin-100 with 50
protons and neutrons, and nickel-48 with 28 protons
and only 20 neutrons. By comparing the behaviour of
these unstable, neutron-starved species with their
close neighbours on the nuclear map, theorists can
test their models of nuclear structure.
:: MIRROR NUCLEITin-100 and nickel-48 are fascinating for another
reason. Although quantum physics prefers the
number of protons and neutrons to be equal, the
electromagnetic force pushes the protons apart,
which is why increasing numbers of neutrons are
needed to hold together the heavier nuclei. Tin-100
does in fact have equal numbers of protons and
neutrons so is rather special; the protons and
neutrons are probably paired off. In the case of
nickel-48, if the numbers of protons and neutrons
are swapped around, then its ‘mirror’ nucleus
calcium-48 is created which is both doubly-magic
and stable. Such a mirror pair can be used to probe
the competing effects of proton and neutron
interactions within the nucleus.
:: NUCLEAR SHAPEAlthough magic nuclei tend to be spherical, just a
small change in energy or number of particles can
cause the nucleons to reorganise, adopting a
radically different shape to achieve stability. Recently,
using the accelerator at GSI, European researchers
uncovered a triple nuclear shape-shifter, lead-186.
This isotope has a closed shell of 82 protons which
likes to be ball-shaped; however, when the shell is
broken by exciting a pair of protons, the nucleus
quickly settles into either a pumpkin (oblate) or
melon (prolate) shape. Similarly, just adding, say, a
pair of neutrons to a medium-mass magic nucleus
can cause a drastic change in physique.
:: SHAKE, RATTLE AND ROLLAs nuclei get heavier, nucleons start to lose their
individuality and become more fluid-like. New types
of surprising phenomena appear as the nuclei are
made to vibrate and rotate very quickly, and these
:: CLUSTERS AND NUCLEAR MOLECULESLight nuclei can also form unusual configurations
comprising loosely bound clusters of alpha particles
(two neutrons and two protons), and these continue
to be a theorist’s paradise. Carbon-12 and oxygen-16
can be thought of as clusters of three and four alpha
particles respectively. Such structures are highly
deformed, the most extreme being excited
magnesium-24 – possibly a polymeric chain of six
alpha particles. Other, even more exotic structures
are thought to exist akin to atomic molecules held
together by covalent bonds, for example, silicon-28
made up of an oxygen-carbon conglomerate and
beryllium-9 which can be thought of as two alphas
with a neutron in shared orbits around them. Such
ideas await further spectroscopic exploration.
:: THE SUPERHEAVIESPerhaps the most exotic nuclei, at least to the
general public, are the heaviest at the top end of the
nuclear map. These are entirely artificial and usually
short-lived. Nevertheless, theorists have predicted an
island of stability between element 114 (the isotope
with 184 neutrons is doubly magic so is expected to
be stable) and 126. Much effort in Europe has been
put into reaching this island using carefully planned
fusion reactions. So far, 112 has been made (at GSI)
and there have been sightings of elements of 114 and
116 (at Dubna in Russia). Further work will require
much more intense beams of both stable and
unstable heavy nuclei. In the meantime, researchers
continue to analyse the chemistry of the superheavy
elements made so far, relying on just a few atoms!
Recently, an international team at GSI investigated
the chemical properties of hassium (element 108),
showing that it behaved similarly to Group 8
elements in the Periodic Table.
Investigating nuclear structure and shape:: MAGIC NUCLEIA key area of study are the magic nuclei with stable,
filled shells of neutrons or protons. Particularly
intriguing are doubly-magic nuclei with full shells of
both protons and neutrons tightly held together. In
the past few years, researchers working with facilities
N U C L E A R S T R U C T U R E ::
>>
Magnesium-24
can be regarded
as a cluster of six
alpha particles
Nulear structure can be
studied by creating exotic
nuclei containing a kaon, for
example. Such experiments
are carried out using the
DEAR apparatus at FNL
N A T U R E A T T H E F E M T O - S C A L E 11
offer important insights into structure. Centrifugal
and Coriolis forces may induce elongated shapes
with axial proportions of 2 to 1 – called
superdeformation – revealed by the emission of a
characteristic gamma-ray cascade, or nuclei may
start to wobble as in the case of lutetium-163. Theory
predicts even more extreme ‘hyperdeformed’ nuclei
with an axis ratio of 3 to 1, and researchers are
searching for them among barium, xenon and tin
isotopes. Another topic of intense interest are the
collective oscillations of nucleons called giant
resonances. They also provide information on
structural changes in excited nuclei, and throw light
on what happens when a nucleus is squeezed.
:: EXOTIC NUCLEI A completely different approach to probing nuclear
structure is to introduce an ‘impurity’ into the
nucleus. Protons and neutrons are made up of two
kinds of quark – ‘up’ and ‘down’ (p.22). Four other
quarks exist in Nature, and so a neutron can be
replaced with an exotic nucleon containing another
quark flavour, such as a lambda (Λ). This has a
strange quark as well as an up and a down quark.
Recently experimenters converted lithium-7 (with
three protons and four neutrons) into lithium-6-Λ.
The effect was extraordinary: the nucleus shrank in
size by 20 per cent as the lambda became more
tightly bound than the outer neutron and proton.
The need for advanced facilitiesTo continue the exciting work highlighted here will
require new advanced facilities for generating a wide
range of nuclei far from stability (p.26) not possible
at the moment – whether to explore the driplines,
probe unusual or significant proton-neutron
combinations, or try to make new elements that have
never existed before. There is still a lot to learn in this
fertile field of scientific exploration and we can
expect many more surprises.
3025201510β2cos(γ+30)
β2 sin(γ+30)
En
erg
y (
Me
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Potential energy surface for 186Pb
5200
-200
0.5
1
1.5
2
2.5
3
3.5
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(MeV)
186Pb
6+
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2+
0+
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Similar nuclei can adopt quite
different shapes to achieve stability.
Experiments at JYFL have been
probing such shape coexistence in
heavy nuclei close to the proton
dripline (left), while work at GSI
(below) has uncovered an isotope of
lead which has three different
shapes depending on energy
Researchers at JYFL measure the masses
of rare neutron-rich heavy nuclei
12 N A T U R E A T T H E F E M T O - S C A L E
fewer neutrinos (elementary particles produced in
nuclear reactions) than predicted theoretically. One
suggestion was that, on their way to the Earth, the
solar neutrinos were changing, or oscillating into
other kinds of neutrino (p.22) not detectable by the
instruments then used. Recent measurements made
at the Sudbury Neutrino Observatory in Canada
confirm the existence of these neutrino oscillations –
a fundamental phenomenon which looks to have
momentous implications for particle physics theory.
In the meantime, our standard nuclear model of the
Sun remains reassuringly intact.
OBSERVATIONS AND MEASUREMENTSNot all the elements are made in stars like the Sun.
To explain the abundances of elements we see today,
physicists have proposed a series of complex but
coherent networks of nucleosynthetic reactions
thought to operate in certain astrophysical
environments such as red giants and supernovae.
These abundances can now be checked in much
more detail against spectroscopic observations
available from powerful telescopes such as the
Hubble Space Telescope and the Very Large
Telescopes of the European Southern Observatory.
The new generation of X-ray and gamma-ray
telescopes, including the European Space Agency’s
XMM-Newton and Integral, will provide important
new data on element-building and distribution in the
Universe. Integral, in particular, will be able to home
in on the explosive processes in supernovae thought
One of the deepest questions on which humans have
always pondered is how did the world and ourselves
come into existence. Clearly our planetary
environment, and the life it supports, have been
greatly shaped by the constituent proportions of
elements. We know that the first, lightest elements,
hydrogen, helium and some lithium, were created in
primordial processes just after the Big Bang – the
observed proportions indeed support those predicted
theoretically. All the heavier elements were and are
still being made by nuclear reactions in stars.
Unravelling the pathways by which elements are
synthesised is a key factor in understanding the early
Universe, the evolution of galaxies and stars, and the
development of planetary systems, such as our Solar
System, which are hospitable to life. Of particular
relevance to us, of course, are the mechanisms
responsible for building up the elements necessary
for our existence.
SOLAR NUCLEAR PHYSICSAnother issue gaining increasing interest, in which
nuclear physics plays a part, are the effects of the
Sun’s inherent variability on the Earth. The Sun is very
much an average star, and its energy output upon
which we depend is driven by those very reactions in
which the elements are made, so understanding
their role in solar dynamics is of clear importance.
Solar nuclear physics has also come under scrutiny
for another reason. In recent decades, researchers
discovered that the Sun appeared to be emitting
N U C L E A R A S T R O P H Y S I C S ::
Probing the nuclear processes that drive the engines of stars is now oneof the most important areas of nuclear physics research
Nuclear physicsin the Universe
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-Au
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lia
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tory
Gamma-ray emission across the
Universe from aluminium-26,
a signature isotope of
element-building in stars
The heavier elements are
synthesised in dying stars
(see the Cat's Eye planetary
nebula, above), and in
supernovae (1987A, below)
to be responsible for creating the heaviest elements.
Meteorites also act as cosmic probes. Most of them
are left-over debris from the construction of the Solar
System, and thus provide pristine ‘fossil’ evidence, in
the form of elemental abundances, on stellar processes.
Another crucially important approach is to study
nuclei and nuclear reactions of astrophysical
relevance in the laboratory using particle and nuclear
beams. In this way, it’s possible to determine
significant parameters of a nucleus such as mass,
lifetime and the characteristic excited states, or the
likelihood of a particular reaction. Nucleosynthetic
pathways may involve intermediate unstable nuclei
with extreme ratios of protons and neutrons which
are far away from the valley of stability on the
nuclear map (p.8), and these can be studied with
radioactive beams. Another aim is to simulate the
real physical conditions found in stars, perhaps using
high-intensity beams at low energies.
Cooking up the elementsAlthough we now have quite robust models for the
various reaction pathways that occur during a star’s
evolution, observational and experimental evidence
is still sparse. Many of the standard reactions are
notoriously difficult to measure. Nuclear astrophysics
is clearly a burgeoning area where many discoveries
are waiting to be made.
:: GENTLY HEATFor most of their lives, stars produce energy by
fusing hydrogen into helium by the pp chain in
which helium-4 is effectively produced from four
protons. In second-generation stars like the Sun, the
transformation of hydrogen to helium can also be
catalysed by carbon-12 (the CNO cycle). We still don’t
know the relative importance of the two mechanisms;
a recent suggestion is that the CNO cycle could be
responsible for up to half the energy production in
the Sun. Measuring the fusion rates is extremely
difficult because they occur at relatively
low energies with low probabilities
(because of the mutually repulsive effects
of the positively charged nuclei).
However, these reactions have recently
been carried out underground (to avoid
interfering background radiation) in the
LUNA laboratory at the Gran Sasso facility
in Italy using a dedicated accelerator.
Other important reactions are the fusion of
helium-3 and helium-4 into beryllium-7 and its
conversion to boron-8 by the capture of a proton. The
latter reaction is largely responsible for neutrinos
emitted from the Sun whose flux measurements led
to the confirmation of neutrino oscillations
mentioned earlier, so understanding fully the
neutrino-generating process is vital to further study
of this newly discovered phenomenon.
In stars more massive than the Sun, there are still
many uncertainties about the rates of the reactions
that follow the onset of helium burning. One of the
most significant reactions is the fusion of helium-4
and carbon-12 to oxygen-16, partly because it
influences all the future stages of nuclear burning
and the final core collapse. Its rate also determines
the abundances of the two foundation stones of life
– carbon and oxygen. Much more work is needed on
this very important reaction. Another significant
uncertainty to investigate is the rate of conversion of
magnesium-25 to aluminium-26. The latter isotope is
quite long-lived and provides a useful marker across
the Galaxy for the distribution of
nucleosynthetic processes (p.12) with
implications for our understanding
of stellar evolution and the origin
of planets. >>
Equipment used at
CYCLONE in Louvain-la-Neuve
to study nuclear reactions of
astrophysical importance
The blast wave from a
supernova explosion
(Cygnus Loop nebula)
ground state which has a half-life of only a month,
thus changing the effective half-life by 15 orders of
magnitude. These kinds of changes have implications
for radioactive ‘clocks’ used to measure astrophysical
ages. The slow decay of lutetium-176 to hafnium-176
is a perfect clock in the laboratory (the principle is
the same as for carbon-dating) and since lutetium-176
is produced only by the s-process, it could be used to
determine the age of s-elements. Again at the high
temperatures in red giants, the isotope can decay via
an alternative shorter-lived state.
At high stellar temperatures, virtually all the
electrons are stripped off atoms, which also reduces
the half-life since nuclear electrons can escape more
easily, and this also has to be taken into account in
isotope-dating. An intriguing additional effect was
recently discovered at GSI in the decay of some
isotopes such as rhenium-187 to osmium-187 (used
to measure the age of the Universe). When atomic
electrons are stripped off, the emitted electron
remains tightly bound in close orbit around the atom
which further modifies the half-life.
:: ALLOW TO BOILThe s-process cannot produce all the heaviest
elements. Instead, another process – the rapid
neutron capture, or r-process – is thought to produce
elements heavier than iron all the way up to uranium
and beyond. The free neutrons must be in great
abundance and stick to nuclei in quick succession
before they have a chance to decay by beta emission,
or be knocked loose by gamma-rays. In this way
elements are rapidly built up (in seconds) through
neutron-rich isotopes far from the valley of stability.
The process may get held-up when especially stable,
‘magic’ nuclei (p.9) are reached – so-called waiting
points – and these regulate its rate. The effects of
these waiting-point nuclei are indeed reflected in the
relative abundances of the heavy elements such as
strontium-88, barium-138 and lead-208.
Unravelling the r-process requires an
understanding of nuclear structure in heavy,
neutron-rich species, and this depends on measuring
lifetimes, masses and spectroscopic properties. The
beta-decays of about 30 neutron-rich nuclei were
:: SIMMER AND STIRAbout half the elements heavier than iron are made
in red giants, starting with iron ‘seed’ nuclei. A free
neutron is captured by a nucleus and decays into a
proton, emitting an electron (beta particle) at the
same time, to give the next heavier element. This
neutron-capture process is extremely slow compared
with the beta decay, and so is called the slow
neutron-capture, or s-process. Successive neutron
captures produce elements all the way up to lead,
following the valley of stability in the nuclear
landscape on p.8. Some 30 elements are produced
only by the s-process. One source of evidence for the
s-process abundances are meteorites which contain
grains with small amounts of elements thought to
originate from the atmospheres of red giants. High-
precision measurements of ratios of specific isotopes
in meteorites can then be compared with theoretical
predictions and experimental results.
Although we have a good, general model of the
s-process and the accompanying mixing of material
in red giants, there are still uncertainties and
anomalies. The two main sources of the
neutrons for the s-process are a reaction in
which carbon-13 captures a helium
nucleus (alpha particle), then spits out a
neutron to form oxygen-16, and a
similar process converting neon-22
into magnesium-25. Their reaction
rates still need to be determined
more accurately, as do the rates of
many s-process reactions. Recently
the neutron-capture rate for the
rarest stable nucleus in Nature,
tantalum-180, was measured.
Tantalum-180 is an example of
where the laboratory-measured
half-life may be drastically
changed by the stellar
environment. This isotope normally
sits in an unusual long-lived high-
energy state lasting a thousand
million million years; but experiments
show that at red-giant temperatures, it
is most likely pushed down into the
N U C L E A R A S T R O P H Y S I C S ::
e+ν
ν e+
4 11H 4
2He 3 4
2He 12
6C 12
6C + 4
2He 16
8O
Nuclear fusion reactions at the heart of the Sun
Protons: red;Neutrons: blue
>>
Detector arrays of the
Sudbury Neutrino
Observatory to detect
solar neutrinos
14 N A T U R E A T T H E F E M T O - S C A L E
N A T U R E A T T H E F E M T O - S C A L E 15
recently measured at the ISOLDE facility including
those of waiting-point nuclei, cadmium-30 and
silver-129. GSI has measured a large number of
masses of short-lived neutron-rich nuclei and hopes
to measure hundreds more in the future. It is not yet
possible to measure directly rates of neutron capture
of these highly unstable nuclei.
So far, our current understanding of the r-process
is supported by accurate measurement of the
abundances of r-process elements in very old stars,
in the Sun and in meteorites, but we still don’t know
for sure where it takes place. Clearly, it has to be an
explosive event, and the most likely site is the hot
wind, driven by neutrinos, gusting off the surface of a
neutron star newly-formed in a supernova explosion.
Neutrinos themselves may also trigger
nucleosynthetic reactions.
Re-heatIn binary systems, the transfer of matter from one
star to another causes violent nuclear reactions
(see box) leading to a different set of abundances.
The hydrogen that accumulates ignites via the ppand CNO cycles which is followed by the rapid
capture of protons (the rp-process). These reactions
scramble up the proton-rich side of the valley of
stability to isotopes around mass 40. Pioneering
experiments on these reactions are carried out using
radioactive beams at CYCLONE. If the accreting star is
a neutron star, then proton capture goes further,
resulting in nuclei up to mass 100.
There is clearly strong interplay between nuclear
astrophysics and nuclear structure studies, and with
the advent of new radioactive beam facilities we can
expect exciting developments in this area.
Star life Although most of the matter in theUniverse is invisible, and of anuncertain nature, a large part of thatwhich we can observe consists of largeglowing spheres held together bygravity – stars. These objects mostlycomprise hydrogen and helium withsmaller amounts of other elements,and are hot and dense enough totrigger nuclear ‘burning’ – the fusion ofthe lighter elements into heavier ones.The energy liberated creates pressure,with the result that a star’s life is anongoing tussle between this outwardpressure and gravity pulling matter tothe star’s centre.
Stars start by fusing hydrogen intohelium which settles into the stellarcore and eventually starts burning aswell. The heat from the burning,shrinking core causes the star to swellinto a red giant (which will happen to
the Sun in another five billion years orso). Stars at least eight times moremassive than the Sun carry on burning,starting with carbon and oxygen, goingthrough, neon and magnesium, siliconand sulfur, creating elements withmasses up to iron. The result is a starwith onion-like layers of elementssurrounding an iron core. Smallamounts of heavier elements are alsoproduced in the inner layers.
When a medium-sized star like theSun has used up all its helium, itshrinks under gravity and settles to adense ‘white dwarf’ while blowing offits outer gaseous envelope to form adiffuse shell called a planetary nebula.Much heavier stars end their lives morespectacularly as supernovae. Theirdense iron cores collapse after thenuclear fuel has run out. The outerlayers fall onto the core and bounce offagain, creating a gigantic shock wave
that rapidly travels outward inducingfurther nuclear reactions to create theheaviest elements. The exploding starbecomes brighter than a million Sunsas it spills its guts into the surroundingspace. Left behind is a compact object –a neutron star or a black hole.
If a dead star is part of a binarysystem, then its nuclear life might besporadically resurrected by suckinghydrogen-rich material from its stellarcompanion. In the case of a typicalwhite dwarf, the accreted mattertriggers a nova explosion and thehydrogen ignites leading to furthernuclear burning. A more massive whitedwarf might destroy itself in asupernova type I explosion, ejecting allthe products of nuclear burning. If thestellar remnant is a neutron star, thenaccretion may lead to thermonuclearrunaway on its surface, resulting inperiodic bursts of X-rays.
Part of GSI’s accelerator system
used to study neutron-rich nuclei
important in nucleosynthesis
16 N A T U R E A T T H E F E M T O - S C A L E
NUCLEAR FISSION AND TRANSMUTATIONTwo of the main problems associated with current
nuclear-energy production are those of long-lived
radioactive waste (transuranium elements, plutonium,
neptunium, americium and curium, and fission
products such as iodine-129 and technetium-99),
and nuclear proliferation. The next generation
of advanced reactors is being designed to be
considerably safer and to produce much less waste.
They can also utilise plutonium and other waste
components as fuel, and could be deployed in
combination with an approach that is gaining
interest: that of transmutation.
Here, the principle is to create a source of
neutrons by firing a proton beam from a high-power
accelerator at a heavy-metal target. The neutrons
knocked out of the target material (a process called
spallation) would then react with waste products in
a surrounding assembly, either being absorbed by
them or causing them to fission, thus converting the
long-lived nuclides into stable or short-lived isotopes.
An important plus is that the process produces more
energy than it consumes, so could also be used to
generate electricity.
SUBCRITICAL NUCLEAR REACTORSSuch so-called accelerator-driven systems (ADS)
could further be deployed in a novel nuclear reactor
burning the element thorium (plentiful in the Earth’s
crust) instead of uranium. Thorium doesn’t readily
fission, but when bombarded with neutrons, it is
transformed into uranium-233 which also fissions.
Significantly, the reaction is subcritical so can be
switched off, and it also produces less waste. The idea
has been developed in Europe by Carlo Rubbia at
CERN, who calls it the Energy Amplifier.
Research into the feasibility of ADS is still at a very
early stage but there are a number of EU-supported
programmes across Europe looking at the neutron
production process (for example, at GANIL and CERN)
and neutron-induced fission (using the neutron beam
facility at The Svedburg Laboratory in Sweden).
Much of our advanced technology derives from
exploiting the complex behaviour of atoms. However,
we have hardly begun to utilise the rich and intricate
properties of nuclei for the benefit of society.
Nevertheless, nuclei and their components,
protons and neutrons – and nuclear processes such
as radioactive decay – are already vital analytical
tools for laboratories and industry, and have also
become an essential part of many medical treatments.
More controversial is whether we can safely
harness the powerful forces binding the nucleus for
energy production. Regrettably, most people
associate nuclear physics studies with just one
nuclear reaction – the chain-reaction fission of
uranium-235, the basis of the atomic bomb. Today’s
nuclear-power generation, which depends on the
same reaction, produces one-third of the EU’s
electricity (2000 figures) but is regarded by many as
environmentally hazardous and uneconomic.
However, some exciting work is going on in European
laboratories to study novel methods of producing
safer, cheaper nuclear energy.
Safe nuclear energyMuch of the world’s energy comes from another
environmentally hazardous chain-reaction – chemical
combustion (mostly of oil and gas). Alternative
sources of energy such as wind and solar power are
not likely to satisfy future energy demands, whereas
harvesting the huge amount of energy locked up in
the nucleus would secure the world’s energy needs
for many thousands of years.
A P P L I C A T I O N S ::
N UCLEAR PHYSICS HAS THE POTENTIAL TO OFFER NEW TYPES OF 21st-CENTURYTECHNOLOGY FOR ENERGY PRODUCTION, INDUSTRY AND HEALTHCARE
Law
ren
ce B
erk
ele
y L
ab
ora
tory
Inertial confinement
fusion could be the route
to clean energy
Fo
rsch
un
gsz
en
tru
m J
üli
ch
PET images of the
brain using iodine-124
(left) and fluorine-18 as
positron-emitters
Exploitingthe nucleusThe energy amplifier –
a route to subcritical
nuclear energy being
developed at CERN
N A T U R E A T T H E F E M T O - S C A L E 17
NUCLEAR FUSION Another nuclear reaction being studied for energy
production is that mimicking the main nuclear
process powering stars – nuclear fusion (p.12).
Isotopes of hydrogen – deuterium and tritium –
combine to form helium and energy-carrying
neutrons at extremely high pressures and
temperatures. The process is very clean, uses a readily
available fuel and produces little waste. One way of
achieving the right conditions for fusion is to fire a
high-power laser, or a beam of light or heavy ions,
at an encapsulated solid pellet of the isotopes. The
intense radiation heats and compresses the fuel so
much that fusion occurs. Work is being carried out on
the heavy ion approach by the HIDIF collaboration
(European Study Group on Heavy Ion Driven Inertial
Fusion) using facilities at GSI, and a large laser facility
is being constructed in France.
ENERGY STORAGEHigh-energy lasers also offer intriguing ways of
tapping into the nucleus’s rich energy stores. Some
nuclear species are readily excited into higher energy
states that are long-lived (nuclear isomers). Indeed,
the naturally occurring form of tantalum-180 is a
stable isomer. If this energy could be released in the
form of gamma-rays by ‘tickling’ the isomer with a
laser, then we would have the basis of a nuclear
battery or even a gamma-ray laser. There is much
potential in investigating these and other more
subtle aspects of nuclear behaviour.
The nucleus as microscope and cameraEver since radioactivity was discovered, the nucleus
has been used as a probe to analyse materials, and
to obtain images of objects otherwise too difficult
to investigate. Today, there are a huge variety of
techniques based on nuclear properties used in areas
as diverse as industry, medicine, environmental
protection and archaeology.
NUCLEAR MAGNETIC RESONANCE AND MRIOne of the most powerful analytical techniques to
come out of nuclear physics exploits the magnetic
properties of certain nuclei – hydrogen (single
proton) or phosphorus-31 have magnetic moments
(they act like bar magnets) which can be oriented by
magnetic and radio-frequency fields. The interaction
is influenced by the nucleus’s chemical environment
so offering a clever method for analysing chemical
structure. Nuclear magnetic resonance (NMR) is now
an indispensable tool for chemists and molecular
biologists alike. Over the past 20 years, the technique
has been successfully taken further to construct
images of regions inside the body, in particular the
brain, the heart and lungs, and tumours. This is
known as magnetic resonance imaging, or MRI, and
MRI scanners are now found in most major hospitals.
RADIOACTIVE PROBESFor many years, radioactive isotopes have been
applied in the clinic as diagnostic tools, and also as
tracers in biological studies following, for example,
metabolic pathways or the uptake of drugs. They
are similarly employed to monitor waste and
pollutants in the environment. Extremely small
amounts of radioactive and stable isotopes can then
be analysed using the technique of accelerator mass
spectrometry (AMS) in which samples are converted
into a beam of ions. The different kinds of ions are
separated by magnetic and electric fields, and their
signature masses measured. The sensitivity of AMS,
whereby single atoms can be detected, has opened
up a much broader use of radioactive labelling in
medical diagnosis and in measuring environmental
pollution. One of the most successful uses of AMS is
in carbon-dating. Instead of relying on measuring the
radioactivity of carbon-14, the ratio of carbon-14 to
stable carbon-12 can be obtained directly by counting
the carbon atoms.
Remarkable in vivo images of the body can be
reconstructed by detecting the distribution of
radioactivity. Positron emission tomography (PET) is a
sensitive technique of growing importance employed
to study blood flow and metabolic activity especially
in the brain. Positrons (positively charged electrons)
emitted from an injected radioisotope such as
fluorine-18 reveal their position when they annihilate
to release pairs of gamma-rays which are detected by
a camera. A similar technique, single photon emission >>
Making measurements related
to the transmutation of nuclear
waste at TSL in Sweden
18 N A T U R E A T T H E F E M T O - S C A L E
nucleus of a target atom so that X-rays are emitted.
This technique – particle induced X-ray emission
(PIXE) – offers a nondestructive way of analysing, for
example, pigments in paintings. Two other techniques,
nuclear reaction analysis (NRA) and particle-induced
gamma-ray emission (PIGE), rely on an ion beam
exciting – or reacting with – a nucleus to emit
gamma-rays. Radioactive ion beams, such as those
produced at the ISOLDE facility at CERN, can also be
used to study materials. When the ions become
implanted on a target surface, their pattern of decay
is perturbed by the surrounding atoms, so revealing
the nature of their immediate environment.
Creating new materialsHigh-energy ion beams also modify materials in
interesting ways. Ions can be implanted into surfaces,
or they may push their way through a material
drilling a nanometre-sized track which can then be
etched away. In this way, new microstructures can
be created, for example, arrays of micropores in
membranes for filtration or drug release, or
semiconductor devices using the tracks as a template.
Curing cancerEvery year, more than 1 million people in the
European Union are diagnosed with cancer, and
while surgery, chemotherapy and conventional
radiotherapy with X-rays cure about 50 per cent of
patients, a large proportion of tumorous cancers are
rather resistant to radiation, or are sited in locations,
such as the head and neck, which are difficult to
target without damaging surrounding tissue. An
alternative is to irradiate tumours with nuclear
beams – neutrons, protons or light nuclei. Neutrons
have been employed for a number of years but are
less interesting because they are not very
penetrating and tend to scatter into surrounding
tissue. Proton and ion beams, however, have the
advantage of depositing all their energy near the end
of their range, so delivering the maximum possible
dose while sparing traversed and surrounding tissue.
Because the ions are electrically charged, they can be
focused with magnetic fields into a thin pencil beam
of variable penetration depth, which can then be
computed tomography (SPECT), relies on radiotracers
that emit a single gamma-ray to follow tracer
distribution in the body and reconstruct an image.
THE POWER OF NEUTRONSOne of the main constituents of the nucleus, the
neutron, offers a powerful probe of atomic and
molecular structure. Neutron beams, generated
either in a nuclear reactor or by spallation, behave as
waves as well as particles, and can scatter off arrays
of atoms to create a diffraction pattern similar to
that from X-rays. Neutron scattering provides
complementary information to X-rays, and has
become a vital tool in molecular biology and
materials studies. With major neutron facilities at the
Institut Laue Langevin in Grenoble, France, and at the
Rutherford Appleton Laboratory in the UK, Europe
leads the world in neutron experiments. Portable
neutron emitters have also been developed which
can detect explosive materials, for example, in
landmines and terrorist bombs.
SEEING INSIDE MATERIALS WITH ION BEAMS Atoms stripped of some of their electrons and
accelerated to form an ion beam is another way of
delving just below the surface of a material, and
analysing its composition and structure. There’s now
a whole range of techniques using fine ion beams,
which are used to investigate thin-film structures for
integrated circuits, corrosion, and delicate
archaeological artefacts (see box). The ions (usually
hydrogen or helium) may bounce off atoms allowing
their mass to be measured, or they may lose a
characteristic amount of energy that indicates the
composition of the target material. These ion-beam
techniques include Rutherford backscattering (RBS)
and medium-energy ion scattering (MEIS). The ion
beam may excite electron energy levels close to the
A P P L I C A T I O N S ::
>>
Microscopic copper
needles made using
tracks fabricated by ion
beams at GSI
PS
I
Some 200 patients
have been treated so
far with GSI’s novel
ion beam therapy
Ion-track irradiation for the
preparation of nano-wires (right)
and testing electronic devices (far
right) at TSL; the SPECT technique
used to image a pancreatic
tumour (inset below)
N A T U R E A T T H E F E M T O - S C A L E 19
drawn back and forth just across the tumour volume,
while avoiding the surrounding tissue. A 3D image of
the tumour is first prepared from a CT scan and used
to program the precise dose distribution. So far,
about 30,000 patents around the world have
undergone proton therapy, which works well for
large tumours unsuitable for X-ray treatment.
NEW ION THERAPYFor deep-seated tumours resistant to X-rays and
protons, carbon-ion beams promise to be the most
effective. Ion beams are three times as damaging to
tumour-DNA than protons or X-rays, allowing little
chance of cell repair. This also means that far fewer
treatment sessions are needed. An additional
advantage is that carbon ions also undergo nuclear
reactions at their target site to form positron-
emitting carbon-11 and oxygen-15. This offers the
opportunity of using simultaneous 3D PET imaging
as a treatment planning tool. GSI is currently carrying
out clinical ion therapy trials with promising results.
Head and neck cancers are particularly suitable for
treatment, but the Laboratory is hoping to make the
equipment responsive enough to compensate for
movement – from breathing, for example.
A TREATMENT FOR LIVER CANCERFinally, another ingenious nuclear therapy exploits
the reaction between boron-10 and low-energy
neutrons to give lithium-7 and an extremely lethal
alpha particle. The combined range of nuclear
products roughly corresponds to the size of a cell so
the dose is very localised. This is the basis of boron
neutron capture therapy (BNCT): boron-10 is
delivered to the target site by a tumour-seeking
boron compound, and the tumour area is then
irradiated with neutrons. Although early trials were
not very successful, the development of new
chemicals and treatment regimes has resurrected
interest in this approach. BNCT is especially suitable
for treating metastases and diffuse tumours.
Recently, a research group in Pavia, Italy treated a
patient with liver metastases by first removing the
liver, irradiating it and then putting it back into the
patient. The patient is now fully cured!
Nuclear science in art and archeology Science and art forge a rewarding relationship when exploringour cultural heritage. Ion-beam analysis is now commonly usedby museums, art galleries and archeological laboratories to dateand establish the provenance of all kinds of artefacts.
For instance, Italian researchers employed PIXE to analyse thecomposition of inks on Galileo’s handwritten, undated notes onastronomy. They were then compared with his dated documentssuch as domestic bills, so helping to understand the evolution ofhis ideas during his life.
The Louvre has its own ion-beam accelerator (AccelerateurGrand Louvre pour l’Analyse Elementaire, AGLAE), and recentlyshowed that one Renaissance portrait in its collection containedchromium and lead pigments introduced only after 1850, thusinferring that the painting was probably a forgery.
The laboratory has also been analysing the crowns and jewelsof the Visigoth kings found at Guarrazar in Spain, in particular todiscover the provenance of their magnificent emeralds. UsingPIGE, and by comparing the emeralds with those mined all overthe world, the researchers showed that they actually came fromthe Austrian Alps, and not from Egypt or central Asia aspreviously thought.
One of the most intriguing archeological finds of recentyears was the discovery of a frozen body high in the Ötztal Alpsnear the Austrian-Tyrol border. It was soon realised that this‘Iceman’ was thousands of years old; in fact, AMS carbon-14measurements carried out in Zurich and Oxford established thathe lived about 4500 years ago.
The Louvre has its own
accelerator (main) for
studying artefacts (top).
It was to used ascertain
the provenance of the
crown jewels of the
Visigoth kings (inset left)
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20 N A T U R E A T T H E F E M T O - S C A L E
P H A S E S O F N U C L E A R M A T T E R ::
further, and may be significant for understanding
how neutron stars form in collapsing supernovae.
As the temperature rises, the nucleons are excited
into higher states of energy called baryon
resonances, and new particles form – mesons (which
contain two quarks as opposed to three in baryons
like protons and neutrons, p.24).
At much higher energies, a transition to a new
type of matter happens. The hadrons (baryons and
mesons) dissolve into a ‘soup’ of quarks and gluons –
the quark-gluon plasma which is thought to have
existed in the earliest moments of creation. This
transition is predicted to occur at densities 10 times
that of ordinary nuclear matter, and at a critical
temperature of about 170 MeV, which is 130,000 times
hotter than the Sun’s interior. In this same region,
theorists believe that the quarks also undergo another
transition when they lose their mass through a
process called chiral symmetry restoration (p.24).
Another intriguing area of the phase diagram is
the area of high densities and low temperatures.
Here, theorists predict that, instead of forming
colourless baryons and mesons, quarks pair up so
that their spins cancel out – in the same way that
electrons do in a typical superconducting material.
For this reason, the new phase of matter is called a
colour superconductor. This cold, dense medium may
actually exist in the heart of neutron stars. And
between the hadron-vapour state, the quark-gluon
plasma and the colour superconducting state, there
may be a critical point where the phases meet.
Exploring nuclear phasesNuclear physicists investigate the various parts of
the phase diagram, using both computer simulations
based on the theories and models described on p.8,
and experiments which compress the nucleus at high
energies. This compression is achieved by colliding
nuclei under a wide variety of conditions, for example,
by firing beams of heavy ions at a nuclear target.
Careful analysis of how they break up and the energies
of the particles will reveal various kinds of information
– on the flow of particles, whether or not the nuclei
are easily compressed, or on particle distributions that
may be a signature of an imminent phase change.
Just as water changes phase from a liquid to a gas at
a certain temperature and pressure, so does nuclear
matter. Studies of nuclear behaviour over ranges of
different temperatures and pressures – or densities –
are a major endeavour in nuclear physics. They
provide information needed to interpret the results
of nuclear experiments such as those colliding heavy
ions, and also to understand the behaviour of matter
under the most extreme conditions in the Universe –
just after the Big Bang when matter was at its
densest and hottest, and in supernova explosions
and neutron stars.
The phase diagramAs for everyday materials, nuclear phases are
characterised by an equation of state (EOS), and can
be plotted on a graph of temperature (energy)
against pressure (density) – the phase diagram, as
shown opposite.
At low temperatures and normal densities, the
nucleus can be regarded as a drop of liquid. At
temperatures of around 20 megaelectronvolts (MeV)
and at two or three times the normal nuclear
density, the liquid bound-state
of protons and neutrons in a
nucleus undergoes a phase
change, boiling to a gas of
freely moving nucleons. In fact,
protons and neutrons can be
regarded as separate liquid
phases, giving rise to a gas
with different proton and
neutron concentrations. This
complicates the phase diagram
What happens when nuclei are heated to hightemperatures or compressed to high densities?
t=-16.34 fm/c
>
A lead-lead
collision used
to create the
conditions for
de-confining
quarks at CERN
Part of the ALICE
experiment being built
at CERN to detect a
quark-gluon plasma
nuclear liquidto quark soupFrom
N A T U R E A T T H E F E M T O - S C A L E 21
NUCLEI ON THE BOILBeams of moderate energies of around 100 MeV will
gently heat up and compress the nucleus. At higher
energies the nucleus vaporises. At intermediate
energies, it may suddenly expand, become unstable
and break up into fragments of all sizes. Studies of
this multifragmentation pattern can be used to
ascertain the heat capacity over a range of energies.
A recent experiment at GANIL colliding a xenon
beam on a tin target revealed a negative heat
capacity (latent heat of vaporisation) which was the
first sign of a nuclear liquid-gas transition.
HOT HADRONSBeam energies of a few hundred MeV reach
temperatures and pressures that convert nuclei into
a hot, dense gas of excited hadrons. New hadrons
such as pions and kaons (mesons containing a
strange quark) are created, and their energy patterns
give some insight into nuclear compressibility
relevant to the creation of black holes from neutron
stars. Recent experiments at GSI using gold-gold
collisions have been using kaons as probes of this
high-density state.
Theory shows further interesting kaon behaviour.
The kaons and antikaons produced behave slightly
differently: positive kaons are slightly repelled by the
other nucleons, while the negative antikaons are
attracted, respectively increasing and reducing their
effective masses. This may explain why neutron stars
are always less than two solar masses. A neutron star
is 10 per cent protons and electrons, but it is thought
that the electrons are converted into the lighter
negative kaons. Unlike electrons, kaons can all exist
in the same quantum energy state, so condense out
thus reducing the repulsive pressure between particles
in the neutron star. This means that gravity can more
easily take over at lower masses than expected to
pull in the matter and create a black hole.
Changes in the expected masses of mesons at
very high energies are also signatures of the next
impending phase change when chiral symmetry is
restored: quarks no longer interact with virtual
quark-antiquark pairs in the vacuum and lose their
mass. A short-lived high-energy version of the pion
called the rho heralds this phase change as its mass
drops and becomes less certain. An experiment at
GSI has been measuring the mass of the rho by
detecting the pairs of electrons and positrons
emitted when the rho decays.
RECREATING EMBRYONIC MATTERAn important aim of nuclear physics is to understand
how primordial matter created in the Big Bang
condensed into hadrons. Experiments using colliders
aim to recreate these conditions, and investigate the
high-energy region when quarks are no longer
trapped in their hadron prisons. Experiments using
the Super Proton Synchrotron (SPS) at CERN to collide
lead nuclei have been exploring the high-density
fireball-state that forms prior to the transition to
the free quark-gluon phase. A key signature of the
transition is the production of the heavier mesons:
theory predicts that as the quark-gluon plasma cools
and condenses, an avalanche of mesons containing
strange quarks is produced, while the formation
of those containing the lighter charmed quarks –
J/Psi particles – remain blocked by the sea of still
de-confined quarks and gluons. CERN experiments,
indeed, observed an enhancement of strangeness
and a reduction of charm, thus catching the first
glimpses of a quark-gluon plasma.
Nudging even closer to the transition-energy
range should reveal even heavier mesons made from
bottom quarks. These will be seen in the Large
Hadron Collider, which will start up at CERN in 2007.
An experiment called ALICE, again colliding beams
of lead ions but at incredibly high energies of
5.5 teraelectronvolts per beam should give the
clearest picture yet of a quark-gluon plasma.
t=-00.34 fm/c t= 07.66 fm/c t= 16.46 fm/c t= 31.66 fm/c t= 182.06 fm/c
> > > > >The phase changes during a high-energy
collision of uranium nuclei
A phase diagram showing
the behaviour of hadrons
at different pressures and
temperatures. At low
temperatures, hadrons
(protons and neutrons)
condense into everyday
nuclei. At higher
temperatures, the nuclei
vaporise into various kinds
of hadrons, eventually
dissolving into separate
quarks and gluons at
extreme temperatures and
pressures. At lower
temperatures and high
pressures, an unusual
superconducting ‘colour’
state forms that might
exist in neutron stars
About 10 times normal nuclear density
Atomic nuclei
170
100
Hadron gas
Quark-gluon plasma
EARLY UNIVERSE
DENSITY
TEM
PERA
TURE
(M
eV)
250
Colour superconductor
Neutron stars
H. W
eb
er/
J. W
. Go
eth
e-U
niv
ers
ity
, Fra
nk
furt
22 N A T U R E A T T H E F E M T O - S C A L E
Mysterious neutrinosThe strange properties of neutrinos are now
uncovering physics beyond the SM. These ghostly
particles are emitted, alongside electrons, in nuclear
reactions when a neutron is converted into a proton
via the weak interaction – radioactive beta-decay. The
Sun emits vast numbers of electron-neutrinos as a
result of this process. However, physicists noted that
far fewer solar neutrinos were being detected than
expected from solar theory (p.12). One explanation
was that they were ‘oscillating’ into the other kinds
of neutrino, which were not detected. This can
happen only if neutrinos have mass; however, the SM
was established assuming they would be massless.
Recent solar neutrino observations have indeed
confirmed the existence of oscillations. A major task
of the next generation of neutrino experiments will
be to measure their masses so as to probe the
theoretical implications further.
:: MEASURING NEUTRINO MASS DIRECTLYOne approach is to try to measure the mass of the
neutrino directly by analysing the energies of the
particles emitted in the beta-decay of tritium
(hydrogen with two additional neutrons) to give
helium-3, an electron and an antineutrino. The
electron may carry away almost, but not quite, all the
energy if the neutrino has mass, and this tiny effect
can be measured from the electron’s energy spectrum.
Such experiments in Troitzk, Russia, and Mainz,
Germany, are being followed up with an international
project, the Karlsruhe Tritium Neutrino experiment
(KATRIN) which is 10 times more sensitive.
A very rare kind of beta-decay in which a nucleus
emits two electrons but no neutrinos is also being
investigated by several experiments based in
underground laboratories in France (Fréjus
Underground Laboratory) and Italy (Gran Sasso
National Laboratory). Neutrinoless double beta-decay
can happen only if the neutrino is its own
antiparticle (the emitted neutrinos then cancel each
other out), for which the neutrino must have mass.
By studying the subtle behaviour of nuclei we can
find out more about the basic properties of the
matter and energy that make up the Universe.
The building blocks of the UniverseOver the past 50 years, physicists have pieced
together a description, called the Standard Model
(SM) based on quantum theory, of the building
blocks of matter and known forces by which they
interact (see box). Although a wonderfully successful
framework for predicting particle interactions, the
SM is not complete: it doesn’t predict the masses of
particles and many other properties that have to be
put in ‘by hand’; it doesn’t explain why there are just
three particle families – and it doesn’t include the
fourth fundamental force, gravity.
To take the SM further, theorists rely on the
underlying mathematical principle of symmetry. A
powerful symmetry would be that which unifies all
the forces into one description – Grand Unification.
Particle theorists believe that when the Universe
came into existence in the Big Bang there was one
symmetrical ‘superforce’ which then broke up into
the forces we see today, as the Universe cooled. So
far, the SM unifies only the electromagnetic and
weak forces as the electroweak interaction. There are
now a number of ‘Grand Unification Theories’ going
beyond the SM, which need to be tested
experimentally. One way is to look for subtle nuclear
behaviour that deviates from SM predictions.
F U N D A M E N T A L I N T E R A C T I O N S ::
e know that nuclei are composed of protons and neutronswhich themselves are made up of more fundamental particlesheld together by a combination of powerful interactions
universal and the Nuclei
W
The cell used at
the Institut Laue
Langevin (above)
to store ultra-cold
neutrons while
measuring their
electric dipole
moment, which is
significant in
understanding
fundamental
interactions
The KATRIN experiment
to measure the mass of
the electron neutrino
N A T U R E A T T H E F E M T O - S C A L E 23
Strange behaviour of quarksBeta-decays and rare decays also give us information
about quarks, leptons and yet-unknown forces that
may throw light on beyond-the-SM physics, and on
how symmetries broke in the early Universe. Quark
flavours, like neutrinos, can also ‘mix’ but according
to the SM, the total mixing must be a zero sum – an
effect called unitarity. Already small deviations from
unitarity have been discovered in experiments at the
Institut Laue Langevin in France measuring the decay
and the lifetime of the neutron (free neutrons
change into protons via beta-decay). Similar precise
experiments in laboratories around Europe are also
probing the decay of mesons (these contain just two
quarks) such as pions and other more exotic particles.
Looking into a mirrorA set of symmetries that are a sensitive probe of
the SM and its extensions describe what happens
when certain particle properties are reflected as
though in a mirror. There’s the charge mirror (C)
which changes particles into antiparticles of opposite
charge, the parity mirror (P) which changes the spin,
or handedness, of a particle, and the time (T) mirror
which reverses a particle interaction or process,
like rewinding a video.
Surprisingly, these mirrors don’t work perfectly.
For instance, electrons emitted in the beta-decay of
cobalt-60 always spin in the same direction even
when the spin of the cobalt nucleus is reversed.
Cracks in the C and P mirrors (CP-violation) also
appear in the decay of certain exotic mesons – the
kaon and B-meson. Electroweak theory within the
SM does, in fact, predict CP-violation.
:: SOME EXPERIMENTAL EXAMPLES■ Parity violation is seen in rare electronic
transitions in atoms. Although atoms are mostly
ruled by electromagnetism, the weak force makes
itself felt through the neutral Z particle which allows
a transition between states with the same spin,
otherwise forbidden. Precise measurements of this
transition in well-understood atoms such as
caesium-133 and other heavy isotopes will look for
tell-tale signs of beyond-SM phenomena.
■ The weak interaction responsible for beta-decay
recognises only left-handedness. More sensitive
measurements in other isotopes of different atoms
and of beta-decays may uncover traces of right-
handed effects that would have existed when the
forces were unified.
■ Violation of time-reversal symmetry is observed in
the beta-decay of neutrons and nuclei, and while a
small amount of T-violation is tolerated by the SM,
a large effect would indicate the need for a new
particle model.
■ Connected to CP and T-violation is the existence
of permanent electric dipole moments (EDMs) in
fundamental particles, nuclei and atoms: EDMs are
forbidden by P, T and CP symmetries, but might be
essential to explain the predominance of matter over
antimatter in the Universe. Laboratories worldwide
are actively searching for these EDMs.
These kinds of experiments typify the kinds of
precise measurements that can be made using
nuclear particles at low and medium energies. They
are playing a vital role in furthering our knowledge
of fundamental interactions.
The Standard ModelThe current picture describes matter as consisting of sixtypes, or ‘flavours’ of quark – called up, down, charm, strange,bottom and top – and six very light particles, or leptons – theelectron, muon, and tau – and their three neutrino partners.The 12 particles are divided into three families of increasingmass, each containing two quarks and two leptons. Eachparticle also has an antiparticle of opposite electric charge.
Everyday protons andneutrons comprise threequarks – two ups and adown, and two downsand an up, respectively.The Standard Model alsoincludes three of thefour fundamental forces,the electromagneticforce, and the weak andstrong interactions.These are carried by particles calledintermediate vectorbosons – respectively,the photon, the W and Zparticles and the gluon.
u c td s b
up
νe νµ ντe-neutrino µ-neutrino τ-neutrino
eelectron
µmuon
τtau
charm top
down strange bottom
lept
ons
quar
ks
w± z°photon
ggluon
?graviton
F E R M I O N S
STRONGWEAK
ELECTROWEAK
ELECTRO-MAGNETIC GRAVITY
γB O S O N S
forc
e ca
rrie
rs
forces
Preparing components for
the NEMO-3 neutrinoless
double beta-decay
experiment in the Fréjus
Underground Laboratory
in France
24 N A T U R E A T T H E F E M T O - S C A L E
they interact with themselves! Yet a further level of
complexity comes from quantum theory which
allows ‘virtual’ particles to pop in and out of
existence from the vacuum. So, even though we
think of nucleons as being made of the lightest,
up and down quarks (as well as interacting gluons),
there are also contributions from virtual quark-
antiquark pairs. The result is that it’s very hard to get
a clear picture of protons and neutrons – let alone a
nucleus – in terms of fundamental interactions.
COMPUTATIONAL STRATEGIES AND MODELSFor this reason, theorists have developed a series of
theoretical pictures that portray what is going on at
a particular scale – and therefore energy – from
about one-tenth of a femtometre (the scale of
quarks) to 1 femtometre (the scale of nucleons). For
instance, one increasingly successful approach, which
rides on the back of the huge numerical power of
supercomputers, regards quarks and gluons as
interacting points and links on a lattice in space-
time. At lower energies, and thus longer scales, this
approach becomes increasingly difficult because the
interactions are so strong and complex. Other, more
approximate pictures are used to describe
interactions between the hadrons as involving clouds
of virtual pions. Eventually, theorists want to link up
the models into a complete QCD theory that will be
able to explain the confinement of quarks into
hadrons, how the pion clouds arise and hadrons
interact, and predict important properties such as
the masses and spins of the proton and neutron. In
this way, we will be able to explain the generation of
nuclear structure.
Inside the protonThese ideas, of course, need to be explored in
experiments. One of the most important ways is to
look at the quarks inside a hadron such as a proton,
by smashing it with another particle. In fact, this was
how evidence for quarks was first uncovered in such
‘deep inelastic scattering’ (DIS) experiments at the
Stanford Linear Accelerator Laboratory in California.
This work has been carried on using the HERA
How exactly does the strong force act to cause
quarks and gluons to ‘condense’ into protons and
neutrons, which then arrange themselves into
nuclei of everyday matter?
QCD, the theory of the strong forceTo start to answer this question, theorists have
developed a mathematical description of the strong
force called quantum chromodynamics (QCD). It is
what is called a quantum field theory similar to that
of electromagnetism describing the behaviour of the
electron and light – quantum electrodynamics (QED).
While QED is well understood, explaining electronic
phenomena in terms of positive and negative electric
charges, QCD requires another kind of charge called
colour, which can have three (rather than two) values
and which have, rather confusingly been named red,
green and blue. The attractive colour charges are
powerful, and actually increase with distance, so that
quarks are always tightly bound into colourless
entities called hadrons. These are either three-quark
systems (combinations of red, green and blue) such
as protons and neutrons, or quark-antiquark pairs (in
which the colours cancel out). The latter are known
as mesons of which the lightest is the pion.
However, the workings of the strong force are
more complicated because the gluons, which flit
between the quarks, also have colour charge, and
Q C D ::
N uclei are ultimately made of fundamental particles called quarks.These point-like particles prefer to cling together in twos and threes,tightly held by the strong force mediated by the exchange of gluons
nuclei together?What holds
u u
d
The spin structure of
a proton – quarks,
gluons and virtual
quarks all play a part
The COMPASS
experiment at CERN to
study the role of gluons
in the spin of a proton
N A T U R E A T T H E F E M T O - S C A L E 25
collider machine (colliding
electrons with protons) at
the DESY Laboratory in
Hamburg. The aim is to map
the distributions of the
quarks, gluons and virtual
particles inside the proton.
WHERE DOES THE SPIN OF THEPROTON COME FROM?One particular problem being investigated is the spin
of the proton or neutron. Surprisingly, the quarks in
the proton account for only a fraction of its spin. One
experiment at DESY, called HERMES, has been using
polarised electrons and target nuclei (with spins all
in the same direction) to investigate spin. The results
seem to confirm that the proton’s spin is largely
made up of contributions from gluons. Further
experiments at DESY and CERN are also looking at
how the spins move in relation to the overall
direction of hadron spin.
GLUONS GET TOGETHERBecause gluons interact with each other, QCD
predicts that they can gather together to form
gluon-rich exotic entities called glueballs. Over the
past decade, experiments testing this idea with
beams of protons and antiprotons at CERN suggest
that there may be a whole spectrum of glueballs.
These will be investigated further with antiproton
beams made in the new High Energy Storage Ring
(HESR) to be built at GSI. Other possibilities are more
complex hybrids of quarks and gluons to form quark
‘molecules’. Just recently several laboratories
reported discovering a bound system of five quarks –
a ‘pentaquark’.
WHERE DOES THE MASS OF THE PROTON COME FROM?Quarks and gluons on their own can be considered as
simple, non-interacting massless points, so how are
the masses of quarks, when confined in hadrons,
generated? QCD predicts that, below a certain
energy, they acquire mass by
interacting with strong gluon
fields and with quarks and
antiquarks condensing out of vacuum
– a process called chiral symmetry
breaking. This idea is being tested in various ways.
One approach is to study how heavier quarks (which
give simpler information than light quarks) bind with
each other. Mesons consisting of charmed quark-
antiquark pairs, known as charmonium, will be made
in abundance with HESR, and will provide a unique
testing-ground for this and other QCD predictions.
This research will complement observations of the
signatures produced by heavy mesons in high-energy
experiments designed to liberate quarks completely
from their gluon chains. The ALICE experiment using
the Large Hadron Collider being built at CERN will
collide heavy ions at high energies to create a soup of
free quarks and gluons (p.20).
Applying QCD to nucleiThe effective force between nucleons in nuclei are
currently described in terms of pion exchange, but a
long-term aim of nuclear theorists is to predict
nucleon interactions at a more basic level
of QCD theory. This will give us a better
understanding of nuclear structure (p.8)
and the build-up of elements in stars
(p.12). Unique information about the
strong force can be obtained by studying
‘hypernuclei’ containing a strange or
charmed quark. These are just some of the
exciting experiments planned to test QCD
at lower energies at which the strong
force is least understood.
The consequences of QCD at low energies are
studied in a variety of experiments that probe
the properties of hadrons, for example, the
KLOE experiment (left) using the DAΦNE
electron-positron accelerator complex (inset) at
FNL in Italy and the MAMI electron accelerator
(below) in Mainz, Germany
Detection of glueballs in
the Crystal Barrel
Experiment at CERN
Fe
de
rici
/ L
NF
IN
FN
26 N A T U R E A T T H E F E M T O - S C A L E
arrangement of the nucleons they contain. Theories
of nuclear structure (p.9) can be tested by preparing
nuclear species with extreme ratios of protons and
neutrons that are teetering on the edge of existence
(the neutron and proton driplines), or are as heavy
as possible. Unstable nuclei with particular
configurations, such as equal numbers of protons
and neutrons, may also reveal some of the more
complex aspects of nuclear forces.
:: ASTROPHYSICSConditions in stars are very different from those on
Earth; their hot, turbulent environments host the
nuclear reactions that build up the elements. The
reaction pathways are thought to involve highly
unstable nuclei – perhaps very neutron-rich, or even
proton-rich (p.12). Radioactive beams are the ideal
experimental tool for testing current ideas about
nucleosynthetic pathways.
:: FUNDAMENTAL FORCESOne way of probing the fundamental forces that
govern matter is to measure the subtleties of decay
processes in various nuclei, such as beta-decay (p.22).
Radioactive beams can provide pure sources of useful
unstable nuclei, which can then be stored and
studied in special magnetic ‘traps’.
:: APPLICATIONS A rewarding use of beams of radioactive ions is as
diagnostic and imaging tools in a range of areas such
as medicine and electronics (p.16). Cancer therapy
with radioactive-ion beams is an exciting new area.
Radioactive-beam generation is also key to
developing new methods of destroying nuclear
waste by transmutation – a new approach that could
transform society’s view of nuclear physics.
R A D I O A C T I V E I O N B E A M S ::
exoticnucleiProducing
The previous pages have shown how our
understanding of nuclei has developed by testing
their behaviour under extreme conditions. For example,
many experiments involve observing nuclei with
unusual ratios of protons and neutrons, or subjecting
nuclear systems to high temperatures and pressures.
Until a few years ago, experiments were mostly limited
to studies of nuclei that were stable or nearly so.
Today, nuclear physics studies, as well as applications,
are being dramatically extended by facilities that
generate unstable nuclei in intense beams.
Why we need radioactive beams :: NUCLEAR STRUCTURE The nuclear map on p.8 shows that while only a
limited number of stable nuclei exist naturally on
Earth, a much larger number can survive for varying
amounts of time depending on the numbers and
urope is developing a new generation of radioactiveion beams that will lead to a deeper understandingof Nature and new technologies E
GANIL’s SPIRAL accelerator
(above) and the source for
producing exotic ions and
the VAMOS detector (inset)
M. D
ésa
un
ay
/ G
AN
IL
N A T U R E A T T H E F E M T O - S C A L E 27
How to produce radioactive beamsThe first dedicated source of accelerated radioactive
beams was at CYCLONE in Louvain-la-Neuve,
Belgium. Using innovative technologies, the
laboratory started in 1989 with a nitrogen-13 beam
producing oxygen-14, important in astrophysical
processes. The underlying method to produce the
isotopes was developed at CERN, with the ISOLDE
facility, which has delivered more than 600 isotopes
of 70 elements in the past 30 years. A new
radioactive beam facility at GANIL, France called
SPIRAL started in 2001.
These facilities employ the ISOL (isotope
separation online) method to produce the beams.
Particles such as protons or ions are accelerated
(for example, in a cyclotron) and directed at a thick
target of suitable material where they produce new
radioactive atomic species. These are then ionised
and separated according to mass by various electro-
magnetic devices. The selected isotope is then re-
accelerated and directed towards the experiment.
There is an alternative approach called the
in-flight method in which an energetic beam of
heavy ions undergoes fragmentation or fission while
passing through a thin target. The radioactive ions
produced are then separated and kept in a storage
ring for experiments. GSI in Germany uses its heavy-
ion accelerator complex to produce radioactive
beams in this way.
The two systems are considered to be
complementary. The ISOL method generates pure,
high-intensity, high-quality beams of isotopes near
stability, which are suitable for a very wide range of
applications – from measurements of astrophysical
importance to sensitive low-energy experiments and
medical applications. The in-flight method produces
less intense beams of nuclei at higher energies. In
theory, it can be used to produce isotopes of any
element. It is a faster method so is suitable for
producing very short-lived nuclei, either as single
beams, or as cocktails of several similar nuclei; their
masses and other properties can be measured
simultaneously in the storage ring. Single beams can
also be prepared for collider experiments.
The radioactive beam layout
at CYCLONE, Louvain-la-Neuve
ISOL method In-flight method
Accelerator
Thickproduction
target
Thinproduction
target
Accelerator
Fragmentseparator
Storage ring
Experiments
Ion source
Isotopeseparator
Postaccelerator
Experiments
The two main methods for
producing radioactive ion beams
Radioactivelaboratory
ISOLDE CERN
Robot1-1.4 GeV protons
GPS
HRS
REX-ISOLDE
New extension
Experimental hall
Controlroom
>>
28 N A T U R E A T T H E F E M T O - S C A L E
Future European facilities Europe has a long tradition and leadership in
radioactive beam production, and its nuclear physics
community is now planning the next generation of
facilities. The aim is to generate much more intense
beams and improve detector technology. Post-
accelerated radioactive beams at ISOLDE (REX)
at CERN and SPIRAL at GANIL now offer new
opportunities in terms of secondary-beam energies
and intensities for nuclei with light-to-medium masses.
The EXCYT (Exotics with Cyclotron and Tandem)
project at Catania will use a tandem post-accelerator.
Another approach being developed at the University
of Munich (MAFF) will exploit the high flux of
neutrons from a new research nuclear reactor in
Garching to fission uranium-235 into neutron-rich
fragments. These are then ionised, separated and
cooled, and then accelerated in a linear accelerator.
Further ahead, two major European projects are in
progress. One will be sited at GSI. It will produce
secondary beams, through the fragmentation of
primary ion beams of all the elements up to
uranium, which are 100 times more intense than is
possible at the moment. New-generation storage
and cooler rings will offer unique world-wide
experimental opportunities especially for the
shortest-lived nuclei.
The other large project is EURISOL, the next-
generation ISOL facility with the most intense,
high quality, post-accelerated secondary beams ever
produced in the energy range up to 100 MeV. This
challenging project is a joint undertaking of many
laboratories. Planned intermediate facilities, SPIRAL II
at GANIL and SPES at INFN-LNL Legnaro, together
with further upgrades of the REX-ISOLDE facility,
represent demonstration steps towards the
construction of EURISOL.
In this way, the European science community will
have access to a network of advanced radioactive
beam facilities that will cover the wide range of
experiments needed to push back the frontiers not
only in nuclear physics research but also in other
areas that depend upon the science and technology
that radioactive beams can deliver.
R A D I O A C T I V E I O N B E A M S ::
Detectors for radioactive beamsSince radioactive beams are not as intense as stable ones, highly efficientdetectors are needed to gather experimental results. A range of detectorsis being developed for the next generation of radioactive beam facilities.Amongst the most important are those that detect gamma-rays. Themost efficient are crystals of the semiconducting material germanium,which are arranged in position-sensitive spherical arrays around theexperiment. Individual detectors are often built by internationalcollaborations and shared between several laboratories.
The EXOGAM detector
at GANIL (inset)
The DEMON neutron detector
(below) is shared among several
exotic-ion beam facilities
GA
NIL
N A T U R E A T T H E F E M T O - S C A L E 29
nuclear physics gave her a golden opportunity to do
just that. “I preferred nuclear physics because it was
related to what I was interested in: philosophical
questions such as what the Universe is built of, but
also applications,” says Konstanze. GSI’s radiobiology
department offered her the chance to work on its
heavy-ion tumour therapy project (p.18). Konstanze’s
project involved measuring the radiation dose
contributed by fast neutrons when a beam of
carbon-12 ions is used to treat a tumour. Using a
volume of water as a ‘phantom’ target, she could
measure the production rates and energy
distributions of neutrons generated by nuclear
fragmentation – a classical nuclear physics
investigation. She also compared the light particles
produced during the treatment of patients with
those from the phantom target. The results showed
that the dose contribution from fast neutrons was
less than 1 per cent of the total dose applied to the
tumour volume, so did not affect the success of the
carbon-12-ion tumour therapy. Konstanze hopes to
continue working in medical physics, either in
academia or in the clinic. “The great satisfaction is
knowing that I am helping people as well as carrying
out research,” she says.
:: Richard Woolliscroft, from York University in the
UK, decided to do nuclear physics because he wanted
the opportunity to work at a fundamental level,
where he could carry out original measurements and
calculations that would be used to test and expand
on the known models. “The main benefit was being
able to set my work standards at a high level. I am
Many young people go into physics because they are
fascinated by questions such as: how did the
Universe evolve and what is it comprised of? Nuclear
science not only aims to answer these questions but
also covers a very wide range of applications, from
medical therapies and energy production to art
conservation. Because it touches on a wide range of
disciplines – for example, particle physics, astrophysics,
condensed matter physics, engineering, computing,
and biology – PhD students have the opportunity to
widen their scientific expertise. Another advantage is
that nuclear research generally involves well-defined
experiments that can be completed within the time-
span of a doctorate. At the same time, students get
the chance to travel to international laboratories
and work within interdisciplinary (often multi-
institutional and multi-national) teams.
The result is that young nuclear physicists develop
an excellent range of transferrable skills:
■ General problem-solving abilities needed in
management and industry;
■ Mathematical and computer skills which can be
applied in the financial and commercial sectors;
■ Engineering expertise in areas such as vacuum
technology, semiconductors and data-processing;
■ The ability to work in a team, and the experience
and skills required to work in a multidisciplinary,
international environment, which is essential in
business today.
:: Konstanze Gunzert-Marx wanted to combine
her physics studies at the Technical University of
Darmstadt in Germany with biology, and a PhD in
Why become a nuclear physicist?N uclear physics offers an attractive
research area for graduate students
>>
E D U C A T I O N A N D T R A I N I N G::
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30 N A T U R E A T T H E F E M T O - S C A L E
:: Jean-Sébastien Graulich carried out his PhD at
Louvain-la-Neuve in Belgium, using the radioactive
beam facility there to study nuclear reactions
thought to happen in explosive stellar
nucleosynthesis, in particular on the conversion of
fluorine-18 to oxygen-15. Afterwards he went to CERN
to work on a new detector for the HARP experiment
there, which will study hadron production in
preparation for the construction of a neutrino
factory. “My PhD in nuclear physics built up a good
basic knowledge in physics, and the ability to work
within international collaboration. It also gave me
the experience of working on a smaller-scale project
from the very beginning up to the end,” says Jean-
Sébastien. He then spent a year working in a
company developing digital X-ray imaging systems
for medical and industrial applications. He is now
back at CERN, working again on detector
development and production.
:: Zoran Radivojevic found that the broad skills
developed through his PhD research on nuclear
structure and neutron detectors were invaluable
to the electronics industry. Working at the Finnish
Accelerator Laboratory in Jyväskylä (JYFL) and in
collaboration with laboratories across Europe, he
worked on the design of a wide range of detection
systems. Part of the research for his PhD thesis
involved developing computer simulations for
describing the transport of neutrons in different
materials to optimise the detection processes and
detector design. In 2001, Zoran started work on novel
silicon detectors for the ALICE experiment at CERN
(p.21). The flexible miniaturised technology involved
has proved to be attractive to the commercial
microelectronics industry, and Zoran has now
joined Nokia Research, carrying out simulations to
optimise mobile phone design. He also organises
collaborations between several large industrial
semiconductor laboratories and universities in
experimental and simulation research. “This kind of
job demands a multi-skilled background in physics,
computation and organisational capabilities,”
says Zoran.
also appreciative of the time spent at international
conferences and laboratories, meeting other
physicists and improving my team-working and
communication skills,” says Richard. After his PhD,
Richard used the experience gained of programming
and data-handling, in going to work as a scientific
programmer for a biotechnology company. He
worked with biologists and chemists, finding that
his scientific background allowed him to understand
rapidly the particular needs of the company and the
scientists he worked with. More recently he has
returned to the field of physics and now works as a
developer of data acquisition software at the new UK
synchrotron facility, Diamond, which will be used
largely for applications in biology and chemistry.
E D U C A T I O N A N D T R A I N I N G ::
Public awareness of nuclear scienceMany people associate nuclear science just with the negative aspects ofnuclear weapons and nuclear waste, but as the highlights in this booklet haveshown, nuclear research is not only concerned with the most exciting scientificendeavours – that of understanding Nature at the deepest levels, but alsounderpins some important applications.
The European nuclear physics community aims to improve public awarenessof these studies – through open days at laboratories and teaching projects forschools, and via an outreach programme, Public Awareness of Nuclear Science(PANS). The PANS team has a travelling exhibition on radioactivity, andsupported the publication of a unique coffee-table book, Nucleus – A Trip to theHeart of Matter, written by Ray Mackintosh, Jim Al-Khalili, Bjorn Jonson andTeresa Pena. PANS also has an EU-funded project, NUPEX, to produce a web-based resource on nuclear physics for schools. It will cover many aspects fromfundamental physics and astrophysics to nuclear medicine. Further informationabout PANS can be obtained at www.pans-info.org
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Konstanze Gunzert-Marx
Jean-Sébastien Graulich
Zoran Radivojevic
Richard Woolliscroft
Accelerator Laboratory (JYFL)JyväskyläFinland
Kernfysisch Versneller Instituut(KVI)GroningenThe Netherlands
The Svedberg Laboratory (TSL)UppsalaSweden
Forschungszentrum Jülich (FZJ)JülichGermany
Centre de Recherches du Cyclotron(CRC)Louvain-la-NeuveBelgium
Grand Accélérateur Nationald'Ions Lourds (GANIL)CaenFrance
Gesellschaft fürSchwerionenforschung (GSI)DarmstadtGermany
FINUPHYCoordinator Professor Jean VervierTel +32-10-473273 (secretary)Fax +32-10-452183E-mail [email protected] www.finuphy.org
P A R T I C I P A T I N G L A B O R A T O R I E S
Institut de RecherchesSubatomiques (IReS)StrasbourgFrance
ISOLDE collaborationCERNGenèveSwitzerland
European Center for TheoreticalStudies in Nuclear Physics andRelated Areas (ECT)TrentoItaly
Laboratori Nazionali di Legnaro(LNL)PadovaItaly
Laboratori Nazionali di Frascati(LNF)FrascatiItaly
N A T U R E A T T H E F E M T O - S C A L E 31
contacts
GANIL
ISOLDEIReS
GSIFZJ
KVI
LNF
LNLECT
TSL
JYFL
CRCGANIL
ISOLDEIReS
GSIFZJ
KVI
LNF
LNLECT
TSL
JYFL
CRC
This booklet was produced byFINUPHY, an InfrastructureCollaboration Network supportedby the EU Fifth FrameworkProgramme. The Networkoperated from 1 October 2000 to30 September 2004.
Editor Professor Brian Fulton,University of York
E-mail [email protected] Nina HallE-mail [email protected] Pete Hodkinson, SpacedE-mail [email protected]
Further information and copies of thebooklet can be obtained through DieterMüller at GSI, Darmstadt in Germany.Dieter MüllerGesellschaft für SchwerionenforschungPlanckstraße 164291 DarmstadtGermanyE-mail [email protected]
32 N A T U R E A T T H E F E M T O - S C A L E
FINUPHYAbout
rontiers In NUclear PHYsics (FINUPHY) is an
Infrastructure Cooperation Network supported by
the European Commission under its Fifth Framework
Programme(FP5). It includes representatives from 12
research institutes in nuclear physics.
FINUPHY aims to promote collaboration and
coordination between the research institutes
through regular round table meetings, and through
joint scientific and technological activities and
studies. It also promotes information on advances in
research and technical development programmes in
nuclear physics.
FINUPHY is particularly interested in education and
public outreach, and supports the PANS (Public
Awareness of Nuclear Science) activity, p.30. This
booklet is a further contribution by FINUPHY to
communicate to a wider audience the wide-ranging
and exciting programme of nuclear science carried
out in Europe.
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