Neutrons and the Universe - ILL · Neutrons and the Universe ... about these building blocks of the...

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a review of ILL research in fundamental and nuclear physics H I G H L I G H T S O F I L L R E S E A R C H Neutrons and the Universe u u d u d d ν e W e PROTON NEUTRON

Transcript of Neutrons and the Universe - ILL · Neutrons and the Universe ... about these building blocks of the...

a review of ILL research infundamental and nuclear physics

H I G H L I G H T S O F I L L R E S E A R C H

Neutrons and the Universe

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ILL
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This booklet belongs to a series

devoted to the application of neutron

techniques in different research areas.

We have already published:

Exploring matter with neutrons (May 2000)

Neutrons and life (May 2001)

Neutrons and new materials (May 2002)

The next one will be published in 2004.

ILL
This PDF file contains an active contents page that links directly to the desired page when selected by clicking on the text. To return to the contents page simply click on the horizontal strip at the top of the page.

1

ur views about the building blocks of Nature –fundamental particles and forces – have evolved

dramatically over the past decades. We now havemodels that attempt to unify the forces and particles,and describe how they came into existence in the veryearly Universe. Similarly at the next level, theories havebeen developed which explain how particles interactthrough forces to form the highly complex nucleicomprising the atoms of everyday matter.

The existing fundamental particles and relevantforces can be described within the framework of theso-called Standard Model. Although highly successful,the Standard Model leaves many questionsunanswered and various extensions to the theory havebeen put forward. To test these models, particlephysicists have designed experiments over a widerange of energies. The most ambitious projects aim tocollide particles at TeV energies, trying to mimicconditions thought to prevail in the early Universe. Thestructure of nuclei can also be investigated throughcollisions, but at a lower energy scale.

There is another approach, however – which is tosearch for subtle physical behaviour that reveals itselfonly at very low energies. This is what the ILL does inuniquely sensitive experiments using neutrons withenergies in the sub-eV range. The cold or ultra-coldneutrons produced at the ILL can tell us a great dealabout the 'symmetry' characteristics of particles andtheir interactions – perhaps helping to explain, forexample, how the Universe came to contain mainlymatter and not antimatter, even though created inequal amounts.

Neutrons at the ILL are also used to investigate thestructure and behaviour of nuclei by generatingexcited nuclear states. Although atomic nuclei have afinite number of constituents – neutrons and protons –they display extremely diverse modes of excitationsassociated with both single-particle and collectivebehaviour, and can be regarded as miniaturelaboratories for studying complex, strongly interactingsystems. The ILL is also able to create exotic nuclei withhigh numbers of neutrons to explore the pathways bywhich elements are made in the stars.

Because neutrons can behave as waves, they canprobe the subtleties of quantum behaviour such asnonclassical 'Schrödinger-cat' states – of greatsignificance in current condensed matter andfundamental physics research. As another example, therecent observation at the ILL of the quantisation ofneutron states due to the Earth's gravitational field hasexperimentally confirmed the coexistence of thegravitational force with quantum mechanics, and hasopened up a new field of pico-eV spectroscopy.

This brochure explains some of the achievementsmade at the ILL in nuclear and particle physics usingneutrons at low energies. The ILL has maintained adiverse suite of neutron sources and instruments toengage in a wide range of scientific topics, and is proudof the advances made in this field. The developmentand renewal of the ILL's infrastructure will enhance thelow-energy neutron sources to contribute to a betterunderstanding and unification of the fundamentalforces in Nature.

Dr Christian VettierAssociate DirectorHead of Science Division

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Foreword

N E U T R O N S A N D T H E U N I V E R S E

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Contents

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Foreword Christian Vettier

The neutron, the Universe and the fundamental forces

The dancing neutron Helmut Rauch

Neutrons are not forever James Butterworth and Peter Geltenbort

Quark-mixing in neutrons Hartmut Abele

When neutrons look into a mirror Michael Kreuz

How to go backwards in time Torsten Soldner and Klaus Schreckenbach

Living in a material world Maurits van der Grinten

Quantum states of matter in a gravitational field Valery Nesvizhevsky

Understanding the nucleus Hans Börner

From shells to liquid drops Jan Jolie and Hans Börner

Probing exotic nuclei Gary Simpson

Phase transitions in nuclei Rick Casten and Victor Zamfir

A stellar thermometer Franz Käppeler and Hans Börner

Where do the heavy elements come from? Herbert Faust and Jean-Alain Pinston

Glossary

Contacts

N E U T R O N S A N D T H E U N I V E R S E

Torsten Soldner and Dirk Dubbers

ost people have heard of the huge machines thataccelerate and collide subatomic particles at

incredibly high energies in order to study theelementary units of matter and the forces governingtheir behaviour. However, some important questionsabout these building blocks of the Universe can bestudied by carrying out subtle, very preciseexperiments at extremely low energies. This is what wedo at the Institut Laue Langevin (ILL), using neutronsproduced by the research reactor. We make hot,thermal, cold, and ultra-cold neutrons with velocities of between a few kilometres and a few metres persecond, corresponding to kinetic energies in theelectronvolt-to-nanoelectronvolt range. This is some 20orders of magnitude less than the energies generatedby accelerators.

Here, we describe how the neutron is a suitable toolfor investigating the four known fundamental forces ofNature – electromagnetism, the weak and strongforces, and gravity. The first three forces, as well as allthe known fundamental particles, are described by avery successful theory called the Standard Model ofParticle Physics. The matter particles are divided into aseries of three families, each consisting of two quarks

(such as the up and down quarks found in protons andneutrons) and two leptons (such as electrons andneutrinos). The forces are mediated by carrier particles– the photon (the carrier of light) for theelectromagnetic force, the W and Z particles whichcarry the weak force, and the gluon which carries thestrong force. However, the theory is by no meanscomplete; for instance, gravity is not yet included.Physicists would like to unify all the forces into a singletheoretical description that would explain fully how theparticles and forces came into being when the Universewas created in the Big Bang about 15 billion years ago.Experimentalists are looking for phenomena thatprovide evidence of physics going beyond thatexplained by the Standard Model. Neutron studies arenot only providing us with precise information on theideas we already have but may well also shed light onthese new theories.

ElectromagnetismThe electromagnetic force acts between all electricallycharged particles, such as the negatively chargedelectrons and positively charged protons making upthe atoms of everyday matter. This force is responsible

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Protons and neutrons form

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Atoms form

Galaxies form

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The neutron,the Universe and the fundamental forcesMeasurements using low-energy neutrons are leading to a better

understanding of the fundamental forces and the nature of the Universe

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The evolution of matter in

the Universe

for chemical bonding and electrical conduction, andcontrols all living processes – even how our musclesmove. Neutrons, also found in atoms, are themselveselectrically neutral; nevertheless they provide someimportant results concerning electromagnetism.

In fact, according to the Standard Model, there is noreason why the neutron’s charge should be zero.However, new theories unifying the forces do requirethe neutron to be exactly neutral. Indeed, a high-precision experiment carried out at the ILL has provedthis to be the case (up to 21 decimal places after thezero!), as required by the new theories beyond theStandard Model.

The neutron can also be used to determine thebasic strength of the electromagnetic interaction,which is given by a quantity called the fine-structureconstant α. This has to be determined experimentallyby linking very precise data from atomic and nuclearphysics to a similarly accurate measurement of thevelocity of free neutrons with a well-definedwavelength. The product of velocity and wavelength isequal to the ratio of Planck’s constant h (anotherimportant constant in fundamental physics, whichrelates the energy of a particle to its wavelength) to themass of the neutron, h/mn. From this ratio and theother data, α can be derived. Such a measurement ofh/mn at the ILL gave a relative accuracy of 3.9 x 10- 8

for α. This result can be compared with othermeasurements of the fine-structure constant, forexample, based on the magnetic moments of electronsand muons (the latter are particles similar to electronsbut heavier). Such comparisons make it possible toexamine closely the theory of electromagnetism in theframework of the Standard Model.

The weak forceThe weak force does not appear to play a noticeablepart in everyday life, mainly because it has anextremely short range. Nevertheless, the weakinteraction is actually vital to our existence. Itdetermines the rate of the nuclear fusion processesthat make the Sun and other stars burn, and thus theirtemperature.

We have known for about 20 years that the weakforce and the electromagnetic force are components of

a more fundamental, unified force called theelectroweak interaction. We believe that the two forcesseparated when the Universe cooled down just afterthe Big Bang (see the ‘electroweak phase transition’ inthe diagram opposite). Unlike the electromagneticinteraction, the weak force retained a number of exoticproperties. For example, it is the only fundamental forcethat distinguishes between left and right-handedness,and between particles and their ‘antimatter’ partners(which are identical but have the opposite charge): itexclusively acts on left-handed particles and right-handed antiparticles.

These peculiar characteristics of the weakinteraction manifest themselves in the behaviour ofneutrons, so they make an excellent probe of thisfundamental force. A neutron is stable when bound inthe nuclei of atoms, but when free, it decays on averageafter about a quarter of an hour into a proton, anelectron and a particle called an antineutrino (see thediagram above). This decay is based purely on the weakinteraction, and the free neutrons produced in largenumbers at the ILL can be used in experiments todetermine precisely the lifetime of the neutron (see p.9)and several other neutron decay parameters, which tellus about the strength and structure of the weak force.

The decay of the free neutron also plays animportant role in precision tests of the weak interactionthat may reveal physics beyond the Standard Model.One topical example examines whether the StandardModel’s description of the effect of the weakinteraction on the various quark families (‘quarkmixing’) is correct (see p.10).

ULTRA-COLD NEUTRONS

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FISSION

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>> T O R S T E N S O L D N E R A N D D I R K D U B B E R S

The range ofneutron energies

produced at the ILL

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The neutron’s lifetime is significant in cosmologicaltheories seeking to explain the origin and evolution ofthe Universe. The lightest elements, hydrogen, heliumand lithium, are thought to have formed in the firstthree minutes after the Big Bang, and their predictedrelative amounts compared with what is actuallyobserved in the Universe is one of the strongest piecesof evidence for the Big Bang model. The neutronlifetime influences significantly the abundances of the light elements. It is only in recent years thatmeasurements of the lifetime have been sufficientlyaccurate to allow researchers to make reliablecalculations.

Two other important values can be derived fromthese element abundances. The first relates to thenumber of particle families that exist. This numberstrongly influences the expansion of the early Universeand with it the abundances of the lightest elements.Calculations of the abundances showed that thenumber of particle families is limited to three, and onlythree – a figure confirmed with high precision shortlyafterwards in high-energy particle physics experiments.

The second significant quantity is the density ofbaryonic matter in the Universe. This is the ordinarymatter we know – the nuclear matter consisting ofprotons and neutrons – which makes up all the starsand planets in galaxies. Research in this area hasproduced surprising results, namely that only aroundone-tenth of this baryonic matter is visible in the stars,and that the rest must be hidden in exotic invisibleobjects such as black holes or neutron stars, or inintergalactic space. Furthermore, various recent

astrophysical observations indicate that the baryonicmatter makes up only about 4 per cent of the totalmatter-energy density of the Universe. The rest seemsto consist of about one-third unknown ‘dark exoticmatter’ and two-thirds unknown ‘dark energy’.

The strong forceThe strong interaction affects only the quarks, bindingthem into particles like protons and neutrons. It too hasa short range and has one peculiar characteristic: theparticles that carry the strong force, gluons, alsointeract with one another via the strong force. As aresult, quantitative predictions about the stronginteraction are extremely difficult. Such calculationshave to be verified experimentally. For example, wewould like to know the strength of the force needed topull quarks apart (they are never found singly). This canbe done by measuring the electric polarisability of theneutron. Here, the quarks are pulled apart by a strongelectric field which acts on the electric charges of thequarks. Nuclear structure studies also provide verydetailed qualitative information about the properties ofthe strong force (see p.18).

GravitationAlthough gravitation is the most familiar of thefundamental forces, its theoretical description does notas yet fit with that of the other interactions. The mostsuccessful theory to date is Albert Einstein’s GeneralTheory of Relativity, which interprets gravitation as thecurvature of space-time. This geometrical approach isfundamentally different from the description of theother three forces which is based on quantum fieldtheories. Ideally, theorists would like to find a quantumtheory of gravity.

Neutrons have a mass so feel the gravitational force.They are ideal particles for studying gravity at themicroscopic level: they are electrically neutral; easy todetect; have a relatively long lifetime; and reflect welloff mirrors. Recently at the ILL, quantum states of theneutron in the Earth’s gravitational field weresuccessfully observed for the first time (see p.14).

Why are such experiments important? Until now,gravitation has been tested only on a large scale byobserving the movements of stars and galaxies or in

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The neutron,the Universe and the fundamental forces

New equipment formeasuring the

neutron electricdipole moment

Earth-bound experiments such as Galileo’s famous free-fall tests. However, some theories for unifying gravitywith the other forces predict some curious effects onextremely small length-scales which may throw newlight on the nature of the Universe. The ILL experimentstudied gravity in the free fall of single particles at alength-scale of micrometres and an energy scale ofpicoelectronvolts, in other words, in a significantregime not accessible with conventional gravitationexperiments.

The neutron and new physicsUnification is an important concept in understandingphysical laws and the existence of the Universe. It hasbeen successfully achieved for the electromagnetic andweak forces. However, unification of all the forces wouldtake us far beyond current particle theories. Beyond-the-Standard-Model physics is a highly active researcharea and there are several classes of theories beingconsidered. These are based on a concept favouredmuch by physicists, that of symmetry.

In the Standard Model we have two symmetries thatare completely broken by the weak interaction, namelyparity P (between left and right-handed particles) andcharge conjugation C (between matter and antimatter).How we can understand the origin of these symmetryviolations? Most popular today are theories that startwith a highly symmetric Universe, in which left andright-handed particles are equivalent at the prevailingvery high energies. The asymmetries in our presentlow-energy Universe are then explained by ‘spontaneoussymmetry breaking’ during one of the ‘phase transitions’between different states of the Universe, as depicted inthe diagram on p.4. Experiments searching for evidenceof such left-right symmetric theories can be carried outusing neutrons (see p.11).

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Another symmetry, time reversal T, is broken only toa very small degree in the Standard Model.T-violation turns out to be necessary to explain thesurvival of matter at the expense of antimatter after theBig Bang (thus securing our existence). Thismechanism, called baryogenesis, however, cannot beaccommodated into present-day theories. It may evenrequire as yet unknown T-violating effects and is a verystrong motivation for probing ‘new physics’ beyond theStandard Model. With neutrons, hypothetical newchannels of T-violation are investigated by searchingfor an electric dipole moment (see p.13) and for smallasymmetries in the neutron decay (see p.12).

These neutron experiments may reveal the fainttraces of ‘new physics’. Thus the neutron can be usednot only to test very precisely our current knowledgeof fundamental physics but also as a delicate probe inuncovering a deeper understanding of how ourUniverse works.

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other beam. The beams are now in different states yetstill entangled, so when they are then recombined theyreveal a quantum superposition which can be detectedand measured.

The quantum states can be visualised as a kind ofprobability distribution called a Wigner function(above). It shows the two Schrödinger-cat states asseparate peaks, and in between a series of wiggles,called ‘the smile of the wave function,’ which point tohow the states are coupled. By varying the phase shiftsbetween the beams and the timing of recombination,the wiggles and the position of the peaks can be variedin a wide variety of ways.

This ‘dancing’ Wigner function of the neutronsshows how much information can be stored in thesedead-and-alive Schrödinger-cat states. More complexWigner functions can be produced with more smileregions, so providing a pathway to the production of quantum-engineered neutron states. These‘nonclassical’ states behave quite differently from thenormal neutron states and give us new insights intothe behaviour of matter.

Another closely related phenomenon has recentlybeen explored at the ILL which also shows the nonlocalnature of quantum mechanics. When a neutron, tinythough it is (confinement radius about 0.7 femtometres),passes through a narrow slit system it seems like theneutron ‘feels’ the walls.The explanation of this intriguingeffect is as follows. The slits cause the transversemotion of the particle to be quantised (forming a seriesof energy levels as would happen if a particle wereconfined in a box or an atom). This produces alongitudinal phase shift which can be measured byneutron interferometry. This effect exists even if theneutron classically does not touch the walls at all.

These two examples offer a small glimpse into thebroad range of neutron interferometry experimentspossible. And even after almost 30 years of research inthis exciting field, still new ways of exploring questionsof fundamental physics with neutrons open up.

uantum theory is one of the most successfultheories of Nature that we have. It can be used to

describe the behaviour of a variety of objects:elementary particles such as electrons and photons,nuclear constituents (protons and neutrons), atoms andeven molecules. Some characteristics predicted byquantum theory are not easy to understand however.For example, microscopic objects can behave as eitherparticles or waves. Since quantum mechanics is ageneral theory, the wave-like characteristics of aparticle can be thought of as representing theprobability distribution of where the particle can befound. Another peculiar aspect is that a quantumsystem changing from one state to another actuallyexists as an entangled state, or ‘coherent superposition’,of those states until measured in some way. This idea isfamously illustrated by Schrödinger’s famous paradox:a cat locked in a box with a lethal chemical poisontriggered by a radioactive source (whose decay isgoverned by quantum probability) remains in anindeterminate, alive-and-dead superposition of statesuntil the box is opened.

Schrödinger-cat states of neutrons Can these so-called Schrödinger-cat states be studiedfor particles like neutrons? Even though neutrons aremassive, they do show wave-like behaviour, whichmeans that neutron waves out of phase can eitherreinforce or cancel each other to produce aninterference pattern. Using neutron interferometry,a coherent superposition of neutron states can becreated and observed. A neutron (wave) is split intotwo beams by diffraction from a perfect silicon crystal.

Even though the beams are separated by severalcentimetres, their quantumstates remain inextricably linked(another peculiarity of quantumsystems). One of the beams ispassed though devices (markedin green opposite) that delay oraccelerate the neutron wave sothat it is out of phase with the

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Research team H. Rauch, G. Badurek and M. Baron (Atomic Institute of the University of Vienna)

The dancing neutron

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Interferometry experiments

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lthough most of the neutrons in the worldaround us are bound into the nuclei of atoms

which have been stable for billions of years, oncereleased from this environment, they rapidly decay into protons, electrons and antineutrinos (called beta decay). The lifetime of the free neutron is about 15 minutes (or more precisely, 886 seconds).

Knowing this number is important forunderstanding the creation of elements in theUniverse. Neutrons, with protons, would have beenmade in the first instances after the Big Bang. Had theneutron lifetime been much smaller, the Universewould consist almost entirely of hydrogen – muchlarger and it would contain only helium. Luckily, thehalf-life is such that the early Universe contained bothhydrogen and helium, which were then able tocombine inside stars to form all the other elementswhich are vital for life. The precision to which therelative abundance of these elements can be calculated(see pp.16 and 17) is limited by the uncertainty in themeasurements of the neutron lifetime, currently about2 seconds.

Beyond the Big Bang, the neutron lifetime also hasimplications for our lives today. It is an importantparameter governing the rate at which two hydrogennuclei can fuse to form deuterium – a crucial step inthe energy-releasing cycle that fuels the Sun.

Finally, precise measurement of the neutron lifetimecombined with results obtained from other neutronbeta-decay experiments (see p.11) can give usinformation about the fundamental forces acting insidethe neutron itself.

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The lifetime of the neutron is a key

factor in explaining the evolution of

the Universe – and our existence Neutrons are not forever

PA R T I C L E P H Y S I C S

Research team J. Butterworth, P. Geltenbort and Th. Brenner (ILL), in collaboration with TU Munich,PNPI (Gatchina, Russia) and the Kurchatov Institute (Moscow)

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>> J A M E S B U T T E R W O R T HA N D P E T E R G E L T E N B O R T

Measuring the lifetime The first neutron lifetime measurements wereperformed with neutron beams obtained from nuclearreactors such as the ILL. Although it is almostimpossible to observe the antineutrino emitted duringneutron decay, the electron and proton can be readilydetected. If we know the number of neutrons in a givenvolume at a given moment, the rate at which electronsand protons are emitted from this volume gives usdirectly the neutron lifetime. The problem arises inknowing exactly how many neutrons are in the volumeand even the size and shape of the volume to whichour detectors are sensitive.

In the 1970s, techniques were developed forobtaining ultracold neutrons with such low energiesthat they could be confined inside a bottle for periodsapproaching their lifetime. In a perfect bottle, relativemeasurements of the number of neutrons remainingafter two different storage times are sufficient todetermine the lifetime.

In reality, a few neutrons are lost when theyencounter the bottle wall, so that their numberdecreases slightly faster than expected from betadecay alone. Several methods have been employed toadjust for this error. One involves performing theexperiment several times using bottles of differentsizes. As the size of the bottle is increased, the rate atwhich the neutrons collide with the walls is reduced. Byextrapolating the data to the point where the collisionrate is zero (an infinitely big bottle!) we can eliminatethe effect of the walls.

A more recent approach is to confine ultra-coldneutrons using magnetic fields (neutrons have amagnetic moment so respond to a magnetic field).In the absence of material walls the losses can beeliminated and the decay can be followed in real time.This method has been demonstrated at the ILL and theNational Institute of Standards and Technology in theUS, and will probably form the next generation ofmeasurements.

Peter Geltenbort andThomas Brenner

Fomblin-coatedstorage volume(made of glass) ofthe MAMBO IIlifetime experiment

Part of the neutron decayexperiment, PERKEO II

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Research team H. Abele, M. Astruc-Hoffmann, D. Dubbers, U. Müller and J. Reich (University of Heidelberg),V. Nesvizhevsky (ILL), S. Bässler and F. Glück (University of Mainz), O. Zimmer (TU Munich)

as a zero-sum – in other words, each quark gives asmuch as it takes (technically, the CKM-matrix is said tobe unitary). Probing this unitarity condition is thus animportant test of the Standard Model.

Measuring asymmetryAt the ILL, we have been doing just this, by studyingthe mixing of the down quark (first element in the CKMmatrix) in the beta decay of cold, free neutrons. Theprinciple is to prepare a highly-polarised neutron beam(with 100 per cent of the neutron spins aligned). Asexplained in the article opposite, the electrons arepreferentially emitted in a direction opposite to thespin of neutron in a proportion predicted by theStandard Model. Applying a magnetic field separatesthe differently oriented electrons, sending one kind to detectors located on the left and the other todetectors on the right. The count rate of electronsmeasured by the left and right detectors differs byseveral per cent (beta-asymmetry). Combining thisasymmetry, which in effect measures the strength ofthe weak interaction, with the neutron lifetime givesvalues directly related to the CKM quark-mixing matrix,in particular the first element.

Using recently upgraded equipment – the PERKEO IIspectrometer which employs superconductingmagnets – we were surprised to find that the dataindicated a significant 1-per-cent deviation from therequisite zero-sum value for quark-mixing. This effectcannot be explained by the Standard Model. Similarresults had been seen in experiments examining betadecay in nuclei, but no-one believed them because ofthe complication of having to introduce correctionsrelating to nuclear structure. However, our experimentavoids this issue, relying solely on neutrons andelectrons to give a cleaner result.

Again, the experiment nicely shows how low-energymeasurements of neutrons can point the way to newfundamental physics.

ccording to the accepted theory of particles andfields, the Standard Model, matter is built from

two types of fundamental particles – quarks andleptons (see p.4). Quarks come in six varieties orflavours called up (u), down (d), charm (c), strange (s),top (t) and bottom (b). The familiar neutrons are builtfrom two d-quarks and one u-quark, whereas protonsare built from two u-quarks and one d-quark. Thus,ordinary matter is made exclusively from up and downquarks, whereas the other quark flavours are observedin particles made in high-energy collider experiments.

Quarks interact, among other things, via the strongand weak forces (see p.4). The weak interaction has thepeculiar effect of allowing one type of quark to changeinto another type. This is what happens when aneutron decays via beta decay into a proton, anelectron and an antineutrino (see p.5) – a d-quark (inthe neutron) changes into a u-quark (in the proton). Inthis process, quantum theory predicts that the quarkstates are not ‘pure’ – the decaying down quark carriessmall contributions from strange and bottom quarks(similarly, a decaying bottom quark has smallcontributions from a down and a strange-quark, and soon). The strengths of the ‘quark mixings’ can be laid outin a useful mathematical way in a three-by-three arraycalled the Cabibbo-Koyabishi-Maskawa (CKM) matrix.The Standard Model requires that the mixing ends up

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probing the strength of the weak interaction – and

looking for new physics beyond the Standard Model

>> H A R T M U T A B E L E

Quark mixing in neutrons

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Looking for right-handedness The neutron, through its decay, offers an ideallaboratory for investigating the weak force and left-right symmetry breaking. At the ILL, we conducted anexperiment using polarised neutrons and the samespectrometer, PERKEO II, as described opposite, tocount the decay particles in the right and left detectors,but in a slightly different configuration, allowing boththe electron and the proton from the decay to bedetected. The principle of the experiment is simple. Itconsists of measuring the asymmetry in the neutrinoemission. Parity violation means that the neutron decayproducts – the electron, the proton and theantineutrino – are not emitted in just any direction. Theantineutrino tends to be emitted in the direction of theneutron spin, while the electron tends to go in theopposite direction. Because the neutrino has virtuallyno mass and interacts only weakly with matter, itcannot be detected directly, but its orientation andenergy can be derived from the measurements of theother two particles.

The count rates of the two detectors will differdepending on the direction of the neutron spin. Thesecount rates can be compared with those predicted bythe Standard Model. If different, then they mightindicate right-handed contributions as predicted bytheories going beyond the Standard Model. Withincreasing accuracy, theexperiments have drawncloser to the outer edge ofthe Standard Model, and newmeasurements with evenhigher accuracy are nowbeing planned.

hen we look into a mirror, we see an exact imageof our world, but with everything reversed – for

example, a right-handed person becomes a left-handedone. What happens if a particle like a neutron looks intoa mirror? Like all fundamental particles, the neutroncan also be left or right-handed, depending on thedirection of its spin to its momentum. We might expect,therefore, the spin to reverse in the mirror – a right-handed particle becoming left-handed and vice versa.

However, Nature is not so simple. The mirror relatingto particle spin, which is called parity (P), is notsymmetrical for particle behaviour governed by theweak force, such as neutron beta decay (see p.9).Whether right-handed or left-handed, a neutron alwaysproduces a right-handed antineutrino when it decays.So the symmetry is said to be broken.

This ‘parity violation’ distinguishes the weak forcefrom the three other fundamental forces. Physicistshave never been happy with this concept becausethere is no reason why Nature should prefer a particularhanded particle. It had to be artificially introduced intothe Standard Model of particle physics (see p.4).

Many theories going beyond the Standard Model,which attempt to explain how the forces arose in theearly Universe suggest, however, that in the high-energy environment just after the Big Bang, the weakinteraction was completely left-right symmetric. As theUniverse cooled, this symmetry was broken. Althoughthe left-handed W particle responsible for carrying theweak force has been discovered in high-energyexperiments mimicking the conditions in the earlyUniverse, no right-handed W particle has been seen,probably because it is too massive to be detected atthe energies so far available. It might be possible,however, to detect its ghostly traces in neutron betadecay. A discovery of such remnants would then offer a natural basis for the asymmetry in the StandardModel and help us to understand the processes in the early Universe.

11

PA R T I C L E P H Y S I C S

Research team M. Kreuz (ILL and University of Heidelberg), H. Abele, C. Vogel, D. Mund, U. Mayer and M. Brehm (University of Heidelberg),T. Soldner, A. Petoukhov and V. Nesvizhevsky (ILL)

When neutrons look into a mirror

W

No-one has seen a right-handed neutrino but highly

accurate neutron decay experiments may reveal

what happened to them in the early Universe

>> M I C H A E L K R E U Ze

Magnetic field

Dete

ctor

2

Dete

ctor

1

νe

p

p proton e electron νe neutrino

The inner detectorchamber of PERKEO II.You can see the carrier

for one of the protondetectors and the

electric shielding of thedecay volume. The

electron detector had tobe removed to gain

access to the installation

The mountedand alignedspectrometerPERKEO II

e know that in daily life time only goes forward –we get older, no matter what we do to stay

young. Time never runs backward for us. Nevertheless,the laws of physics at a fundamental level seem happyfor time to go in both directions. Take a process likeone particle bouncing off another: like a movie, thereaction can usually be run forwards or backwards.Such a process cannot be used to identify whether thetime is running forwards or backwards and so issymmetrical with respect to time reversal.

This behaviour, called time-reversal symmetry (T),is related to two other so-called discrete symmetries,and they are all vitally important in our understandingof elementary particles and forces. One symmetry,parity (P), states that Nature does not distinguishbetween the behaviour of a particle and that of itsmirror image (as described on p.11). The other is calledcharge conjugation (C) and exchanges particles bytheir antiparticles – it converts matter to antimatterand vice versa. Nevertheless, the weak force violatesthese symmetries. For example, the decay of theneutron, which is a weak process (see p.4), violates Cand P. The violation of these first two symmetries hasbeen known for 40 years, but T-violation, discovered inaccelerators with particles called kaons, has not yetbeen observed in neutron decay.

Reversing time in a spin We tested for time reversal when a neutron decays(into a proton, an electron and an antineutrino) byeffectively reversing the motions of the particlesinvolved. This is done in a subtle way by measuring thenumbers of electron-proton couples emitted in certaindirections for two sets of neutrons spinning in oppositesenses. Flipping the spin in this way is equivalent toreversing the motions of all particles involved in thedecay and thus reversing time (if the film ranbackwards the spins would reverse). Of course, we arenot reversing the decay process itself but theoreticianssay that this does not matter at our current level ofexperimental precision.

The actual experiment counts the electron–protoncouples in a fixed detector geometry, and comparesthe count rates for the two opposite directions of theneutron spin. If they do not agree, then this wouldindicate time-reversal violation. Highly sophisticateddetectors have had to be developed to ensure that thetwo sets of count rates are measured under identicalconditions and that other effects like the known parityviolation cannot adulterate the measurement. Toachieve sufficient precision, a huge number of decayshave to be counted – only possible using a high-fluxreactor such as that at the ILL. Our experiments have sofar shown that Nature’s preference for one direction oftime is less than one in a thousand events.

Should we go further? Although, the StandardModel of Particle Physics (see p.4 ) describes mostphysical phenomena quite precisely, there are still openquestions. For example, in a symmetric world ofparticles and forces, equal amounts of matter andantimatter would have been created in the Big Bang,but we see no signs of antimatter now. Theorists thinkthat a process involving the violations of C and T-symmetry was the key factor. However, this processcannot be explained by the tiny breaking of time-reversal symmetry already incorporated in theStandard Model, so alternative models have beenproposed which need to be tested experimentally.Some of these models contain a violation of T-symmetry that may show up in neutron decay.

12

N E U T R O N S A N D T H E U N I V E R S E

Research team C. Plonka, K. Schreckenbach and O. Zimmer (Technical University of Munich), L. Beck (University of Munich),P. Liaud (ISN Grenoble), A. Bussière and R. Kossakowski (LAPP Annecy), A. Petoukhov and T. Soldner (ILL)

W

An experiment at the ILL is looking for tiny differences in

processes that can go forwards and backwards in time

>> T O R S T E N S O L D N E R A N DK L A U S S C H R E C K E N B A C H

How to go backwards in time

Equivalent whenrotated aroundneutron spin

p4 p1

e1e4

e3

p3 p2

Motion reversalequivalent totime reversal

p

n

p

n

e e

p

n

Wire chamber

Neutron beam

PIN diode

Plastic scintillator

Neutron spin flip

e2

Direction of proton

Direction of electron

Direction of neutron spin

A neutron decays into aproton, electron and

neutrino. Time reversalinverts the directions ofparticle movements and

spins. This is equivalent to asimple spin flip of theneutron. The detector

scheme (below) has electrondetectors left and right,

e1-e4 (plastic scintillatorsand wire chambers), proton

detectors up and down,p1-p4 (PIN diodes),

and the neutron spin isperpendicular to the planeof the drawing. Comparingthe concidence count rates

of the protons and electronsdetected for the two

directions of the neutronspin can uncover whether

the system of the decayingneutron is symmetric under

time reversal

Installing the detectorused in the experiment(the TRINE detector)

e

Searching for the EDMHow do you measure the neutron EDM? First, theneutrons are placed inside a quartz storage cell in aweak magnetic field. The neutrons are spin-polarised,and precess in the magnetic field with a frequency thatdepends on the neutron magnetic moment and themagnetic field strength. In addition to the weakmagnetic field, a very high electric field is applied overthe quartz storage cell by putting 100,000 volts on itslid. The interaction of the neutron EDM with the electricfield will shift the precession frequency slightly – byjust a few parts per billion. Measuring this tiny shift tohigh precision thus gives us evidence for a neutronEDM. The precision with which we are searching for the EDM is unprecedented: if you can imagine scaling up the neutron to the size of the Earth, our experimentis equivalent to looking for a single positive andnegative electric charge separated by only a fewmicrometres (less than one-tenth of the thickness of ahuman hair) at its centre.

To achieve the required experimental sensitivity, weemploy a variety of technologies. The experiment relieson an atomic mercury magnetometer that can measurethe magnetic field in the storage cell to a precision ofone-billionth of the Earth’s magnetic field. Nuclearmagnetic resonance techniques are used to measurethe neutron precession frequency. Advanced materialsare also exploited: diamond-like coatings to optimiseneutron-storage properties; and a special alloy,mu-metal, to shield the experiment from the Earth’smagnetic field.

Searches for particle electric dipole moments take aprominent place in modern particle physics because oftheir bearing on the origin of symmetry violations andthe implication these have on understanding theUniverse as we experience it today. The puzzle of whatexactly did happen to matter and antimatter after theearly days of the Universe remains yet to be solved. Thissearch for the neutron EDM may well bring us a stepcloser to the understanding of why we are living in amaterial world.

13

A project at the ILL led by a team of UK researchers is addressing

some of the most fundamental questions of our existence

PA R T I C L E P H Y S I C S

Research team K. Green, P. S. Iaydjiev and S. N. Ivanov (RAL), M. G. D. van der Grinten (RAL/Sussex),P. G. Harris, J. M. Pendlebury and D. B. Shiers (Sussex), P. Geltenbort (ILL)

>> M A U R I T S V A N D E R G R I N T E N

he Big Bang is believed to have generatedequal populations of particles and anti-

particles but our Universe contains predominantlymatter, so why is our Universe made of matter? Is thestandard theory of particles and forces used todescribe our world complete, or are there, for example,additional particles yet to be discovered?

Experimental searches to answer these questionsare normally the privilege of accelerator laboratoriesoperating at extremely high energies. At the ILL,however, we are seeking to address these issues fromthe opposite extreme: a unique source of very lowenergy neutrons, so-called ultra-cold neutrons, is beingexploited to observe the behaviour of neutrons as theyare submitted to a combination of weak magnetic andstrong electric fields. In these extreme conditions, theneutron can expose a violation of time-reversalsymmetry (T-violation, see p.4) to a level that couldexplain the particle nature of our Universe. T-violationwould be revealed by the presence of a permanentneutron electric dipole moment (EDM). This experimentis pursuing the search for the neutron EDM. Althoughthe neutron is an electrically neutral particle, there aresmall positive and negative charges deep within it. AnEDM would arise if the average positions of the positiveand negative charge do not coincide. The neutrons also have a spin, and while an EDM would remainunchanged under T-reversal, the spin would bereversed – in other words, time symmetry is broken.

T

Living in a material world

The quartz storage cell for themeasurement ofthe neutron EDM

Equipment usedto measure theneutron EDM: anarray of ultra-coldneutron detectorsand the highvoltage stack

uantum, or discrete, properties of matter aremanifest in a variety of phenomena in Nature,particularly at the microscopic level. Quantum

mechanics, the theory that describes the very small,predicts that subatomic particles can have only certainenergy values, described as quantum states. This ruleshould hold for all matter under the influence ofNature’s four fundamental forces – the strong and weaknuclear forces, electromagnetism and gravity (see p.4).We can, for example, observe the quantum states ofelectrons in an electromagnetic field, and these indeedgive rise to the well-defined structure of atoms.Similarly, the quantum states of nucleons (protons andneutrons) in the strong nuclear field, responsible for thestructure of atomic nuclei, can be detected.

By analogy, gravity should also lead to the formationof gravitational quantum states in particles, whichaffect their behaviour in a gravitational field such asthat of the Earth. However, it is by far the weakest ofthe four forces (for an electron in an atom, for instance,the electromagnetic field is about 40 orders ofmagnitude stronger than the gravitational field), someasuring such phenomena is extremely challenging.

Participants in the ILLexperiment: from the left,Valery Nesvizhevsky,Hans Börner and A. K. Petoukhov

Bowling ultra-cold neutronsNevertheless, we have for the first time observed suchstates in an experiment with ultra-cold neutronscarried out at the ILL. We demonstrated that thevertical motion of very slow neutrons in the Earth’sgravitational field proceeds in discrete steps. In theexperiment, the ultra-cold neutrons were sent on agentle parabolic trajectory through a baffle and onto ahorizontal mirror. They had horizontal and verticalvelocity components of metres per second andcentimetres per second, respectively.

Neutrons at such low energy are totally reflected bythe mirror. They arrive at grazing angles (in trajectoriesvery close to the mirror’s surface) and are reflectedupwards until gravity forces them to descend again.Gravity acts on the vertical velocity component only.The experiment shows that there is a minimum energycorresponding to the vertical motion of matter in thegravitational field. For neutrons at the surface of theEarth, this was measured to be as small as about onepicoelectronvolt, which corresponds to the jumpingheight of about 10 micrometres. This energy is 13orders of magnitude smaller than the binding energyof an electron in the hydrogen atom, which mightdemonstrate why it is not easy to observe this effect.Thus, the classical motion of ultra-cold neutronsactually becomes discrete, when you observe them atsufficiently low energies.

14

N E U T R O N S A N D T H E U N I V E R S E

Research team V. V. Nesvizhevsky, H. G. Börner and A. K. Petoukhov (ILL), A. M. Gagarski (PNPI, Gatchina),H. Abele, S. Bässler and A. Westphal (University of Heidelberg), A. V. Strelkov (JINR, Dubna)

Q

>> V A L E R Y N E S V I Z H E V S K Y

Quantum states of matter in a gravitational field

For the first time, ultra-cold neutrons have

revealed quantum states due to gravity

Serg

e C

lais

se

The equipment for measuringquantum states of a neutron ina gravitational field

Nuclear modelsWhy are these more extreme nuclei interesting? Theinterplay of the forces between many nucleons withinthese minuscule systems ensures that they behave invery complex ways – we have really only just scratchedthe surface in our understanding of nuclear structure!Nuclei, like atoms, obey the laws of quantummechanics but there is as yet no single quantumdescription of nuclear structure. One characteristic thatresearchers first noticed in the 1930s and 1940s wasthat nuclei with certain numbers of protons andneutrons were particularly stable. These co-calledmagic numbers were explained by a nuclear model,introduced In 1948 by Otto Haxel, Hans Jensen, HansSuess and Maria Meyer, in which each nucleon movesin an orbit under the influence of a sphericallysymmetrical central field of force calculated from the

15

N U C L E A R P H Y S I C S

The intense source of neutrons offered by the ILL is an excellent tool for

investigating nuclei and nuclear reactions, leading to a better understanding

of the structure of nuclei and how they are made in the stars

>> H A N S B Ö R N E R

Understanding the nucleus

ost of the visible matter in the Universe is madeup of nuclei – collections of protons and neutrons

which form the tiny, but dense cores of all atoms.Although nuclei are extremely small, only about onemillion millionth of a centimetre across, they can containup to a couple of hundred protons and neutrons(collectively called nucleons) which interact throughthe electromagnetic and nuclear forces (see p.4).

The familiar elements of everyday life are built fromnuclei with a characteristic number of protons (atomicnumber) and at least an equal but often slightlyvarying number of neutrons (to give isotopes of theelement). It is also possible, however, to create nucleicontaining vastly varying proportions of protons andneutrons which can survive for a short time. Physicistsbelieve that around 7000 different proton-neutroncombinations are possible. These can be plotted on akind of nuclear landscape (see opposite). The chartshows a long valley of stability marked in black goingdiagonally across, which encompasses the isotopesmaking up everyday matter, but on either side areareas inhabited by unstable nuclei with increasingnumbers of protons or neutrons. These areas arebounded by so-called driplines, marking where theproportions of protons or neutrons in the nucleus areso high that they just leak away. We know where theproton dripline is but only the lower part of theneutron dripline has so far been investigated.

82

82

126

50

50

28

2820

20

2

2

8

8

The nuclear landscape

PROT

ON N

UMBE

R Z

NEUTRON NUMBER N

proton drip line

neutron drip lineneutron drip line

r-process path

unknown nuclei

known nuclei

stable nuclei

The lifetimes of nuclear states can be measured usingthe GRID technique. The top image is a simulation of

atoms recoiling in a single crystal of sodium chloride (therecoil is induced by gamma emission); the bottom imageshows the measured shift in energy induced by the recoil

M

<100>

Doppler shift (eV)

-700.0 -350.0 0.0 350.0 700.0

Sodiumchloride

Inte

nsity

0.8

0.6

0.4

0.2

0.0

as sets of vibrationally and rotationally excitedquantum states. They also showed that both protonsand neutrons like to couple in pairs. Later in the 1970s,Akito Arima and Francesco Iachello built on the idea ofpaired nucleons to develop a model in which an inertcore is surrounded by pairs of valence nucleons, whichare treated as ‘bosons’ (see p.18). This ‘interactingboson’ model describes nuclei in terms of dynamicsymmetries corresponding to particular nuclear shapessuch as spherical, deformed axial symmetric (rugbyball-shaped), or deformed axial asymmetric. A smallchange in the number of nucleons in a nucleus canindeed result in a rapid change in its shape (see p.20).

Not surprisingly, nuclear quantum states areextremely complex. Measuring their lifetimes mayindicate which model best predicts the properties of aparticular class of nuclei, and the often subtle interplaybetween single-particle and collective behaviour (seep.18). Excited states which are quite long-lived (calledisomers) are good probes of nuclear models. They mayarise because certain quantum rules (which are model-sensitive) do not allow the excited nucleus readily tofall back to its lowest energy level, or becausecompatibility between collective and single-particlebehaviour impedes the transition (see p.19).

NucleosynthesisStudies of extreme nuclei provide stringent tests notonly for these models but also for the theories of theunderlying nuclear forces. Another importantapplication is in understanding how the elementscame to be made in Nature. We know that all elementsheavier than boron are made in stars through nuclearreactions. The lighter elements are synthesised vianuclear burning during the normal lifetime of stars,while the elements heavier than iron are made either in

16

N E U T R O N S A N D T H E U N I V E R S E

average effects of all the nucleons. As for electrons inatoms, the nucleons build up in shells according toquantum mechanical principles. This approach, the so-called shell model, successfully predicted that themagic numbers represented fully occupied ‘closed’stable shells of protons or neutrons. Any nucleonsorbiting outside the outermost closed shell behave as‘valence’ nucleons. Like valence electrons in atoms, theycan be excited through a range of higher energy states,as predicted by quantum theory.

This ‘single-particle’ approach works effectively onlyfor light nuclei and those with numbers of neutronsand protons near a closed shell. Because nucleons,unlike atomic electrons, interact strongly with eachother through the nuclear forces, individualinteractions become too complicated to calculate forheavier nuclei with nucleons not near a magic number.Theorists have therefore resorted to another class ofmodels in which the nucleons are treated collectivelyas a liquid drop, held together by surface tension. Whenexcited, the nucleons all move together causing thenucleus to vibrate or rotate, or even change shape.

The shell and liquid drop models represent twoextreme pictures of nuclear behaviour. In the 1950s,Aage Bohr, Ben Mottelson and James Rainwatercombined aspects of both descriptions, pioneering acollective model which couples the motions ofindividual nucleons to nuclear surface oscillations. Thetheory accounts for the complex spectra seen in nuclei

Understanding the nucleus

Supernova 1987A

Merging neutron starscould be one of the sites ofheavy element synthesis

ESA

/M.R

uff

ert

& H

.Jan

ka

Hu

bb

le S

pac

e Te

lesc

op

e/ES

A

17

N U C L E A R P H Y S I C S

gaseous envelopes of ageing red giants or insupernova explosions. The main processes involve anucleus capturing a neutron. Nuclei with an excess ofneutrons are ‘beta-unstable’. This means that a neutronfrom the nucleus decays into a proton, emitting a betaparticle (electron) and a neutrino in the process, soincreasing atomic number by one, thus climbing up the nuclear chart by one element. The capture reactionmay go very slowly as in red giants (the slow, or s-process, see p.21) or explosively fast as in supernovae(the rapid, or r-process, see p.22).

These processes involve the creation of unstableneutron-rich nuclei. Experiments studying the spectraof such nuclei, measuring lifetimes of their excitedstates and determining the probability of capturing aneutron, allow us to contribute to predictions aboutthe expected abundances of the elements and thus themost likely route by which they are made.

The ILL’s unique roleThe ILL has been carrying out nuclear research for thepast 30 years. Its reactor provides an intense neutronsource for studying neutron-capture reactions and thestructure of the nuclei they form. When a nucleuscaptures a neutron, it is excited to the binding energyof the neutron (typically about 10 megaelectronvolts).As the excited nucleus returns to lower energy levels,it emits a cascade of characteristic gamma rays. Precisemeasurement of these gamma rays allows us todetermine the sequence of the excited states throughwhich the nucleus descends and so determine the pathof de-excitation.

It is also possible to measure the lifetimes of theexcited nuclear states using technology pioneered atthe ILL, called GRID (Gamma Ray Induced Dopplerbroadening). Gamma rays emitted when the nuclei arein flight shift slightly in wavelength due to the Dopplereffect. This results from nuclear motion induced by theemission of gamma rays preceding the ones beingmeasured. The recoils are extremely small in energyand the Doppler effect can only be detected byspectrometers with a resolution of one part per million.This is possible with the ILL’s crystal spectrometers GAMSwhich have 1000 times better resolving power thanconventional semiconductor gamma-ray detectors.

With the GAMS spectrometers we can study nucleiin the region of stability, or at least close to it. If wewant to do gamma-ray spectroscopy of nuclei far fromstability, however, we need an additional tool. By firingneutrons at heavy nuclei such as uranium they can beinduced to fission into neutron-rich fragments. Not onlycan we then study the fission process itself, but we canalso investigate the properties of very neutron-richnuclei. The fission fragments are directly extracted fromthe reactor using the ILL’s unique mass separatorLohengrin in about a microsecond. Electromagneticfields separate the charged fragments according totheir mass, charge and kinetic energies, and an array of germanium detectors then detects the emittedgamma rays. These instruments are used to study awide variety of nuclei, giving insights into nuclearstructure and nucleosynthesis as is shown in examplesin the following pages.

Finally, nuclear physics techniques have manyapplications in areas such as medical imaging and cancertherapy. In addition, accurate knowledge of fissionyields studied with a special set-up (called Mini Inca) atLohengrin are essential ingredients for studies of thedestruction by transmutation of long-lived radioactivewaste produced by nuclear power stations.

The interferometers used in the GAMS5 high-resolution gamma-ray spectrometer

The liquid drop also has defined quantum stateswhich appear as surface vibrations and rotations of thedeformed droplet. These quantum excitations, calledphonons, behave as bosons, and can exist collectivelyin the same vibrational mode. By studying thesemultiphonon states, nuclear physicists hope to find outhow the bosonic collective excitations are related totheir single-particle shell model constituents – thefermionic nucleons.

One approach is to expose atomic nuclei toneutrons from the ILL reactor core so that they capturethe neutrons, forming a statistical population of excited(higher energy) nuclear states. When they return totheir lowest energy state they emit gamma radiationwhich can be measured using the world’s most precisegamma-ray spectrometers, the ILL’s crystalspectrometers – GAMS.

Over the past decade, we have observed the firstmultiphonon states in the heavy nucleus erbium-168,followed up by studies of further multiphononexcitations in other heavy, deformed nuclei. In classicalsolids, multiple vibrations pile on top of one another atwill. In nuclei, however, the finite number of particlesand the Pauli exclusion principle significantly impedethe creation of such modes. Indeed, the existence ofmultiphonon vibrational excitations in nonsphericalnuclei is a long-standing issue. To characterise thesestates involves measuring their very short lifetimeswhich last less than one million millionth of a second.A powerful method known as the GRID technique (see p.17) pioneered at the ILL has enabled us tomeasure their lifetimes. In this way, we can probe how inherent loners, the fermionic nucleons, aretransformed into collective excitations which are thejovial phonons. This provides a deeper understandingof the residual interactions mentioned earlier.

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N E U T R O N S A N D T H E U N I V E R S E

Research team J. Jolie (University of Cologne), H. G. Börner (ILL), R. F. Casten and V. Zamfir (Yale University, USA),A. Aprahamian (University of Notre Dame, USA).

he atomic nucleus is made up of densely packednucleons – protons and neutrons, held in the firm

grip of the strong nuclear force (see p.6) so that theyinteract strongly with each other. Because of thestrength and complexity of the strong force, physicistshave had to rely on a series of approximate pictures of what atomic nuclei look like and how they behave(see p.16).

These models are, of course, based on quantummechanics which means that the nucleons in thenucleus exist in defined quantum energy states. Theway these states are filled up depends on anotherquantum property by which all particles are dividedinto two kinds called fermions and bosons. Fermionsobey the Pauli exclusion principle whereby no twoparticles can occupy the same quantum state, whereasbosons will happily all sit in the same energy level.

Nucleons, like electrons, are fermions, and so can beconsidered to sit in orbits of defined energy in thenucleus – in the same way that electrons do in atoms.However, for the nucleus there are two sets of orbits –one for protons, one for neutrons. This description,which works well with nuclei containing not too manynucleons, is called the shell model. The orbits formshells – or groups of orbits having similar energies –with large energy gaps between shells. Nuclei with aclosed (full) shell of protons, or a closed shell ofneutrons (and especially those with both) are muchmore stable. For nuclei with a few additional nucleonsbeyond a closed shell, the nucleons in the closed shellcan mostly be neglected, and only the interactionsbetween the outermost nucleons – called residualinteractions – must be taken into account.

Quantum liquidIn heavy nuclei with many nucleons outside the lastclosed shell, the calculations become prohibitivelycomplex even with modern computers, so a differentmodel is used called the collective model or liquid dropmodel. The model views the nucleus as a droplet ofquantum liquid with typical features such as densityand surface tension.

>> J A N J O L I E A N D H A N S B Ö R N E R

T

(a) A typicalnucleus consisting

of protons (yellow)and neutrons

(blue), (b) the shellmodel of the

nucleus and (c) theliquid drop model

Hans Börner, head ofthe Nuclear and Particle

Physics Groups

(b)

(a)

(c)

Groundbreaking experiments at the ILL are

probing the complex interplay between single-

particle and collective behaviour in heavy nuclei

From shells to liquid drops

Very neutron-rich nuclei far from stability

are excellent testbeds for nuclear theory

uclei make up more than 90 per cent of theknown mass of the Universe, so it is important to

understand their properties and structure. Althoughresearchers have been studying the nucleus for about50 years, we are still a long way from having an all-encompassing description – a ‘Standard Model’. Atthe moment, nuclear behaviour is explained in terms oftwo extreme types of model, shell models which treatthe constituent protons and neutrons as singleparticles, and collective models which treat the nucleusas a macroscopic system (see p.16). The behaviour ofmany nuclei can be explained in terms of the interplayof single-particle and collective behaviour.

These theories have so far been rigorously testedonly with stable or near-stable nuclei, of roughly similarnumbers of neutrons and protons. However, theseconstitute less than 10 per cent of all nuclei that couldexist (see the nuclear chart on p.15). Nevertheless, bystudying exotic species with extreme neutron-to-proton ratios, we can verify the validity of our nuclearmodels, and advance nuclear theory.

Nuclei with high ratios of protons to neutrons canbe obtained relatively straightforwardly usingaccelerators. Obtaining neutron-rich nuclei is moredifficult, and only a handful of facilities worldwide canproduce reasonable amounts. One of these isLohengrin at the ILL. A target of a fissile isotope isplaced close to the ILL reactor core, in this uniqueinstrument. The isotope fissions into a range ofneutron-rich nuclei which are separated by Lohengrin(see p.17). Decays of excited nuclear states can then be

detected by germanium gamma-ray detectors, to giveinformation about the nucleus. The properties of theseneutron-rich, unstable species are expected to deviatefrom those near stability. For instance, neutron-richnuclei with a mass of around 100 rapidly change shapefrom being spherical to deformed by adding just twoneutrons. Similarly, shell structure changes and newclosed shells appear, and other shells disappear, in veryneutron-rich regions.

Neutron-rich tin-132One example is the structure of the neutron-rich tin-132 nucleus which has 50 protons and 82 neutrons– eight beyond stability; its lifetime is about 40seconds. It is ‘doubly magic’, meaning it has both fullproton and neutron shells. Doubly-magic nuclei aretightly bound, so tin-132 and similar nuclei wereexpected to have a relatively simple structure, makingthem excellent for testing nuclear models on theexotic, neutron-rich side of the nuclear landscape.

Studies at Lohengrin on nuclei in the region of tin-132 have measured the lifetimes of long-livedexcited (isomeric) states of these nuclei, to verifywhether the excited states in the doubly-magic tin-132region do have a pure single-particle nature.

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N U C L E A R P H Y S I C S

Research team G.S. Simpson (ILL), J. Genevey and J. A. Pinston (ISN Grenoble, France), W. Urban and T. Rzaca-Urban (University of Warsaw, Poland),J. Jolie (University of Cologne), J. Durrell (University of Manchester)

>> G A R Y S I M P S O N

N

Probing exotic nuclei

Side-on view of the Lohengrin mass separator when

it was being constructed

The Lohengrinteam (clockwise

from the left:Antonella Scherillo,

Gary Simpson,Herbert Faust, IgorTsekhanovich andRiccardo Orlandi)

The array of gamma-ray detectorspositioned at theLohengrin focal point

The equilibriumphase diagram fornuclei. Nuclearmodels typicallyspan the triangleusing two variableswhich are analogousto pressure andtemperature inLandau theory

Members of the Yale NuclearStructure Group (left to right)Rick Casten, Libby McCutchan,Victor Zamfir and Mark Caprio,looking at a GRID spectrum for

an isotope of samarium

he structure of atomic nuclei depends sensitivelyon the numbers of protons and neutrons in the

outermost orbits – active nucleons – near the surfaceof the nucleus. Physicists have known for half a centurythat, in many regions of the nuclear chart (see p.15),nuclei change structure and shape from spherical todeformed (ellipsoidal) as the number of activenucleons increases. However, nuclei in these shape-transitional regions have been the most difficult todescribe theoretically, because they involve intensecompetition between single-particle and collectivebehaviour (see p.18). Nevertheless, a useful approach isto think of the change in shape as a phase transition,rather like that between water and ice, or between twodifferent crystal structures. We can apply this idea eventhough nuclei are small, finite objects with a limitednumber of particles.

Precision studies of shape changesThis perspective has recently been explored andenhanced in exciting new ways by studying shapetransitions in a particular group of nuclei, throughprecision gamma-ray studies at the ILL and otherlaboratories. Using the GRID method (p.17), ILLresearchers measured the lifetimes of nuclearexcitations in the picosecond (million millionth of asecond) range of the heavy nucleus samarium-152,considerably revising our knowledge of this and similarnuclei with 90 neutrons. The studies showed that thestructure changes abruptly at that point – the nucleican simultaneously take on both spherical anddeformed shapes in different quantum states. Theseobservations confirm that finite nuclei do undergophase transitional behaviour depending on thenumber of nucleonic constituents.

This work has inspired two ground-breakingdevelopments. In the first, Francesco Iachello at Yale

20

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Research team R. F. Casten, V. Zamfir and M. Caprio (Yale University, USA), J. Jolie (University of Cologne) and H. G. Börner (ILL)

>> R I C K C A S T E N A N D V I C T O R Z A M F I R

Phase transitions in nuclei

T

University developed a new concept for describingnuclear phase transitions. Again, just as the shapes ofdifferent crystal structures can be describedmathematically in terms of symmetries, so can nuclearshapes. This leads to the idea of ‘critical point’ nuclearsymmetries where the shape changes from onesymmetry to another depending on a small change inthe number of nucleons that the nucleus contains.These symmetries are simple, easy to calculate and areparameter-free. They are classified according to thetype of phase transition, and have an underpinning inthe mathematical language of group theory. One ofthese symmetries called X(5) describes a rapidspherical-to-deformed phase transition. Supported byrecent ILL data, samarium-152 emerges as the firstempirical example of this symmetry. This is not the firsttime that the ILL has pioneered the study ofsymmetries in nuclei, however. In 1978, ILL researchersdiscovered the first example of the then newlyproposed symmetry, O(6), in platinum-196.

The second development in this area is theapplication, by Jan Jolie, Pavel Cejnar and theircolleagues, to nuclei in their lowest energy state, of awell-known theory in physics – Landau theory. This isused classically to describe changes in, for example,crystal structure in terms of parameters like pressureand temperature which can be shown as a phasediagram. When applied to nuclear shape, the resultingphase diagram (see above) shows that nuclei havethree shape phases at low energy – spherical (denotedby β = 0), prolate deformed (rugby ball-shaped, βgreater than 0), and oblate deformed (disc-like, β lessthan 0) and that the phase transitions that separatethese phases meet at the triple point t.

U(5)Symmetry

SU(3)Symmetry

SU(3)Symmetry

ββ <0

E(5)

ββ =0 ββ >0

t

O(6)Symmetry/Transition

X(5)Transition

X(5)Transition

Pioneering work at the ILL has led to the development of

new ideas for describing nuclear structure, in particular

phase transitions associated with shape changes

he heaviest elements, from iron to uranium,are made when a star becomes a red giant or

when, if very massive, it explodes as a supernova. Theabundances of elements we see in Nature are roughlyan equal mix of the ashes of these two scenarios.

The origin of lutetium-176 and of about 30 othernuclei, however, can be completely ascribed to redgiants. The process involves the slow capture of aneutron (the s-process) by a nucleus followed by betadecay in which one of its neutrons converts into aproton with the emission of an electron and a neutrinoto give the element with the next highest atomicnumber. The nucleosynthesis associated with thisprocess is now quite well understood, and bydetermining experimentally the rates at which neutronsare captured, the resulting abundances of these so-called s isotopes can be quite accurately described.

Having a clear picture of how lutetium-176 is madeand being able to predict its natural abundance is ofparticular interest because it has a half-life of 36 billionyears decaying, via beta emission, to hafnium-176. Thisdecay could, in principle therefore, be used as a clock todetermine the age of the s elements – the original s-production of lutetium-176 could be compared with its abundance today (as in the familiar radioactivecarbon-dating process).

However, attempts along these lines encounteredan unexpected obstacle. Whilst behaving as a perfectclock under laboratory conditions, the decay oflutetium-176 was suspected to change at theextremely high temperatures inside red giants. Thereason for this exotic behaviour is subtle. Lutetium-176is formed, through neutron capture, in an excited state.This, however, can de-excite back to the lowest energystate (ground-state) by emitting gamma-rays, or toanother so-called isomeric state which decays with ahalf-life of only 3.68 hours. The overall lifetime oflutetium appears therefore shorter. Nevertheless, underlaboratory conditions, the rates of the two processescan be measured and taken into account.

Normally the two families of energy levelsassociated with these two processes do not influenceeach other. However, the question was whether thisremained the case in the hot, energetic red giantenvironment. The ground-state would be excited

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N U C L E A R P H Y S I C S

Research team N. Klay, H. Beer and F. Käppeler (Karlsruhe Research Centre, Germany), H. G. Börner and C. Doll (ILL)

T

A stellar thermometer

Hafnium-176

Lutetium-176 isomeric stateHalf-life = 3.68 hours

High precision gamma-ray spectroscopy

of lutetium-176 reveals the temperature

inside red giant stars

thermally to a series of higher states, which may decayto the short-lived isomer. The exact population of thesehigher states depends on the temperature, which theninfluences the rate at which lutetium-176 is lost via theshort-lived isomer. The overall, apparent decay rate oflutetium-176 would thus also depend on temperature(see figure below).

Probing the stellar interiorWe explored this question using high-resolutiongamma spectroscopy, which allowed us to study thetwo families of states with unsurpassed sensitivity. Wefound that, while the half-life of lutetium-176 remainsat its laboratory value up to 150 million degrees, it dropsto about 1 year at its production site in a red giant.

To correct this defect, we would need to know thetemperature history to which lutetium-176 is exposed.Since this is not currently possible, the idea of usinglutetium-176 as a cosmic clock has had to be abandonedfor now. Instead, the argument can be turned around toobtain an estimate for the temperature at whichlutetium-176 is produced – between 200 and 300million degrees. This information thus provides a probe for the interior of red-giant stars completelyindependent of the yet uncertain stellar models.

Lutetium-176 ground stateHalf-life = 36 billion years

Thermalexcitation

mediating level

Gammadecay

Excitation of a mediatingstate in the hot stellarphoton bath provides alink between theincompatible isomer andthe ground state oflutetium-176, resultingin a drastic reduction ofthe half-life

A red giant (left) isthe final stage of astar like the Sun. Itejects its outer layersinto space to form abeautiful planetarynebula (right)

>> F R A N Z K Ä P P E L E R A N D H A N S B Ö R N E R

Hu

bb

le S

pac

e Te

lesc

op

e/ST

ScI

The remnants of asupernova explosion inwhich heavy elementsare built up and spreadacross the Galaxy

Researchers at the ILL are investigating how

the heaviest elements are made in the Universe

e know that all but the lightest elements inNature are made in stars via nuclear reactions.

Understanding the path by which the heaviestelements are synthesised is a current challenge. Half ofthe elements above iron are thought to be created by aprocess called the rapid neutron capture (r) process,whereby a nucleus captures a large number ofneutrons and decays to the element of the nexthighest atomic number, by emitting an electron and anantineutrino (beta decay).

Our observational knowledge of the r-process isbased on the relative abundances of elements asanalysed in the Sun and distant stars. They reveal thatthe process must occur in environments where neutrondensities are extremely high – at least 1020 neutronsper cubic centimetre – and at temperatures of morethan a billion degrees. The site of the r-process is notyet known, despite huge efforts on the astrophysicsside. It may happen in explosive events of so-calledtype II supernovae, or in the merger of two neutronstars. The resulting debris of the r-process goes on toform the next generation of stars such as our Sun.

The path of the r-process is very exotic. Iron nuclei,first made in stars by other processes which stop atiron, undergo successive neutron captures, going up tonuclear masses where the high temperatures result inneutrons being knocked loose from nuclei by gamma-rays at the same rate as they are captured. The nucleithen have to wait for beta decay to happen to continuethe climb up the elemental ladder. Eventually, elementsup to the actinides are created.

Even though we don’t yet know the astrophysicalcircumstances of the r-process, in nuclear physics termsits path must occur on the very neutron-rich side of thenuclear chart, almost 10 to 20 mass units away from the

valley of stability (see p.15). We can indeed calculatethe path, assuming particular neutron densities andtemperatures, and using what we know about neutron-rich nuclei.

Investigating r-process nucleiThe ILL is investigating neutron-rich nuclei located nearto the r-process path. These nuclei are created bynuclear fission, and then separated and analysed by theLohengrin spectrometer (see p.17). Their mass rangecovers a considerable part of the region where the r-process is thought to proceed.

An important area of the nuclear chart toinvestigate is that surrounding neutron-rich nuclei withso-called magic numbers of protons and neutrons (seep.15). These have increased stability due to full shells ofnucleons. For example, we investigated the excitedhigh-spin states in antimony isotopes which have anearly closed shell with about 82 neutrons and 50protons. These long-lived nuclear states are single-proton excitations, and such measurements areimportant for testing aspects of nuclear structureneeded to fix parameters relevant to the r-process.More experiments are planned in this direction.

The fission process itself is also a rich source ofinformation: the abundances of the fission fragmentsproduced and their excited states depend on theirnuclear structure. Recently, we have proposed anempirical model in which the excitation and kineticenergies of the fragments are calculated from theirground-state masses and the so-called level densityparameter, which determine how the nuclei behave athigher temperatures. These parameters, which are easyto measure, also probe nuclear models, and are directlylinked to values needed in r-process calculations.

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Research team H. Faust, G. Simpson and I. Tsekhanovich (ILL), J. A. Pinston and J. Genevey (ISN Grenoble)

>> H E R B E R T F A U S T A N D J E A N - A L A I N P I N S T O N

W

Where do the heavy elements come from?

Inside the Lohengrinspectrometer

Glossary

AntimatterThe equation in quantummechanics describingsubatomic particlespredicts that each matterparticle has an antimatterpartner (of oppositeelectrical charge, if theparticle is charged).

AntineutrinoThe antimatter partner ofthe neutrino; sometheories suggest theneutral neutrino is its ownantiparticle.

Baryonic matterMatter made of baryons –particles consisting ofthree quarks, such asneutrons and protons.

Beta decayA type of radioactivedecay in which a neutronconverts into a proton, anelectron and anantineutrino.

Big BangThe Universe is believedto have been born in afireball explosion about15 billion years ago.

BosonA quantum particle which has a spin ofinteger value (0,1,2....etc).Bosons include theparticles that mediate thefundamental forces.

Critical pointThe point at whichphases are in equilibrium(see Phase transition).

C-violation (violation of charge conjugation)The phenomenon inwhich matter andantimatter particlesbehave differently.

DeuteriumAn isotope of hydrogencontaining a neutron aswell as a proton.

DriplineThe boundary on thenuclear chart marking theedge of nuclear stabilityat which neutrons orprotons ‘drip out’ of thenucleus.

Electric dipole momentThe quantity describingthe electric field of anobject due to the spatialseparation of positive andnegative charges insidethe object.

ElectronAn elementary particlewhich is a significantconstituent of atoms andis also emitted byunstable nuclei as betaradiation.

ElectronvoltThe energy required toaccelerate an electronthrough a potential of 1 volt.

23

FermionA particle which has half-integer spin. Examples arequarks and leptons aswell as compositeparticles like neutronsand protons.

Femto Unit of scale10-15.

Fine structure constantA fundamental constantof Nature that describesthe strength of theelectromagnetic force.

Fundamental forcesAll known forces presentin the Universe can bereduced to fourfundamental forces: thegravitational andelectromagnetic force weknow from everyday life,and the weak and strongforces which areresponsible for theexistence of the materialworld we live in.

Gamma rayVery high energyelectromagneticradiation. Gamma rays areemitted in nucleartransitions.

Group theoryA mathematicalapproach, based on theconcept of symmetry, andapplied extensively in thephysical sciences toclassify the properties ofsystems.

GluonThe particle (boson) thatmediates the strong force.

IsomerAn excited nuclear statewhich decays with arelatively long half-life.

LeptonA type of elementaryparticle predicted by theStandard Model. Theycomprise the electron,muon, tau and theircorresponding neutrinopartners. They interact viathe electromagnetic andweak forces.

Liquid drop modelOne of the theoreticaldescriptions of thenucleus which regardsthe constituent nucleonsas acting collectively in afluid drop with densityand surface tension.

Magic numberThe number of protons orneutrons in a nucleuswhich denotes aparticularly stableconfiguration with aclosed outer shell ofprotons or neutrons.

Magnetic momentProperty characterisingthe magnetic field of aspinning electric charge.

MuonAn elementary particlebelonging to the leptonfamily – a heavier versionof the electron with ahalf-life of 2.2microseconds.

NeutrinoA neutral elementaryparticle with hardly any mass.

NeutronOne of the two particlesmaking up the nucleus.Free neutrons decay intoa proton, an electron andan antineutrino.

Neutron starA small star consistinglargely of densely packedneutrons that is the end-product of a supernovaexplosion.

Nuclear chartA map showing the rangeof possible nuclei in termsof numbers of protonsand neutrons. It marksboth stable and short-lived nuclei.

Nuclear fissionProcess by which heavynuclei break up toproduce smaller nuclei.

Nuclear magneticresonanceA process in which thespin of a nucleus interactswith magnetic andelectromagnetic fieldscausing it to precess like aspinning top does in theEarth’s gravitational field.

NucleonA constituent of thenucleus, usually a protonor a neutron.

NucleosynthesisThe processes by whichincreasingly heavy nucleiare built up from protonsand neutrons (mainly instars) to create theelements of Nature.

NucleusA constituent of atomicmatter consisting ofprotons and neutrons.

Pauli exclusion principleFundamental quantumlaw which distinguishesfermions from bosons. Itsays that no two fermionscan occupy the samequantum state; bosonscan occupy the samequantum state.

Phase transitionThe point in terms of, forexample, temperature orpressure at which adefined structure or set ofproperties of a systemsuddenly changes.

PhononName used for avibrational mode inquantum mechanics;phonons behave as bosons.

Pico Unit of scale, 10-12.

Planck’s constantThe fundamentalconstant of Nature thatrelates the energy andwavelength of quantumparticles.

ProtonOne of the constituents ofthe nucleus. The numberof protons characterisesthe chemical and physicalproperties of an element.

P-violation (parityviolation)A phenomenon by whichright and left-handedparticles behavedifferently.

Quantum mechanicsThe theory that describesmatter and energy interms of particles withwave-like behaviour.

Quantum stateSystems obeying the lawsof quantum mechanics,such as particles or nuclei,exist in defined energystates.

QuarkThe elementary particlemaking up particles suchas protons and neutrons.There are six kinds ofquark and they interactvia the strong, weak andelectromagnetic forces.

Red giantAn ageing star that hasexpanded to form a largevolume of cool, tenuousgas (the volume of ourinner Solar System) with asmall core, after all itshydrogen fuel has beenexhausted, and it hasbegun to burn heliuminto carbon and oxygen.

r-Process (rapid process)The explosive processthought to occur insupernovae (type II)whereby nuclei rapidlycapture neutrons on atime-scale that is fastcompared with theprocess of beta decay sothat several neutrons arecaptured before betadecay occurs to give theelement of the nexthighest atomic number.

Shell modelA theoretical descriptionof the nucleus in whichthe constituent nucleonsare arranged in filledshells under the influenceof an averaged centralforce. Outer ‘valence’nucleons in theoutermost shell controlthe behaviour of thenucleus.

s-ProcessThe process ofnucleosynthesis in redgiants whereby nucleicapture neutrons on atimescale that is slowcompared with thesubsequent beta decay togive the element of thenext highest atomicnumber.

Standard Model ofParticle PhysicsThe accepted theoreticaldescription of theelementary buildingblocks of matter andthree of the four forces ofNature in terms of matterparticles – six quarks andsix leptons and forceparticles – the photon(electromagnetism),W and Z bosons (weakforce) and the gluon(strong force).

Strong interactionOne of the fourfundamental forces; itaffects only quarks andgluons.

Supernova type IIMassive stars (10 to 30times the mass of ourSun) end their life in agigantic explosion asbright as an entire galaxy.

Symmetry A mathematical conceptused throughout physicsto describe the state anddegree of order in systems.

Symmetry breakingA process by which asystem changes to a stateof lower symmetry withmore complex structuralor dynamic characteristics.

T-violation (time-reversal violation) The phenomenonwhereby a forward-goingprocess differs from thesame process going inreverse.

Ultra-cold neutronsNeutrons cooled toenergies so low that theycan not penetratethrough material walls.

Uncertainty Principle An intrinsic characteristicof quantum behaviourwhereby the precisionwith which linkedvariables such as positionand momentum, orenergy and time, can be determinedsimultaneously is limited.

Weak interactionOne of the fourfundamental forces;it is responsible forphenomena such as beta decay.

W particleOne of the particles that mediate the weakinteraction.

Z particleOne of the particles that mediate the weakinteraction.

Glossary

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25

Contacts

Scientific Coordination Office (SCO) Institut Laue Langevin6 rue Jules HorowitzBP 156F-38042 Grenoble Cedex 9FranceEmail: [email protected] or [email protected]

Page 10

Dr Hartmut AbelePhysics InstituteUniversity of HeidelbergPhilosophenweg 12D-69120 HeidelbergGermanyEmail: [email protected]

Page 15, 18 and 21

Dr Hans BörnerEmail: [email protected]

Page 20

Professor Richard CastenWright Nuclear Structure LaboratoryYale University272 Whitney AvenueP.O. Box 208124New HavenCT 06520-8124 USAEmail: [email protected]

Page 4

Professor Dirk DubbersPhysics InstituteUniversity of HeidelbergPhilosophenweg 12D-69120 HeidelbergGermanyEmail: [email protected]

Page 22

Dr Herbert FaustEmail: [email protected]

Page 9

Dr Peter GeltenbortEmail: [email protected]

Page 18

Professor Jan JolieIKP, University of CologneZuelpicherstr. 77D-50937 CologneGermanyEmail: [email protected]

Page 18

Dr Franz KäppelerForschungszentrum Karlsruhe, IK IIIHermann von Helmholtz Platz 1D-76344 Eggenstein-LeopoldshafenGermanyEmail: [email protected]

Page 11

Dr Michael Kreuz Email: [email protected]

Page 14

Dr Valery Nesvizhevsky Email: [email protected]

Page 8

Professor Helmut RauchAtomic Institute of the University of ViennaStadionallee 2A-1020 ViennaAustriaEmail: [email protected]

Page 12

Professor Klaus SchreckenbachTechnical University of MunichLichtenbergstraße 1D-85747 GarchingGermanyEmail: [email protected]

Page 19

Dr Gary Simpson Email: [email protected]

Page 4 and 12

Dr Torsten Soldner Email: [email protected]

Page 13

Dr Maurits van der GrintenDepartment of Physics and AstronomySchool of CPESUniversity of SussexBrighton BN1 9QJUK Email: [email protected]

Page 20

Dr Victor ZamfirWright Nuclear Structure LaboratoryYale University272 Whitney AvenueP.O. Box 208124New HavenCT 06520-8124 USAEmail: [email protected]

N E U T R O N S A N D T H E U N I V E R S E

Published by the Scientific

Coordination Office

Institut Laue Langevin

Email: [email protected]

Editor: Nina Hall

Tel: 44 (0)20 8248 8565

Fax: 44 (0)20 8248 8568

Email: [email protected]

Design: Spaced

Email: [email protected]

www.spaced-out.biz

Photography (unless shown otherwise):

ILL and Artechnique – [email protected]

Cover photo: Hubble Space Telescope/NASA/ESA

Institut Laue Langevin6, rue Jules Horowitz

BP 15638042 Grenoble Cedex 9

Franceweb: www.ill.fr

May 2003

The Boomerang Nebula – a planetarynebula formed by a fierce wind blowing

ultra-cold gas at half a million kilometres anhour from a dying central star.The heavier

elements are created by the process ofnucleosynthesis (see p. 16) in such stars

The inset shows the path of neutron decay