The gateway to exotic matter

Twenty first century nuclear physics involves bursting through nature's boundaries to produce matter never before observed in the laboratory. New physics, new medical and industrial appli:ca­ tions, and new paradigms are emerging from this challenging venture in which we produce nuclei not normally seen outside such exotic objects as super­ novae and x-ray bursters. To create these exotic nuclei requires new tech"nology - the ability to generate radioactive beams. Europe has taken the lead in this field and is now preparing a new generation of radioactive ion beam accelerators to explore exotic matter. Unexplored Regions

The differen t shapes of the nucleus: collect ivit y vers us individuality

The nucleus is a fascinat ing mesoscopic It is also possible to st udy shape coexis­ system , giving the possibility to link col­ tence at very low excitation energy, and lective behaviour to individual par ticles. some specific nuclei outof the 5000 pos­ Studying the shape of the nucleus can sib ly existing ones could provid e the key almo st be done naturally using heavy- ion to the ori gin of collective behaviour. A reactions, wh ich confer high angu lar striking examp le is 186Pb, where the first momentum on the fo rm ed nucleus. three states have three diffe rent shapes. Combined with powerful detector arrays, This is unique in nuclear physics, and a ri ch var iety of shapes has been maybe also in other fields, sin ce it is eas­ observed. However, certain region s of the ier for this quan tu m system to change nuclear chart are not avail able for stu dy, shape from spheric al through oblate to since the right·combination of stable pro­ prolate than to build up excitation energy jectile and stable target to produce th e thro ugh vibrations or rotations. By usin g nuclei of interest does not exist. This sit­ spectrosco pic methods, it is even possi­ uati on wi ll change dr amati cally as new ble to relate the different shape s to the radioactive beam s becom e available. excitation of som e specific nucleon s.

Isospi n dependence of the nuclear force

Alth ough the nucle ar force is to first order independent of the exact nature of the inter­ acting protons and neutrons, strong nuclear struct ure effects are observed whe n the rat io of the number of neutrons IN) to the number of protons IZI changes. The nuclei best suited for stu dying t his so-called isospin dependence of the nucle ar force are those where the protons fill the same orbitals as th e neutrons. These are nuclei tha t lie near the N = Z line , which for heavier nuclei is far fr om sta bility. It is in this region that , along with neutron- neutron and proton-proton pairi ng, proto n-neut ron pair ing might also lead to a new superconducting phase cons ist ing of proton-neutron Cooper pairs.

Explaining comp lex nuclei from basic const it uent

Nuclear physics trea ts the atom ic nucleu s as a collection of nucleons ­ neutrons and protons - but th is is only part of the story. Nucleons are built up of quarks and gluons, and th ese have an effe ct beyond the confines of z their nucleon cages. Interactions between nucleons are diff erent inside a nucleus than they are for two free nucleons, and depend particularly on neutron and proton densiti es. There is no analytical form of the effective interactio n betw een them, and first pri ncip les calculations can only be used for the lig htest nuclei . The fun damental goal of nuclear physics is to derive th e cor rect form of the eff ective inte raction and to present a unified til C description of the nucl eus. The strategy is to amplify the deficiencies of a a cur rent nuclear models by testing them in nuclei with ext reme proto n/ neu­ L 0.. tron rati os. This wi ll allow us to discriminate between models. "'---"~N neutrons )f the Nuclear Chart - A Ri

In this chart , stable nuclei occuPY the cent ral ground - t he valley of stability, but t here is . , much interesting physics wait ing t o be explo'l"'ed for those who vent ure beyond...

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", ;fne size of the nucleus: halos and skins

Textbook nuclear physics relates the size that of 48C a and a halo neutron rad ius as of the nucleus to its number of con­ large as that of the outermost neut rons in stituent part icl es [AI thro ugh the simple 208 Pb. relation R = RoA1/3 with R the rad ius and Diff erent techn iques are needed to meas­ Ro a constant. This can be understood ure the rad ius of a nucleus. By using low energy radioactive ion beams and fr om the liquid drop mod el of the nucleus, and small deviations from this relation coll inear laser light, for example, it is are a measure of unde rly ing structure. possible to measure th e distribution of th e protons using atom ic spectroscopy. High­ energy rad ioactive ion beams, on the When moving far from stability, however, ~~- 'L; large deviations can occur: the binding oth er hand, can be used to determine the "Li ~ "'---.: ) distribution of all nucleons. : -0 : energies of the last -bound particle s can become so small that halos or ski ns can ~ be formed . The most studied halo nucleus is 11Li, wi th a matter radiu s as large as Physics Potential

l~~ -:-e nd of Mendeleev's table: superheavy elements

, 0 0 o c:::J About ninety eleme nts fr om hydrogen to Prod uci ng sup er heavy el em ents is a 0 0 II::J uranium are found natur ally on Ear th, but com plex task, but intens e beam s of ] o 0 by usi ng heavy- ion fusion reacti ons it has unstable neutron-rich nuclei w ith stable o 0 been possible to 'synthesize new ones . neutron-rich targ ets could provide the 0 ultimate pat h towards the Island of 1 Mendeleev's table 'of the elements has been exte nded in this way to element 112, Stab ili ty. Superheavy ele ments 'also pres­ and there are even cla ims up to elem ent ent a major ch allenge to quantum 116. These heavies t nuclei experience chemists, since relativistic effects could extremely large Coulom b repulsion but play a major role. Modern tec hniques are stabilized through shell effe cts. An allow studies to be perf ormed wi th very I Island of Stability is predicted at the neu­ few ato ms. Chem ical analysi s of Bohri um tron numb er N = 184 and proton number [ele me nt 1081. for exam ple, has recently Z = 114, alth ough the exact locati on of th e been performed w ith just a couple of sta bilizing shells is still und er heavy atom s. , debat e.

Measuring and predicting ·the limits of nuclear existence

Atomic mass, a fundam ent al propert y of Accurate mass measurements, together nuclei, is com pose d of the individual with precise decay character izations of a masses of the nucleons along with a con­ set of exotic nuclei, can als o be used to tribution fr om the binding energy hol din g deter mine string ent constraints on some the nucleus together. It th erefore gives a basic assumptions of the Standard Model valuable hand le on nuclear struct ure . of particle physics. Measurem ents with relative precisi on of the ord er of 10-7 are required, and can be Masses can only be calculated by cur rent achiev ed w ith both very slow beam s and theoretical mode ls with a pre cision less so ~ with relativist ic radioactiv e ion beams . than one hundredth of tha t experiment al­ ~ 650 '" In the first case, rad ioactive ions fr om a ly achieved. Their predictive power is thus .2 limited, leaving fund amental questi ons ::.. 600 low- energy ISOL facility are inj ected into e> a precision ion trap wh ere unprecedented such as the limits of nuclear stability and ~ .~ ;: UJ 550 accuracy of the order of 10-8 has been th e exact location of cert ain paths of achieved. Trap expe r iment s have also nucleosynthe sis still to be resolved. 500 ~ljH "'A measured nuclid es w it h half -lives as 450 short as 65 m il liseconds. In the relativis ­ ~ . tic case, ion s are inje cted into a storage 400 ring.

252.5 255 257.5 260 262.5 265 267,5 270 272,5 Time of flight (ns)

Doubly-magic nuclei and shell structure far from stability

Nuclei with th e magic number of 2, 8, 20, magic nuclei, in wh ich the numbers of nucle i is of top priority for pushing the 28, 50, 82, or 126 protons or neutrons are protons and neutrons are both magic, are shell mod el to its limits. particularly stable compared to their the anchor points of the shell model. Only neighb ours. This observation forms th e five doubly-magic nucle i are stable and basis for the benchmark shel l model can be found on earth, but five mo re can again st w hich all other nuclear models be synthe sized in the labo rato ry. These are com pared. In the shell model, nucle­ exotic doubl y-magic nuclei are of crucial ons orbit in distinct shells. The mag ic importance since they can give evidence I numbers cor respond to nuclei with just for the per sistence of shell str uct ure far enough protons or neu trons to com plete ­ from sta bility. Production of intense 48Ni 56Ni 78Ni ly fill a given num ber of shells. Doubly­ beams of ra dio active , doubly-magic Understanding the ol"igi n of the elements

In the different burni ng stages of stars, new into the rapi d proton-capture process has chemical elements are synthesised . Some to be yet achieved. of the nucleosynthesis paths close ly follow the valley of stab ili ty. Others, such as the To imp leme nt the complex mode ll ing of rapi d- neutron capture path lr-process}, nucleosynthesis mec han isms, react ion proce ed through unknown territory. rate s of cruc ial steps in ast rop hysical cycles and pathways have to be meas­ Nuclear reacti ons involving unstable nuclei ured , as do the decay characte ristics of Nuclear astrophysics plays a vital role in can only be measured directly with radi oac­ ma ny as yet unknown nuclei. These understanding the structure and comp osi­ tive beams. This means t hat the new essenti al inputs include the half-lives, tion of the Universe and its constituents. generation of radioac tive ion beams will masses, and main decay channels of a Stars generate energy through nuclear hel p in understanding energy generation numb er of key nuclei far from stability. As reactions betwee n nuclei - stable as well and element 'synthesis in the Universe. A these nuclei are expected to have inter­ as radioactiv e. Sometimes this burning of number of recent pioneering experiments esting nuclear struct ure properties, they nuclear fuel is slow and steady and lasts were devoted to key reacti ons in th e so­ are also a target for nuclea r physics stu d­ for billions of years , sometimes it is explo­ calle d hot CNG cycle, but the full cycle of ies. sive and lasts only seconds or mi nutes. ste ll ar reactions and a possible extension

N,Z N-1 ,Z+1 Testing the Standard Mod el

assump tions , Elaborat.e experimental and new generation of rad ioactive ion-beam theoretical effo rts in part icle physics are facilit ies. devoted to cla rifying these assum pti ons and Precise tests need the ri ght nuclei and to unifying the fundamental int eractions. precise measuri ng too ls. Radioactive ion beam s can provid e clean sources of very Precise measurements of the decay proper­ unstabl e nuclei and deliver them to preci­ ties of certain nucle i provide stringent and sion instr ume nts. Ion and ato m tr aps are com plementary tests of the Standard Model exce llent environments fo r pr ecis ion The Standard Model provides the best cur ­ and may also uncover new physics. exper iments since they are able to collect rent description of the building blocks of and manipulate a sma ll nu mber of nuclei Nature by describing the interactions of Superallowed beta-decay is one example, wi thout disturbing interacti ons wi th the quarks and leptons. Althoug h very suc­ and with stable beams it has been possible sur roundings. Ion traps , for examp le, can cessfu l, the Standard Model is not fully sat ­ to obtain precise results from lOC to 54Co. precisely measure the mass of a nucleus isfactory as it depen ds heavily on ad-hoc The extension to heavier nucle i lying around with just a few nuclei in the trap. the N=Z line wi ll become possible with the

Applications in materials and life sciences

Nuclear scien ce has br ought many applica­ neling tec hnique. Radioacti ve ion beams ti ons in other resear ch fields, as well as in are also used in com bination with th e dai ly life. In par ti cu lar, radioactive ion Positron Emi ssion Tom ography [PET] beams can act as a superior diagnostic tool techniq ue to local ize tumors precisely. in a wide range of materials rang ing from semiconductors to brain tissue . A radi oac­ tive nucle us, precise ly chosen out of th e whole nuclear chart. is implanted at th e right depth and posit ion in the sam ple unde r investigation. The radiation em itte d after decay reveals info rmation on th e immedia te environment of the imp lanted nucle us, and can be detected very effi ciently in detector - 2° 0° 2° a-Itanglearound arrays, also develo ped in nuclear physics < 110,. axis laboratories. An exam ple of such a study in solid- state researc h is the localisation of a probe atom in the crystalline structure of a host material by using the emission chan ­ First Generation Radioactive Beam Projects in Europe

l.ouvaln-Ia-Neuve, Belgium: delivering ISOL beams since 1989 http://www.cyc.ucl.ac.be/RIB.html

~ ~'-'~ '; "'>~ 'Y~ ' l " r:! j \~ . Daresbury, United Kingdom: r http://www.casim.ac.uk/

under 00 truction Caen, France: http://www.ha.physik.uni­ delivering IF beams muenchen.de/maff/ since 1984, delivering ISOL beams since 200 ~ Legnaro, It aly: http://www~ganil.fr/ I~ '", prc3j e ..,.",~ ~ ~~ p: // www. l n l.i nf n . i t/ s p e s / ' ~~:' . rA­ 'i!. ' - ~ A q ) "1\ - -' ;;'":"'- .,,' "VI.(\!, ;- . ~~~~ "" J~ • Geneva, Switzerland: delivering beams since 2001 Ca t ania, Italy: http://isolde.web.cern.ch/ISOLDE/ proje t t t p://www.lns.inf n.it/

NuPECC member countries ~:...I,LI•.alue, ISOL facilities ' green and IF in I T an

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Strengths of the IN-FLIGHT method

Projectile fragmentation and fission in-flight provide relativistic beams of exotic nuclei aLong the whole chart of nuclei. Furthermore, the separation method is universal as it is neither hindered by the chemicaL properties of the nuclide under investigation nor by its haLf-life.The In-Flight method is therefore very weLL suited to expLor­ ing the Limits of nuclear existence and obtaining exploratory information on the most exotic nuclei. Due to the kinematics of the production mechanism, the radioactive ions obtained can naturaLly be injected into separators, beam lines and storage rings. Due to unambiguous Z and A identification, it is possibLe to obtain both pure beams of a particuLar isotope and beam cocktails where neighboring nuclei can simultaneously be investigated. The high particle velocity leads naturally to the use of the exotic nuclei for reaction studies in the high-energy regime. CooLed beams can be obtained by injection into heavy-ion storage rings. This makes possible a full range of precision experiments, such as mass measurements or the study of the decay characterist ics of fuLLy­ stripped ions. Colliding beam experiments become possible when the stored radioactive ions interact with eLectrcns from an electron ring. In an electron-heavy-ion coLlider it is possibLe to scatter electrons from exotic nuclei, open ing up a compLetely new field with a high discovery potentiaL.

A bridge between the

The use of a high-temper<: m eth od is currently limited f '.l endeLeev·s table due to Low .hese isotopes, a possibLe 5 .e-53Lproduction of the In-F ce sm quaLity of the ISOL IT 5: :J opi g the relativistic rad i. in 2 [1 SOL system . OL complementary approaches for the «D

elerator ~ ., . . High t empera ure thick t arget --·.. ... pped and y e-accelerated beams (lSOL) Ion' source Megawatt Light-ion beams ms y ms to s

St ren gt hs of the ISOL y method

Due to unprecedented driver -b eam intensiti es and the use of thick target s, it Ion ooling becomes possi ble to manipulate a large meV• numb er of radioactive nuclei into beams with ion-optical qua lities (purity, emit­ y tance , intensityl sim ila r to those of sta ­ ble beams now used in acc elerator centres. The seq uence of sepa rato r, cooling and High res ~b1 10 mass separator accelera tor modu les makes it possible to obtai n rad ioact ive ion beam s with keY energies rang ing from less than one keV to several tens of MeV. y The low- energy beams can fu rthe r be injected into ion traps for prec ision stu d­ ies such as mass measurements or p­ ~ . decay studi es te sting th e Standard Charge ate breeder Mode l. few• keY The high-qual ity keV beams make it possible to produce penc il-like beams y and poi nt -like so urce s for comp lex /0 methods decay stud ies and for materials research. e thick target in the ISOL ~ The radioactive ion beams, post -accel ­ number of elements out of erated in the MeV energy domain, allow iciency or slow release. For MeV reacti on studies ranging from the ast ro­ ion is to combine the uni ­ physical regime up to high energy. Of t meth od wit h t he excellent particula r interest ar e beams of neu­ od by using a gas cell for tron-r ich nuclei since they can reach ive ions and injecting them out into territory hardly access ible wit h sta bl e beam s, for examp le the Superheavy Island . all this?

Decisive m oments for the future of the cial for conducting successful exper i­ field of Radi oactive Nuclear Beams in ments with these rare beam s are the Europe are coming : subje ct of R&D networks. Last but not - several facilities ar e operat iona l or are least, in the past decad e, Europe has reaching the commissioni ng stage ; been at th e forefront in conducti ng pilot - a num ber of new proposals are in th e experiments wit h radioa ct ive ion beams fund -raisin g phase; r <,l flgin9 from therma l energies through - several European networks addressing energ ies in th e astrophysical domain up the necessary R&D for the next genera­ to re lativistic energ ies . This all mea ns tion of Radioactive Nuclear Bea m s are that wor king on the next" generation of gai ning mo me nt um. radi oact ive ion bea m s doe s not require a leap wi th research gro ups w ithin -Euro pe Although Euro pe has taken th e lead in so lely orient ing th eir effo rts to the proj ­ th is field, strong competit ion is expected ect. Rath er, it consists of co-ordinating in the near future from North America on-going efforts in m any places.The and Japan. Eur ope has the great advan­ radioactive ion beam comm unity is well tage that many lab oratories have devote d equipped to do this, with many R&D proj ­ thei r efforts to th e pr oducti on of radioac­ ects already obta ining European Uni on tiye ion beam s ' by both the In - Flight funding . method and the ISOL technique. The w hole arsenal of different methods of The training of young resea rchers in the producing radioactive io n beams has fie ld of rad ioact ive ion beams is ensured been pioneer ed in Europe and vigorous by organisi ng the yearly " EUROSUMMER R& D pro gra mmes are bein g pu rsu ed. SCHOOL ON EXOTIC BEAMS", funded by Moreover, the detection techn iqu es cru­ th e EU pr ogramme " High- leve l Scie nt ific Confe re nces" ations and adopted strategy

Nu PECC recognises the cru cial position European research infrastructu res. of ra dioactive ion beam s in nuclear Furthe rm ore , vigorous R&D pro­ physi cs research and endorses the stra t­ grammes, focussing on basic prob lems, egy to aim for two complementa ry faci li­ should be pursued with in a European ties. One facility should be base d on the conte xt. The syne rgy with ot her areas of ISOL scheme and have bea m intensities research shou ld be fully explored to 1000 times higher tha n t he facilities define common R&D goals, and perhaps presently running or at the com mission­ to ar rive at infrastructure sharing. in g stage. The other facility should be This strategy wo uld lead to the constr uc­ based on the IF technique and the pri­ tion of the m ost powerful nuclear physics m ar y beam accelerator sh ould provide research complex, read y at the start of 1012 particles per second at 1 GeV/a m u the next deca de, to force a substa ntial up to Urani um. The w ay to reali se th ese breakthro ugh in our und er standin g of the ambitious goals is to support the prese nt ato mic nucleus and to provid e vita l infor­ radioactive ion beam fac ilities in Europe ma ti on for rel ated fields su ch as astro­ as they wi ll provide the necessary expert­ physics and particle physics. These faci l­ ise in both exper imental techn iqu es and iti es would not only consolidate the pr i­ physics understandi ng. These facilities, mary position of Europe in th is research along with othe r cen tres of nuclear fi eld but al so ens ure knowledge flow to phy sics re search, will provide vitally applied fields generating many ap pl ica ­ important train ing of young researchers tions and new tec hnologies. who will be the future users of these new ~with Other Projects ,------,

Ttie effort needed to realize the next generation of radioactive ion beams is so extensive that not only will the whole European nuclear structure community be mobilize d, but there will also be a need to share ideas and developments with other area s of science and technol­ ogy. From the design goals of the ISOL-based facility, it is clear that a new generation of high-power proton accelerators (HPPA), potentially capable of producing beams of several MegaWatts should be developed. Such an HPPA also forms the heart of a new scheme for the transmutation of nuclear waste and for the production of energy [Accelerator Driven Systems}, for Neutron Spallation Sources used in materials research, for intense material irradiation tools, and for producing intense neutrino sources. The technical requirements set by the IF facility can only be met with new technological solutions such as the development of rapidly cycling, superconducting magnets and beam cooling for ions over a broad energy range. All this requires a boost of ------, more than an order of magnitude in the performance of 'j , ':; I II ~ . present-day accelerators and therefore common R&D 1 1II I I I : I T .. Pw efforts are crucial for the future of all these differ­ ent fields.

Further Information

NuPECC, the Nuclear Physics European Collabo­ This brochure has been written by a NuPECC wor k­ ration Committee is an Expert Committee of the ing group consisting of Daniel Guerreau [IN2P3, European Science Foundation [ESF]. Paris, France l. Bjorn Jonson [Chalmers University The obj ective of NuPECC is to strengthen European of Technology, Goteborq, Sweden) and John Durell Collaboration in nuclear science through the promo­ [The University of Man chester, Manchester, United ti on of nuclear physics and its trans-disciplinary use Kingdom) chaired by Mark Huyse [K.U.Leuven, and app licat ion in collaborative ventu res between Leuven, Belg ium]. Many thanks to James Gillies research groups within Europe and particularly and Fabienne Marcastel [CERN, Sw itzer landl. th ose from countries linked to th e ESF. More information can be found at the web site: Further reading: http ://www.nupecc.org • NuPECC Report, December 1997, " Nucl ear Physi cs in Europe: Highlights and Opportunities" • NuPECC Report, Apri l 2000, " Radioactive Nuclear Chairperson: Prof. Juha Aysto, Beam Facilities" CERN, PPE/ISOLDE, 1211 Geneva 23,Switzerland • Project Report "Frontiers in Nuclear Physics", " Phone : [41] 22 767 5825, [3581 505649011 European Communities 2000, ISBN 92-894-0026-9 Fax: [41] 227678990 Copies are available from the NuPECC Scientific emai l: juha.aystofdcern.ch Secretariat.

Scientific Secretariat: Dr. Gabrie le-Elis abeth Kor ner Photos: NuP ECC clo Physik-Department E12,Technische We would lik e to th ank the followi ng people and Universitat Miinchen. 85748 Garching, Germany institutes for the use of photographic material: Phone : (49) 89 2891 - 2293, 1491 172 89 15 011 GSI, Darmstadt IG. Otto and A. Zschaul : CERN, Fax: (49) 89 2891 - 2297 Geneva; LMU Miinchen. CRS4, Italy; J. Hughes, email: sissy.koe rn er0phys ik.tu-muenchen .de Rutgers; Steve Mandl, Hidden Valley Observatory; NASA.