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... ,­- ", ;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.