FEATURES
Questfor superheavynuclei
2 P.H.Heenenl and W Nazarewicz -4 IServicede PhysiqueNucleaireTheorique, U.L.B.-C.P.229,B-1050 Brussels,Belgium 2Department ofPhysics,UniversityofTennessee,Knoxville, Tennessee37996 3PhysicsDivision, Oak Ridge National Laboratory,Oak Ridge, Tennessee37831 4Institute ofTheoreticalPhysics,UniversityofWarsaw,ul. Ho\.za 69, PL-OO-681Warsaw,Poland
he discovery of new superheavy nuclei has brought much The superheavy elements mark the limit of nuclear mass and T excitement to the atomic and nuclear physics communities. charge; they inhabit the upper right corner of the nuclear land Hopes of finding regions of long-lived superheavy nuclei, pre scape, but the borderlines of their territory are unknown. The dicted in the early 1960s, have reemerged. Why is this search so stability ofthe superheavy elements has been a longstanding fun important and what newknowledge can it bring? damental question in nuclear science. How can they survive the Not every combination of neutrons and protons makes a sta huge electrostatic repulsion? What are their properties? How ble nucleus. Our Earth is home to 81 stable elements, including large is the region of superheavy elements? We do not know yet slightly fewer than 300 stable nuclei. Other nuclei found in all the answers to these questions. This short article presents the nature, although bound to the emission of protons and neutrons, current status of research in this field. are radioactive. That is, they eventually capture or emit electrons and positrons, alpha particles, or undergo spontaneous fission. Historical Background Each unstable isotope is characterized by its half-life (T1/2)- the The quest for superheavy nuclei began in the 1940s with the syn time it takes for half of the sample to decay. Isotope half-lives thesis of atomic nuclei with a number of protons greater than range from less than a thousandth ofa second to billions ofyears. uranium (Z=92). In 1940, neptunium (Z=93) and plutonium Many radioactive nuclei may have half-lives comparable to or (Z=94) were discovered. This was followed by the synthesis of longer than the age of the Earth (4.5 billion years). Examples are americium (Z=95) and curium (Z=96) in 1944, berkelium 1o 8 the actinide nuclei 232Th (TI/2=1·10 years), 23SU (TI/2=7·10 (Z=97) in 1949, californium (Z=98), einsteinium (Z=99) and fer 9 years), or 239U(TI/2=4·10 years). Short-lived isotopes cannot be mium (Z=100) in 1952, and mendelevium (Z=lOl) in 1955. All found naturally on Earth because they have long decayed since these elements were produced by intense neutron irradiations or our planet was formed. Yet, thousands of short-lived isotopes are by proton, deuteron, or helium (alpha particle) bombardment in continually created in the cosmos. Their existence may be fleet a cyclotron. Some ofthese isotopes have been producedin sizable ing, but they play a crucial role in the ongoing formation of the amounts. Einsteinium and fermium were discovered in the elements in the Universe. The properties of most rare isotopes are debris from the thermonuclear explosion conducted at Eniwetok unknown and can be only inferred,with considerable uncertain Atoll in the Pacific Ocean. Amazingly, the fermium nucleus was ty, from theoretical calculations. made through the capture of 17 neutrons by 23BUfollowed by the Figure 1 shows bound nuclear systems as a function of their subsequent beta decays. In nature, a similar process (the so-called proton number Z (vertical axis) and their neutron number N r-process, believed to be taking place in supernova) is responsible (horizontal axis). The black squares represent the stable or very for the synthesis of heavy elements. long-lived nuclei. They form the so-called "valley of stability". Still heavier elements (transuraniums) were produced in The yellow region indicates shorter-lived nuclei that have been heavy-ion accelerators by fusing heavy actinide targets (plutoni produced and studied in laboratories. By adding either protons um-californium) with light ions of carbon (nobelium, Z=102, or neutrons, one moves away from the valley of stability, finally 1958; ruthefordium, Z=104, 1969), boron (lawrencium, Z=103, reaching the drip lines where nuclear binding ends because the 1961), neon (dubnium, Z=105, 1967), and oxygen (seaborgium, forces between neutrons and protons are no longer strong Z=106, 1974). In order to go heavier, to compensate the decrease enough to hold these particles together. of the proton-to-neutron ratio with mass, fusion of nuclei with the largest possible surplus ofneutrons had to be used. In the 70s, Nuclear Landscape "cold fusion" reactions involving medium-mass projectiles with Z ~24and lead or bismuth targets were introduced and replaced "hot fusion" reaction with actinide targets. This enabled the dis stable nuclei covery of bohrium (Z=107, 1976), hassium (Z=108, 1984), 82· ~ meitnerium (Z=109, 1982), ununnilium (Z=1l0, 1994), unununium (Z=1l1, 1994), and ununbium (Z=1l2, 1996). The known nuclei .3 - ~-\\ r# . last three elements are still unnamed. So far, they carry tempo -t••• rary names derived directly from the atomic number according Q~ 50 \ to the systematic nomenclature of the International Union of e Pure and Applied Chemistry (IUPAC). 10.. Most of the heaviest elements were found in three"heavy ele unknown nuclei ment factories": Lawrence Berkeley Laboratory in Berkeley (USA), Joint Institute for Nuclear Research in Dubna (Russia), and Gesellschaft fur Schwerionenforschung (GSI) near Darm neutrons Fig. 1: Nuclearlandscape stadt (Germany). After much discussion about the priority for 1 8 their synthesis, IUPAC 1997 has accepted names up to Z=109. At
europhysics news JANUARY/FEBRUARY2002 5 Article available at http://www.europhysicsnews.org or http://dx.doi.org/10.1051/epn:2002102 FEATURES I~. ~~- ~ • .4 -r - --'t "- ...-(.=-...... -l ...... =------_ = _: that time, nuclei with Z=110, 111, and 112, discovered at GSI, were already known. The lifetimes of the heaviest elements were found to be o Pb, Si - Targets very short. For instance, element if' U, Pu, Cm - Targets Z=112, investigated using the reac tion 70Zn + 208Pb...... 277Uub + In, turned out to have a half life of only Fig. 2: Crosssectionsfor 10-~ about 280 JJsec. The corresponding C the productionofthe ~ heaviestelements.The fusion cross section is extremely '0-10 small, 1 pb. Since the production ""- resultsofrecenthot fusionexperimentswith cross section was found to be rapidly I:) '0- 11 decreasing with the atomic number, j------jhot fu ion the 48Cabeamand actinidetargetscarried see Fig. 2, it was then concluded that :.....t8ea ! out inDubnaarealso itwouldbe very difficult to reach still !l . showninthe box. heavier elements. " ._------',(FromRef.[1] and As far as theory is concerned, Yu.T.Oganessian,private major advances were made in 1966, '0-14 r-'-',---r--,----,-,---r--,-----,-.,--....--,----r-,--..,---,--! communication.) when the first realistic theoretical 102 104 106 108 110 112 114 116 calculations for the superheavy Flement nlJmbp.r nuclei were carried out. These early calculations were' based on the shell correction (or: macroscopic-microscopic) method in which the New discoveries total binding energy was decomposed into a smooth part The last three years (1999-2001) have brought a number of (approximated by a macroscopic liquid drop model) and a experimental surprises which have trulyrejuvenated the field [1]. microscopic quantal shell correction term strongly oscillating In January 1999, scientists from Dubna reported the synthesis of with the number of nucleons and reflecting the energy stabiliza one atom of element 114
Chart of the Nuclides 2001
event #5: May 8, 2001 I=-j 291116 .....- 1O.54MeV 4n 55m. /n El 114 ~ -, ..- r 9.80MeV 113113 IM7, );:--I . , m:ff ._---!- :;78 176 ell . , 'U'.N '12112 i __. ! 9.1J M.V "'m 111:m: : 152.62. 111 111 'Urn.: I
lUilP 1H1'"'110 t10 ~10Ut": OJ'~IIl.j\OO,:1.1 51 ----JP I.l'~ s 1 : a, ~:0<,.1li C '10110 , ,I ~:IIlI'm.mtj ----'IIttW 7~~ 1,'. 172 Fig. 3: Theupperend ofthe nuclearchart inthe regionofthe heaviestelements.Thea chain associatedwiththe elementmUuh,recentlydiscovered 161 la 1113 , ... (May8,2001) inDubnainthe 248Cm(48Ca,4n)reaction,is showninthe upperleftcorner.(FromRef.[1]andYu.T. 1511 180 Oganessian,privatecommunication.)
6 europhysics news JANUARY/FEBRUARY2002 FEATURES came as a great surprise that hot fusion reactions with a 48Ca slightly different spin-orbit interaction. The experimental deter beam couldproduce a significant number ofevents. Another sur mination of superheavy nuclei from this region will thus be of prising (and exciting) result of the Dubna experiments was that extreme importance for pinning down the fundamental question nuclei which were produced have fairly long lifetimes. For ofthe spin-orbit force. instance, the lifetimes reported for the elements 284Uub and Does it actually make sense to talk about "magic superheavy 280Uunare many orders of magnitude longer than those of the nuclei"? A recent theoretical work [3] sheds a new light on this isotopes with Z ~112 previously discovered at GSI. question. According to calculations, the patterns of single-parti Progress has also been made in the region previouslyexplored cle levels are significantly modified in the superheavy elements. by GSI. The GSI group found a new even-even isotope ofZ70Uun, Firstly; the overall level density grows with mass number A, as ex and also mUuu and 271Uub.A group led by scientists from the A113.Secondly, no pronounced and uniquely preferred energy Paul Scherrer Institute (PSI) observed the Z69,Z70Hs decay chains gaps appear in the spectrum. This shows that shell closures which at GSI, thus convincingly confirming the data on z77Uub for are to be associated with large gaps in the spectrum are not which z69Hs is a member of the decay chain. New isotopes of robust in superheavy nuclei. Indeed, the theory predicts that z66,Z67Bh have been found at Berkeley. As far as chemistry is con beyond Z =82 and N =126 the usual localization of shell effects cerned, a tremendous achievement and a true tour-de-force were at magic numbers is basically gone. Instead the theory predicts the first chemical studies of seaborgium, bohrium, and hassium fairly wide areas of large shell stabilization without magic gaps. (see below). Figure 3 shows the upper end.ofthe nuclear chart as This is good news for experimentalists: there is a good chance to of2001. reach shell-stabilized superheavy nuclei using a range of beam Physics at the limit of cross sections is difficult and challeng target combinations. ing. The synthesis of the new element Z=118 (Z93UUO), Some of the observed a-decay chains attributed to elements announced in 1999 bythe Berkeley group, has been retracted two 112 and 114 are believed to linkodd-Nnuclei. The presence ofan years later. The history of heavy element research remembers odd neutron adds to a theoretical uncertainty. Since the single other cases ofelements discovered and gone, and sometimes dis particle level density of transactinides is large, there are many covered again. A team working in Stockholm at the Nobel candidates for low-lying one-quasiparticle states in parent and Institute of Physics reported in 1957 an isotope with atomic daughter nuclei. Moreover, manyalpha transitions are structural number 102. The group proposed the name Nobelium in honor ly forbidden since the properties of one-quasiparticle excitations to Alfred Nobel. In 1958 a group at Berkeley reported that they in parent and daughter nuclei often differ dramatically. As an were unable to reproduce this work, findings agreed bya Russian example, Fig. 4 shows the predicted a-chains for the nucleus group at Dubna. But the name stuck... 271 112 found at GSI. The Hartree-Fock-Bogoliubov calculations explain two observed a branches, attributed experimentally to Structure of the 5uperheavy nuclei z73110, in terms of transitions between negative parity and posi The structure of superheavy elements results from the interplay tive paritysingle-neutron orbitals. between the strong and electromagnetic forces. Unlike in lighter nuclei, the Coulomb interaction cannot be treated as a small per turbation atop the dominating nuclear interaction; it does a decay of 277112165 112' (611] influence significantly proton and neutron distributions[2]. 512' 161J1 312'f611J == The stability of heavy and superheavy elements has been a 1312' 716 mU2 160 longstanding fundamental question in nuclear science. Theoreti Experiment: 208Pb+'OZn ~:{ml cally, it has been shown that the mere existence of nuclei with Theory: HFB+SLy4 1312' 1716] = 0..=11.9MeV Z > 102 is entirely due to quantal shell effects. Indeed, in the clas 3121:[611"'110 1"1>:n.7 MeV 112' 16201 163 11.9MeV sicalliquid drop picture, the shape ofa nucleus is governed byan 3/z+ 6ZZJ __ Q..=I J.I M.V interplay between surface tension (proportional to N/3,trying to 7/Z+ [6131 -= 10.3MeV 9/Z+~16151_ Exp: ll.3M<\' make the nucleus spherical) and Coulomb repulsion (propor lI!Z'I72S] ""108'61 9.9 M.V tional to ZZIA1I3,trying to make the nucleus deformed). At large 1fi'162Oj Q.,=9.1 MeV values of ZZIA, the Coulomb force is so strong that the nuclear ~n:tm_ Exp: 9.4 MeV 712+[613 liquid drop becomes unstable to surface distortions and it fis 1112'[725 "'106,5'+ sions spontaneously. It is the stabilization brought by quantal Qa=8.7M.V 112' 1620] 8.3 MeV shell effects that is responsible for the superheavies. 1112'[725) = Exp: 8.9 M europhysics news JANUARY/FEBRUARY2002 7 FEATURES Chemical properties of superheavy elements First predictions of the chemistry of superheavyelements were Since atomic relativistic effects scale approximately with Z2, made in 1971, and the first relativistic molecular calculations for chemical properties of transactinides cannot be properly systems involving transactinide elementswere published in 1977. described by non-relativistic quantum mechanics. However, Since then, calculations based on the relativistic quantum theory experimental tests of relativistic effects and of anticipated devia have been considerably improved and applied to many atoms tions from the periodic table of the elements in the superheavy and molecules [5]. Relativistic effects intransactinides are crucial region are extremely difficult because the single atom chemistry and show up in several ways. Most importantly, 5 andP1l2atomic requires half-lives of the order of one second, i.e., much longer orbitals contract relativistically. The shrinking of the inner shells than those ofthe heaviest elements found so far. Another obstacle results in an increased screening of the nuclear charge, and this is very small production rates: a few events per day for hassium gives rise to an expansion of the P3/2 and of higher angular and less for heavier elements. In spite of these difficulties, there momentum orbitals. Another relativistic effect is a change in the has been major progress in the chemical characterization of spin-orbit coupling. Both can produce drastic rearrangements of transactinides. Special techniques, such as gas phase separation, orbital levels. That is what is predicted to happen for element 112. have been devised to perform one atom-at-a-time chemistryand Recent calculations indicate that the 75 orbital should be shifted to deduce thermodynamical properties of new elements [4]. below the 6ds/2 orbital due to relativistic effects. It is the large rel In 1993, chemical studies of dubnium demonstrated that it ativistic stabilization of its valence 75 orbital, combined with its does not behave like tantalum, the element from the group to closed shell electron configuration, that has led to the prediction which dubnium was supposed to belong. On the contrary, the fol that element 112 is chemically inert. lowing chemical studies of Sg (year 1997), Bh (2000), and Hs Chemistryof the superheavyelementswith Z > 118 is believed (2001) confirmed relativistic calculations predicting their behav to show relativistic effects that are so large that comparison with ior to coincide with that expected on the basis of their positions lighter elements or nonrelativistic results is meaningless. in the periodic table. Hassium, for instance, forms a gaseous oxide similar to that of osmium, which places it in group 8. As of Borders of the superheavy region today, the periodic table has been experimentally established up Where are the borders of the superheavy region? What are the to element 108. However, preparations cue under way to carry out properties of the heaviest nuclei that can be bound (at least, in gas chemistry of element 112, which instead of behaving like its theory), in spite of the huge disruptive Coulomb force? lighter transition metal homologs (Zn, Cd, Hg) is expected to As a consequence of their saturation properties, nuclear forces have properties of a noble gas like Rn or Xe; a first attempt to favor values of the internal density close to the saturation density chemically identify element Z=112 has recently been carried out ofnuclear matter. On the contrary, since the Coulomb interaction in Dubna. Still heavier elements, predicted to be noble volatile tends to increase the average distance between protons, the metals, are also excellent candidates for gas chemical studies, pro Coulomb energy is significantly lowered by either the creation of vided that their half-lives are sufficiently long. In particular, there a central depression or by deformation, or byboth. Based on this is hope of employing chemical methods for the firm identifica general argument, one expects the formation of voids in heavy tion of the alpha-chains oflong-lived isotopes of Z=114 and 116 nuclei. Recently, the subject of exotic (bubble,toroidal, band-like) observed in Dubna. configurations in nuclei withverylarge atomic numbers has been revisited by self-consistent calculations. Box 1: The nucleus is a complicated Since the main decay mode for the many-body system. The strong forces known heaviest elements is a decay, one 20 describing heavy nuclei are strongly of the crucial quantities determining affected by the in-medium effects; lifetimes ofthe heaviest and superheavy 124 ( they are usually approximated by elements is the a-decay energy. The Qa 18 1 1L~ . phenomenological effective density values for the heaviest elements calcu ·126• ,1 ?R .1J'1" dependent interactions fitted to a few lated with the self-consistent model J22 1I H .. key nuclear properties. This strategy, with the SLy4 effective interaction are 11G PO K well known in condensed matter displayed in Fig. 5. They are compared 114' .\" physics, turned out to be extremely with known a-decay energies. It is seen JJ2, successful. The self-consistent density that the agreement with experimental functional theories (relativistic and data(includingtherecentdatashownin non-relativistic) describe bulk nuclear Fig. 3) is very good, and this suggests properties such as masses, radii, shapes, that calculations such as those shown as well as nuclear excitations.A number on Fig. 5 canbe used to predict proper of effective interactions have been ties ofunknown superheavy nuclei. introduced and their capability to describe properties of exotic Fig. 5: Qa valuesfor even-evennucleiwith radioactive nuclei is one of the major 96 s Zs 118obtained inthe self-consistent challenges in present-day nuclear calculations.Theyare comparedto structure. experimentaldata (closedsymbols) Any theoretical model aiming at includingthe recent data forZ=114and 134 144 154 164 174 184 makingpredictions in an unknown ter 116.(Takenf(omS.(wiok et al.,Phys.Rev. ritory should first be tested in known Lett.83(1999)1108,and to be published.) Neutron Number regions of the chart of the nuclides. 8 europhysics news JANUARY/FEBRUARY2002 FEATURES Box 2: In spite ofthe fact that the number of protons and (201 ----T----- (2841) nucleons in the nucleus is rather small, and nucleonic I Spinning the heaviest elements I'" velocities are fast, atomic nuclei exhibit collective modes I suchas rotations andvibrations. Manynucleiemitphotons 2!!.18-+_-+ __ 2342 characteristic ofthe sequential de-excitation of rotational ------_ .._--- 4562 0.28 bands. In quantum mechanics, collective rotation canonly 130 " 014 "'-'--+--'886 arise for systems that have a deformed shape, Le., 124 ~133 nonspherical distribution of the nucleonic density. It has 010 118 .!'-14+_-+ __ ,,,, been only recently that rotational photons could be 0.16 N 112 3666(7) detected iJispectroscopic studies of the transactinides. 0.:2 ';.0..2+_-4- __ 1106 First results on the collective properties of nobelium 106 O.ll8 318ol1(4) isotopes 252.254Nohave been obtained at Argonne (USA) 100 ',,-0'_--1- "0 0.04 __ and Jyvaskyla (Finland). It hasbeenfound thatthe ground "7.3(3. 0.00 state rotional band of 254No,the best deformed nucleus 2...··--m--- S1• 21.. 1 (2) above 208Pb,resists fission up to a spin at least 20 1i(see ~2 6' 305 of N 4+ 11 1159.1 (3) _+_~-::- 146 Fig. 6). Detailedmicroscopic calculations haveshownthat ! ==~~p------«thefission barrier ofthis nucleus changes veryweaklywith 254 angular momentum,thus explainingthis amazing stability 102 No152 ofthe nobeIiums as a combinedeffect ofthe Coulomb and Coriolis-plus-centrifugal forces. Fig. 6: Experimentalconfirmationof the roleof deformation in transactinides was recently obtained for 254No.Thefigure (right portion) shows a cascade of transitions characteristic of the rotation of a deformed nucleus.Fromthe preciseenergy differencebetween successive states, it is inferredthat 254Nohas a football-likeshape with an axisratio of 4:3,in agreement with theory.Thefact that states with angular momenta as high as 201iwere detected underscoresthe remarkable resilienceof the shell effects against the centrifugalforce and fission.Modern nuclear structure calculations- such as the one labeled HF+Sly4(left)- tell that stability comes in specificcases fromthe ability ofthe nucleus to deform.Here,the deformation parameter fhcharacterizesthe elongation ofthe nuclear shape (for(32=0nucleus isspherical). The calculations predict the competition between several of describe heavy-ion fusion are not reliable when extrapolating these forms for the central nuclear density due to the huge elec outside the regions of known nuclei. The situation is not better tric charge. It is difficult to say at present whether these exotic for the description of static fission barriers and of dynamics of topologies can occur as metastable states. Will the spherical bub fission. Slightly better is the situation in the theoretical modeling ble minimum undergo the shape transition to the "band-like" of nuclear structure of transactinides, where the microscopic configuration? What is the stability of all these shapes to reflec self-consistent models can make quantitative predictions. Here, tion-asymmetric and triaxial deformations? It is possible that the main problem is how to extrapolate density-dependent effec there exist isolated islands of nuclear stability, associated with tive interactions to the superheavy "terra incognita" and how to very exotic topologies of nuclear density, stabilized by shell describe exotic forms of nuclear existence created thanks to the effects. huge Coulomb force and the borders of the superheavy region. The advances in computer technology and in numerical algo Perspectives rithms will certainly be crucial for the success ofthis task. The heavy element research has been traditionally concentrated in three laboratories: Berkeley, Dubna, and GSI. But the exciting This research was supported in part by the US. Department of new discoveries brought new players to the "superheavy" com EnergyunderContractNos. DE-FG02-96ER40963 (University of munity: RlKEN (Japan) and GANIL (France). Chemical studies Tennessee), DE-AC05-000R22725 with UT-Battelle, LLC (Oak of transactinides are also being carried out at PSI and JAERl Ridge National Laboratory), the Polish Committee for Scientific (Japan). Research (KBN) under Contract No. 2 P03B 040 14, and NATO One of the most important and urgent tasks facing experi grant PST.CLG.977613. mentalists will be to confirm the new elements found in Dubna. This will not be easy, since these nuclei form an isolated island, References disconnected from the known region of the nuclear chart. In [1] S. Hofmann and G. Miinzenberg, Rev.Mod. Phys. 72 (2000) 733. order to identify the elements beyond Z=112, new tools, such as direct mass measurements or chemical studies must be devel [2] S. Cwioketal.•Nucl.Phys.A611 (1996) 211. oped. How to reach even heavier nuclei? The new-generation [3) M. Bender, W.Nazarewicz, and P.G.Reinhard, Phys. Lett. B515 high-current stable-beam accelerators will certainly be helpful in (2001) 42. making new discoveries at a picobarn level. However, in order to [4] J.v.Kratz, in Heavy Elements and RelatedNew Phenomena, Eds.W. explore new, more neutron-rich superheavy regions, accelerated Greiner and R.K. Gupta (World Scientific, Singapore, 1999) p. 129. beams ofradioactive neutron-rich nuclei will have to be utilized. [5] P.Schwerdtfeger and M. Seth, RelativisticEffectsofthe Superheavy It is hoped that the new high-powered ISOL facilities will enable Elements,Encyclopedia ofComputational Chemistry; Vo!'4 (Wiley. us to explore the theoretically crucial region of nuclei between New York,1998). p. 2480;V.Pershina et al. in Heavy Elements and the deformed N=162 shell and the spherical N=184 shell. Related New Phenomena,Eds. W.Greiner and R.K. Gupta (World There are also many challenges facing nuclear theory ofsuper Scientific, Singapore, 1999) p. 194. heavies. The currently highly phenomenological models used to europhysics news JANUARY/FEBRUARY 2002 9