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A beachhead on the Yuri Ts. Oganessian, and Krzysztof P. Rykaczewski

Citation: Physics Today 68, 8, 32 (2015); doi: 10.1063/PT.3.2880 View online: https://doi.org/10.1063/PT.3.2880 View Table of Contents: https://physicstoday.scitation.org/toc/pto/68/8 Published by the American Institute of Physics

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The image of scientists in The Theory Physics Today 70, 40 (2017); https://doi.org/10.1063/PT.3.3427 A beachhead on the island of stability

Yuri Ts. Oganessian and Krzysztof P. Rykaczewski

Recent experiments to synthesize the heaviest elements have dramatically expanded the and the Segrè chart of . emember learning the periodic table of el- Despite much recent progress, scientists are ements in high school? Our chemistry still learning how to consistently describe the struc- teachers explained that the chemical prop- ture and properties of and nuclei. Experimen- R erties of elements come from the electronic tal studies of the heaviest elements and nuclei con- shell structure of atoms. Our physics tinue to yield new data that challenge and inform teachers enriched that picture of the atomic world theoretical models of atomic and nuclear structures. by introducing us to and the Segrè chart of The overarching questions are far from trivial. nuclides, which arranges them by number Z Where is the end of the periodic table? What is the and number N. heaviest nucleus? How do properties evolve for ex- Nuclear binding energies and nuclear shell treme numbers of , , and ? structures provide organizing principles for under- How do relativistic electrons in outer atomic orbits standing the properties of nuclei and the directions influence the size and chemical behavior of the heav- of . Protons and neutrons occupy a iest atoms? Do superheavy elements (SHEs), say range of nuclear states with unevenly spread ener- those beyond Z = 100, follow the well-established gies. Groups of those states with relatively close en- group structure of the periodic table, or do they ex- ergies form so-called shells separated by large energy hibit unexpected chemical and nuclear properties? gaps, analogous to shells in atomic physics. In the 1960s, developments by Vilen Strutinsky1 At certain numbers of protons and neutrons, and others in the understanding of the nuclear struc- called magic numbers, nuclear shell closures occur, ture of lighter nuclei led to predictions by Adam and consequently, a large amount of energy is Sobiczewski and his collaborators on new nuclear needed to excite a to the distant higher shell closures.2 At about the same time, two groups— level. Closed nuclear shells enhance nuclear stabil- William Myers and Władysław Świątecki, and ity. (See the article by David Dean, PHYSICS TODAY, Victor Viola Jr and Glenn Seaborg—independently November 2007, page 48.) predicted the existence of heavy nuclei that would occupy a so-called island of stability on the Segrè Yuri Ts. Oganessian is the scientific leader of the Flerov chart.3 Since that time, the concept of an island of Laboratory of Nuclear Reactions at the Joint Institute for stability has dominated the physics of superheavy Nuclear Research in , Russia. Krzysztof P. Rykaczewski nuclei. is a senior researcher at Oak Ridge National Laboratory in Figure 1 shows the grand nuclear landscape as Oak Ridge, Tennessee. it is understood today. A handful of magic numbers 32 August 2015 Physics Today www.physicstoday.org Figure 1. The grand nuclear landscape. Nuclei that have been experimentally identified are inside the yellow region, whereas nuclei only predicted to exist are roughly indicated by the green area. Black squares mark stable isotopes. Magic proton and neutron numbers, at which nuclei have enhanced stability, are indicated by red lines. The star labeled SHE indicates the region of superheavy elements. (Courtesy of Witold Nazarewicz).

2015, page 40), researchers have artificially created 26 new elements and hundreds of isotopes, all by using nuclear reactions to modify the properties of WINSLOW HOMER existing nuclei. Attempts to change the nuclear properties of materials are not new. Historically, the have been well established: 2, 8, 20, 28, 50, and 126 hope of creating precious like out of (neutrons only). Modern theoretical approaches, more common ones like tin or drove such al- supported by new experimental data, also point to chemical ambitions. the existence of long-lived nuclei at and around the The dreams of medieval alchemists have nearly magic N = 184. Presently, 177 is the come true in the modern era of accelerators and nu- largest experimentally observed neutron number4,5— clear reactions.7 For example, one can fuse two 294 293 74 in isotopes 117 and -293 (116 Lv). metallic atoms, a germanium-74 ( 32Ge) projectile 124 74 Although there is no consensus on the next with a tin-124 ( 50Sn) target. When the 32Ge has a magic Z, theoretical predictions kinetic energy of 300 MeV, which corresponds to 198 * place it between 114 and 126. In fact, in the vicinity about 9% of the speed of , a 82Pb nucleus is of N = 184 and Z = 114 to 126, with very high neu- created in the . 198 * tron and proton numbers and large densities of That initial 82Pb nucleus has a neutron and proton states, the stability of nuclei equal to the sum of the Ge projectile’s and the Sn tar- might be enhanced without sizeable energy gaps.6 get’s mass numbers. The asterisk indicates that the Current experimental results are consistent nucleus—which is created “hot” with an excitation with the existence of an extended island of super- energy of about 50 MeV, corresponding to a temper- heavy nuclei that are more resistant to radioactive ature of more than 1010 K—is a compound nucleus, decay and have much longer half-lives than some- one that is not fully bound. It promptly evaporates what lighter isotopes, as shown in figure 2. Indeed, several neutrons to cool down, and different Pb iso- some models predict half-lives up to a million years topes, called fusion-evaporation residues, are cre- for new superheavy nuclei, and some even calculate ated in the process. 198 * them at around Earth’s age. In the 82Pb example, the evaporation of four 194 Extensive chemistry studies have searched for neutrons to produce 82Pb accounts for about 60% traces of SHEs in geothermal waters, rare ores, and of the evaporation residues. Within a few tens of 194 cosmic rays. However, to date all known elements minutes, transforms the 82Pb into thal- 194 beyond (Z = 92) have been manmade first, lium-194 ( 81Tl). Then a second beta decay turns 194 194 in laboratories using large-scale devices like nuclear 81Tl into mercury-194 ( 80Hg), a nucleus with a reactors and particle accelerators or in the violent 520-year half-life. The nuclear alchemist seeking Au conditions of nuclear explosions. has to wait quite some time for the next beta decay 194 194 into 79Au. Unfortunately, 79Au is an unstable iso- Fusion evaporation tope, with a half-life of only 38 hours. 194 Since the discovery of in 1938 (see the The beta decay of 79Au does create stable and 194 article by Michael Pearson, PHYSICS TODAY, June even more precious platinum-194 ( 78Pt), but at www.physicstoday.org August 2015 Physics Today 33 Island of stability

104 for each channel, 1 h drops exponentially with increasing projectile 103 atomic number. N = 162111 N = 184 107 The last and most challenging cold-fusion ex- 2 10 209 70 FE (s) 113 1 min periment, which used a Bi target and Zn projec- N = 152 83 30 101 tiles, ran for nine years, from 2003 to 2012, at the 109 115 RIKEN research institute in Japan, with nearly 600 100 1 s days of beam on target. That tour-de-force effort re-

Y HALF-LI sulted in the observation8 of three decay chains of −1 117 10 the 278113. The deduced cross section for the production of 278113 via cold-fusion reaction is ex- 10−2 tremely small, about 20 fb (1 fb = 10−39 cm2), which is 10−3 1 ms less than 1/1012 of the total reaction cross section.

ALPHA-DECA In the late 1990s, researchers at the Joint Insti- 10−4 tute for Nuclear Research (JINR) in Russia success- fully applied a new method to create the heaviest el- 10−5 ements and nuclei to date.5 Instead of stable target 150 155 160 165 170 175 180 185 208 209 NEUTRON NUMBER N nuclei, like 82Pb and 83Bi, radioactive ma- 238 249 terials from 92U to -249 ( 98Cf) were Figure 2. The dependence of half-lives on neutron number. used as targets and irradiated with calcium-48 48 Solid and open symbols correspond to nuclei with odd and even numbers (20Ca) ions, which have both magic proton (Z = 20) of neutrons, respectively. The symbols indicate atomic numbers— and neutron (N = 28) numbers. Only about 0.2% of 48 pentagons for 107, triangles for 109, upside-down triangles for 111, naturally occurring Ca is 20Ca, and substantial en- for 113, circles for 115, and squares for 117. The dashed lines richment is required to obtain a few grams of the are guides to the eye and labeled with corresponding atomic numbers. isotope for performing long experiments. Nuclei synthesized in hot-fusion reactions with calcium-48 projectiles are Improvements in ion-source technology en- shown in orange; in cold-fusion reactions, blue; and in other reactions, abled high beam intensities, up to 1013 projectiles black. For nuclei with neutron number N > 165, half-lives grow by orders per second on target, at low Ca consumption, about of magnitude with increasing N. The stability of those nuclei against alpha 0.5 mg/h, during many months of continuous target decay is enhanced for larger neutron numbers. The positions of neutron irradiation (see figure 3a). The compound nuclei magic numbers, the established N = 152 and N = 162 and the predicted created by the fusion of actinide target nuclei and 48 N = 184, are indicated by gray dot-dashed vertical lines. 20Ca projectiles typically cool through the evapora- tion of three or four neutrons. That novel approach, called hot fusion, led to the first observations4,5,9 of typical beam intensities used in current SHE exper- elements 113–118. 124 74 iments, continuous irradiation of 50Sn with 32Ge The cross section for the synthesis of isotope 194 293 48 would produce only 1 g of stable 78Pt in about 100 117 in the hot-fusion reaction between a 20Ca beam 249 million years. So nuclear alchemy is possible in prin- and a berkelium-249 ( 97Bk) target is approximately ciple, but it is definitely not a wise capital venture. 100 times as large as the one for the production of However, experiments creating superheavy nuclei 278113 in . Elements 113–117, first ob- are much more rewarding in terms of scientific gain. served in experiments at the JINR, were recently The development of powerful heavy-ion accel- confirmed at the GSI, Lawrence Berkeley National erators, from the 1970s on, gave a significant boost Laboratory (LBNL) in the US, and RIKEN.10–12 In to experiments aiming to create new elements. By 2012 the International Union of Pure and Applied 208 209 using 82 Pb and -209 ( 83 Bi) targets and Chemistry, the world body in charge of chemical 54 heavy-ion projectiles, from chromium-54 (24Cr) up nomenclature, recognized the discoveries of ele- 70 to zinc-70 ( 30Zn), researchers created and identified ments 114 and 116, and the names (Fl, the isotopes of new elements up to (Cn, 114) and livermorium (Lv, 116) now appear in the Z = 112). periodic table. In particular, elements 107–112 were discov- The JINR experiments used a series of actinide ered at the separator for heavy-ion products (SHIP) targets to create elements 113–118 and their isotopes. facility at the GSI Helmholtz Centre for Heavy Ion The manmade target materials, having atomic num- Research in Germany. The beam energy in those ex- bers between Z = 93 () and Z = 98 (Cf), periments was kept low to create compound nuclei, were made at Oak Ridge National Laboratory 278 such as 112Cn, at low excitation energy and to allow (ORNL) in the US (see figures 3b–d) and at the the evaporation of only one neutron—a method Research Institute of Atomic Reactors in Russia. nicknamed cold fusion. Increasing beam energy in- At ORNL, the High Flux Isotope Reactor can creases the fusion cross section, but compound nu- produce the heaviest isotopes of the transuranium clei at higher excitation energies tend to fission im- elements in quantities of tens to hundreds of mil- mediately, and superheavy evaporation residues ligrams by neutron irradiation of americium (Am, are not created at measurable rates. Z = 95) and (Cm, Z = 96) isotopes.13 The Ra- Each new step down the periodic table re- diochemical Engineering Development Center next quired greater projectile charge and mass, higher door separates individual elements from the irradi- beam intensity, and longer irradiation time to fuse ated Am and Cm material. The production and 249 the colliding nuclei. The probability for creating fu- purification of 97Bk, with a half-life of only 327 days, sion-evaporation residues, measured in terms of the led to the most recent discovery4 of two isotopes of 34 August 2015 Physics Today www.physicstoday.org element 117, with mass numbers 293 and 294. (See PHYSICS TODAY, June 2010, page 11.) Ion separators When heavy-ion projectiles smash into a target, various c reaction products, of which only a few are fusion- evaporation residues, are ejected. All those particles, and the primary beam, enter an ion separator that uses a combination of mag- b a netic and electric fields to d select ions of interest and guide them to detectors. Figure 3. Superheavy elements require heavyweight facilities. The intensity of heavy-ion beams used for the (a) The heavy-ion U 400 at the Joint Institute for Nuclear production of superheavy nuclei is on the order of Research in Russia is among the few accelerators in the world capable 1013 projectiles per second. In contrast, the rate of of providing intense beams of calcium-48 projectiles for research on SHE events produced at the target varies from a sin- superheavy nuclei. (Courtesy of the Joint Institute for Nuclear Research.) gle event every few days to a single event in several (b) Radioactive actinide target materials are made at the High Flux months. Obviously, the main purposes of a separa- Isotope Reactor at Oak Ridge National Laboratory in the US. tor designed for SHE studies are to efficiently trans- (c) At the Radiochemical Engineering Development Center next door mit SHE nuclei that are only rarely produced in the to the reactor, the are chemically separated from irradiated target and to reject the massive amount of primary- pellets inside heavily shielded hot cells. Program associate Karen Murphy beam particles and other unwanted reaction prod- is shown at a hot-cell window, working with manipulators inside. ucts that can cause false signals. (d) Shown here is 22 mg of highly purified berkelium-249, the target The two most successful separators are the material used for the discovery of element 117. (Courtesy of Oak Ridge GSI’s SHIP, in operation since the late 1970s, and the National Laboratory.) Dubna gas-filled recoil separator (DGFRS), in ser- vice since the late 1990s at the JINR. Six new ele- ments, from (Bh, Z = 107) to Cn (Z = 112), Unwanted particles—such as transfer-reaction were discovered at SHIP, and six new elements from products produced when a target nucleus absorbs Z = 113 to Z = 118 were first detected at the DGFRS. one or more from a projectile, light parti- To pick out the evaporation residues from the cles emitted from the target, and scattered beam primary beam and other reaction products, SHIP op- particles—still reach the detectors at a rate of a erates in vacuum with a velocity filter, which takes few hundred per second, compared with about one advantage of the different velocities of recoiling nu- event per week (about 2 × 10−6/s) for SHE evapora- clei entering the separator. Because the residues re- tion residues. Other recoil mass separators have ceive all their momenta from the projectiles in the pri- been built following the same principle as SHIP— mary beam but are four to six times as heavy as those for example at ORNL and Argonne National Labo- projectiles, they move more slowly. In the velocity ratory in the US, at the National Large Heavy Ion filter, which is made of two electric deflectors and Accelerator in France, and at the JINR. several dipole and quadrupole magnets, the ratio of The DGFRS utilizes a different concept for electric and magnetic fields can be set to allow only transmitting superheavy recoils from the target area particles with a particular velocity to pass through. and rejecting primary beam particles. The separator Typical transmission efficiency at SHIP for fusion- is filled with hydrogen at a pressure of about 1 torr evaporation residues is about 30%, and the primary and consists of a 23° dipole bending magnet and a pair beam suppression factor is more than 1010. of quadrupole magnets (see figure 4). All recoiling

Detector station Figure 4. The Dubna gas-filled recoil Recoils separator is outfitted with a dipole Gas-filled Primary beam bending magnet (D) and two ion- chamber Q Position-sensitive Q 23° silicon detector focusing quadrupole magnets (Q) Calcium-48 D to select and guide the superheavy beam TOF recoils (red) from collisions between detectors calcium-48 projectiles (blue) and a rotating actinide target to a set of detectors. The inset shows the Rotating titanium detector station with two time-of- entrance window Side Rotating Recoils detectors flight detectors and silicon-stack actinide target detectors. (Adapted from Y. T. Oganessian et al., Phys. Rev. C 83, 054315, 2011.)

www.physicstoday.org August 2015 Physics Today 35 Island of stability

Figure 5. Decay chains for isotopes 293117 and 294118, showing the half-life and alpha- of each nucleus in the chains. Black arrows indicate alpha decay and gray arrows indicate . In both cases, hot-fusion reactions between calcium-48 projectiles and actinide target materials, either berkelium-249 or californium-249, produce compound nuclei, labeled with asterisks, that promptly evaporate off several neutrons. Toward the ends of the chains, -281 (from 293117) and flerovium-286 (from 294118) can spontaneously fission or alpha decay into -277 and copernicium-282, respectively. Both separator and chemistry apparatus at the GSI, the Berkeley gas-filled separator at end-chain nuclei LBNL, and the gas-filled recoil ion sep- undergo arator at RIKEN are similar spectrome- spontaneous ters used for SHE studies. fission. Spotting the heaviest nuclei The fusion of a projectile ion and a target actinide into a super- heavy nucleus in its ground state is an extremely rare event. To identify such an event and distinguish it from the back- ground of scattered multi nucleon transfer products, researchers rely on the measured charac- teristics of signals generated by the recoiling evap- oration residue itself and of its chain of several se- quential decays. The superheavy nucleus triggers the TOF de- tectors and creates a valid TOF signal. Then it be- comes lodged in the implantation counter and, for fusion-evaporation residues have their ionic gas-filled separators, deposits around 8–15 MeV of charges reset to an average value through collisions energy. The implantation signal is followed seconds with hydrogen atoms. The transmission efficien- or even milliseconds later by the detection of an 48 cies—typically about 35% for the products of a 20Ca at the same position, with a resolution 249 2 beam on a heavy actinide target like 97Bk—depend of a few square millimeters down to 1 mm . on the difference between the projectile and target If the alpha particle’s energy is close to the ex- masses. The actual rate of scattered beam particles pected one for a superheavy nucleus, the signal trig- and unwanted reaction products reaching the detec- gers a beam stop to block the primary beam. That way, tors is a few hundred per second. subsequent decays are observed without beam-asso- The detection of superheavy nuclei is similar at ciated background. When another alpha decay is ob- SHIP and the DGFRS. The nuclei selected by the served at the same position, the beam pause is pro- electromagnetic fields and transmitted to the focal longed, sometimes to several minutes or even many plane of the separator pass through time-of-flight hours, to detect the final decay—a spontaneous fis- (TOF) detectors that give timing signals. For the sion signal. Such low-background operation mode DGFRS, they are two multiwire proportional coun- causes losses of about 5–10% in real experiment time ters filled with 1.7 torr of pentane gas and operated as a tradeoff for significant gain in sensitivity to rare at 1500 V. The combined efficiency of the TOF coun- events, in particular for longer-lived daughter nuclei. ters is about 99.9%. So far, all decay chains of nuclei at the so-called The ions are shallowly implanted into an approx- hot-fusion island, the region of superheavy nuclei 2 48 imately 5000-mm position-sensitive silicon counter created using actinide targets and 20Ca beams at that is surrounded by several Si side detectors. about 240-MeV energy, have terminated in sponta- When an alpha emission indicates an implanted ra- neous fission. Figure 5 shows the decay chains of dioactive superheavy nucleus, the alpha energy and isotopes 293117 and 294118. The half-lives of nuclei at the position and time of implantation are recorded. the ends of decay chains can range from a fraction 282 Side detectors catch about 37% of the alpha particles of a millisecond (for example, 112Cn) to more than 268 emitted at backward angles, whereas those going one day (-268, 105Db). forward are fully absorbed in the implantation de- The probability for a random sequence of tector with nearly 100% efficiency. The transactinide events to simulate a long is extremely 36 August 2015 Physics Today www.physicstoday.org 294118 295118 296118 118 0.7 ms ? ? 293117 294117 117 20 ms 50 ms 290Lv 291Lv 292Lv 293Lv 116 8 ms 18 ms 12 ms 60 ms Figure 6. The top of the 287115 288115 289115 290115 115 40 ms 0.2 s 0.3 s 0.8 s nuclear landscape, with 284Fl 285Fl 286Fl 287Fl 288Fl 289Fl the heaviest identified nuclei 114 2ms 0.15 s 0.12 s 0.5 s 0.6 s 2s and their half-lives and decay 278113 282113 283113 284113 285113 286113 modes—yellow for alpha 113 1.4 ms 70 ms 0.1 s 1s 4s 8s

Z decay and green for 277Cn 281Cn 282Cn 283Cn 284Cn 285Cn 112 0.69 ms 0.13 s 0.9 ms 4 s 0.1s 30 s spontaneous fission. 274Rg 278Rg 279Rg 280Rg 281Rg 282Rg The contoured color 111 12 ms 4ms 0.1 s 4s 17 s 2m background indicates 273Ds 277Ds 279Ds 281Ds 110 the predicted stability 0.17 ms 4 ms 0.2 s 14 s of nuclei—the darker the 274Mt 275Mt 276Mt 277Mt 278Mt 109 0.4 s 20 ms 0.6 s 5 ms 4 s color, the more stable the

PROTON NUMBER nucleus. A Russia–US team 270Hs 271Hs 273Hs 275Hs 277Hs 108 10 s 4 s 0.8 s 0.2 s 30 s is currently focusing on 270 271 272 274 new isotopes 295118 and 107 Bh Bh Bh Bh 60 s 1 s 10 s 40 s 296118 (gray squares) using 269Sg 271Sg a calcium-48 beam and an 106 3 m 2 m isotopically mixed californium 267Db 268Db 270Db 105 1 h 30 h 1 h target. (Background contour 267 plot courtesy of the GSI 104 Rf 1.3 h Helmholtz Centre for 266 103 Lr Heavy Ion Research.) 10 h 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176177 178 179

NEUTRON NUMBER N small,14 usually at the level of 10−6 to 10−12. Such a se- The alpha decay occurs with orbital angular quence starts with a nucleus triggering a valid TOF l = 0, which means that the alpha parti- signal, followed by implantation at the right energy cle has to tunnel through only a coulomb barrier and and a chain of alpha-like signals terminating in not an additional orbital angular momentum bar- spontaneous fission, all occurring at the same place rier. That scenario results simultaneously in fast and within a relatively limited time frame. alpha decay and high fission probability. Conse- Decay studies involving superheavy ion im- quently, we observe relatively short decay chains plantation and alpha spectroscopy allow re- that end in spontaneous fission in even–even nuclei. searchers to deduce only the half-life and energy of Fission probabilities are about 1000 times lower for the emitted particle. Decay chains originating from odd-mass nuclei and more limited for those with some odd-mass nuclei and from odd-Z and odd-N both odd Z and odd N, so they have longer alpha- isotopes display richer spectra of alpha energies, decay chains. However, they also end in fission of which indicate the population of low-energy excited the terminating nuclei. states in daughter nuclei. Recent studies of decay chains that start from Discovery and exploration 288115 nuclei complemented alpha spectroscopy During the past 15 years, more than 50 superheavy with gamma- and x-ray detectors.11 The experi- nuclei and six new elements, from Z = 113 to ments detected photons emitted from excited states Z = 118, have been synthesized in hot-fusion reac- 276 48 of the daughter nuclei meitnerium-276 (109Mt) and tions between 20Ca beams and radioactive actinide 272 107Bh that were populated by alpha decay from re- targets. The experimental proof of the existence of 280 spective parent nuclei roentgenium-280 (111Rg) and superheavy nuclei with strongly enhanced stability 276 109Mt. Although the search for characteristic x rays against radioactive decay, the long-sought island of that define the atomic number of the emitting nu- stability, is the most important result of those stud- cleus was not conclusive, the experiments nonethe- ies. They confirm our general understanding of the less mark the first time researchers have observed, of the heaviest atoms. That im- with the high energy resolution of germanium de- pressive success came out of experimental develop- tectors, gamma transitions from excited states of ments that were undertaken at a time when the path isotopes at the island of stability. to new elements via cold-fusion reactions seemed to Nuclei with both even Z and even N at the is- be at a dead end. land of stability are characterized by relatively large All decay chains observed at the hot-fusion rates of fission and of alpha decay. The daughter island end in spontaneous fission and do not con- nuclei, after alpha decay, also have even proton and nect to the nuclear mainland. In 2013 the alpha 271 267 neutron numbers. In such nuclei, all the protons decay of 107Bh into 105Db and the subsequent spon- 267 and neutrons are paired, so the ground states have taneous fission of 105Db were independently ob- 9,11 271 spin 0 and even parity. served at the JINR and the GSI. Therefore, 107Bh www.physicstoday.org August 2015 Physics Today 37 Island of stability

technically connects the mainland and island nuclei, New techniques in the physics of superheavy but researchers have not yet observed a decay chain nuclei include next-generation accelerator facilities connecting both territories. like the Factory under construc- The currently accepted assignments of mass tion at the JINR, new technologies for the production and atomic numbers for nuclei at the hot-fusion is- of robust targets, and the creation of sophisticated land are the only consistent ones possible. The nu- detector arrays. Such developments should allow clei are connected by decay chains, and the assign- the production of the heaviest nuclei in larger quan- ments are cross checked by producing the involved tities and advance our understanding of the funda- nuclei in different nuclear reactions, a procedure mental properties of nuclear structure and atomic called cross bombardment. properties at the top of nuclear and atomic worlds. In addition, the measured excitation functions, References the beam-energy dependence of the cross sections 1. V. M. Strutinsky, Nucl. Phys. A 95, 420 (1967); 122, 1 for different reaction channels, offer a consistent (1968). picture of identified nuclei synthesized in different 2. A. Sobiczewski, F. A. Gareev, B. N. Kalinkin, Phys. laboratories. The new nuclei synthesized at the is- Lett. 22, 500 (1966). land of stability are presented in figure 6. 3. W. D. Myers, W. Świątecki, Nucl. Phys. 81, 1 (1966); In addition to developing improvements in V. E. Viola Jr, G. T. Seaborg, J. Inorg. Nucl. Chem. 28, 741 (1966). decay spectroscopy of nuclei, several groups are at- 104 tempting to directly measure the mass numbers of 4. Y. T. Oganessian et al., Phys. Rev. Lett. , 142502 (2010). nuclei at the hot-fusion island. -252 J. Phys. G 34 252 254 5. Y. T. Oganessian, , R165 (2007). (102No) through Nobelium-254 (102No), - 6. M. Bender, W. Nazarewicz, P.-G. Reinhard, Phys. Lett. 256 257 256 (103Lr), and lawrencium-257 (103Lr) are the heav- B 515, 42 (2001). iest nuclei with directly and precisely measured 7. G. Münzenberg, M. Schädel, Moderne Alchemie: Die masses. Those measurements were made at the Jagd nach den schwersten Elementen, Vieweg (1996). SHIPTRAP facility at the GSI.15 Two new instru- 8. K. Morita et al., J. Phys. Soc. Jpn. 81, 103201 (2012). 16 17 9. Y. T. Oganessian et al., Phys. Rev. C 87, 054621 (2013). ments, MASHA at the JINR, and FIONA at LBNL, 104 are expected to operate soon. 10. C. E. Düllmann et al., Phys. Rev. Lett. , 252701 (2009); S. Hofmann et al., Eur. Phys. J. A 48, 62 (2012); The quest continues for new elements and nu- J. Khuyagbaatar et al., Phys. Rev. Lett. 112, 172501 clei closer to the next magic neutron number pre- (2014). dicted at N = 184. Recently, researchers at ORNL 11. D. Rudolph et al., Phys. Rev. Lett. 111, 112502 (2013). 249 104 produced a target containing a mix of 98Cf (50%), 12. L. Stavsetra et al., Phys. Rev. Lett. , 132502 (2009); 250 251 K. Morita et al., in RIKEN Accelerator Progress Report 98Cf (15%), and 98Cf (35%) to be used at the JINR. That target enables the search for isotopes 295118 and 2013, vol. 47, RIKEN Nishina Center for Accelerator- 296118, heavier than any nucleus synthesized so far. Based (2014), p. xi. 13. J. B. Roberto et al., in Fission and Properties of Neutron- An expansion of the periodic table beyond ele- Rich Nuclei: Proceedings of the Fifth International Confer- ment 118 will require projectiles with atomic num- ence on ICFN5, Sanibel Island, Florida, USA, 4–10 No- 48 bers greater than 20 that are heavier than 20Ca. Re- vember 2012, J. H. Hamilton, A. V. Ramayya, eds., cent attempts to synthesize heavier elements using World Scientific (2014), p. 287. 50 54 18 14. K.-H. Schmidt et al., Z. Phys. A 316, 19 (1984). titanium-50 (22Ti) and 24Cr and earlier experiments 58 64 15. E. Minaya Ramirez et al., Science 337, 1207 (2012). using iron-58 (26Fe) and -64 (28Ni) were not 57 successful. Evidently, new approaches that facilitate 16. A. M. Rodin et al., Instrum. Exp. Tech. , 386 (2014). 17. N. Esker et al., Bull. Am. Phys. Soc. 59(10), 54 (2014). about a 10-fold increase in beam intensities and total 79 20 18. Y. T. Oganessian et al., Phys. Rev. C , 024603 (2009); beam doses greater than 10 projectiles on target S. Hofmann et al., in GSI Scientific Report 2008, are needed to reach new elements at the island of K. Grosse, ed., GSI Report 2009-1, GSI Helmholtz Cen- stability. tre for Heavy Ion Research (2009), p. 131. ■