SYNTHESIS AND STUDY OF SUPERHEAVY ELEMENTS · 2011-12-16 · upper limits of superheavy elements produc-tion cross sections reached by the use of on-line recoil separators, fast on-line

Proceedings of the DAE Symp. on Nucl. Phys. 56 (2011) 24

Available online at www.sympnp.org/proceedings

SYNTHESIS AND STUDY OF SUPERHEAVYELEMENTS

A. Popeko∗Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research,

Joliot-Curie str., 6, Dubna, 141980, Russia

Results of experiments on the synthesis of superheavy nuclei in 48Ca-induced reactionsare presented. The experiments were carried out at the Flerov Laboratory of NuclearReactions (FLNR) Dubna heavy ion cyclotron U400 in the framework of a large collabo-ration: FLNR ( JINR, Dubna, Russia), IAR (Dimitrovgrad, Russia), LLNL (Livermore,USA), ORNL (Oak-Ridge, USA) and Vanderbilt University (Nashville, USA).

Enriched isotopes of U÷Cf were used as targets. In the reactions studied in 2000–2011, decays of the heaviest isotopes of Rf÷Cn and isotopes of six new elements 113÷118were observed. The discovery of the elements 114 and 116 has been recognized by theInternational Union of Pure and Applied Chemistry (IUPAC) in June 2011.

1. Introduction

First qualitative predictions of the positionof an ”island of stability” of superheavy el-ements (SHE) were done after analyzing thelevel diagrams of heavy nuclei [1, 2].

As in recent times, present-day theoreti-cal predictions on the next closed proton andneutron shells numbers vary strongly depend-ing on the model. Following the well-knownshells with Z=82 and N=126 (208Pb), theshell correction amplitude has a maximum atZ=114, N=184 (spherical shells) in macro-microscopic (MM) models. An interestingconsequence from these calculations was therevealing of a remarkable gap in the level den-sity of deformed nuclei around N=162 (de-formed shell). After calculations performedusing the Hartree–Fock–Bogoliubov (HFB)-model or self-consistent relativistic models,the next closed spherical proton shell is pre-dicted at Z=120 or 126 [3].

At the beginning of 70-th the discovery ofsuperheavy elements seemed to be reachablein the closest future without serious problems.A large number of projectile-target combina-tion have been studied in attempts to pro-duce superheavy elements around predictednew nuclear shell closures.

∗Electronic address: [email protected]

Multifarious sophisticated physical andchemical methods were employed for the iso-lation and detection of superheavy elements.Among the studied in 70–80-th reactions,there were fusion reactions: 232Th+86Kr,248Cm+40Ar, 248Cm+48Ca, 254Es+48Ca, anddeep inelastic transfer reactions 76Ge+238U,136Xe+238U, 238U+248Cm [4–8].

No evidence for the formation of superheavynuclei has been obtained. Figure 1 shows theupper limits of superheavy elements produc-tion cross sections reached by the use of on-line recoil separators, fast on-line and off-linechemical techniques.

Recoil fragmentseparators

Fast on-linechemical

separation

Off-linechemistry

Region of interest

FIG. 1: Reached limits of superheavy elementsproduction cross sections (envelope from [7]).



First transfermium elements - Md–Sg havebeen produced in fusion reactions of U, Pu andlater heavier targets up to Cf with ions from Bto Ne. This type of reactions has been called”hot” fusion reactions.

The elements - Bh–Cn [9], were synthesizedusing the so called ”cold” fusion reactions - fu-sion reactions between 40Ar–70Zn projectileswith magic nuclei 208Pb and 209Bi. Com-pound nuclei produced in this type of reac-tions have a minimum excitation energy com-pared with other target-projectile combina-tions [10], giving a substantial rise in the sur-vival probability. But, as it has been found outlater [9], the overall production cross-sectionsof evaporation residues, drastic decreased ap-proximately by a factor of 3 with the increasein Z of a compound nucleus per unit charge.Figure 2 shows the production cross-sectionsof transfermium nuclei in ”hot” and ”cold” fu-sion reactions.

100 102 104 106 108 110 112 114 116 11810

-6

10-5

10-4

10-3

10-2

10-1

100

101

102

103

"hot" fusion

1 nb

1 mb

1 pb

Cro

ss-s

ecti

on

/ n

b

Atomic number / Z

1 fb

"cold" fusion

FIG. 2: Production cross-sections for ”hot” and”cold” fusion reactions.

During 2003–2008, the RIKEN group(Japan) performed an experiment to synthe-size element 113 in a 209Bi+70Zn reaction us-ing the gas-filled separator GARIS [11]. Thebeam-on-target time amounted 7477 h andtotal accumulated dose of 70Zn projectileswas 8.48·1019. Two observed α-decay chainswere assigned to the subsequent decays from278113. The production cross section corre-sponding to these two events was deduced tobe 23+30

−15 fb (!).

It seems to be, the natural limit for produc-tion of superheavy elements in ”cold” fusionreactions had been reached in this experiment.

The progress in development of acceleratortechnic, especially that, of ion sources, newdata on reaction mechanisms and on prop-erties of transactinide nuclei obtained duringpast 20 years allowed one to return by the endof 90-th to the use of double magic nucleus48Ca for the synthesis of superheavy elements.

2. Experimental approach andset-ups

The expected half-lives of heaviest nucleiproduced in fusion reactions of 48Ca with neu-tron rich actinides can vary in a wide range:from a few µs up to tens of hours. The ex-pected cross-sections are calculated to be ofthe order of picobarns (10−36 cm2) (see ’Re-gion of interest” on the Fig. 1).

Because of extremely low expected produc-tion cross-sections of superheavy elements, thecornerstone in the experiments was the pro-duction of a stable and intense ion beam ofthe rare and expensive isotope 48Ca at a min-imal material consumption [12].

The low energy 48Ca5+-beam from the ex-ternal 14 GHz ECR-ion source was axially in-jected into the center of the U400 heavy ioncyclotron’s chamber. In all experiments theaccelerator was operated in continuous beammode (DC). The long-term average ion beamsintensity on the target was ∼ 0.6 pµA.

The evaporation residues (ER) were sepa-rated in-flight from beam particles and otherreaction products by the Dubna Gas-filled Re-coil Separator (DGFRS) [13]. The separationefficiency of ERs from reaction Act.+48Ca wasestimated from preparatory experiments. Ithas been determined that about 35÷45% ofERs, produced with the 48Ca projectiles couldreach the separator’s focal plane detector.

The background from a primary beam atthe focal plane was eliminated by a factor of> 1017 and target-like products of incompletefusion reactions were suppressed by a factorof > 105. Due to high suppression factors, thedirect implantation of ERs into the focal planedetector was possible.



0

1

2

3

4

Targetwheel

Quadrupolelenses

Time of flightdetector

Beam

Focal planedetector

Magneticdipole 22.5o

Distance inmetre

s

FIG. 3: Layout of the gas-filled recoil separator.

In the focal plane of the separator a systemof multi-wire proportional chambers (TOF-detectors) and silicon position-sensitive stopand backward detector arrays were installedfor the registration of ERs and their decays.

The signals from the TOF detectors wereused both, for measurement of the recoils ve-locity and for distinguishing decays of thepreviously implanted nuclei (anticoincidence),from arriving ions signals (coincidence).

The time window for measuring decaychains could be widened up to several hours.A special ”chopper” switched out the beamafter a hardware request with selected param-eters, coming from the data tacking system,allowing practically background free detectionof correlated decays with a long half-life at theabsence of a beam.

The data on energy, deposited in the stop-or/and backward detectors, time and positionof appearing signals and auxiliary data werestored in a reference list mode. The analysisof events collected in an experiment has beenperformed to find generic decay links of theimplants in certain time interval depending onthe supposed half-lives.

3. Experimental resultsThe targets consisted of enriched isotopes of

233,238U, 237Np, 242,244Pu, 243Am, 245,248Cm,249Bk and 249Cf in a form of oxides depositedwith a thickness of ≈ 0.35 mg/cm2 on a 1.5-µm Ti foil.

TABLE I: Targets and experimental conditions inirradiations Act.+48Ca.

Target Excitation Beam ERsenergy (MeV) dose (1019)

233U 32.7÷37.1 0.7 no events238U 29.3÷41.9 1.8 282,283Cn

237Np 36.9÷41.2 1.1 282113242Pu 30.4÷47.2 1.8 286,287,288114244Pu 29.8÷54.7 3.0 287,288,289114243Am 38.0÷46.5 3.0 287,288,289115245Cm 30.9÷44.8 2.6 290,291116248Cm 31.2÷41.1 3.0 292,293116249Bk 32.9÷41.1 4.4 293,294117249Cf 26.6÷36.1 4.1 294118

The elementary targets in a shape of an arcsegment were mounted on a disk, that wasrotated at ≈ 2000 rpm. Experimental con-ditions: excitation energy ranges covered bytargets, beam doses in 1019 and observed evap-oration residues (ERs) are listed in Table I.

In reactions studied in 2000–2011 [14–16],decays of the heaviest isotopes of Rf ÷ Cnand 20 isotopes of 6 new elements with Z=113÷ 118 were observed among the products ofcomplete fusion reactions involving 48Ca pro-jectiles and actinide targets U ÷ Cf. Decayproperties of 45 new nuclide, produced in ex-periments with 48Ca projectiles and actinidetargets, are listed in Table II.

4. Studying chemical propertiesof superheavy elements

The investigation of chemical properties ofsuperheavy elements is of fundamental signif-icance. Some of the newly discovered isotopeshave half-lives ranging from seconds to ≈ 1 d,times - reachable by radiochemical methods.

The most suitable for direct chemical stud-ies (see Table II) are the isotopes of Cn: 283Cn(T1/2 ≈ 4 s) and 285Cn (T1/2 ≈ 30 s).

According to the atomic configuration inthe ground state, Cn should belong to the 12-th group of the Periodic Table of the Elementsas a heavier homologue of Hg, Cd and Zn. Towhat extent Cn is a homologue of Hg, dependson the so-called ”relativistic effect” in the elec-tronic structure of the superheavy atom.



TABLE II: Decay properties of the heaviest iso-topes produced in Act.+48Ca reactions [14–16].

Isotope Decay Eα (MeV) T1/2 N ofmode events

267Rf SF — 1.3 h 2266Db SF/EC — 22 min 1267Db SF — 1.2 h 1268Db SF/EC — 27.9 h 39270Db SF — 23.1 h 1271Sg α/SF 8.54± 0.08 1.9 min 3270Bh α 8.93± 0.08 1.0 min 1272Bh α 9.01± 0.06 8.2 s 24274Bh α 8.80± 0.10 0.9 m 1275Hs α 9.30± 0.06 0.19 s 3274Mt α 9.76± 0.10 0.45 s 2275Mt α 10.33± 0.09 9.7 ms 1276Mt α 9.70± 0.06 1.6 s 24278Mt α 9.55± 0.19 7.6 s 1279Ds α/SF 9.7± 0.06 0.20 s 26281Ds SF — 11.1 s 10278Rg α 10.69± 0.08 4.2 ms 2279Rg α 10.37± 0.16 0.17 s 1280Rg α 9.75± 0.06 3.5 s 24281Rg SF — 26.3 s 6282Rg α 9.00± 0.10 0.5 s 1282Cn SF — 0.8 ms 12283Cn α/SF 9.54± 0.06 3.8 s 22284Cn SF — 0.1 s 19285Cn α 9.15± 0.05 29.0 s 10282113 α 10.63± 0.06 73.0 ms 2283113 α 10.12± 0.09 0.1 s 1284113 α 10.00± 0.06 0.95 s 24285113 α 9.74± 0.08 5.5 s 6286113 α 9.63± 0.10 20 s 1286114 α/SF 10.19± 0.06 0.13 s 24287114 α 10.02± 0.06 0.48 s 16288114 α 9.94± 0.06 0.80 s 18289114 α 9.82± 0.05 2.6 s 10287115 α 10.59± 0.09 32 ms 1288115 α 10.48± 0.06 170 ms 24289115 α 10.31± 0.09 0.22 s 6290115 α 9.95± 0.40 16 ms 1290116 α 10.84± 0.08 7.1 ms 10291116 α 10.74± 0.07 18 ms 3292116 α 10.66± 0.07 18 ms 5293116 α 10.54± 0.06 61 ms 4293117 α 11.03± 0.08 14.5 ms 5294117 α 10.81± 0.10 77.5 ms 1294118 α 11.65± 0.06 0.89 ms 3

As it has been shown in [17], Hg atoms canbe transported with a neutral carrier gas (e.g.,He, Ne) to a distance of more than 30 m, witha velocity of up to 5 m/s. Therefore, inves-tigation of Cn adsorption on metal surfaceshas been undertaken. To produce the isotope283Cn the reaction 242Pu(48Ca,3n)287114 α→283Cn has been used.

The chemical setup applied in these exper-iments (Fig. 4) was based on the thermochro-matographic in situ volatilization technique(IVO) combined with the cryo-on-line detec-tor ”COLD” [18].

FIG. 4: Schematic experimental set-up used toinvestigate adsorption properties of Cn on a goldsurface.

To the 242Pu-target natNd has been addedto produce the α-radioactive Hg isotopes,which served to monitor the production andseparation processes.

The recoil nuclei leaving the targetstopped in a high-purity gaseous medium:He(70%)+Ar(30%). A self-drying closed gasloop system was developed to keep the amountof trace gases such as oxygen and water in thiscarrier gas mixture as low as possible.

The stopped nuclei were transported to de-tectors by means of a 8 m capillary tube . Thetotal transport time from the reaction cham-ber to detectors was 3.6 s. This time is longenough for the decay 287114 α→ 283Cn.

The detector array ”COLD” consisted of 32pairs of ion-implanted planar silicon detectorsfacing each other, forming a narrow chromato-graphic channel.



One side of the channel was covered witha 50 nm thick gold layer, deposited directlyon the silicon detector surface. A temperaturegradient was established along the detector ar-ray using a thermostat at the entrance and aliquid nitrogen cryostat at the exit, spanninga range from room temperature to -184oC.

In control experiments α-particles from de-cays of isotopes of high volatile elements181−188Hg (from natNd+48Ca reaction) and219,220Rn (descendants of transfer products)have been detected. Hg atoms are registeredby the first detectors, and decays of the chem-ically neutral Rn atoms by last ones, whichare at the lowest temperature.

During the runs [18, 19], experimental con-dition: gas flow rates and temperature gradi-ents were varied.

One of the obtained thermochromato-graphic deposition patterns for 185Hg, 219Rn,and 283Cn are depicted in Fig. 5 and representa characteristic example for the gas chromato-graphic behavior of single atoms.

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32

0

10

20

30

40

50+50

0

-50

-100

-150

-200

Gold Ice

185Hg

283

Cn

219Rn

Detector number

Te

mp

era

ture

(C

)o

Re

lative

yie

ld /

de

tecto

r %

FIG. 5: Thermochromatographic deposition pat-terns of 185Hg, 219Rn, and 283Cn.

The statistical analysis of the deposition be-havior of 283Cn (Fig. 5) revealed a standardadsorption enthalpy of element 112 on goldsurfaces of ∆HAu

abs(Cn) = 52+4−3 kJ·mol−1 [18].

The observed enhanced adsorption enthalpyindicates a metallic-bond character involvedin the adsorption interaction between Cn andAu.

By applying the estimated values for thesublimation entropy ∆Ssubl = (106.5± 2.0) J ·mol−1K, Cn can be presumed to have a boilingpoint of (357 ± 110) K. These values indicatethat element 112 is considerably more volatilecompared to its lighter homologues Zn, Cd,and Hg.

In experiment [19], aimed at the investiga-tion of chemical properties of Cn rather unex-pectedly, one decay chain was observed, whichwas unambiguously attributed to the decay of287114. Even more surprising was the obser-vation of this decay chain on the detector 19,held at a temperature of -88 oC.

There were special thermochromatographyexperiments devoted to the element 114 [20].From the observation of three atoms of ele-ment 114 adsorbed on gold surfaces of theCOLD detector, its most probable standardadsorption enthalpy on gold was determinedas ∆HAu

abs(E114) = 34+20−3 kJ·mol−1.

So, the experimental data point to ”Hg-like” behavior of Cn and rather ”noble gaslike” behavior of the element 114. This obser-vation is the first indication on the influenceof relativistic effects on properties of super-heavy atoms. This problem is fundamentalfor the modern chemistry. Experiments are inprogress.

5. Summary and outlook

What can we learn from the analysis of thewhole set of the data?

In reactions Act. + 48Ca 20 isotopes of sixnew elements 113 ÷ 118 have been producedand among their decay products 25 heaviestisotopes of the known elements Rf ÷ Cn iden-tified. Figure 6 shows the ”north-east” cornerof the chart of the experimentally investigatednuclides.

The discovery of the elements 114 and 116has been recognized by the IUPAC in June2011 [21]. The IUPAC/IUPAP Joint WorkingParty (JWP) on the priority of claims to thediscovery of new elements 113–116 and 118has reviewed the relevant literature pertainingto several claims. In accordance with the cri-teria for the discovery of elements previouslyestablished by the 1992 IUPAC/IUPAP Trans-fermium Working Group (TWG), and rein-forced in subsequent IUPAC/IUPAP JWP dis-cussions, it was determined that the Dubna-Livermore collaborations share in the fulfill-ment of those criteria both for elements Z =114 and 116. A synopsis of experiments andrelated efforts is presented.



Ds

Rg

Cn

118

117

116

115

114

113

3 sm

1.7 ms

Mt 266

a

8.84;8.76;8.94;8.69 SF

a

8.93SF

a

8.45;8.63SF

0.2 s7.4 s

34 s 27 s

Db 262 Db 263

Sg 265 Sg 266

a

e

9.179.08

4.4 s1.9 s

Db 258Db 256a

9.049.079.12

1.5 s

Db 260Db 2571.5 s0.8 s

SF

a

e

9.01;8.97;8.78

4.7 s

Rf 26021 ms

a 8.28

Rf 26178 s

SF

Rf 2622.1 s

a 8.77- 8.87

Rf 2593.0 s

Rf 2566.2 msa

8.72;8.77SF

Rf 2551.6 s

SF

Rf 25812 ms

a

10.40;10.10;10.03

Bh 26111.8 ms

10.37;10.24

a a

10.06;9.91;

8.0 ms 102 ms

Bh 262

Rf 257

Db 2611.8 s

a

9.77;9.72SF

Sg 2603.6 ms

Sg 2610.23 s

a

9.25a

9.06

a 8.36SF

Sg 2630.9 s0.3 s

a

9.16

a 9.62...

Sg 2590.48 s

Bh 264440 ms

a 9.48; 9.62

Cn 277240 sm

a -11.17 11.65

Db 26622 m

SF

Db 2671.2 h

SF

Db 26828 h

SF

Db 27023.1 h

SF

Sg 2711.9 m

a 8.54

Bh 27061 s

a 8.93

Bh 2728.2 s

a 9.02

Bh 27453 s

a 8.80

113 2781.8 ms

a -11.52 11.68

113 2830.1 s

a 10.12

113 28273 ms

a 10.63

114 2870.48 s

a 10.02

Hs 27010 s

a 9.08

Hs 2750.19 s

a 9.30

Mt 2740.5 s

a 9.76

Mt 2759.7 s

a 10.33

Mt 2761.6 s

a 9.71

Mt 2787.6 s

a 9.55

Ds 2790.2 s

a 9.70

Ds 28111 s

SF

Rg 279170 ms

a 10.37

Rg 2784.2 ms

a 10.69

Rg 2803.6 s

a 9.75

Rg 28126 s

SF

Rg 2820.5 s

a 9.00

117 29314 ms

a 11.03

117 29478 ms

a 10.81

115 28732 ms

a 10.59

115 288175 ms

a 10.46

115 2890.22 s

a 10.31

115 29016 ms

a 9.95

116 2907.1 ms

a 10.84

116 29118 ms

a 10.74

116 29218 ms

a 10.66

116 29361 ms

a 10.54

118 2940.89 ms

a 11.65

114 2880.8 s

a 9.94

114 2892.6 s

a 9.82

113 2855.5 s

a 9.74

113 28620 s

a 9.63

113 2840.95 s

a 10.00

Cn 28529 s

a 9.15

Cn 28497 ms

SF

Cn 2833.8 ms

a 9.54

Bh

Hs

Mt

Ds 267

a 11.6

Sg 2582.9 ms

a

9.07;8.97...

56 ms1.1 ms

Ds 271

a

10.7410.68

a

10.71

Rg 2721.5 ms

10.82

164162 168

174 176

172

166

pro

ton

n

um

ber

neutron number

160

152 158156154150

Db

Rf

Sg

a

e

9.02…

SF

8.79a

SF

Hs 2699.3 s

a 9.23;9.17

a

9.88;9.83;9.75

59 ms

Hs 267

a

10.30;10.37

a

10.57;10.34

0.8 ms 2.0 ms

Hs 2650.45 ms 8 ms

SF

6.3 ms

Sg 262

a

10.95

180 sm

a

10.97

Ds 270 Ds 27376 sm 118 ms

9.7311.08a a

a

a 11.46-11.74

5.1 ms

Hs 266

a 10.16sf

Hs 264

a 10.43SF

Bh 2661 s

a 9.29

Bh 26717 s

a 8.83

a

9.56;9.52

Ds 269170 sm

a 11.11

Mt 26870 ms

a 10.10; 10.24

170

SF

114 2860.13 s

a 10.19SF

Cn 2820.82 ms

SF

SF

SF

Act + Ca48

FIG. 6: The ”north-east” corner of the chart of the nuclides.

In reactions with 48Ca at a bombarding en-ergy close to the Coulomb barrier the maxi-mum yield for 3n- and 4n-evaporation chan-nels has been observed. Products of evapora-tion channels accompanied by the emission ofcharged particles (protons, α-particles) havebeen not observed.

For all events of sequential α-decays the en-ergies and decay probabilities obey the basicrule of Geiger—Nuttall (e.g. in the form [22]),which connects the α-decay energy Qα and thehalf-life Tα and imply decays of nuclei withlarge atomic numbers Z = 110 – 118.

According to existing systematics, sponta-neous fission events with TKE ∼ 200 MeV arerelated to the decay of considerably long-livednuclei with Z ≥ 104 which for one’s turn are”issues” of even heavier nuclei.

Comparing properties of ”light” isotopes,produced in ”cold” fusion reactions [9, 11],with those of ”heavy” ones, produced in Act.+ 48Ca reactions (Table II), one can see, thataddition of several neutrons leads to a signifi-

TABLE III: Decay properties of the heaviest iso-topes produced in Pb,Bi + 48Ca and Act.+48Careactions.

”Light” T1/2 ∆n ”Heavy” T1/2 Gainisotope (ms) isotope (s)269Ds 0.2 14 281Ds 11.1 6 · 104

272Rg 3.8 9 281Rg 26.3 7 · 103

277Cn 0.7 8 285Cn 29.0 4 · 104

278113 1.8 7 285113 20.0 1 · 104

cant increase in lifetimes (Table III). Thesedata indicate on the presence of a neutronshell at higher neutron numbers.

Comparing the half-lives of even-even mostlong living isotopes 284Cn - T1/2 = 0.1 s,288114 - T1/2 = 0.8 s, 292116 - T1/2 = 18 msand 294118 - T1/2 = 0.9 ms, one can suppose,that Z = 114 is probably a proton shell.

Similar conclusion can be drawn after ana-lyzing production cross-sections of superheavyisotopes [14–16] (see Fig. 7).



“hot” fusion

reactions

48

Ca induced

reactions

FIG. 7: Measured excitation functions maxima ofxn-evaporation channels of Act.+48Ca reactions.

Regardless of the facts that there are inter-plays of odd-even effects, mass-asymmetriesin entrance channels, differences in formationand survival probabilities of compound nuclei,the clear maximum in the vicinity of Z=114 isalso an indication on the presence of a closedshell around this proton number.

There is also an indication, that this shellcould be a spherical one. In the limits ofthe detector energy resolution all α-decays of288114 can be attributed to the decay fromthe ground state, which is populated after de-cay of the evaporation residue or after the α-decay of the parent nucleus 292116. This canbe compared with the decays of 277Cn, whichpopulate different levels in deformed (due tothe neutron shell N=162) 273110. Similar,in the two decays of 278113 [11], observed α-transition differ by 0.15 MeV .

Relatively long half-lives of isotopes with Z= 108–114, obtained in 48Ca induced reac-tions, open up new opportunities for the in-vestigation of the influence of relativistic ef-fects on chemical properties of superheavy el-ements.

Due to relatively long life-times on newisotopes, the experimental approach to theirinvestigation can be changed – quasi-on-linemass separation or chemical separation can beemployed. These methods have sufficient ad-vantages in the effective target thickness (fac-tor of ∼15) and in the beam acceptability.

6. Investigation of reactions per-spective for the synthesis of SHE

The heaviest isotope, that can be used inreality as a target for the synthesis of SHE,is 251Cf. The advance to isotopes of heav-ier, than Z=118 elements, requires using ofheavier projectiles e.g. 50Ti, 54Cr, 58Fe andso forth.

Three reactions, leading to the same com-pound nucleus – 302120 (N=182) have beenstudied at the velocity filter SHIP (GSI,(238U+64Ni and 248Cm+54Cr) and DGFRS(FLNR, 244Pu+58Fe). No events which couldbe attributed to formation and decay ofZ=120–isotopes were detected. The sensitiv-ity of these experiments corresponded approx-imately to 0.4 pb for the detection of a sin-gle decay. The study of 249Cf+50Ti reactionstarted at the TASCA (GSI) gas-filled separa-tor in autumn 2011.

The most neutron rich nuclide producedso far in 48Ca induced reactions are 293116and 294117. A step to more neutron rich nu-clei could be done using the isotopes 250Cm(T1/2=9700 a) and 251Cf (T1/2=898 a). How-ever, for the separation of these isotopesone needs special electromagnetic separators,which are yet not available.

Thus, the obtaining of nuclei close to theneutron N=184 shell in Act.+HI reactions re-quires studies of the reaction mechanisms, soas to determine optimal conditions and toprovide realistic estimates of the probabilityof producing compound nuclei in such reac-tions. Another possibility is to look for reac-tion other than Act.+HI.

7. Reactions other than HI + ActReactions of 82Se+208Pb and 86Kr+208Pb

were studied using the velocity filter SHIP atGSI with a sensitivity of ≈ 1 pb, but no eventswhich could be attributed to the formation ofisotopes with Z=116 or else 118 were observed.

A very attractive possibility could be theuse of neutron-rich radioactive magic nuclei asbeams, e.g. fission fragments 132Sn (T1/2=40s); 133Sb (2.5 min) and 134Te (42 min), andof complementary fission fragments like 93Kr(T1/2=1.3 s).



Fusion reactions with stable nuclei, for in-stance 132Sn+176Yb could lead to compoundnuclei with ZCN=120 and NCN=188. In re-action 226Ra+93Kr, compound nucleus withZCN=124 and NCN=195, decaying down tothe isotope 296114182 (3n channel), could beformed.

In principle, the use of symmetric reactionsbetween deformed nuclei like 150Nd+150Ndshould also be considered. In this case theeffect of orientation of interacting nuclei in atouching point can play an important role inthe rise of compound nucleus formation cross-section.

However, from the experimental data it isknown, that the increased Coulomb repul-sion in symmetric reactions, will enhance thequasi-fission channel and thus hinder the for-mation of a compound nucleus. As long ascalculations of these limitations are very un-certain, it seems reasonable to estimate themusing test reactions with known nuclei.

In this connection first experimentsaimed at the study of symmetric re-actions - 136Xe(136Xe,xn)272−xHs and136Xe(124Sn,xn)260−xRf were conductedradiochemically at the FLNR and usingLISE-III spectrometer at GANIL. In bothcases the upper limit of only ≈ 20–100 pb hasbeen reached.

8. Search for superheavy elementsin nature

The possibility to exist in Nature of ele-ments heavier than 238U depends on two de-terminatives: in the Universe should exist amechanism leading to the formation of su-perheavy elements, and it is necessary thatone of superheavy nuclides would have a life-time comparable with the age of the Earth (ofabout (4−5) ·109 y). The search for heavy el-ements in terrestrial samples, meteorites andin cosmic rays in 70’s and 80’s was one of theextensive experimental investigations.

When choosing the objects of such stud-ies, it was assumed that the most stable nu-clei were located in the vicinity of the closedshells Z=114 and N=184 (corresponding tothe chemical behavior of Pb) [24].

In all experiments only upper limits of thesuperheavy element concentration in the stud-ied samples have been determined. All thisresulted in a pessimistic view on the possibleexistence of SHE in nature, and the searchingexperiments were practically stopped in themid of 80-th.

The experimental data accumulated, anddevelopment of modern microscopic modelsduring passed 30 years, simulated new ap-proach to the search for perspective objects.

So, a noticeable increase in T1/2α andT1/2SF may be expected in the region of nu-clei with Z ≤ 110 [14], which has not been yetlooked for. Calculated half-lives T1/2 of iso-topes of elements 108 and 114 against sponta-neous fission and α-decay, obtained in a MM-model [23], are shown in Fig. 8.

Neutron number

150 160 170 180 190140150 160 170 180 190140

Z=114

Z=108

a - decay

Lo

g1

0T 1

/2(s

)

15

10

5

0

-5

-10

Z=114

Z=108

spontaneousfission

FIG. 8: Predicted half-lives T1/2 of isotopes ofelements 108 and 114 against spontaneous fissionand α-decay.

Considering different nuclei as objects forsuch studies, it turns out that for element108 - Hs[14], the chemical homologue of Os,the chances to be found in terrestrial samplescould be favorable.

The search for rare decays may be under-taken with a raw metallic sample of Os, whereatoms of Hs can be present. Spontaneous fis-sion in an Os sample (up to 1 kg) can beregistered by detecting multiple emission ofprompt neutrons [25].

Such an experiment has been now run-ning in the underground laboratory in Modane(France), protected by a 4000-m water-equivalent layer and it will be continued forseveral years.



Another possibility for choosing of objects,perspective for SHE search, can follow fromthe discovered high volatility of Cn and ele-ment Z=114. These elements can be gases(noble) at normal conditions – the boilingtemperature of Cn is (360 ± 100) K. Thusone can look for SHE in heavy fractions of Xeproduction.

The problem of the existence of superheavyelements belongs to the must fundamental innatural sciences, because it affects nuclear andatomic physics, quantum chemistry, electro-dynamic of strong fields, astrophysics, cosmol-ogy, and undoubtedly the efforts to solve itwill be actively continued.

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