Journal of Nuclear and Radiochemical Sciences, Vol. 8, No. 2, pp. 39-45, 2007

Study of Heavy Nuclei at FLNR (Dubna)
Yu. Ts. Oganessian*Flerov Laboratory of Nuclear Reactions (JINR), 141980 Dubna, Moscow region, Russia
Received: November 13, 2006; In Final Form: February 2, 2007
The synthesis of new nuclides has been performed with the use of the gas-filled recoil separator; in a number ofexperiments some radio-chemical methods have been employed. The experimental data point to a substantial
increase in the nuclear stability with a growing neutron number. For all the nuclei in decay chains undergoing α
decay or spontaneous fission the energies and half-lives have been measured. The obtained data are in agreementwith predictions of microscopic theory on the deciding role of the nuclear shells in the stability of heaviest
elements.

1. Introductionof new “magic” nuclei in heavy-ion induced reactions.

It is well known that the main field of application of themacroscopic, liquid drop (LD) model has been the descriptionof the average properties of the nuclear masses and deformation energies. From the analysis of the detail properties ofheavy nuclei it had become obvious that the macroscopic
approaches needed corrections by means of the shell model.1,2
The main achievements of the macro-microscopic theory areconnected with the development of the calculation method ofthese corrections to the nuclear ground and highly deformed
states.3 In many publications (e.g., see the reviews4,7–13 and references therein), a number of the existing disagreements
between the macroscopic description and experiment wereexplained by taking into account the shell effect when calculating the nuclear energy in the ground and highly-deformed
states (nuclear fission). One important consequence of thesecalculations was the disclosure of a significant gap in the spectrum of low-lying levels in the region of hypothetical superheavy nuclei, viz. a new (following N = 126) closed sphericalneutron shell N = 184. It was also shown that the considerable
variations of the binding energy of spherical nuclei were due tothe nuclear shells, and that shell effects might be present also
in deformed “magic nuclei” (deformed shells). And finally, atfurther and quite significant increase of the deformation arising in fission, the shell effects continued to play an importantrole in defining the potential energy and the nuclear inertial
masses. The theoretical predictions of above mentioned macromicroscopic approaches, as well as purely microscopic selfconsistent models for the new shells, which in fact are not too
far from the well known region of the actinides (it is a questionof nuclei with mass ~ 280–300), push far away the limits ofnuclear masses and extend the region of existing elements atleast as far as Z ~ 120 and even more.
Of main interest to us are the basic consequences of thesemodels from the point of view of their experimental verification. The remarkable success in the past few years achieved in
the synthesis of heavy nuclei in cold fusion reactions are related
basically to isotopes in the vicinity of the N = 162 shell, mainly
at N < 162.14 But in order to probe the effect of the next, spherical shells, which influence a much wider charge and mass
region of heavier nuclei, it is necessary to synthesize nuclei
with Z ≥ 112 and N ≥ 172. This is hard to achieve in cold fusion
reactions. One of the key questions pertains to the production

2. Reactions of Synthesis*Corresponding author. E-mail: oganessian@jinr.ru.

In the standard fusion theory, tested in many experimentswith light projectiles (from 4He to 26Mg), the evaporation-residues (EVR’s) cross section, σEVR(Ex) = Σσxn (Ex) is determinedasσxn (Ex) = σCN(Ex) · Pxn (Ex),where σCN (E x) is the cross section for the formation of thecompound nucleus with excitation energy Ex, and Pxn (Ex) is theprobability of its survival during de-excitation by emission of
x-nucleons (for the heavy nuclei — mainly neutrons) and γ
rays.
It is then assumed that for all collisions with l ≤ l crit, fusion(amalgamation of the interacting nuclei) takes place automatically in a very short time after overcoming the Coulomb barrier. Indeed, if the nuclear attractive force was stronger than the
Coulomb repulsion, this simple fusion pattern would be quite
defensible. It has been shown also that when the two magic
nuclei 208 Pb and 48 Ca fuse the maximum cross sections forevaporation residues are reached with low excitation energy(cold fusion) and small number of evaporated neutrons (x = 1–
3).15 When the projectile becomes more and more heavy, theexcitation energy of the compound nuclei decreases (down toEx ≈ 15–10 MeV) and the transition to the ground state takes
place by the emission of only one neutron and γ rays.16–18 As aresult, the survivability of the compound nucleus Pxn (Ex) significantly increases, this being the main advantage of the cold
fusion reactions.
Another peculiarity of cold fusion reactions of the nuclei
208
Pb or 209Bi with stable isotopes from 54Cr to 70Zn as projectiles lead to the formation of compound nuclei with small neutron excess. The EVR’s are some 10–15 mass units shiftedfrom the β -stability line. This, in turn, leads to a considerable
decrease in their half-lives. Finally, in cold fusion reactions the
six heaviest elements with Z = 107–112 were synthesized (seealso RIKEN experiment on the synthesis of element 113).19
As can be seen from Figure 1(a), the cross section σ1n (Z CN)— of the main channel of the synthesis reaction — exponentially decreases with the increase of Z CN. When Z CN changes
from 102 to 113 the cross section decreases by almost a factorof 107. The observed strong decrease in the cross section with
the increase of Z CN in cold fusion points out that strong obstacles arise on the way of formation of the cold compound© 2007 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 10/19/2007

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J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007

Figure 1. (a) Maximal cross sections of the 1n-evaporation channel
in cold fusion reactions. (b) Upper panel: experimental cross sections at the maximum of 4n-evaporation channels in hot fusion reactions. Lower panel: calculated values of (B n – B f ) for isotopes of
elements 102–120 with different neutron number.

nucleus itself.
It is clear that the mechanism of fusion of massive nuclei,
such as 208Pb and 70 Zn, significantly differs from the aboveconsidered simple scenario of formation of compound nuclei
using light projectiles. The formation of a composite system ofsummed mass at the contact point and the following its evolution does not guarantee the formation more compact shape with
deformation close to the ground state. This transition is theresult of a collective motion of the system in conditions ofstrong Coulomb repulsion, and the process of formation of the
EVR’s takes place now via three stages:

σEVR = σcapt · Pdyn · Psur.
Experimental data and transport models, describing thestage of collective motion in different assumptions about the
dynamical properties of the nuclear system, indicate a strong
decrease of Pdyn with the increase of the proton number.
In order to decrease the factors hindering fusion, it is desirable to make use of more asymmetric reactions, and to obtainan increase in the neutron number of the EVR’s by using bothtarget and projectile nuclei with maximum neutron excess. As
target material, it is reasonable to use neutron-rich isotopes of
the actinides (Act.), such as 244Pu, 248Cm, and 249Cf, and as projectiles — doubly magic nucleus 48Ca. The compound nucleus
292
114, produced, for example, in the fusion of 244Pu and 48Ca,
acquires 8 additional neutrons compared to the case of the208
Pb + 76Ge cold fusion reaction. These 8 neutrons, as will beshown below, play a key role in the production and the decay
properties of superheavy nuclei. Compared to the cold fusion
reaction 208Pb + 76Ge (Z P·ZT = 2624), the Coulomb repulsion in
the reaction 244Pu + 48Ca (Z P·ZT = 1880) decreases by almost
40%, which, in turn, should lead to the decrease of the factors
hindering the formation of a compound nucleus. On the other
hand, due to the magic structure of 48Ca, the excitation energy
at the Coulomb barrier E xmin of the compound nucleus 292114
amounts to approximately 30 MeV, a value by 10–15 MeV
lower than in typical hot fusion reactions induced by lighter
projectiles.
The last stage — the survival of the compound nucleus — isthe decisive one in the given method of synthesis of the heaviest nuclei.
The estimations of E xm in and the following experiments,
aimed to measure the excitation functions for evaporation products, have shown that the compound nuclei with Z CN = 112–118,

Figure 2. Layout of the Gas-filled Separator.

may attain excitation energy from 30 to 55 MeV. This energy
will be released by a cascade emission of 2 to 5 neutrons (theevaporation of charged particles is significantly less probable)and γ rays. Indeed the excitation functions correspond to theevaporation mainly of 3 or 4 neutrons from the excited nucleus,
the maximum cross sections for evaporation residues areobserved at Ex ≈ 40 MeV (Reference 20) (hot fusion). The crosssections of nuclei with Z = 102–110, produced in the 4n-evaporation channel of the fusion reactions Act. + 22Ne, 26Mg, and 36S(5n), are presented in Figure 1(b) (the neutron number of thecorresponding compound nuclei is shown on the horizontalaxis). Since there is no significant hindrance for fusion in suchmass-asymmetric reactions (Z P ·Z T = 920–1500), the strong
decrease in the cross section σ4n is connected mainly with thesurvivability of the nuclei. The relatively high production crosssection of isotopes with Z ≤ 105 is a consequence of the highfission barrier, which is almost completely determined by theshell effect of the two closed neutron shells N = 152 and N =162. At neutron numbers NCN > 162, as can be seen from Figure1(b) (lower panel) the fission probability significantly increases
with the decrease of Bf. However, if the predictions of the theoretical models (see above) about the existence of the next closed
shell N = 184 is justified, the fission barrier height will again
increase when advancing to the region where N CN ≥ 174 andZ CN ≥ 112. In turn, the nuclear survivability will increase too
and as a result, one can expect even a rise in the σEVR for heavy
nuclei with large neutron excess. Indeed, as can be seen fromthe experimental data presented in Figure 1(b), when increasing the number of neutrons from NCN = 169 (233U + 48Ca) to NCN= 172 (237Np + 48Ca) and then to NCN = 178–180 (244Pu,248Cm +48Ca), σEVR grows by more than one order of magnitude. For
this reason, the observed increase in the survivability of theexcited nuclei with neutron number appears to be, to our opinion, evidence for the existence of the closed neutron shell in theregion of N ≥ 180.
3. Setting the Experiments
The Gas-Filled Recoil Separator (DGFRS) used in theexperiments with 48Ca projectiles is schematically presented in
Figure 2. The calculated and measured in the test experiments
transmission efficiency of the separator for Z = 112–118 nuclei
is about 35–40%,21 whereas full-energy 48Ca projectiles, projectile-like ions, and target-like nuclei are suppressed by factors~1017, 6 × 1014, and 104 –106, respectively.

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J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007

41

Figure 3. Chart of the heaviest nuclides. The squares contain the half-lives (without errors) and the maximal α-transition energy (E α in MeV).

The typical beam intensity of 48Ca ions at the target was 1.0–
1.2 pµA. The consumption of the 48Ca material amounted toabout 0.5 mg/h. In the experiments, targets of actinide oxidesof the highly enriched isotopes of U, Np, Pu, Am, Cm, and Cf(thickness of ≈0.35 mg/cm2) were used.
EVRs passing through the separator were implanted in a 4 ×
12-cm2 semiconductor detector with 12 vertical position-sensitive strips. The detection efficiency of the focal-plane detector
array for α -particles is 87% of 4 π ; for detection one fission
fragment — close to 100%, for two fission fragment — about
40%. For α-particles, emitted by the parent or daughter nuclei,
it is possible to choose wide enough energy and time gatesδE α1, δt α1 and δE α2, δt α2 and employ a special low-backgrounddetection scheme. An example: during the irradiation of the243
Am target, the beam was switched off after a recoil signal
was detected with parameters of implantation energy and TOF
expected for evaporation residues, followed by an α-like signal
with an energy in the interval δE α1 in the same strip/position
and a time interval δt α1 of up to 8 s. If the first α-particle was
not detected (the probability being about 13%), then the switching off the beam was done when a second α-particle in the corresponding δE α2 and δ t α2 intervals was detected. At theregistration of the second α -particle the beam-off period is
extended to 12 min, whereas of the third — to 3 hours.
Such running condition allowed detection of rare events anddecay characteristics of heaviest nuclei with decay time of up
to 1 day and even longer. 22 The most short-lived nuclei aredetected in DGFRS corresponding to t ≥ 5 µs. In this way, thesetup allows investigation of nuclei in a wide range of half-lives—from 10 –5 s to more than 105 s.
From the characteristics of the DGFRS, which are given
above, it follows that with a 48Ca-beam intensity of 1.2 pµA,
0.35 mg/cm 2 target thickness and a beam dose 5 × 1018 (realized for 200 hours of operation) the observation of one decay
event corresponds to the production cross section of about 0.7
pb.
One of the direct methods of atomic-number identification isbased on classical chemical methods, which were long ago usedin the first identification and characterization of many of theartificial elements heavier than uranium (see, e.g., Reference
23 and references therein). Obviously, the investigation of thechemical properties of the new nuclides is of separate interestin connection with the study of the structure of superheavy
atoms and of the chemical behaviour of the heavy and superheavy elements. Such a possibility is now opened for a series ofneutron-rich relatively long-lived nuclei synthesized in Act. +48
Ca reactions. It is seen from Figure 3 that some of nuclei inthe decay chains of isotopes with Z ≥ 114 with odd proton number and/or odd neutron number has a half-lives ranging fromseveral seconds to ~1 day, times — reachable by radiochemical

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J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007

TABLE 1: Results obtained with DGFRS and in the chemical experiment (243Am + 48Ca)
Separation method
Separation efficiency
Detection method
48
Ca beam energy
Total 48Ca beam doseThickness of the 243Am target
Number of detected SF events
Formation cross section
Half-life
TKE
Neutron multiplicity / fission
Identification method of SF-decaying nuclei

DGFRS

Chemical experiment

Recoil separator
35%
Decay chains Z = 115
246 MeV
4.3 × 1018
0.36 mg/cm2
3

Radiochemical separation
80%
SF of nuclei with Z = 105
247 MeV
3.4 × 1018
1.2 mg/cm2
15

2.7+4.8–1.6 pb
16+19–6 h≈225 MeV

4.0+1.4–1.1 pb
32+11–7 h≈230 MeV≈4.2
Isolation of group 5 elements(Z = 105)

Excitation functions and decay
properties (Z = 115)

methods. Further below we shall present the results on thechemical isolation of two nuclides: 268 Db (TSF ≈ 1.2 d) and
283
112 (Tα ≈ 4 s). These results are compared with the data fromDGFRS.
4. Experimental Results
For the synthesis of superheavy nuclei at DGFRS, the fusion
reactions of 48Ca with target nuclei, the isotopes of U, Np, Pu,
Am, Cm, and Cf (9 isotopes of six actinide elements), wereused. The decay chains are presented in Figure 3. In the investigations carried out at different 48Ca energies, 29 new nuclides(34 including preliminary data for the decay chain of 282113)
were detected, all of them being evaporation products and theirdaughter nuclei in the region of Z = 104–118 and A = 266–
294.20,24
4.1. Chemical separation of 268Db. As shown above, thelonger-lived R- α1-...- α5 -SF decay chains terminated by a SF
nuclide (TSF = 16+19–6 h) were associated with the decay of the
odd-odd isotope 288115, produced in the 3n-evaporation channelof the 243Am + 48Ca reaction. Since all consecutive α -decays
and SF are strongly correlated with each other and the order ofoccurrence of the nuclei in the decay chains is determined, theidentification of the atomic number of any nucleus in thischain would independently prove the synthesis of the previously unknown elements 115 and 113.
According to the atomic configuration in the ground state,
Db should belong to the 5th group of the Periodic Table, as aheavier homologue of Nb and Ta. The chemical behaviour of
Db has been investigated through the study of the 34-s 262Db
both in a solution as chloride or fluoride and in the gas phaseas volatile bromides and chlorides.25,26
For the purpose of chemical identification, Db can be separated, along with the members of chemical group 5, from theother elements. For this case, we developed a method of sorption extraction for the group 5 elements as anionic fluoride
complexes. Bearing in mind that the Z = 105 isotope of interest
undergoes spontaneous fission, special attention was paid toseparating the group 5 elements from the actinides and, mostimportant, from SF isotopes, e.g., 252Cf and 254Cf. Here we shallnot give details of the chemical method used for separation of
the elements of group 5 (they can be found in the original
papers27–29), but rather we shall give the main results. It is seenfrom Table 1 that the decay properties of the long-lived SF
nuclei after chemical isolation of Ta fraction from 243Am + 48Ca
reaction, in all measured values (TSF, TKE), are consistent withdata obtained previously with DGFRS.22 In this case atomic
number of the nuclei of decay chain 288115- α → 284113- α →280
111-α → 276109-α → 272107-α → 268105 (SF) was confirmed

by chemical identification of the long-lived isotope 268Db.
4.2. Chemistry of element 112. The reaction 242Pu(48Ca,3n)287114-α → 283112 was used to produce the isotope 283112. Its
cross section, as it follows from experiment (see above), ishigher than for the 238U(48Ca,3n) 283112 reaction.
The recoil nuclei leaving the 242Pu target stopped in a highpurity gaseous medium: He (70%) + Ar (30%). The energy of
the 48Ca beam at the middle of the target was E Lab = 245 MeV,
which for a 1.4 mg/cm 2 thick target corresponded to the maximum yield of the isotope 287114 (T1/2 ≈ 0.5 s) — the product of
the 3n-evaporation channel of the fusion reaction 242Pu + 48Ca.
To the 242 Pu target (99.93%) about 15 µg/cm 2 of nat Nd wasadded; this allowed to simultaneously produce the neutrondeficient short-lived α-radioactive 185Hg isotope having a halflife of 49 s, which served to monitor the production andseparation processes. The recoiling nuclei, which stopped in
the He/Ar medium, were transported to the detectors by meansof an 8-m capillary tube (the inner diameter was 1.5 mm). The
total transport time from the reaction chamber to the detectors
was 3.6 s. This time is long enough for the decay 287114 (0.5 s)α → 283112. Also, only about 50% of the daughter nuclides
283
112 reached the detector chamber.
The setup COLD30 consists of 32 pairs detectors, about 1
cm 2 each, placed one opposite the other with a 1.5 mm gap in
between, through which the He/Ar gas flows. One of the detectors of each pair was covered with a 30–50-nm gold layer. The
temperature gradient along the whole length of the detectors
spanned a range from –24 °C to –184 °C in the first experimentand from +35 °C to –180 °C in the second one. The energy resolution for decay α-particles amounted to 120 keV. The SF fission fragment energy was calibrated using a thin 248Cm source.
In the control experiments, only α particles from the decayof 181–188Hg (the fusion natNd + 48Ca reaction), 211At and 219,220Rn
were observed. As it was expected, only nuclei with high volatility were transported to the detectors. It was shown that all
Hg atoms are registered by the first detectors with the Au coating. This can be explained by the strong adsorption on thedetector Au-surface, which is due to the chemical reaction
leading to the production of the Hg/Au compound. On the contrary, the decay of the chemically neutral Rn atoms is observedin the region of the last detectors, which are at the lowest temperatures (Figure 4). In these conditions, the atoms of element
112 must be between these two extreme cases, their position(detector number) depending on their chemical properties.
Two events from the decay of 283112 were observed in thedetector chamber (Figure 4). In the first case, the sequence αSF with E α = 9.38 ± 0.12 MeV and then 0.59 s later the two
fragments with total kinetic energy TKE = 231 MeV were registered by the second pair of detectors with the Au surface atStudy of Heavy Nuclei at FLNR (Dubna)Hg
Rn

9.38±0.12 MeV
110 >3sGold
SF 231 MeV
0.53 s

40

1

30

50
0

Ice

-50

Hg

20

Rn

-100

10

Relative yield %

0

-150
1

4

8

12

b)

50

16
20
Detector number

283

Gold

α 112

40

24

28

32

30

Hg

20

0

0

-50

Rn

-100

2

10

-200

50

Ice

9.47 ± 0.12 MeV
110 >3s
SF 232 MeV
0.59 s
279

-150
1

4

8

12

16
20
Detector number

24

28

32

Temperature 0C

283

α 112
279

43

Temperature 0C

Relative yield %a)50

J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007

-200

Figure 4. Results of thermo-chromatographic separation of element
112, produced in the reaction 242Pu(48Ca,3n) 287114- α → 283112. The
dotted lines (right-hand axis) denote the temperature distribution
along the 32 detectors from the two experiments. The relative yieldsof the 49 s-185Hg and 3.7 s-219Rn are shown together with the observed
decays of 3.8 s-283112 (black arrows) as a function of the detector
number. The Monte Carlo simulation of the adsorption process of Hg
and Rn on the gold surface and ice are shown by the red dashed line.
(a) the temperature gradient is –24 °C to –184 °C; (b) the temperature
gradient +35 °C to –180 °C. Figure is taken from Reference 31.

temperature –28 °C (the temperature conditions for detectors 1
to 32 were in the range –24 °C to –184 °C). In the second case,the same α -SF sequence (E α = 9.47 ± 0.12 MeV, t SF = 0.54 s,
TKE = 232 MeV) was registered by the seventh detector pairagain on the Au surface, which was at –5 °C (the temperature
conditions for detectors 1 to 32 were in the range +35 °C to –180 °C). Both events are consistent with the properties of the
decay of the 283112 nucleus. While the first event practicallyfollows the location of the Hg atoms the second one somewhat
differs (the probability of Hg hitting the 7th detector is about3%). This may mean weaker adsorption of element 112 withgold compared to Hg, but it is obviously stronger than expected
for noble-gas like behaviour. Avoiding the details, they are
given in a separate publication31 (experiments with the isotopes
283
112 and 285112 are continued), even now from the obtained
data a conclusion can be drawn that the isotope 283112 by its
chemical properties is related to the group 12 elements. The
p r o d u c t io n c r o s s s e c t io n of 2 8 3 112 i n t h e r e a c t io n
242
Pu(48Ca,3n)287114-α → 283112 is estimated as 2–4 pb depending on the adsorption properties of the parent nucleus 287114 atroom temperature (from the DGFRS measurements σ3n = 3.6 +3.4
–1.7
pb).28 The results of the given experiment in an independent
way confirm the identification of the atomic numbers of thenuclides in the even-Z nuclear decay chain 291116 → 287114 →
283
112 → 279110 → 275108 → 271106 → 267104.
5. Discussions
Actually the identification of the atomic numbers of thenuclides was performed by:- the mechanism of fusion reactions (excitation functions
and cross bombardments ensuring variation of the protonand neutron numbers of the compound nucleus);- decay properties of the nuclei in the decay sequences(half-lives Tα and α-decay energies Qα of even-even (andfor many even-odd) isotopes; see Figure 5;- radiochemical identification of the atomic number of thenuclides 268Db and 283112 — links of the decay chains of
the parent nuclei: 288115 and 291116.27–29,31
All methods give the same identification of the atomic number of the synthesized nuclei. When the atomic numbers of
the parent nuclei are determined (showing that they are theproducts of xn-evaporation channels), the identification of themass of an isotope comes to the quantification of evaporated

Figure 5. Half-lives Tα as a function of the α-decay energy Q α for
nuclei with even atomic numbers Z ≥ 100 (indicated in the figure).
The solid lines represent calculations using the Viola-Seaborg formula (given in the figure). The black symbols denote even-even isotopes, the open symbols — even-odd.

neutrons at various excitation energies. This is achieved:- by means of the measured excitation functions ensuring
variation of the neutron number in the compound nucleus;- by producing the same nuclei in different ways: as evaporation residues and as α-decay products of heavier nuclei.
The adjoining four isotopes of the elements with Z = 112,
114, and 116, genetically connected with the daughter nuclei byconsecutive α decays give a self-consistent picture of theatomic and mass numbers of all nuclides, synthesized in the48
Ca-induced experiments.
Further verification of the identification of the mass numberof the isotopes follows from the decay properties. Because of
the high suppression of spontaneous fission of nuclei with odd
neutron numbers, their decay chains are longer and the total
decay time is noticeably higher than those in the neighbouring
even-N isotopes (see chains on the Figure 3).
5.1. Alpha decay. As can be seen from Figure 3, the odd
isotopes of element 112 and all isotopes (even and odd) with Z≥113 predominantly undergo α decay. As known from the theory of α decay, in this case the probability for the decay (or thehalf-life Tα) is directly connected to the decay energy Q α andthe atomic number of the nucleus. The experimental values
obtained earlier in hot and cold fusion reactions and belonging
to the α decay of even-even nuclei with 100 ≤ Z ≤ 110 with newdata for all isotopes with even proton numbers from Z = 106 to
118, produced in 48Ca-induced reactions, are shown in Figure 5.
The experimental values Qα (exp) and Tα (exp) shows steep raiseof the Tα with increase the neutron number in heaviest nuclei.
They can be used also for the calculation of the atomic numbers of nuclei comprising the chains of correlated decays. For
example the probability that the consecutive α transitions
observed in the 245Cm,248Cm + 48Ca reaction take place in the
nuclei with atomic numbers 116 → 114 → 112 → 110 amounts
to 0.992.
The values of Qα (th), obtained in the framework of the macroscopic-microscopic (MM) model in the version of Reference
32 for the isotopes of all elements with even atomic numbers
from Z = 100 to 118 and with odd atomic numbers from Z =103 to 115, are presented in Figures 6(a) and 6(b), respectively.
The predicted Qα (th) values for the heaviest nuclei, observed inour experiments are systematically larger than the experimental data.
At the same time, the trends of the predictions are in good
agreement for the 23 nuclides with Z = 106–118 and N = 165–

44
12Alpha-decay energy Qα (MeV)

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J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007
Ds

a)

112

114

b)

118

Rg

115

116

Hs

11

Mt

Bh

Sg

10

114

Db

113

Rf

9

109

No
112

108

107
Lr

110

8

111

105

106

Fm
104

even-even
even-odd

160
165
170
Neutron number

175

Z=103

odd-odd
odd-even

7
150

155

180

150

155

160
165
170
Neutron number

175

Figure 6. Alpha-decay energy vs. neutron number for: (a) the isotopes of even-Z elements with Z ≥ 100, (b) isotopes of odd-Z elementswith Z ≥ 103. Squares correspond to the nuclei, produced in 48Cainduced reactions. Dashed lines are long sequences of correlated
decays of the nuclei 288115 and 291116. The solid lines are Qα (th), calculated in the MM model.

177. The trend of the Q α (N) systematics, predicted by theory
and confirmed by experimental data can be considered as
direct evidence for the deformed neutron shell closure at N =162.
The comparison of Q α (exp) with the values Q α (th), calculated within the Skyrme-Hartree-Fock Bogoliubov (HFB) and
the Relativistic Mean Field models (RMF), was carried out,
too. In the HFB model a better agreement is obtained with
masses from Reference 33 calculated with 18 parameters.
Finally, in the RMF model the agreement between theory andexperiment is least satisfactory. But it cannot be excluded that abetter agreement can be achieved in this model also by using a
different set of parameters.
As a whole, the measured values of Q α (exp) are in agreement with theory, because the model calculations do not claim
to be more precise in determining Q α (th) than 0.4–0.6 MeV.
We must recall, that all three models predict the same sphericalneutron shell at N = 184, but different proton shells, Z = 114(MM) and Z = 120, 124, or 126 (HFB, RMF). Yet, all describe
the experimental data equally well. Such insensitivity withrespect to the various models in this region of Z and N can be
explained either by the remoteness of the nuclei under consideration from the closed shell at N = 184 or by the weaker influence of the proton shells at Z = 114 or higher, compared to thatof the neutron shell at N = 184.
5.2. Spontaneous fission. For 8 out of the 34 synthesized
nuclei spontaneous fission is the predominant mode of decay.
In two more nuclei, 271Sg and 286114, spontaneous fission competes with α decay. For the remaining nuclides spontaneous
fission was not observed. The partial SF half-lives of nuclei
with N ≥ 163, produced in fusion reactions with 48Ca, together
with the half-lives of SF nuclides with N ≤ 160, are shown in
Figure 7.
Four isotopes of element 112 with N = 170–173 are locatedin a region, where a steep rise of TSF(N) is expected. Indeed, inthe even-even isotopes 282112 and 284112 the difference of two
neutrons increases the partial half-life TSF by two orders ofmagnitude.
The neighbouring odd isotopes 283112 and 285112 undergo α
decay. For them, only lower limits of TSF can be determined(shown in the figure). Such a picture is observed also for theeven-even isotopes of element 114: the additional two neutrons
in the nucleus 286114 (TSF ≈ 0.13 s) lead to increase of the stability relative to spontaneous fission. It is significant that the riseof stability relative to spontaneous fission is observed for the
nuclei are by 10–12 neutrons away from the closed neutron
shell N = 184.
For the nuclei with Z < 110 and N < 170 the probability for

Figure 7. Partial half-lives for spontaneous fission TSF vs. N for
nuclei with even Z = 98–114. Solid symbols and crosses denote eveneven nuclei, open symbols — even-odd. Solid lines are drawn
through the experimental points of even-even nuclei, the dashed lines— calculated TSF (th).

spontaneous fission decreases again when the deformed shellclosure N = 162 is approached. The stabilizing effect of the N =162 shell manifests itself in the properties of the even-even isotopes of Rf, Sg, and Hs with N ≤ 160, which, as seen from
Figure 7, are well described by the mentioned model calculations. The odd SF-isotopes with Z = 104–110, produced in the48
Ca-induced reactions, are located in the transition region,
where the larger the neutron number, the smaller the effect of
the N = 162 shell. In this region, the N = 184 shell comes into
effect. Such a behaviour of TSF (exp) as a function of Z and N
correlates with the SHE fission barrier heights and has been
predicted by all models: MM, HFB, and RMF. For the isotopesof element 115, due to the strong hindrances to spontaneous
fission of nuclei with odd proton (or/and neutron) number, αdecay predominates as far as the N = 162 shell, where, similarly to the previous case, the sequences terminates by spontaneous fission.
The decay properties of the nuclei obtained in Act. + 48Ca
reactions show that the basic theoretical concept on the existence of closed shells in the region of the hypothetical superheavy elements and their decisive role in defining the limits ofnuclear mass has received its experimental confirmation.
The experiments were performed at U-400 heavy ion cyclotron of the FLNR (JIINR) in collaboration with Analytical andRadiochemical Division of LLNL (USA); the experiments on
the chemical identification of the isotopes 268 Db and 283112
within the collaboration: Paul Scherrer Institute (PSI, Villigen),Department for Chemistry and Biochemistry of the Universityof Bern, Institute of Electronic Technology (IET, Warsaw) withthe participation of Dr. M. Hussonois from the Institute of
Nuclear Physics (IPN, Orsay).
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