Startup of superheavy element chemistry at RIKEN

A review is given on the startup of the superheavy element (SHE) chemistry at RIKEN. A gas-jet transport system for the SHE chemistry has been coupled to the gas-filled recoil ion separator GARIS at the RIKEN Linear Accelerator. The performance of the system was appraised using ²⁰⁶Fr and ²⁴⁵Fm produced in the ¹⁶⁹Tm (⁴⁰Ar, 3n) ²⁰⁶Fr and ²⁰⁸Pb (⁴⁰Ar, 3n) ²⁴⁵Fm reactions, respectively. The α particles of ²⁰⁶Fr and ²⁴⁵Fm separated with GARIS and transported by the gas-jet were identified with a rotating wheel system for α spectrometry under desired low background condition. The high gas-jet efficiencies over 80% were independent of the beam intensities up to 2 particle μA. A gas-jet coupled target system for the production of SHEs was also installed on the beam line of the RIKEN K70 AVF cyclotron. The gas-jet transport of ²⁵⁵No and ²⁶¹Rf produced in the ²³⁸U (²²Ne, 5n) ²⁵⁵No and ²⁴⁸Cm (¹⁸O, 5n) ²⁶¹Rf reactions, respectively, was conducted for the future chemical studies of ²⁶⁵Sg via the ²⁴⁸Cm (²²Ne, 5n) ²⁶⁵Sg reaction.     

Cluster approach to the structure of nuclei with <Emphasis Type="Italic">Z</Emphasis> ≥ 96

The properties of alternating parity bands in heavy nuclei ²³⁴Th, ^239–242U, ^241–245Pu, ^243–248Cm, ^245–250Cf, ^248–251Fm, ^249–254No, ^253–256Rf, and ²⁵⁸Sg are analyzed within the dinuclearsystem model. The model is based on the assumption that the cluster-type shapes are produced by the motion of the nuclear system in the mass-asymmetry coordinate. The energies of the low-lying states whose parity is opposite to the parity of the ground state are predicted for the first time. The text was submitted by the authors in English.     

Observation of a shell gap at N = 152 in the 246 Cm(t,p) 248 Cm reaction

Previous reports of two-neutron stripping reactions in the actinide nuclei have not indicated any excited 0⁺ strength below an excitation energy of 1.5 MeV. In contrast, the ²⁴⁶ Cm(t,p) ²⁴⁸Cm reaction shows a relatively strong 0⁺ state at 1.084 MeV. We believe that this result, coupled with a marked change in ground-state transition strenghts between ²⁴⁶ Cm and ²⁴⁸ Cm targets, indicates a gap in the single-particle orbitals that is larger than the pairing gap.     

Synthesis of 292116 in the 248Cm + 48Ca reaction

We report on the observation of the first decay event of the new nuclide ²⁹²116 produced in an experiment devoted to the synthesis of Z=116 nuclei in the ²⁴⁸Cm + ⁴⁸Ca reaction. The implantation of a heavy recoil in the focal-plane detector was followed in 46.9 ms by an α particle with E _α=10.56MeV. The energies and decay times of the descendant nuclei are in agreement with those observed in the decay chains of the even-even isotope ²⁸⁸114, previously produced in the ²⁴⁴Pu + ⁴⁸Ca reaction. Thus, the first α decay should be attributed to the parent nuclide ²⁹²116 produced via the evaporation of four neutrons in the complete fusion of ²⁴⁸Cm and ⁴⁸Ca. The experiment is in progress at Flerov Laboratory for Nuclear Reactions (FLNR, JINR, Dubna).     

The synthesis and properties of superheavy nuclei produced in the 48Ca+244Pu, 248Cm reactions

We present the results of the experiments aimed at producing hypothetical long-lived superheavy elements located near the spherical shell closures with Z>-114 and N>-172. For the synthesis of superheavy nuclei a combination of neutron-rich reaction partners, such as ²⁴⁴Pu and ²⁴⁸Cm targets and a ⁴⁸Ca projectile have been used. The sensitivity of the present experiment exceeded by more than two orders of magnitude previous attempts to synthesize superheavy nuclides in reactions of ⁴⁸Ca projectiles with actinide targets. We observed new decay sequences of genetically linked α-decays terminated by spontaneous fission. The decay properties of the synthesized nuclei are consistent with the consecutive α-decays originating from the parent nuclides ^288,289114, produced in the 3n and 4n-evaporation channels, and ²⁹²116 — in the 4n-evaporation channel with cross sections of about a picobarn. The present observations can be considered an experimental evidence of the existence of the “island of stability” of superheavy elements and are discussed in terms of modern theoretical approaches.     

Synthesis and properties of even-even nuclei with Z=114−116 produced in 48Ca induced reactions

In the irradiation of targets made from enriched ²⁴⁴Pu and ²⁴⁸Cm isotopes with beam doses of 1.5×10¹⁹ and 2.3×10¹⁹, respectively, the detector array situated in the focal plane of the gas-filled separator registered heavy atoms of new elements undergoing sequential α decays terminated by spontaneous fission. The time of the decay chains is approximately 1 min. Decay properties of the synthesized nuclei are consistent with the consecutive α decays originating from the parent nuclides ²⁸⁸114 and ²⁹²116 produced with the cross section of about 0.5 picobarn. Comparison of T _SF and T _α values for the nuclei with Z=110 and 112 with those obtained earlier for more light isotopes of these elements points to an enhanced stability of heavy nuclei with an increase in the neutron number. The α-decay energies Q _α, measured experimentally in the chains $116 \underrightarrow {\alpha _1 } 114 \underrightarrow {\alpha _2 } 112 \underrightarrow {\alpha _3 } 110$ , are compared with-theoretical predictions of different nuclear models. From this it follows that the theoretical models predicting the decisive influence of the nuclear structure on the stability of superheavy elements are well-founded not only qualitatively but in some sense also quantitatively. Some preliminary data, obtained in the first experiment aimed at the synthesis of element 118 in the reaction ²⁴⁹Cf+⁴⁸Ca, are presented in the paper. The prospects of further investigations in the field of superheavy nuclei are discussed briefly.     

Dependence of light charged particles parameters on Z2/A in ternary fission

Emission of light charged particles from ternary fission of²⁴⁸Cm,²⁵²Cf and²³⁵U+n _th has been measured by a CsI(T1) counter. Relative probabilities and parameters of p, t,α energy spectra in triple fission were extracted and compared with earlier reference results.     

Dependence of light charged particles parameters on Z2/A in ternary fission

Emission of light charged particles from ternary fission of ²⁴⁸Cm, ²⁵²Cf and ²³⁵U+n_th has been measured by a CsI(Tl) counter. Relative probabilities and parameters of p, t, α energy spectra in triple fission were extracted and compared with earlier reference results.     

Few-valence-particle excitations around doubly magic 132Sn

Prompt γ-ray cascades in neutron-rich nuclei around doubly-magic ¹³²Sn have been studied using a ²⁴⁸Cm fission source. Yrast states located in the N = 82 isotones ¹³⁴Te and ¹³⁵I are interpreted as valence proton and neutron particle-hole core excitations with the help of shell model calculations employing empirical nucleon-nucleon interactions from both ¹³²Sn and ²⁰⁸Pb regions.     

Measurement of 248Cm/252Cf Cf Spontaneous Prompt Fission Neutron Spectrum

In order to treat the waste material of nuclear power and develop new type of clean nuclear power,it is necessary to measure the neutron adta of long half life nuclei existed in the waste material.The prompt spontaneous neutron spectrum is one of the most important unclear data for new type nuclear power facilities as well as for understanding the mechanism of fission neutron emission.The measurements of ²⁴⁸Cm/²⁵²Cf spontaneous prompt fission neutron spectrum in the neutron energy range form 200keV to 12MeV wer performed by using TOF method.A micro-ionization chamber aws used as fission fragment detector and stibene crystal as neutron detector.The flying paths of neutrons for the measurements were 30cm,50cm and 100cm respectively.The spontaneous prompt fission neutron spectrum of ²⁴⁸Cm was fitted by the Maxwellian distribution and the temperature was determined as (1.401±0.006)MeV in the corresponding neutron energy range.     

Synthesis of superheavy nuclei in the reactions of 244Pu and 248Cm with 48Ca

This paper presents results of the experiments aimed at producing long-lived superheavy elements located near the spherical-shell closures with Z ⩾ 114 and N ⩾ 172 in the ²⁴⁴Pu + ⁴⁸Ca and ²⁴⁸Cm + ⁴⁸Ca reactions. The large measured α-particle energies of the newly observed nuclei, together with the long decay times and spontaneous fission terminating the chains, offer evidence of the decay of nuclei with high atomic numbers. The decay properties of the synthesized nuclei are consistent with the consecutive α-decays originating from the parent nuclides ^{288, 289}114 and ²⁹²116, produced in the 3n and 4n evaporation channels with cross-sections of about a picobarn. The present observations can be considered as experimental evidence of the existence of the “island of stability” of superheavy elements. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: utyonkov@sungns.jinr.ru     

Synthesis of heavy and superheavy elements by reactions of massive nuclei

By comparing theoretical and experimental excitation functions of evaporation residues resulting from the same compound nucleus or heavy and superheavy nuclei, it is possible to understand the effect of the entrance channel and the shell structure of reacting nuclei on the fusion mechanism. The competition of complete fusion with the quasifission process is strongly related to the intrinsic fusion barrier B _fus ^* and the quasifission barrier B _qf as well as the size of the well in the nucleus-nucleus potential. In our calculations of the excitation functions for capture, fusion, and evaporation residues, we use the relevant variables such as mass asymmetry of nuclei in the entrance channel, potential energy surface, driving potential, spin distribution, and surviving probability of compound nucleus that are responsible for the mechanism of the fusion-fission process. As a result, we obtain a beam energy window for the capture of the nuclei before the system fuses and the Γ_n/Γ_f ratio at each step along the deexcitation cascade of the compound nucleus. Calculations performed in the framework of the model taking into account the nuclear shell effect and shape of colliding nuclei allow us to reach useful conclusions about the mechanism of the fusion-fission process and the production of the evaporation residues. We analyze the ⁴⁰Ar + ¹⁷⁶Hf, ⁸⁶Kr + ¹³⁰Xe, and ¹²⁴Sn + ⁹²Zr reactions leading to ²¹⁶Th*; the ³²S + ¹⁸²W and ⁶⁰Ni + ¹⁵⁴Sm reactions leading to ²¹⁴Th*; the ⁴⁸Ca + ²⁴⁸Cm reaction leading to the ²⁹⁶116 compound nucleus; and the ⁴⁸Ca + ²⁴⁹Cf reaction leading to the ²⁹⁷118 compound nucleus.     

Search for octupole deformation in neutron-rich Xe isotopes

A search for octupole deformation in neutron-rich Xe isotopes has been conducted through prompt gammaray spectroscopy of secondary fragments produced in the spontaneous fission of²⁴⁸Cm. The spectrometer consisted of the Eurogam 1 array and a set of 5 LEPS detectors. Level schemes were constructed for Xe isotopes with mass number ranging from 140 to 144 and excited states for^143,144Xe nuclei were observed for the first time. None of the level schemes exhibit an alternating parity quasimolecular band, a feature usually expected in nuclei in which octupole correlation effects are strong enough to produce stable octupole deformation. For several isotopes, structures observed in the level schemes are consistent with an octupole softness of the nuclei.     

Stochastic model of tilting mode in nuclear fission

A model of induced nuclear fission was developed with consideration of thermodynamically fluctuating orientation degree of freedom (tilting) of deformed nuclei. This model was applied to analysis of the experimental angular anisotropy of fission fragments in the ¹⁶O + ²³²Th, ²³⁸U, ²⁴⁸Cm, ²⁰⁸Pb, ²⁰⁹Bi; ¹²C + ²³⁶U; ¹⁹F + ²⁰⁸Pb; and ¹¹B + ²³⁷Np reactions. Information on the equilibrating time of the tilting mode was obtained. The text was submitted by the authors in English.     

Evaluating the yield of transuranium nuclides with masses of up to A = 270 via pulsed nucleosynthesis

A model of the creation of transuranium isotopes of up to A = 270 under conditions of pulse nucleosynthesis in a neutron flux with densities of up to ??10²⁵ neutron cm^?2 is considered. The pulse process allows us to divide it in time into two stages: the process of multiple neutron captures (t < 10^?6 s) and the subsequent ??-decay of neutron-excess nuclei. The modeling of the transuranium yields takes into account the adiabatic character of the process, the probability of delayed fission, and the emission of delayed neutrons. A target with a binary composition of ²³⁸U and ²³⁹Pu, ²⁴⁸Cm, and ²⁵¹Cf isotopes is used to predict the yields of heavy and superheavy isotopes.     

Nuclear spectroscopic studies of the 23.7 h 248Bk

Main features of the decay of the 23.7 h ²⁴⁸Bk have been determined by spectroscopy with scintillation and semiconductor detectors. The results are as follows: half-life 23.7 ± 0.2 h; branching is (30 ± 5)% to ²⁴⁸Cm (≈ 23% EC decay to 0⁺ ground state and ≈ 7% to first 2⁺ state) and (70 ± 5)%β^? decay to ²⁴⁸Cf(5 % to 592 keV K^π = 2^? state and (65 ± 5)% to the ground-state band). The β^? decay energy, Q_β^?, has been measured to be 860 ± 20 keV, and the electron capture decay energy, Q_EC, has been derived from closed cycle to be 705 ± 25 keV. The deduced logft values of β^? and EC transitions restrict the spin of the ²⁴⁸Bk ground state to 1, with configuration assignment $\text{{}\text{n}\text{[734]}\text{9}\text{2}{}^{\mathrm{?}}$; $\text{p}\text{[633]}\text{7}\text{2}{}^{\mathrm{+}}$. It has also been deduced that the long lived isomer lies 65 ± 40 keV abovethe 23.7h ²⁴⁸Bk ground state.     

Stochastic model of angular distributions of fragments originating from the fission of excited compound nuclei

The anisotropy of angular distributions of fission fragments and the average multiplicity of prescission neutrons were calculated within a stochastic approach to fission dynamics on the basis of three-dimensional Langevin equations. This approach was combined with a Monte Carlo algorithm for the degree of freedom K (projection of the total angular momentum I onto the fission axis). The relaxation time τ _K in the coordinate K was considered as a free parameter of the model; it was estimated on the basis of a fit to experimental data on the anisotropy of angular distributions. Specifically, the relaxation time τ _K was estimated at 2 × 10^?21 s for the compound nuclei ²²⁴Th and ²²⁵Pa and at 4 × 10^?21 s for the heavier nuclei ²⁴⁸Cf, ²⁵⁴Fm, and ²⁶⁴Rf. The potential energy was calculated on the basis of the liquid-drop model with allowance for finiteness of the range of nuclear forces and for the diffuseness of the nuclear surface. A modified one-body viscosity mechanism featuring a coefficient k _s that takes into account the reduction of the contribution from the wall formula was used to describe collective-energy dissipation. The coefficient k _s was also treated as a free parameter and was estimated at 0.5 on the basis of a fit to experimental data on the average prescission multiplicity of neutrons.     

Hints of giant halo in Zr isotopes by resonant RMF+ACCC+BCS approach

In experiments for the synthesis of superheavy elements at the velocity filter SHIP, GSI, we observed fission events, which could not be attributed to decay chains of superheavy isotopes from fusion reactions. Usually, the observation of spontaneous fission is a crucial first step for the detection of decay chains. In order to avoid random correlations and misidentifications of superheavy isotopes, it is therefore essential to know the features and cross-sections of fission events not originating from decay chains of superheavy nuclei. The special properties of the velocity filter allowed us to identify and study the ??background?? fission events as decay products of heavy target-like nuclides populated in nucleon transfer reactions. Here, we will discuss the results obtained in collisions of ₂₀ ⁴⁸ Ca + ₉₆ ²⁴⁸ Cm, ₂₄ ⁵⁴ Cr + ₉₆ ²⁴⁸ Cm and ₂₈ ⁶⁴ Ni + ₉₂ ²³⁸ U, which were applied for the synthesis of elements Z = 116 and 120, respectively.     

Formation of Superheavy Elements: Study Based on Dynamical Approach

Using multi-dimensional Langevin equations for the probability distribution of the distance between the surfaces of two approaching nuclei, we have studied the formation of superheavy elements via calculation of evaporation and fission cross sections of these elements. Evaporation residue cross sections have been calculated for the 1n, 2n, 3n, 4n, and 5n evaporation channels using one and four dimensional Langevin equations for the ⁴⁸Ca+²²⁶Ra, ²³²Th, ²³⁸U, ²³⁷Np, ^{239,240,242,244}Pu, ²⁴³Am, ^245,248Cm, ²⁴⁹Bk, and ²⁴⁹Cf reactions. Our results show that with increasing dimension of Langevin equations the evaporation residue cross section is increased. Also, obtained results based on fourdimensional Langevin are in better agreement with experimental data in comparison with one-dimensional model.     

原子核的大质量转移反应和丰中子超重核的产生(英文)

迄今为止,人们合成的超重核都是缺中子的,无论是熔合-裂变反应还是碎化过程都无法使产物达到周期表的“东北区域”。而重核之间(如U核之间) 的近垒大质量转移反应则可能是目前生成丰中子超重核和达到未知丰中子重核区域的唯一途径。在改进的量子分子动力学(ImQMD) 模型结合统计模型框架内,研究了U+U等反应体系的大质量转移反应,计算了反应产生的初生态碎片的质量和电荷分布,并成功再现了产物的终态质量和电荷分布。通过比较3 个反应136Xe+248Cm,48Ca+248Cm和238U+248Cm 产生的106号元素的截面大小,揭示了U+U等重核大质量转移反应对产生丰中子超重核是非常有利的。For elements with Z > 100 only neutron deficient isotopes have been synthesized so far. The “northeast area” of the nuclear map can be reached neither in fusion-fission reactions nor in fragmentation processes.The large mass transfer reactions in near barrier collisions of heavy (U-like) ions seem to be the only reaction mechanism allowing us to produce neutron rich heavy nuclei including those located at the superheavy(SH) island of stability and unexplored area of heavy neutron-rich nuclides. This study is extremely important for nuclear astrophysical investigations and, in particular, for the understanding of the r process. In this paper within the Improved Quantum Molecular Dynamics (ImQMD) model combining with the statistical-evaporation model, the large mass transfer reactions, like 238U+238U have been studied. The charge and mass distributions of transiently formed primary fragments are investigated within the ImQMD model and de-excitation processes of those primary fragments are described by the statistical decay model. The mass distribution of the final products in 238U+238U collisions is obtained and compared with the recent experimental data. Through compared the formation cross sections of transfermium element 106 by three reactions of 136Xe+248Cm, 48Ca+248Cm and 238U+248Cm, it is explored that the large mass transfer reactions, like U+U are very benefit for the production of SH nuclei.