R. N. Sagaidak

Synthesis of nuclei of the superheavy element 114 in reactions induced by 48Ca

The stability of heavy nuclides, which tend to decay by α-emission and spontaneous fission, is determined by the structural properties of nuclear matter. Nuclear binding energies and lifetimes increase markedly in the vicinity of closed shells of neutrons or protons (nucleons), corresponding to ‘magic’ numbers of nucleons; these give rise to the most stable (spherical) nuclear shapes in the ground state. For example, with a proton number of Z = 82 and a neutron number of N = 126, the nucleus 208Pb is ‘doubly-magic’ and also exceptionally stable. The next closed neutron shell is expected at N = 184, leading to the prediction of an ‘island of stability’ of superheavy nuclei, for a broad range of isotopes with Z = 104 to 120 (refs 1, 2). The heaviest known nuclei have lifetimes of less than a millisecond, but nuclei near the top of the island of stability are predicted to exist for many years. (In contrast, nuclear matter consisting of about 300 nucleons with no shell structure would undergo fission within about 10−20 seconds.) Calculations indicate that nuclei with N > 168 should already benefit from the stabilizing influence of the closed shell at N = 184. Here we report the synthesis of an isotope containing 114 protons and 173 neutrons, through fusion of intense beams of 48Ca ions with 242Pu targets. The isotope decays by α-emission with a half-life of about five seconds, providing experimental confirmation of the island of stability.

Modernization of the detector system at the recoil separator Vassilissa

Abstract Since the past 10 years, the recoil separator VASSILISSA has been used for the investigations of evaporation residues produced in heavy-ion-induced complete fusion reactions. In the course of experimental work in the region of the elements with 92⩽Z⩽94 , 14 new isotopes have been identified by the parent–daughter correlations. For further experiments aimed at the synthesis of the superheavy element isotopes ( Z⩾110 ) with intensive 48 Ca extracted beams, improvements in the focal plane detector system have been made. As a result, energy and position resolutions of the detectors have been significantly improved, the detection efficiency for the α -particles, emitted from the implanted into the focal plane detector recoil nuclei, has been increased to 85%. The decay properties of the new isotope of element 112 and the isotope of the new element 114 with mass 287 have been measured in two-nuclei decays.

In-beam separation and mass determination of superheavy nuclei. Part I

Within the past 15 years, the recoil separator VASSILISSA has been used for the investigations of evaporation residues produced in complete fusion reactions induced by heavy ions. The study of decay properties and formation of cross-sections of the isotopes of elements 110, 112 and 114 was performed using high-intensity Ca beams and Th; U; Pu targets. For further experiments aimed at the synthesis of the superheavy element isotopes ðZX110Þ with the use of intense Ca extracted beams, improvements in the ion optical system of the separator and the focal plane detector system have been made. The results from the test reactions and new results for the isotope 112 are presented. r 2003 Elsevier B.V. All rights reserved. PACS: 23.60.+e; 25.85.Ca; 29.30.Cm; 29.40.Pe

The upgrade of the kinematic separator VASSILISSA - Experimental results and plans

Within the past twelve years, the recoil separator VASSILISSA has been used for the investigations of evaporation residues produced in heavy-ion-induced complete fusion reactions. In the course of the experimental work, fourteen new isotopes have been identified by the parent-daughter correlations in the region of elements with 92≤Z≤94. The study of the decay properties and formation cross sections of the isotopes of elements 110, 112, and 114 was performed using high-intensity 48Ca beams and 232Th, 238U, and 242Pu targets. At the beam energies corresponding to the calculated cross-section maxima of the 3n evaporation channels, the isotopes 277110, 283112, and 287114 were produced and identified. For further experiments aimed at the synthesis of the superheavy element isotopes (Z≥110) with the use of intensive 48Ca extracted beams, improvements in the ion optical system of the separator and the focal plane detector system have been made.

Decay widths of highly excited Ra compound nuclei

Abstract Excitation functions were obtained for the x n, p x n and αx n decay channels of 216,218,220 Ra compound nuclei produced in the reactions 22 Ne+ 194,196,198 Pt in the excitation energy range of 40–160 MeV. Due to the employment of three platinum isotope targets, the cross section ratios were measured with a high degree of accuracy for evaporation reaction channels resulting in the formation of fixed cold products after the termination of evaporation cascades of a different numbers of stages. Experimental values for the reduced neutron widths of highly excited compound nuclei were obtained from the measured cross section ratios. Evaporation reaction cross sections were well described in the framework of the statistical model taking into account shell effects according to Ignatyuk. Our present, and also earlier, investigations showed that calculations making use of one single set of the model parameters (the scaling factor to the liquid-drop fission barrier of Cohen, Plasil and Swiatecki, C = 0.6–0.7 and the ratio of the level density parameters, a f a v = 1.0 ) reproduces correctly the cross sections of the evaporation reactions in a wide range of compound nuclei extending from Bi to U. Moreover, the measured cross section ratios are exceptionally sensitive to the value of the parameter a f a v , i.e. are very suitable for a high accuracy estimation of this value. By fitting two sets of the experimental data — the absolute cross section values and cross section ratios — we were able to independently estimate the values of two principal parameters of the model, C and a f a y . Our calculations showed that a large part of the pre-fission neutrons is evaporated by hot pre-actinide compound nuclei before they reach the saddle point (50 to 100% of the total number of pre-fission neutrons are pre-saddle).

Cross sections of 102 element isotopes formation in the reactions of22Ne+236U and26Mg+232Th

The production cross sections of the isotopes252102,253102, and254102 were measured for the heavy ion fusion reactions of22Ne+236U and26Mg+232Th by using the kinematic separator VASSILISSA. The obtained excitation functions and the maximum production cross sections are compared with the ones for more asymmetric reactions leading to the same compound nucleus258102. The experimental cross sections and the results of statistical model calculations are compared and discussed.

Study of the stability of the ground states and K-isomeric states of 250Fm and 254102 against spontaneous fission

By employing the 249Cf(4He, 3n) and 208Pb(48Ca, 2n) reactions, experiments to study the stability against spontaneous fission of the nuclides 250Fm and 254102 as well as of the two-quasi-particle (2q-p) K isomers 250mFm (T1/2 = 1.8 ± 0.1 s) and 254m102 (T1/2 = 0.28 ± 0.04s) have been performed. The ground-state spontaneous fission of the two nuclides has been discovered and the corresponding branching ratios bsf and partial half-lives Tsf, respectively, have been determined to be: (6.9 ± 1.0) × 10−5, 0.83 ± 0.15 yr for 250Fm; (1.7 ± 0.5) × 10−3, (3.2 ± 0.9) × 104s for 254102. As a by-product of these studies, new data about cross sections of the 206,208Pb(48Ca, xn) reactions have been obtained. Experiments designed to search for the spontaneous fission decay of the 2q-p K-isomeric states in 250Fm and 254102 have not revealed the effect in question. The lower limits of the ratios of the partial spontaneous fission half-lives for the 2q-p K-isomeric states to those for the respective ground states, Tsf*/Tsf, have been established to be ≥ 10−1 for 250mFm/250Fm and ≥ 5 × 10−3 for 254m102/254102. This means that the stability of the 2q-p K-isomeric states in 250Fm and 254102 against spontaneous fission is practically not inferior to that of the ground states of these nuclei. In accord with the experimental findings, the theoretical estimates of Tsf*/Tsf made in the present paper show that, due to the influence of the specialization and blocking effects on the potential energy and the effective mass associated with fission, spontaneous fission from 2q-p K-isomeric states cannot be facilitated but, on the contrary, should be essentially hindered compared with ground-state spontaneous fission.

Monte Carlo simulation of ion trajectories in the kinematic recoil separator “VASSILISSA”

Abstract In order to calculate the transmission of ions through the kinematic separator VASSILISSA a Monte Carlo computer code was developed. The program considers the effects from reaction kinematics, momentum transfer from the evaporated particles to the evaporation residues (ER), scattering and energy losses of ERs in the target and foils and the ion charge distribution of ERs. The results of calculations are further combined with the first-order ion optics to simulate the trajectory of each ion. Simulation of the time-of-flight, energy, position, charge distribution spectra for different reaction products is also possible.

The new isotope219U

In continuation of our experimental program on investigation of evaporation residues (ERs) in the region close to N=126 [1, 2], the new neutron-deficient isotope219U has been produced in the irradiations of an197Au target with a beam of27A1 ions. The identification of219U was made establishing genetic position and time correlations between the implanted ERs with their subsequentα-decays. Theα-decay energy and half-life were measured to be (9680±40) keV and (42−13+34)μs, respectively. To separate the evaporation residues in-flight the kinematic separator VASSILISSA [3] was used.

New nuclides228,229Pu

In continuation of our work on investigation of the neutron-deficient plutonium isotopes [1] we carried out the experiments aimed to produce new isotopes228,229Pu. The isotope228Pu has been produced in the bombardment of the208Pb target with a beam of24Mg and the isotope229Pu — in the reactions of207,208Pb targets with26Mg projectiles. The isotopes were identified after in-flight separation from the beam and target-like particles by the VASSILISSA electrostatic separator [2] and implantation into silicon strip detectors. The assignment of unknown α-decays was made by establishing their genetic position and time correlations with those belonging to known α-decay chains. The α-decay energies of228,229Pu were measured to be (7810±20) keV and (7460±30) keV, respectively.