Yu. A. Lazarev

Development of a new SARA/IGISOL technique for the study of short-lived products from heavy-ion-induced fusion—evaporation reactions

Abstract A new ion guide chamber has been designed for heavy-ion (HI) induced fusion—evaporation reactions. Its principle is based on the beam projectiles-evaporation residues separation which is achieved by taking advantage of their very different angular distributions after passing a thick target. In this work it is proved that the IGISOL technique can be applied with an overall efficiency > 10 −3 for the study of mass-separated exotic nuclei produced in (HI, x n) reactions.

Fission dynamics for capture reactions in 58,64Ni + 208Pb systems: New results in terms of thermal energy and neutron multiplicity correlated distributions

Abstract The neutron multidetector DeMoN has been used to investigate the symmetric splitting dynamics in the reactions 58,64 Ni + 208 Pb with excitation energies ranging from 65 to 186 MeV for the composite system. An analysis based on the new backtracing technique has been applied on the neutron data to determine the two-dimensional correlations between the parent composite system initial thermal energy (ECNth) and the total neutron multiplicity (νtot), and between pre- and post-scission neutron multiplicities (νpre and νpost, respectively). The νpre distribution shape indicates the possible coexistence of fast-fission and fusion-fission for the system 58 Ni + 208 Pb (Ebeam = 8.86 A MeV). The analysis of the neutron multiplicities in the framework of the combined dynamical statistical model (CDSM) gives a reduced friction coefficient β = 23 ± 1225 × 1021 s−1, above the one-body dissipation limit. The corresponding fission time is τf = 40 ± 2046 × 10−21s.

Reconstruction of particle multiplicity distributions using the method of statistical regularization

Abstract The problem of reconstructing particle multiplicity distributions from experimental data is discussed. Because of statistical errors involved in the data, the problem of reconstruction is “incorrectly posed”, which results in the oscillatory behaviour of the direct solution when the detection efficiency e is substantially lower than 100%. It is shown that the method of statistical regularization used reconstructs the real distribution and allows one to estimate the rms errors of the results for e ≳ 25%. The possibilities of the method are examined on the basis of the measurements of the multiplicity distribution of neutrons from spontaneous fission of 244 Cm. The application of the method to the determination of the multiplicity distributions for three Fm isotopes is presented.

Observation of Delayed Nuclear Fission in the Region of 180Hg

The formation of nuclides undergoing delayed fission with half-lives in the range of T1/2 1 s has been observed in fusion-evaporation reactions induced by 230 MeV 40Ca and 40Ar projectiles on isotopically enriched targets of 144Sm, 147Sm and 150Sm. The fission activity with T1/2 = 0.70+0.12-0.09 s produced in the 144Sm + 40Ca reaction has been found to be the most striking one. The origin of this activity is explained as being due to the β-delayed fission occurring in the chain . The discovery of β-delayed fission in the Hg region opens up valuable prospects for extending low-energy (E* ≤ Qβ+(EC)) fission studies to neutron-deficient nuclei of many elements belonging to the translanthanide part of the Mendeleev periodic table.

Multiplicity of prompt neutrons from spontaneous fission of the isotope 252102

Abstract The average number of prompt neutrons, v , emitted in the spontaneous fission of the isotope of element 102 with mass number 252 and a half-life of 2.4 s has been measured to be 4.15±0.30 relative to v = 2.69±0.01 for spontaneous fission of 244 Cm.

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.

Observation of enhanced nuclear stability near the 162 neutron shell

In bombardments of {sup 248}Cm with {sup 22}Ne the authors discovered two new isotopes, {sup 265}106 and {sup 266}106, by establishing genetic links between {alpha} decays of the 106 nuclides and SF or {alpha} decays of the daughter (grand-daughter) nuclides. For {sup 266}106 they measured E{sub {alpha}}=8.62{+-}0.06 MeV followed by the SF decay of {sup 262}104 for which they measured a half-life value of 1.2{sup +1.0}{sub {minus}0.5} s. For {sup 265}106 they measured E{sub {alpha}}=8.82{+-}0.06 MeV. They estimated {alpha} half-lives of 10-30 s for {sup 266}106 and 2-30 s for {sup 265}106 with SF branches of {approximately}50% or less. The decay properties of {sup 266}106 indicate a large enhancement in the SF stability of this N=160 nuclide and confirm the existence of the predicted neutron-deformed shell N=162.

Spontaneous fission of light californium isotopes produced in 206,207,208Pb + 34,36S reactions; new nuclide 238Cf

Abstract In bombardments of 206,207,208Pb with 34S and 206Pb with 36S, we identified a new spontaneously fissioning isotope 238Cf with T sf ≈ T 1 2 = 21 ± 2 ms and obtained evidence of the production of a new isotope 237Cf with T 1 2 = 2.1 ± 0.3 s . The spontaneous-fission (SF) decay mode was established for 240Cf; its SF branch was estimated to be bsf ∼ 2 × 10−2. We measured also bsf ⩽ 1.4 × 10−4 for 242Cf and estimated bsf ∼ 10−1 for 237Cf. The production cross sections of 238Cf in the 206,207,208 Pb + 34 S reactions were measured to be in the range of 0.3 to 1.1 nb. Finally, we probed the influence of the neutron excess in the N = 20 projectile 36S on cross sections of fusion-evaporation reactions occurring on lead targets.

Influence of Pairing Correlations on the Probability and Dynamics of Tunnelling Through the Barrier in Fission and Fusion of Complex Nuclei

An analytically solvable model is used to study the barrier penetrability pattern for subbarrier fission and fusion of nuclei in the case when the pairing gap parameter Δ is treated as an additional dynamical variable whose behaviour in tunnelling is governed by the least action principle. It is found that, as compared to the standard (BCS) approach, the dynamical treatment of pairing correlations essentially modifies (considerably weakens) the dependence of the fission barrier penetrability on the intensity of pairing correlations in the initial state (Δ0), on the barrier height, and on the energy of the initial state. On this basis, a more adequate explanation is proposed for typical order-of-magnitude values of the empirical hindrance factors associated with ground-state spontaneous fission of odd nuclei. It is also shown that a large enhancement of superfluidity in tunnelling - the inherent effect of the dynamical treatment of pairing correlations - strongly facilitates deeply subbarrier fusion of complex nuclei and results in increasing fusion cross sections by several orders of magnitude. Finally, an analysis is given for the probability of spontaneous fission decay from K-isomeric quasi-particle (q-p) states in even-even heavy nuclei. It is found that the relative change of the partial spontaneous fission half-life in going from the ground-state to a high-spin q-p isomeric state, Tsf*/Tsf, is strongly dependent on whether or not there takes place the dynamically induced enhancement of superfluidity in the tunnelling process. Measurements of Tsf*/Tsf provide thus a unique possibility of verifying theoretical predictions about the strong, inversesquare Δ dependence of the effective inertia associated with large-scale cold rearrangements of nuclei in fission and fusion.

Problems involved in the synthesis of new elements

The present status and perspectives of the development of the synthesis of new elements in the nuclear reactions induced by heavy ions are reviewed. The efficiency of various reactions leading to the synthesis is analysed, the radioactive properties of heavy and hypothetical super— heavy nuclei are considered and the physical methods of identification of new elements are discussed.