N. A. Kondratiev

Evidences for resonance states in 5H

Resonance states of H-5 were investigated through the two-neutron transfer reaction t(t, P)(5) H. A triton beam at 57.5 MeV and a cryogenic liquid tritium target were used. The H-5 missing mass spectrum in triple coincidence, proton + triton + neutron, shows a resonance at 1.8 +/- 0.1 MeV above the t + 2n decay threshold. This energy is in good agreement with the result reported in Phys. Rev. Lett. 87 (2001) 092501. The resonance width, Gamma(intr) less than or equal to 0.5 MeV, is surprisingly small and difficult to reconcile with theory predictions

The CORSET time-of-flight spectrometer for measuring binary products of nuclear reactions

The CORSET time-of-flight spectrometer has been developed at the Flerov Laboratory of Nuclear Reactions of the Joint Institute for Nuclear Research (Dubna, Russia) for investigating binary products of nuclear reactions. The spectrometer has been used to study the dynamics of fusion-fission and quasi-fission of superheavy elements. The design and the main characteristics of the spectrometer, as well as the algorithms for deducing the mass-energy distributions of fragments and the cross sections of nuclear reactions, are presented. The spectrometer contains two time-of-flight arms based on microchannel-plate detectors and three telescopes, each of which is composed of two microchannel-plate detectors and one semiconductor detector. A system of four semiconductor detectors is used to obtain the absolute value of a cross section. The time resolution of the time-of-flight arms is 150 ps, which allows the time-of-flight distances to be set at 10–20 cm, thus providing a mass resolution of 3 amu and an angular resolution of 0.3°. Owing to these characteristics, the spectrometer can be used as a trigger in multidetector setups for measuring light charged particles, neutrons, and γ rays in coincidence with reaction fragments.

Fission modes in the reaction 208Pb(18O,f)

Results of fission fragment mass-energy distributions of the compound 226Th nucleus formed in the subbarrier fusion reaction 18O+H208Pb at an energy of 18O ions Elab=78 MeV are reported. The reaction has been studied twice using two different accelerators, and both sets of experimental data agree quite well. Performed analysis of the experimental data with the use of a new multicomponent method has shown that alongside the well-known modes, i.e., the symmetric (S) and two asymmetric modes standard-one and standard-two, a high-energy mode standard-three has manifested itself. The last named mode appears due to the influence of the close-to-sphere neutron shell with N≈50 in the light fission fragment group. Theoretical calculations of the prescission shapes of the fissioning nuclei 224,226Th confirm this conclusion.

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.

Resonance states of hydrogen nuclei 4H and 5H obtained in transfer reactions with exotic beams

Abstract Experimental search for 4 H and 5 H has been performed in the reactions 3 H(t,p) 5 H, 2 H( 6 He, 3 He) 5 H and 2 H(t,p) 4 H. A resonance state situated at 1.8±0.1 MeV above the t+n+n decay threshold is obtained in the missing mass energy spectrum of the 5 H nucleus. Also, there is an indication that another resonance located at 2.7±0.1 MeV presents in this spectrum. Visible peak widths are governed mostly by the instrumental resolution, which came to about 0.4 MeV. We set an upper limit of 0.5 MeV on the true width of any of the two states. The resonance state of 4 H with E res = 3.3 ± 0.2 MeV and Γ res = 4.1 ± 0.3 Mev was obtained in the 2 H(t,p) 4 H reaction.

Fission of heavy and superheavy nuclei at low excitation energies

The talk presents the results of an investigation of the main characteristics (mass and energy distributions of fission fragments and multiplicity of neutrons) of the fission of the nuclei of 220 Ra, 226 Th, 256 No, 270 Sg, 286 112 produced in reactions with ions of 18 O, 22 Ne and 48 Ca at energies close to and essentially below the Coulomb barrier. The data obtained show that the form of the mass and energy distributions of the fission fragments of 226 Th and 270 Sg is accounted for by the multimodal nature of the fission. In addition, for 226 Th, a new phenomenon was established there is a significant difference between the numbers of prescission neutrons for symmetric and asymmetric fission modes. It was found that, for the low-energy fission of the nucleus of 286 112, the mass distribution of the fragments is of a clear-cut asymmetric form, contrary to what is observed for the spotaneous fission of the nuclei with Z = 100–104 and for the induced fission of 270 Sg.

Investigation of the 208Pb(18O, f) fission reaction: Mass-energy distributions of fission fragments and their correlation with the gamma-ray multiplicity

The mass-energy distributions of fragments originating from the fission of the compound nucleus 226Th and their correlations with the multiplicity of gamma rays emitted from these fragments are measured and analyzed in 18O + 208Pb interaction induced by projectile oxygen ions of energy in the range Elab = 78–198.5 MeV. Manifestations of an asymmetric fission mode, which is damped exponentially with increasing Elab, are demonstrated. Theoretical calculations of fission valleys reveal that only two independent valleys, symmetric and asymmetric, exist in the vicinity of the scission point. The dependence of the multiplicity of gamma rays emitted from both fission fragments on their mass, Mγ(M), has a complicated structure and is highly sensitive to shell effects in both primary and final fragments. A two-component analysis of the dependence Mγ(M) shows that the asymmetric mode survives in fission only at low partial-wave orbital angular momenta of compound nuclei. It is found that, for all Elab, the gamma-ray multiplicity Mγ as a function of the total kinetic energy (TKE) of fragments, Mγ(TKE), decreases linearly with increasing TKE. An analysis of the energy balance in the fission process at the laboratory energy of Elab = 78 MeV revealed the region of cold fission of fragments whose total kinetic energy is TKE ∼Qmax.

Probing fission time scales with neutrons and GDR gamma rays

The time scales for nuclear fission have been explored using both pre-and postfission neutrons and GDR gamma rays. Four systems were investigated: 133-MeV 16O + 176Yb and 208Pb and 104-MeV 4He + 188Os and 209Bi. Fission fragments were measured in coincidence with PPACs. The neutrons were detected using eight detectors from the DEMON array, while gamma rays were measured using the US BaF2 array. The pre-and postfission gamma rays were determined using moving source fits parallel and perpendicular to the fission fragment emission directions. The time scales for fission for the neutrons were determined using the neutron clock technique. The gamma-ray data were fitted using a statistical model calculation based on the code CASCADE. The results of the fits from both data types were used to extract nuclear friction coefficients, γ, and fission time scales. The γ values ranged from 7 to 20, while the fission times were (31–105)×10−21 s.

Hydrogen-4 and Hydrogen-5 from t+t and t+d transfer reactions studied with a 57.5-MeV triton beam

An environmentally safe liquid/solid tritium target suitable for beams of radioactive nuclei was created and used in the study of t+d and t+t transfer reactions with putting emphasis on the observation of 4H and 5H resonance states. A state of 4H with Eres=3.22±0.15 MeV and Γobs=3.33±0.25 MeV was obtained in t+d reaction from the spectra of protons leaving the target at θlab=18°–32° and detected in coincidence with tritons. A valuable fraction of protons detected in t+t reaction at θlab=18°–32° in ptn coincidence events was attributed to the states of 5H nucleus. At ∼2.5 MeV above the tnn decay threshold the 5H spectrum shows up a narrow maximum followed by a wide structure at 4–7 MeV. One can not exclude that interference effects could modify the observed 5H resonance maximum resulting in its energy shift and width reduction.

MASHA separator on the heavy ion beam for determining masses and nuclear physical properties of isotopes of heavy and superheavy elements

The MASHA mass spectrometer designed for identifying superheavy elements by their masses is described. The separation efficiency has been measured in the autonomous mode using four calibrated leakages of noble gases. The total separation efficiency of the mass spectrometer with a hot catcher and an ion source based on the electron cyclotron resonance has been determined using the 40Ar beam. Test experiments have been carried out, in which α-active Hg isotopes produced in complete fusion reaction 40Ar + 144Sm → 184 − xnHg + xn, have been detected in the focal plane of the mass spectrometer. The separation time and efficiency have been determined for short-lived Hg isotopes.