Production of Transuranium Nuclides in Pulsed Neutron Fluxes from Thermonuclear Explosions

The production of transuranium nuclides in pulsed neutron fluxes from thermonuclear explosions has been studied within the kinetic model of the astrophysical r-process taking into account the time dependence of external parameters and processes accompanying the beta decay of neutron-rich nuclei. Neutron fluxes depending on the time in the range of ~10^–6 s have been simulated within the developed adiabatic binary model. The probabilities of beta-delayed processes have been calculated within the microscopic theory of finite Fermi systems. The yields of transuranium nuclides Y(A) have been calculated for three experimental thermonuclear explosions Mike (Y_M), Par (Y_P), and Barbel (Y_B) (United States). The rms deviations of the calculations from experimental data are 91, 33, and 29% for Y_M, Y_P, and Y_B, respectively. These deviations are much smaller than those for other known calculations and are comparable with the proposed exponential approximation ensuring rms deviations of 56, 86.8, and 60.2% for Y_M, Y_P, and Y_B, respectively. The even–odd anomaly in the observed yields of heavy nuclei is explained by the dominant effect of processes accompanying the beta decay of heavy neutron-rich isotopes.     

Effect of properties of superheavy nuclei on their production and decay

Properties and stability of superheavy nuclei resulting from hot fusion are discussed. It is shown that the microscopic–macroscopic approach allows obtaining the closed proton shell at Z ≥ 120. Isotopic trends of K-isomeric states in superheavy nuclei are predicted. Evaporation residue cross sections in hot fusion reactions are calculated using the predicted properties of superheavy nuclei. Interruption of α decay chains by spontaneous fission is analyzed. Alpha decay chains through isomeric states are considered. Internal level densities in superheavy nuclei are microscopically calculated.     

Special features of the <Emphasis Type="Italic">K</Emphasis> = 0 channel in nuclear fission

The opinion that the K = 0 fission channel is completely closed if the spin J and the parity π of the nucleus undergoing fission do not satisfy the condition (?1) ^J = π is widespread. On the basis of a detailed analysis of quantum numbers characterizing the rotational states of deformed nuclei, it is shown that this opinion is erroneous. In fact, the K = 0 channel may be partly open. Its suppression is caused by special features of fission barriers in the state being considered. It is also shown that factors that suppress the K = 0channel may exist even in states characterized by J and π values such that they satisfy the condition (?1) ^J = π. More precise information about the contribution of the K = 0 channel may be obtained by measuring the hexadecapole component of the angular distribution of fragments originating from the slow-neutron-induced fission of aligned nuclei.     

Winkelverteilungen elastisch an Edelgas-Atomstrahlen gestreuter Elektronen; Spinpolarisation eines an Argon gestreuten 40 eV-Elektronenstrahls

Various non-leptonic decay modes of baryons are calculated in a simple quark model. Form factors for various matrix elements are taken both from experiment and the quark model. Additionally theK→2π andK→3π decay modes are computed in the same model. The theory has theΔ I=1/2 rule and static SU⁶ built-in. A relation between the∑ ⁺→N ⁺ π ⁺ decay, not calculable in the model, and theK→3π decay is given via an effective six quark interaction. Agreement with experiment is order of magnitude for the baryonic decays and worse for theK decays.     

Coriolis interaction in the second well of deformation potential and low-energy fission

It is shown that, because of sufficiently large energy spacing between neutron resonance states (NRS-II) in the second well of the deformation potential for actinide nuclei, the Coriolis interaction, mixing the states of an axially symmetric deformed nuclei with different values of the projection K of the nuclear spin J onto the symmetry axis, is week, and the K value in the wave functions of NRS-II is a good quantum number. It is concluded that the K distribution for the states of fissile actinide nuclei in the vicinity of their scission point into fission fragments is determined by simultaneous influence of the internal and external fission barriers, which allows to coordinate the experimental data on subbarrier photofission with the P-odd and P-even correlations in the angular distributions of fission fragments.     

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.     

Probable Decay Modes at Limits of Nuclear Stability of the Superheavy Nuclei

The modes of decay for the even–even isotopes of superheavy nuclei of Z = 118 and 120 with neutron number 160 ≤ N ≤ 204 are investigated in the framework of the axially deformed relativistic mean field model. The asymmetry parameter η and the relative neutron–proton asymmetry of the surface to the center (R _η ) are estimated from the ground state density distributions of the nucleus. We analyze the resulting asymmetry parameter η and the relative neutron–proton asymmetry R _η of the density play a crucial role in the mode(s) of decay and its half-life. Moreover, the excess neutron richness on the surface, facets a superheavy nucleus for β^? decays.     

Cluster decay of <Superscript>112−122</Superscript>Ba isotopes from ground state and as an excited compound system

The decay properties of various even–even isotopes of barium in the range 112 ≤A ≤ 122 is studied by modifying the Coulomb and proximity potential model for both the ground and excited state decays, using recent mass tables. Most of the values predicted for ground state decays are within the experimental limit for measurements (T_1/2 < 10³⁰ s). The minimum T_1/2 value refers to doubly magic or nearly doubly magic Sn (Z= 50) as the daughter nuclei. A comparison of log ₁₀(T_1/2) value reveals that the exotic cluster decay process slows down due to the presence of excess neutrons in the parent nuclei. The half-lives are also computed using the Universal formula for cluster decay (UNIV) of Poenaru et al and the Universal decay law (UDL) of Qi et al, and are compared with CPPM values and found to be in good agreement. A comparison of half-life for ground and excited systems reveals that probability of decay increases with a rise in temperature or otherwise, inclusion of excitation energy decreases the T_1/2 values.     

Survivability of excited superheavy nuclei

The survivability of even-even and odd superheavy nuclei is analyzed on the basis of a statistical model and various theoretical predictions for nuclear properties. In this analysis, use is made of various methods for computing level densities. For Z<114 nuclei, calculations on the basis of all models predicting nuclear properties lead to close values for the ratio of the width with respect to the neutron channel to the width with respect to the fission channel. For Z≥114 nuclei, different values are obtained for this ratio. The dependence of the results on model parameters is discussed. The collective-enhancement factor in the level density is taken into account in the calculations.     

Angular-anisotropy coefficients for fragment originating from the resonance-neutron-induced fission of <Superscript>235</Superscript>U oriented nuclei

Statistical distributions of the coefficients measured for the angular distribution of fragments originating from the fission of ²³⁵U oriented nuclei that was induced by resonance neutrons obtained by using booster targets at the electron accelerator in Harwell and at a pulsed reactor in Dubna were approximated by a curve that was calculated under the assumption of a normal distribution of partial-wave fission amplitudes. A cutoff from below at a level of one-half of the average partial-wave width was introduced in this distribution. The calculation was performed with allowance for the K = 0, 1 and 2 channels for J = 3 and the K = 1 and 2 channels for J =4. The contributions of the K channels to the total probability were in the ratio 0.15 : 0.53 : 0.32 for J = 3 and in the ratio 0.625 : 0.375 for J = 4. A strong suppression of the K = 0 channel in the J = 3 spin subsystem in contrast to the situation observed in photofission can be interpreted as an indication of the possible partial conservation of K in resonance states formed from the entrance channel, which features only maximum values of K equal to J and J ? 1.     

Formation of Superheavy Elements in Nature

It is shown that superheavy elements may also be formed in the main r process responsible for the formation of the heaviest elements observed in nature. Under conditions of a high neutron density, the nucleosynthesis region lies close to the neutron drip line, so that the r process may circumvent the region where nuclei undergo spontaneous fissions and therefore have short lifetimes. However, a high induced-fission rate, which increases with the charge number, may prevent the nucleosynthesis wave from overcoming the region of isotopes heavier than curium, and the beta-decay chain leading to an increase in the charge number of product elements inevitably results in the spontaneous fission of the majority of product nuclei. Calculations of nucleosynthesis that were performed with available nuclear data within the scenario of a neutron-star merger reveal that only Z < 106 superheavy elements are formed. Their abundance at the end of the r process is commensurate with the abundance of uranium, but their lifetime does not exceed several years, so that they fast undergo decay.     

Independent yields of Kr and Xe isotopes in the photofission of heavy nuclei

The yields of Kr (A = 87–93) and Xe (A = 138–143) primary fission fragments produced in ²³²Th, ²³⁸U, and ²⁴⁴Pu photofission upon the scission of a target nucleus and neutron emission were measured in an experiment with bremsstrahlung from electrons accelerated to 25 MeV by a microtron, and the results of these measurements are presented. The experimental procedure used involved the transportation of fragments that escaped from the target by a gas flow through a capillary and the condensation of Kr and Xe inert gases in a cryostat at liquid-nitrogen temperature. The fragments of all other elements were retained with a filter at the capillary inlet. The isotopes of Kr and Xe were identified by the γ spectra of their daughter products. The mass-number distributions of the independent yields of Kr and Xe isotopes are obtained and compared with similar data on fission induced by thermal and fast neutrons; the shifts of the fragment charges with respect to the undistorted charge distribution are determined. Prospects for using photofission fragments in studying the structure of highly neutron-rich nuclei are discussed.     

Investigation of anomalousK,L andM shell conversion in electric dipole transitions

Anomalous experimental conversion coefficients of theK, L andM subshells are investigated for thirteen highly hindered electric dipole transition in six deformed odd mass nuclei. For a distinct nuclear transition, all experimental anomalies are consistent with one parameter, containing all nuclear information. The values of this parameter are determined and a simple connection with the hindrance of the transition seems to exist. TheK andL shell conversion coefficients have been recalculated, while theM shell conversion data were newly computed including all finite nuclear size effects as well as screening by a Thomas-Fermi-Dirac model.     

Investigations of fission characteristics and correlation effects

We review the experimental results on the P-even and P-odd angular correlations of fission fragments in the fission of the ²³⁵U and ²³⁹Pu nuclei induced by unpolarized and polarized resonance neutrons, and on the TRI and ROT effects in the ternary and binary fission of actinides induced by polarized thermal neutrons. Also reported are the measured yields of prompt and delayed neutrons per fission event. The experimental data are analyzed within a novel theoretical framework developed by the JINR—RNC KI Collaboration, whereby the reduction of the multidimensional phase space of fission fragments to the JπK-channel space is consistently validated and the role of resonance interference in the observed correlation effects is revealed.     

Preliminary results of the experiment on search for neutron nuclei on a nuclear reactor

An experiment on search for neutron nuclei in the reaction of neutron-induced fission of ²³⁵U nuclei has been performed on a nuclear reactor. The hypothetical reaction ¹²²Te(^xn, (x ? k)n)^{122 + k} Te → (β^?) → ^{122 + k} I (x = k ≥ 10) has been investigated. The radiochemical method for selecting iodine isotopes from tellurium was used. The upper limit on the probability of formation of neutron clusters has been obtained: P _k ≤ 10^?8 (fission)^?1 for ¹¹n clusters and P _k ≤ 10^?9 (fission)^?1 for ¹¹n clusters.     

Analysis of single-particle energies of doubly magic <Superscript>100,132</Superscript>Sn nuclei within the dispersive optical model

Extrapolation of the single-particle energies E _nlj of the bound states of neutrons and protons in the ^{112,116,118,120,124}Sn isotopes has been performed to estimate the values of E _nlj for unstable doubly magic ₅₀ ¹⁰⁰ Sn₅₀ and ₅₀ ¹³² Sn₈₂ nuclei. The estimates obtained are compared with the data derived from the analysis of the decay spectra of neighboring radioactive nuclei and with the results of the calculation within the dispersive optical model.     

Decay Angular Distributions of <Emphasis Type="Italic">K</Emphasis>* and <Emphasis Type="Italic">D</Emphasis>* Mesons as a Tool for the Dynamics of Open Strange and Charm Production

We analyze decay spin-density matrix elements of K* and D* vector mesons and find that the corresponding vector meson decay distributions are sensitive to the production mechanisms. This suggests that the measurement of these quantities can be used to unravel the production mechanisms.     

New multi-phonon gamma vibrational bands in A∼110 neutron-rich nuclei

The high spin states of neutron-rich ¹⁰³Nb, ¹⁰⁷Tc and ¹⁰⁹Tc nuclei in A～110 region have been investigated by measuring prompt γ-γ-γ coincident measurements populated with the spontaneous fission of ²⁵²Cf with the Gammasphere detector array. In ¹⁰³Nb, one-phonon K = 9/2 and two-phonon K = 13/2 γ-vibrational bands have been identified. In ¹⁰⁷Tc and ¹⁰⁹Tc, one-phonon K = 11/2 and two-phonon K = 15/2 γ-vibrational bands, in which the zero-phonon bands are based on K=7/2 excited states, have also been identified. The two-phonon bands are first observed in odd-Z nuclei. The characteristics for these band structures have been discussed.     

Role of (<Emphasis Type="Italic">n</Emphasis>,2<Emphasis Type="Italic">n</Emphasis>) reactions in transmutation of long-lived fission products

The conditions under which (n,γ) and (n,2n) reactions can help or hinder each other in neutron transmutation of long-lived fission products (LLFPs) are considered. Isotopic and elemental transmutation for the main long-lived fission products, ⁷⁹Se, ⁹³Zr, ⁹⁹Tc, ¹⁰⁷Pd, ¹²⁶Sn, ¹²⁹I, and ¹³⁵Cs, are considered. The effect of (n,2n) reactions on the equilibrium amount of nuclei of the transmuted isotope and the neutron consumption required for the isotope processing is estimated. The aim of the study is to estimate the influence of (n,2n) reactions on efficiency of neutron LLFP transmutation. The code TIME26 and the libraries of evaluated nuclear data ABBN-93, JEF-PC, and JANIS system are applied. The following results are obtained: (1) The effect of (n,2n) reactions on the minimum number of neutrons required for transmutation and the equilibrium amount of LLFP nuclei is estimated. (2) It is demonstrated that, for three LLFP isotopes (¹²⁶Sn, ¹²⁹I, and ¹³⁵Cs), (n,γ) and (n,2n) reactions are partners facilitating neutron transmutation. The strongest effect of (n,2n) reaction is found for ¹²⁶Sn transmutation (reduction of the neutron consumption by 49% and the equilibrium amount of nuclei by 19%).