Том 82, № 6 (2018)

Photonuclear reactions on a natural mixture of tin isotopes are studied by means of gamma activation. The experiment is performed using the bremsstrahlung γ-beam of an RTM55 racetrack microtron with a maximum electron energy of 55.6 MeV. The measured yields of photoproton and photoneutron reactions are compared to results from other experimental studies, and to calculations using the TALYS code and the combined model of photonucleon reactions (CMPR).

The demand for more precise nuclear data on beryllium interactions is growing along with the development of controlled fusion facilities, where beryllium is already used or is planned to be used as a structural material in the coating of the first inner wall of the tokamak (the tritium breeding blanket). Evaluated cross sections of the ⁹Be(d, x)¹⁰Be_gs reaction at deuteron energies ranging from 0 to 22 MeV are presented. All available experimental data and the mathematical tools of the SaBa [1] electronic library developed at the Russian Federal Nuclear Center All-Russian Research Institute of Experimental Physics are used to obtain these cross sections.

series of experiments performed in 2014–2015 using the SHELS separator and a beam of multicharged ions from the U-400 accelerator at the Flerov Laboratory of Nuclear Reactions (Joint Institute for Nuclear Research) in Dubna is described. In preparation for a large program of experiments with a beam of highly charged ions of ⁵⁰Ti, an experiment is performed to study the spontaneous fissioning (SF) of short-lived neutron-deficient isotope ²⁵⁶Rf formed in complete fusion reaction⁵⁰Ti + ²⁰⁸Pb = ²⁵⁶Rf + 2n. The half-life of ²⁵⁶Rf is measured along with the probability of its SF decay. The average number of neutrons per one event of ²⁵⁶Rf SF (\(\bar v\)= 4.47 ± 0.09) is obtained for the first time. The results from experiments to study the SF of short-lived neutron-deficient isotopes synthesized in complete fusion reaction ⁴⁸Ca + ²⁰⁴Pb → ²⁵²No* are presented as well. Two short-lived activities attributed to the SF of the ground and isomeric states of ²⁵⁰No are revealed. The half-life, total kinetic energies of fission fragments, and neutron multiplicities are measured for the obtained short-lived nuclei. The average numbers of neutrons per one SF event are \(\bar v\)= 4.38 ± 0.13 and \(\bar v\)= 3.90 ± 0.20, respectively, for SF activity with t_1/2 = 5.1 ± 0.3 and t_1/2 = 36 ± 3 μs.

To investigate the impact shell effects have in the formation of neutron-rich fragments in multinucleon transfer reactions, a series of experiments to explore the binary channel in ^156,160Gd + ¹⁸⁶W reactions at energies near and above the Coulomb barrier is performed at the Flerov Laboratory’s U-400 accelerator using the CORSET setup. These experiments are aimed mainly at obtaining the production cross sections of leadlike fragments in the process of inverse quasifission. The mass, energy, and angular distributions of the binary reaction products are measured at energies of 860 and 935 MeV of ¹⁶⁰Gd ions and 878MeV in the case of ¹⁵⁶Gd ions. The excitation energies of primary fragments are estimated using their measured mass–energy distributions. Enhanced yields of products with masses of 200–215 amu are observed for both reactions. At energies above the barrier for side-to-side collisions (935 MeV), the yield of lead-like fragments is an order of magnitude larger than at energies near the Coulomb barrier, due possibly to the influence of orientation effects. The enhancement observed in the yield of reaction products with masses heavier than the target mass confirms that multinucleon transfer reactions can be used to obtain new neutron-rich isotopes, and to synthesize new superheavy elements.

New computational schemes, symbolic-numerical algorithms and programs implementing the high-accuracy finite element method (FEM) for the solution of quantum mechanical boundary-value problems (BVPs) are reviewed. The elliptic BVPs in 2D and 3D domains are solved using the multivariable FEM and Kantorovich method using parametric basis functions. We demonstrate and compare the efficiency of the proposed calculation schemes, algorithms, and software by solving the benchmark BVPs that describe the scattering on a barrier and a well, the bound states of a helium atom, and the quadrupole vibration in a collective nuclear model.

Many problems of numerically solving the Schrödinger equation require that we choose asymptotic distances many times greater than the characteristic size of the region of interaction. If the solution to one-dimensional equations can be immediately chosen in a form that preserves unitarity, the invariance of probability (in the form of, e.g., fulfilling an optical theorem) is a real problem for two-dimensional equations. An addition that does not exceed the discretization error and ensures a high degree of unitarity is proposed as a result of studying the properties of a discrete two-dimensional equation.

Helium ⁵Hе states are sought in missing-mass (MM) spectra from reactions of the absorption of stopped pions: ¹⁰В(π⁻, dt)Х and ¹¹В(π⁻, tt)Х. The experiment is conducted using the LANL accelerator equipped with a two-arm semiconductor charged-particle spectrometer. The parameters of the ground state agree with the literature data. The best fit for the first excited level is obtained at E_r = 1.8(5) MeV and Γ = 4.7(7) MeV. Neither of the two reactions produce statistically significant evidence for the presence of highly excited (E_x ≥ 20 MeV) states of ⁵Hе.

Different estimates of the even–odd effect for the nuclear mass surface are discussed. The proton-number dependence of the energy gap is derived from the measured masses of the N = 20, 50, and 82 isotones with closed neutron shells. Its interrelation with the properties of external proton shells is demonstrated, and the effects of proton pairing are considered along with the microscopic nuclear structure.

The emergence of the pairing effect of identical nucleons in the j = 9/2 state in low-lying excited states of nuclei near ⁹⁰Zr (N = 50, Z = 40) is discussed. Multiplets of states with seniority s ≥ 2, the splitting of which is determined by the proton pairing energy, are clearly visible in the nuclear spectra for a chain of N = 50 isotones. A comparison of the spectra of ground state multiplets, calculated in the δ-interaction approximation, with experimental data and results from other theoretical calculations shows this approach can be used to describe the structure of spectra and level positions with high J values.

Double beta decay (β + EC, EC/EC) of ⁵⁸Ni is investigated at France’s Modane Underground Laboratory (4800 m water equivalent) using the OBELIX ultralow-background HPGe detector with a sensitive volume of 600 cm³ and a natural nickel sample of ~68% 58Ni with a mass of ~21.7 kg. After preliminary analysis of the experimental data accumulated over ~144 days, new experimental limits are obtained for the 2νβ⁺EC decay of ⁵⁸Ni to the 0⁺ ground state and the 2₁⁺, 811 keV excited state of ⁵⁸Fe, and for the 2νEC/EC decay of ⁵⁸Ni to the 2₁⁺, 811 keV and 2₂⁺, 1675 keV excited states of ⁵⁸Fe. The limits are T_1/2(β⁺EC,0→0⁺) > 1.7 × 10²² yr, T_1/2(β⁺EC,0→2₁⁺) > 2.3 × 10²² yr, T_1/2(EC/EC,0→2₁⁺) > 3.3 × 10²² yr, and T_1/2(EC/EC,0→2₂⁺) > 3.4 × 10²² yr. Experimental limit T_1/2(0νEC/EC–res, 1918 keV > 4.1 × 10²² yr is obtained for resonant neutrinoless radiative EC/EC decay with an energy of 1918.3 keV. All limits are at 90% CL.

The mass and energy distributions of fission fragments in reactions ¹⁶O + ²³²Th, ¹⁶O + ²³⁸U, ²²Ne + ²³²Th, and ²²Ne + ²³⁸U are measured to investigate the role of closed proton and neutron shells in the fission of ²⁴⁸Cf, ²⁵⁴Fm, and ²⁶⁰No nuclei at excitation energies of 40–45 MeV. Experiments are performed on the U-400 cyclotron at the Flerov Laboratory of Nuclear Reactions (Dubna, Russia) using the CORSET double-arm time-of-flight spectrometer. For compound nucleus ²⁶⁰No* formed in the reaction ²²Ne + ²³⁸U, an increase in the fragment yields is observed in the superasymmetric mass region 52/208 amu, which corresponds to the formation of fissioning pairs of two double-magic nuclei, ⁴⁸Ca and ²⁰⁸Pb. Superasymmetric fission at a mass yield level of 10−5 was not observed for compound nuclei ²⁴⁸Cf and ²⁵⁴Fm* formed in reactions ¹⁶O + ²³²Th, ¹⁶O + ²³⁸U, and ²²Ne + ²³²Th at an initial excitation energy of 40−45 MeV. Bimodal fission was observed for the ²⁶⁰No*compound nucleus at an initial excitation energy of 41 MeV. In the symmetric mass split, both fission fragments are in this case close to double-magic ¹³²Sn.

Data obtained on the IN-LUE photoneutron source at the Russian Academy of Sciences’ Institute for Nuclear Research are analyzed. The data are acquired using a position-sensitive neutron detector with an active ¹⁰B layer and a ³He SNM-18 counter. The efficiency of the ¹⁰B detector relative to the ³He counter ranges from 2 to 4% for different modes of accelerator operation. Waveforms of the ³He counter’s pulses are recorded using a built-in L-CARD L-783 multifunctional high speed signal processor with sampling frequencies of up to 3 MHz. The amplitude and time spectra of thermal neutrons are obtained from the waveforms using induced electromagnetic signals from the accelerator dump and neutron pulses. Using a cadmium mask with slits and a position-sensitive detector with a low (<10⁻⁷) sensitivity to electromagnetic interference and the gamma-ray background allows the spatial resolution of the¹⁰B detector to be determined and the contribution from neutrons with different energies to b estimated.

Ways of reconstructing the form of the energy spectrum of the W–Be photoneutron source at the Russian Academy of Sciences’ Institute for Nuclear Research are considered, along with contributions to the spectrum of thermal and epithermal neutrons. A comparative analysis of neutron capture cross sections for various nuclei is performed, accompanied by convolutions of these cross sections with different energy regions of the model neutron spectrum. A number of elements are selected as samples for irradiation by source neutrons and subsequent neutron activation analysis. Only thermal and epithermal neutrons or combinations of them make an appreciable contribution to the measured activities of these elements. The samples are irradiated and the activation spectra are measured using a low-background gamma spectrometer. The shape of the source neutron spectrum in the low-energy part of the spectrum is reconstructed. The values of the neutron flux density for these regions of the spectrum are estimated.

Experimental studies on the radiation treatment of food products by various types of ionizing radiation are conducted at Moscow State University’s Faculty of Physics. The effect different doses of X-ray radiation have on the biochemical characteristics of potatoes is considered as an alternative to gamma radiation and accelerated electrons. The effect different doses of accelerated electrons have on the microbiological parameters of refrigerated fish products is also considered. Results are presented from studies on the radiation sterilization of bioimplants in combination with chemical action. The proposed technique of combined sterilization based on the effect of an ozone–oxygen mixture and a beam of accelerated electrons allows the radiation dose of bioimplants to be reduced.

A charge division algorithm is proposed for determining coordinates in a positional sensitive neutron detector based on a layer of ¹⁰B and an ion chamber. Experimental impedances of the input resistances of amplifiers and their gain factors are considered in order to improve spatial resolution. The best spatial resolution of ~2 mm with respect to the horizontal X-coordinate and ~4 mm with respect to the vertical Y-coordinate is achieved at a power supply voltage of 700 V. The efficiency of thermal neutron registration is ~3%. A helium-3 counter of known efficiency is used to monitor thermal neutrons.

A way of describing lepton families in the context of a scheme with charge and neutral quantum numbers is considered. The possibilities of studying the structure of weak lepton currents on the basis of the total cross sections of (anti)neutrino scattering by electrons are discussed.