Bulletin of the Russian Academy of Sciences: Physics最新文献

Probabilities that are close for the (s, f) and (n_th, f) fission reactions of the corresponding nuclei are estimated for the formation of third particles in the neck of a fissile nucleus. The probabilities are obtained using formulas for calculating the widths of spontaneous and thermal neutron-induced ternary fission of atomic nuclei with the emission of light charged particles. The formulas are based on an approach that views ternary fission as a virtual process, in combination with experimental energy distributions of α particles, hydrogen isotopes, and ⁶He nuclei in the ternary fission of actinide nuclei. It is shown that spontaneous and induced ternary fissions of actinide nuclei with the emission of light charged particles and nucleons occurs from close configurations of the fissile nucleus. The bonding energy of the thermal neutron introduced into the fissile compound nucleus in reactions (n_th, f) is added to the energy of deformation the fissile nucleus but not to the kinetic energy of the third light particle.

Time measurements using a fast beam–beam collisions monitor based on chevron assemblies of microchannel plates (MCPs) were simulated in the Quartus software environment. The measurement algorithm was based on the delayed coincidence method. Four-channel electronics for an MCP detector based on high-speed comparators and an EPM240 FPGA (ALTERA) were created. The test prototype was tested using a 4-channel nanosecond pulse generator with an adjustable delay between channels. It was determined that, using these comparators and FPGAs, the time of detection of particles by a detector can be found with an error of 100 ps, and the calculated speed of the readout circuit coincides with the measured one and is no more than 10 ns for each event for the 4-channel system.

Total cross sections σ_d,xt of reaction ⁹Be(d, xt) are measured in the 1.5–12 МeV range of deuteron energies with an error of 8.3% using activation reaction ⁹Be + t → α + ⁸Li + 2.927 МeV (T_1/2 = 0.84 s, E_β max = 13 МeV) according to B.Ya. Guzhovskii’s method on the EGP-10 electrostatic tandem accelerator at the All-Russian Research Institute of Experimental Physics (VNIIEF). The primary target and convertor functions are combined in a beryllium layer 1 mm thick, which can be done if the layer is thicker than the sum of the paths of primary deuterons and secondary tritons. The main problem in measuring the cross sections of activation is the lack of reliable data on the mean energy of tritons produced in the considered reaction. This value is derived from measured triton energy spectra. Spectrometric differential cross sections are obtained for the first time and serve as the basis for calculating total reaction cross sections. Good agreement is shown between the total activation and spectrometric cross sections.

The authors consider the problem of describing theoretically the dynamics of nucleon clustering inside a fissile nucleus. The approach is based on the microscopic modeling of clustering as a new type of collective particle motion. The use of a dynamic clustering algorithm in the region of heavy nuclei requires effective multiparticle interaction to be developed for a distributed microscopic model. Calculations are performed for a double magic cluster that plays an important role in the formation of the second minimum of the fission barrier observed in the multimodal fission of heavy nuclei.

The energy and the squared modulus of the wave function of the ground state of the ⁹Be nucleus as a system of two alpha-clusters and an outer neutron were calculated using hyperspherical functions. A system of hyperradial equations was solved using cubic splines. The charge distribution and the root-mean-square charge radius for the ⁹Be nucleus were calculated and were found to agree with the experimental data.

Using the experimental-theoretical method for evaluation of partial photoneutron reaction cross sections basing on objective physical criteria the reliability of data on the (γ, 1n) and (γ, 2n) reaction cross sections of ⁵¹V and ⁵⁹Co nuclei obtained on bremsstrahlung γ-rays was investigated. It is found that partial reaction cross sections obtained by adding corrections to neutron yield cross sections σ(γ, xn) = σ(γ, 1n) + 2σ(γ, 2n) calculated within statistical theory do not satisfy the reliability criteria. Cross sections of the (γ, 1n) reaction are significantly underestimated in the considered experiments, while those of the (γ, 2n) reaction are overestimated, due obviously to the way information is obtained on the partial reaction cross sections by applying statistical theory corrections to the yield cross section of the σ(γ, xn). When describing the (γ, 1n) reaction cross section at photon energies below threshold B2n of the (γ, 2n) reaction, such corrections produce substantial systematic uncertainties in the ranges of energy where both partial reactions compete.

The correlation between the variations of multiplicity of prompt fission neutrons and the total kinetic energy of fission fragments was studied using a double ionization chamber in combination with a prompt fission neutron detector. An estimate was made of the systematic errors introduced by crosstalk caused by the multi-module structure of the neutron detector.

A method based on Bayes’ formula was used to estimate the relative error in determining the number of spectator nucleons from the energy measured in a calorimeter during the collision of relativistic nuclei. The minimum possible relative error in determining the number of spectator nucleons in an event is obtained for an arbitrary distribution of spectators.

A method of testing of threshold SiO₂ aerogel Cherenkov detectors using cosmic radiation is described. The method requires a minimum amount of additional electronics and does not involve massive absorber layers (e.g., lead). The method makes it possible to directly take into account the threshold in determining the efficiency of the detectors under study and to test their operation in different energy ranges for relativistic charged particles. Good agreement between the results of the simulation and measurements was obtained.

To study the cluster structure of light nuclei in the reactions of interaction of fast neutrons with the ⁹Be nucleus, the kinematics of the reactions n + ⁹Be → α + ⁶He and n + ⁹Be → ⁸Be + 2n → 2α + 2n at energies of 1–3 MeV were modeled. It was shown that the characteristics of reaction channels in the interaction of a neutron with a ⁹Be nucleus can be found by measuring the ionization losses of charged reaction fragments in a multilayer gas-filled charged particle detector with a beryllium converter.