[en] At the linear electron accelerator AANL (YerPhI), at mean electron energies Ê_e = 28 and 30 MeV (with a Gaussian width σ_e ≅ 0.7 MeV), an attempt is undertaken to search for near threshold production of four-neutron system in the reaction ²⁰⁹Bi(γ,4n) ²⁰⁵Bi (with the threshold energy Ε_γ ^th = 29.5 MeV) induced by Bremsstrahlung photons. The induced activity method was applied in the experiment. The gamma-spectroscopic measurements were carried out in the underground laboratory of the AANL. Owing to low-background conditions in the laboratory, one achieves, at Ê_e = 30 MeV, a reliably identification of ²⁰⁵Bi radionuclide and first determination of the near-threshold cross sections of this rare process, namely, the weighted by the spectrum of bremsstrahlung photons cross section σ_w = (4.42 ± 0.48)×10^-4 mb and the averaged over the spectrum of bremsstrahlung photons cross section <σ> = (1.95 ± 0.22) mb. No ²⁰⁵Bi production was observed at Ê_e = 28 MeV. Low-background conditions allow one also to identify the rare decays of the ²⁰⁸Bi nucleus produced in the reaction ²⁰⁹Bi(γ,n) ²⁰⁸Bi, and measure its weighted and averaged cross sections at Ê_e = 28 and 30 MeV: σ_w = (19.6 ± 3.5) and (16.4 ± 2.2) mb and <σ> = (139 ± 25) and (142 ± 19) mb, respectively. From the data at Ê_e = 28 MeV, an upper limit of the yield of a hypothetical four-neutron bound state (tetraneutron) production reaction relative to the yield of the reaction ²⁰⁹Bi(γ, n) ²⁰⁸Bi is estimated to be 10^-5. The obtained experimental data are compared with predictions of the TALYS1.9 and FLUKA models. It is shown that the measured at Ê_e = 30 MeV cross section of the reaction ²⁰⁹Bi(γ,4n) ²⁰⁵Bi strongly overestimates (by more than one order of magnitude) the theoretical value. This disagreement can be caused by the fact that the models do not foresee the production of correlated neutron clusters, in particular, four-neutron resonance state with 2.4 MeV excitation energy observed recently in the reaction 1^H(⁸He,¹H+⁴He)4n. The near-threshold production of four-neutron system, observed in present work, can, probably, serve as an indirect indication on its resonance nature

[en] Within the framework of the method of hyperspherical functions we study the continuous spectrum states of the tetraneutron on the basis of ideas of ''truly'' four-particle scattering. We show that ⁴n has a resonance state because of existence of a kinematical barrier. Parameters of this state are estimated

[en] The four-neutron system is studied using exact continuum equations for transition operators and solving them in the momentum-space framework. A resonant behavior is found for strongly enhanced interaction but not a the physical strength, indicating the absence of an observable tetraneutron resonance, in contrast to a number of earlier works. As the transition operators acquire large values at low energies, it is conjectured that this behavior may explain peaks in many-body reactions even without a resonance.

No abstract available

[en] Short note. 3 refs

No abstract available

[en] The trineutron and tetraneutron binding energies are calculated using an interaction which reproduces ³H, ³He, ⁴H, ⁴He, and ⁴Li ground-state properties. A 4 h/2πω model space and completely symmetrized translationally invariant harmonic oscillator eigenstates are used in the analysis. The calculation yields a ³n eigenvalue within the range suggested in an experiment by Ohlsen et al. The ⁴n eigenvalue is consistent with the location of the first T = 2 level in the ⁴He system. (orig.)

[en] The SS-HORSE method is applied to describe the scattering of nucleons by light nuclei based on calculations within the No-Core Shell Model with JISP16 and Daejeon16 NN interactions. The resonant states in ⁵He, ⁵Li, and ⁷He nuclei have been investigated. The SS-HORSE method generalized to the case of the democratic decay is also applied to study a four-neutron system (tetraneutron). The calculations with JISP16 and Daejeon16 and with chiral NN interactions indicate the existence of a rather narrow low-energy tetraneutron resonance.

No abstract available

[en] The tetraneutron has attracted the attention of nuclear physicists during the past decades, but there is still no unambiguous confirmation of its existence or non-existence. A new experiment based on ⁸He(p, 2p)⁷H(t+⁴n) reaction, with direct detection of the four neutrons, has been carried out at RIBF, which can hopefully help to draw a definite conclusion on the tetraneutron system. (author)