Searching for Tetraneutron in Bismuth Nucleus Photodisintegration Reaction

At the linear electron accelerator AANL (YerPhI), at mean electron energies Ê_е = 28 and 30 MeV (with a Gaussian width σ_е ≈ 0.7 MeV), an attempt is undertaken to search for near-threshold production of the four-neutron system in the reaction ²⁰⁹Bi(γ,4n)²⁰⁵Bi (with the threshold energy \(E_{\gamma }^{{{\text{th}}}}\) = 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 low background laboratory of the AANL. Owing to low-background conditions in the laboratory, one achieves, at Ê_е = 30 MeV, reliable identification of ²⁰⁵Bi radionuclide and the 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 \(\left\langle \sigma \right\rangle \) = (1.95 ± 0.22) mb. No ²⁰⁵Bi production was observed at Ê_е = 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_е = 28 and 30 MeV: σ_w = (19.6 ± 3.5) and (16.4 ± 2.2) mb and \(\left\langle \sigma \right\rangle \) = (139 ± 25) and (142 ± 19) mb, respectively. From the data at Ê_е= 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_е = 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, the four-neutron resonance state with 2.4 MeV excitation energy observed recently in the reaction ¹H(⁸He, ¹H + ⁴He)4n. The near-threshold production of the four-neutron system, observed in the present work, can, probably, serve as an indirect indication of its resonance nature.