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For the first time in Armenia, gamma activation analysis of geological samples of obsidian was carried out using beams of bremsstrahlung photons at the linear electron accelerator of the A. Alikhanyan National Scientific Laboratory (AANL). It is shown that the results of the chemical composition of the obsidian sample are comparable with the results of instrumental neutron activation analysis (INAA) of the same sample, carried out at the Curt-Engelhom-Center for Archaeometry (Mannheim, Germany), after irradiation with neutrons in a specialized nuclear reactor TRIGA at the Institute of Nuclear Chemistry of the Mainz University. It is also shown that gamma activation analysis makes it possible to determine the content of a number of elements for which the use of INAA is impossible or difficult. [ABSTRACT FROM AUTHOR]

At the linear electron accelerator AANL (YerPhI), at mean electron energies Êе = 28 and 30 MeV (with a Gaussian width σе ≈ 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 = 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 Êе = 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 σw = (4.42 ± 0.48) × 10–4 mb and the averaged over the spectrum of bremsstrahlung photons cross-section = (1.95 ± 0.22) mb. No 205Bi production was observed at Êе = 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 Êе = 28 and 30 MeV: σw = (19.6 ± 3.5) and (16.4 ± 2.2) mb and = (139 ± 25) and (142 ± 19) mb, respectively. From the data at Êе= 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 Êе = 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. [ABSTRACT FROM AUTHOR]

Using the induced activity method, the inclusive process 16O(γ, X)7Be of the 7Ве isotope photoproduction from oxygen nuclei was studied at the linear electron accelerator AANL (YerPhI) at the boundary energies of bremsstrahlung photons = 40 and 70 MeV. This process was first observed in the near-threshold energy range Eγ < 40 MeV in this work owing to low-background conditions in the underground laboratory of the AANL where spectroscopic measurements were carried out. The cross-section averaged over the spectrum of bremsstrahlung photons and the cross-section per equivalent photon has been measured. Estimates are also obtained for the integral cross-section of this process. The measured cross-sections are compared with the available experimental data and predictions of the TALYS1.9, GEANT4, and FLUKA models. It is shown that the predictions of TALYS and GEANT4 are strongly underestimated (especially at = 40 MeV) as compared with our data, which is associated with a strong underestimation in model computations of the role of two main near-threshold reaction channels: the 16O(γ, 9Be)7Be channel, which proceeds through the two-particle photofission mechanism and the 16O(γ, n + α + α)7Be spallation channel. The FLUKA predictions are qualitatively comparable with experimental data at Eγ < 40 MeV and Eγ = (50–60) MeV, however, they greatly exceed them in the energy range Eγ = (40–50) MeV, this excess is almost entirely caused by the overestimated contribution of the 16O(γ, n + α + α)7Be spallation reaction cross-section. The data obtained may turn out to be useful in attempts to improve theoretical models of nuclear reactions, in the study of the radioisotope content of the earth's atmosphere, and also be of interest in connection with the so-called "primary lithium problem". [ABSTRACT FROM AUTHOR]