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.
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
209
Bi(γ,4
n
)
205
Bi (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
Ê
е
= 30 MeV, reliable identification of
205
Bi 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
= (1.95 ± 0.22) mb. No
205
Bi production was observed at
Ê
е
= 28 MeV. Low-background conditions allow one also to identify the rare decays of the
208
Bi nucleus produced in the reaction
209
Bi(γ,
n
)
208
Bi, 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
= (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
209
Bi(γ,
n
)
208
Bi 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
209
Bi(γ,4
n
)
205
Bi 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
1
H(
8
He,
1
H +
4
He)4
n
. 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.
We present a new measurement of $J/\psi$ production in Pb-Pb collisions at 158 GeV/nucleon, from the data sample collected in year 2000 by the NA50 Collaboration, under improved experimental ...conditions with respect to previous years. With the target system placed in vacuum, the setup was better adapted to study, in particular, the most peripheral nuclear collisions with unprecedented accuracy. The analysis of this data sample shows that the ($J/\psi$)/Drell-Yan cross-sections ratio measured in the most peripheral Pb-Pb interactions is in good agreement with the nuclear absorption pattern extrapolated from the studies of proton-nucleus collisions. Furthermore, this new measurement confirms our previous observation that the ($J/\psi$)/Drell-Yan cross-sections ratio departs from the normal nuclear absorption pattern for semi-central Pb-Pb collisions and that this ratio persistently decreases up to the most central collisions.
Using the induced activity method, the inclusive process
16
O(γ, X)
7
Be of the
7
Ве 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
E
γ
< 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
16
O(γ,
9
Be)
7
Be channel, which proceeds through the two-particle photofission mechanism and the
16
O(γ, n + α + α)
7
Be spallation channel. The FLUKA predictions are qualitatively comparable with experimental data at
E
γ
< 40 MeV and
E
γ
= (50–60) MeV, however, they greatly exceed them in the energy range
E
γ
= (40–50) MeV, this excess is almost entirely caused by the overestimated contribution of the
16
O(γ, n + α + α)
7
Be 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”.
The results of recent years work, performed at the scientific electron linear accelerator LUE-75 of the A.I. Alikhanyan National Science Laboratory (AANL) on accelerator physics and low-energy ...nuclear physics, are briefly described. The operating installation allows varying the beam current and energy over a wide range 10
–18
–10
–5
A and 10–75 MeV, respectively. The experiments were carried out jointly with research groups both from scientific centers in Armenia and other countries. The main directions of research with the use of electron beams of LUE-75 are outlined.
The process of the photoemission of one, two, three and four neutrons from the
209
Bi nuclei with producing, respectively,
208
Bi,
207
Bi,
206
Bi and
205
Bi daughter radioisotopes was investigated at ...the end-point energies of
= 30 and 40 MeV of bremsstrahlung photons at the AANL (YerPhI) linear electron accelerator. Particularly, for the relative yields of
208
Bi/
207
Bi,
206
Bi/
207
Bi and
205
Bi/
207
Bi the following values were obtained: 10.8 ± 2.2, 0.060 ± 0.002 and (1.45 ± 0.53)×10
–4
at
= 30 MeV and 6.1 ± 0.5, 0.120 ± 0.006 and 0.0216 ± 0.0016 at
= 40 MeV, respectively. It is shown that the predictions of the TALYS model, as well as the model which is contained in the GEANT4 package are in a contradiction with the experimental data. For instance
,
the predictions of GEANT4 at
= 40 MeV for relative yields of
208
Bi/
207
Bi and
205
Bi/
207
Bi and the prediction of TALYS for
205
Bi/
207
Bi about twice are smaller than the experimental values. At
= 30 MeV, the predictions of both models for
205
Bi/
207
Bi about two order of magnitude are smaller than the experimental values.
The production of alpha particles in photonuclear reactions on
65
Cu,
115
In,
92
Mo and
207
Pb targets in the beam of accelerated electrons with energy of 21 and 40 MeV was studied. The experiments ...were run on the Linear Electron Accelerator NLA (The Yerevan Physics Institute). The measurement results represent the yields of reactions studied as functions of the mass number of nuclei and the photon energy. The data obtained enable one to suggest a change in the mechanism of alpha particle photoproduction with increasing energy of incident photons and target mass.
We report a new measurement of J/ψ, ψ′ and Drell–Yan cross-sections, in the kinematical domain -0.425<ycm<0.575 and -0.5<cosθCS<0.5, performed at the CERN-SPS using 400 GeV/c incident protons on Be, ...Al, Cu, Ag, W and Pb targets. The dependence of the charmonia production cross-sections on the size of the target nucleus allows to quantify the so-called normal nuclear absorption. In the framework of the Glauber model, this new measurement is combined with results previously obtained with the same apparatus, under different experimental conditions, and leads to a precise determination of the J/ψ and ψ′ absorption cross-sections in the surrounding nuclear matter.
ψ′ production is studied in Pb–Pb collisions at 158 GeV/c per nucleon incident momentum. Absolute cross-sections are measured and production rates are investigated as a function of the centrality of ...the collision. The results are compared with those obtained for lighter colliding systems and also for the J/ψ meson produced under identical conditions.
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