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.
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.
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.
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 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 process of photoproduction of a series of radioisotopes from copper nuclei at the bremsstrahlung end-point energies
= 21, 30, and 40 MeV was studied at the AANL (YerPhI) linear electron ...accelerator. The relative yields of
61
Co,
60
Co,
58
Co,
57
Co with respect to the
61
Cu are measured; in particular, at
= 40 MeV they are equal to (2.6 ± 0.3) × 10
–2
, (2.67 ± 0.04) × 10
–2
, (9.3 ± 0.6) × 10
–2
, and (0.76 ± 0.04) × 10
–2
, respectively. It is shown that the predictions of the TALYS model, as well as the model inserted in the GEANT4 software package are in a contradiction (especially in the case of the GEANT4 code) with the measured relative yields. A continuously decreasing energy dependence was observed for the ratio of unfolded (weighted with the bremsstrahlung spectrum) cross-sections of reactions
65
Cu(γ,α)
61
Co and
63
Cu (γ,2n)
61
Cu, similarly to that observed earlier for the ratio of cross-sections of electronuclear reactions
65
Cu(e,e′ + α)
61
Co and
63
Cu (e, e′ + 2n)
61
Cu. The ratio of the
61
Co and
64
Cu yields is also measured, being equal to (2. 6± 0.2)×10
–3
, (3.8 ± 0.6) × 10
–3
, and (3.2 ± 0.4) × 10
–3
, respectively, at
= 21 MeV, 30 MeV, and 40 MeV; its value (2.6 ± 0.2) × 10
–3
at
= 21 MeV is consistent with the general trend of the dependence of this ratio on the atomic number
Z
, measured recently at
= 23 MeV for heavier nuclei with the atomic numbers from Z = 47 to
Z
= 82.
Search for rare decay modes of Californium Ayvazyan, G. M.; Gulkanyan, H. R.; Pogosov, V. S. ...
Journal of contemporary physics,
2016/1, Letnik:
51, Številka:
1
Journal Article
Recenzirano
The results ofmeasurements on the search for the cluster decay of
252
Cfnucleus with the emission ofthe bound state of eight neutrons are presented. The value of an upper limit ofthe partial ...probabilities for this process is obtained, which contradicts to the results ofthe recently published work, in which the possible existence of octaneutron was declared. Based on the results of our measurements, the estimations for the upper limits ofpartial probabilities for the emission of
20
C,
24
O,
28
Ne, 32Mg,
36
Si,
40
S,
40
P, and
40
Si clusters are given. The estimation for the upper limit of the partial probability of
250
Cf decay with the emission of the bound state of six neutrons is also obtained.
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.
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