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Search for the population of the low-energy continuum of tetraneutron system was performed for reactions of the $^{8}$He beam on deuterium target. These studies are performed for the data [I.A. Muzalevskii et al., Phys. Rev. C 103, 044313 (2021)], previously used for the studies of $^{7}$H and $^{6}$H in the $^2\text{H}(^8\text{He},{^3\text{He}})^{7}$H and $^2\text{H}(^8\text{He},{^4\text{He}})^{6}$H reactions. Evidence for a hump in the $^4n$ continuum at $3.5 \pm 0.7$ and $3.2 \pm 0.8$ MeV, was observed in the $^2$H($^8$He,$^6$Li)$^4$n and $^2$H($^8$He,$^3$He)$^7$H$\rightarrow ^3$H+$^4$n reactions, respectively. The observed statistics is quite low (6 and up to 40 events) corresponding to very low cross sections of few microbarns or tens of microbarns. The background conditions for the $^2$H($^8$He,$^6$Li)$^4n$ reaction are shown to be good, favoring the physical nature of the observed events. The $^2$H($^8$He,$^3$He)$^7$H$\rightarrow ^3$H+$^4n$ process transforms to the $^2$H($^8$He,$^6$Li$^{\ast})^4n$ reaction in the limit of the highest $^7$H decay energies. The population of the low-energy region in the $^{4}$n spectrum is found to be perfectly correlated with the population of the lowest $^{6}$Li states in the $^{3}$He+$^{3}$H continuum. Theoretical calculations of $^{8}$He in a five-body $\alpha$+$4n$ and of $^{4}$n in a four-body hyperspherical models are presented. The $^{8}$He wave function is shown to contain strong specific correlations, which may give rise to very low-energy structures in tetraneutron continuum in extreme-peripheral reaction scenarios., Comment: 13 pages, 9 figures

Coulomb and nuclear dissociation of $^{17}$Ne on light and heavy targets are studied theoretically. The dipole E1 strength function is determined in a broad energy range including energies of astrophysical interest. Dependence of the strength function on different parameters of the $^{17}$Ne ground state structure and continuum dynamics is analyzed in a three-body model. The discovered dependence plays an important role for studies of the strength functions for the three-body E1 dissociation and radiative capture. The constraints on the $[s^2]/[d^2]$ configuration mixing in $^{17}$Ne and on $p$-wave interaction in the $^{15}$O+$p$ channel are imposed based on experimental data for $^{17}$Ne Coulomb dissociation on heavy target., Comment: 12 pages, 13 figures

The $^{6}$Be continuum states were populated in the charge-exchange reaction $^1$H($^{6}$Li,$^{6}$Be)$n$ collecting very high statistics data ($\sim 5 \times 10^6$ events) on the three-body $\alpha$+$p$+$p$ correlations. The $^{6}$Be excitation energy region below $\sim 3$ MeV is considered, where the data are dominated by contributions from the $0^+$ and $2^+$ states. It is demonstrated how the high-statistics few-body correlation data can be used to extract detailed information on the reaction mechanism. Such a derivation is based on the fact that highly spin-aligned states are typically populated in the direct reactions., Comment: submitted to Physical Review C

The interaction of an $E/A$=57.6-MeV $^{17}$Ne beam with a Be target was used to populate levels in $^{16}$Ne following neutron knockout reactions. The decay of $^{16}$Ne states into the three-body $^{14}$O+$p$+$p$ continuum was observed in the High Resolution Array (HiRA). For the first time for a 2p emitter, correlations between the momenta of the three decay products were measured with sufficient resolution and statistics to allow for an unambiguous demonstration of their dependence on the long-range nature of the Coulomb interaction. Contrary to previous experiments, the intrinsic decay width of the $^{16}$Ne ground state was found to be narrow ($\Gamma<60$~keV), consistent with theoretical estimates., Comment: 6 pages, 5 figures

Structure with the lowest energy observed in the $^{10}$He spectrum populated in the proton knockout reaction with $^{11}$Li beam has a peak at $1.2-1.5$ MeV. This peak is usually interpreted as a resonant $0^+$ ground state of $^{10}$He. Our theoretical calculations indicate that this peak is likely to be a pileup of $1^-$, $0^+$, and $2^+$ excitations with very similar shapes. %We predict a very specific nature of the $1^-$ excitation in $^{10}$He. Moreover, the ``soft'' $1^-$ excitation appears to be the lowest one in energy. Such an anomalous continuum response is traced to the halo structure of $^{11}$Li providing extreme low energy shift to all the expected continuum excitations. Competitions of the initial state structure (ISS) and the final state interaction (FSI) effects on the spectrum and three-body correlations in $^{10}$He are discussed. Analogous effect of the extreme low-energy shift could also be expected in other cases of $2n$ emitters populated in reactions with halo nuclei. Simplified example of the $^{10}$He spectrum in $\alpha$ knockout from $^{14}$Be, is given. We also discuss limits on the properties of $^{9}$He stemming from the observed $^{10}$He spectrum., Comment: 10 pages, 13 figures

The nucleosynthesis of elements beyond iron is dominated by neutron captures in the s and r processes. However, 32 stable, proton-rich isotopes cannot be formed during those processes, because they are shielded from the s-process flow and r-process beta-decay chains. These nuclei are attributed to the p and rp process. For all those processes, current research in nuclear astrophysics addresses the need for more precise reaction data involving radioactive isotopes. Depending on the particular reaction, direct or inverse kinematics, forward or time-reversed direction are investigated to determine or at least to constrain the desired reaction cross sections. The Facility for Antiproton and Ion Research (FAIR) will offer unique, unprecedented opportunities to investigate many of the important reactions. The high yield of radioactive isotopes, even far away from the valley of stability, allows the investigation of isotopes involved in processes as exotic as the r or rp processes., Comment: Invited Talk given at the Conference Nuclear Physice in Astrophysics VI (NPA6), Lisbon, Portugal, May 19-24 2013. To appear in the NPA6 Proceedings in Journal of Physics: Conference Series (JPCS)

The influence of the initial-state structure and the reaction mechanism on three-body decays is investigated by example of the 6Be continuum populated in neutron-knockout reactions on 7Be. The sensitivity of the 6Be excitation spectrum and the three-body correlations to the different components of the model is demonstrated. It is shown that the spin composition of the initial state may have an overwhelming effect on the three-body continuum. The characteristics and structure of the second $0^+_2$ and $2^+_2$ states in 6Be are predicted for the first time and the conditions for their reliable observations are formulated. The effects of interference and the alignment of three-body states on the three-body correlations are demonstrated., Comment: 6 pages, 7 figures

The interaction of an E/A=70 - MeV 7Be beam with a Be target was used to populate levels in 6Be following neutron knockout reactions. The three-body decay of the ground and first excited states into the alpha+p+p exit channel were detected in the High Resolution Array. Precise three-body correlations extracted from the experimental data allowed us to obtain insight into the mechanism of the three-body democratic decay. The correlation data are in a good agreement with a three-cluster-model calculation and thus validate this theoretical approach over a broad energy range., Comment: 5 pages, 7 figures; Corrected typos

The 0+ ground state of the 10He nucleus produced in the 3H(8He,p)10He reaction was found at about $2.1\pm0.2$ MeV (\Gamma ~ 2 MeV) above the three-body 8He+n+n breakup threshold. Angular correlations observed for 10He decay products show prominent interference patterns allowing to draw conclusions about the structure of low-energy excited states. We interpret the observed correlations as a coherent superposition of the broad 1- state having a maximum at energy 4-6 MeV and the 2+ state above 6 MeV, setting both on top of the 0+ state "tail". This anomalous level ordering indicates that the breakdown of the N=8 shell known in 12Be thus extends also to the 10He system., Comment: 5 pages, 5 figures

By using the 1H(6Li,6Be)n charge-exchange reaction, continuum states in 6Be were populated up to E_t=16 MeV, E_t being the 6Be energy above its three-body decay threshold. In kinematically complete measurements performed by detecting alpha+p+p coincidences, an E_t spectrum of high statistics was obtained, containing approximately ~5x10^6 events. The spectrum provides detailed correlation information about the well-known 0^+ ground state of 6Be at E_t=1.37 MeV and its 2^+ state at E_t=3.05 MeV. Moreover, a broad structure extending from 4 to 16 MeV was observed. It contains negative parity states populated by Delta L=1 angular momentum transfer without other significant contributions. This structure can be interpreted as a novel phenomenon, i.e. the isovector soft dipole mode associated with the 6Li ground state. The population of this mode in the charge-exchange reaction is a dominant phenomenon for this reaction, being responsible for about 60% of the cross section obtained in the measured energy range., Comment: 8 pages, 7 figures

Unknown isotope 26S, expected to decay by two-proton (2p) emission, was studied theoretically and was searched experimentally. The structure of this nucleus was examined within the relativistic mean field (RMF) approach. A method for taking into account the many-body structure in the three-body decay calculations was developed. The results of the RMF calculations were used as an input for the three-cluster decay model worked out to study a possible 2p decay branch of this nucleus. The experimental search for 26S was performed in fragmentation reactions of a 50.3 A MeV 32S beam. No events of 26S or 25P (a presumably proton-unstable subsystem of 26S) were observed. Based on the obtained production systematics an upper half-life limit of T_{1/2}<79 ns was established from the time-of-flight through the fragment separator. Together with the theoretical lifetime estimates for two-proton decay this gives a decay energy limit of Q_{2p}>640 keV for 26S. Analogous limits for 25P are found as T_{1/2}<38 ns and Q_{p}>110 keV. In the case that the one-proton emission is the main branch of the 26S decay a limit Q_{2p}>230 keV would follow for this nucleus. It is likely that 26S resides in the picosecond lifetime range and the further search for this isotope is prospective for the decay-in-flight technique., Comment: 15 pages, 7 figures

Nowadays quantum-mechanical theory allows one to reliably calculate the processes of 2p radioactivity (true three-body decays) and the corresponding energy and angular correlations up to distances of the order of 1000 fm. However, the precision of modern experiments has now become sufficient to indicate some deficiency of the predicted theoretical distributions. In this paper we discuss the extrapolation along the classical trajectories as a method to improve the convergence of the theoretical energy and angular correlations at very large distances (of the order of atomic distances), where only the long-range Coulomb forces are still operating. The precision of this approach is demonstrated using the "exactly" solvable semianalytical models with simplified three-body Hamiltonians. It is also demonstrated that for heavy 2p emitters, the 2p decay momentum distributions can be sensitive to the effect of the screening by atomic electrons. We compare theoretical results with available experimental data., Comment: 13 pages, 18 figures

We consider methods of charged particle acceleration by means of high-intensity lasers. As an application we discuss a laser booster for heavy ion beams provided, e.g. by the Dubna nuclotron. Simple estimates show that a cascade of crossed laser beams would be necessary to provide additional acceleration to gold ions of the order of GeV/nucleon., Comment: 4 pages, 4 figures, Talk at the Helmholtz International Summer School "Dense Matter in heavy Ion Collisions and Astrophysics", August 21 - September 1, 2006, JINR Dubna, Russia; v2, misprints corrected

We study spectrum inclusion regions for complex Jacobi matrices which are compact perturbations of real periodic Jacobi matrices. The condition sufficient for the lack of discrete spectrum for such matrices is given, Comment: LaTeX, 16 pages

The discrete spectrum of complex Jacobi matrices that are compact perturbations of the discrete laplacian is under consideration. The rate of stabilization for the the matrix entries which provides finiteness of the discrete spectrum and is sharp in the sense of order is found. The Jacobi matrix with the discrete spectrum having a single limit point is constructed. The results can be viewed as the discrete analogs of the well known theorems by B.S. Pavlov about Schr\"odinger operators on the half line with a complex potential., Comment: LaTeX, 24 pages

We study spectrum inclusion regions for complex Jacobi matrices which are compact perturbations of the discrete laplacian. The condition sufficient for the lack of discrete spectrum for such matrices is given., Comment: LaTeX, 7 pages