Background: A double-$K$-electron capture is a rare nuclear-atomic process in which two $K$ electrons are captured simultaneously from the atomic shell. A “hollow atom” is created as a result of this process. In single-$K$-shell electron-capture decays, there is a small probability that the second electron in the $K$ shell is excited to an unoccupied level or can (mostly) be ejected to the continuum. In either case, a double vacancy is created in the $K$ shell. The relaxation of the double-$K$-shell vacancy, accompanied by the emission of two $K$-fluorescence photons, makes it possible to perform experimental studies of such rare processes with the large-volume proportional gas chamber.

Purpose: The purpose of the present analysis is to estimate a double-$K$-shell vacancy creation probability per $K$-shell electron capture $P_{KK}$ of ${}^{81}\mathrm{Kr}$, as well as to measure the half-life of ${}^{78}\mathrm{Kr}$ relative to $2\nu 2K$ capture.

Method: Time-resolving current pulse from the large low-background proportional counter (LPC), filled with the krypton sample, was applied to detect triple coincidences of “shaked” electrons and two fluorescence photons.

Results: The number of $K$-shell vacancies per the $K$-electron capture, produced as a result of the shake-off process, has been measured for the decay of ${}^{81}\mathrm{Kr}$. The probability for this decay was found to be $P_{KK}=(5.7\pm 0.8)\times {10}^{-5}$ with a systematic error of ${\left(\mathrm{\Delta }{P}_{KK}\right)}_{\text{syst}}=\pm 0.4\times {10}^{-5}$. For the ${}^{78}\mathrm{Kr}\left(2\nu 2K\right)$ decay, the comparative study of single- and double-capture decays allowed us to obtain the signal-to-background ratio up to 15/1. The half-life $T_{1/2}^{2\nu 2K}(g.s.\to g.s.)=[1.9_{-0.7}^{+1.3}\left(\text{stat}\right)\pm 0.3\left(\text{syst}\right)]\times {10}^{22}y$ is determined from the analysis of data that have been accumulated over 782 days of live measurements in the experiment that used samples consisted of 170.6 g of ${}^{78}\mathrm{Kr}$.

Conclusions: The data collected during low background measurements using the LPC were analyzed to search the rare atomic and nuclear processes. We have determined $P_{KK}^{\text{exp}}$ for the $EC$ decay of ${}^{81}\mathrm{Kr}$, which are in satisfactory agreement with $Z^{-2}$ dependence of $P_{KK}$ predicted by Primakoff and Porter. This made possible to more accurately determine the background contribution in the energy region of our interest for the search for the $2K$ capture in ${}^{78}\mathrm{Kr}$. The general procedure of data analysis allowed us to determine the half-life of ${}^{78}\mathrm{Kr}$ relative to $2\nu 2K$ transition with a greater statistical accuracy than in our previous works.

Background: Fission is a fundamental decay mode of heavy atomic nuclei. The prevalent theoretical approach is based on mean-field theory and its extensions where fission is modeled as a large amplitude motion of a nucleus in a multidimensional collective space. One of the important observables characterizing fission is the charge and mass distribution of fission fragments.

Purpose: The goal of this Rapid Communication is to better understand the structure of fission fragment distributions by investigating the competition between the static structure of the collective manifold and the stochastic dynamics. In particular, we study the characteristics of the tails of yield distributions, which correspond to very asymmetric fission into a very heavy and a very light fragment.

Methods: We use the stochastic Langevin framework to simulate the nuclear evolution after the system tunnels through the multidimensional potential barrier. For a representative sample of different initial configurations along the outer turning-point line, we define effective fission paths by computing a large number of Langevin trajectories. We extract the relative contribution of each such path to the fragment distribution. We then use nucleon localization functions along effective fission pathways to analyze the characteristics of prefragments at prescission configurations.

Results: We find that non-Newtonian Langevin trajectories, strongly impacted by the random force, produce the tails of the fission fragment distribution of ${}^{240}\mathrm{Pu}$. The prefragments deduced from nucleon localizations are formed early and change little as the nucleus evolves towards scission. On the other hand, the system contains many nucleons that are not localized in the prefragments even near the scission point. Such nucleons are distributed rapidly at scission to form the final fragments. Fission prefragments extracted from direct integration of the density and from the localization functions typically differ by more than 30 nucleons even near scission.

Conclusions: Our Rapid Communication shows that only theoretical models of fission that account for some form of stochastic dynamics can give an accurate description of the structure of fragment distributions. In particular, it should be nearly impossible to predict the tails of these distributions within the standard formulation of time-dependent density-functional theory. At the same time, the large number of nonlocalized nucleons during fission suggests that adiabatic approaches where the interplay between intrinsic excitations and collective dynamics is neglected are ill suited to describe fission fragment properties, in particular, their excitation energy.

We study the production of charged particles ($K^{+}$, $K^{-}$, $p$, and $\overline{p}$) in proton-proton ($pp$) collisions at $\sqrt{s}=0.9\mathrm{TeV}$ within the paciae model whose parameters we fix by comparing the yield of charged particles with experimental data from ALICE. We analyze the production of $K^{-}p$ and $K^{+}\overline{p}$ bound states with the paciae+dcpc model. Results of our work indicate that in $pp$ collisions at $\sqrt{s}=0.9\mathrm{TeV}$ the $\mathrm{\Lambda }\left(1405\right)$ and its antiparticle may be produced at almost the same rate if the $\mathrm{\Lambda }\left(1405\right)$ is a $K^{-}p$ bound state formed during the hadron rescattering period. The combined yield of $K^{-}p$ and $K^{+}\overline{p}$ bound states is found to be of the order of ${10}^{-3}$, but the experimental results indicate that the combined yield of $\mathrm{\Lambda }\left(1405\right)+\overline{\mathrm{\Lambda }}\left(1405\right)$ is of the order of ${10}^{-2}$ if $\mathrm{\Lambda }\left(1405\right)$ is a standard three-quark baryon. Since there are no experimental data available on this observable at present, our work may provide a guide for future experiments.

I have computed the width for sequential $2p$ decay of the first $2^{+}$ state of ${}^{16}\mathrm{Ne}$, using a potential model and convolution over the first two resonances in ${}^{15}\mathrm{F}$. The result is 83 keV, somewhat larger than a recent computed value of 56 keV and smaller than the experimental value of 150(50) keV.

The very neutron-deficient isobars ${}^{179}\mathrm{Pb}$ and ${}^{179}\mathrm{Tl}$ have been produced using the fusion-evaporation reactions ${}^{104}\mathrm{Pd}({}^{78}\mathrm{Kr},xpyn$), where $x\le 1$ and $y\ge 2$. The gas-filled separator RITU was employed to transport and separate the recoiling nuclei of interest from the scattered beam and unwanted products. The GREAT spectrometer was used to study the decay properties through $\alpha \text{−}\alpha$ and $\alpha \text{−}\gamma$ correlations, which has allowed the ground state of ${}^{179}\mathrm{Pb}$ to be assigned as $I^{\pi }=9/2^{-}$. The decay of ${}^{179}\mathrm{Pb}$ was measured to have an $\alpha$-particle energy and half-life of $E_{\alpha }=7348\left(5\right)\mathrm{keV}$ and $t_{1/2}=2.7\left(2\right)$ ms, respectively. A search for a $\nu i_{13/2}$ state in ${}^{179}\mathrm{Pb}$ was performed, but only a limit of excitation energy and half-life was obtained. In ${}^{179}\mathrm{Tl}$ a $t_{1/2}={114}_{-10}^{+18}$ ns isomeric state, likely at an excitation energy of 904.5(9) keV, was identified and is tentatively assigned to be a $9/2^{-}$ proton intruder state.

Background: The influence of shell effect on the dynamics of the fusion fission process and its evolution with excitation energy in the preactinide Hg-Pb region in general is a matter of intense research in recent years. In particular, a strong ambiguity remains for the neutron shell closed ${}^{210}\mathrm{Po}$ nucleus regarding the role of shell effect in fission around $\approx 30–40\mathrm{MeV}$ of excitation energy.

Purpose: We have measured the fission fragment mass distribution of ${}^{210}\mathrm{Po}$ populated using fusion of ${}^4\mathrm{He}+{}^{206}\mathrm{Pb}$ at different excitation energies and compare the result with recent theoretical predictions as well as with our previous measurement for the same nucleus populated through a different entrance channel. Mass distribution in the fission of the neighboring nuclei ${}^{213}\mathrm{At}$ is also studied for comparison.

Methods: Two large area multiwire proportional counters (MWPC) were used for complete kinematical measurement of the coincident fission fragments. The time of flight differences of the coincident fission fragments were used to directly extract the fission fragment mass distributions.

Results: The measured fragment mass distribution for the reactions ${}^4\mathrm{He}+{}^{206}\mathrm{Pb}$ and ${}^4\mathrm{He}+{}^{209}\mathrm{Bi}$ were symmetric and the width of the mass distributions were found to increase monotonically with excitation energy above 36.7 MeV and 32.9 MeV, respectively, indicating the absence of shell effects at the saddle. However, in the fission of ${}^{210}\mathrm{Po}$, we find minor deviation from symmetric mass distributions at the lowest excitation energy (30.8 MeV).

Conclusion: Persistence of shell effect in fission fragment mass distribution of ${}^{210}\mathrm{Po}$ was observed at the excitation energy $\approx 31\mathrm{MeV}$ as predicted by the theory; at higher excitation energy, however, the present study reaffirms the absence of any shell correction in the fission of ${}^{210}\mathrm{Po}$.

An empirical parametrization for the production cross sections of ${}^{238}\mathrm{U}$ photofission fragments at low energies ($E_{\gamma }<30$ MeV) is developed. This parametrization, $\mathrm{GIF}{}^{238}\mathrm{U}$, consists of three parts, namely, the photofission cross section, the mass yield, and the isobaric charge distribution. The photofission cross section is parametrized to reproduce measured data over a wide energy range. The mass yield distribution is described by the energy-dependent multimodal fission model. The isobaric charge distribution is improved, relative to other parametrizations, to describe many experimental data sets over a broad mass range. Comparisons with different measured ${}^{238}\mathrm{U}$ photofission data sets at various energies reveal that $\mathrm{GIF}{}^{238}\mathrm{U}$ is in very good agreement with measured elemental and mass yields and can accurately reproduce experimental isotopic yields. Production cross sections (yields) of photofission fragments calculated by this parametrization indicate that many neutron-rich nuclei approaching the $r$-process path can be accessed via photofission of ${}^{238}\mathrm{U}$ at radioactive-beam facilities.

In this paper heavy quark energy loss models are embedded in full event-by-event viscous hydrodynamic simulations to investigate the nuclear suppression factor and the azimuthal anisotropy of $D^0$ mesons in $\text{PbPb}$ collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$ in the $p_{\text{T}}$ range $8–40\mathrm{GeV}$. In our model calculations, the $R_{AA}$ of $D^0$ mesons is consistent with experimental data from the CMS experiment. We present the first calculations of heavy flavor cumulants $v_2\left\{2\right\}$ and $v_3\left\{2\right\}$ (and also discuss $v_2\left\{4\right\}$), which is also consistent with experimental data. Event-shape engineering techniques are used to compute the event-by-event correlation between the soft hadron $v_n$ and the heavy meson $v_n$. We predict a linear correlation between these observables on an event-by-event basis.

The existence of neutron stars with $2M_{\odot }$ requires strong stiffness of the equation of state (EoS) of neutron-star matter. We introduce a multi-Pomeron exchange potential (MPP) working universally among three- and four-baryon systems to stiffen the EoS. Its strength is restricted by analyzing the nucleus-nucleus scattering with the $G$-matrix folding model. The EoSs are derived using the Brueckner-Hartree-Fock (BHF) theory and the cluster variational method (CVM) with the nuclear interactions ESC and AV18. The mass-radius relations are derived by solving the Tolmann-Oppenheimer-Volkoff (TOV) equation, where the maximum masses over $2M_{\odot }$ are obtained on the basis of terrestrial data. Neutron-star radii $R$ at a typical mass $1.5M_{\odot }$ are predicted to be $12.3\text{–}13.1$ km. The uncertainty of calculated radii is mainly from the ratio of three- and four-Pomeron coupling constants, which cannot be fixed by any terrestrial experiment. Though values of $R\left(1.5M_{\odot }\right)$ are not influenced by hyperon-mixing effects, finely observed values for them indicate degrees of EoS softening by hyperon mixing in the region of $M\sim 2M_{\odot }$. If $R\left(1.5M_{\odot }\right)$ is less than about 12.3 km, the softening of EoS by hyperon mixing has to be weak. Useful information can be expected by the space mission NICER, offering precise measurements for neutron-star radii within $\pm 5\%$.

The question of correlations among empirical equation of state (EoS) parameters constrained by nuclear observables is addressed in a Thomas-Fermi meta-modeling approach. A recently proposed meta-modeling for the nuclear EoS in nuclear matter is augmented with a single finite size term to produce a minimal unified EoS functional able to describe the smooth part of the nuclear ground state properties. This meta-model can reproduce the predictions of a large variety of models, and interpolate continuously between them. An analytical approximation to the full Thomas-Fermi integrals is further proposed giving a fully analytical meta-model for nuclear masses. The parameter space is sampled and filtered through the constraint of nuclear mass reproduction with Bayesian statistical tools. We show that this simple analytical meta-modeling has a predictive power on masses, radii, and skins comparable to full Hartree-Fock or extended Thomas-Fermi calculations with realistic energy functionals. The covariance analysis on the posterior distribution shows that no physical correlation is present between the different EoS parameters. Concerning nuclear observables, a strong correlation between the slope of the symmetry energy and the neutron skin is observed, in agreement with previous studies.

We discuss the role of pairing antihalo effect in the observed odd-even staggering in reaction cross sections for ${}^{30,31,32}\mathrm{Ne}$ and ${}^{36,37,38}\mathrm{Mg}$ isotopes by taking into account the ground-state deformation of these nuclei. To this end, we construct the ground-state density for the ${}^{30,31}\mathrm{Ne}$ and ${}^{36,37}\mathrm{Mg}$ nuclei based on a deformed Woods-Saxon potential, while for the ${}^{32}\mathrm{Ne}$ and ${}^{38}\mathrm{Mg}$ nuclei we also take into account the pairing correlation using the Hartree-Fock-Bogoliubov method. We demonstrate that, when the one-neutron separation energy is small for the odd-mass nuclei, a significant odd-even staggering still appears even with finite deformation, although the degree of staggering is somewhat reduced compared to the spherical case. This implies that the pairing antihalo effect in general plays an important role in generating the odd-even staggering in reaction cross sections for weakly bound nuclei.

The interplay between isoscalar and isovector correlations in the $1^{-}$ states in neutron-rich ($N\ne Z$) even-even nuclei is studied, taking as examples of the nuclei, ${}_8{}^{22}\mathrm{O}_{14}$ and ${}_8{}^{24}\mathrm{O}_{16}$. The excitation modes explored are isovector dipole and isoscalar compression dipole modes. The self-consistent Hartree-Fock plus the random-phase approximation with the Skyrme interaction, SLy4, is solved in coordinate space so as to take properly into account the continuum effect. The isovector peak induced by isoscalar correlation, the isoscalar peak induced by isovector correlation, and the possible collective states made by both isoscalar and isovector correlations (“iS-iV pigmy resonance”) are shown. The strong neutron-proton interaction in nuclei can be responsible for controlling the isospin structure of normal modes. It is explicitly shown that in the scattering by isoscalar (isovector) particles on $N\ne Z$ even-even nuclei isovector (isoscalar) strength in addition to isoscalar (isovector) strength may be populated.

Starting from the variational principle, the time-dependent pairing equations are generalized by including the Landau-Zener effect and the Coriolis coupling. A system of microscopic equations of motion for configuration mixing is deduced, allowing the determination of quantities that have the same meaning as the preformation factors of the $\alpha$ particle. These equations are solved in order to reproduce the hindrance factors of the $\alpha$ decay of an odd-$A$ mass nucleus. The $\alpha$ decay of ${}^{211}\mathrm{Po}$ is treated as a superasymmetric fission process, by following the rearrangement of the nuclear orbitals from the parent ground state up to the scission configuration. The probabilities of finding the excited states of the daughter at scission are obtained from the microscopic equations of motion. The intensities of the transitions to the excited states of the daughter were evaluated theoretically. The experimental data were compared with the theoretical findings. A very good agreement was obtained. A mean value of the tunneling velocity of about $2\times {10}^4$ fm/fs was extracted.

The semi-inclusive deep-inelastic electron scattering off transversely polarized ${}^3\mathrm{He}$, i.e., the process $e+\overrightarrow{{}^3\mathrm{He}}\to e^{\prime }+h+X$, with $h$ being a detected fast hadron, is studied beyond the plane-wave impulse approximation. To this end, a distorted spin-dependent spectral function of a nucleon inside an $A=3$ nucleus is actually evaluated through a generalized eikonal approximation, in order to take into account the final state interactions between the hadronizing system and the $\left(A-1\right)$ nucleon spectator one. Our realistic description of both nuclear target and final state is a substantial step forward for achieving a reliable extraction of the Sivers and Collins single spin asymmetries of the free neutron. To illustrate how and to what extent the model dependence due to the treatment of the nuclear effects is under control, we apply our approach to the extraction procedure of the neutron single spin asymmetries from those measured for ${}^3\mathrm{He}$ for values of the kinematical variables relevant both for forthcoming experiments at Jefferson Laboratory and, with an exploratory purpose, for the future Electron Ion Collider.

Oscillation modes of isolated compact stars can, in principle, be a fingerprint of the equation of state (EoS) of dense matter. We study the non-radial high-frequency $l=2$ spheroidal modes of neutron stars and strange quark stars, adopting a two-component model (core and crust) for these two types of stars. Using perturbed fluid equations in the relativistic Cowling approximation, we explore the effect of a strangelet or hadronic crust on the oscillation modes of strange stars. The results differ from the case of neutron stars with a crust. In comparison to fluid-only configurations, we find that a solid crust on top of a neutron star increases the $p$-mode frequency slightly with little effect on the $f$-mode frequency, whereas for strange stars, a strangelet crust on top of a quark core significantly increases the $f$-mode frequency with little effect on the $p$-mode frequency.

The low-lying structure of ${}^{55}\mathrm{Sc}$ has been investigated using in-beam $\gamma$-ray spectroscopy with the ${}^9\mathrm{Be}({}^{56}\mathrm{Ti},{}^{55}\mathrm{Sc}+\gamma )X$ one-proton removal and ${}^9\mathrm{Be}({}^{55}\mathrm{Sc},{}^{55}\mathrm{Sc}+\gamma )X$ inelastic-scattering reactions at the RIKEN Radioactive Isotope Beam Factory. Transitions with energies of 572(4), 695(5), 1539(10), 1730(20), 1854(27), 2091(19), 2452(26), and 3241(39) keV are reported, and a level scheme has been constructed using $\gamma \gamma$ coincidence relationships and $\gamma$-ray relative intensities. The results are compared to large-scale shell-model calculations in the $sd\text{−}pf$ model space, which account for positive-parity states from proton-hole cross-shell excitations, and to ab initio shell-model calculations from the in-medium similarity renormalization group that includes three-nucleon forces explicitly. The results of proton-removal reaction theory with the eikonal model approach were adopted to aid identification of positive-parity states in the level scheme; experimental counterparts of theoretical $1/2_1^{+}$ and $3/2_1^{+}$ states are suggested from measured decay patterns. The energy of the first $3/2^{-}$ state, which is sensitive to the neutron shell gap at the Fermi surface, was determined. The result indicates a rapid weakening of the $N=34$ subshell closure in $pf$-shell nuclei at $Z>20$, even when only a single proton occupies the $\pi f_{7/2}$ orbital.

Background: In a series of experiments at different time-of-flight spectrometers of heavy ions we have observed manifestations of a new at least ternary decay channel of low excited heavy nuclei. Due to specific features of the effect, it was called collinear cluster tri-partition (CCT). The obtained experimental results have initiated a number of theoretical articles dedicated to different aspects of the CCT. Special attention was paid to kinematics constraints and stability of collinearity.

Purpose: To compare theoretical predictions with our experimental data, only partially published so far. To develop the model of one of the most populated CCT modes that gives rise to the so-called “Ni-bump.”

Method: The fission events under analysis form regular two-dimensional linear structures in the mass correlation distributions of the fission fragments. The structures were revealed both at a highly statistically reliable level but on the background substrate, and at the low statistics in almost noiseless distribution. The structures are bounded by the known magic fragments and were reproduced at different spectrometers. All this provides high reliability of our experimental findings. The model of the CCT proposed here is based on theoretical results, published recently, and the detailed analysis of all available experimental data.

Results: Under our model, the CCT mode giving rise to the Ni bump occurs as a two-stage breakup of the initial three body chain like the nuclear configuration with an elongated central cluster. After the first scission at the touching point with one of the side clusters, the predominantly heavier one, the deformation energy of the central cluster allows the emission of up to four neutrons flying apart isotropically. The heavy side cluster and a dinuclear system, consisting of the light side cluster and the central one, relaxed to a less elongated shape, are accelerated in the mutual Coulomb field. The “tip” of the dinuclear system at the moment of its rupture faces the heavy fragment or the opposite direction due to a single turn of the system around its center of gravity.

Conclusions: Additional experimental information regarding the energies of the CCT partners and the proposed model of the process respond to criticisms concerning the kinematic constraints and the stability of collinearity in the CCT. The octupole deformed system formed after the first scission is oriented along the fission axis, and its rupture occurs predominantly after the full acceleration. Noncollinear true ternary fission and far asymmetric binary fission, observed earlier, appear to be the special cases of the decay of the prescission configuration leading to the CCT.

Detection of the ${}^{68–72}\mathrm{Ni}$ fission fragments with a kinetic energy $E<25\mathrm{MeV}$ at the mass-separator Lohengrin is proposed for an independent experimental verification of the CCT.

The half-life of ${}^{19}\mathrm{Ne}$ was measured using a real-time digital multiparametric acquisition system, providing an accurate time stamp and relevant information on the detector signals for each decay event. An exhaustive offline analysis of the data gave unique access to experimental effects potentially biasing the measurement. After establishing the influence factors impacting the measurement such as after-pulses, pile-up, gain, and baseline fluctuations, their effects were accurately estimated and the event selection optimized. The resulting half-life, $17.2569\pm 0.{0019}_{\left(\text{stat}\right)}\pm 0.{0009}_{\left(\text{syst}\right)}$ s, is the most precise up to now for ${}^{19}\mathrm{Ne}$. It is found to be in agreement with two recent precise measurements and not consistent with the most recent one [L. J. Broussard et al., Phys. Rev. Lett. 112, 212301 (2014)] by 3.0 standard deviations. The full potential of the technique for nuclei with half-lives of a few seconds is discussed.

A microscopic approach to description of radiative capture reactions based on a multiscale algebraic version of the resonating group model is developed. The main idea of the approach is to expand wave functions of discrete spectrum and continuum for a nuclear system over different bases of the algebraic version of the resonating group model. These bases differ from each other by values of oscillator radius playing a role of scale parameter. This allows us in a unified way to calculate total and partial cross sections (astrophysical $S$ factors) as well as branching ratio for the radiative capture reaction, to describe phase shifts for the colliding nuclei in the initial channel of the reaction, and at the same time to reproduce breakup thresholds of the final nucleus. The approach is applied to the theoretical study of the mirror ${}^3\mathrm{H}(\alpha ,\gamma ){}^7\mathrm{Li}$ and ${}^3\mathrm{He}(\alpha ,\gamma ){}^7\mathrm{Be}$ reactions, which are of great interest to nuclear astrophysics. The calculated results are compared with existing experimental data and with our previous calculations in the framework of the single-scale algebraic version of the resonating group model.