Inelastic interactions between nuclei at high energies

A theory of nucleus-nucleus collisions has been developed for kinetic energies substantially in excess of the binding energy. The very high pressure produced in the compound system as a result of the fusion of the two colliding nuclei is the reason for the subsequent hydrodynamic expansion of the nuclear medium. The energy and angular distributions of the reaction products are investigated. The charge distribution is also determined in the case where the nucleon and ion components of the reaction products are predominant. A solution is found for the expansion into vacuum of a sphere in which the initially uniformly distributed material is initially at rest and at an ultrarelativistic temperature.Comment: 18 pages, 2 figures in GIF forma

Thermodynamic inequalities for nuclear rotation

The strictly reversible, thermodynamically equilibrium nature of the free rotation of a body makes it possible to obtain a number of bounds on the rotational characteristics within individual rotational bands of nonspherical nuclei. As a result, the bounds between which the possible values of the critical spin $J_c$ lie can be expressed exclusively in terms of a restricted number of the experimentally most accessible data on the lower phase $J<J_c$ for a given nuclide. The bounds are tested on the ground-state rotational bands (yrast lines) of even-even nuclei, in which the corresponding phase transition (backbending) has already been observed experimentally. For nuclei with pronounced non-sphericity, all the bounds are invariably confirmed. For the ground-state rotational bands for which the phase transition point $J =J_c$ has not yet been reached, predictions are made for the corresponding values of $J_c^{min}$ and, especially, $J_c^{max}$. The specific features of excited rotational bands, and also the bands of odd nuclei are discussed.Comment: 14 pages, 5 figure

Theory of the radiative width of a highly excited nucleus

A microscopic theory of the electromagnetic radiation emitted by a highly excited nucleus is developed on the basis of the Landau theory of a Fermi liquid. Closed formulae are obtained for the mean radiative width and its mean square fluctuation from level to level. The temperatures of many nuclei are found from the observed widths. The relaxation time is estimated from the experimental data on the radiative-width fluctuations. The regions of applicability of the various types of relations between the relaxation time and the lifetime of the compound nucleus, as well as the relevant physical consequences, are discussed.Comment: 14 page

Theory of shell structure and of the "magic" effect in spherical nuclei

A consistent theory is developed of the volume energy oscillations of spherical nuclei due to sharpness of the Fermi distribution boundary for quasiparticles. The lowest value of the oscillating part of the energy corresponds to a magic nucleus. A formula is obtained for the corresponding limiting momentum of a quasiparticle and it is shown that we have here an isolated point of a temperatureless second-order phase transition. An expression for the discontinuity of the derivative of the energy of the body with respect to the number of particles is obtained in the case of a sharp (step-like) Fermi distribution limit. Comparison with experimental nuclear-mass data permits some conclusions to be drawn regarding the true structure of the boundary layer of the Fermi distribution and regarding its variation with increasing nuclear size. In the region of magic nuclei actually accessible up to the present time, apparently no signs are observed of any appreciably expressed residual phenomenon, such as the Cooper phenomenon, which would result in instability of the energy spectrum of infinite nuclear matter with an absolutely sharp Fermi limit for quasiparticles.Comment: 16 pages, 4 figures in GIF forma

Theory of rapid (nonadiabatic) rotation of nonspherical nuclei

On the basis of the concept of the growing role of nonadiabatic effects of the non-conservation of the quantum number $K,$ a theory has been developed of the phenomenon which has been given the name of backbending. Above the transition point, for $J\geq J_c$, all the values $-J\leq K\leq J$ are equally probable. An investigation is made of the singularities possessed by the ordering parameter (proportional to the spectroscopic quadrupole moment of a nonspherical nucleus), the rotational angular velocity and the moment of inertia of a nucleus at the Curie point. Formulas have been derived for the intensity of quadrupole radiation in the more symmetric $n$-phase $J> J_c$. By analyzing the experimental values of the moments of inertia belonging to the $n$-phase, the radius of the mass distribution in the nucleus was determined. It agrees with the radius of the proton distribution derived from data on the scattering of electrons by nuclei. On the basis of the simplest form of the singularity of the parametric derivative of the Hamiltonian of the system a general theory of zero-temperature second-order phase transitions is developed in the Appendix.Comment: 18 pages, 2 figures in GIF forma

Relation between the critical spin and angular velocity of a nucleus immediately after backbending

In nonspherical nuclei at $J = J_c + 0$ the relationship between the angular momentum and angular velocity immediately after backbending is the same as in the limiting case $J - J_c\to\infty$. This indicates that there is a unique type of cancellation of the deviations from a rigid-body moment of inertia in the upper phase $J>J_c$. An integral relationship is found which expresses this cancellation quantitatively. This formula permits $J_c$ to be calculated for the rotational bands of the even-even nuclei studied and the results are in agreement with those obtained by other methods of locating the Curie point. For the ground state band of W$^{170}$ the cancellation of the reciprocals of the true and rigid-body moments of inertia can be verified directly. The condition for the stability of the rotation of a nonspherical nucleus is analyzed in the Appendix in close connection with the problem of a reasonable definition of the concept of a variable moment of inertia.Comment: 9 pages, 1 figure in GIF forma

Change in the Nuclear Moment of Inertia at the Number of Neutrons N = 98 and Isotopic Shift of Atomic Levels

From analysis of experimental data on nuclear rotation, it is found that the so-called rigid-body value of the nuclear moment of inertia and the nuclear-radius values associated with the moment of inertia as functions of the number of nucleons change their behavior at the number of neutrons $N = 98$. This phenomenon is confirmed by experimental data on the isotopic shifts of atomic levels.Comment: 4 pages, 2 figure

Structure of residual interaction in spherical nuclei

The effect of residual interaction between nucleons (quasiparticles) on shell oscillations of the masses of spherical nuclei is considered. The singularity of the ground state energy of the system in the vicinity of nucleon magic numbers is analyzed for various types of the dependence of residual interaction on orbital momentum of the quasiparticle. It is shown that only the perturbation band width of the Fermi distribution due to residual interaction which is proportional to the square of the angular momentum vector is consistent with the character of the magic cusps. The coupling constants between the quasiparticles are determined on the basis of the available data. The constant decreases rapidly with increase of nuclear radius. Possible consequences pertaining to the energy spectrum of infinite nuclear matter are discussed.Comment: 16 pages, 6 figures in GIF forma

Characterization of photo-multiplier tubes for the Cryogenic Avalanche Detector

New Cryogenic Avalanche Detector (CRAD) with ultimate sensitivity, that will be able to detect one primary electron released in the cryogenic liquid, is under development in the Laboratory of Cosmology and Particle Physics of the Novosibirsk State University jointly with the Budker Institute of Nuclear Physics. The CRAD will use two sets of cryogenic PMTs in order to get trigger signal either from primary scintillations in liquid Ar or from secondary scintillations in high field gap above the liquid. Two types of cryogenic PMTs produced by Hamamatsu Photonics were tested and the results are presented in this paper. Low background 3 inch PMT R11065- 10 demonstrated excellent performance according to its specifications provided by the producer. The gain measured with single electron response (SER) in liquid Ar reached 10^7, dark count rate rate did not exceed 300 Hz and pulse height resolution of single electron signals was close to 50%(FWHM). However, two R11065-10 PMTs out of 7 tested stopped functioning after several tens minutes of operation immersed completely into liquid Ar. The remaining 5 devices and one R11065-MOD were operated successfully for several hours each with all the parameters according to the producer specifications. Compact 2 inch PMT R6041-506-MOD with metal-channel dynode structure is a candidate for side wall PMT system that will look at electroluminescence in high field region above liquid. Four of these PMTs were tested in liquid Ar and demonstrated gain up to 2x10^7, dark count rate rate below 100 Hz and pulse height resolution of single electron signals of about 110% (FWHM).Comment: Submitted to JINS

Neutral bremsstrahlung in two-phase argon electroluminescence: further studies and possible applications

We further study the effect of neutral bremsstrahlung (NBrS) in two-phase argon electroluminescence (EL), revealed recently in [1]. The absolute EL yield due to NBrS effect, in the visible and NIR range, was remeasured in pure gaseous argon in the two-phase mode, using a two-phase detector with EL gap read out directly by cryogenic PMTs and SiPMs. Possible applications of the NBrS effect in detection science are discussed, including those in two-phase dark matter detectors.Comment: 4 pages, 5 figures, presented at VCI2019 conference, to be published in NIM