A. V. Rusakov

Analysis of commercial fish population size oscillations in Lake Peipus

With the use of recurrence plots, we show that the irregular oscillations in time, which commercial fish populations in Lake Peipus undergo, are deterministic.

A conceptual mathematical model of the aquatic communities of lakes Naroch and Myastro (Belarus)

A conceptual mathematical model of the dynamics of fish and zooplankton (rotifer) populations of connected lakes Naroch and Myastro (Belarus) is built and examined with parameters based on field data. It is shown that community coupling and trophic interactions give rise to both regular and irregular oscillations in population numbers.

{alpha}-decay half-life of {sup 253}Es in metallic Fe at temperatures between 4 K and 50 mK

It has been claimed recently that half-lives of radioactive nuclei embedded in metals would be significantly affected by electron screening provided by the metal. The effect would further be strengthened at low temperatures. We have determined the half-life-of {sup 253}Es nuclei embedded in iron at temperatures between 4 K and 50 mK. Our results agree with the room temperature literature value within about 2% and show no dependence on temperature over a range of almost two orders of magnitude.

Modelling aquatic communities: Trophic interactions and the body mass-and-age structure of fish populations give rise to long-period variations in fish population size

Abstract We present a mathematical model of an aquatic community, which includes zooplankton and fish reproduction, and age-weight-structured trophic relationships. We show that interactions between separate components of the aquatic community can give rise to long-period oscillations in fish population size. The period of these oscillations is on the order of decades.We show that predatory fish can be an element, which gives rise to the long-period oscillations. With this model we also show that an increase in the zooplankton growth rate may entail a sequence of bifurcations in the fish population dynamics: steady states → regular oscillations → quasicycles → dynamic chaos. Since aquatic, and in particular, lake ecosystems are spatially heterogeneous, they are often considered as consisting of separate habitats, which are distinguished by their hydrophysical and ecological characteristics. We show that interhabitat fish migration can lead to dramatic changes in the fish population dynamics. In particular, the fish migration can destabilize both stationary states and chaotic regimes giving rise to regular and quasiregular oscillations in the fish population size.

The divergence of chlorophyll dynamics in the Naroch lakes

We present the results of an analysis of the long-term chlorophyll a dynamics in the Naroch lakes. It was shown that an increase in nutrient load in the 1970s resulted in progressive eutrophication of the Naroch lakes. Then, starting from the mid-1980s, the water transparency began to increase and the concentration of phosphorus and nitrogen began to decrease due to the implementation of an environmental improvement program. In the 1980s, the Naroch lakes experienced an invasion by the zebra mussel (Dreissena polymorpha Pallas). Our analysis showed that the responses of all three lakes to the intensive nutrient load and a further decrease in the nutrient concentration as a result of the environmental-protection measures are correlated. At the same time, the invasion of the zebra mussel Dreissena polymorpha, which causes significant transformation of ecosystem processes, led to divergence in the chlorophyll dynamics; this was manifested as a drastic decline in the correlation between the fluctuations in the chlorophyll concentration in each lake.

Investigation of giant-dipole-resonance decay in the (γ, n) reactions on 52Cr and 51V nuclei

The cross sections for the emission of fast neutrons (ɛn > 3.7 MeV) in the reactions 52Cr(γ, n)51Cr and 51V(γ, n)50V at incident-photon energies in the range between 16.0 and 25.8 MeV were studied. The neutron energy spectra were measured at the bremsstrahlung-photon endpoint energies of 18.5, 21.0, and 23.0 MeV for the 52Cr and 51V nuclei and at the bremsstrahlung-photon energy of 25.5 MeV for the 51V nucleus. Special features of giant-dipole-resonance decay that are associated with the existence of a structure in photoneutron cross sections and spectra are discussed.

Deterministic chaоs and the problem of predictability in population dynamics

The studies of the processes that can significantly influence the predictability in population dynamics are reviewed and the results of mathematical simulations of population dynamics are compared to the time series obtained in field observations. Considerable attention is given to the chaotic changes in population abundance. Some methods of numerical analysis of chaoticity and predictability of the time series are considered. The importance of comparing the results of mathematical simulation and observation data is tightly linked to problems in detecting chaos in the dynamics of natural populations and estimating the prevalence of chaotic regimes in nature. Insight into these problems can allow identification of the functional role of chaotic regimes in population dynamics.

Discrimination of particles by the scintillation pulse shape in the range of low energies (6–100 keV for electrons) using the linear filters

The use of the linear filters for particle discrimination by the scintillation pulse shape is considered. The particle separation process has been simulated by the Monte Carlo method, taking into account that the slow scintillation component decays according to the hyperbolic law and that its relative contribution is energy dependent. The best figures of merit of particle discrimination attainable with this technique have been obtained assuming that the PMT and electronic noises are zero. It is shown that, by contrast to the zero crossing method, pulse shape discrimination of particles using the linear filters can ensure rejection of the γ-ray background to a level of ∼10−4 at particle energies of up to 12 keV of the electron equivalent. For energies of <24 keV, it is expedient that the signal acquisition time be increased to a few microseconds.

Applying the photonuclear technique to fissile materials detection

Application of a photonuclear technique is considered as a nondestructive method for detecting the unauthorized transporting of camouflaged fissile nuclear material in cargo containers. The advantages of the technique (the use of time selection and the analysis of energy distributions of delayed neutrons) are demonstrated. The operating modes of the accelerator were optimized by modeling. A version of a neutron scintillation spectrometer based on a stylbene monocrystal is presented along with its schematic and main parameters.

Suppression of pileup events by pulse shape discrimination using the Gatti filter

An algorithm is proposed that provides reliable discrimination between neutrons and γ rays in mixed fields and reduces the influence of γ-ray pileups at a slight complication of the hardware and the algorithms. The algorithm uses a stepwise Gatti filter for two detection periods (a short period of several hundreds of nanoseconds and a long period of a few microseconds) for the same event. Using mathematical simulation of 12000 neutron and γ-ray events, it has been demonstrated that at particle energies of ~128 keV in the electron equivalent (ee), the algorithm suppresses γ-ray pileups on a level of 104 with a neutron loss of only approximately 1%. Due to the proposed algorithm, the counting ability is of the order of 105 events/s.