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Observation of high energy and very high emission from Gamma Ray Bursts (GRBs) is crucial to study the gigantic explosion and the underline processes. With a large field-of-view and almost full duty cycle, the Water Cherenkov Detector Array (WCDA), a sub-array of the Large High Altitude Air Shower Observatory (LHAASO), is appropriate to monitor the very high energy emission from unpredictable transients such as GRBs. Nevertheless, the main issue for an extensive air shower array is the high energy threshold which limits the horizon of the detector. To address this issue a new trigger method is developed in this article to lower the energy threshold of WCDA for GRB observation. The proposed method significantly improves the detection efficiency of WCDA for gamma-rays around the GRB direction at 10-300 GeV. The sensitivity of the WCDA for GRB detection with the new trigger method is estimated. The achieved sensitivity of the quarter WCDA array above 10 GeV is comparable with that of Fermi-LAT. The data analysis process and corresponding fluence upper limit for GRB 190719C is presented as an example.

The extension of the cosmic-ray spectrum beyond 1 petaelectronvolt (PeV; 1015 electronvolts) indicates the existence of the so-called PeVatrons—cosmic-ray factories that accelerate particles to PeV energies. We need to locate and identify such objects to find the origin of Galactic cosmic rays1. The principal signature of both electron and proton PeVatrons is ultrahigh-energy (exceeding 100 TeV) γ radiation. Evidence of the presence of a proton PeVatron has been found in the Galactic Centre, according to the detection of a hard-spectrum radiation extending to 0.04 PeV (ref. 2). Although γ-rays with energies slightly higher than 0.1 PeV have been reported from a few objects in the Galactic plane3–6, unbiased identification and in-depth exploration of PeVatrons requires detection of γ-rays with energies well above 0.1 PeV. Here we report the detection of more than 530 photons at energies above 100 teraelectronvolts and up to 1.4 PeV from 12 ultrahigh-energy γ-ray sources with a statistical significance greater than seven standard deviations. Despite having several potential counterparts in their proximity, including pulsar wind nebulae, supernova remnants and star-forming regions, the PeVatrons responsible for the ultrahigh-energy γ-rays have not yet been firmly localized and identified (except for the Crab Nebula), leaving open the origin of these extreme accelerators. Observations of γ-rays with energies up to 1.4 PeV find that 12 sources in the Galaxy are PeVatrons, one of which is the Crab Nebula.

The extrusion process advantage of agricultural raw materials processing into food and feed over traditional technologies is the high specific productivity, energy efficiency and versatility of use. The presence of a large number of control factors and influence on the raw materials quality, such as operating parameters and structural elements, make extrusion a difficult reproducible process when scaling or moving from one installation to another. But this aspect is the reason for the maximum flexibility of extrusion systems and the possibility of their continuous improvement. One of the ways for the extrusion technology development is the improvement of equipment and design solutions in the field of energy saving. Investigations of the thermoplastic extrusion process of wheat grinding with the application of the recovery of steam heat generated by a special configuration of screw elements and its degassing with the feeding into the processed material were carried out in the work. The modernized extruder Werner & Phleiderer Continua 37, supplemented by a developed recuperative unit and a steam line, was used to achieve this goal. An assessment of changes in energy consumption during steam recovery was carried out. The physicochemical properties of the experimental and control samples of wheat grain extrudates obtained under various operating conditions were investigated and determined. It was found out that the steam recovery mode is a statistically significant reason for increasing the humidity of the extruded product from 3.8 to 5.0 bulk density - by 15% relative to the mode without recovery. A decrease in the explosion coefficient and the water-holding capacity of the extrudates during operation in the regenerative mode of extrusion was noticed. The presented method with additional thermal effects on the processed raw materials allows us to expand the technological capabilities of the process of thermoplastic extrusion in obtaining products of various functional and technological purposes.

We report the discovery of a new unidentified extended $\gamma$-ray source in the Galactic plane named LHAASO J0341+5258 with a pre-trial significance of 8.2 standard deviations above 25 TeV. The best fit position is R.A.$=55.34^{\circ}\pm0.11^{\circ}$ and Dec$=52.97^{\circ}\pm0.07^{\circ}$. The angular size of LHAASO J0341+5258 is $0.29^\circ \pm 0.06^\circ_{stat} \pm0.02^\circ_{sys}$. The flux above 25 TeV is about $20\%$ of the flux of Crab Nebula. Although a power-law fit of the spectrum from 10 TeV to 200 TeV with the photon index $\alpha=2.98 \pm 0.19_{stat} \pm 0.02_{sys}$ is not excluded, the LHAASO data together with the flux upper limit at 10 GeV set by the Fermi LAT observation, indicate a noticeable steepening of an initially hard power-law spectrum %($\alpha \leq 1.75$) spectrum with a cutoff at $\approx 50$ TeV. We briefly discuss the origin of UHE gamma-rays. The lack of an energetic pulsar and a young SNR inside or in the vicinity of LHAASO J0341+5258 challenge, but do not exclude both the leptonic and hadronic scenarios of gamma-ray production., Comment: Accepted by APJL

Interesting results are obtained using a setup with electron-neutron detectors (EN-detectors) developed for the PRISMA (PRImary Spectrum Measuring Array) project to study extensive atmospheric showers. A small installation of four EN-detectors (PRISMA-YBJ) has been in operation for three and a half years on the Earth’s surface in Tibet, 4300 m above sea level, constantly measuring the natural flux of thermal neutrons. Neutrons are produced in soil during (α, n)-reactions with naturally radioactive α-particles, mainly from the decay of radon and daughter heavy nuclides. The neutrons are thermalized in the medium and, once in equilibrium with it, become sensitive to its state. They then can indicate many geophysical processes in the crust, including earthquakes. Results illustrating the sensitivity of EN-detectors to earthquakes are considered.

Exchange bias properties of MnFe$$_2$$ 2 O$$_4$$ 4 @$$\gamma$$ γ –Fe$$_2$$ 2 O$$_3$$ 3 core–shell nanoparticles are investigated. The measured field and temperature dependencies of the magnetization point out a well-ordered ferrimagnetic core surrounded by a layer with spin glass-like arrangement. Quasi-static SQUID magnetization measurements are presented along with high-amplitude pulse ones and are cross-analyzed by comparison against ferromagnetic resonance experiments at 9 GHz. These measurements allow one to discern three types of magnetic anisotropies affecting the dynamics of the magnetic moment of the well-ordered ferrimagnetic NP’s core viz. the easy-axis (uniaxial) anisotropy, the unidirectional exchange-bias anisotropy and the rotatable anisotropy. The uniaxial anisotropy originates from the structural core–shell interface. The unidirectional exchange-bias anisotropy is associated with the spin-coupling at the ferrimagnetic/spin glass-like interface; it is observable only at low temperatures after a field-cooling process. The rotatable anisotropy is caused by partially-pinned spins at the core/shell interface; it manifests itself as an intrinsic field always parallel to the external applied magnetic field. The whole set of experimental results is interpreted in the framework of superparamagnetic theory, i.e., essentially taking into account the effect of thermal fluctuations on the magnetic moment of the particle core. In particular, it is found that the rotatable anisotropy of our system is of a uniaxial type.

The main scientific goal of the LHAASO WFCTA experiment is to measure the cosmic ray energy spectra and composition from 10 TeV to 1 EeV. Cherenkov photons in the extensive air shower measured by the SiPM camera of Cherenkov telescopes can be used to reconstruct the cosmic ray energy. The absolute calibration of the camera is a crucial step to achieve the accurate measurement of the cosmic ray energy spectrum. A multi-wavelength cylindrical illuminator based on LEDs is developed and mounted inside the telescope to calibrate and monitor the camera, and the illuminator’s stability is better than 0.5% under the temperature variation from -26 to 26 ° C. A portable probe with a single photoelectron resolution of 21.6% is developed. After calibration by National Institute of Metrology, China (NIM), the probe is taken to the LHAASO site to measure the absolute photon density of the cylindrical illuminator inside the telescope. Based on the illuminator with known photon density, the photon conversion factor of the camera can be calibrated, and the overall calibration uncertainty is less than 2.6%.

We demonstrate that a ferronematic (a dilute suspension of ferromagnet nanoparticles in a nematogenic matrix) may display a direct analog of Neel superparamagnetism entailed by the orienting effect of the matrix on the embedded particles. The latter are assumed to be the objects with pronounced anisometricity; in magnetic aspect, they are single-domains with strong magnetic hardness (e.g. imposed by their rod-like shape) so that the magnetic moment is fixed inside the particle body. Above the isotropic–nematic transition point, the considered system is just a ferrofluid with random distribution of the particle axes. Below the transition, the particle–matrix coupling emerges that sets the axis of each particle (and, hence, its magnetic moment) under the same angle to the director. If this alignment is along the director, then each particle falls under the action of orientational potential with two equal wells (0° and 180°) separated by the energy barrier whose height is defined by the intensity of the surface particle–matrix interaction. Provided the thermal energy is of the order of the barrier height (the material estimates readily admit that), the Brownian motion makes the particle to randomly rotate between the orientational minima. This mechanism entails spontaneous inversions of the magnetic moment as well, thus ensuring relaxation of any initially established magnetization of the ferronematic; the reference time of this process depends exponentially on the hight of the energy barrier scaled with thermal energy. Treating the nematogenic matrix with the aid of mean field model and using the linear response theory to describe the magnetodynamics of the particles, we show that the “liquid-crystalline” superparamagnetism produces an easily identifiable signature in the dynamic magnetic susceptibility spectrum of a ferronematic.

The mass composition of primary cosmic rays remains an unresolved problem of physics in the region of energies above the knee. Results from different experiments are contradictory in assessments of the mean mass number and its variation as the primary energy grows. The PRISMA project is designed to study the energy spectrum and mass composition of cosmic rays at energies of 1015–1017 eV. The project is based on a detector capable of simultaneously detecting the electromagnetic and hadron components of a shower. The results are presented from measurements with the PRISMA-YBJ prototype located at 4300 m a.s.l., made over 3.5 years of operation. A new way of estimating the average mass composition from the electron/neutron ratio is tested.

A consistent theory of ferromagnetic resonance in a dilute suspension of superparamagnetic particles with uniaxial anisotropy of arbitrary strength is presented. The developed approach is used for studying the high-frequency response of a magnetic fluid at different temperatures. It is shown that in a certain temperature interval the absorption line splits into two components. The width of this interval is essentially dependent on the magnitude of the particle anisotropy.

Some unexpected sporadic increases of an environmental radioactive background have been recorded at mountain level at Baksan Neutrino Observatory (BNO, 1700 m above sea level) using electron-neutron detectors (en-detectors), which could be explained by radioactive aerosol enhancements. The large area inorganic scintillator en-detectors developed for cosmic ray study are continuously monitoring environmental thermal neutron fluxes at various geophysical conditions. Application of the pulse shape discrimination method allows us to select and separately measure both thermal neutrons and radioactive beta-decay nuclides being products of radon decays in air (mostly Rn-222 and Rn-220). There are two en-detector setups running now at BNO, one deep underground while another one at surface. Both installations had recorded some strange sporadic increases of radioactive nuclides in air. In this paper, we present results and the most probable explanation of the significant increases joint by radioactive aerosols production but caused by different reasons: Baksan river floods or nearby underground experiment with powerful Cr-51 radioactive source.

Primary cosmic ray energy spectrum above 1 PeV has been measured with PRISMA-YBJ being a prototype of PRISMA array at altitude of 4300 m a.s.l. It realized a novel type of EAS recording method measuring hadronic EAS component over the total array area through thermal neutron detection with a specially developed so-called en-detectors sensitive to electron and thermal neutron EAS components. Primary c. r. spectrum was recovered in 2 ways: through usual Ne procedure as well as through measurements of EAS size spectrum in thermal neutrons. Monte-Carlo simulations of the experiments allowed us recover the primary spectrum above 1 PeV. The both obtained spectra follow power law function with index close that measured by direct measurements below 1 PeV.

Seismic hazard forecasts submitted in real-time mode to the Kamchatkan Branch of the Russian Expert Council (KB REC) on Earthquake Prediction, Seismic Hazard, and Risk Assessment are described. In the first long-term forecasts (September 7 and November 9, 2009), based on the GLOBAS_place technique (GLOBal Activation of Seismicity), we forecasted the occurrence of earthquakes with M ≥ 6.6 in the region of 35° N–40° N (Japan) and in three other regions of the world. In the forecasts submitted on February 25, and March 2 and 3, 2011, we indicated the possibility of more than one earthquake with Mw ≥ 8 occurring starting from noon, March 3. Forecasts were based on the GLOBAS techniques and recording of remote precursors using instrumental methods. Precursors were recorded before the Tohoku earthquake with a preceding time of up to 28 days based on the dynamics of molecular hydrogen (Kamchatka) and less than 8 days based on the bulk deformations in the area of town of Aleksandrov (Moscow Syneclise, East European Craton). A complex analysis of the data, performed within the framework of the techniques used, confirms the possibility of forecasting the time of occurrence, magnitude, and place of strong earthquakes with M ≥ 8.

This article is devoted to the development of the production of 99Mo used in nuclear medicine. The stages in the development of the production of 99Mo and molybdenum-technetium generators are shown. Production has increased several-fold over the period 2009–2013 and meets the demand of domestic nuclear medicine. The problem of recycling the uranium from an irradiated target after 99Mo is extracted from the target is important and strongly influences the price of 99Mo. A technology for reprocessing uranium-containing tailings with the uranium obtained being returned into the 99Mo production cycle has been developed at the Scientific-Research Institute for Physical Chemistry. This has eliminated the need to purchase expensive high-enrichment uranium for targets and has made it possible to use the uranium remaining from the reprocessing of irradiated targets.

Primary cosmic ray mass composition above 1 PeV has been measured with PRISMA-YBJ being a prototype of PRISMA array at altitude of 4300 m a.s.l. It realized a novel type of EAS recording method measuring hadronic EAS component over the total array area through thermal neutron detection with a specially developed so-called en-detectors sensitive to electron and thermal neutron EAS components. Primary c. r. mass composition was recovered through measurements of two EAS components with the en-detectors: electromagnetic (energy deposit) and the number of secondary thermal neutrons produced locally by high energy hadrons. Monte-Carlo simulations of the experiments allowed us to find a parameter highly sensitive to the primary particle mass (atomic number) A above 1 PeV. The preliminary obtained mass composition is consistent with light composition and does not show any significant change at higher energies.

A uniaxially anisotropic superparamagnetic particle suspended in a viscous fluid and subjected to an ac field is considered. Consistently taking into account both internal (Neel) and external (Brownian) magnetic relaxations, a simple expression for the dynamic susceptibility is obtained. This result, with regard to the ac field energy absorption, is compared to the common heuristic approach. This is done for a model polydisperse colloid containing maghemite nanoparticles, which are assumed to posses either bulk or surface magnetic anisotropy. It is shown that viscous losses caused by the particle motion in a fluid matrix make important contribution to the full magnetic response of a ferrocolloid and, thus, its ability to absorb the ac field energy. The obtained exact expression, which takes in both dissipation mechanisms, paves the way to correct optimization of the nanoparticle-mediated heating effect.

The semi-adiabatic theory of magnetic response of a uniaxial single-domain ferromagnetic particle is presented. The approach is developed in the context of the kinetic theory and allows for any orientation of the external field. Within this approximation, the dynamic magnetic hysteresis loops in an ac field are calculated. It is demonstrated that they very closely resemble those obtained by the full kinetic theory. The behavior of the effective coercive force is analyzed in detail, and for it a simple formula is proposed. This relation accounts not only for the temperature behavior of the coercive force, as the previous ones do, but also yields the dependence on the frequency and amplitude of the applied field.

The results from the registration and analysis of sporadic variations of atmospheric thermal neutron flux during thunderstorms are reported. Measurements were performed in Moscow and at the Baksan Neutrino Observatory of the Institute for Nuclear Research of the Russian Academy of Sciences. The neutron flux was detected by unscreened scintillation en-detectors based on 6LiF + ZnS (Ag) compound targets with signals selected according to pulse shape. Reductions of 5–10% in the neutron flux due to shower precipitates during thunderstorms are detected. No incidents of increased neutron flux during thunderstorms were detected over four summer seasons. The upper limits for the integrated and pulsed flux of neutrons from thunderbolts are estimated.

The results from a 29-month investigation into the EAS electromagnetic component using the PRISMA-32 array are presented, notably for the central density spectrum and energy deposit spectrum. The array consists of 32 en-detectors in which specialized inorganic scintillators (SL6-5 phosphorus composition) based on silver-activated zinc sulfide (ZnS) with LiF enriched to 90% 6Li (ZnS(Ag) + 6LiF) are used as detecting layers. The possibility of detecting the electromagnetic component using a thin scintillation neutron detector with the multiple passage of charged particles is shown. The experimental data are compared to the results from simulations.

Heat generation by viscous dissipation in a dilute suspension of single-domain ferromagnetic particles in a rotating magnetic field is analyzed by assuming that the suspended particles have a high magnetic rigidity. The problem is solved by using a kinetic approach based on a rotational diffusion equation. Behavior of specific loss power (SLP) as a function of field strength H and frequency ω is examined at constant temperature. SLP increases as either of these parameters squared when the other is constant, eventually approaching a saturation value. The function SLP(H, ω) can be used to determine optimal and admissible ranges of magnetically induced heating.

Ferrihydrite nanoparticles (2–5 nm in size) produced by bacteria Klebsiella oxytoca in the course of biomineralization of iron salt solutions from a natural medium exhibit unique magnetic properties: they are characterized by both the antiferromagnetic order inherent in a bulk ferrihydrite and the spontaneous magnetic moment due to the decompensation of spins in sublattices of a nanoparticle. The magnetic susceptibility enhanced by the superantiferromagnetism effect and the magnetic moment independent of the magnetic field provide the possibility of magnetically controlling these natural objects. This has opened up the possibilities for their use in nanomedicine and bioengineering. The results obtained from measurements of the magnetic properties of the ferrihydrite produced by Klebsiella oxytoca in its two main crystalline modifications are reported, and the data obtained are analyzed theoretically. This has made it possible to determine numerical values of the magnetic parameters of real biomineral nanoparticles.

EAS (extensive air shower) thermal neutron measurement gives advantages to study energy and mass composition of primary cosmic rays especially in the knee region. After the success of the PRISMA-YBJ experiment, we build a new EAS thermal neutron detection array at Tibet University, Lhasa, China (3700 m a.s.l.) in March, 2017. This prototype array so called "PRISMA-LHAASO-16" consists of 16 EAS EN-detectors ("EN" is abbreviation for electron and neutron) measuring two main EAS components: hadronic and electromagnetic ones. Different from PRISMA-YBJ, these detectors use a thin layer of a novel type of ZnS(Ag) scintillator alloyed with natural boron compound for thermal neutron capture. PRISMA-LHAASO-16 will be moved to the LHAASO site in the near future. In this paper, we introduce principle of the detection technique, deployment of the array, and the test results of the array.

Energy absorption produced by non-interacting single-domain ferromagnetic particles suspended in a liquid matrix is studied. The particles are assumed to be rigid dipoles so that all the dissipation in the system is due to the viscous flow induced in the matrix by orientational motion of the particles in response to a linearly polarized uniform AC field. The kinetic approach employed is valid for arbitrary values of the field frequency and amplitude. Comparison of the results obtained against those of the conventional phenomenological description reveals that the amplitude–frequency dependencies delivered by the latter are limited to weak field regimes.

A kinetic model is proposed to describe the low-frequency magnetodynamics of antiferromagnetic nanoparticles suspended in a fluid. Because of their small size, apart from an anisotropic magnetic susceptibility typical of antiferromagnets, these particles also have a constant magnetic moment caused by sublattice decompensation. An orientational crossover takes place in such a nanosuspension (colloid) when magnetized by a constant field: the axes of easy particle magnetization that were initially aligned along the field become oriented perpendicularly. This effect changes significantly the characteristics of the system’s magnetic response: the dynamic susceptibility spectrum and the relaxation time in a pulsed field.

The state of the art in the theory of dynamic response properties of single-domain particles (macrospins) is presented. If allowing for thermofluctuational effects, one admits temperature as an essential parameter. This implies substantial modification of classical results of magnetodynamic phenomenology from quasi-static hysteresis of magnetization to FMR. With a number of examples we demonstrate how this work is done on the basis of Brown's kinetic equation.

The coefficient of reflection of microwave radiation from a plane layer of a magnetic fluid magne- tized in a field of 11.5 Oe is measured in a frequency range of 24 MHz in temperature intervals 293-333 K; the volume concentration of the magnetic phase (magnetite) is 15 vol %. In this range, the dynamic susceptibility of nanoparticles exhibits strong dispersion (ferromagnetic resonance). The experimental results are interpreted on the basis of the theory of high-frequency magnetization for an ensemble of noninteracting isotropic super- paramagnetic particles. It is found that this simple model provides a satisfactory description of the temperature and frequency dependences of the reflection coefficient.

Dynamic magnetic hysteresis in a nanoparticle is treated with the aid of Brown's kinetic equation. The case in question is the one where the period of the external field falls in between the short intrawell and long interwell magnetic relaxation times. Dependencies of the hysteresis loop on the frequency and amplitude of the field and on temperature are found and analyzed.

The prototype of a novel type EAS (extensive air shower) recording and investigation array (the PRISMA project) is created on the basis of the NEVOD experimental complex through cooperation between Institute for Nuclear Research, Russian Academy of Sciences, and the National Research Nuclear University MEPhI. The prototype (ProtoPRISMA) consists of thirty-two en-detectors sensitive to EAS electromagnetic (e) and hadronic (via the registration of thermal neutrons) (n) components. The array is designed to develop and to test a new method of EAS investigation using neutron and electromagnetic components and to obtain preliminary data on the spectrum and composition of cosmic rays.

Field dependence of dynamic magneto-orientational birefringence in a ferrocolloid based on the nickel-ferrite nanoparticles is examined. The nanoparticles are electrostatically stabilized and suspended in glycerin at low-volume fractions Φ⩽0.75%. The colloids are tested under crossed magnetic fields: an alternating weak (probing) and a constant strong (bias) one. By comparison to a theoretical model of the birefringence relaxation, an evaluation of the mean nanoparticle magnetic anisotropy energy, Ea, is done. We get σ=Ea∕kT∼10, given that the particles are rather polydisperse: a log-normal distribution with exp[⟨lnd⟩]=6.5nm and the width s=0.55. With the parameter σ in such a range, the NiFe2O4 grains make a first example of a nanodisperse material (ferrofluid) whose anisotropy properties differ substantially from both magnetically soft (e.g., maghemite with σ 50) nanosubstances of the same grain size.

In this article, the derivation of a non-linear superparamagnetic theory for suspensions and random assemblies of superparamagnetic nanoparticles is outlined. The basic differences between the low-frequency linear and cubic dynamic susceptibilities of magnetic suspensions (ferrofluids) and solid systems are discussed.

Odd-order dynamic magnetic susceptibilities of an assembly of single-domain uniaxial particles in the low-temperature limit are considered. The solution of the micromagnetic Fokker-Planck equation is presented in the form of asymptotic series with respect to the parameter σ that is the ratio of the anisotropy energy barrier height to the thermal energy. The expressions for the set of susceptibilities up to the fifth order are presented for the case of an assembly of particles randomly oriented in a solid matrix. The validity of the derived asymptotic representation in a wide range of σ is proved by comparison with the numerical solution of the Fokker-Planck equation.

A dynamic birefringence in a ferrocolloid placed in constant and variable crossed magnetic fields is considered. It is assumed that the energy of magnetic anisotropy of colloid nanoparticles is finite so that a magnetic moment can rotate both inside the particle and together with the particle. This "two-channel" pattern of a process significantly affects the kinetics of colloid macroscopic birefringence, that is the character of low- frequency dispersion of this value. Theoretical model is based on the generalized Fokker-Planck equation describing the interaction of internal and external orientational diffusion in a magnetic particle. Calculation results are presented in the form of so-called integral relaxation time; the convenience of this value for the char- acterization of multimode transient processes is shown.

Some results on the EAS neutron component measurements by means of the PRISMA-32 array are presented. The array consists of 32 electron-neutron detectors (en-detectors) capable to detect two main EAS components: electromagnetic one consisting of charged particles, and hadronic one by measuring delayed thermal neutrons accompanying the showers. For thermal neutrons detection, a compound of a well-known inorganic scintillator ZnS(Ag) and LiF, enriched to 90 % with 6Li isotope is used. The setup allows us to record neutron component over the whole array area.

An innovative method for studying the properties of extensive air showers (EASes) that is based on the detection of thermal neutrons generated by high energy hadrons under a soil absorber of 500 g cm−2 is proposed and tested. It is shown that underground hadronic calorimeter can be performed with special scintillation detectors developed by the authors. The method could be quite useful in gamma shower selection and investigations of EAS properties, primary cosmic ray mass composition, EAS core location, and so on.

Nonlinear response of a uniaxially anisotropic superparamagnetic particle subjected to a dc bias and strong ac field is evaluated. For the third harmonic, a non-monotonic dependence of the response amplitude on the strength of the oscillating field (noise-induced resonance) is found. Introduction Studies on stochastic resonance (SR), the number of which multiplies impressively during last years, have revealed a variety of remarkable properties of noisy dynamic systems. In particular, noise-induced resonance (NIR) [1] has been found for both symmetrical [1] and non-symmetrical [2] potentials. Unlike the ordinary stochastic resonance (see the review [3]), the description of NIR does not involve the signal-to-noise ratio and is given in terms of higher harmonic generation. The phenomenon manifests itself as occurrence of “dips” on the amplitudes of higher harmonics plotted as the functions of the noise power or external field strength. Under imposing of a bias field, complementary odd/even harmonics turn up in the system spectrum. Both these effects are now under intense investigation [4,5] in view of their application to detection/enhancement of weak signals. Assemblies of superparamagnetic particles provide a very clear physical example of an essentially noise-affected system. Conventional SR in them, i.e., the response to a weak magnetic field on the thermofluctuational background, has been studied in Refs. [6–10]. In Refs. [11,12] a step has been made to the case of the finite-amplitude excitation. However, NIR-like phenomena have not been addressed. In the original papers [1,2] the existence of NIR had been established only in a weak-field limit, i.e., in terms of power series expansions. The basic demerit of this method is that when evaluating the amplitude of an l-th harmonic, all the contributions of the order (ξ ) are neglected; here ξ is the ratio of the intensity of the driving field amplitude to the reference thermal (noise) energy. Obviously, the higher the field the greater the expansion results deviate from the true ones. Here we describe a magnetic response of a superparamagnetic particle subjected to a dc (bias) field and an ac harmonic field of an arbitrary strength. The novelty is twofold: (1) we prove the NIR-like effects in superparamagnetic systems and (2) we move far beyond the limits of a small parameter approach. Solution of the micromagnetic Fokker-Planck equation We assume a particle to be single-domain so that its magnetic state is that of a magnetic dipole of the magnitude μ = IV, where I is the magnetization of the particle material, and V its volume. Being well below the Curie temperature, we treat I as a constant. Then all the motions of the dipole are reduced to its rotations, and are described by a unit vector e or the set of spherical coordinates θ and φ, with the polar axis pointing along the particle easy axis. Performing any angular motion, the dipole μ = μe effectively experiences the bulk magnetic anisotropy (a symmetrical two-well potential), a dc bias field co-aligned with the particle easy axis (a one well potential) and a ac field of frequency ω imposed along the same axis. The driving frequency is assumed to be small in comparison with that of the Larmor precession in the effective internal (anisotropy + bias) field. Therefore, in the magnetodynamic equation (taken in the Gilbert form) only the relaxation part must be retained. The system resides in a thermal bath of temperature T, the Boltzmann constant is set to unity. Thermally-affected Gilbert magnetodynamics of a single-domain particle is described by the micromagnetic Fokker-Planck equation derived by Brown [14]. It is written for the orientation distribution function W(e,t) of the particle magnetic moment and reads ( ) ( ) { } 1 1 div 2 W E W W E W t β λ τ − ∂   = ∇ ×∇ + ∇ + ∆   ∂ e , (1) where ∆ is the Laplacian on the surface of a unit sphere, E is the free energy density. The parameter τ = βI(1 + λ)/2γλ (2) is the characteristic relaxation time, where β = V/T, γ is the gyromagnetic ratio for electrons, and η is the damping parameter of the Larmor precession used in the Gilbert equation, so that the combination λ = γηI makes a dimensionless dissipation constant. Interactions between different particles of the system as well as any memory effects are neglected. With Eq. (1), as in any statistical approach, the physically observed quantities are the averages fn(t) = 〈Pn(cos θ)〉(t) = 0 π ∫ Pn(cos θ) W(θ,t) sinθ dθ . (3) In particular, the issue of prime interest is f1(t), that is the dimensionless magnetization of the system. Under superimposed constant uniform magnetic field H0 and an ac field H1 = H1 cosωt (both applied along the polar axis) the energy density takes the form E = –K cos 2 θ – (H0+ H1 cosωt) I cosθ , (4) where K is the uniaxial anisotropy constant. Allowing the magnitudes of both ac and dc fields to be large enough so that the Zeeman energies of the magnetic moment are comparable with or higher than the thermal energy T, one faces an essentially nonlinear problem. To solve it, we use the method developed in Refs. [15,16]. Substituting the energy (4) in Eq. (1) and expanding W(θ,t) in a series of Legendre polynomials Pn(cosθ), one arrives at the set of differential-recurrence equations [12]: τ dt d fn(t) + 2 1 n(n + 1) fn(t) = e(t) [an fn–1(t) + bn fn+1(t)] + σ[cn fn–2(t) + dn fn(t) + gn fn +2(t)], (5) where the following notations for the coefficients are used ( 1) 2(2 1) n n n n a b n + = − = + , ( 1)( 1) (2 1)(2 1) n n n n c n n + − = − + , ( 1) (2 1)(2 3) n n n d n n + = − + , ( 1)( 2) (2 1)(2 3) n n n n g n n + + = − + + , (6) and the dimensionless material parameters are e(t) = ξ0 + ξ cosωt, ξ0 = βIH0, ξ = βIH1 , σ = βK . (7) Here we solely concern ourselves with the steady ac response, which is independent of the initial conditions, so that one needs only the stationary solution of Eq. (3). In view of that, we present the sought for asymptotic (with respect to time) relaxation functions in the form

The low-frequency dynamic magnetization of a superparamagnetic particle is studied. It is shown that selective suppression of higher harmonics in the spectrum of the system can occur with a variation in both the temperature (noise-induced resonance) and the excitation intensity (force-induced resonance). © 2001 MAIK "Nauka/Interperiodica".

Dynamic magnetic hysteresis in a single-domain uniaxial particle was studied on the basis of the kinetic approach for the base of an arbitrary angle between the easy magnetization axis and an applied field. Hysteresis loops and absorbed power were calculated, and the dependences of the cyclic magnetization process on the temperature, particle characteristics, and the frequency and amplitude of the magnetizing field were analyzed.

The first experimental data on detection of thermal neutrons accompanying extensive air showers (EASs) which were obtained at two prototypes of the future novel PRISMA EAS array are reported. The operation of this array (even in the minimum prototype version) and its ability to yield completely new information on the hadron and electron components of the EAS were experimentally checked.

The first results obtained during one year of measurements at the upgraded shower Carpet-2 installation at the Baksan Neutrino Observatory of the Institute for Nuclear Research, Russian Academy of Sciences, are presented. The results of calculation of the energy response of the installation are given. The results of data analysis using the standard method and the East-West method are compared. The constraints on amplitudes of harmonics of solar anisotropy for three values of primary energy: E > 12 TeV, E > 40 TeV, and E > 70 TeV, are obtained.

Ferromagnetic resonance (FMR) experiments on non-interacting maghemite (c-Fe 2 O 3 ) nanoparticles in ferro#uids, are performed in X- and Q-bands as a function of the particle diameter (4.8}10 nm) and the temperature (3.5}300 K). The colloidal stability and grain size are controlled through a chemical synthesis, the polydispersity being reduced by a phase separation method. The dependencies of spectral characteristics with temperature „, particle volume< and experimental frequency u, reveal a generic behavior of the FMR lines due to the e!ect of thermal #uctuations. Their scaling with the Langevin parameter m 0 "M

Time-dependent orientational response of a nematic liquid crystal doped with single-domain ferromagnetic particles is studied. A set of coupled macroscopic equations describing evolution of the director texture and the magnetization distribution under the change of the external field is derived. For the case of a plane layer, the relaxation rates are obtained and analyzed.

Semi-adiabatic approximation which accounts fairly well for the macroscopic fluctuational magnetodynamics of an isolated superparamagnetic particle in the intermediate-to-low temperature range is employed to describe ZFC and FC magnetization tests.

Magnetic birefringence experiments under static eld are performed on ionic ferrofluid samples based on Fe2O3 . A colloidal size-sorting of the particles allows to obtain narrow size distributions. The optical birefringence of the solutions is found positive and can be described by a Langevin formalism. It scales as H 2 in the low eld limit. In the high eld limit the particles size dependence of the saturation birefringence is compatible with a surface anisotropy constant KS =2 : 810 2 erg cm 2 associated with a particle elongation of 1.25 both coherent with N eel predictions of surface anisotropy in small grains.

Ferromagnetic resonance (FMR) experiments at 9.26 GHz on non-interacting maghemite (γ-Fe2O3) nanoparticles of ferrofluids are performed as a function of temperature (3.5–300 K) and particle diameter (4.8–10 nm). The orientational mobility of the particles inside the fluid is employed to monitor the orientational distribution of the anisotropy axes by solidifying the MF matrix under the external field. On those textured suspensions, angular analysis is performed allowing to determine the anisotropy of the system, which turns out to be uniaxial and positive. The dependence of angular variations with respect to particle size leads to the conclusion that this anisotropy is induced by the particle surface, its density being Ks=2.7×10−2 erg/cm2 at 3.5 K. Extra isotropic contributions to internal fields at low temperature appear as the signature of the disordered magnetic structure of the surface layer.