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The nuclear stopping power of hydrogen and helium isotopes in Be, C, and W materials that are promising for use as the first wall of a tokamak reactor are calculated. It is shown that the presence of an attracting potential well has a substantial effect on the dependence of the nuclear stopping power on the collision energy. The use of potentials calculated in the approximation of the density functional theory with an attracting potential well has allowed us to obtain more accurate values of the nuclear stopping power for hydrogen isotopes, which differ by 27–60% from the tabular data at low energies. The results for different hydrogen isotopes are well described by a universal curve.

The depth distribution of the energy release (linear energy transfer) occurring upon bombardment of a tungsten target by deuterium atoms with energies ranging from 100 eV to 10 MeV is calculated. It is demonstrated that, contrary to common belief, the maximum energy release at energies below 100 keV is localized near the surface of the solid. At energies above 100 keV, the shape of the distribution changes, and a Bragg maximum emerges near the particle stopping point. The depth distribution of the energy release in tungsten under conditions typical of the ITER tokamak reactor is determined. This distribution provides an opportunity to estimate the wall heating by incident plasma atoms.

OBJECTIVE. The authors created an effective system of prediction of acute erosive-ulcerative lesions of the stomach and duodenum in victims with extensive burns. MATERIALS AND METHODS. The patients (85) were divided into two groups. The first group included 45 patients (53 %) with erosive ulcerative lesions of the stomach and duodenum which developed during 14 days after injury. The second group numbered 40 patients (47 %) without complications. The clinical, laboratory and instrumental methods of investigations were applied. RESULTS. According to data analysis of patient’s examination, the scale of assessment of sign rates was developed. The amount of rates corresponded to the index of prediction (IP). The values of IP were calculated at the interval from -15 to +15 units. The high possibility (85 %) of development of acute erosive and ulcerative lesions of the stomach and duodenum could be expected in cases when IP was about +15 units. CONCLUSIONS. The developed predictive scale of erosive-ulcerative complications of gastrointestinal tract appeared to be highly reliable and allowed doctors to use preventive measures on time.

It has been shown by computer simulation that the coefficient of reflection of argon atoms scattered by crystalline aluminum and germanium targets at glancing angles of less than 4° is close to unity and the beam of scattered particles exhibits focusing (the angular distributions of particles are strongly compressed). Whereas beam focusing with respect to the azimuth is well known and has already been studied, sharp focusing in the surface-normal direction at small glancing angles has not been studied so far. This effect is confirmed by the experimental results. It is associated with multiple scattering of incident particles by the atomic chain. The simulation results allowed finding quite accurately the amplitude of thermal vibrations of surface atoms ((0.123 ± 0.007) A for aluminum), which agrees well with the experiment.

A mathematical model of helium absorption by a sorbent consisting of hollow spherical particles with allowance for the distributions of permeability coefficients and particle sizes is constructed. A possibility of reconstructing the permeability coefficients of the microsphere materials from the known sorption curves and particle size distribution is demonstrated. The model is analyzed and tested by examples of two different sorbents.

The mechanism of the interaction of coherent terahertz radiation with a probe-nanoobject system has been experimentally investigated in a terahertz apertureless near-field microscope. It has been found that the type of the material of a sample under the probe, as well as the geometry of the probe, determines the form of the dependence of the differential signal of the terahertz field on the distance between the probe and sample. The amplitude of the modulation of the probe height significantly affects the spectral composition of the differential terahertz signal, which is directly related to the amplitude and phase of a terahertz wave scattered by the probe-nanoobject system.

The data on interatomic interaction potentials obtained from experiments are compared with the well-known theoretical models of potentials. A new analytical form of a potential that provides the best fit for experiments is proposed. The accuracy of its application is estimated as a function of the internuclear distance under consideration. Individual potentials for systems consisting of inert gases are discussed, and an algorithm for constructing potentials is proposed for cases not yet studied.

Upon collisions of multiply charged ions with a surface, the electric field of the image charge causes Stark splitting of the ion levels; in this case, the Stark states whose energy approaches the electron binding energy in the metal with a decrease in the distance to the surface are selectively occupied. It is shown that consideration of the electric field effect leads to an increase in the probability of Auger transitions in the presence of field. This effect changes our representations about the scheme of occupation of lower levels of multiply charged ions.

We report on the first principle calculations of photocarriers kinetics in a photoconductive antenna excited by an ultrashort optical laser pulse. The solution of non-equilibrium Boltzmann equation is used to derive the expression for the irradiated electric field. The analysis reveals the important role of non-uniform photocarrier distribution inside the active layer in the formation of the terahertz radiation from the emitter in both collinear and anti-collinear directions.

The interaction potentials for systems Ar+, C+, Cd+, Kr+, Ne+, Xe+, Zn+-Xe, Ar+-Ar, and Ne+-Ne are obtained from experimental data on scattering of atomic particles with energies 1–300 keV. A model potential is proposed, which describes the experimental data most correctly, and the error in its application for the description of particle scattering for various nearest approach distances is estimated.

Radiative processes in a nonequilibrium e-h plasma are theoretically studied using a self-consistent solution of the kinetic equation and Maxwell’s equations. The terahertz emission from a finite-thickness semiconductor sample is due to the retardation and nonstationarity of the electromagnetic interaction of the photocurrent in the e-h plasma and the radiation field. The duplex waveform of the terahertz electromagnetic pulse for an arbitrary ratio of the radiation formation length and the plate thickness originates due to coherent radiative processes accompanying the generation of the e-h plasma at the input boundary and its extinction at the output boundary of a semiconductor plate through which a weakly absorbed ultrashort laser pulse propagates. The theoretical conclusions show analogies with the radiative phenomena accompanying the start-stop motion of external currents (Tamm problem) and the nonlinear interaction of optical waves in a finite-thickness medium.

The polarization spectra of spontaneous terahertz radiation in uniaxially deformed germanium have been measured upon the electric breakdown of shallow acceptors. Lines with various degrees of polarization with respect to the compression axis have been observed in the radiation spectrum. These lines are associated with the optical transitions of holes between the excited and ground states of the acceptor, as well as with the transitions of holes from the valence band to the ground state of the impurity. At a pressure of about 3 ± 0.3 kbar in the [111] direction near the impurity breakdown, the linear polarization degree reaches ∼80–90% in the main lines of terahertz radiation. As the electric field intensity increases, the depolarization of radiation is observed, which is caused by the heating of nonequilibrium holes by the electric field.

Terahertz pulsed imaging is a spectroscopic imaging modality using pulses of electromagnetic radiation (100 GHz-10 THz), and there has been recent interest in studying biomedical specimens. It is usual to display parametric images derived from the measured pulses. In this work, classification was achieved by applying multispectral clustering techniques to sets of parametric images. It was hypothesised that adequate information for clustering was carried in a small number of parametric images, providing these were weighted by complementary physical properties. Materials prepared for histopathological examination were chosen because their condition remained stable during long imaging periods and because their dehydrated state led to greater penetration of the radiation. Two specimens were examined in this pilot study, one of basal cell carcinoma and one of melanoma. Unsupervised ISODATA classification using three selected parametric terahertz pulsed images was compared qualitatively with k-means classification using the shape of the whole time series, and with conventional stained microscope slides. There was good qualitative agreement between the classifications. Classifications were consistent with the morphological appearances expected, but further work is required to determine if tumour discrimination is possible. The results have implications for the future development of the technique as the need for only a small number of features could lead to considerably reduced acquisition times.

Techniques for the coherent generation and detection of electromagnetic radiation in the far infrared, or terahertz, region of the electromagnetic spectrum have recently developed rapidly and may soon be applied for in vivo medical imaging. Both continuous wave and pulsed imaging systems are under development, with terahertz pulsed imaging being the more common method. Typically a pump and probe technique is used, with picosecond pulses of terahertz radiation generated from femtosecond infrared laser pulses, using an antenna or nonlinear crystal. After interaction with the subject either by transmission or reflection, coherent detection is achieved when the terahertz beam is combined with the probe laser beam. Raster scanning of the subject leads to an image data set comprising a time series representing the pulse at each pixel. A set of parametric images may be calculated, mapping the values of various parameters calculated from the shape of the pulses. A safety analysis has been performed, based on current guidelines for skin exposure to radiation of wavelengths 2.6 μm–20 mm (15 GHz–115 THz), to determine the maximum permissible exposure (MPE) for such a terahertz imaging system. The international guidelines for this range of wavelengths are drawn from two U.S. standards documents. The method for this analysis was taken from the American National Standard for the Safe Use of Lasers (ANSI Z136.1), and to ensure a conservative analysis, parameters were drawn from both this standard and from the IEEE Standard for Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields (C95.1). The calculated maximum permissible average beam power was 3 mW, indicating that typical terahertz imaging systems are safe according to the current guidelines. Further developments may however result in systems that will exceed the calculated limit. Furthermore, the published MPEs for pulsed exposures are based on measurements at shorter wavelengths and with pulses of longer duration than those used in terahertz pulsed imaging systems, so the results should be treated with caution.

Recently published studies suggest thatterahertz pulsed imaging will have applications inmedicine and biology, but there iscurrently very little information about the opticalproperties of human tissue at terahertzfrequencies. Such information would be useful forpredicting the feasibility of proposedapplications, optimising acquisition protocols,providing information about variability ofhealthy tissue and supplying data for studies of theinteraction mechanisms. Research ethicscommittee approval was obtained, andmeasurements made from samples of freshlyexcised human tissue, using a broadbandterahertz pulsed imaging system comprisingfrequencies approximately 0.5 to 2.5 THz.Refractive index and linear absorptioncoefficient were found. Reproducibility wasdetermined using blood from one volunteer,which was drawn and measured on consecutivedays. Skin, adipose tissue, striatedmuscle, vein and nerve were measured (to date, from oneindividual). Water had a higher refractiveindex (2.04 ± 0.07) than any tissue.The linear absorption coefficient was higher formuscle than adipose tissue, as expectedfrom the higher hydration of muscle. As these samples camefrom a single subject, there is currentlyinsufficient statistical power to draw firmconclusions, but results suggest that in vivo clinical imaging will be feasible in certainapplications.

As with other imaging modalities, the performance of terahertz (THz) imaging systems is limited by factors of spatial resolution, contrast and noise. The purpose of this paper is to introduce test objects and image analysis methods to evaluate and compare THz image quality in a quantitative and objective way, so that alternative terahertz imaging system configurations and acquisition techniques can be compared, and the range of image parameters can be assessed. Two test objects were designed and manufactured, one to determine the modulation transfer functions (MTF) and the other to derive image signal to noise ratio (SNR) at a range of contrasts. As expected the higher THz frequencies had larger MTFs, and better spatial resolution as determined by the spatial frequency at which the MTF dropped below the 20% threshold. Image SNR was compared for time domain and frequency domain image parameters and time delay based images consistently demonstrated higher SNR than intensity based parameters such as relative transmittance because the latter are more strongly affected by the sources of noise in the THz system such as laser fluctuations and detector shot noise.

The low-temperature time-resolved photoluminescence of polycrystalline GaN layers grown by molecular beam epitaxy on metal substrates (Mo and Ta) was investigated. The photoluminescence spectra observed include two emission bands in the ultraviolet spectral region. We assign one of these bands to recombination processes inside cubic nanocrystallites, which are formed in the hexagonal polycrystalline GaN host. The recombination radiation of cubic nanocrystallites is enhanced due to predominant trapping of the nonequilibrium electron-hole pairs in these crystallites.

The results of the investigation of low-temperature time-resolved photoluminescence in undoped and Si-doped In0.2Ga0.8N/GaN structures, which contain 12 quantum wells of width 60 A separated by barriers of width 60 A, are reported. The structures were grown by the MOCVD technique on sapphire substrates. The photoluminescence properties observed are explained by the manifestation of two-dimensional donor-acceptor recombination. These properties are the high-energy shift of the peak upon increasing the pumping intensity, a low-energy shift with increasing delay time, and a power law of luminescence decay of the t -γ type. The estimates of the total binding energy for donor and acceptor centers are given. This energy is 340 and 250 meV for Si-doped and undoped quantum wells, respectively. The role of the mosaic structure, which is typical for Group III hexagonal nitrides, is discussed as a factor favorable for the formation of donor-acceptor pairs.

Electroluminescence from a quantum-cascade structure comprising 40 periods of GaAs/Al0.15Ga0.85As tunnel-coupled quantum wells (QW) was studied. A terahertz emission band in the range 1.0–1.8 THz is observed under bias exceeding 1.5–2.0 V. The emission band peak shifts linearly to higher frequency with the increasing bias. The effect is accounted for by spatially indirect electron transitions between states in the neighboring QWs.

The problem of interpretation of hot‐wire measurements of acoustic fields in compressible flows is considered. Relations between mass‐flow and total‐temperature fluctuations registered by a hot‐wire anemometer and pressure and velocity fluctuations are found. The relations obtained are applicable in the general case for measurement of resultant acoustic fluctuations at some point of the flow, which are generated by arbitrary distributed sources of sound with prior‐unknown properties.

We present the terahertz (THz) scanning probe microscope which combines a THz coherent spectrometer and a scanning probe microscope. It detects forward-scattered radiation and employs harmonic signal demodulation to extract the signal of near-field contribution to scattering of THz electromagnetic waves

An analysis of the time-resolved photoluminescence spectra of magnesium-doped layers of gallium nitride grown by molecular epitaxy on GaAs and sapphire substrates is presented. New characteristic features of Mg impurities and their interaction with intrinsic defects, which are manifested via a mechanism of donor-acceptor recombination, are discovered. The donor-acceptor optical transitions in a doped, weakly compensated broad-band semiconductor are considered theoretically under the assumption of a nonrandom distribution of the impurities. A quantitative analysis of the donor-acceptor photoluminescence spectra of diverse samples of GaN:Mg epilayers reveals depletion at short distances in the distribution of the donors and acceptors and/or the presence of a predominant distance between impurities (singularities), depending on the growth conditions. The following are determined from a comparison of the calculated and experimental dependences of the time-resolved photoluminescence: the form of the interimpurity distribution function, the dopant concentration, and the binding energies of the acceptors (Mg atoms) and active donors.

The Russian Academy of Sciences and Federal Space Agency, together with the participation of many international organizations, worked toward the launch of the RadioAstron orbiting space observatory with its onboard 10-m reflector radio telescope from the Baikonur cosmodrome on July 18, 2011. Together with some of the largest ground-based radio telescopes and a set of stations for tracking, collecting, and reducing the data obtained, this space radio telescope forms a multi-antenna ground-space radio interferometer with extremely long baselines, making it possible for the first time to study various objects in the Universe with angular resolutions a million times better than is possible with the human eye. The project is targeted at systematic studies of compact radio-emitting sources and their dynamics. Objects to be studied include supermassive black holes, accretion disks, and relativistic jets in active galactic nuclei, stellar-mass black holes, neutron stars and hypothetical quark stars, regions of formation of stars and planetary systems in our and other galaxies, interplanetary and interstellar plasma, and the gravitational field of the Earth. The results of ground-based and inflight tests of the space radio telescope carried out in both autonomous and ground-space interferometric regimes are reported. The derived characteristics are in agreement with the main requirements of the project. The astrophysical science program has begun., 54 pages, 11 figures; published by Astronomicheskij Zhurnal (in Russian) and Astronomy Reports (in English)

Recently published studies suggest thatterahertz pulsed imaging will have applications inmedicine and biology, but there iscurrently very little information about the opticalproperties of human tissue at terahertzfrequencies. Such information would be useful forpredicting the feasibility of proposedapplications, optimising acquisition protocols,providing information about variability ofhealthy tissue and supplying data for studies of theinteraction mechanisms. Research ethicscommittee approval was obtained, andmeasurements made from samples of freshlyexcised human tissue, using a broadbandterahertz pulsed imaging system comprisingfrequencies approximately 0.5 to 2.5 THz.Refractive index and linear absorptioncoefficient were found. Reproducibility wasdetermined using blood from one volunteer,which was drawn and measured on consecutivedays. Skin, adipose tissue, striatedmuscle, vein and nerve were measured (to date, from oneindividual). Water had a higher refractiveindex (2.04 ± 0.07) than any tissue.The linear absorption coefficient was higher formuscle than adipose tissue, as expectedfrom the higher hydration of muscle. As these samples camefrom a single subject, there is currentlyinsufficient statistical power to draw firmconclusions, but results suggest that in vivo clinical imaging will be feasible in certainapplications.

We report PL studies of the kinetics of current-induced non-equilibrium Landau level (LL) populations in 2DEGs. The B −2 field dependence of the relaxation rate is inconsistent with theoretical rates for one- and two-phonon inter-LL processes, suggesting relaxation takes place by extrinsic processes.

The spectrum of spontaneous terahertz electroluminescence was obtained near the breakdown threshold of a shallow acceptor (Ga) in germanium. The emission spectra were recorded by the Fourier spectroscopy method at a temperature of ∼5.5–5.6 K. The emission spectrum exhibits narrow lines with maxima at ∼1.99 THz (8.2 meV) and ∼2.36 THz (9.7 meV), corresponding to the optical transitions of nonequilibrium holes from the excited impurity states to the ground state of impurity center. A broad line with a maximum at ∼3.15 THz (13 meV) corresponding to the hole transitions from the valence band to the impurity ground state is also seen in the spectrum. The contribution of the hole transitions from the states of the valence band increases upon an increase in the electric-field strength. Simultaneously, the optical transitions of nonequilibrium holes between the subbands of the valence band appear in the emission spectrum. The integral terahertz-emission power is ∼17 nW per 1 W of the input power.

We report comprehensive photoluminescence, Raman scattering and photoluminescence excitation data on GaN layers grown by MBE on GaAs and GaP substrates. The main photoluminescence feature of GaN layers corresponds to the recombination of free carriers from conduction and valence bands. It is supported by both photoluminescence and photoluminescence excitation spectroscopic data. The origin of impurity-assisted photoluminescence is discussed. We also report the Raman spectra on cubic GaN films.

Terahertz electroluminescence in the range ≈1.5 THz was observed in a quantum-cascade GaAs/AlGaAs structure containing 40 periods of tunnel-coupled wells. The luminescence is caused by the spatially indirect optical electron transitions between the ground states of neighboring quantum wells.

We report on the observation of efficient terahertz emission from semiconductors (n-GaAs) under continuous-wave interband excitation at liquid helium temperatures. The radiative transitions occur under relaxation and trapping of photoexcited electrons to charged donor centers. The external quantum yield of the emission is up to 0.3%.

We report results of spectroscopic studies of terahertz (THz) spontaneous emission at conditions of electrical breakdown of impurity centers in Ge(Ga) with Na-Nd~ 1.6x1014 cm-3. The samples were deformation-free or subjected to compressive strain along [111] axis. The experiments were performed at T~5 K with using a step-scan Fourier spectrometer. The maximum level of the compressive strain was 3 kbar.

We report on a new electrically pumped terahertz (THz) source based on GaAs1-xNx/GaAs heterojunction doped with shallow (Be) acceptors. The spectra of observed efficient THz electroluminescence in the spectral range of 2.4-6.0 THz (10-25 meV) suggest the scheme of intra-center optical transitions employing resonant and localized energy levels of acceptor states split by built-in stress at the heterojunction interface.

We report on the first measurements of spectra of the terahertz (THz) emission occurring near the threshold of impact breakdown of the shallow acceptor (Ga) in Ge at zero deformation and also at the uniaxial stress of order of 3 kbar in the [111] direction. A strong level of linear polarization of the THz emission is found for the deformed p‐Ge.

Terahertz (THz) frequency radiation, 0.1 THz to 20 THz, is being investigated for biomedical imaging applications following the introduction of pulsed THz sources that produce picosecond pulses and function at room temperature. Owing to the broadband nature of the radiation, spectral and temporal information is available from radiation that has interacted with a sample; this information is exploited in the development of biomedical imaging tools and sensors. In this work, models to aid interpretation of broadband THz spectra were developed and evaluated. THz radiation lies on the boundary between regions best considered using a deterministic electromagnetic approach and those better analysed using a stochastic approach incorporating quantum mechanical effects, so two computational models to simulate the propagation of THz radiation in an absorbing medium were compared. The first was a thin film analysis and the second a stochastic Monte Carlo model. The Cole–Cole model was used to predict the variation with frequency of the physical properties of the sample and scattering was neglected. The two models were compared with measurements from a highly absorbing water-based phantom. The Monte Carlo model gave a prediction closer to experiment over 0.1 to 3 THz. Knowledge of the frequency-dependent physical properties, including the scattering characteristics, of the absorbing media is necessary. The thin film model is computationally simple to implement but is restricted by the geometry of the sample it can describe. The Monte Carlo framework, despite being initially more complex, provides greater flexibility to investigate more complicated sample geometries.

A pulsed terahertz imaging system has been developed for potential use in vivo. Few data are available regarding the optical properties of human tissue at terahertz frequencies. This work demonstrates transmission measurements through human ex vivo tissue sections, and determines broadband refractive indices, and broadband and frequency dependent absorption coefficients. The data presented here are the first systematic measurements of this type. Significant differences were found between a numbers of human tissue types.

Terahertz (THz) frequency spectroscopic imaging studies of teeth are reported. The aim is to establish the characteristic spectroscopic properties of the enamel and dentine at these high frequencies. Changes to the THz characteristics as a result of various types of tooth decay are reported showing the potential of this technique for dental diagnosis.

Terahertz (THz) frequency spectroscopic imaging studies of teeth are reported. The aim is to establish the characteristic properties of the enamel and dentine at these high frequencies. Changes to the THz characteristics as a result of various types of tooth decay are reported showing the potential of this technique for dental diagnosis.

This contribution attempts to answer the questions of how THz images are best processed, analyzed, presented, classified and interpreted, especially within a clinical setting. The solutions to these problems are reviewed within the context of recent imaging work on tumours and dental tissue, largely conducted by the Teravision group. It is concluded that compound images, especially those involving pulse broadening as an image parameter, are highly promising routes to visualisation. The possibility of using computer vision and data compression techniques is also discussed.

Modelling the interaction of terahertz(THz) radiation with biological tissueposes many interesting problems. THzradiation is neither obviously described byan electric field distribution or anensemble of photons and biological tissueis an inhomogeneous medium with anelectronic permittivity that is bothspatially and frequency dependent making ita complex system to model.A three-layer system of parallel-sidedslabs has been used as the system throughwhich the passage of THz radiation has beensimulated. Two modelling approaches havebeen developed a thin film matrix model anda Monte Carlo model. The source data foreach of these methods, taken at the sametime as the data recorded to experimentallyverify them, was a THz spectrum that hadpassed though air only.Experimental verification of these twomodels was carried out using athree-layered in vitro phantom. Simulatedtransmission spectrum data was compared toexperimental transmission spectrum datafirst to determine and then to compare theaccuracy of the two methods. Goodagreement was found, with typical resultshaving a correlation coefficient of 0.90for the thin film matrix model and 0.78 forthe Monte Carlo model over the full THzspectrum. Further work is underway toimprove the models above 1 THz.

The first demonstrations of terahertz imaging in biomedicine were made several years ago, but few data are available on the optical properties of human tissue at terahertz frequencies. A catalogue of these properties has been established to estimate variability and determine the practicality of proposed medical applications in terms of penetration depth, image contrast and reflection at boundaries. A pulsed terahertz imaging system with a useful bandwidth 0.5-2.5 THz was used. Local ethical committee approval was obtained. Transmission measurements were made through tissue slices of thickness 0.08 to 1 mm, including tooth enamel and dentine, cortical bone, skin, adipose tissue and striated muscle. The mean and standard deviation for refractive index and linear attenuation coefficient, both broadband and as a function of frequency, were calculated. The measurements were used in simple models of the transmission, reflection and propagation of terahertz radiation in potential medical applications. Refractive indices ranged from 1.5 ± 0.5 for adipose tissue to 3.06 ± 0.09 for tooth enamel. Significant differences (P < 0.05) were found between the broadband refractive indices of a number of tissues. Terahertz radiation is strongly absorbed in tissue so reflection imaging, which has lower penetration requirements than transmission, shows promise for dental or dermatological applications.

A pulsed terahertz imaging system has been developed for potential use in vivo. Few data are available regarding the optical properties of human tissue at terahertz frequencies. This work demonstrates transmission measurements through human ex vivo tissue sections, and determines broadband refractive indices, and broadband and frequency dependent absorption coefficients. The data presented here are the first systematic measurements of this type. Significant differences were found between a numbers of human tissue types.