Remote sensing by nuclear quadrupole resonance

Detection of explosives has the flavor of those mathematical problems that are not invertible. It is easier to hide explosives than to find them. Many approaches have been proposed and executed for the remote detection of explosives, contraband materials, weapons of mass destruction, currency, etc. Most detection technologies suffer from a common problem: the features they look for, such as discontinuities in electrical conductivity, are not unique properties of the target but are contained, to some degree, in the more benign surroundings. Such a degeneracy leads to “clutter” in the response. For example, resolving the false alarms generated by this clutter can determine the rate of advance of a conventional electromagnetic metal detector employed as a landmine detector. One approach that provides a “unique” signature is nuclear quadrupole resonance (NQR) (the technique is also called QR, to avoid confusion with strictly nuclear techniques). This paper outlines the important physical principles behind the use of NQR for remote detection, indicates areas of applicability, and presents results of field trials of a prototype landmine detection system     

Frequency-selective detection of nuclear quadrupole resonance signals

Nuclear quadrupole resonance (NQR) offers an unequivocal method of detecting and identifying both hidden explosives, such as land mines, and a variety of narcotics. Unfortunately, the practical use of NQR is restricted by a low signal-to-noise ratio (SNR), and means to improve the SNR are vital to enable a rapid, reliable, and convenient system. In this paper, we introduce a frequency-selective approximate maximum-likelihood (FSAML) detector, operating on a subset of the available frequencies, making it robust to the typically present narrow-band interference. The method exploits the inherent temperature dependency of the NQR frequencies as a way to enhance the SNR. Numerical evaluations, using both simulated and real NQR data, indicate a significant gain in probability of accurate detection as compared to a current state-of-the-art approach.     

Spectra of nuclear quadrupole resonance in vitreous semiconductors

The method of nuclear quadrupole resonance is used to study the chalcogenide semiconductors with compositions As₂Se₃ and As₁₄Sb₄Se₂₇. It is shown that partial crystallization occurs in a sample of vitreous As₂Se₃ kept for a long time at room temperature; as a result, a change in the shape of the spectrum of the nuclear quadrupole resonance is observed. The ⁷⁵As spectrum in vitreous As₁₄Sb₄Se₂₇ at a temperature of 77 K is measured for the first time. It is assumed that the ¹²¹Sb or ¹²³Sb nuclei can contribute to the broad line of the quadrupole resonance. It is shown that the use of the nuclear spin-echo Fourier-transform mapping spectroscopy for reconstruction of very broad lines of the nuclear quadrupole resonance provides no advantages compared to the method of reconstruction based on the points in the integrated intensity of the echo signals.     

爆炸物检测中微弱NQR信号的处理

介绍了用核四极矩共振技术检测爆炸物品的原理及检测过程中信号的处理方法。根据NQR测量中接收信号极其微弱的特点,针对以数字累加技术处理NQR信号的常规方法,提出了利用相关检测的方法来处理核四极矩共振（NQR）信号,该方法可以更有效地提高信噪比,并消除随机噪声和外界干扰对接收信号的干扰,从而提高了检测微弱NQR信号的准确性。     

Production and investigation of AgIn5S8/(InSe, GaSe) heterojunctions

It is shown that photosensitive AgIn₅S₈/(InSe, GaSe) heterojunctions can be fabricated using bulk crystals grown from the melt and from the vapor phase or polycrystalline thin films of the ternary compound prepared by pulsed laser evaporation. The spectral curves of the photosensitivity of the heterojunctions are investigated as a function of the photodetection geometry. It is concluded that the resulting structure have a promising potential as wideband and selective photodetectors. Fiz. Tekh. Poluprovodn. 33, 805–809 (July 1999)     

Structure and magnetic properties of InSe single crystals intercalated by nickel

A technology of intercalation of layered A^IIIB^VI single crystals by elements of the Fe 3d group during their growth has been developed. The X-ray structural and X-ray phase analyses of the thus-obtained Ni _x InSe samples confirmed their homogeneity and showed a nonmonotonic dependence of the InSe lattice parameter along the anisotropy axis on the incorporated Ni concentration. The dependence of the magnetic susceptibility on magnetic field, χ(H), has been studied by the Faraday method, and a nonmonotonic change in χ with an increase in the incorporated nickel concentration has been revealed for Ni _x InSe crystals. This nonmonotonicity can be qualitatively explained within a modification of the Anderson model for layered crystals, which takes into account the main specific features of layered crystals and the intercalation mechanism.     

Van der Waals Heteroepitaxy of GaSe and InSe,Quantum Wells,and Superlattices

Bandgap engineering and quantum confinement in semiconductor heterostructures provide the means to fine-tune material response to electromagnetic fields and light in a wide range of the spectrum. Nonetheless, forming semiconductor heterostructures on lattice-mismatched substrates is a challenge for several decades, leading to restrictions for device integration and the lack of efficient devices in important wavelength bands. Here, it is shown that the van der Waals epitaxy of 2D GaSe and InSe heterostructures occur on substrates with substantially different lattice parameters, namely silicon and sapphire. The GaSe/InSe heteroepitaxy is applied in the growth of quantum wells and superlattices presenting photoluminescence and absorption related to interband transitions.     

Raman spectra of the laser-irradiated GaSe single crystals

The specific features of the crystal structure of the lamellar GaSe crystals of various polytypes are analyzed. The possibility of identifying the phase transitions, which occur as a result of variations in temperature, pressure, and composition, is considered from the standpoint of resonance Raman scattering. The effects of high-temperature thermal annealing and pulsed laser irradiation on Raman spectra is investigated. It is found that pulsed laser radiation gives rise to regions with different polytype composition and regions with residual stress.     

Investigation of the promising thermoelectric compound CuAlO2 by the method of nuclear quadrupole resonance in Cu

The results of investigation of the promising thermoelectric compound CuAlO₂ by the nuclear-quadrupole-resonance spectroscopy of the copper nuclei ^{63, 65}Cu are presented. An assumption regarding the defect structure of the crystals is made on the basis of analysis of the asymmetric shape of the resonance lines. The experiments with the nutational nuclear quadrupole resonance of ⁶⁴Cu are indicative of the axial symmetry of the electric-field-gradient tensor on copper nuclei in the investigated samples.     

Er掺杂GaSe晶体特性的研究

对改进型布里奇曼方法生长的Er掺杂GaSe晶体的成 分、光学特性和倍频特性进行了测量和分 析。研究发现,掺杂晶体中Er的分布不均匀且浓度远低于装料浓度,高浓度掺杂使得晶体光 学质量下降。通 过对掺杂晶体光学特性测量发现,Er的掺杂没有明显改变晶体的光谱特性,掺杂浓度为0.07mass.%的 GaSe:Er晶体在可见光波段表现出了比较好的光学性质。通过对晶体拉曼光谱的测量发现, 在GaSe:Er (0.09 mass.%)和GaSe:Er (0.10mass.%)晶体中出现了新的拉曼散射带,它 是由Er³⁺的⁴F9/2→⁴I9/2跃迁产生的。 通过飞秒Ti:sapphire激光和CO₂激光泵浦下I类倍频实验发现,掺杂晶体相位匹配角与纯G aSe晶体相比没有明显变 化,实验结果与理论曲线符合得较好,最佳浓度掺杂晶体GaSe:Er(0.07mass.%)将CO₂倍 频转换效率提高了43%。     

Growth and crystal quality of inp crystals by the liquid encapsulated czochralski technique

The growth of InP single crystals by the liquid encapsulated Czochralski (LEC) technique has been studied from the standpoint of improving crystal quality. Twin-free crystals have been grown reproducibly in the <lll>P direction under the following conditions; (1) using starting material which does not contain fine InP particles, (2) controlling the cone shape of the crystals such that the angle with the growth axis is less than 19.68°, (3) arranging the.hot zone to produce a temperature at the top surface of the B₂O₃ encapsulant layer below 550°C. It has been confirmed that electrical properties of nominally undoped crystals are dominated by the impurity, Si, and the concentration of Si in an LEC crystal corresponds to that of the starting material. The dislocation densities of undoped LEC InP crystals depend on thermal stresses during the growth process. This knowledge has led to the growth of dislocation-free crystals.     

Quasi-localized states and resonance scattering of particles by defects in semiconductor crystals with band spectrum structure

The interaction of particles with planar defects in semiconductor crystal was studied on the basis of a model of the band energy-spectrum structure. Generalized localized states are shown to exist in the vicinity of defects. It is also shown that nonquadratic dispersion law gives rise to quasi-localized states. Particle scattering by the semiconductor boundary was analyzed. The condition for the total reflection from the interface is formulated as follows: the incident-particle energy should be equal to the energy of a quasi-localized state. It was found that a weak dissipation of particle energy in the crystal causes instability of the resonance condition for the total reflection.     

Effect of low-temperature annealing on the quality of InSe layered single crystals and the characteristics of n-InSe/p-InSe heterojunctions

The effect of the low-temperature annealing of n- and p-type InSe single crystals on the photoelectric characteristics of n-InSe/p-InSe heterojunctions is investigated. It is found that the most pronounced enhancement of these characteristics takes place for annealing temperatures of 150–200°C. Improvement in the quality of single-crystal samples upon annealing is confirmed by the observation of multiplet nuclear quadrupole resonance spectra, which reflect ordering in the system of polytypes of layered InSe crystals. For annealed materials, n-InSe/p-InSe structures exhibit an increase in the intensity of the exciton peak and an increase in the open-circuit voltage from 0.29 to 0.56 V and short-circuit current from 350 to 840 μA/cm².     

Electrical and photoelectric characteristics of structures based on InSe and GaSe layered semiconductors irradiated with 12.5-MeV electrons

The effect of irradiation with 12.5-MeV electrons on the electrical and photoelectric parameters of layered photoconverters based on p-InSe-n-InSe and p-GaSe-n-InSe structures is studied. The observed variations in the current-voltage characteristics, photoresponse spectra, open-circuit voltage, and short-circuit current are caused by the formation of point defects. The absence of pronounced changes in the characteristics of the homojunctions and heterojunctions even after irradiation at the highest dose makes it possible to recommend these junctions for use in the fabrication of radiation-resistant photodetectors.     

Photoconductivity and luminescence in GaSe crystals at high levels of optical excitation

The photoconductivity and luminescence of GaSe layered crystals at high levels of optical excitation are studied experimentally. The specific features observed in the photoconductivity and photoluminescence spectra are controlled by the nonlinear optical absorption in the region of excitonic resonance.     

温梯法生长的优质Nd:YAG晶体的激光性质

用温梯法生长的Nd:YAG晶体制成φ5×50毫米激光棒,光学均匀性很好,晶体干涉条纹为零级,重复率输出的平均效率～1％,且极易获得单横模输出。     

Effect of annealing on the spectra of nuclear quadrupole resonance in gallium-indium selenides and characteristics of structures based on these materials

The results of studying the effect of low-temperature annealing (at temperatures no higher than 250°C) on the spectra of nuclear quadrupole resonance in layered GaSe and InSe single-crystal semiconductors are reported. The electrical and photoelectric characteristics of the p-GaSe-n-InSe structures have also been studied. It is shown that, as the annealing temperature is lowered to room temperature, the quality of the samples improves due to a decrease in the defect concentration in the crystals and due to ordering in a system of polytypes. The temperature conditions for the heat treatments, at which the main parameters of the heterojunction are improved, are determined. Mechanisms affecting the behavior of the p-GaSe-n-InSe structure during the course of annealing are discussed.     

Simultaneous optical and nuclear magnetic resonance spectroscopy for monitoring cardiac energetics in vivo

There are a number of applications in which it is useful to simultaneously collect data from what are traditionally separate instrumentation modalities. In particular, in vivo physiological investigations in which data from parallel experiments must be correlated would benefit from simultaneous data collection through 1) elimination of subject variability, 2) elimination of treatment variability, and 3) a reduction in the number of animal preparations required. Here we describe the simultaneous collection of fluo-3 optical fluorescence and 31P nuclear magnetic resonance (NMR) spectra to measure intracellular calcium levels and high-energy phosphate metabolism, respectively, in vivo. This work is part of ongoing research into the profound anoxia tolerance exhibited by the hearts of certain turtle species. An NMR compatible optical fluorescence spectrometer was constructed and tested. In the 31-cm bore of a 2 T superconducting magnet, NMR and optical spectra were collected every 10-15 min from the in situ, in vivo hearts of anesthetized turtle subjects prior to and during one to three hours of anoxia. It was found that while PCr stores became significantly depleted during anoxia, beta-adenosine triphosphate (ATP) levels remained within 20% of control values, and intracellular diastolic calcium levels did not vary by more than 10%. The ability to make simultaneous phosphorus and calcium measurements on a single subject is important to understanding the exact relationship between phosphorus energy state and maintenance of calcium homeostasis.     

Films of degenerate intrinsic oxides of InSe and In<Subscript>4</Subscript>Se<Subscript>3</Subscript> semiconductor crystals

The results of investigation of electric properties of oxide-p-InSe and oxide-p-In₄Se₃ heterojunctions, the front layer in which is obtained by the thermal oxidation of crystalline substrates, are presented. It is established that the forward portions of I–V characteristics of heterojunctions are similar to the forward portions of the I–V characteristics of metal-semiconductor diodes in which the majority carriers are involved in the charge transport. Therefore, the electric properties of heterojunctions are interpreted under the assumption that the intrinsic oxide is a degenerate semiconductor. For confirmation of the assumption, the data were compared to the results of the investigation carried out on the ITO-GaTe heterojunctions in which the ITO film was intentionally made degenerate.     

Specific features of the spectra of nonlinear optical absorption in nonstoichiometric and Ni doped GaSe single crystals

The method of two radiation sources is used to measure the spectra of nonlinear optical absorption in the GaSe, GaSe:Ni, Ca_1.05Se_0.95, and Ca_0.95Se_1.05 single crystals. The levels with the depths of 1.38, 1.45, 1.55, 1.65, and 1.80 eV are detected. Absorption by nonequilibrium charge carriers, induced absorption, and induced transmission caused by a variation in the occupation of impurity centers due to laser radiation are observed in the spectra of nonlinear optical absorption.