Pulsed 14N NQR Device Designed to Detect Substances in the Presence of Environmental Noise

A new device designed for both volumetric and surface NQR detection of substances spatially located in several positions, and in the presence of environmental interference, is described. The device consists of two probe coils, placed on the same detection plane, for excitation and detection of NQR signals. Experimental results obtained using Strong Off Resonance Comb (SORC) pulse sequences, for the excitation of the ν- transition in samples of Sodium Nitrite (NaNO ₂), are presented. It is shown that, when the total signals induced in each coil are properly combined, the interference commonly detected in both coils is attenuated relative to the NQR signal detected by either one or both probe coils. NQR signal can be detected by either one or both coils, but in both cases the noise induced by distant environmental sources is attenuated. These files correspond to the revised version of the paper N° O-G-6, entitled “Pulsed ¹⁴N NQR device designed to detect substances in the presence of environmental noise” presented at the XVII International symposium on Nuclear Quadrupole Interactions, held in Bonn, Germany in 2004. Argentinian and International patents pending.     

Nuclear quadrupole resonance echoes from hexamethylenetetramine

We investigated the echo phenomenon of nuclear quadrupole resonance (NQR) from hexamethylenetetramine (HMT). We detected the pure NQR echo signal of HMT with a short pulse interval. The intensity of the echo signal increased as the pulse interval time was decreased. We observed that a clean echo signal was generated even when the pulse interval was shorter than the decay time constant T₂^*. Since the short interval time gives a strong echo, our result insists that shorter interval time is preferred for the NQR detection.     

Pulse EPR, ENDOR, and ELDOR Study of Anionic Flavin Radicals in Na+-Translocating NADH:Quinone Oxidoreductase

The Na⁺-translocating nicotinamide adenine dinucleotide (NADH):quinine oxidoreductase (Na⁺–NQR) is a component of respiratory chain of various bacteria and it generates a redox-driven transmembrane electrochemical Na⁺ potential. It contains four different flavin prosthetic groups, including two flavin mononucleotide (FMN) residues covalently bound to the subunits NqrB and NqrC. Na⁺–NQR from Vibrio harveyi was poised at different redox potentials to prepare two samples, containing either both FMN_NqrB and FMN_NqrC or only FMN_NqrB in a paramagnetic state. These two samples were comparatively studied using pulse electron paramagnetic resonance (EPR), electron-nuclear double resonance (ENDOR), and electron-electron double resonance (ELDOR) spectroscopy. The echo-detected EPR spectra and electron spin relaxation properties were very similar for flavin radicals in both samples. The splitting of the outer peaks in the proton ENDOR spectra, assigned to the C(8α) methyl protons, allows to identify both radicals as anionic flavosemiquinones. The mean interspin distance of 20.7 Å between these radicals was determined by pulse ELDOR experiment, which allows to estimate the edge-to-edge distance (r _e) between these flavin centers as: 11.7 Å < r _e < 20.7 Å. The direct electron transfer between FMN_NqrB and FMN_NqrC during the physiological turnover of the Na⁺–NQR complex is suggested.     

Detection of plastic explosives in luggage with14N nuclear quadrupole resonance spectroscopy

Nuclear quadrupole resonance (NQR) of¹⁴N nuclei has many advantages as a method for detecting nitrogen-containing explosives, the most important are very high chemical specificity, true noninvasive operation and detection of bulk explosive in situ only (no vapor or particular capture needed). One of the most high explosives is hexogen (RDX) often used by terrorists in plasticized forms. The ring nitrogen nuclei in an RDX molecule generate three sets of NQR frequencies corresponding to three physically nonequivalent positions of the molecule in the crystal lattice. The prototype device we have constructed is intended for inspection of suitcases for the presence of plastic explosives containing RDX or octogen by¹⁴N quadrupole resonance. It is essentially a fully automated PC-controlled pulsed FT NQR spectrometer equipped with a large volume (70 1) radio-frequency (RF) sample coil to accommodate a typical suitcase. The device consists of a measure chamber with an RF coil, tuning and matching box, an RF pulse transmitter and a control PC with dedicated cards like digital receiver, frequency synthesizer, pulse programmer and probe-tuning controller. The control software finds the NQR lines and measures their frequencies. An alarm is produced if any of these frequencies matches the characteristic NQR frequency of the explosive and the signal-tonoise ratio exceeds the preset threshold. Multipulse sequences of the type SORC (strong off-resonance comb) or SLSE (spin-locked spin echo) were used in order to increase the allowed data acquisition rate. We could detect 230 g of PMW-8, a plastic explosive (containing 81% of RDX) in 10 s or 100 g in 30 s. Detection probability was not less than 90%.     

Three-Frequency Composite Multipulse Nuclear Quadrupole Resonance Technique for Explosive Detection

New method of multifrequency nuclear quadrupole resonance (NQR) for the explosive detection has been proposed. This technique consists of application of the series of composite excitation circles, each consisting of two or three successive pulses of different frequencies. In this work, we study in detail the multipulse sequence consisting of n excitation sets, each set consists of three pulses. The first pulse is applied with frequency ?? _?, the second pulse with frequency ?? ₀, and the third pulse with frequency ?? _?C, but with a shifted phase. The NQR signal is detected at the frequency ?? ₊. The maximal amplitude of the detected signal is obtained by tuning the pulse parameters at frequencies ?? _? and ?? ₀. We have shown that the phase of the NQR signal at the frequency ?? ₊ second part of the composite pulse with the frequency ?? ₀ the signals with different phases to suppress the spurious signals. The method could be used for increasing the NQR signal, avoiding the spurious signals and improving the reliability of NQR detection. Possible applications of the method for the explosive detection are also discussed.     

Br NQR relaxation and successive phase transitions of CH3NH3HgBr3

The measurement of ⁸¹Br NQR in CH₃NH₃HgBr₃ has been carried out in the temperature range between 80 and 300 K using a pulse NQR method. The temperature dependence of ⁸¹Br NQR frequencies in CH₃NH₃HgBr₃ has revealed that it undergoes three characteristic successive phase transitions at T?=?123, 184 and 239 K. The phase transition temperature at T?=?239 K is the second-order type, whereas those at T?=?184 and 123 K are the first-order nature of the phase transitions. Each phase transition seems to be closely related to the motions of methyl ammonium cation as a partial or whole. The enhancement of 1/T ₁ at T?=?239 K indicates the onset of the molecular motion of the cation as a whole with increasing temperatures.     

NQR investigations of high-T c superconducting oxides

Nuclear quadrupole resonance (NQR) of⁶³Cu has been performed to study the electronic and magnetic properties of YBa₂Cu₃O_6.1 ceramics, treated in iodine or bromine vapours. An analysis of the NQR shows undistorted CuO₂ planes and the absence of an increase of the oxygen content in chains of Cu(1)-O in halogen treated Y-Ba-Cu-O ceramics.¹³⁹La NQR spin-lattice relaxation measurements in powdered La_2?x CuO_4?y compounds are also reported. The present results indicate that the¹³⁹La nuclear relaxation is dominated by the Cu⁺⁺-Cu⁺⁺ magnetic correlations.     

Relaxation effects in two-dimensional zero-field nutation NQR spectroscopy

The relaxation and off-resonance effects on the NQR (nuclear quadrupole resonance) nutation frequency and on the damping constant of the nutation spectrum for spinsI = 3/2 are considered. Using the solution of dynamic equations for nuclear magnetisation in the rotating frame, we obtained a general expression for determination of the asymmetry parameter η, which includes the relaxation time and frequency offset. Specific requirements for the RF (radio frequency) field allowing the presence of nutation and the sufficient resolution of nutation spectra are determined. It is shown that at low RF field it is necessary to take into account theT ₂ relaxation effect on the location of frequency singularities in the nutation spectrum. The values ofH ₁ andT ₂ must be exactly known in nutation NQR spectroscopy. The 2D NQR spin-echo separation technique was used for determination of transverse relaxation of individual spin packets in case of inhomogeneously broadened NQR lines, so that the asymmetry parameter could then be determined by the nutation experiment with consideration of relaxation effects and frequency offset. The theoretical results are confirmed experimentally for the narrow and inhomogeneously broadened 2D-nutation NQR spectra in several molecular compounds containing the³⁵Cl and⁷⁵As nuclei.     

Differential thermal analysis and nuclear quadrupole resonance study on polymorphic 1,4-dichlorobutane

Differential thermal analysis (DTA), and the ³⁵Cl nuclear quadrupole resonance (NQR) frequency (v _Q ) and line width (Δv _Q ) were measured as a function of the temperature in solid 1,4-Dichlorobutane (DCB). Two crystalline modifications forms I and II, stables above and below T _c = 210 K respectively were found according to the thermal history of the sample. A comparative analysis of the NQR frequency in both phases suggests that the crystal structure in form II is closer-packed than in form I. The NQR line width shows the existence of a thermal activated process with an activation energy E _a ? 20 kJ mol^?1, probably related with reorientations of the CH₂Cl end groups. Some orientational disorder is expected in the low temperature modification since the line width is twice as much as that in the high temperature phase.     

Application of the spherical tensor method for Two-Frequency pure NQR of spinI=1 nuclei

The results of a theoretical investigation of two-frequency excitation in pure nuclear quadrupole resonance (NQR) for a spinI=1 nucleus with a nonaxial elecric field gradient are presented. The multipole tensor operator technique is used for the treatment of the one- and two-frequency pulse excitations. The results are applied to the characterization of the two-frequency signal of nitrogen¹⁴N nuclei. The experiments on sodium nitrite, NaNO₂, confirm the presence of additional (two-frequency) echo in the NQR signal. The effect of resonance offsets on two-frequency NQR is also considered.     

35Cl NQR and XRD Studies on Cs2[Ag(I) x Au(I)1 − x Cl2][Au(III)Cl4]

³⁵Cl NQR and XRD have been investigated on the solid solution Cs₂[Ag(I) _x Au(I)_1???x Cl₂][Au(III)Cl₄] which yielded NQR lines by annealing. The crystal forms a tetragonal cell in the whole range of x. However, a change of the modification occurs at x?≈?0.85. The increase of c and decrease of a were observed in the range of 0.0?≤?x?≤?0.7.     

Pressure dependence of phase transitions in K2SnCl6 from NQR frequency measurements

The influence of hydrostatic pressure 0 ? p ? 4 kbar on the ³⁵Cl NQR in K₂SnCl₆ was studied in the temperature range 238 K ? T ? 300 K. The phase transition temperatures T_C1 and T_C2 were determined from changes in the NQR line pattern.The phase boundaries in the p-T diagram are straight lines in the region studied. The pressure coefficients are given by dT_C1/dP = 1.35 (10) K kbar^?1 and dT_C2/dP=?1.25 (20) K kbar^?1.     

Application of the Wavelet Transform for Detecting Signals of Nuclear Quadrupole Resonance

In the present study, the wavelet transform is used to increase the signal-to-noise ratio of nuclear quadrupole resonance (NQR) signals in the direct pulse method. The efficiency of the wavelet and Fourier transforms used for detecting the NQR signal component is compared. The signal consists of noise, magnetoacoustic and piezoelectric signals, noise from external sources, and NQR signal. Signals from ¹⁴ N nuclei in hexahydro-1,3,5-trinitro-s-triazine C₃H₆N₆O₆ (RDX) are investigated at a temperature of 298 K.__________Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 47–50, January, 2005.     

Metastable chaotic state in K2ZnCl4

The ³⁵Cl NQR frequencies of K₂ZnCl₄ have been measured close to the incommensurate-commensurate transition. A calculation of the temperature dependence of the soliton density n_s from the NQR data showed that in K₂ZnCl₄ close to T_c exists a metastable chaotic state.     

Solid-state NMR/NQR and first-principles study of two niobium halide cluster compounds

Two hexanuclear niobium halide cluster compounds with a [Nb₆X₁₂]²⁺ (X=Cl, Br) diamagnetic cluster core, have been studied by a combination of experimental solid-state NMR/NQR techniques and PAW/GIPAW calculations. For niobium sites the NMR parameters were determined by using variable B_o field static broadband NMR measurements and additional NQR measurements. It was found that they possess large positive chemical shifts, contrary to majority of niobium compounds studied so far by solid-state NMR, but in accordance with chemical shifts of ⁹⁵Mo nuclei in structurally related compounds containing [Mo₆Br₈]⁴⁺ cluster cores. Experimentally determined δ_iso(⁹³Nb) values are in the range from 2400 to 3000 ppm. A detailed analysis of geometrical relations between computed electric field gradient (EFG) and chemical shift (CS) tensors with respect to structural features of cluster units was carried out. These tensors on niobium sites are almost axially symmetric with parallel orientation of the largest EFG and the smallest CS principal axes (V_zz and δ₃₃) coinciding with the molecular four-fold axis of the [Nb₆X₁₂]²⁺ unit. Bridging halogen sites are characterized by large asymmetry of EFG and CS tensors, the largest EFG principal axis (V_zz) is perpendicular to the X-Nb bonds, while intermediate EFG principal axis (V_yy) and the largest CS principal axis (δ₁₁) are oriented in the radial direction with respect to the center of the cluster unit. For more symmetrical bromide compound the PAW predictions for EFG parameters are in better correspondence with the NMR/NQR measurements than in the less symmetrical chlorine compound. Theoretically predicted NMR parameters of bridging halogen sites were checked by ^79/81Br NQR and ³⁵Cl solid-state NMR measurements.     

Effect of <Superscript>14</Superscript>N Nucleus Capture in Nuclear Quadrupole Resonance at Small Relaxation Parameters

Experiments on detecting induction signals of nuclear quadrupole resonance (NQR) on ¹⁴N nuclei in hexahydro-1,3,5-trinitro-s-triazine C₃H₆N₆O₆ (RDX) with preliminary changed population densities of NQR energy levels upon exposure to a saturating pulse are described. It is demonstrated that this actually causes the induction signal amplitude to change; however, complete saturation of the transition cannot be reached because of the NQR relaxation time. It is also demonstrated that the results of calculations without regard for the relaxation processes in RDX lack experimental support. Experimental data demonstrate the feasibility of application of the methods of preliminary change of the population densities (by pumping) for multifrequency experiments in NQR and explosive detection.__________Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 49–52, April, 2005.     

Non-cyclic geometric phase of nuclear quadrupole resonance signals of powdered samples

The non-cyclic geometric phase of ¹⁴N and ³⁵Cl NQR signals induced by the character of trajectory of nuclear magnetization motion upon pulse r.f. excitation of powdered samples is studied. Analytical expressions for the geometric phases of NQR signals of the nuclei of spins I=1 and 3/2 upon nuclear magnetization rotation induced by means of r.f. pulses with frequency detuned from the resonance and for any impulse duration for a separate crystallite are obtained. It is shown that the geometric phase recorded for the signal from a powdered sample at Δω=0 can be different from zero and can oscillate upon changes in duration of the r.f. excitation pulse. An alternative variant of the nutation experiment aimed at obtaining the asymmetry parameter η from locations of frequency singularities in the nutation phase spectrum for nuclei of spin I=3/2 in powder substances is proposed.     

Local Magnetic Fields in Some Bismuth-Based Diamagnets

Evidence that local ordered magnetic fields from 30 to 250 G exist in bismuth-based diamagnetic compounds comprising neither d- nor f-elements was given by ²⁰⁹Bi NQR spectroscopy and supported by SQUID measurements of α-Bi₂O₃. The NQR experiments involved a study of the zero-field line shapes, analysis of the Zeeman-perturbed patterns, and examination of the zero-field spin-echo envelopes in single crystals and powders. The results of the experiments followed by computer modeling of the observed spectra were interpreted assuming that ordered magnetic fields are located at the bismuth sites in α-Bi₂O₃, Bi₃O₄Br, Bi₂Al₄O₉, Bi₄Ge₃O₁₂, Bi₂Ge₃O₉ and perhaps in Bi₃B₅O₁₂. A survey of the related ²⁰⁹Bi NQR data is here presented.     

Magnetic structure of the Sr<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>4</Subscript>Cl<Subscript>2</Subscript> two-subsystem antiferromagnet according to nuclear quadrupole resonance data

The magnetic structure of the Sr₂Cu₃O₄Cl₂ two-subsystem antiferromagnet is studied by the nuclear quadrupole resonance (NQR) method on the ^{63, 65}Cu and ³⁵Cl nuclei. The resonance spectrum above T _N2 = 40 K is determined by the Zeeman splitting of the levels of the ^{63, 65}Cu nuclei of the copper atoms at the Cu1 site with the first-order quadrupole perturbation. The magnetic field on the copper nuclei is equal to 93 kOe. The spectrum below n is significantly different: it includes a low-frequency part, which is associated with the ordering of the second magnetic subsystem Cu2. The splitting of the NQR lines of ³⁵Cl is observed above and below T _N2. This fact indicates the ferromagnetic ordering of the moments of the Cu1 subsystem, which are located along the c axis of the crystal, and makes it possible to determine the direction of the magnetic field on Cu1 copper as (110).     

Br NQR relaxation of the mixed-valence compound (NH4)4SbIIISbVBr12

⁸¹Br NQR measurements of the mixed-valence compound (NH₄)₄Sb^III Sb^VBr₁₂, which has Sb^VBr₆ ^? and Sb^IIIBr₆ ^3? octahedra in different oxidation states in the tetragonal pseudo-K₂PtCl₆ structure, were carried out by pulse method in the temperature range between 80 and 300 K. The phase transition temperature of T _c = 212 K was redetermined. The T ₁ values are quite different between Sb^VBr₆ ^? and Sb^IIIBr₆ ^3? octahedra. For all resonance lines T ₁ minima were observed at T _c. The T ₁ behavior at T _c was explained by a softening of the rotary lattice mode around a principal axis of each octahedron along the c axis of the crystal. The distinctive feature in the temperature dependence of both ⁸¹Br NQR frequencies and T ₁ values for each anion indicates that the static rotation may occur for Sb^VBr₆ ^? but not for Sb^IIIBr₆ ^3? at T _c in the low temperature phase.