<Superscript>14</Superscript>N Nuclear Quadrupole Resonance Study of Polymorphism in Trinitrotoluene Samples Obtained from Old Ordnances

The field application of a ¹⁴N nuclear quadrupole resonance (NQR) detector needed for the detection of different explosives, including trinitrotoluene (TNT), requires the examination of the distribution of ¹⁴N NQR lines stemming from the monoclinic and/or orthorhombic modifications of TNT, as well as from a mixture of both. In this work, 30 different TNT samples up to 70 years old were measured. The main result of this study is that the measured ¹⁴N NQR spectrum is strongly influenced by the environmental conditions to which the explosive was subject during its history.     

14N nuclear quadrupole resonance in several liquid crystals in their solid state

Nitrogen-14 nuclear quadrupole resonance (NQR) spectra in several liquid crystals in their solid state have been detected at 77 K, using a pulsed spectrometer. The quadrupole coupling constants (e²qQ) and asymmetry parameters (η) for the nitrogens have been calculated.     

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.     

利用核四极矩共振技术探测炸药

介绍了用核四极矩共振的原理进行爆炸危险物品检测的方法及实验测试系统. 该系统运用特殊的激励信号来激发被检测样品中所含的特殊元素¹⁴N（炸药的主要成分）,使其产生核四极矩共振,并产生辐射,从而达到检测的目的. 由于该辐射信号的能量非常小(10 nV量级）,在通常的环境中很难检测该信号, 这就对此弱信号的检测和提取提出了很高的要求. 本测试系统使用了一系列有效的组合激励信号及弱信号采集、处理技术,在通常环境中成功地检测到了RDX（黑索金）信号.     

The Influence of Pressure in Paracetamol Tablet Compaction on 14N Nuclear Quadrupole Resonance Signal

¹⁴N nuclear quadrupole resonance (¹⁴N NQR) of several commercially available paracetamol tablets was measured. The spectra of two polymorphs are presented. The linewidths of the correspondent ¹⁴N NQR lines in all the measured samples containing the room-temperature stable monoclinic polymorph were noticeably different. We proved experimentally that the linewidth differences are the consequence of different compacting pressure in the production of tablets.     

Nuclear quadrupole resonance in C-nitro compounds

The nitrogen-14 nuclear quadrupole resonance spectra of CH₃NO₂ and of C₆H₅NO₂ were measured at 77°K. The results are: CH₃NO₂: e²qQ/h = 1694.6 kHz, η = 0.4240 and C₆H₅NO₂: e²qQ/h = 1424.9 kHz, η = 0.4039.     

The two-frequency nuclear quadrupole resonance for explosives detection

The two-frequency nuclear quadrupole resonance (NQR) of¹⁴N nuclei is described for purposes of explosives detection. Two applications are known: two-frequency NQR for increasing the signal intensity, two-frequency NQR for improved reliability of explosives detection. The two-frequency experiments were carried out in hexahydro-1,3,5-trinitro-s-triazine C₃H₆N₆O₆ and sodium nitrite NaNO₂ as a substitute for octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocycine C₄H₈N₈O₈. The two-frequency sequences for NQR are proposed for increasing the amplitude of NQR signal and improvement of detection condition.     

NQR炸药检测系统中激励脉冲拖尾的抑制

介绍了核四极矩共振（Nuclear Quadrupole Resonance简称NQR）炸药检测系统的基本原理及检测过程中面临的如何消除激励脉冲信号拖尾对NQR检测信号的影响问题. 文中采用了在发射电路中增加消振铃电路来消除激励脉冲拖尾的方法，并给出了电路分析和实验结果. 经实际应用验证，该种方法对消除激励脉冲拖尾具有较好的效果，提高了系统的探测能力.     

New Methods for Detection of 14N NQR Frequencies

Nitrogen atoms are present in a number of solid explosives and illicit substances. The nuclear quadrupole resonance (NQR) spectra and spin?Clattice relaxation of the nitrogen atomic nucleus ¹⁴N can be used to characterize these compounds and to distinguish between possible crystal polymorphs. After the characteristic ¹⁴N NQR frequencies and spin?Clattice relaxation rates in a compound are determined, NQR can be used to detect this compounds and, in case of crystal polymorphs, also to determine the method of preparation. The ¹⁴N NQR frequencies and spin?Clattice relaxation rates are measured either by pulse NQR or by nuclear quadrupole double resonance (NQDR) based on magnetic field cycling. Here, we discuss several ¹H?C¹⁴N NQDR techniques which can be used to measure the ¹⁴N NQR frequencies and spin?Clattice relaxation rates under various experimental conditions. Some characteristic applications of these techniques are presented and discussed in details.     

Factors affecting the NQR line width in nitramine explosives

A number of factors associated with crystal quality contribute to the nuclear quadrupole resonance (NQR) line width. Imperfections such as dislocations, voids, strain and impurities can be electrical sources that distort the electric field gradient at nearby quadrupolar nuclei and broaden the observed NQR line. We measured the¹⁴N NQR line widths in powdered samples of the nitramine explosives hexahydro-1,3,5-trinitro-s-triazine (RDX), octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) and 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane and show correlations with sample purity, particle size distribution and density. Cast plastic-bonded explosives containing either RDX or HMX were also studied and their line widths compared with those of the powdered samples.     

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.     

NQR炸药探测系统中射频线圈的设计研究

基于核四极矩共振(Nuclear Quadrupole Resonance, NQR)炸药探测原理,对探测系统中拾取信号的关键部件射频线圈进行了优化设计,并确定了小型螺线管型线圈的直径、长度和匝数. 试验测试表明,此线圈射频场均匀性好、信噪比大、灵敏度高,可以快速准确探测到NQR信号,验证了线圈设计理论的有效性和制作方法的可行性. 该设计方法对提高隐藏炸药探测的准确率以及不同试验条件下射频线圈的设计具有重要意义.     

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.     

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.     

Nuclear quadrupole resonance of explosives: Simultaneous detection of RDX and PETN in semtex

We have used nuclear quadrupole resonance (NQR) for the detection of¹⁴N in explosives and introduce a new method for the simultaneous detection of the explosives cyclotrimethylene trinitramine (RDX) and pentaerythritol tetranitrate (PETN). We have developed an interleaved pulse sequence, which provides efficient excitation of both RDX and PETN, to drive a solenoid coil that is doubly resonant at 3.41 and 0.89 MHz (suitable resonant frequencies of RDX and PETN, respectively). The excitation of RDX and PETN is carried out alternately within the pulse sequence via frequency hops in the spectrometer; no electronic switching is required to alternately returne the doubly resonant probe to the resonant frequency of each material. The signals from RDX and PETN, which are separated by about 2.52 MHz, are brought into the same audio window by mixing with a suitable intermediate frequency. We have tested the doubly resonant probe and interleaved pulse sequence on plastic formulations of RDX and PETN, on their own (PE-4 and Detasheet respectively) and as a mixture in Semtex.     

Nuclear quadrupole resonance of norephedrine

Toward searching for illegal drugs, we investigated the pulsed nuclear quadrupole resonance (NQR) response of ¹⁴N in (1R,2S)-(-)-norephedrine, based on the predictions of quantum chemical calculations. Two pairs of spectral lines (ν₊=3.089, 3.093 MHz and ν₋=2.594, 2.608 MHz) were observed despite its molecule structure having only a single nitrogen atom. This indicates that the molecular crystal has two nonequivalent nitrogen atoms in the unit cell. The temperature dependence of the NQR frequencies and relaxation properties were investigated for the purpose of accurate remote sensing of the drugs. The NQR frequency shift was approximately 0.23 kHz/K around room temperature. The spin-lattice relaxation and spin-phase memory times were 5.2–10.2 ms and 0.6–1.5 ms, respectively.     

Temperature dependence of17O and14N NQR frequencies in commercial TNT

The temperature dependence of the¹⁷O nuclear quadrupole resonance (NQR) frequencies has been measured in commercial trinitrotoluene by highly sensitive nuclear quadrupole double resonance techniques. The¹⁴N NQR frequencies have been measured in the same sample for comparison. In contrast to the¹⁴N quadrupole coupling constant, the¹⁷O quadrupole constant increases with increasing temperature.     

Nuclear cooling in PrIn3

Adiabatic demagnetization of PrIn₃ was carried out and a lowest temperature of 2.1 mK was obtained. The observed temperature as a function of the demagnetizing field is well reproduced by assuming a value of -230 MHz for the quadrupole interaction constant of In, e²qQ/h. This value also gives a reasonable account of the recent specific heat data.     

Quadrupole interaction at Ta and lattice location of Hf in Be

The time-differential perturbed angular correlation technique (TDPAC) has been applied to the 482 keV excited state of¹⁸¹Ta, to determine the nuclear electric quadrupole interaction (QI) at Ta in Be. Sources were prepared by ion-implantation of¹⁸¹Hf in Be. Particle channeling measurements have revealed that. Hf impurities implanted in Be reside primarily at the tetrahedral interstitial site. The fundamental QI frequency obtained for Ta at this interstitial site in Be is |v _Q| =e ² qQ/h = 227.0 ± 2.2 MHz which corresponds to an electric field gradient |eq|=(3.71±0.15)×10¹⁷ V/cm². This result is discussed in terms of results for Cd impurities in the same system and the systematics of the impurity QI in metals.     

Study of 14N NQR response to SORC pulse sequence

The behavior of nuclear quadrupole resonance (NQR) signals between RF pulses of the strong off-resonance comb (SORC) as well as the spin-locking spin-echo (SLSE) pulse sequences was studied as for ¹⁴N NQR line ν _?+? of dimethylnitramine (CH₃)₂NNO₂ at 77 K. The periodic variation of the signal amplitude observed by using SORC pulse sequence could be reasonably explained by the theoretical expression reported in the literature.