爆炸物检测中NQR信号的处理及判识

介绍了基于核四极矩技术的爆炸物检测原理及信号处理方法.系统分析了NQR信号检测中主要误差来源,提出多种方法消除NQR信号处理中检测误差,主要包括相关累加消除随机噪声;单通道自适应消除振铃影响;相关性检测抑制同频干扰.实验表明相关检测可有效提高NQR信号信噪比,提高微弱NQR信号检测的准确性.     

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.     

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.     

Circularly Polarized RF Magnetic Fields for Spin-1 NQR

The low sensitivity of nuclear quadrupole resonance (NQR) of powders is due, in part, to the inability to efficiently excite and detect nuclei at all crystal orientations. Here we describe the use of circularly polarized RF magnetic fields for excitation followed by detection of the resultant circular RF magnetization in I=1 NQR to increase the fraction of nuclei excited and detected. We show that the technique can greatly improve the effective RF field homogeneity and increase the largest signal amplitude by a factor of 1.72. In favorable cases, the resulting circularly polarized NQR signal can be separated from linearly polarized magnetoacoustic and piezoelectric ringing artifacts that occur in some NQR materials detection applications.     

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.     

Modified steady-state free precession pulse sequences for the detection of pure nuclear quadrupole resonance

Modifications of the steady-state free precession multi-pulse technique for the effective detection of the nuclear quadrupole resonance (NQR) signals are proposed. These modifications are based on the use of composite pulses and enable the suppression of the coherent noise signals such as the magneto-acoustic and piezo-electric signals or the ringing signal from the NQR probe. Experimental results of applying the proposed technique to nitrogen-14 NQR in the sample of C6H12N4 are also presented and convincingly demonstrate its effectiveness.     

Some Aspects of the Effective Detection of Ammonium Nitrate-Based Explosives by Pulsed NQR Method

The nuclear quadrupole resonance (NQR) method has been applied to ammonium nitrate (AN)-based explosives, ANFO and REOGEL+, to find an optimized multi-pulse technique for effective detection. The relaxation parameters and signal intensities for nitrogen-14 lines of these substances were measured. The experimental results of applying the proposed multi-pulse technique in these samples are presented and convincingly demonstrate a path toward efficient detection. A detection using sequences of this character could be achieved over real-world scan volumes for screening of goods. It was proven that the cross-polarization technique can significantly enhance the NQR signal for AN-based explosives.     

Optimal filtering in multipulse sequences for nuclear quadrupole resonance detection

The application of the multipulse sequences in nuclear quadrupole resonance (NQR) detection of explosive and narcotic substances has been studied. Various approaches to increase the signal to noise ratio (SNR) of signal detection are considered. We discussed two modifications of the phase-alternated multiple-pulse sequence (PAMS): the 180° pulse sequence with a preparatory pulse and the 90° pulse sequence. The advantages of optimal filtering to detect NQR in the case of the coherent steady-state precession have been analyzed. It has been shown that this technique is effective in filtering high-frequency and low-frequency noise and increasing the reliability of NQR detection. Our analysis also shows the PAMS with 180° pulses is more effective than PSL sequence from point of view of the application of optimal filtering procedure to the steady-state NQR signal.     

Correlation-based method for improvement of NQR signals utilizing signal shape information

In the correlation method the signal shape information can be utilized for detection of weak signals, hidden in the noise. We tested the advantages, drawbacks and some practical features of the method applied to magnetic resonance detection. The influence of the signal width, shape and similarity to the template on the correlation function was studied by numerical simulation. The findings were applied to the optimization of the template for improved localization of a noisy¹⁴N nuclear quadrupole resonance (NQR) line in NaNO₂ during repetitive scans by a superregenerative spectrometer. The value of the correlation method in faster NQR intensity evaluation was also confirmed.     

相位可控的核四极矩共振激励脉冲发生器设计

在核四极矩共振（NQR）领域，射频激励脉冲信号的优劣对NQR响应信号有重要影响.针对常规方法中射频激励脉冲参数不可控的问题，本文基于32位闪存微型控制器STM32和直接数字频率合成（DDS）芯片AD9910设计了一种相位可控激励脉冲发生器.采用STM32控制AD9910产生波形参数（脉冲宽度、脉冲间隔、脉冲个数和共振频率等）可控的射频激励脉冲，利用LabVIEW软件平台设计脉冲参数设置界面，并建立计算机与微控制器通信，实现波形参数的精确优化控制.实验结果表明，该方法实现了相位可控的NQR激励脉冲序列，可为后续NQR信号检测提供有效激励源.     

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.     

Modified multipulse technique for the effective detection of pure nuclear quadrupole resonance

Modification of the steady-state free precession (SSFP) multipulse technique for the effective detection of nuclear quadrupole resonance (NQR) signals was studied. This modification is based on the use of the preparatory pulse in the SSFP pulse sequences. It was shown that under certain conditions the total intensity of accumulated NQR signal can be increased. Experimental results of applying the proposed technique to nitrogen-14 NQR in the sample of sodium nitrite (NaNO₂) are presented and convincingly demonstrate the technique’s effectiveness.     

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.     

Capacitor-based detection of nuclear magnetization: nuclear quadrupole resonance of surfaces

We demonstrate excitation and detection of nuclear magnetization in a nuclear quadrupole resonance (NQR) experiment with a parallel plate capacitor, where the sample is located between the two capacitor plates and not in a coil as usually. While the sensitivity of this capacitor-based detection is found lower compared to an optimal coil-based detection of the same amount of sample, it becomes comparable in the case of very thin samples and even advantageous in the proximity of conducting bodies. This capacitor-based setup may find its application in acquisition of NQR signals from the surface layers on conducting bodies or in a portable tightly integrated nuclear magnetic resonance sensor.     

Thermoactivated molecular motions in solids and determination of the energy of their activation by NQR spectroscopy methods

The activation energy of thermoactivated molecular motions in solids is determined by examining the influence of these motions on the temperature dependence of the nuclear quadrupole spin-lattice relaxation rate and quadrupole resonance (NQR) signal intensity for chlorine-35. In the latter case, the heating of the crystals is accompanied by the fading of resonance signals, which is analyzed using a linear correlation between the activation energy of this motion and the fade-out temperature. The correlation parameters are demonstrated to be dependant on the type of molecular motion. The relaxation method is shown to be more effective in studying molecular motion and evaluating its activation energy as compared to the NQR signal fading technique.     

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.     

Special features of remote observations of weak signals from 14N nuclear quadrupole resonance

The case of excitation of nuclear quadrupole resonance (NQR) in nitrogen-containing compounds is examined for significant sample removal under conditions of strong dispersion of the registered signal. Principal limitations on the sensitivity of remote NQR observations are demonstrated. A method of registering resonant lines after fading of the main signal is suggested. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 8–11, August, 2008.     

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.     

Optimised NQR pulse technique for the effective detection of Heroin Base

The nuclear quadrupole resonance (NQR) method has been applied to Heroin Base (HB) to find an optimised multi-pulse technique for effective detection of HB. Experimental results of applying the proposed spin-locking multi-pulse (SLMP) technique to nitrogen-14 NQR in this sample are presented and convincingly demonstrate as a path towards efficient detection. A detection using a sequence of this character could be achieved over real-world scan volumes for screening of goods. All experiments were carried out at room temperature.     

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.