The detection of trinitrotoluene by pure nuclear ... - Springer Link

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G. Zhang 2, and H. Guo 2 ... capacitor of 5-400 pF, 7.5 kV, 20 A, anda radio-frequency coil. We used a solenoid coil with a diameter of 10 cm anda length of 6 cm.
Appl. Magn. Reson. 29, 293-298 (2005)

Applled Magnetic Resonance 9 Springer-Verlag 2005 Printed in Austria

The Detection of Trinitrotoluene by Pure Nuclear Quadrupole Resonance G. V. Mozzhukhin ~, A. V. B o d n y a ~, V. V. Fedotov 1, C. Chen 2, G. Z h a n g 2, and H. G u o 2 ~Kaliningrad State University, Kaliningrad, Russian Federation 2China Research Institute of Radiowave Propagation, Xinxiang, Henan, China Received August 27, 2004; revised November 30, 2004

Abstraet. The data of the observation of the nuclear quadrupole resonance signal in trinitrotoluene

C6H2(NO2)3CH3 are reported. The measurements are carried out on a crude industrial sample of the Chinese manufacturing at 293 K. It is shown that the lines of the monoclinic form and probably orthorhombic forms are present in our spectra, but the most suitable lines for the detection of trinitrotoluene belong to the monoclinic form. The results of our experiments specify an opportunity of application of the pulse method of nuclear quadrupole resonance for the detection of trinitrotoluene in crude industrial samples.

As it is k n o w n [1], one o f the basic explosives used in industry and for military purposes is trinitrotoluene (TNT) C6H2(NO2)3CH 3. T N T has a wide range o f applications in industry, especially in the mining industry. Hence, the industrial T N T is more accessible for probable terrorists in comparison with other explosives. However, the quality and parameters o f the military and industrial TNT m a y differ [2] depending on the manufacturer. The detection o f T N T by different methods is an important p r o b l e m for humanitarian mine clearing and struggle against terrorism. One o f the detection methods m a y be nuclear quadrupole resonance (NQR) [3]. One can use two methods for the N Q R detection o f TNT. The first meth0d is the pure pulse N Q R method [1, 4]. The other is the double nuclear magnetic-nuclear quadrupole resonance (double N M R - N Q R ) [3]. F o r the application o f double N M R - N Q R the presence o f the static magnetic field for proton polarization is necessary. During this experiment in the field o f 0.8 T the longitudinal relaxation time o f protons is T~ ~ 30 s [5]. The duration o f the registration o f signals for a 0.5 g sample is large despite the high sensitivity o f the method [5]. However, the main defect is the necessity o f the creation o f a strong homogeneous magnetic fiel& These features set limits on the application o f the double N M R - N Q R for the purposes mentioned above.

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G.V. Mozzhukhin et al.

The problems of pure NQR for TNT detection are the large number o f NQR frequencies in the frequency band up to 0.9 MHz, and the large relaxation parameters (A.N. Garroway and A.D. Hibbs, U.S. Naval Research Laboratory, Washington D.C., USA, pers. commun.) [1] (2.2-9.8 s). Moreover, at pulse NQR there are problems connected with a relatively long "dead time" of the receiver after the action of the radio-frequency pulses and the presence of spurious signals [6]. In some cases these difficulties of the TNT detection have resulted in a failure to detect the signals at room temperature in industrial samples a n d a s consequence in the unlikelihood to apply the direct pulse NQR in real conditions [2]. In the given work we present the results of the detection of the crude industrial TNT of the Chinese manufacturing at 293 K by the pure pulse NQR. The samples used for the experiment are squamuliform TNT, used for conducting explosive works in open-mining development. The TNT sample is a yellow sand-colored flake. The temperature of hardening is not less than 80.2 ~ The mass fraction of moisture and flying substances does not exceed 0.07%. Acidity in recalculation on a sulphuric acid does not exceed 0.01%. The mass fraction o f substances that are insoluble in organic solvent does not exceed 0.05%. The mass fraction of sodium sulphite is absent. The weight o f the used samples ranges from 150 to 500 g. The major part of measurements was carried out on the sample weighing 500 g. Later we used the sample of 150 g to check the efficiency of our TNT detection. In the series resonance circuit of the probe head we used a variable Jennings capacitor o f 5-400 pF, 7.5 kV, 20 A, a n d a radio-frequency coil. We used a solenoid coil with a diameter of 10 cm a n d a length of 6 cm. The probe head of the spectrometer also includes a low-noise preamplifier a n d a system to improve the recovery of the preamplifier. The preamplifier has the following parameters: the gain is not less than 30 db, the noise electro-motive force is not more than 2.7 nV. The recovery period is approximately 500 gs. The variable impedance of the spectrometer is connected with the series resonance circuit. The spectrometer causes the changes of the total Q factor of the circuit during the action of radio-frequency pulses and the short time period after pulses. Q factor ranges from 100 to 150. We used the following sequences of radio-frequency pulses: the phase alternated pulse sequence (PAPS) [7], the multiple pulse spin locking echo [4] and combinations of these pulses for decreasing the recovery time. The durations of the radio-frequency pulses are between 50 and 200 gs. We used a power amplifier with a wide band of 0.6-6.0 MHz and output power of 1.25 kW. The results of our measurements are shown in Table 1. Ir is necessary to note that lines referring to the monoclinic form are more convenient for the TNT detection by NQR if compared with other lines which we denote as the orthorhombic forro. The errors of these measurements are not more than 1 kHz. Hence, we suggest that our data coincide with the results of the measurements in ref. 1 and by A. N. Garroway and A. D. Hibbs (U.S. Naval Research Laboratory, Washington D.C, USA, pers. commun.) The estimations of the relaxation parameters coincide with the data presented in these works too. We observed the set of lines

The Detection of TNT by Pure NQR

295

Tahle 1, NQR frequencies (kHz) in TNT. Measurements by A. Garroway and A. Hibbs (Washington, D.C., pers. commun.) and in ref. 1 for:

Our measurements at 293 R

Monoclinic form, 293 K 836.4 841.7 843.4 845.2 848.2 859.7 867.7 871.6

Orthorhombic form, 279.6 K

837 842 844 848 859 870

839 844.6 848 850.5 864.7 870

w h e n the Q factor was near 100. T h e result o f this o b s e r v a t i o n is p r e s e n t e d in Fig. 1. T h e use o f the PAPS s e q u e n c e has p r o b l e m s o f the f r e q u e n c y offset dep e n d e n c e . H o w e v e r , the m u l t i p u l s e s e q u e n c e has a shorter d e t e c t i o n t i m e i f c o m p a r e d w i t h the o t h e r sequences, s u c h as the spin e c h o s e q u e n c e . T h e v i s i b l e line in the s p e c t r u m at the f r e q u e n c y c l o s e to 846 k H z p r o b a b l y indicates the prese n c e o f the o r t h o r h o m b i c forro o f T N T in the sample. T h e results o f Table 1 w e r e o b t a i n e d f r o m the m e a s u r e m e n t s w i t h Q o f 150. F i g u r e 1 is o n l y the sketch o f the c o m m o n spectrum, b e c a u s e the high Q o f 50 does not p e r m i t us to see a f e w lines together. We o b t a i n e d the s p e c t r u m in Fig. 1 with Q less than 100. We s u p p o s e that the use Of the P A P S s e q u e n c e does not p r o d u c e the e x a c t line position. M o r e o v e r , it is k n o w n f r o m analyses that the s u p e r p o s i t i o n o f the free

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