RU2249202C1 - Nuclear quadrupole resonance based method of detecting explosives and drugs - Google Patents

Abstract

FIELD: radiospectroscopy.

SUBSTANCE: according to the method of detecting explosives and drugs the spot being suspected in storing matters kept in non-metal container is influenced by high-frequency magnetic field radio-frequency variable duration pulses. Frequency of carrier is close to frequency of quadrupole nuclear resonance of detected matter. Induced electromotive force is measured during periods of absence of pulses from phase-sensitive detection circuit. Response input signals are summed subsequently which signals are detected at special moments of time. Matters to be found have preset known parameters of nuclear quadrupole resonance. Duration of pulses of illuminating pulses is changed to compensate drop in sensitivity induced by non-uniformity of radio-frequency filed of illuminating coil.

EFFECT: improved capability of detection of matters in tested area.

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Description

The invention relates to the field of radio spectroscopy. The primary area of its use is detectors of explosive and narcotic substances (BB and HB) based on nuclear quadrupole resonance (NQR).

Known similar methods for detecting explosives and HB based on the NQR phenomenon / 1-5 /, where the studied sample is irradiated using radio frequency (RF) pulses filled with a resonant frequency or close to the resonant frequency of the NQR of the desired substance.

A device for detecting explosives and HB with the implementation of the pulse identification method / 1 / contains several measuring chambers, the first of which contains the analyzed object, and the rest contain reference substances that coincide in their chemical composition with the substances that need to be identified, and identification is carried out by comparing the spectra of the test and reference samples. The device contains a synthesizer, modulator, power amplifier, loaded on two transmitting antennas, a pulse generator, a microprocessor controller, synthesizer, indicator, electronic switch, analog-to-digital converter. Based on the comparison of the spectra, a decision is made on the presence or absence of narcotic and explosive substances.

A device / 2 / for detecting the presence of selected nuclei in a larger product of a specific substance containing quadrupole nuclei. It includes researching sections of the product by placing the product in a coil, excitation using a radio frequency source of the winding during the cyclic sequence, connecting the winding in the remaining periods of the sequence with a phase-sensitive circuit for detecting and measuring response output signals associated with nuclear quadrupole resonances. Summation of the response output signals detected at the corresponding time points of the cyclic sequence.

The system for detecting mines and controlling baggage / 3 / allows you to detect small quantities of explosives and HB in the ground, walls and baggage. The equipment contains a radio head, which is a system for irradiating a substance with pulsed signals with a filling frequency equal to the resonance frequency in an explosive or drug. This system contains a series resonant circuit with a surface coil located near the plastic bottom of the cylindrical screen. The signal from the substance is received by ferrite antennas, which are also inside the named screen.

A device for detecting a substance using NQR / 4 / makes it possible to detect the presence in a sample of predetermined substances having known characteristic frequencies of NQR. It contains a sequence controller, a source of high-frequency signals with a variable frequency, many capacitors of constant capacitance and many individually controlled switching means for selectively connecting these capacitors to a circuit of a high-frequency coil. The single-turn distributed high-frequency coil has such a shape that it limits the cavity of a predetermined volume for receiving the test sample and for supplying high-frequency signals to it from the specified high-frequency source, while high-frequency signals are applied to the sample located in the cavity, forming a field inside the cavity. The specified coil is also designed to receive NQR signals from the sample with the formation of the output signal.

Closest to the proposed is a smuggling detection system using NQR / 5 /. Designed to detect a given substance from a class of explosives and HB in a sample. It contains a sequence controller, a radio frequency subsystem containing a variable frequency generator, a radio frequency coil for supplying radio frequency excitation, which is also a receiving coil for receiving NQR signals from the test sample to form an output, a device for tuning the radio frequency coil, a signal receiving and data processing subsystem, and a device display for receiving the final output signal from the subsystem for receiving and processing data. The detector head subsystem that forms the scanner contains a flat distributed radio-frequency coil bounding the two ends of the cavity of a given volume for receiving the entire sample to be studied and for supplying the radio frequency signal to it from the radio frequency generator, which is fed to the sample to be studied and creates an alternating magnetic field inside the cavity . An electrically conductive radiofrequency screen that has such a shape and construction as to provide electromagnetic and radiofrequency shielding from external noise and to prevent the radiofrequency magnetic field from escaping from the cavity of the radiofrequency coil with the shield, while the radiofrequency shield enhances field uniformity. Known characteristics of the NQR signals for given substances are entered into the memory of the processor of the data signal in the detection system, the test sample is inserted into the cavity formed in the radio frequency coil. The RF pulses are transmitted to the cavity formed by the RF coil and create the field inside the RF coil that the sample is exposed to, the NQR signals are processed and compared with the characteristics of the signals stored in the memory to determine whether the detected NQR signals indicate the presence of a given substance.

The disadvantage of such devices / 1-5 / is that they have uneven sensitivity of the detector in different areas of the volume of the working chamber, the presence of "dead" areas that sharply reduce the likelihood of detecting small volumes of explosives and HB. This is a consequence of the fact that when developing devices for detecting explosives and HBs, no attention is paid to the effect on the intensity of the NQR signal of the heterogeneity of the exciting RF field of the sensor. At the same time, such heterogeneity of the alternating magnetic field (axial and transverse) is significant for NQR detectors with a large coil / 1, 2, 4, 5 / and in particular for single-sided mine detectors with a surface coil / 3 /, which does not allow the creation of maximum nuclear magnetization and thereby reduces the efficiency of the device.

The present invention provides a method of compensating for a drop in signal intensity caused by an inhomogeneity of the RF field, taking into account the used mismatch of the carrier frequency from the resonance frequency. This is achieved by the fact that to compensate for the drop in the sensitivity of the sensor in the areas of the inspected volume with high and low values of the amplitude of the RF field during signal accumulation, the duration of the probe pulses is changed by the programming device so that the optimal excitation conditions are fulfilled in the measurement process alternately for all areas of the inspected volume.

The invention is based on the obtained dependences of the intensity of the NQR signal induced by the explosive in the sensor coil on the amplitude of the RF field, the duration of the exciting pulses, and the magnitude of the detuning of the frequency of the detector from resonance. The solution of the unsteady Schrödinger equation in the time interval 0≤ t≤ t _w gives for describing the reaction of the system of nuclear spins ¹⁴ N (I = 1) to an RF pulse of duration t _w with frequency detuning Δ ω relative to the exact resonance ω _{+ the} following expression for nuclear magnetization:

Averaging the magnetization over the angles θ and φ for the powder in the volume of a ball of unit radius for zero detuning Δ ω = 0 leads to the Bessel function J ₁ (z), which is the essence - an elementary transcendental function:

In the presence of a frequency detuning (Δ ω ≠ 0), averaging over the powder using a computer allows one to obtain the dependence of the intensity of the ¹⁴ N NQR signal on the duration of the RF pulse and frequency detuning on resonance.

Let the condition of “90-degree” of the exciting pulse (maximum of expression (2)) γ B ₁₀ t _w0 ≈ 0.66π be satisfied for a zero detuning Δ ω = 0. Here t _w0 is the optimal pulse duration, and B ₁₀ is the amplitude of the RF field at zero detuning. The value proportional to the nutation rate of the nuclear magnetization is denoted by ω _r0 = γ B ₁₀ . If you do not take into account the resonance properties of the transceiver oscillatory circuit, then the dependence of the signal intensity on the relative duration of the RF pulses t _w / t _w0 and the relative detuning of their carrier frequency Δ ω / ω _r0 has the form shown in Fig. 1.

It is easy to see that a decrease in duration compared to t _{w0 is} accompanied by a decrease in the signal amplitude, which cannot be compensated by an increase in the frequency detuning. An increase in the pulse duration compared to t _w0 also causes a decrease in the signal, but up to t _w ≈ 2t _w0 this decrease can be fully compensated by increasing the frequency detuning to Δω ≈ 0.8ω _r0 .

Let us now consider the possibilities of compensating for the signal intensity with a decrease in the amplitude of the RF field by increasing the pulse duration in the presence of a frequency detuning. Figure 2 illustrates the relationship between the optimal relative pulse widths and the relative values of the field B ₁ , leading to compensation of the magnitude of the signal. Line 1 corresponds to zero detuning, where the duration and amplitude of the RF pulse are inversely proportional to each other.

Let the pulse duration have the value t _w0 = 10 μs, which is typical for ¹⁴ NQR. Then, ω _r0 ≈ 2π · 33 kHz and, with the Q factor of the sensor loop Q = 100, the detuning value cannot be greater than Δ ω _max / ω _r0 = 0.7-0.8, or Δ ω _max ≈ 2π · 25 kHz. If Δ ω / ω _r0 = 0.1, i.e. Δ ν = 3.3 kHz (a typical value of the detuning used in measurements in ¹⁴ N NQR), then the dependence for the optimal RF pulse duration is shown by curve 2. The larger the detuning, the faster the optimal duration increases when the level of the RF field B ₁ decreases.

However, as can be seen from the graphs, with large detunings, the decrease in the radio frequency field B ₁ cannot be compensated for by an increase in the pulse duration t _w (in the graphs the regions where only partial compensation occurs are shown by a dotted line).

Thus, to obtain the maximum signal intensity from a sample located in an inhomogeneous irradiating RF field, pulses of different durations that are optimal for different field levels B ₁ should be used. The conditions for compensating for signal intensity given here will increase the probability of detecting small volumes of matter and equalize the sensitivity of the detector for different areas of the sensor’s working space.

Examples of specific embodiments of the invention

The method is as follows. Let there be an arbitrary detector based on the pulsed NQR method with a large coil. The dimensions of the coil in the form of a parallelepiped, for example, 12 × 24 × 33 cm, which corresponds to a standard inspection chamber for postal items / 6 /. The coil contains 8 turns, with a winding pitch of 4.7 cm. Let the explosive be a trotyl block weighing 50 g with dimensions of 1 × 6 × 5 cm in the shape of a parallelepiped (standard “IKAO suitcase”,

developed by the experts of the group of experts of the International Aviation Security Commission / 7 /). Fig. 3a illustrates various possible positions of explosives in the working volume of the inspection chamber. Figure 3b shows the dependence of the emf induced in the coil on the explosive at different positions of the checker in the chamber, depending on the duration of the probe RF pulse. Irradiation chamber space field "90 degree" RF pulse with a duration _{(t w) opt = 0.66π /} γ _10/8 / gives maximum signal for the location of embodiment VV 3, 4 and 5. Here, γ - gyromagnetic ratio nuclei ¹⁴ N , ₁₀ - the amplitude of the RF field in the center of the coil. When choosing the threshold device trigger level of 0.5, the explosive checker located in position 5 will not be detected under these conditions. When the duration of the irradiating pulse is reduced to a value of t _w = 0.7 (t _w ) _opt , the EMF induced in the coil increases to a level sufficient for reliable operation of the threshold detector device.

The proposed method involves irradiating the camera with RF pulses of not constant duration, but to change them in this particular case in the range from 0.6 (t _w ) _opt to 1.2 (t _w ) _opt using a programming device in the process of signal accumulation in order to eliminate the negative effect of heterogeneity RF fields for detection probability.

All of the above applies to any pulse sequences used for detection (single pulses, spin-locking sequences, SORC, etc.). The need for the proposed method becomes inevitable with an increase in the size of the inspection chamber (field heterogeneity increases) and with a decrease in the size of explosives). Flat field coils for one-sided detection (mine detectors) provide even greater heterogeneity of the RF field. The limits of variation of the duration t _w here can be significant and are determined by the size of the active region adjacent to the surface coil.

Similar results can be obtained for HB.

Thus, the invention allows to achieve equalization of sensitivity for different areas of the test volume without the use of long-term accumulation, to exclude “dead” zones from the test volume. The use of the invention will reduce the detection time and increase the likelihood of detecting explosive and narcotic substances.

Sources of information

1. Patent RU 99102859 A, 7 G 01 N 24/00, 12.20.2000.

2. Patent GB 92/00577 A, 6 G 01 R 33/44, 04/01/1992.

3. Patent RU 2165104 C2, 7 G 08 B 13/00, 04/10/2001.

4. Patent SU 96/03075 A, 6 G 01 R 33/36, 03/05/1996.

5. Patent SU 96/03286 A, 7 G 01 R 33/44, 03/05/1996.

6. The device “OVV - YAKR-10”. www.mtu-net./logis.

7. Research report "Research on the properties of marking additives and their application" FSUE GosNII "Crystal", 2002, Dzherzhinsk, 26 pp.

8. Grechishkin B.C., Sinyavsky N.Ya. Local NQR in solids. Advances in Physical Sciences, 1993, vol. 163, No. 10, pp. 95-120.

Claims (1)

A method for detecting explosive and narcotic substances based on nuclear quadrupole resonance, which consists in the fact that the alleged bookmark of a substance located in a nonmetallic shell is exposed to radio frequency pulses of a high-frequency magnetic field with a carrier frequency close to the frequency of nuclear quadrupole resonance of the detected substance, characterized in that that with simultaneous detuning of the carrier frequency from the resonance frequency, the duration of the high-frequency pulses is changed in such a way that In order to reach the maximum value of the signal of the nuclear magnetization of a substance at its location in an inhomogeneous high-frequency magnetic field.

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