Application of DNAR for detection of explosives.

Application of DYAKR for detection of explosives..

GRECHISHKIN Vadim Sergeevich, Doctor of Physical and Mathematical Sciences, Professor,
SHPILEVOI Andrey Alekseevich, Candidate of Physical and Mathematical Sciences, Associate Professor,
PERSICHKIN Andrey Andreevich.

USE OF DYAKR FOR DETECTING EXPLOSIVES  

One of the most reliable methods for detecting narcotics and explosives is the nuclear quadrupole resonance (NQR) method. From a technological point of view, in addition to reliability, NQR-based explosive and drug detectors are convenient in that the spectra of these substances are localized in the low-frequency region (below 6 MHz) and detection is carried out by the resonance of nitrogen nuclei 14N, which is included in almost all of the specified substances.

However, there are difficulties in detecting NQR signals of light quadrupole nuclei (including 14N). This is due to the fact that light nuclei have a small quadrupole moment, and the NQR frequency and spectral line intensity depend on its magnitude. In addition, the amplitude of the signal induced in the receiving coil is proportional to the square of the frequency, and at frequencies less than 1 MHz, difficulties in detection arise.

The biggest problem in this regard is the detection of TNT-based explosives by NQR methods, with NQR frequencies of 840 and 740 kHz. The line intensity at these frequencies is low, and TNT is also subject to biotransformation, which further broadens the spectral lines.

To increase the sensitivity of detection of such light nuclei, indirect methods such as adiabatic demagnetization and various methods of double nuclear quadrupole resonance (DNQR) are successfully used.

 Adiabatic demagnetization

The essence of adiabatic demagnetization [2] is “cooling the quadrupole system due to thermal contact with a pre-cooled magnetic system.

The sample is initially placed in an external magnetic field H0, where the NMR system spins are polarized in the direction of the magnetic field. After this, the sample is adiabatically demagnetized in ~0.1 s. Adiabatic demagnetization can be achieved by mechanically ejecting the sample from the gap of a permanent magnet.

During adiabatic demagnetization, the temperature of the spin system decreases to a value

, (1)

where Hl is the local field in the region of the magnetic system spins.

When the sample enters a zero magnetic field at a certain value of the magnetic field strength the NMR and NQR levels intersect, and thermal contact occurs between the spin systems of nuclei and protons, resulting in cooling of the quadrupole system to the temperature of the magnetic system, since the heat capacity of the magnetic system is usually much greater than the quadrupole. The frequencies of the quadrupole and magnetic systems are equalized:

, (2)

where g is the gyromagnetic ratio of the spins of the magnetic system, wQ is the NQR frequency. In this case, for small concentrations of quadrupole nuclei, the signal-to-noise ratio increases by a factor of vp/vQ.

 Double nuclear quadrupole resonance

The phenomenon of DNQR [6] is observed in samples with two spin systems coupled to each other via dipole-dipole interactions. A weak NQR signal of one spin system is registered indirectly by a change in the strong signal (double NQR-NQR) or NMR signal (double NQR-NMR) of the other spin system.

Double NQR-NQR, or pure quadrupole double resonance, is subdivided into spin-echo double resonance, double resonance in a rotating coordinate system, stationary double resonance, and non-resonant double resonance.

In double NMR-NQR, regardless of the method used, the magnetic P-system is first cooled by adiabatic demagnetization, then the quadrupole Q-system is subjected to the maximum possible perturbation and is brought into contact with the P-system. After this, the residual magnetization of the P-system is measured.

When using the above methods, the detection sensitivity increases by one or two orders of magnitude. However, there are a number of serious difficulties in their practical application for remote detection of substances, the main one being the remote generation of a relatively large magnetic field (about 0.2 T) and the need to switch it off in a very short period of time. In stationary DNQR spectrometers, this is achieved by magnetizing the sample inside a solenoid powered by a current source of more than 100 A, and rapid switching of the field is performed by mechanically or pneumatically ejecting the sample from the solenoid gap. It is clear that this method is unacceptable for remote detection.

As a solution to this problem, the group headed by Professor

V.S. Grechishkin proposed to use a semi-toroidal solenoid [1], and to obtain a magnetic field of greater magnitude, to use soft magnetic materials in the design of the solenoid core.

Figure 1 shows a block diagram of the DYAKR remote spectrometer.

Fig. 1 Block diagram of the DNaKR remote spectrometer

The one-toroidal solenoid 1 faces the ground with its ends and is powered by a current source 5. The solenoid creates a closing magnetic field parallel to the surface of the earth at points located on the axis of symmetry of the magnet. The field causes polarization of the magnetic moments of the protons in the sample 4.

For the DNQR experiment, the quadrupole system, which is a set of nitrogen nuclei of the sample, is saturated with the electromagnetic radio frequency field using the coil 2 from the powerful generator 6, which leads to the equalization of the levels of nitrogen nuclei and to an increase in the spin temperature of the spin system. After turning on the magnetic field B0 of the solenoid 1, at the moment of intersection of the energy levels of the NQR and NMR systems, thermal contact occurs. In the receiving coil 3, an induction signal at the proton frequency (NMR system) is induced and processed by the pulsed NMR spectrometer 7. The NQR signal is detected by a decrease in the amplitude of the NMR induction signal. The entire system is controlled by the programmer 8, the results are displayed on the recording device 9.

To calculate the magnetic system, we will use the technique given in [8].

a)

б)
Fig. 2. a – electromagnet; b – axonometry

When determining the characteristics of the magnetic system field, we will make a number of assumptions that facilitate the solution of our problem:

• we will consider the width b (Fig. 2b) of the magnetic system and the depth h to be infinite, which allows us to consider the picture of the electromagnet field as two-dimensional instead of three-dimensional;
• the magnetic field strength of the magnetic system will be considered independent of time, i.e. consider a static magnetic field;
• the magnetic permeability of the core will be considered infinitely large.

The magnetic field strength can be approximately found according to the total current law from the expression:

, (4)

where w is the number of turns of the winding, Im is the amplitude of the current in the winding, bh is the cross-sectional area of ​​the core in the working gap, mнач is the initial magnetic permeability of the core, li is the length of the average line of force of the i-th section of the core magnetic circuit, si is the average value of the cross-sectional area of ​​the i-th section of the core magnetic circuit.

It can be shown that for values ​​of y (Fig. 2a) exceeding half the width of the working gap (y > 0.5l0), the lines of equal moduli of field strengths take the form of semicircles. Such a field can be calculated as the field of a single conductor with current:

, (5)

, (6)

, (7)

where Hx(0, y) is the field strength at point x = 0, y > 0.

We get:

. (8)

Photo 1 shows an experimental semi-toroidal solenoid. Electrical steel is used as the core material.

Photo 1.

The experimental electromagnet has the following parameters:

  — number of turns W = 743;
    — number of winding layers N = 15;
    — wire diameter d = 1.4 mm;
    — cross-sectional area of ​​magnetic poles S = 4 x 8 cm = 32 sq. cm.;
    — outer diameter of electromagnet D1 = 17 cm;
    — inner diameter of electromagnet D2 = 7 cm;
    — distance between poles l0 = 7 cm.

Fig. 3 shows the magnetic field lines generated by the magnetic system.

 Fig. 3. Magnetic field lines

The discrepancy between theory and experiment is due to the fact that in the calculations the length of the poles was assumed to be quite large (tends to infinity compared to the width of the working gap). The tested design is a semi-toroid with very short poles, due to which there is a “deflection of the magnetic lines from the y direction (Fig. 3), which leads to an increase in the lengths of the magnetic lines, and accordingly to a faster decrease in the value of the magnetic field with distance.

Fig. 5. DNQR in TNT(9,10)

Conclusion

From the above calculations and experimental results, the following conclusions can be drawn regarding the design of the magnetic system and the possibility of its use for remote detection of nuclear resonance spectra

1. The most optimal configuration of an electromagnet for remotely obtaining a magnetic field is a one-and-a-half.

2. To obtain the best characteristics of the magnetic field and the possibility of an accurate theoretical calculation of the parameters in the design of a system based on a one-and-a-half, the ends of the poles should be extended by a certain value L, as shown in the following Fig. 6.

Fig. 6. Core with extended ends

This will reduce the distortion of the power lines from the calculated values. In addition, such a design will allow, under the condition of a multilayer winding, to increase the number of turns of the coil, which will lead to an increase in the magnetic flux.

3. The most effective material for the magnetic system core is soft magnetic materials with high magnetic permeability — permalloys, however, due to the magnetic saturation effect, the maximum possible value of the magnetic field obtained on such a core is about 0.8 T. In reality, for design reasons (gap size, geometric dimensions of the core, etc.), this value will be 4 — 5 times less. For example, for the core used in our experiment at a distance of 3 cm, at a current of 50 amperes, the magnetic field strength will be about 0.2 T, which will correspond to a proton resonance frequency of about 8.5 MHz. This value is most likely the limiting frequency at which experiments can be carried out to study nuclear resonances with the specified design of the magnetic system.

4. To increase the homogeneity of the magnetic field of the system, it is necessary to select the width of the magnetic circuit as large as possible compared to the working gap l of the magnetic system.

Literature

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