SI21715A - Triple resonance enhanced nuclear quadropole resonance detection of tnt and other explosives - Google Patents

Abstract

The subject of the invention is a new triple-resonant polarization amplified NQR method for trinitrotoluene detection (TNT) and other explosives as well as narcotics and rods containing low-frequency NQR cores such as are 14N cores, in addition to high frequency cores such as chlorine, bromine, 17 O. The invention is based on use high frequency cores such as chlorine, bromine or 17 O for polarization of protons through the crossing of levels or "solid and low - frequency NQR detection (e.g., 14 N) signal with pulsed NQR at a distance. Compared to the previously proposed quadrupole-quadrupole polarization reinforced technique is avoided by the new method the problem of slow spin diffusion in polarization in 14 N is a quadrupole system with double resonance replaced by a three-resonance scheme. On this mode is the spin diffusion of the polarization amplifications in the proton system rather than the 14 N system.

Description

Triple-resonance amplified nuclear quadrupole resonance detection of TNT and other explosives

The subject of the invention is a new tri-resonant polarized amplified nuclear quadrupole resonance (NQR) detection method for trinitrotoluene (TNT) and other explosives as well as narcotics and drugs containing low-frequency NQR nuclei such as e.g. ¹⁴ V core - in addition to high frequency cores such as ³⁵ Cl, ^il Br, ¹⁷ O.

The invention is based on the polarization of protons via level crossing or "solid effect" with high-frequency NQR nuclei such as chlorine, bromine or ¹⁷ O, transfer of proton polarization to low-frequency NQR nuclei (such as nitrogen) by "solid effect" and detection of a low-frequency NQR signal ( e.g., nuclei ¹⁴ N) using a pulsed NQR at a distance. Compared to the previously presented dual (quadrupole-quadrupole) polarization-enhanced technique, the new method bypasses the problem of slow spin diffusion in a ¹⁴ / V quadrupole nucleus system by replacing the double resonance with the triple-resonance scheme. Thus, spin diffusion of enhanced polarization is achieved in the proton system rather than in the ¹⁴ / V core system. The new method of the invention allows for a multiple increase of signal / noise ratio and reduction of measurement time by one or two size classes. The new method is particularly useful in cases where the polarization amplification of high-frequency nuclei by a direct quadrupole quadrupole solid effect fails because the spin diffusion rate for the polarization of low-frequency NQR nuclei is too small. This is the case with TNT, where rare quadrupole nuclei of ¹⁷ O (1 = 5/2) polarize the surrounding quadrupole nuclei of ¹⁴ V (1 = 1) through a quadrupole-quadrupole solid effect only in a limited radius around ¹⁷ O, and therefore polarization does not transmit to the entire ¹⁴ V core system, which makes it impossible to significantly increase the ¹⁴ N core signal with a quadrupole-quadrupole solid-state effect. The proposed new method is also applicable for the detection of explosives in smuggled luggage and for the detection of anti-personnel mines.

FIELD OF THE INVENTION It relates to the measurement and testing of the electrical and magnetic characteristics of materials using triple resonance techniques.

Detection and removal of field mines is one of the most challenging tasks in over 70 countries worldwide. In view of the recent rise in global terrorism, it is also of great importance to find explosives that are hidden at airports or carried in luggage. A big problem is also the detection of illegal drugs and narcotics. NQR offers an acceptable method for the detection of long-range explosives, mines, drugs and narcotics, as described in articles: V. S. Grechishkin, N. J. Sinayavsky, Physics-Uspekhi 40, 393 (1997); J. P. Yesinovski, L. L. Buess, A. N. Garoway, Anal. Chem. 67, 2256 (1995); G. I. Allen, P. Czipott, R. Mathews, R. H. Koch, Proc. SPIE 3711, 103 (1999).

NQR detection is possible because all major explosives (TNT, RDX, ΗΜΧ, PETN, etc.) contain nitrogen, which quadrupoles the nucleus. These explosives are also used in field mines. The main feature of the NQR method is that it is molecular specific for the detection of compounds. The method does not depend on any other property of the mine (such as a metal minefield) as it detects the presence of explosives directly through the detection of quadrupole nuclei in them. The same is true for narcotics detection. Also, the presence of other nitrogen compounds (eg in soil fertilizers) does not interfere with the detection of e.g. mines - precisely because of the large difference in the resonant frequencies of nitrogen in explosives and in other compounds in the earth. Of course, the intensity of the measured signal is proportional to the number of NQR nuclei in the object being measured.

The NQR method cannot detect explosives in mines with a metal casing because radio waves cannot penetrate metal. This is certainly not a limitation on the use of the NQR method, since the NQR detector can always be adjusted to act as a metal detector by having the presence of metal affect the frequency tuning of the NQR probe. The main disadvantage of the NQR method is its low sensitivity, which is approximately proportional to the square of the NQR frequency of the measured kernel. The method is particularly successful in detecting ¹⁴ V NQRs in an RDX explosive where the resonant NQR frequencies are in the range of 3 to 5 MHz. However, for TNT, which is most commonly used as an explosive in field mines, the ¹⁴ V NQR frequencies in the range below 0.9 MHz are unfortunately resonant. In this case, the NQR signal is too weak to be measured within a reasonable measurement time, so in practical application of the NQR method this signal must be amplified in any way. This problem is described in the following works: Robert Blinc, Janez Seliger, Denis Arčon, Pavel Cevc, Veselko Žagar, Phys. Status Solids A 180, 541 (2000); Janez Seliger, Robert Blinc, Gojmir Lahajnar, Denis Arčon, Pavel Cevc, Slovenian Patent no. 20551, shared 5/11/2001; M. Nolte, A. Privalov, J. Altmann, V. Anferov, F. Fujara, J. Phys. D: Appl. Phys. 35, 939 (2002). Janko Luznik, Janez Pirnat, Zvonko Trontelj, Solid State Communications 121, 653 (2002); Zvonko Trontelj, Janko Lužnik, Robert Blinc, Gojmir Lahajnar, Janez Seliger, Slovenian Patent no. 20595, shared on 31.12. 2001; and RE Slusher, E.

L. Hahn, Phys. Rev. 128, 2042 (1962). The above observation also applies to the rapid detection of explosives and narcotics when screening passengers and baggage at airports.

In the absence of an external magnetic field, the core ¹⁴ N (1 = 1) has three NQR frequencies:

⁽ⁱ⁾ < ² >

V _o = v ₊ -v_ (3) which are characteristic of a given type of substance. In the above equations, e ² qQ / h is a quadrupole coupling constant and an η asymmetry parameter.

The signal-to-noise ratio of the NQR method can be improved in three different ways. These are:

- Nuclear quadrupole double resonance (NQDR), in which a weak signal is measured by its effect on a strong (usually ^ NMR) signal.

This mode is successful in laboratory conditions but not in the field. In detecting ¹⁴ A NQR of military TNT, it was possible to improve the signal to noise ratio by a factor of 30 according to the standard ¹⁴ / V NQR method (R. Blinc, J. Seliger, D. Arčon, P. Cevc, V. Žagar, Phys. Status Solidi A 180, 541 (2000); Janez Seliger, Robert Blinc, Gojmir Lahajnar, Denis Arčon, Pavel Cevc, Slovenian Patent No. 20551, granted 5 November 2001).

- Zeeman polarized amplified NQR, which measures the signal of pure NQR in the external magnetic field zero after it has been amplified by transferring nuclear polarization from a system of previously polarized nuclei ^l H. This approach does not require a homogeneous magnetic field as in the standard double resonance method , and is therefore suitable for off-site practical use.

- Quadrupole-quadrupole-amplified NQR, where the pure NQR signal in the external magnetic field is measured to be zero after it has been amplified by transmitting the polarization of other high-frequency quadrupole nuclei via a "solid effect".

The basic ^l H- ^l4 N nuclear quadrupole resonance experiment is described in the paper: RE Slusher, EL Hahn, Phys. Rev. 128, 2042 (1962). In this experiment, the magnetic field cycles between a high field (eg 0.5 T) and a field of zero, and the initial polarization of the proton spin system is adiabatically demagnetized. In the zero field, the sample is then irradiated with a long radio frequency (rf) pulse. After irradiation with an rf field in an external magnetic field, zero the magnetic field to its original high value (adiabatic remagnetization), and measure the proton NMR signal. Under the resonance condition, when the rf radiation frequency and the ¹⁴ ANQR frequency in _N0R are the same, ¹⁴ A NQR transients are saturated and the ^l H NMR signal is reduced due to the transfer of polarization from the proton NMR system to the nitrogen NQR system. This process can be represented by the flow of energy from a saturated system of nitrogen nuclei (with an infinite "spin temperature") into an adiabatically cooled proton system.

The sensitivity of the NQDR technique can be further enhanced by achieving a better energy flow between the two nuclear subsystems. This possibility was experimentally realized by the method of "NQDR with multiple level crossing" (R. Blinc, J. Seliger et al., J. Chem. Phys. 51, 5082 (1972)), in which proton NMR levels and nitrogen NQR levels they cross several times during the rf irradiation time interval (see Figure 1). The method is described in part by: R. Blinc, J. Seliger and others, J. Chem. Phys. 51, 5082 (1972). In order to maintain the high spin temperature of the ¹⁴ A nuclei, rapid phase changes of the rf field phase by 180 ° were used in this method.

The spectrum is wide (~ 30 kHz) and the six nitrogen resonance lines of the TNT sample are indistinguishable (Figure 2). The reason for the indistinguishability of lines across a wide spectrum is due to the elimination of dipole coupling between protons and nitrogen, which enables thermal contact between the two core systems (spinquenching effect).

In order to increase the resolution of the ¹⁴ A NQR spectra, we used the dual resonance techniques described in the works: J. Seliger, V. Žagar, R. Blinc, Z. Naturforsch. 49A, 31 (1993); J. Seliger, V. Zagar, R. Blinc, J. Magn. Reson. Al 06,214 (1994).

In addition to providing high resolution and higher sensitivity, it also has the potential to assign v_ and v ₊ transitions (see Figure 3) to a given ¹⁴ A core, which is far from trivial for the complex ¹⁴ ANQR spectrum.

In the aforementioned experiment, the magnetic field cycles adiabatically between the high and low sizes of the Bo field determined by the resonant condition χ _Η Β ₀ / 2π = ν _θ ( ¹⁴ a).

When the low magnetic field is equal to Bo, the sample is irradiated with a long rf pulse whose frequency is continuously varied in the range that covers the frequencies of both v ₊ and v_. During each change in the frequency of rf radiation, transitions v_ are excited first and then transitions in ₊ . This results in a very effective flow of thermal energy from the quadrupole ¹⁴ A system into the proton system with high spin temperature, which is reflected in the large drop ("minimum") of the proton signal (see Figure 4). A proton signal is greatly reduced only if the frequency change covers both transitions v_ and r ₊ . This also makes it possible to determine both v_ and v ₊ via the variation of the upper and lower bounds of the frequency variation of the rf origin.

Even higher resolution is achieved by using dual resonance irradiation instead of changing the frequency rf of the origin. Here the first frequency in _{l is} approximately chosen so that Vj «v ₊ , then the second frequency in _{2 is} varied until the exact condition v ₂ = v_ In the next step, then we leave the frequency in ₂ unchanged and vary Vj until it is exact the condition v, = v _{+ is} fulfilled. The resolution of this method is about 0.1 kHz. The ¹⁴ VNQR frequencies for TNT at room temperature are given in Table 1 and their temperature dependencies are shown in Figure 5.

Table 1

in ₀ (kHz) (kHz) v ₊ (kHz) 1 160 712 872 2 92.5 769 861.5 3 110.5 742.5 853 4 132 718 850 5 92 755 847 6 95 742 837 7 -0 -807 -807

The nuclear quadrupole dual-resonance techniques described above enable the rapid and effective determination of ¹⁴ tVNQR parameters on less than a gram of heavy samples of explosives and narcotics. However, the requirement for a highly homogeneous magnetic field for the detection of protons and the expected masking of the sample signal by non-sample protons do not allow this method to be used in out-of-laboratory practice.

In the Zeeman-polarized amplified NQR (PE-NQR) experiment (Fig. 6), the signal of pure NQR in the magnetic field is zero amplified by the transfer of polarization from the nucleus ^l H system previously polarized in the magnetic field. In this experiment, the sample is first in a strong magnetic field, which is not required to be homogeneous, since it serves only to polarize the protons and not to detect the proton NMR signal.

In the next stage of the experiment, the magnetic field is reduced adiabatically, either by switching off the magnet or by moving it away. During this adiabatic demagnetization, the cleavage of proton NMR lines is reduced and finally equated to a small cleavage of ¹⁴ 2VNQR lines when energy begins to flow from the "hot" nitrogen NQR system to the "cold" proton NMR system. At the same time, the nitrogen NQR system is "cooled" (polarized), which is detected as an ⁱ⁴ tV NQR signal with standard NQR measurement techniques in the magnetic field of zero (if we neglect the earth's magnetic field). Detection begins immediately after the magnetic field is switched off. The ¹⁴ jVNQR signal is amplified by a factor

F = K _e "(" vK! (N "+ N _n ) if there is no decrease in the ¹⁴ / V polarization due to the fast quadrupole relaxation.

The Zeeman PE-NQR technique was tested at low temperature (77 K) on a ¹⁴ jV NQR 4-nitrobenzoic acid ((.NOi ^ eH ^ COOH) signal in a NdFeB permanent magnet (cube (7cm) ³ )) (Figs. 7 and 8).

The experimental parameters were:

vJ'M = 982kHz

M = 8.5 ± 2 MHz (magnetic field 200 ± 50 mT)

N _h / (jV _h + N _n ) = 0.85

F (predicted) = 7.4

The modified gain factor F (measured) «7 fits well with the predicted magnitude of 7.4. One PE-NQR cycle improves the signal-to-noise ratio as much as an average of 1000 times the repeated sequence for the non-polarization method (Figure 8). As expected, the homogeneity of the polarizing magnetic field is not critical, but the increase in the signal / noise ratio is only proportional to the size of the average field.

In summary, the Zeeman PE-NQR method is effective if:

- Proton NMR to ¹⁴ N NQR ratio high

- ¹⁴ TV NQR spin network relaxation time long enough for the polarization magnetic field to fall to zero and the ¹⁴ N cores remain polarized

- the number of hydrogen atoms N _H in the sample is at least comparable to the number of nitrogen atoms N _n in it.

In addition, the time required for the proton polarization in the external magnetic field (spin-network relaxation time) to be taken into account may also be the cause of the time constraints in the experiment.

The transfer of proton polarization by means of the crossing of ¹⁴ N- ¹ H levels in a magnetic field is technically and economically very demanding, since it requires the establishment of a static magnetic field of size 0.1 T or more at the location of the sample being tested, such as. anti-personnel landmines. In practice, this poses an additional problem as the investigator must carry a magnet or magnetization coil to generate a static magnetic field at the investigated site. This problem can in some cases be remedied by quadrupole-quadrupole polarization-amplified nuclear quadrupole resonance detection of nuclei with a low NQR frequency of ¹⁴ N, by means of a "solid effect" if another nucleus with a high NQR frequency is present in the molecule. The solution is described in Slovenian patent no. 20995, awarded February 28, 2003, by Robert Blinc, John Seliger, Tomaž Apih, and Gojmir Lahajnar.

For a description of the method, let's look at an example of a sample containing two types of quadrupole nuclei Q ₁ and Q ₂ . Furthermore, the kernels should have 0, much higher NQR frequency than the kernels Q ₂ . By incorporating a strong radio frequency field oscillating at a frequency equal to the frequency difference of transitions between the quadrupole levels of the nuclei Q _x and Q ₂ , we induce simultaneous transitions in both spin systems, thus transferring polarization from system Q _} to systemQ ₂ . Due to the coupling dipole between Q ₂ nuclei, the amplified polarization is also transferred to other Q ₂ nuclei, which are not necessarily near the nucleus (Figure 9).

In equilibrium, the polarization of S nucleiQ ₂ , which is proportional to the nitrogen NQR signal at frequency, is increased by a factor by comparison with the polarization S _o without the quadrupole quadrupole "solid effect".

S / S _o = 1 +

Yes where are:

S _o equilibrium g ₂ NQR signal without additional rf irradiation, equilibrium (Ž, NQR signal with rf irradiation, / ^ probability for simultaneous transition to a time unit in the case of a quadrupole-quadrupole solid effect,

W _{Qi is the} speed of spin-network relaxation of oxygen nuclei Q,

W _{Qi is the} rate of spin-mesh relaxation of Q ₂ nuclei, and ε is the natural frequency of Q _x nuclei.

For the condition W _s = <»D ω _Γ , where ω _{β is the} dipole width of the line, ω _ο = Ίπν.

and ω _χ = γΒ _χ , relatively high amplitudes of the radio frequency B _x (greater than or equal to 100 Gauss) are required if the method is to be effective. Of course, it should be noted that the amplitude B _x is not critical in itself, since the smaller size B _x corresponds only to the lower amplification, and the effect itself is not eliminated, since it is not associated with a resonant condition.

A further condition for the successful operation of this method is that the spin-mesh relaxation time (Tj) of nuclei g ₂ must be sufficiently long, whereas the spin-mesh relaxation time of another type of nucleus Q _{x must be} sufficiently short. Furthermore, the spin diffusion of the polarization of Q ₂ nuclei must be sufficient enough to polarize the entire system of Q ₂ nuclei. It is not enough to polarize only one of the core Q _2, which are directly linked to the group containing cores Q _x.

Many times the characteristic spin diffusion time of Q ₂ nuclei is not short enough to polarize the entire Q ₂ nucleus system. The amplification of the NQR signal is then relatively small. To overcome this problem, we present the invention of a new three-resonance method.

The object and object of the invention is a three-resonance method for amplifying pulsed NQR signals into a low-frequency NQR with the following characteristics:

- Highly effective polarization of the proton spin system via Qi -H solid effect,

- Rapid (less than 1 ms duration) transfer of proton polarization from the nucleus surroundings ^ to the rest of the proton spin system,

- Transfer of polarization from the proton spin system to the spin system Q _by either crossing the levels or by the β / -'H "solid effect".

The following is a detailed description of the invention by means of Figures 1-10.

Figure 1. NQDR experiment with multiple level crossing. The proton NMR signal is measured after an adiabatic demagnetization-remagnetization cycle. The NQRs of the nitrogen crossings are saturated by rf irradiation in a zero field, which allows energy to flow between the two subsystems.

Figure 2. NQDR spectrum of TNT in the external magnetic field zero as a function of frequency rf origin.

Figure 3. ¹⁴ In quadrupole (1 = 1) nuclear energy levels and frequency of transitions between levels. Here K = eqV _2z / h is a quadrupole coupling constant and 7 = (^ -1 ^) / ^ asymmetric parameter for a given location of nuclei ¹⁴ N. Each type of nucleus ¹⁴ V in TNT has 3 resonance lines in ₀ , v_ and v ₊ . therefore, in the TNT spectrum, 6 3 = 18 lines.

Figure 4. Proton signal as a function of external magnetic field in the range v _Q = v ₊ -v_.

¹⁴ In NQR, the TNT gateway in the presence of an rf field that moves freely across the area ("rf sweep"), covering both v ₊ and v_ transitions. For comparison, the intensity of the proton signal in the absence of this frequency-varying rf field is shown.

Figure 5. Temperature dependence of ¹⁴ N quadrupole resonant frequencies v_ and v ₊ in TNT.

Figure 6. Schematic of the PE-NQR experiment. The external magnetic field is polarized by the proton nuclear subsystem. After reducing the magnetic field to zero and crossing ¹⁴ V NQR and ^X H split NMR levels, a ¹⁴ N NQR signal in the magnetic field zero is measured.

Figure 7. Magnetic field profile of the NdFeB magnet used in the experiment.

Figure 8. Polarization amplification of a ¹⁴ N NQR signal of a sample (NO ^ JHjCOOH (4nitrobenzoic acid) in a 200 mT magnetic field (J. Lužnik, J. Pirnat, Z. Trontelj, Solid State Commun. 121,653 (2002)).

a) after one non-polarization method sequence (no signal)

b) after 1000 sequences of the method without polarization

c) after one PE-NQR sequence (polarization in 200 mT field)

Figure 9. Time dependence of the "static" magnetic field B _o and rf of the magnetic fields B, (0,) and B _l (Q ₂ ) in the triple-resonance measurement cycle.

Figure 10. Determination of ¹⁴ N NQR frequency in _Q = v ₊ = v_ n sample CH ₂ NH ₂ Br.

The method requires three core systems in the sample:

- a quadrupole nucleus system (Q) with a multi-MHz NQR frequency and high spin relaxation rate (chlorine, bromine, ¹⁷ O, ...), proton spin system ( ¹ H), and

- system of quadrupole nuclei (Q ₂ ) of interest (nitrogen); the method consists of three steps (Figure 9), namely:

- Polarization of the proton spin system by the "solid effect" Q _x - H,

- Transfer of polarization of the proton spin system to the Q ₂ spin system by means of level crossing or solid effect, and

- Detection of Q ₂ nuclei with pulsed NQR.

In the first step, the system is irradiated in a weak static magnetic field with a strong rf magnetic field (several mT) at a frequency ω _ΰι ~ ω _Η . Here, <y _{ft is the} NQR frequency of nuclei <2, and ω _{Η is the} proton resonance frequency in a low static magnetic field. This low static magnetic field is typically lmT or smaller in size. The choice of such a field is a trade-off between the proton spin-mesh relaxation time, which usually increases sharply with the increasing external static magnetic field in the mT region, and the extension of NQR lines due to the magnetic field. The kinetic equations for the spin temperatures of the spin systems Q _x and H are:

(6) dNJdt = -W _Q / N, + ir _a N ₂ +

Here JVjin jV _{2 are the} occupations of the upper and lower energy levels of the nuclei Q _l . Similarly, n, and n _{2 of} the occupancy of the upper and lower energy levels of protons. W _H is the speed of the proton spinmrežne relaxation, _W JQ probability per unit time for simultaneous "solid effect" transition in the two spin systems, JT _g and W _Q and ^d probability of transition per unit time between the energy levels spin system Q _x, which induces crystal lattice. The symbol "indicates the upward transition and the d downstream symbol. Both transition probabilities can be given by the expressions:

(7)

The foreign W _Qi spin spin relaxation rate of the spin system g ,, T is the net temperature, and γ measures the difference in the occupancy of the two energy levels in equilibrium. In a proton spin system, at relatively low resonance frequency, we do not distinguish between probabilities per time unit for upward and downward passage.

In equilibrium, when dN _x ! Dt = Q> and dn, / di = 0, the occupations of both proton energy levels are given by:

(8)

N,

Foreign (9)

+ -2LS_ + AL £ L

In this case, ε = N _Q JN _{H is the} number of nuclei Q ,, N _& , divided by the number of protons, N _H. When W _H «W _SE and at the same time W _H « W _Qi , we obtain β -γ. The magnetization of the proton spin system is equal to the equilibrium proton magnetization in a static magnetic field, at which the proton Larmor frequency is equal to ω _Οι . Relaxation time, which leads to the dynamic balance of the two spin systems, is approximately determined by the longer of the two times, and _(ε1Ρ ΕΕ) \ This relaxation time is typically much shorter than the proton spinmrežnega relaxation time at the corresponding magnetic field when _ω Η = a> _Q.

In the second step, the proton magnetization is transmitted through the level-crossing (level-crossing) to the quadrupole spin system Q ₂ . When the resonant frequency is below 1 MHz, the external magnetic field is first raised to a size of about 25 mT and then lowered to zero. During the rise and fall of the external magnetic field, the proton energy levels intersect with the energy levels of the Q ₂ nuclei, thus polarizing the Q ₂ spin system. For simplicity, we assume that the Q ₂ core has only two energy levels. The occupations of these two energy levels Ρ _λ and P ₂ are initially equal to the equilibrium occupations:

(10)

N _o tx ^p 2 = —- (! + «) ·

Foreign a = ha) _Q2 / 2kT.

After level-crossing, the occupations P, and P ₂ change to (11) (12)

Ρ ₂ = ^ (ι ₊ s).

Foreign e Nq ₂ n H δ = a-— + β --— ^Ν Ά ^{+ Ν} Η

The increase in polarization of the spin system Q ₂ equals δ! a. In the optimal case, when β ~ γ, we obtain:

δ / α = —— + - ^ -. (13)

Nq ₂ + N _H co _Qi Nq ₂ + N _H

In the model compound CH ^ NH ^ The fast nucleus Q _{x is} either ^8I 5r (va = 10804 £ Zfe) or ⁷⁹ 5r (va = 12933 £ / Zz). Q ₂ cores are ¹⁴ N with a resonant frequency of 680 kHz (Figure 10).

⁸¹ Br was used to polarize protons in an external magnetic field of 0.8 mT. The maximum proton signal size measured immediately after the sample was transferred to a normal magnetic field (y _HO = 32MHz) was approximately equal to one-third of the full proton NMR signal in that magnetic field. Since the ratio in _a lv _{H 0} = \ Q.% Q4MHz / 32MHz is close to one-third, it is clear that the condition β = γ was obtained experimentally (see expression (9)). If we calculate the expected increase in the nitrogen NQR signal by the three-resonance technique (expression (13) at N _H / N _& = 6), we obtain b / a «14. (14)

The three-resonance technique therefore allows a significant increase in the NQR signal in low-frequency NQR spectroscopy. The time between two consecutive measurements with this technique is equal to several times the factor longer than the time (fJF ^ J ^ or required for proton polarization, plus the few tens of seconds required for the levelcrossing process, and is typically less than 1 second. Q ₂ refers to ¹⁴ N with a spin-mesh relaxation time of a few seconds, obtained simultaneously at signal intensity and repetition rate. Limitations of this technique occur when the concentration of nuclei Q _{l is} low and when the spin-mesh relaxation rate is not high enough. be greater than or at least approximately equal to W _H if the new technique is to be effective) and when Q ₂ nuclei relax rapidly.

Claims (3)

Patent claims 1. Triple-resonance amplified nuclear quadrupole resonant TNT detection in mines and other explosives, as well as narcotics and drug detection, characterized in that low-frequency NQR nuclei such as ¹⁴ N are polarized (i) by transferring polarization from the proton system via solid effect "(for example," solid effect "protons ^l H ¹⁴ 77), (ii) where the proton system previously polarized by a second" solid effect ", for example ¹⁷ O ^X H" solid effect ", with the transfer of polarization from high frequency NQR cores, such as ¹⁷ O, by "solid effect" or crossing levels on a proton system, the "solid effect" being obtained by (i) irradiating the system with radio frequency fields at a frequency corresponding to the frequency difference of the selected quadrupole transients ( ¹⁴ JV), and proton transitions in a dipole field or in the earth's magnetic field, while the "solid effect" of (ii) was obtained by irradiating the system with an rf field with frequencies corresponding to the frequency difference, the associated NQR frequency ( ¹⁷ O) of the high-frequency nucleus and the proton frequency in the dipole field or in the earth's magnetic field, and where low-frequency NQR nuclei, such as nuclei ¹⁴ 77, are detected remotely by conventional pulse NQR spectroscopy after the inclusion of additional irradiation with rf fields . 2. Triple-resonance amplified nuclear quadrupole resonance detection of TNT in mines and other explosives, as well as the detection of narcotics and drugs, characterized in that the detection of low-frequency NQR nuclei such as nuclei ¹⁴ 77 according to claim 1 is characterized by being "solid effect ”in step (ii) is replaced by polarization transfer by means of level crossing. 3. Triple-resonance amplified nuclear quadrupole resonance detection of TNT in mines and other explosives, as well as narcotics and drug detection, characterized in that the detection of low-frequency NQR nuclei such as cores ¹⁴ 77 according to claims 1 and 2 is characterized in that the polarization and detection cycles are repeated.