RU2444003C1 - Method of identifying concealed explosives and narcotic drugs - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: analysed substance containing nitrogen and/or carbon is irradiated with a pulse of gamma-radiation. Secondary radiation from decay products formed by nitrogen-12 and boron-12 isotopes is picked up. The time spectrum of secondary radiation signals is recorded. The abundance ratio of nitrogen and carbon is determined from the obtained time spectrum analysis results. The analysed substance is identified based on its carbon and nitrogen composition through comparison with standard values from a data base. The time spectrum of the secondary radiation is recorded in a limited time interval, which starts from the end of the radiation pulse with time delay. The spectrum is processed by converting the measured differential time spectrum into an integral time spectrum. Determination of the abundance ratio of nitrogen and/or carbon in the sample is carried out for several time intervals of the measured spectrum. Accuracy of detecting substances containing nitrogen or carbon is determined by comparing values of abundance ratio of nitrogen and oxygen in the sample, calculated for different time intervals.

EFFECT: high accuracy and reliability of a photonuclear detector of explosive and narcotic substances with simultaneous increase in efficiency thereof.

3 cl, 3 dwg

Description

The invention relates to the field of detection of hidden explosives (BB) and narcotic drugs (NS) by the method of photonuclear detection and can be used in stationary and mobile installations, for example, when inspecting baggage of air passengers, customs inspection or clearing territories in the framework of humanitarian actions.

The well-known method of photonuclear detection of hidden explosives and NS was proposed in [1] and developed in [2-4]. This method relates to effective and high-speed methods for direct detection of explosives and NS and consists in the registration of decay products of short-lived ¹² V (boron-12) and ¹² N (nitrogen-12) isotopes with half-lives of T _{1/2 ,} 220.2 and 11 , 0 ms, respectively. These isotopes are formed on nuclei ¹⁴ N (isotope content in the natural mixture η≅99.63%) and ¹³ С (η≅1.107%), whose chemical elements nitrogen and carbon form the basis of all modern explosives and natural NSs when they are irradiated with incident gamma quanta with an energy of E _γ greater than the threshold values of E _{γ p} as a result of the following photonuclear reactions:

The choice of these reactions as marking provides high selectivity for the detection of substances containing nitrogen and / or carbon, because upon irradiation of any other chemical elements with a gamma beam with E _γ <~ 55 MeV, practically no other isotopes are formed with T _1/2 ≈ (1 ÷ 100) ms. The ¹² V and ¹² N isotopes are β-active and in the process of decay emit electrons and positrons with a maximum energy of ~ 13 MeV and ~ 17 MeV, respectively, which, braking in the substance, in turn emit secondary gamma rays. All secondary gamma quanta, just indicated as inhibitory ones, as well as from de-excitation of ¹² C nuclei formed in decays, as well as from annihilation of ¹² N positrons emitted during decays, together with electrons and positrons make up the secondary decay products and can be detected by a detector. Therefore, if the object under investigation is irradiated with a pulse of the order of several microseconds of sufficiently energetic gamma radiation, then in the subsequent time interval of up to ~ 100 ms, it will respond if there is a sufficient concentration of nitrogen and / or carbon in it with the flow of secondary particles from the decay of ¹² V isotopes and ¹² N. Otherwise, this flow will not be in the measured period of time.

Thus, in view of the uniqueness and small half-lives of the radionuclides formed during the indicated irradiation of nitrogen and / or carbon, and taking into account the fact that both the initiating irradiation and the useful signal are formed by gamma radiation streams with high penetrating power, the BB and NS detectors using the photonuclear detection method must have high operational characteristics: high sensitivity for the depth of detection in the hiding substance and speed, which is confirmed by the result and computer modeling of its work [5].

The success of any method of detecting explosives and NS is largely determined by its selectivity. When using the photonuclear technique, along with “useful” signals from explosives and NSs, “false” signals from substances that are not explosives or NSs but contain nitrogen or carbon may appear. Of particular importance is the suppression of false positives in installations for screening baggage of air passengers, when the sources of false signals can be various objects containing primarily carbon (polymeric materials, natural fabrics or food products). When a photo-nuclear detector is used in humanitarian mine action, various biological objects in the soil and nitrogen-containing fertilizers can become sources of false signals.

In [4], a method for identifying hidden explosives and NSs was selected, selected as a prototype, based on an analysis of the temporal distribution of secondary radiation signals using the photonuclear detection method.

The essence of this method is as follows. Upon a single irradiation of the test substance containing nitrogen and / or carbon with a gamma ray beam with an energy of more than 31 MeV, nuclei of short-lived isotopes ¹² V and ¹² N are formed in it as a result of reactions (1) - (3). The relative concentration of these isotopes is uniquely related with the relative concentration of nitrogen and carbon in the irradiated substance and defines a unique "portrait" of the investigated substance. When decaying, the ¹² V and ¹² N isotopes form a secondary radiation flux, the shape of the time dependence of which is determined by the half-lives of the isotopes and their initial relative concentration. As you know, the law of change in the number of radioactive nuclei of one element over time is:

where N (t) is the number of non-decayed nuclei at time t; N ₀ is the initial number of nuclei formed (at t = 0); λ is the decay constant (λ = ln (2) / T _1/2 ).

For the decay of the isotopes of two elements of interest, we obtain:

at t = 0:

where λ _(N-12) and λ _(B-12) are the decay constants for ¹² N and ¹² V, respectively;

N _{0 (N-12)} and N _{0 (B-12)} are the numbers of formed nuclei ¹² N and ¹² V, respectively;

N _0Σ is the total number of formed nuclei of radioactive isotopes ¹² V and ¹² N.

By analyzing after the end of the irradiation pulse the shape and course of the time dependence of the secondary radiation flux on the decays of ¹² V and ¹² N, it is possible to determine the initial relative concentration of isotopes ¹² V and ¹² N and to identify the test substance.

The results of the analysis of the time spectrum of the decay products of isotopes ¹² V and ¹² N are presented in the form of values of the parameter k, defined as:

where k _B-12 = N _{0 (B-12)} / N _0∑ , k _N-12 = N _{0 (N-12)} / N _0∑ .

Knowing the value of k allows one to determine the affiliation of the irradiated substance with explosives and NSs, and by comparing the results obtained when the detector was operated with the results obtained with the irradiation of reference samples, identify explosives and NSs.

However, under real operating conditions, the device declared in [4] records the differential time spectrum, where in each measured time interval (bin) the number of decays of these isotopes is recorded, i.e. n {t} = (dN / dt) × Δt, so that the temporal distribution of events recorded by the detector from decays of isotopes ¹² N and ¹² V, measured during installation, is described by the expression:

where Δt is the channel width of the measured time distribution.

A certain complexity of processing the received signals is due to the fact that the accumulation of signals for constructing a time distribution in a real explosive and NS detector can be carried out only for a certain limited time interval. In the first few milliseconds after the end of the irradiation, numerous gamma quanta are received in the gamma detector, which are primarily associated with the reactions (n, γ) caused by photoneutrons, which can greatly distort the measurement results. Therefore, the analysis of the time spectrum can begin only after the end of the irradiation pulse with a certain delay. On the other hand, the accumulation duration is also limited by the requirements to ensure a given detector performance. The latter is especially relevant in the scanning mode of the test sample, when, in order to obtain a complete picture, it is necessary to irradiate repeatedly in different zones. Thus, due to the restrictions imposed on the time interval for the accumulation of the decay curve, it is impossible to directly measure N _0Σ and directly calculate the value of k, which reduces the performance of the installation, as well as the accuracy and reliability of the results.

The problem solved by the invention is to increase the accuracy and reliability of the photonuclear explosive and NS detection detector while increasing its performance.

The problem is solved by the claimed method, the essence of which is as follows. After the exposure of the object under study with a primary gamma-ray pulse, the registration of the time spectrum of the secondary radiation begins with a time delay of several milliseconds and lasts for a time comparable to the half-lives of the isotopes ¹² V and ¹² N. After that, the recorded time spectrum of the signals is converted from differential to integral form. Then, by analyzing several sections of the integral time spectrum, the values of k corresponding to these time intervals are calculated. Further, by analyzing the totality of the calculated values of k, the membership of the irradiated substance in nitrogen- or carbon-containing substances is determined.

The proposed method allows to determine the value of k, based on two measured values of n (t), corresponding to different times t. In this case, we obtain a system of two equations linear with respect to the desired N _{0 (N-12)} and N _{0 (B-12)} :

The solution of system (9) will give the values N _{0 (N-12)} , N _{0 (B-12)} and k = N _{0 (N-12)} / (N _{0 (N-12)} + N _{0 (B-12)} ) . Applying the described procedure to the combination of values of i and j = i + 1, we can obtain a sequence of values of k _i related to each interval Δt of the time spectrum.

In order to reduce the statistical error in determining the values of N _{0 (N-12)} , N _{0 (B-12),} and accordingly k, caused by the relatively small statistics accumulated in each individual channel of the time distribution, we can go from the differential form of the description of the isotope decay process to its integral description and work with the total number of events accumulated in several channels corresponding, say, to the time interval (t ₂ -t ₁ ). In this case, the difference between the number of non-decayed nuclei at time t ₁ and t ₂ is equal to:

- сумма событий, зарегистрированных в каналах, соответствующих интервалу (t₂-t₁). А если взять два временных промежутка (t₂-t₁) и (t₄-t₃), то из соответствующих уравнений типа (10) получим систему, аналогичную системе (9):Where

- the sum of the events recorded in the channels corresponding to the interval (t ₂ -t ₁ ). And if we take two time intervals (t ₂ -t ₁ ) and (t ₄ -t ₃ ), then from the corresponding equations of type (10) we get a system similar to system (9):

и

- суммы сигналов, накопленные в каналах, соответствующих временным интервалам (t₂-t₁) и (t₄-t₃). Решение системы (11) относительно значений N_0(N-12) и N_0(B-12) позволяет найти и значения k.Where

and

- the sum of the signals accumulated in the channels corresponding to the time intervals (t ₂ -t ₁ ) and (t ₄ -t ₃ ). The solution of system (11) with respect to the values of N _{0 (N-12)} and N _{0 (B-12)} allows us to find the values of k.

With the appropriate choice of t ₁ , t ₂ , t ₃ and t _{4, the} integrated description allows to obtain the values of N _{0 (N-12)} , N _{0 (B-12)} and k (we call them “optimal” here) associated with the total number of events accumulated in all channels of the measured time spectrum and given with the best available statistical measurement accuracy (or close to it).

At the same time, the integral description also allows one to obtain, with comparatively high statistical accuracy, the values of N _{0 (N-12)} , N _{0 (B-12)} , which can be attributed in one way or another to the time intervals of the measured spectrum. One of these methods, which is proposed here, is the following: for the measured time spectrum in the interval from the initial moment of time t ₁ to the final t _kon are found above the "optimal" integral values of N _{0 (N-12)} and N _{0 (B-12 )} , as well as N (t ₁ ) = N _{0 (N-12)} × exp (-λ _(N-12) × t ₁ ) + N _{0 (B-12)} × exp (-λ _(B-12) × t ₁ ) (in accordance with equation (5)), then by sequential per-channel subtraction are:

etc. As a result, the integral distribution N (t) for a limited time interval is constructed from the experimental data on measuring the differential spectrum. This distribution has a significantly greater statistical security and allows one to determine the parameter k (using (5) - (7)) with a much better accuracy as a function of k (t) in the entire analyzed time interval.

Obtaining such a sequence of k (t) values at each measured point gives information about the reliability of the result of the identification of a substance for the presence of nitrogen and / or carbon or their absence. In the ideal case, in the presence of nitrogen and / or carbon, the function k (t) is described by a straight line with an angle of inclination to the time axis equal to zero. Otherwise, the straight line approximating the function k (t) has an angle of inclination to the time axis that is very different from zero. This description may serve as an additional criterion for the reliability of the result. In addition, the above “optimal” value of k gives the value of “photonuclear portrait of a substance”, i.e. characterizes the presence in the substance of nitrogen and / or carbon and their relative concentrations.

Thus, the claimed invention, in contrast to the prototype [4], offers a method for analyzing data measured by a photonuclear detector obtained in real conditions, and introduces a new criterion that determines the reliability of detection of substances containing nitrogen or carbon, namely:

1. A method is proposed for transitioning to the description of the measured differential spectra in an integral form, which significantly increases the statistical accuracy of the measurements.

2. A method for analyzing time spectra in a limited time interval is proposed. The limitation from the side of small times makes it possible to reduce the influence of the background processes on the time spectrum, which are primarily caused by the photoneutrons formed during the interaction of the electron beam with individual parts of the setup. The limitation on the part of large times increases the overall performance of the detector.

3. A criterion for the reliability of the result based on the analysis of the time dependence of k values calculated for various time intervals is proposed.

The invention is illustrated by the following example.

To confirm the effectiveness of the proposed method, its application to the search and identification of hidden explosives in conditions simulating baggage inspection was carried out in control installations at airports, customs terminals, etc.

Using computer simulation, taking into account the processes occurring in the irradiated sample, the output of the secondary radiation and its registration by the secondary radiation detector, the procedure for detection and identification of explosives was simulated. We determined the value of the parameter k _source equal to the ratio of the nuclei of the isotope ¹² N (N _{0 (N-12)} ) formed in the sample to the total number of formed nuclei of radionuclides N _0∑ , and the time spectrum of events recorded by the detector was constructed. Further, this spectrum was processed using the above procedure, and the values of k _ISM corresponding to the selected time intervals were determined.

The following computer simulation results were obtained under the following initial conditions: the sample was irradiated for 6 μs by a gamma-ray beam generated in a tungsten inhibitory target 0.35 mm thick when electrons with an energy of 55 MeV and a current of 30 mA in a pulse of duration 6 were incident on it μs. The secondary radiation detector with an area of 1 m ² was located at a distance of 60 cm from the sample, and the detector efficiency was 100%. For modeling, we used TNT, weighing 100 g, graphite, weighing 100 g, as targets, and generated a background spectrum that was adequate to loading the detector with the background from nuclear reactions of the type (n, γ) caused by photoneutrons. Below are the results obtained by simulating target irradiation with a single accelerator pulse.

Figure 1 shows the differential time spectrum of the secondary radiation recorded by the detector obtained by simulating the irradiation of a sample containing TNT (curve 1), and a typical spectrum caused by background processes (curve 2). In this case, it was assumed that the secondary radiation detector was switched on 5 ms after the end of the irradiation pulse, the events were recorded for 15 ms, and the channel width of the time analyzer was 1 ms.

Figure 2 shows the integral time spectrum N (t) of the secondary radiation for TNT obtained from the differential time spectrum (curve 1 in figure 1) using procedure (12). The physical meaning of the spectrum shown in FIG. 2 is a curve describing the number of non-decayed nuclei ¹² N and ¹² B in the irradiated substance at each time t _i . From a comparison of the corresponding curves in FIGS. 1 and 2, it can be seen that, in comparison with the differential spectrum, the integrated spectrum is characterized by a significantly greater uniformity of the calculated values, which ultimately leads to greater accuracy in determining the parameter k.

Using the proposed method, it is possible not only to identify substances having a close elemental composition, but also to separate samples whose elemental composition differs from the previously assumed. If radionuclides with half-lives other than expected are formed during irradiation, then equation (11) for this case ceases to correctly describe the occurring decay processes, which will be expressed in the fact that the values of k calculated for different time intervals cease to be constant, which may serve as the most important qualification attribute.

An example of ambiguous determination of the parameter k using the proposed method can be the data presented in figure 3, which shows the dependence of k (t) for TNT (3), graphite (4) and the background process (5). It can be seen that for the background process there is a significant instability of the values of k, which allows one to reliably separate the false object from the desired explosives and NS.

The table presents the total data on modeling registration processes, where for the various detected substances the “true” values of k _source are calculated, which are calculated from the known number of radionuclide nuclei formed, the measured values of k _ISM obtained during the processing of time spectra by the present method, and the tangents of the angle the slope of the lines approximating the k (t) -dependence.

Substance k _east k _rev Slope tangent k (t) TNT 0.72 0.696 ± 0.062 0.003643 Graphite 0,0 -0.04 ± 0.07 0.004143 Background - 2,962 ± 0,062 -0.173105

Thus, the above example indicates the efficiency of the proposed method, which is based, in contrast to the prototype [4], on:

1. The method of transition from the measured differential spectra to the integral form of representing the decrease over time of the number of nuclei of radionuclides, which significantly increases the statistical accuracy of the measurements.

2. A method for analyzing time spectra in a limited time interval. The restriction from the side of small times allows to exclude the influence on the time spectrum of background processes. The limitation on the part of large times increases the overall performance of the detector.

3. Offering a reliability criterion for the result of the detection of nitrogen and / or carbon-containing substances, which is associated with the analysis of k-dependence and increasing the accuracy of determining the value of k.

Information sources

1. US patent No. 4756866, 376/157.

2. E.A. Knapp, A.W. Saunders, and W.P. Trower. Direct Imaging of Explosives. Proceedings Euroconference on: Sensor systems and signal processing techniques applied to the detection of mines and unexploded ordnance, MINE'99, October 1-3, 1999. Villa Agape, Firenze, Italy Laboratorio Ultrasuoni e Controlli Non Distruttivi Università di Firenze, Italy. http.

3. A device for detecting hidden explosives. RF patent No. 2185614. BI No. 20, 2002

4. A method and apparatus for detecting and identifying hidden explosives and drugs. RF patent No. 2226686. (Bull. No. 10) / 14, 2004

5. AIKarev, VGRaevsky, JAKonyaev, ASRumyantsev, RRIliutschenko. The High Efficiency Complex for Detection of Landmines. In Proceeding of NATO Advanced Research Workshop # 977941 Detection of Explosives and Land Mines: Methods and Field Experience, St. Petersburg, 11 ^th -14 ^th of September 2001, Russia, NATO-Series book, Kluwer Academic Publisher, Netherlands.

Claims (3)

1. A method for identifying hidden explosives and narcotic drugs, including irradiating a test substance containing nitrogen and / or carbon with a gamma-ray pulse, recording secondary radiation from decay products formed by nitrogen-12 and boron-12 isotopes, recording the time spectrum of secondary signals radiation, determination by the analysis of the time spectrum of the relative content of nitrogen and carbon, identification of the test substance by its carbon and nitrogen composition by comparison with the reference values of the database, characterized in that the time spectrum of the secondary radiation is recorded in a limited duration of a time interval beginning after the end of irradiation of pulse with a time delay; spectrum processing is carried out by converting the measured differential time spectrum into integral; the determination of the relative content of nitrogen and / or carbon in the sample is carried out for several time intervals of the measured spectrum; the reliability of detection of substances containing nitrogen or carbon is determined by comparing the values of the relative amounts of nitrogen and carbon in the sample, calculated for different time intervals. 2. The method according to claim 1, characterized in that the time delay of recording the time spectrum of the secondary radiation is several milliseconds. 3. The method according to claim 1, characterized in that the duration of the time interval in which the time spectrum of the secondary radiation is recorded is comparable to the half-lives of the nitrogen-12 and boron-12 isotopes.

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