RU2710606C1 - Method for remote detection of radioactive substances in field conditions - Google Patents

Abstract

FIELD: measurement technology.SUBSTANCE: invention relates to measurement of ionizing radiation and a method for remote detection of radioactive substances in field conditions based on double-beam laser-induced breakdown of air. Method involves exposure of the investigated area with two lasers, detection of a signal reflected from the ionized region. Photon-ionizing laser is a compact fiber erbium laser. As probing, powerful focusing laser for avalanche breakdown of air pulsed CO-laser is used. To detect a laser beam reflected from an ionized region formed near radioactive substances, a compact non-inertial infrared photodetector is used based on the photon drag effect with a time constant of less than 1 ns. Reflected laser beam is used to measure the delay time of air breakdown, which depends on the level of radiation.EFFECT: reduction of power consumption and weight and dimension parameters of equipment, improvement of safety, reliability and speed of remote probing.1 cl, 3 dwg

Description

The invention relates to techniques for measuring ionizing radiation and can be used for remote sensing and detection of radioactive substances in the field.

In the case of industrial accidents associated with environmental pollution by radioactive materials, it is necessary to minimize the time of detection and localization of places of radioactive contamination. There are methods and devices for direct and indirect measurements of ionizing radiation (alpha particles, beta electrons, neutrons and gamma rays). Direct measurements are more accurate and reliable. But they are dangerous to the health of personnel and can lead to contamination of devices. In addition, direct measurements using radiation reconnaissance devices are time consuming. The range of high-energy particles with energies of the order of several MeV in the atmosphere is approximately equal to several centimeters for alpha particles, several meters for beta electrons, several tens and hundreds of meters for neutrons and gamma rays. Thus, remotely direct methods can detect only high-energy neutrons and gamma rays. Indirect methods can be divided into passive and active. Passive methods are based on the effect of radioluminescence of the air near radioactive substances. Photodetectors detect the excess characteristic ultraviolet (UV) radiation of nitrogen molecules. Active methods use either laser-induced fluorescence or laser-induced breakdown of the air near radioactive sources. At the same time, lasers (masers) can operate in various spectral ranges: from UV to infrared (IR) and sub-hertz. Methods of remote detection of radioactive substances can significantly reduce the time of detection of the most dangerous sources of radioactive contamination in large areas.

Known passive method for the remote detection of radioactive sources by induced UV radioluminescence (US 7317191 B1, "Standoff radiation imaging detector", IPC G01J 1/42, publ. 08.01.2008) [1]. This method uses the Maksutov telescopic system with mirror-lens optical elements, a set of 6 UV cameras on charge-coupled devices, and a personal computer. The experiments confirmed the possibility of remote detection of radioactive sources in the presence of a weak night background in winter (moonlight and diffused light from street lighting on a snowy surface). The disadvantages of this method include the bulkiness of the optical system (122 kg, including a tripod, cardan unit, computer, power supplies), sensitivity to vibrations, and the difficulty of estimating the mass (activity) of a radiation source.

There is an active method of remote detection of radioactive sources, based on laser-induced fluorescence (US 8890077 B2, "Remote detection of radiation", IPC G01T 1/205, publ. 11/18/2014) [2]. This method uses lidar technology. The lidar includes a pulsed laser, a telescopic receiver, a computer unit. A laser beam is directed to an ionized region (plasma) near a radiation source and induces UV fluorescence of nitrogen ions. The beam reflected from the plasma makes it possible to estimate the ion concentration, which is associated with the activity of the source. This method allows you to detect radioactive sources at significant distances (up to several km depending on weather conditions). The disadvantages include the difficulty of alignment of the optical system, high power consumption, low accuracy of determining the activity of sources.

Closest to the claimed is an active method for the remote detection of radioactive sources, based on laser-induced breakdown of air (US 20160377761 A1, "Active remote detection of radioactivity based of electromagnetic signatures", G01V 8/00, publ. 12.29.2016) [3] . The system includes a powerful pulsed photo-ionizing laser (based on Nd: YAG, a wavelength of 1.06 μm, a peak intensity of 160 GW / cm ² , a pulse duration of 1 ns), a source of probing sub-terahertz radiation (frequency 90-110 GHz), spectrometer for measuring the frequency modulation of the reflected sub terahertz beam. A high-power laser causes an avalanche breakdown of air near a radioactive source, a sub-terahertz beam is also directed to an area near the source. The spectrometer measures the frequency of the reflected beam, which is modulated due to ionization near the source. This method allows you to detect and evaluate the mass of a radioactive source at distances up to several hundred meters, if its composition is known (specific activity). The disadvantages include the dependence on weather conditions, the complexity of spectrometric measurements, high energy consumption, and a laser wavelength (1.06 μm) that is hazardous to the eyes.

The objective of the invention is the development of a safe method for the remote detection of radioactive substances in the field based on a two-beam laser-induced breakdown of air, which allows to provide a technical result, which consists in reducing energy consumption and weight and size parameters, increasing safety, reliability and speed.

The essence of the invention consists in the development of a safe method for the remote detection of radioactive substances in the field based on double-beam laser-induced breakdown of air. First, it is proposed to use a compact erbium fiber laser at a wavelength of 1.55 μm as a photo-ionizing laser. Compared with a 1.06 micron Nd: YAG laser, its advantages are: compactness and lower power consumption; eye-safe wavelength; less likely multiphoton ionization, which increases the reliability of measurements due to a decrease in the probability of a false signal; the possibility of using flexible fiber waveguides, which leads to a decrease in weight and size parameters and energy consumption. Secondly, it is proposed to use a powerful pulsed CO ₂ laser at a wavelength of 10.6 μm for avalanche breakdown of air, as well as for probing. Thirdly, to detect a laser beam (10.6 μm) reflected from the ionized region, it is proposed to use a compact inertialess IR photodetector based on the photon drag effect with a time constant of less than 1 ns, which does not require cooling, which increases the speed of the method and also leads to reduce energy consumption and weight and size parameters. Both lasers (1.55 μm, 10.6 μm), as well as the photodetector, are commercial devices and are affordable. If we compare the powerful focusing lasers generating an avalanche breakdown: Nd: YAG (1.06 μm) and CO ₂ (10.6 μm), then the first has an advantage in terms of radiation directivity (diffraction divergence), while the threshold power of the second , which is proportional to the square of the frequency, two orders of magnitude lower, which greatly reduces energy consumption.

The method is illustrated by the following graphic materials:

FIG. 1. Scheme of double-beam laser detection of radionuclides.

FIG. 2. Time dependences of the electron concentration at different values of the ionization enhancement factor α _rad (1 - 0 and 2 - 10 ⁴ ).

FIG. 3. Dependences of the breakdown delay time on the ionization enhancement factor for different intensities of the photo-ionizing laser (1 - 10 ⁶ , 2 - 10 ⁵ , 3 - 10 ⁴ W / cm ² ).

In FIG. 1 shows a two-beam laser detection circuit. Radionuclides 4 emit alpha particles, beta electrons, neutrons or gamma rays, which ionize the surrounding air 5, creating high-energy electrons, which in turn relax through a cascade process, creating low-energy (thermal) electrons. The latter quickly attach to oxygen molecules, forming negative ions. At an average terrestrial level of radiation, the concentration of free electrons is much lower than the concentration of oxygen ions. Near radiation sources, the concentration of free electrons increases. The ionization potential of oxygen ions (electron affinity) is 0.46 eV, which is lower than the quantum energy of a photo-ionizing laser. The rate of photoionization of oxygen ions is proportional to the intensity of the laser beam at a wavelength of 1.55 μm (that is, at a photon energy of 0.8 eV) ν _photo [s ^-1 ] ≈1.4I _photo [W / cm ² ] [4]. When the photo-ionizing laser 2 is turned on, the electron concentration increases until it reaches a new equilibrium value. The resulting equilibrium electron density depends on the level of radiation and laser intensity. For avalanche breakdown, a powerful CO ₂ laser 1 is used, which generates pulses with an intensity of up to 10 ¹⁰ W / cm ² and a duration of 10-100 ns. Avalanche breakdown occurs when the intensity of the laser beam exceeds the threshold intensity I _BD . In this case, free electrons are accelerated and ionize the molecules at a speed exceeding the rate of attachment (formation of ions). The threshold laser intensity is estimated by equating the effective laser electric field (more precisely, the intensity of this field) to the breakdown constant field E _BD = 35 kV / cm. The laser intensity is I ₀ = cE ₀ ² / 8π, in practical units I _o [W / cm ² ] = 1.33 × 10 ³ E _o ² [V / cm] [5]. The effective laser field is E _eff = (1 + ω ² / ν _e ) ^-1/2 E ₀ , where ν _e = ν _en + ν _ei is the frequency of electron collisions. Equating E _BD = E _eff , the threshold intensity I _BD ≈ (s / 2π) (ω ² / ν _e ² ) E _BD ^{2 is} obtained under the condition ω >> ν _e . Assuming that the collision frequency is mainly determined by collisions of electrons with neutral molecules ν _en [с ^-1 ] = 10 ^-7 N _n T _e ^1/2 [eV], where N _n = 2.7 × 10 ¹⁹ cm ^-3 is the number Loshmidt, T _e - electron temperature, get the formula for estimating the threshold intensity I _BD [W / cm ² ] ≈1.63 × 10 ⁶ ω ² / ν _e ² [5]. In particular, for a CO ₂ laser (ω = 1.8 × 10 ¹⁴ s ^-1 ) I _BD [W / cm ² ] ≈7 × 10 ⁹ / T _e [eV]. To initiate breakdown, some electrons must have an energy exceeding the ionization potential of air molecules (15 eV for nitrogen, 12 eV for oxygen). Since these are high-energy tail electrons of the distribution function, the electron temperature is approximately equal to T _e = 5 eV. Then, the threshold intensity of the CO ₂ laser is 1.4 × 10 ⁹ W / cm ² , which is consistent with experimental data [5]. To initiate an avalanche breakdown, at least one free electron must be in the laser volume with a length sufficient for impact ionization of the neutral molecule and the creation of another electron that will continue the process. Breakdown occurs when the rate of impact ionization begins to exceed the rate of attachment of electrons to molecules. These speeds coincide at an intensity I _BD = 4 × 10 ⁹ W / cm ² , which corresponds to an effective electric field E _eff = 32 kV / cm. The time required for breakdown τ _b is the sum of the statistical delay time τ _s and the formation delay time τ _f , i.e., τ _b = τ _s + τ _f . The statistical delay time is the time of the appearance of an electron in the optical volume, in which the laser intensity exceeds the threshold. Formation delay time is the time during which the electron density reaches a critical value. It is estimated under the assumption that the electron density increases exponentially with the velocity ν _ion ≈ν _coll -η, i.e., N _e (t) ≈N _e (0) exp (ν _ion t), where N _e (0) is the initial electron density . The formation delay time is determined from the equality N _e (τ _f ) = N _crit , that is, τ _f ≈ (1 / ν _ion ) ln (N _crit / N _e (0). For laser-induced breakdown, the critical density is the electron density, for which the electron plasma frequency is equal to the laser frequency. When the electron density reaches a critical value, the laser beam reflected from the plasma. The reflected beam may be observed near a laser and provides a measure of the total time delay breakdown τ _b. The proposed delay time detection method of forming a signature presence . I radionuclides breakdown time determined by the parameters of the laser pulse:. peak intensity and shape of Figure 2 shows the time dependence of the electron density for different values of the gain factor ionization α _rad (0 and 10 ⁴⁾ obtained by numerical methods In both cases first a photoionization by the laser.. at a wavelength of 1.55 μm, an intensity of 10 ⁵ W / cm ² , a pulse duration of 10 ns, Then turn on a powerful CO ₂ laser with a square profile and an intensity of 4 × 10 ⁹ W / cm ² . The difference in breakdown time is approximately 2 ns and is a measurable radioactivity signature. For its measurement, a compact inertialess IR photodetector 3 is used based on the photon drag effect, the operation of which is synchronized with the operation of a CO ₂ laser. This effect is due to the transfer of a photon momentum to free electrons or holes in a doped semiconductor. The transfer of momentum leads to the transfer of carriers in the direction of light propagation. As a result, between the contacts on the side of the sample through which exposure is produced, and on the opposite side there is a potential difference - emf photon hobby. Its value depends on carrier mobility, pulse relaxation time, laser intensity, absorption and reflection coefficients, and light frequency. For example, with an intensity of 10 ⁵ W / cm ² , a mobility of 10 ³ cm ² / (V × s), an absorption coefficient of 1 cm ^-1 , a reflection coefficient of 0.3 emf is about 1 mV for a CO ₂ laser [6]. The time constant is less than 1 ns (determined by the pulse relaxation time - 1 ps).

Based on the measurement of the difference in breakdown times, the ionization enhancement factor near the probed source and its mass are estimated. In FIG. Figure 3 shows the dependences of the breakdown delay time on the ionization enhancement factor for different intensities of the photo-ionizing laser (1 - 10 ⁶ , 2 - 10 ⁵ , 3 - 10 ⁴ W / cm ² ). The ionization factor is proportional to the mass of the radioactive material M and the ratio of the average energy of the primary electrons created by gamma rays <E> to the energy required to create one pair of secondary electron and ion, ΔЕ. The latter is approximately equal to 35 eV, while <E> = 0.44 MeV. The dependence of the density of gamma rays on the distance R from the radiation source is determined by the formula n _γ = AMexp (-R / L _γ ) / 2πR ² s [4]. Here A is the specific radioactivity (for Co-60 it is 1.1 × 10 ³ Ci / g, 1 Ci = 3.7 × 10 ¹⁰ decays / s), M is the mass, Lγ is the range of gamma rays in air. Thus, the ionization factor is α _RM Q≈AM <E _e > exp (-R / L _γ ) / 2πL _{γ, a} ΔER ² . Q is the ionization rate due to background radiation at sea level, approximately equal to 20 pairs / cm ³ s. For example, at a distance of 4 m from a source containing 10 mg of Co-60, the ionization enhancement factor is α _RM = 2.2 × 10 ⁴ . Then the minimum detectable mass is estimated by the formula M (g) ≥4πL _{γ, and} R ² ΔEν _i exp (R / L _γ ) / AVτ <E> ν _{i, eff} . V is the plasma volume, τ is the pulse duration. It should be noted the difference between the range of gamma rays in air, determined by the total interaction cross section σ _T due to Compton scattering, Lγ = 1 / n _a σ _T , from the path determined by the average absorption cross section σ _a responsible for the generation of free electrons, L _{γ, a} = 1 / n _a σ _a . For gamma rays with an energy of 1 MeV, these distances are 130 and 280 m, respectively.

Let us evaluate the detected mass of the Co ⁶⁰ radioactive source with characteristic parameters: plasma volume 2 cm ³ , pulse duration 100 ns, ratio of ionization frequencies 0.82, gamma-ray energy 1 MeV. We get that at a distance of 20 m you can find 2 g of a radioactive substance. If the source is in a lead container with a transmittance of 1%, the minimum detectable mass is 200 g.

The technical result of the invention is to reduce energy consumption and weight and size parameters, increasing safety, reliability and speed due to the use of a compact fiber erbium laser operating at an eye-safe wavelength, as well as a compact inertialess infrared photodetector based on the effect of photon drag.

The technical problem to be solved is the prompt and safe remote detection of radioactive substances in the field. Technical implementation is possible through the use of commercial lasers and an infrared photodetector.

The technical result is achieved by providing a combination of all the essential features of the invention.

List of references

1) US 7317191 B1, "Standoff radiation imaging detector", IPC G01J 1/42, publ. 01/08/2008.

2) US 8890077 B2, "Remote detection of radiation", IPC G01T 1/205, publ. 11/18/2014.

3) US 20160377761 A1, "Active remote detection of radioactivity based of electromagnetic signatures", G01V 8/00, publ. 12/29/2016.

4) P. Sprangle, et al, Active Remote Detection of Radioactivity Based on Electromagnetic Signatures, Physics of Plasmas, 21, 013103, 2014.

5) J. Isaacs, C. Miao, P. Sprangle, Remote monostatic detection of radioactive material by laser-induced breakdown, Physics of Plasmas 23, 033507, 2016.

6) A.H. Pikhtin, “Physical Foundations of Quantum Electronics and Optoelectronics”, M., Higher School, 1983.

Claims (1)

A method for remote detection of radioactive substances in the field based on a two-beam laser-induced breakdown of air, including irradiating the area under study with two lasers, detecting a signal reflected from the ionized region, characterized in that a compact erbium laser is used as the first photo-ionizing laser, as a second probe, focusing a powerful laser to avalanche breakdown of air using pulsed CO ₂ laser, for detecting the laser beam on reflection from the ionized region formed near the radioactive substances used freewheeling compact infrared light detector based on photon entrainment effect with a time constant of less than 1 ns, while using the reflected laser beam is measured delay breakdown of air, which depends on the level of radiation.

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