RU2364890C1 - Method for detection of nuclear materials and radioactive substances - Google Patents

Abstract

FIELD: physics, nuclear physics.

SUBSTANCE: invention is related to the field of radioactive substances and nuclear materials detection in case of their unauthorised transportation by individuals through controlled space. Method includes usage of pedestrian radiation monitor of gamma-radiation, and also radiation channel with scintillating detection units for detection of nuclear materials and radioactive substances, metal detector for detection of metal objects, presence sensor for detection of human being presence in controlled space, electronic processing unit and control and indication panel, which differs by the fact that prior to beginning of objects control, initial detection threshold T₀ of metal detector is introduced from control and indication panel, which corresponds to selected threshold mass of searched metal object, and initial values of counting speed from background radiation n_ok, registered by each of scintillating detection units, then for each scintillating detection unit in sliding time window of fixed duration, current counting speeds are identified from background radiation n_br, current values of parametre are calculated T_K=T₀-(n_ok/n_br)^0.5, and minimum of them is accepted as current threshold T_c for metal detector detection. Current value of amplitude of signal U_m, registered by metal detector, is compared to T_c, if U_m>T_c, then decision is made on detection of searched metal object, otherwise decision is made on non-detection.

EFFECT: expansion of application field of radiation monitor of gamma-radiation and improvement of its alarm reliability.

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Description

The invention - a method for detecting nuclear materials and radioactive substances - relates to the field of environmental protection, and more specifically to the field of registration of radioactive emissions of nuclear materials and radiation hazardous substances, and most effectively it can be used to register radioactive emissions when moving through a controlled space using various objects (for example, by man) of nuclear materials and radiation hazardous substances, as well as for monitoring the radiation environment.

A device for detecting radioactive nuclear materials (see. Description of the invention to the patent of the Russian Federation No. 2129289), unauthorized moving through checkpoints and checkpoints of customs and other objects of national economic purpose.

The known device includes a gamma radiation detection unit, an object presence sensor in a controlled area, a neutron radiation detection unit, an autopsy sensor, an information processing controller, an alarm unit, an electrical power unit and a control panel.

This device has a reduced threshold sensitivity of the neutron radiation detection unit relative to the gamma radiation detection unit, which is caused by fundamental physical processes accompanying the decay of nuclear material. So, nuclear material of ²³⁹ Pu with a mass of 1 g as a source of gamma radiation and neutron flux is equivalent in gamma radiation to a source of ¹³⁷ Cs with an activity of 170 kBq, and for neutron radiation is equivalent to a source of ²⁵² Cf with a neutron flux of 0.60 × 10 ² c ^-1 (see GOST R 51635-2000, p. 28-29), i.e. the number of gamma rays emitted by it per unit time exceeds more than 1000 times the number of emitted neutrons. In this regard, the detection thresholds of the gamma-ray and neutron detection units of the device under consideration are very different from each other. This feature of radiation monitors of nuclear materials is reflected in GOST R 51635-2000 (p.4-5), where, for example, it is indicated that if the detection threshold of a pedestrian monitor by gamma radiation is 1 g ²³⁹ Pu, then by neutron radiation (according to GOST R 51635-2000, pp. 4-5) it is enough that it is equal to 250 g.

In this regard, the known device has the disadvantage that its method does not allow detecting nuclear material whose mass is less than the detection threshold of the neutron radiation detection unit and which moves in a metal container (case), for example, steel or lead, screening its gamma -radiation.

Known portal radiation monitor (see the description of the invention to the patent of the Russian Federation No. 2191408), designed for registration and identification of radionuclides when moving through the monitor using various objects of nuclear materials and radiation hazardous substances, as well as for environmental control.

The well-known portal radiation monitor includes a two-rack portal with located object detection sensors, scintillation detectors and spectrometric amplifiers, as well as an electronic signal processing system, a light and sound alarm unit. Each scintillation detector consists of a plastic scintillator connected to a light to electric converter, equipped with a high-voltage power source. A plastic scintillator is a rectangular block of not less than 10,000 cm ³ , one of the ends of which is made flat and the other in the form of a tapering pyramid, turning its upper part into a cylinder.

The known portal radiation monitor has the disadvantage that the method used by it does not allow detecting nuclear materials transported in metal containers (cases) that shield gamma radiation.

The closest in technical essence to the claimed method is used in a device for detecting nuclear materials during unauthorized movement of individuals through a controlled space (see patent application RU 96102644).

The known device includes blocks for detecting gamma radiation, indicators of the presence of persons in a controlled space, equipment for processing information received from the blocks for detecting gamma radiation, and generating a signal about exceeding a set level of gamma radiation, as well as a metal detector for generating a signal about exceeding a set mass of moving through a controlled space of metal objects.

The device operates as follows. After the device is connected to the network and a software check of its serviceability begins, the simultaneous measurement of the background by all gamma radiation detection units begins. The measurement is made at consecutive time intervals of duration T _f . After each measurement, a moving average is calculated for the count rate of the pulses from the background radiation for each detection unit — n _fc and their total — n _fs .

When a person enters the arch of the device, the background measurement is interrupted and the measurement by all gamma-ray detection units of the "effect" begins - the number of pulses N _eq coming from each detection unit. When a person leaves the arch of the device, the measurement of the number of pulses N _ek stops and the background measurement resumes.

Further, the processing equipment according to a previously entered program performs automatic calculation:

- the time spent by the person in a controlled space t _e

- the sum of the number of pulses from all detection units N _es ;

- threshold values of the number of effect pulses corresponding to the established values of the probability of correct detection P = 0.95 and the probability of false alarm P _lt = 0.0001 -

N _pc = n _fk · t _e + 3.9 · (n _fk · t _e ) ^0.5 +2 for each detection unit,

N _ps = n _fs · t _e + 3.9 · (n _fs · t _e ) ^0.5 +2 for the sum of the readings of all detection units.

After that, the conditions are checked:

N _ec > N _pc

N _es > N _ps .

If at least one of these conditions is fulfilled, an alarm signal is generated and an ALARM message is issued! NUCLEAR MATERIAL! If none of the above conditions is met, then the alarm does not occur.

Simultaneously with the detection of nuclear materials by the gamma radiation emitted by them, a person in a controlled space is subject to monitoring for the presence of metal objects with a metal detector. If the mass of metal objects from the controlled person exceeds the set threshold value, which can be, for example, if the person tries to remove nuclear material shielded from metal from the object’s territory, an alarm is generated and an ALARM message is issued! METAL! If the mass of metal objects from the controlled person does not exceed the established threshold value, then the alarm does not occur.

The disadvantage of the method used by the known device for detecting nuclear materials during unauthorized movement by individuals through a controlled space is the lack of automatic adaptation of the detection threshold of the metal detector depending on the current values of the background radiation detected by the gamma radiation detection units, which reduces the likelihood of detecting nuclear materials transported in metal containers (pencil cases) that shield gamma radiation.

Consider an example. Let the detection threshold of the metal detector be set to 300 g, the current value of the counting rate of pulses from the background radiation n _fk = 64 imp / s, the number of impulses of the effect of a sample of nuclear material from ²³⁵ U is 60 imp / s (N _ek = n _fk + 60 = 124 imp / s) and t _e = 1 s. Then N _pc = 97 pulses / s, the condition N _ec > N _pc is satisfied and nuclear material will be registered.

Now let the current value of the pulse counting rate from the background radiation increase to n _fk = 100 pulses / s, which, for example, can be caused by deliberate actions by covertly placing a special gamma radiation source at a checkpoint. Then N _pc = 141 pulse / s, and N _Ek = n _FC + 60 = 160 pulse / s. In this case, as before, the condition N _ec > N _pc is satisfied and nuclear material will be registered. However, from the same nuclear material, but placed in a steel container weighing less than the detection threshold (for example, 260 g) and with a wall thickness of 8.4 mm, the number of effect pulses will decrease by 1.5 times (see Kimel L.R., Mashkovich V .P. Protection against ionizing radiation. Handbook. Ed. 2. M .: Atomizdat, 1972., p.148, table 4.30, for an energy of ²³⁵ U, which is equal to 0.185 MeV) and will be 40 imp / s. Then we get N _pc = 141 pulse / s, and N _Ek = n _fk + 40 = 140 pulse / s. In this case, the condition N _ek > N _{pc is} no longer fulfilled and the nuclear material will not be registered by the gamma radiation detection unit. It will also not be detected by a metal detector, since the mass of the container is 260 g less than the set detection threshold.

The advantage of the proposed method for the detection of nuclear materials and radioactive substances is to expand the scope and increase its signaling reliability.

These advantages are achieved due to the fact that the inventive method for detecting nuclear materials and radioactive substances involves the use of a pedestrian radiation monitor of gamma radiation containing three physical recording channels (radiation for detecting nuclear materials and radioactive substances, a metal detector for detecting metal objects and a presence detector for detecting human presence in the controlled area), an electronic processing unit and a control and display panel.

The radiation channel contains scintillation detection units equipped with high-voltage power supplies.

The metal detector contains two induction coils, which are flat multi-coil induction frames of a special configuration. One frame is a generator, the second is a reception.

The presence detector contains an infrared emitter and an infrared receiver.

The electronic processing unit consists of an electronic module of the radiation channel, an electronic module of the metal detector, a microprocessor module, an audio module, and a power module.

The control and display panel is an electronic unit that provides the ability to control the operating parameters of the monitor, display current information and issue light and sound actuation signals.

A distinctive feature of the proposed method is that the microprocessor module of the electronic processing unit generates a signal for automatically adjusting the detection threshold of the metal detector depending on the current values of background radiation recorded by the scintillation detection units, and the electronic module of the metal detector is equipped with an input for receiving the specified signal.

The inventive method is illustrated by the drawings shown in figures 1-4.

Figure 1 shows a General view of a pedestrian radiation monitor of gamma radiation in the embodiment with four scintillation detection units.

Figure 2 shows a section along aa in figure 1.

Figure 3 shows a section along BB in figure 1.

Figure 4 presents a block diagram of the proposed device.

The pedestrian gamma radiation monitor consists of a generator panel 1, a receiving panel 2, an upper horizontal panel 3, an electronic processing unit 4 and a control and indication panel 5. The generator panel 1, a receiving panel 2 and an upper horizontal panel 3 constructively form the monitor arch.

The generator panel 1 consists of scintillation detection units 6, a high-voltage power supply 7, a generator frame 8 and an infrared emitter 10.

The receiving panel 2 consists of scintillation detection units 6, a high-voltage power supply 7, a receiving frame 9, and an IR receiver 11.

The electronic processing unit 4 consists of an electronic module of the radiation channel 12, an electronic module of the metal detector 13, a microprocessor module 14, an audio module 15, and a power module 16.

Pedestrian gamma radiation monitor operates as follows.

After applying the mains voltage, the power module 16 and high-voltage power sources 7 begin to generate the voltages necessary for the functioning of all electronic components of the monitor. The microprocessor module 14 monitors the health of the monitor nodes and counts the number of background pulses in a time window of duration T _f for each detection unit 6. After checking and initial background dialing, the monitor indicates that it is ready for operation by lighting the READY indicator on the control and display panel 5. By command from the control panel and display 5 is stored in the memory of the microprocessor module 14 of the primary values of the pulse counting speed for each detection unit 6, which These are used as reference values for the counting speed n _approx . The background calculation is then continued in a sliding time window of duration T _f and moving average values are calculated for the pulse count rate n _f from the background radiation for each detection unit 6. By command from the control and display unit 5, the initial threshold is entered and stored in the microprocessor module 14 detecting a metal detector P ₀ corresponding to a threshold mass of a metal subject of search.

When a controlled person enters the monitor’s arch, the presence sensor is triggered, the background measurement is interrupted, the values of n _{fc are} remembered, and the measurement begins by all gamma-ray detection units of the “effect” - the number of pulses N _eq coming from each detection unit. When a person leaves the arch of the monitor, the measurement of the number of pulses N _ek stops and the background measurement resumes.

Next, the microprocessor module 14 according to a previously entered program performs automatic calculation:

- the time spent by the person in a controlled space t _e

- threshold values of the number of effect pulses corresponding to the established values of the probability of correct detection P = 0.95 and the probability of false alarm P _lt = 0.0001 -

N _pc = n _fk · t _e + 3.9 · (n _fk · t _e ) ^0.5 +2 for each detection unit.

After that, the conditions are checked:

N _ek > N _pc for each detection unit.

If at least one of these conditions is fulfilled, an alarm signal is generated and the ALARM: NUCLEAR MATERIAL message is issued. If none of the above conditions is met, then the alarm does not occur.

Simultaneously with the detection of nuclear materials by the gamma radiation emitted by them, a person in a controlled space is monitored for the presence of metal objects with a metal detector by recording the amplitude of the signal U _m generated by metal objects at its output. In this case, the microprocessor module 14 calculates the current values of the parameter P _k = P ₀ · (n _ok / n _fk ) ^0.5 for each detection unit, the minimum of which is taken as the current detection threshold P _{t of the} metal detector.

After this, the condition U _m > P _{t is} checked, if it is satisfied, which can be, for example, if a person tries to remove nuclear material shielded from metal from the site, an alarm is generated and an ALARM: METAL message is issued. If the mass of metal objects from the controlled person does not exceed the established threshold value, then the alarm does not occur.

Consider the operation of the monitor using the above example.

Let the initial detection threshold of the metal detector be set to P ₀ = 300 g, the reference value of the counting speed n _ok = 64 imp / s, the number of pulses of the effect of a sample of nuclear material from ²³⁵ U is 60 imp / s (N _ec = n _fk + 60 = 124 imp / s) and t _e = 1 s. Now, let the current value of the pulse count rate from the background radiation increase to n _fc = 100 pulses / s and a sample of nuclear material of ²³⁵ U is placed in a steel container weighing 260 g with a wall thickness of 8.4 mm (a steel container in the form of a cylinder has such parameters whose height is equal to the diameter of the base, and inside there is a cavity for accommodating nuclear material, and the cavity has a cylindrical shape with a height of 20 mm and a diameter of the base of 20 mm). The number of effect pulses will decrease by 1.5 times and amount to 40 pulses / s. Then we get

N _pc = 141 pulses / s, and N _ec = n _FC + 40 = 140 pulses / s. In this case, the condition N _ek > N _{pc is} not satisfied and the nuclear material will not be registered by the gamma radiation detection unit.

At the same time, the current value of the detection threshold of the metal detector will be P _k = 240 g and, therefore, the container will be registered by the metal detector, since its mass is 260 g more than the current detection threshold.

Claims (1)

A method for detecting nuclear materials and radioactive substances during unauthorized movement of individuals through a controlled space, including the use of a pedestrian gamma radiation monitor containing a radiation channel with scintillation detection units for detecting nuclear materials and radioactive substances by their gamma radiation, metal detector for detecting metal items, presence sensor for detecting human presence in a controlled space , an electronic processing unit and a control and display panel, characterized in that before starting the monitoring of objects from the control and display panel, an initial detection threshold of the metal detector P ₀ is entered, corresponding to the selected threshold mass of the metal search object, and the initial values of the count rate from the background radiation n _OK , registered by each of the scintillation detection blocks, then for each of the scintillation detection blocks in a moving time window of a fixed duration is determined tsya current values of the counting rate of radiation from background n _FC are computed current parameter value P _K = P ₀ · (n _QA / n _FC) ^{of 0.5} and a minimum one is taken as the current threshold detector detecting P _T, then the current amplitude value the signal U _M detected by the metal detector is compared with P _T , and if U _M > P _T , then a decision is made to detect a metal subject of search, otherwise a decision is made about non-detection.

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