RU2293999C1 - Method of detection and measurement of weak flows of ionizing radiations - Google Patents

Abstract

FIELD: radiation inspection of objects.

SUBSTANCE: method can be used for detection and/or measurement of weak flows of photon and/or corpuscular ionizing radiations. Method of detection and measurement of weak flows of ionizing radiations is based upon measurement of instant speed f_i of pulse count and its second derivative f_i(2) by means of digital filtration of results of registration by i-th channel from n≥2 on base of similar detectors, disposed close to each other in plane being perpendicular to direction of search and measurement. It is also based upon formation of two thresholds of detection, which have to be upper limits of meanings of amplitude and duration of informative part of function √/ⁿΠ_i=1fi₍₂₎/ achieved at the absence of radiation objects inside inspected space. Decision on detection of weak flow of ionizing radiation is made in case if one or both thresholds are prevailed by current meanings of mentioned parameters. Sum of instant meanings is taken as result for n channels deducted from similar sum, calculated at absence of radiation objects inside inspected space.

EFFECT: increased sensitivity; higher truth of detection of weak flows of ionizing radiations; reduced repetition of false operations of radiation inspection equipment.

3 cl, 3 dwg, 1 tbl

Description

The invention relates to the field of radiation monitoring using, for example, ionization, scintillation, semiconductor detectors and is intended to detect and / or measure weak fluxes of photon and / or corpuscular ionizing radiation with an intensity, including substantially lower background, from moving and stationary objects of radiation control. In this case, the moving speed of moving objects can reach technical limits for vehicles.

The inventive method can be used in radiation monitors of nuclear and other radioactive materials, in other indicators of ionizing radiation, in appropriate measuring instruments, such as radiometers, dosimeters, spectrometers, for solving various problems of radiation monitoring, including cross-border movement of goods, vehicles, other objects of foreign economic activity, in industrial and agricultural production, in trade and banking, in medicine and ecology, in science and education, associated with the detection and / or measurement of the fluxes of such emissions, including relatively low intensity.

A known method for detecting weak flows of ionizing radiation used in stationary radiation monitoring equipment of the Yantar type of various modifications (see the Yantar-1A Stationary Customs System for the Detection of Fissile and Radioactive Materials). Technical description and operating instructions. DKTS. 425713.004 TH. G Dubna, 2000). It is based on evaluating the statistical parameters of the natural background radiation and ionizing radiation from the control object.

The specified method provides the following order of operations. Before starting the monitoring of objects moving through the target racks (racks with a limiter of the width of the controlled space) with gamma and neutron radiation detectors, for M adjacent time intervals with a total duration t _b , a sample of samples representing a series of N _i pulses of background ionizing radiation in i m (i = 1, 2, ..., M) interval. Such a sample is used to obtain an estimate of the average value of N _{from the} count of background pulses:

and variance:

After switching by the signal of the presence sensor to the monitoring mode of the object that triggered the operation of such a sensor, the pulse count N _{io is} recorded in each of a similar series of adjacent time intervals and compared with threshold values calculated on the basis of the last background estimate from the expression:

where L _j is the coefficient of tuning of radiation monitoring equipment to the required frequency of false positives;

j = 1, 2, 3.

The comparison procedure with the indicated thresholds is also applied to measurements immediately preceding the inclusion of the presence detector and to measurements following its switching off.

If N _{io is} greater than any of the three q values obtained using expression (3), then a decision is made to detect an increased flux of ionizing radiation with the subsequent formation of an alarm (light and / or sound). Otherwise, a decision is made that there is no cause for alarm.

The disadvantage of the presented method is the receipt and use of incomplete information about the recorded fluxes of ionizing radiation, leading to an overestimation of the thresholds for their detection, the frequency of false positives, and errors in measuring the intensity of such flows. In particular, this method practically does not provide for the receipt and use of information about the time intervals between adjacent pulses arising in the measuring channels, for the proper implementation of the joint processing of data obtained using the same type of detectors available in each rack of the Yantar equipment.

There is also a detection method implemented in the search device PM-1401 (see Search dosimeter microprocessor-based DRS-PM1401. Technical description and operating instructions. - Minsk: JV Polimaster. 1997). It includes the following operations. Before starting work with the desired objects, the count N _{b of} background pulses is recorded for t _{b =} 36 s. Then the device automatically switches to the operational control mode and performs the registration of the pulse count N _o for subsequent adjacent time intervals of duration t _n = 2 s each. The threshold of the device is calculated using the expression:

Where

- среднеквадратическое отклонение величины N_b;

- standard deviation of the value of N _b ;

m is the coefficient of multiplicity for σ (usually m≥4).

The main disadvantage of the presented method, as well as the previous one, is the incomplete obtaining and use of technically quite accessible information about the recorded photon-ionizing radiation fluxes, which also leads to an overestimation of their detection thresholds, false alarm rates, and errors in measuring the intensity of such fluxes.

The closest adopted for the prototype is the "Method for detecting weak flows of ionizing radiation" (see patent for the invention No. 2140660 C1, IPC 6 G 01 T 1/167, publication date 10/27/1999, application No. 98102583/06 of 02/10/1998 ) It provides for the following operations.

Before starting the monitoring of objects, the threshold q ₀ is determined in accordance with the operator-defined probability of false alarms in the tables for normal distribution. Then measure the average count rate b, due to background ionizing radiation. In this case, the background measurement time t _b can be arbitrarily large and is determined by the operational environment or is set by the operator. The radiation control device, in which the described method is implemented, is switched to the operational control mode, thereby ensuring the measurement of the number of radiation pulses N _o from the controlled object during the monitoring interval t _n specified by the operator. Next, calculate the average count rate of the additive mixture of background ionizing radiation and radiation of the control object

determine the value of the parameter

The obtained value of η is compared with the threshold q ₀ . If η> q ₀ , then a decision is made about the detection (the alarm signal is turned on, which can be light, sound, etc.). Otherwise, a decision is made about non-detection (absence of source).

The known method also does not allow to detect and to measure with the required accuracy the intensity of weak flows of ionizing radiation due to the orientation to the only measuring channel of the corresponding equipment and the non-use of the possibility of obtaining and joint processing of information about time intervals between adjacent pulses in the channels, which inevitably lead to an overestimation of the frequency of false alarms, thresholds detection and measurement errors of the intensity of such flows.

The objectives of the invention are to increase the sensitivity, efficiency and reliability of the detection of weak flows of ionizing radiation, reduce the frequency of false alarms of radiation monitoring equipment and measurement errors of the intensity of such flows, expand the range of acceptable values of the speed of movement of objects under control through the controlled space, reduce the actual cost of radiation control.

цифровой фильтрацией текущей серии результатов измерения их времени появления либо значений счета в смежных интервалах экспозиции в i-м канале из числа n не менее двух на основе однотипных детекторов, размещаемых рядом друг относительно друга в плоскости, перпендикулярной направлению поиска и измерения, формируют два порога обнаружения, представляющие собой верхние пределы значений амплитуды и длительности информативной части функцииA method is proposed for detecting and measuring weak fluxes of ionizing radiation using, for example, ionization, scintillation, and semiconductor detectors, which consists in determining the pulse count rate in the measuring channel, in forming a detection threshold and deciding on the detection result, characterized in that the instantaneous speed f _i pulse counting and its second derivative

by digital filtering of the current series of results of measuring their appearance time or counting values in adjacent exposure intervals in the i-th channel from at least two n based on the same type of detectors placed next to each other in a plane perpendicular to the direction of search and measurement, two detection thresholds are formed representing the upper limits of the amplitude and duration of the informative part of the function

obtained in the absence of radiation objects in a controlled space. In this case, the decision to detect a weak ionizing radiation flux is made if one or both thresholds is exceeded by the current values of these parameters. As a result of measuring its counting rate, take the sum of its instantaneous values over n channels minus a similar amount calculated in the absence of radiation objects in the controlled space.

An increase in the sensitivity of detecting a weak flow of ionizing radiation, the accuracy of determining its intensity, and thereby reducing the actual cost of radiation monitoring, is achieved by digitally filtering the results of pulse recording using non-recursive filters formed on the basis of exact solutions of systems of equations of analytical functions.

Increasing the efficiency, expanding the range of acceptable values of the speed of movement of objects of control through the controlled space is facilitated by the measurement, registration and accounting of the time of appearance of each pulse in the channel or by reducing exposure to values corresponding to the systematic manifestation of time intervals between successively recorded pulses.

An increase in the reliability of detection of weak flows of ionizing radiation, an effective reduction in the frequency of false alarms is ensured by the simultaneous registration of such flows through at least two measuring channels based on the same type of detectors, and obtaining and using the values of function (7).

When using the invention, the type and target parameters of the detectors are selected taking into account the data presented in the table. Compared with the prototype, the achievement of the specified signal-to-background ratios for the probability of detection of 0.5 with a confidence level of at least 95% is achieved by registering a significantly smaller number of background pulses required for this. The multiplicity factor of such a decrease may be more than four. At the same time, a reduction in the frequency of false alarms of monitors to values of less than one per thousand passes (passages) and / or no more than one false alarm in eight hours of continuous operation of monitors is also achieved.

Reducing the error in determining the intensity of a weak flow of ionizing radiation compared with the prototype for identical conditions can be more than one and a half times.

The proposed method for detecting and measuring weak fluxes of ionizing radiation is as follows.

The initial value is set to the minimum length of stay of objects of control in a controlled space. For this space, in the absence of objects of control for a time exceeding the above value and meeting the requirement for the total error of the results of measuring the intensity of the weakest of the expected and detected flows of ionizing radiation, estimates of the background count rate of the used detectors and the upper limits of the amplitude and duration of the informative part of the function are obtained (7) through digital filters with time constants numerically equal to the duration of the interval of the actual measurement of pho ovogo account and the expected minimum length of stay of control objects in a monitored space, respectively. The values obtained in this way are used in the implementation of radiation monitoring of objects falling into the controlled space. If the proposed method detects a weak flow of ionizing radiation from an object, its intensity is measured using a digital filter, the time constant of which is numerically equal to the time the object actually was in the control zone.

To clarify the proposed method, the following drawings can serve as the basis for which are measurement results made using the URK-01F “Strizh” instrument (see the State Register of Measuring Instruments of the Russian Federation. Measuring Instrument No. 24125-02):

figure 1 - the results of the registration of ionizing radiation by measuring channel No. 1 of the device URK-01F "Strizh";

figure 2 - the results of the registration of ionizing radiation by measuring channel No. 2 of the device URK-01F "Strizh";

figure 3 - the results of the detection of a weak flow of ionizing radiation with the device URK-01F "Strizh".

Figures 1 and 2 show the initial data obtained by recording the ionizing radiation of the background and the radionuclide source based on California-252 with an activity of not more than 3 kBq by two (No. 1 and No. 2, respectively) from the twelve measuring channels of the above device for a time exceeding the time finding this source in a device-controlled space.

Figure 3 presents the results of detecting a weak flow of ionizing radiation, indicated in figures 1 and 2, the proposed method for the time interval noted there. It follows from them that the joint use of data obtained even by a part of the measuring channels of the indicated device, thus allows one to almost completely neutralize possible false signals due to background fluctuations.

The revealed features of the invention also give reason to conclude that similar results are obtained when moving to significantly lower compared with the exposure values shown in figures 1 and 2. In such cases, almost every pulse used to form function (7) in FIG. 3 is separated from the neighboring ones by a different number of “empty” intervals, each of which is numerically equal to the exposure.

The use of the invention allows effective radiation monitoring, including of vehicles crossing a controlled space, for example, at a speed of 110 km / h, i.e. with the duration of their stay in the control zone of not more than 0.1 s. And in such a situation, in order to detect a weak flow of ionizing radiation from such objects at the location of the monitor detectors, it is necessary to use at least two detectors of the same type, each of which during the specified period of time allows you to receive and register at least 1 background pulse (10 pulses / from). This makes it possible with a probability of 0.5 for a confidence probability of at least 95% for the indicated time interval to detect two times weaker than the background flow of ionizing radiation by the monitor detectors from the corresponding vehicle passing by them.

If, however, to use detectors in such a monitor with a sensitive area volume that provides an increase in the count of background pulses, for example, six times (i.e., 60 pulses / s) compared with that indicated above, this makes it possible (see table) to reduce the minimum the detected signal-to-background ratio in the presented example to a value of 0.2.

The use of the invention is possible for solving other tasks of radiation control.

For example, in medicine, when conducting diagnostic operations using radiopharmaceuticals, the use of the invention allows, while maintaining the achieved level of error in the results of such operations, to significantly reduce the activity levels of doses of such drugs administered to patients, with a corresponding decrease in the radiation load on them, as well as on the medical staff. A similar reduction in the radiation load on the human body is achieved by using the invention in medical x-ray equipment with digital recording of the information received.

The use of the invention is also possible in the implementation of environmental measures, geological survey and the like, for example, by means of airborne gamma surveys of the underlying territory. This allows, while maintaining the achieved sensitivity level of the applied detectors, to several times increase the speed of carrier movement of the corresponding radiation monitoring equipment. When performing such work in compliance with the previously established standards for the speed of such carriers, a similar increase in the sensitivity of detection of radiation objects located either on the surface of the underlying territory or under it is provided.

Thus, the proposed method allows, in comparison with the prototype, to significantly reduce the thresholds for detecting weak fluxes of ionizing radiation, the error in determining their intensity, the frequency of false alarms of the used radiation monitoring equipment, to expand the range of acceptable values of the speed of movement of objects of control through the controlled space, to reduce the actual cost of radiation control .

Table
Correspondence between the minimum detectable signal / background ratio with a detection probability of 0.5 for a reliability of at least 95% and the minimum number of background pulses in the detector during the monitoring Minimum detectable signal to background ratio The minimum number of background pulses in the detector during the monitoring, imp. 0.5 one 0.2 6 0.1 twenty 0.05 80

Claims (3)

1. A method for detecting and measuring weak fluxes of ionizing radiation using, for example, ionization, scintillation, semiconductor detectors, which consists in determining the pulse count rate in the measuring channel, in forming a detection threshold and deciding on a detection result, characterized in that the instantaneous speed is determined f _i pulse counts and its second derivative

by digital filtering of the results of their recording by the i-th channel from among n at least two based on the same type of detectors placed next to each other in a plane perpendicular to the direction of search and measurement, two detection thresholds are formed, which are the upper limits of the amplitude and duration of the informative part of the function

obtained in the absence of radiation objects in the controlled space, the decision to detect a weak ionizing radiation flux is made if one or both thresholds is exceeded by the current values of the specified parameters, and the sum of its instantaneous values over n channels minus the like the amount calculated in the absence of radiation objects in the controlled space. 2. The method for detecting and measuring weak flows of ionizing radiation according to claim 1, characterized in that the current series of results of measuring the time of occurrence of adjacent pulses is subjected to digital filtering. 3. The method for detecting and measuring weak fluxes of ionizing radiation according to claim 1, characterized in that the current series of samples recorded in adjacent exposure intervals are digitally filtered.

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