RU2436120C2 - Dynamic radiation monitoring method - Google Patents

Abstract

FIELD: ecology.

SUBSTANCE: dynamic radiation monitoring method involves recording of ionisation radiation at least with one detector, continuous measurement at unit time interval of the number of pulses χ_i of detector, which is proportional to flow of recorded radiation, determination of background value χ_b, which differs by the fact that during the monitoring there constantly determined is background value as

where m - positive number meeting the ratio m≤σ/S, where σ -standard deviation of detector signal from minimum possible background value, S - maximum possible curvature value of background variation, there calculated is difference

the values of which are compared with threshold K being dimensionless constant value during monitoring within 1 to 4, and in case the above difference exceeds threshold K, radioactivity in the monitoring object is supposed, and values χ_i at which the above difference exceeds the threshold are excluded from the process of determining χ_b value.

EFFECT: improving detection reliability of radioactive sources in conditions of quickly changing background.

Description

The invention relates to the field of environmental protection, and more particularly to measuring the radioactivity of objects, and more particularly to methods for detecting radioactive sources in the study area and in moving objects. The method will find the greatest application in radiation monitoring of various territories and objects using radiation detection systems installed on vehicles, such as automobiles, as well as using systems installed at checkpoints, points of reception and processing of secondary raw materials, scrap metal, industrial and household waste.

A known method of radiation monitoring of moving objects, including the registration of ionizing radiation by at least one detector installed in the control zone through which these objects are moved, continuous measurement of the current values of the radiation flux detected by the detector, comparing the said values with a threshold threshold, if exceeded which is judged by the flow of detected radiation about the presence of radioactivity in the controlled objects [1].

The disadvantage of this method is the low detection sensitivity due to the fact that the threshold must be set significantly higher (1.5-2 times) the level of natural background in the control zone due to its possible fluctuations in the control process, caused, for example, precipitation, dust instability of equipment, etc.

The closest technical solution to the present invention is a method of dynamic radiation control, including the registration of ionizing radiation by at least one detector, continuous measurement over a unit time interval of the number of pulses χ _{i of the} detector proportional to the flux of the detected radiation, determining the background value χ _f [1] .

In the prototype, the background χ _f is determined by averaging many values of χ _i in the absence of an object in the control zone. The threshold χ _then exceeds the level of the natural background χ _f by Δχ = mσ, where σ is the standard deviation of the detector signal n = 3-8. In the process of monitoring, the threshold is not a constant value and must be adjusted in accordance with a change in the background level (it is necessary to change the threshold due to a shift in the average background value χ _f and the standard deviation of the detector signal σ, depending on the background level).

The disadvantage of the prototype is the low reliability of the detection of low-intensity radioactive sources in a rapidly changing background radiation. This is due to the inconstancy of the threshold value and a certain inertia of its correction during the control process, the inaccuracy of estimating the standard deviation of the detector signal. To detect low-intensity sources, Δχ should not exceed 3-4σ. In the case of rapid changes in the background, the values of χ _f and Δχ may not correspond to the changing background and, due to the inaccuracy of the threshold setting, weak sources may not be detected. On the other hand, the probability of false positives increases due to the possible excess of the fluctuations in the background of a sufficiently low, not exactly defined threshold.

The technical result of the proposed method is to increase the reliability of detection of radioactive sources in a rapidly changing background. Such changes can be associated with a change in the level of the natural background during traffic with a detection system, the appearance of precipitation, dust, the movement of various objects, including objects with radioactivity, near the control zone.

The specified technical result is achieved by the proposed method of dynamic radiation control, including the registration of ionizing radiation by at least one detector, continuous measurement over a unit time interval of the number of pulses χ _{i of the} detector proportional to the flux of the detected radiation, determining the background value χ _f , in the process of monitoring continuously determine background value as

where m is a positive number satisfying the condition m≤σ / S, where σ is the standard deviation of the detector signal for the minimum possible value of the background, S is the maximum possible value of the slope of the background change,

значения которой сравнивают с порогом К, являющимся безразмерной постоянной в процессе контроля величиной в пределах от 1 до 4, и в случае, если указанная разность превышает порог К, судят о наличии радиоактивности в объекте контроля, а значения χ_i, при которых упомянутая разность превышает порог, исключают из процесса определения величины χ_ф.calculate the difference

the values of which are compared with the threshold K, which is a dimensionless constant in the control process, a value ranging from 1 to 4, and if the specified difference exceeds the threshold K, the presence of radioactivity in the control object is judged, and the χ _i values at which the difference exceeds threshold, exclude from the process of determining the value of χ _f .

Distinctive features is that in the process of monitoring continuously determine the value of the background as

where m is a positive number satisfying the condition m≤σ / S, where σ is the standard deviation of the detector signal for the minimum possible value of the background, S is the maximum possible value of the slope of the background change,

значения которой сравнивают с порогом К, отличающимся безразмерной постоянной в процессе контроля величиной в пределах от 1 до 4, и в случае, если указанная разность при наличии объекта в зоне контроля превышает порог К, судят о наличии радиоактивности в объекте контроля, а значения χ_i,, при которых упомянутая разность превышает порог, исключают из процесса определения величины χ_ф.calculate the difference

the values of which are compared with the threshold K, which differs by a dimensionless constant in the control process from 1 to 4, and if the specified difference in the presence of the object in the control zone exceeds threshold K, the presence of radioactivity in the control object is judged, and the values χ _i , at which the mentioned difference exceeds the threshold, exclude from the process of determining the value of χ _f .

New significant features of the proposed method provide an increase in the reliability of detection of radioactive sources in a rapidly changing background. In addition, it ensures the independence of the threshold from the background level and its constancy in the control process, which increases the reliability of the functioning of devices that implement the method.

The invention is illustrated in the drawing, which shows a diagram of the implementation of radiation monitoring of moving objects.

A device that implements the method contains detectors 1, 2, stationary mounted in the control zone, through which follow the controlled objects 3 (vehicles with goods in which there may be radioactive sources). Large plastic scintillators with photoelectronic multipliers can be used as detectors.

The method is implemented as follows. In the absence of object 3 in the control zone, detectors 1, 2 register background radiation. A continuous measurement of the number of pulses of the detectors is performed for a single time interval, for example, in one second. The results of each measurement χ _{i are} sent to a computing device (computer). Given the possible changes in the background in the control zone, established empirically, calculate the number m of measurements of χ _i to determine the background level χ _f according to the formula

where σ is the standard deviation of the detector signal (the number of pulses per unit time interval) for the minimum background value;

S is the maximum value of the steepness of a possible background change, equal to the increment in the number of detector pulses when the background changes over a unit time interval (one measurement).

The computing device summarizes m values of χ _i and determines the value of the background χ _f as follows

where i is the current number of the unit time interval during which the number of detector pulses is counted.

и

и сравнения разности

с порогом К. В случае, если указанная разность не превышает порог, текущие значения χ_i учитываются при определении очередного значения фона χ_ф по выражению (2), при этом величина χ_i - m исключается из вычислений фона χ_ф. Таким образом, величина χ_ф постоянно обновляется, что обеспечивает отслеживание возможных изменений фона в зоне контроля.The first value of χ _f when starting the control method is determined after the m-th measurement of χ _i . In the future, the value of χ _{f is} updated every time after the appearance of the next measurement result of χ _i , calculations of the values

and

and difference comparisons

with threshold K. In case the indicated difference does not exceed the threshold, the current values of χ _i are taken into account when determining the next value of the background χ _f from expression (2), while the value χ _i - m is excluded from the calculation of the background χ _f . Thus, the value of χ _{f is} constantly updated, which provides tracking of possible changes in the background in the control zone.

обусловленная изменяющимся фоном, не превышает 0,25, что значительно ниже устанавливаемого порога К и не оказывает существенного влияния на обнаружения радиоактивности в объекте.Due to the fact that the number m of measurements of χ _i for determining the background is calculated from expression (1), the value of χ _f can differ from the true value of the changing background by no more than half the value of the standard deviation of the detector signal σ. Therefore, in the absence of radioactivity in the object, the difference

due to the changing background, does not exceed 0.25, which is significantly lower than the set threshold K and does not significantly affect the detection of radioactivity in the object.

превышает порог К, то это свидетельствует об обнаружении радиоактивности в объекте. В этом случае величина χ_i не включается в сумму (2) и не влияет на определение значения фона χ_ф, остающегося неизменным до появления нового измерения величины χ_i, при которой разность

не превышает порог. Это значение χ_i включается в сумму (2) и величина χ_ф продолжает обновляться, осуществляя слежение за фоном. Таким образом, наличие радиоактивности в объекте не искажает результаты измерения фона.If the current value of the difference

exceeds threshold K, this indicates the detection of radioactivity in the object. In this case, the value of χ _{i is} not included in the sum (2) and does not affect the determination of the value of the background χ _f , which remains unchanged until a new measurement of the value of χ _i , at which the difference

does not exceed the threshold. This value χ _{i is} included in the sum (2) and the value χ _f continues to be updated, tracking the background. Thus, the presence of radioactivity in an object does not distort the results of background measurements.

Производная этой функции y' равнаWhen radioactivity is detected, not absolute χ values are compared with the threshold, as in the prototype, but the function

The derivative of this function y 'is

Приращение dy, вызывающее срабатывание системы в предлагаемом способе, для эквивалентного прототипу порога с учетом (3) определяется какIn the prototype, the threshold component Δχ is determined by the increment of the signal, causing the detection system to trigger

The increment dy, causing the system to trigger in the proposed method, for the threshold equivalent to the prototype, taking into account (3), is defined as

Thus, the response threshold in the proposed method does not depend on the magnitude of the detector signal and on the background level in the control zone, which reduces the effect of background changes on the process of detecting radioactivity in the object. Significant in this case is that the standard deviation of the signals y _i and y _f is constant over the entire range of the signal x _i , and therefore no adjustment of the response threshold is required when the detector signal level changes.

In the prototype, the background value is determined by averaging a certain number of x _i values that are not related to the standard deviation of the signal σ and the steepness of the background change, while the response threshold depends on the background value and is constantly adjusted in the control process. This leads to the fact that with relatively rapid changes in the background, false alarms can be observed due to the mismatch of the threshold value with the current background value. On the other hand, the detection sensitivity may fall if the threshold is excessively high.

Example 1

When implementing the method, a detector is used, the pulse repetition rate of which at a background level of 0.1 μSv / h is equal to 900 imp / s. A single measurement time is 1 s. The background during the control changes from 0.1 μSv / h to 0.15 μSv / h. The detector pulse frequency at the maximum background value is 1350 imp / s. The rate of change of the S background is 10 pulses per 1 second. The threshold K is set equal to 2, which corresponds to the probability of false alarms of not more than 10 ^-4 . The standard deviation σ of the signal x _{i of the} detector for 1 second at a minimum background level is 30.

составляет 0,25, что существенно ниже порога К=2. В связи с этим при увеличении фона ложные тревоги отсутствуют. В процессе изменения фона порог остается постоянным и вероятность ложных тревог также не изменяется, т.к. стандартное отклонение сигналов

и

при любых значениях x_i является постоянной величиной.The value of m determined by condition (1) is 3. The background value determined by relation (2), when the background changes at a speed of 10 pulses / s, will differ from its value at point i by no more than 15 pulses. Difference

is 0.25, which is significantly below the threshold K = 2. In this regard, with an increase in the background, false alarms are absent. In the process of changing the background, the threshold remains constant and the probability of false alarms also does not change, because standard deviation of signals

and

for any values of x _i is a constant.

For the same background values, detector pulse repetition rate, the number of averaging values x _i equal to 30, the response threshold 4σ = 120 for the prototype, the deviation of the calculated background value x _f when the background changes will be 150 pulses, which will lead to a false alarm. Due to the mismatch of the response threshold to the changing background value and the value of σ, the probability of false alarms will increase with increasing values of x _i and x _f .

Example 2

в сторону уменьшения составит 0,25. С учетом выбранного порога К=2 чувствительность обнаружения уменьшится не более чем на 15%.Under the same conditions for the implementation of the method as in example 1, only the background decreases from 0.15 μSv / h to 0.1 μSv / h. In the prototype, the deviation of the magnitude

downward will be 0.25. Given the selected threshold K = 2, the detection sensitivity will decrease by no more than 15%.

For the prototype for the same conditions, the detection sensitivity will decrease by more than 2 times.

Example 3

составляет 0,12, что существенно ниже порога К=2.Under the same conditions for implementing the method as in example 1, but the background change rate is 5 pulses per 1 second. The background value determined by relation (2), when the background changes at a speed of 5 pulses / s, will differ from its value at point i by no more than 7 pulses. Difference

is 0.12, which is significantly below the threshold K = 2.

The likelihood of false alarm will not exceed 10 ^-4 .

In the prototype, the deviation of the calculated background value x _f when changing the background will be 75 pulses (half of the threshold value). This will lead to an increase in the probability of false alarm to 10 ^-2 , which is significantly higher than for the prototype.

Example 4

If the background change rate is 1 pulse / s, the probability of a false alarm is practically unchanged for the proposed method. For the prototype, the likelihood of false alarm increases approximately 2 times.

Thus, the proposed method provides an increase in the reliability of detection of radioactive sources in a rapidly changing background. In addition, it ensures the independence of the threshold from the background level and its constancy in the control process, which increases the reliability of the functioning of devices that implement the method.

LITERATURE

1. RU 2094821 C 1, Bulletin No. 30 of 2710.97

2. N.P. Valuev, Yu.V. Moish, N.V. Nikonenkov "Problems of radiation monitoring of scrap metal." Bulletin "Ferrous metallurgy", 1997, No. 9, pp. 7-9.

Claims (1)

A method of dynamic radiation control, including the registration of ionizing radiation by at least one detector, continuous measurement over a unit time interval of the number of pulses χ _{i of the} detector proportional to the flux of the detected radiation, determining the background value χ _f , characterized in that the value is continuously determined in the control process background like

where m is a positive number satisfying the condition m≤σ / S, where σ is the standard deviation of the detector signal for the minimum possible background value, S is the maximum possible value of the steepness of the background change, the difference is calculated

, the values of which are compared with the threshold K, which is a dimensionless constant in the control process, a value ranging from 1 to 4, and if the specified difference exceeds the threshold K, the presence of radioactivity in the control object is judged, and the values χ _i at which the difference exceeds the threshold, exclude from the process of determining the value of χ _f .