FEATURES OF CONSTRUCTION OF PEDESTRIAN RADIATION MONITORS..

FEATURES OF CONSTRUCTION OF PEDESTRIAN RADIATION MONITORS..

RUDNICHENKO Valery Alexandrovich

FEATURES OF CONSTRUCTION OF PEDESTRIAN RADIATION MONITORS

Currently, many areas of human practical activity use sources of ionizing radiation. Any area of ​​activity associated with such sources is potentially dangerous, since it leads to the risk of radiological contamination of people and the environment in the event of radioactive products entering the sanitary protection zone or using them for terrorist purposes. In this regard, the problem of reliable radiation monitoring is becoming increasingly important, covering a wide range of tasks, some of which are solved using special technical means integrated into access control and management systems (ACS) and implementing continuous radiation monitoring at pedestrian checkpoints (CP). Continuous radiation monitoring in places where people pass through CPs in large numbers significantly increases the effectiveness of physical protection systems at facilities and plays an important role in countering nuclear and radiation terrorism.For radiation monitoring, pedestrian radiation monitors (RM) are used, which are devices designed to detect nuclear materials (NM) and radioactive substances (RS) by their gamma and/or neutron radiation. Requirements for the detection threshold and other technical characteristics of radiation monitors of nuclear materials are established by GOST R 51635-2000, according to which pedestrian RMs are divided into the categories listed in Table 1.

Table 1. Requirements for the mass of detected NM according to GOST R 51635-2000

Notes:
1. Plutonium reference sample is a standard sample made of plutonium, the content of the mass fraction of plutonium is not less than 98% (the content of 239Pu is not less than 93.5%)
2. Uranium reference sample is a standard sample made of uranium, the content of the mass fraction of uranium is not less than 99.75% (the content of 235U is not less than 89%)

As follows from Table 1, gamma radiation monitors are capable of detecting NM in smaller quantities compared to neutron radiation monitors. In this regard, the main channel for detecting RM is the gamma radiation registration channel. Let us consider some operational features that must be taken into account when choosing the detection threshold of a gamma radiation monitor.
In modern radiation monitors, scintillation detectors are used to register gamma radiation. The detector operates in a counting mode, in which the value of the count at its output at an arbitrary moment in time depends on the number of gamma quanta registered by the detector in a given time interval, called the exposure time. The count rate at the output of the scintillation detector is a random variable and has a Poisson distribution [1]. The count flow created by the radiation background is used to calculate the current threshold level of the count rate, which ensures the false alarm rate RM is not lower than the required value. The false alarm rate depends not only on the threshold value, but also on the implemented processing algorithm. For a simple threshold detector, the false alarm probability PL can be estimated using the formula

PL = tE/TL, (1)

where tE is the exposure time; TL is the operating time between false alarms, at tЭ = 1 s and TL = 8 h (requirement of GOST R 51635-2000) we obtain PL » 3.5×10-5.
Fig. 1 shows a graph of the background count rate probability (curve 1), which shows that with a known count flow intensity l, the above obtained value of PL is achieved at a threshold level value Пg » l + 4s (s is the root-mean-square value of the background count rate).
By selecting the threshold level Pg, the detection threshold of RM can be determined. To do this, follow the following method.

1. First, the minimum sensitivity zone of the monitor is determined: a two-dimensional sensitivity map is constructed by recording the count rate from the detectors of the gamma radiation source. In this case, the gamma radiation source is successively placed at the nodal points of a two-dimensional coordinate grid in a plane located perpendicular to the direction of the pedestrian's movement and corresponding in size to the detection zone of the RM. At each point, the count rate is measured, and the minimum sensitivity zone is determined by the smallest value.
2. In accordance with Table 1, a standard sample (SS) of nuclear material is selected and passes through the RM detection zone in such a way that it moves along the trajectory of minimum sensitivity of the monitor. The number of passes depends on the specified probability of detection of the radiation monitor PRM (selected from the range: 0.50; 0.75; 0.80; 0.85; 0.90; 0.95) and the number of monitor responses, which is regulated by GOST R 51635-2000. For example, to confirm the probability of detection of PRM і 0.95, 100 passes should be performed, of which at least 99 should cause the RM to respond.

Fig. 1. Count rate probability:
1 – depending on the background;
2 – depending on the radionuclide source and background;
l – count rate intensity depending on background (s=Цl)

For a CO corresponding to the RM detection threshold at PRM і 0.95, the graph of the count rate probability when it is carried along the trajectory of minimum sensitivity is shown in Fig. 1 (curve 2). As follows from Fig. 1, at the selected threshold level Пg, the radiation monitor must simultaneously satisfy two requirements: ensure a given false alarm rate from the background and have the required probability of detecting PRM of a gamma radiation source.
Using the described method, the probability of detecting a selected gamma radiation source carried without a protective container is determined. When using a protective container, the gamma radiation flow, passing through its walls, is weakened and, accordingly, the probability of detecting a PRM source decreases. Metal containers are used to transport gamma sources, which are usually steel or lead due to the effectiveness of the protective properties of these metals and their availability.
The attenuation factor depends on the gamma radiation energy, the material and thickness of the protective screen. Fig. 2 shows graphs of the dependence of the attenuation factor of gamma radiation of NM samples on the thickness of the protective screen [2].

Fig. 2. Attenuation factor of gamma radiation:
1 – attenuation of 235U gamma radiation by a lead screen;
2 – attenuation of 239Pu gamma radiation by a lead screen;
3 – attenuation of 235U gamma radiation by a steel screen;
4 – attenuation of 239Pu gamma radiation by a steel screen

Considering that the count rate from the source is distributed according to the Poisson law, it is possible to plot graphs of the dependence of the probability of detecting a gamma radiation source on the attenuation factor. Fig. 3 shows such dependencies for various options that differ in the value of the required PRM, provided that there is no protective screen; it is clear that the use of a protective screen significantly reduces the probability of detecting a gamma radiation source. When it is reduced to the level of PRM ? 0.5, the efficiency of using RM is sharply reduced.
In order to prevent the loss of tactical properties, a combined detection method is used in the ACS, in which the inspection is carried out sequentially by a radiation monitor and a metal detector (MD). In this case, the presence of prohibited materials is recorded when at least one of the devices is triggered. With this inspection method, the choice of detection thresholds for the RM and MD, which ensure the required value of the total probability of detection, is critical.

Fig. 3. Dependence of the probability of detecting a gamma radiation source
on the attenuation factor (in the absence of a protective screen).
Curves 1 – 6: probabilities of detecting a gamma radiation source
respectively 0.95; 0.90; 0.85; 0.80; 0.75; 0.50

Let us consider the procedure for selecting detection thresholds for these means using the example of a typical protective container, the shape of which is shown in Fig. 4. Table 2 shows the mass of such a container (with different wall thicknesses), providing different degrees of gamma radiation attenuation for CO from uranium and plutonium.

Table 2. Mass of a typical container

Fig. 4. Option of a typical protective
container (L is the wall thickness)

When using RM and MO together, the total probability of detection will be equal to [3]

P0 = PPM + PMO – PPM PMO, (2)

where PPM is the probability of detection using RM; PMO is the probability of detection using MO.
The probability of detection by a radiation monitor can be estimated using the data in Fig. 3 and Table 2. Fig. 5 shows, as an example, the dependence of the PPM probability (curves 1–6) on the mass of a steel container for CO from uranium.
To estimate the probability of detecting a protective container, it is advisable to use a technique similar to that described above. Let PMO be the probability of detecting a protective container by a metal detector when it is carried along the trajectory of least sensitivity. The experiment shows that the random value of the signal from the container carried along the trajectory of least sensitivity is distributed according to a truncated normal law, and the range of signal values ​​is such that the maximum value is approximately twice the minimum.
Considering that the signal value is directly proportional to the surface area of ​​the container, Fig. 5 (curve 7) shows the dependence of the probability of detecting a steel container on its mass, provided that the threshold for detecting a protective container corresponds to a mass of a steel object of 300 g with PMO   і 0.95.

Fig. 5. Dependence of the probability of detecting CO from uranium on the mass of the steel protective container
(for RM) and the dependence of the probability of detecting the container on the mass (for MO).
Curves 1 — 6: probabilities of detecting RM of a standard sample from uranium
without a protective container, respectively 0.95; 0.90; 0.85; 0.80; 0.75; 0.50;
curve 7 — probability of detecting a steel protective container
at a detection threshold of 300 g with a probability of 0.95 (for MO)

The graphs of the dependence of PPM and PMO on the mass of the protective container shown in Fig. 5 allow us to calculate the total probability of detection P0. The dependences of the P0 value on the mass of the steel container for uranium CO are shown in Fig. 6, where it is evident that with an increase in the mass of the container, the P0 value changes within the range from PPM (the container is absent) to »1 (the mass of the container, the threshold mass of detection of MO). In this range, the P0 value has a clearly expressed minimum P0min, the position of which on the graph depends on the initial PPM (in the absence of a protective container) and the threshold of detection of MO.
Fig. 7–10 show the dependences of the detection probability P0min on the mass of a typical container for uranium and plutonium CO, allowing the selection of the required MO detection threshold. For example, if the requirement for the total detection probability is P0  і 0.5 when carrying uranium CO in a steel container along a minimum sensitivity trajectory, the MO detection threshold should not exceed 450 g (Fig. 7).
When making the final selection of the MO detection threshold, the probability of detecting CO by a radiation monitor in the absence of a protective container should be taken into account. Thus, if the specified probability is 0.9, then the MO detection threshold should correspond to 350 g (Fig. 7, curve 2).

Fig. 6. Dependence of the probability of detection P0 on the mass of the steel protective container
when carrying CO from uranium (for the RM + MO system with a MO detection threshold of 300 g with PMO і 0.95).
Curves 1 – 6: probabilities of detecting CO from uranium without a protective container, respectively 0.95; 0.90; 0.85; 0.80; 0.75; 0.50

Thus, when using pedestrian gamma radiation monitors, it is necessary to take into account the possibility of carrying radioactive substances placed in a protective metal container. The total probability of detection when using a radiation monitor and a metal detector simultaneously depends on the detection thresholds of individual means (RM and MO) and may be less than the required value.
For the correct joint operation of the RM and MO, it is necessary to select the detection thresholds of individual means in a coordinated manner. The above methodology describes the procedure for such a selection and allows for an assessment of the expected value of the total probability of detection.

Fig. 7. Dependence of the probability P0min on the MO detection threshold
when carrying CO from uranium in a steel container
(for the RM + MO system when the metal detector detects the threshold mass with a probability of 0.95).
Curves 1–6: probabilities of detecting CO from uranium without a protective container, respectively 0.95; 0.90; 0.85; 0.80; 0.75; 0.50

Fig. 8. Dependence of the probability P0min on the MO detection threshold when carrying CO
from uranium in a lead container (for the RM + MO system when the threshold mass is detected by the metal detector with a probability of 0.95).
Curves 1–6: probabilities of detecting CO from uranium without a protective container, respectively 0.95; 0.90; 0.85; 0.80; 0.75; 0.50

Fig. 9. Dependence of the probability P0min on the MO detection threshold when carrying CO
from plutonium in a steel container (for the RM + MO system when the threshold mass is detected by the metal detector with a probability of 0.95).
Curves 1–6: probabilities of detecting CO from plutonium without a protective container, respectively, 0.95; 0.90; 0.85; 0.80; 0.75; 0.50

Fig. 10. Dependence of the probability P0min on the MO detection threshold when carrying CO
from plutonium in a lead container (for the RM + MO system when detecting the threshold mass by a metal detector with a probability of 0.95).
Curves 1–6: probabilities of detecting CO from plutonium without a protective container, respectively, 0.95; 0.90; 9.85; 0.80; 0.75; 0.50

Literature

1. Tarasov G.P. Statistical methods of information processing in systems of measuring ionizing radiation. — M.: Atomizdat, 1980, p. 208.
2. Kimel L.R., Mashkovich V.P. Protection from ionizing radiation: Handbook. 2nd ed. — M.: Atomizdat, 1972, p. 312.
3. Ventzel E.S., Ovcharov L.A. Probability theory. M.: Nauka, 1973, p. 368.