Surface Contamination Measurement by Combining Detectors and Robots

Against the background of the rapid development of nuclear energy and nuclear technology, the number of places with radioactivity has gradually risen, which poses a significant safety hazard to human beings. Aiming at indoor radioactive places, this paper designs a detector that can be mounted on a robot, and experiments prove that this detector can accurately detect α-surface contamination and β-surface contamination. The surface contamination is measured using a combination of a remote-controlled robot and a detector.


Introduction
With the rapid development of nuclear energy and technology in today's world, there are more and more radioactive places, which are more common in our lives, such as the nuclear medicine department of medical science, isotope production workshops, and laboratories that need to do radioactive experiments [1].These radioactive places are more prone to radioactive contamination.According to the national radiation safety management regulations, in places where radioactive contamination is likely to occur, the ground, equipment surfaces, etc., should be routinely monitored for contamination to prevent the spread of contamination [2].
In the routine monitoring of surface contamination of the ground, there are direct [3] and indirect [4] measurement methods.Direct measurement refers to the surface contamination measuring instrument to detect the level of contamination below the limit value of the regulations.The detector should be moved as close to the surface as possible during the measurement.Once the contamination is detected, the detector should be positioned in the area for a sufficient time to confirm the level of contamination.Indirect measurement refers to transferring contamination from a specific area of the object to be measured through wiping or decontamination, etc., to a fixed instrumentation in the laboratory for measurement.Both direct and indirect measurements avoid the risk of exposing personnel to radioactive scenarios and radiation hazards.On the other hand, the means of measurement using robots have been widely used in various industries, such as the inspection of 3D structures using robots [5], inspection of power systems [6], source localization of gas leaks [7], etc. Robots can also be applied to the measurement of surface contamination.
This paper designs a practical surface contamination detector and utilizes a robot to carry the detector out surface contamination measurements.This method not only avoids human contact with potentially radioactive hazardous locations, but also reduces the time that personnel are exposed to risky locations while dealing with surface contamination, and can reach inaccessible places to carry out measurements.

Surface Contamination Detector Design
The detectors used for surface contamination detection in the market are mainly hand-held, such as the CoMo 170 [8] surface contamination meter from Germany, which is unsuitable to be mounted on a robot.Therefore, in this paper, we design a detector suitable for mounting on a robot.
The surface contamination detector is mainly based on the STM32F103C8T6 as the core, the operating voltage is 24 volts, and the data detected on the surface of the detector is counted to characterize the severity of surface contamination.It mainly consists of STM32 and its peripheral circuits, multistage buck circuit, operational amplifier circuit, high voltage power supply module, and comparator circuit module [9].

Buck circuit
Since the detector needs to be mounted on the robot, the robot supplies 24 volts.The high-voltage power supply module operates at 24 volts, so the detector's operating voltage is also set to 24 volts.Since the STM32 operates at 3.3 volts, it also needs to supply 3.5 volts and 7 volts to the operator amplifier circuitry, so it is necessary to set up a buck module to enable the detector to operate normally.
The buck circuit to step down the detector operating voltage from 24 volts to 3.3 volts to make the STM32 work properly is shown in Figure 1.The buck circuits all use multiple bucks during the bucking process, mainly to minimize the ripple effect on the circuit, improve the voltage quality, and ensure that the detector works better.

High voltage circuits
The primary function of the high-voltage module is to power the photomultiplier tube of the detector.When the rays emitted by surface contaminants such as α and β pass through the scintillator and emit photons, the photons pass through the photomultiplier tube, which converts the weak light signal into an electronic signal.The high-voltage module circuit is shown in Figure 3. 3. Circuit diagram of the high-voltage module.

Operational amplifier circuit
When the weak light signal through the photomultiplier tube produces electronic signals, the electronic signal through the ADA4817 voltage feedback amplifier will be converted from current signals to voltage signals.It can amplify the voltage signal to the level that can be processed by subsequent circuits.The circuit diagram is shown in Figure 4. Converted voltage signal after the AD8066ARZ chip output input differential voltage and amplifier gain product for comparator use [10].

Comparator Circuit
This comparator compares the difference between the amplified signal and the preset threshold and controls the output voltage according to the comparison result.In this circuit, the output signal of the preamplifier is compared with the preset voltage to output a pulse signal, which is transmitted to the STM32 for counting.The circuit diagram is shown in Figure 5.

Test and experimental results
The robot uses a mature robot chassis, the detector is mounted on the front of the robot, the detector probe faces the ground to ensure that it can detect surface contamination, and the robot is controlled by remote control.This combination of detector and robot can detect the degree of surface contamination in the forward direction of the robot.If there is contamination in the forward direction, you can use a remote control to avoid contamination of the robot's wheels, and at the same time to avoid surface contamination through the robot's wheels spread to the entire ground, expanding the scope of surface contamination.
The program of STM32 in this detector will count the number of pulses per second in cps, and the designed detector will be tested on α and β radioactive sources.The information on α and β radioactive sources is shown in Table 1 below.Since the nuclides have a half-life, 204 Tl has a half-life of 3.78 years and 239 Pu has a half-life of 24110 years, it is necessary to calculate the actual activity based on the half-life, the activity of 204 Tl varies a lot while 239 Pu remains unchanged, and the calculation of the number of particles emitted per minute is shown in Table 1.
Table 1 Information on α and β radioactive sources.In the same environment, the detector for radiation background, α and β, and other surface contamination of radioactive sources, the different radioactive sources will be under the detector for two minutes of counting.The results of the counts per second will be utilized in the moving average method and smoothed with a window of 30 data.The results of the experiment are shown in Figure 6.Based on the experimental results, the detection efficiency of the detectors was calculated using the average counts per second of the detectors compared to the number of particles emitted per second from the sources.The radiation background was calculated to be 9%, the detection efficiency of the alpha source was 42.72%, and the detection efficiency of the beta source was 19.09%.The detector is mounted in front of the robot for detection.The data are transmitted to the robot through the serial port and then displayed on the computer using the remote desktop, which successfully realizes the detection of surface contamination by the robot.It has good detection efficiency.

Conclusion and outlook
The detector designed in this paper effectively detects the surface contamination of α and β radioactive sources.In remote-controlled robots using this detector, the detector is used for a fixed time for each position on the road to determine whether there is surface contamination in each position, which is proved to be feasible.However, there is the problem of a long time for detection, the robot is not easy to control, and the problem of detection efficiency is low.Next, work will be carried out to allow the robot to move autonomously to carry out the detection so the robot can work autonomously to complete the work.

Figure 1 .
Figure 1.The circuit diagram of 24 volts stepped down twice to 3.3 volts.The circuit diagram of the detector operating voltage of 24 volts stepped down to 7 volts versus 3.5 volts allowing the high voltage module, operational amplifier, and comparator circuits to operate properly is shown in Figure 2.

Figure 2 .
Figure 2. 24-volt to 7-volt with 3.3-volt circuit diagram.The buck circuits all use multiple bucks during the bucking process, mainly to minimize the ripple effect on the circuit, improve the voltage quality, and ensure that the detector works better.

Figure 6 .
Figure 6.Detector counts per second.Based on the experimental results, the detection efficiency of the detectors was calculated using the average counts per second of the detectors compared to the number of particles emitted per second from the sources.The radiation background was calculated to be 9%, the detection efficiency of the alpha source was 42.72%, and the detection efficiency of the beta source was 19.09%.The detector is mounted in front of the robot for detection.The data are transmitted to the robot through the serial port and then displayed on the computer using the remote desktop, which successfully realizes the detection of surface contamination by the robot.It has good detection efficiency.