R dose rate measuring instrument based on single chip microcomputer design

This instrument is a measuring instrument for measuring the concentration of γ-radioactive dose rate in the environment. It is divided into two parts: the probe and the host. The probe consists of a high and low range GM counter tube, a counting unit and a communication unit; the host consists of a communication unit, a processing unit, a keyboard, and a display unit. 1

Probe principle
1.1 Geiger-Muller counter tube principle

The GM tube principle is shown in Figure 1. The Geiger-Muller counter tube (GM tube) is also called a gas discharge counter. A sealed glass tube with an anode in the middle made of tungsten wire material, a layer of conductive material is coated on the inner wall of the glass tube, or a metal round tube is used as the cathode, and the inside is evacuated and filled with inert gas (neon, helium) or halogen gas. The characteristic is low working voltage.

When the radiation enters the counter tube, the gas is ionized. When the negative ions are attracted by the anode and move toward the anode, the ions collide with other gas molecules to produce multiple secondary electrons. When they are close to the anode, the secondary electrons multiply rapidly to produce an avalanche phenomenon. The avalanche causes an avalanche on the entire anode line, and discharge occurs. After the discharge, the space electrons are neutralized again, and many positive ions remain to surround the anode, forming a positive ion sheath. The electric field between the positive ion sheath and the anode is weakened by the presence of positive ions. At this time, if electrons move to this area, an avalanche discharge will also occur. This period of time cannot be counted and is called "dead time". When positive ions hit the cathode, electrons are generated (knocked out). The electrons are accelerated by the electric field, causing the counter tube to discharge and produce a positive ion sheath. This process occurs repeatedly.

When the voltage U on the counter tube is constant, the stronger the incident radiation, the larger the current I, and the larger the number of output pulses N. The a and b segments are called "plateaus". Geiger counter tubes are mainly used to detect beta particles and gamma rays.

1.2 Probe Circuit

The probe consists of a high voltage generating circuit, two Geiger-Mueller counter tubes (GM tubes) with high and low ranges, a single-chip microcomputer SM89C52, and a serial port to 485 communication chip MAX485, as shown in Figure 2.

The high voltage generation circuit provides high voltage for the high and low range GM tubes, so that the GM tubes can work. The GM tube is used to measure radiation. When the rays pass through the GM tube and cause ionization, the GM tube generates a current pulse. The pulse is transformed into a square wave with steep edges after passing through the shaping circuit and the 2-frequency division circuit, and sent to the T0 and T1 timers of the SM89C52 for counting. For the low range, T0 is used for counting and T1 is used for 1 second timing, while for the high range, T1 is used for counting and T0 is used for timing. The host can switch between high and low range measurements by sending commands. When the probe receives the query command sent by the host, it returns the count value per second (CPS) to the host. The probe power is obtained from the host through the cable.

The interface methods of the low range GM tube ZP1210 and the high range GM tube Zp1304 are different. The design according to the data sheet is shown in Figure 3.

2 Working principle of the host

The host consists of a single-chip microcomputer SM89C58, a 12 864-dot LCD, 4 buttons, a MAX 485 chip, an AT24C64, and a real-time clock chip M41T0. The host and the probe are connected by a cable, as shown in Figure 4. The host's job is to query the probe regularly, obtain CPS, convert it into a dose rate and display it on the LCD, and at the same time decide whether to send a command to switch the measurement channel and which of the high and low range GM tubes to apply high voltage based on the data. M41T0 provides a real-time clock for the system. AT24C64 is used to save system parameters and passwords, etc., and is also used to save measurement results with time information. In the measurement interface, users can save the current measurement results for later tracing. The host uses a menu on the LCD with 4 buttons to set parameters, adjust time and date, and manage passwords. It also provides two language switches, Chinese and English, and can be expanded to display any other language. [page]

The counts per second (CPS) obtained by the probe GM tube needs to be calculated to be converted into the dose rate in microgray per hour (μGy/h). The conversion formula is:

 

Where D is the dose rate in μGy/h; K is the probe sensitivity, a constant; t is the dead time in seconds; N is the counts per second, that is, the number of pulses generated by the GM tube being irradiated every second. For the low-range probe (ZP1210), the value range of t is 50-80μs, and the value range of K is 2.5-3.2; for the high-range probe (ZP1304), the value range of t is 21-35 μs, and the value range of K is 0.030-0.040. The ι and K of the high and low GM tubes can be set in the host through the menu, and each set of measuring instruments needs to be calibrated to set the ι and K values. Since ι, K are important parameters of the entire calculation and cannot be modified by the user at will, a password protection function is added when designing the instrument. The user needs to enter the password to enter the ι, K setting menu. Only after the password verification is passed can the ι, K parameters be modified.

Due to the random characteristics of radioactivity, the CPS detected by the instrument will have jitter changes. The host program performs smoothing and filtering on the results of several consecutive measurements to make the measurement results more accurate. If the average measurement results are too few, the smoothing effect is not good; if the average measurement results are too many, the instrument response speed will become very slow. Therefore, the average of 5 points is taken for comprehensive consideration.

3 Range switching

The low-range GM tube ZP1210 can measure radiation doses of 10-1~104μGy/h, while the high-range GM tube ZP1304 can measure radiation doses of 102~107μGy/h.

The range switching is shown in Figure 5. The host calculates the dose rate result in μGy/h based on the real-time CPS data obtained from the slave. The host will detect several consecutive measurement results. When it is found that the change trend of the measurement result is from less than 1200 μGy/h to more than 1200 μGy/h, the host sends a command to the slave to let the slave measure the count value of the high-range GM; when it is found that the change trend of the measurement result is from more than 800 μGy/h to less than 800 μGy/h, the host sends a command to the slave to let the slave measure the count value of the low-range GM. In this way, the high-range and low-range GM tube measurement switching occurs at 800 μGy/h and 1200 μGy/h respectively, thereby avoiding the ringing that is easy to occur when switching at a single position, and the frequent switching of high and low ranges.

When a large dose of radiation occurs, the high voltage of the low-range GM tube should be turned off and stopped working to avoid damaging the low-range GM tube. In addition, in order to save battery power, the probe needs to control the power-consuming high-voltage generating circuit. When the host finds that the measurement result is less than 800μGy/h, the high voltage of the high-range GM tube is turned off and the high voltage of the low-range GM tube is turned on. When it is greater than 1,200μGy/h, the high voltage of the low-range GM tube is turned off and the high voltage of the high-range GM tube is turned on. When it is greater than 800 μGy/h and less than 1,200 μGy/h, the high voltage of the high and low range GM tubes is turned on at the same time. The high and low range GM tube high voltage on and off control switching diagram is shown in Figure 6.

In order to avoid high-dose radiation damaging the low-range GM tube when the machine is turned on, the probe applies high voltage to the high-range GM tube and turns off the high voltage of the low-range GM tube when the machine is turned on, and the measurement starts from the high-channel GM. The probe's single-chip microcomputer SM89C52 decides whether to apply high voltage to the high and low-range GM tubes according to the command of the host.

4 Sound and light alarm

Due to the danger of radiation, when the radiation dose in the environment is greater than a certain value, the instrument needs to give the user a prompt to remind the user to pay attention. The user can set the alarm threshold of the instrument. When the host determines that the dose value is greater than the threshold set by the user, the host displays an alarm prompt of "exceeding the threshold" on the LCD screen and sounds a buzzer.

After the host finds that the communication with the probe fails, it also needs to alarm to remind the user to check the line and probe.

5 Conclusion

The environmental γ dose rate measuring instrument has been successfully developed and delivered to users after calibration, and users have good feedback. This instrument is mainly used to measure the dose rate of gamma rays and give an alarm. It can also be used in industrial non-destructive testing, hospital gamma knife treatment, isotope application, gamma irradiation, hospital X-ray diagnosis, cobalt treatment, nuclear power plants and other radioactive places to remind workers that the radiation source or radiation device is in working or leaking state, thereby protecting the safety of workers.