MEDE3500 Mini-project (Day1)

Transcription

1 MEDE3500 Mini-project (Day1) Geiger counter DIY and ionizing radiation Department of Electrical and Electronic Engineering The University of Hong Kong Location: CYC-102/CB-102 Course Lecturer: Dr. Philip W. T. Pong Lab Demonstrators: Chao Zheng Xu Li Wenchao Miao Lab report submission: the lab report must be submitted through the Moodle system within two weeks from the laboratory session. Late submission will not be marked. 1

2 MEDE3500 Mini-project Geiger counter DIY and ionizing radiation Caution: 1. High voltage (800 V) involved. Exercise extreme caution. 2. Do not touch the radioactive mineral. INTRODUCTION The nuclear leakage disasters in Chernobyl and Fukushima have brought public concerns on the risk to be exposed to the ionizing radiations generated by the leaked radioactive materials. Geiger counter is a device that can effectively measure the strength of radioactive radiations. The core of a Geiger counter is a Geiger-Müller tube (GM tube), which is a gas-filled chamber operated at subbreakdown voltage. In this mini-project, we will apply the electromagnetics knowledge to understand the mechanisms of the GM tube, and build our own Geiger counter from the electronic components (resistors, capacitors, transistors, etc.). EDUCATIONAL OBJECTIVES (1) Gain basic understanding on the ionizing radiation. (2) Learn the operating principles of a GM tube and how it reacts to the incident radiation. (3) Understand the breakdown process and how it is related to the applied electric field. (4) Learn to understand the circuit system diagram and relate it to the components on a PCB board. (5) Learn to build a system with a specific function from various electronics components. EXPERIMENTAL OBJECTIVES (1) Practice how to implement electronic components on a PCB board. (2) Practice how to use various instruments to measure the input and output signals of electronics systems. (3) Learn to determine the radioactive level with a Geiger counter. 2

3 PRE-LAB (1) Learn the basic soldering techniques. (2) Learn the basic mechanisms of a Geiger counter. EQUIPMENT NEEDED Soldering iron, wire solder, multimeter (Max. DC volt: 500 V), fine point tweezer, cable cutter, oscilloscope. INTRODUCTION TO GEIGER COUNTER a. Types of ionizing radiation Ionizing radiation is radiation that carries enough energy to liberate electrons from atoms or molecules, thereby ionizing them. The major ionizing radiation includes beams of subatomic particles, ions, atoms and high-energy electromagnetic waves. Alpha particles are helium nucleus which can cause electrical excitation and ionization of surrounding atoms. Beta particles are electrons or positrons which can cause electrical excitation and ionization by displacing orbital electrons. Gamma rays and X rays are high-energy photons which are capable of causing photoelectric effect, Compton scattering, and pair production. Neutron has nearly the same mass as a proton and has no electrical charge. Neutron beam can cause inelastic scattering, elastic scattering, radiative capture, or fission. b. Units of ionizing radiation There are different units of ionizing radiation depending on the measurement of radiation. 1. Exposure dose is a measure of radiation based on the ability to produce ionization in air. The units for exposure dose are the roentgen (R) and coulomb/kilogram (C/kg). 2. Absorbed dose is the mean energy imparted to matter per unit mass by ionizing radiation. The units for absorbed dose are the radiation absorbed dose (rad) and gray (Gy). 3. Equivalent dose is related to biological damage to human tissue and factor the differences between types of radiation. Units for equivalent dose are the Roentgen equivalent man (rem) and Sievert (Sv). c. Health effect and protection of ionizing radiation 3

4 Ionizing radiation may damage the human cells by decomposing protein and breaking the bonds in DNA molecule, which may result in mutations, chromosome aberrations, or cell death. This effect is commonly used in radiation therapy, which engages low-dose localized ionizing radiation to selectively kill cancer cells. Since cancer cells grow and divide faster than many of the normal cells around them, they have a higher chance to be killed by the radiation-induced DNA damage. However, deterministic effects (or tissue reactions) may be induced when exposed to ionization radiation of high dose (>4 Sv in a short time). According to International Commission on Radiological Protection (ICRP), the safety limit is 1 msv/year, not including the background, medical and occupational exposures. Since ionizing radiations have potential hazard to human health, protections should be applied to avoid direct exposure to these radiations. Generally, alpha particles could be stopped by a piece of paper or skin. Beta particles could be easily shielded by aluminum foil. Neutron beam could be shielded by thick materials (such as hydrogen-rich materials) that scatters or slows the neutrons. While gamma radiation and X ray are highly penetrative, large mass of materials with high atomic ratio and high density are required as shielding. d. Radiation detector: Geiger-Müller tube Radiation is invisible and not directly detectable by human senses. As a result, instruments such as Geiger counters are usually required to detect its presence. A typical Geiger counter consists of a high-voltage supply, a GM tube, and a visual or audio readout (Fig 1). The GM tube is a type of ionization chamber that counts particles of radiation. The tube consists of a chamber filled with inert gas at low-pressure (about 0.1 atmosphere). The inside wall of the tube was coated with thin metal film to act as the cathode, while the anode is a wire in the center of the tube. High voltage (several hundred volts, depending on the tube model) was applied between the cathode and anode. In the example illustrated in the lecture notes (Statistics_D_22-23), the electric field near the anode can be around 3.0 MV/m, which is comparable to the breakdown electric field of the filled gas (~3 MV/m for 4

5 air). So breakdown is expected to be triggered when the incident radiation ionizes a gas molecule and produces positive ions and free electrons: radiation X X e These carriers will be accelerated by the applied voltage, and produce a discharge current pulse between the electrodes. A current pulse is produced each time a radiation particle ionizes the inert gas inside the tube. The function and performance of a GM tube vary significantly with different high voltage (Fig. 2). Fig 1 Schematic of a Geiger counter with a GM tube Fig 2 Voltage-current (pulse) characteristic of a GM tube 1. Recombination region (V<V 1 ) Due to the low applied voltage, the low-energy ions and electrons recombine in the way and do not get enough energy to reach electrodes. In this region, the output current pulse is unstable, so it is not used. 5

6 2. Ionization region (V 1 <V<V 2 ) The applied voltage allows all ions to be collected at the electrodes without amplification (Fig. 3). In this region, the output is stable, so it is used in calibration. Fig 3 Schematic illustration of a GM tube working in ionization region. 3. Proportional region (V 2 <V<V 3 ) The applied voltage is large enough to amplify the signal by causing avalanche ionization (Fig. 4). The incident radiation ionizes the gas molecule, producing positive ions and free electrons. The ions (electrons) were accelerated by the electric field when drifting towards the cathode (anode) until they collide with other molecules and produce more ions and electrons. The process goes on and this avalanche phenomenon produces a discharge current in the tube. In this region, the output pulse height is proportional to incident energy, so it is used in radiation spectroscopy. Fig 4 Schematic illustration of a GM tube working in proportional region. 6

7 4. Geiger-Müller region (V 3 <V<V 4 ) At very large applied voltage, the avalanche produces photons which simultaneously trigger similar chains (Fig. 5). The output pulse is very large, and is approximately the same height. So it is used for counting. The Geiger counter works at this region. Fig 5 Schematic illustration of a GM tube working in Geiger-Müller region. 5. Continuous discharge region (V>V 5 ) The applied voltage is so large that once ionization takes place in the gas, there is a continuous discharge of electricity. This mode cannot be used for detection. DIY GEIGER COUNTER COMPONENTS The overall structure of a Geiger counter is shown in Fig 6. It consists of several modules working together to deliver the function of detecting ionizing radiation. Each module can be implemented with electronics components. 7

8 Fig 6 Overall structure of a Geiger counter a. Power supply As shown in Fig 7, this module consists of a 9 V battery and a voltage regulator. L7805 is a voltage regulator IC. This module provides a regulated 5 V voltage to power the rest modules. Fig 7 Circuit diagram of the power supply module b. High voltage supply As shown in Fig 8, this part contains a wave generator, a step-up transformer, a rectifier, and a voltage stabilizer. A square wave is generated by the wave generator and its amplitude is boosted up by the step- 8

9 up transformer. The AC output is then converted to DC by the rectifier. The high voltage output from this stage is stabilized by the Zener diodes depending upon the voltage required for the GM tube. Fig 8 Circuit diagram of the high voltage supply module Task 1 Please design a rectifier circuit to convert a 5 V peak-to-peak square wave to a 5 V DC output. Use a function generator and an oscilloscope to test the designed rectifier circuit. The available components include: Breadboard, Dupont line Capacitors (10 pf, 1 nf, 10 nf, 100 nf), diode (1N4007), Resistors (1 Ω 1 MΩ) Fig 9 Circuit diagram of the GM tube module 9

10 c. GM tube Fig 10 Circuit diagram of the signal processing and output module As shown in Fig 9, a 400 V DC output is fed to the anode of the GM tube through a current limiting 10 MΩ resistor (R16). This 10 MΩ resistor limits the current through the GM tube and helps quench the 10

11 avalanched ionization when a radioactive particle is detected. The cathode of the tube is connected to a transistor and a 10 kω resistor (R9). Each time a particle is detected, a voltage pulse appears across this resistor. d. Signal processing and output As shown in Fig 10, the signal across the 10 kω resistor is fed to the comparator #1 in the quad differential comparator IC LM339 which contains four comparators. The signal pulse was changed into a square pulse after passing through the Comparator #1. Task 2 Consider a typical comparator circuit shown in Fig. (a). If a noisy signal pulse was inputted in the Vin port, we will get an output signal with multi-square-pulse at the Vout instead of one single square pulse (Fig. (b)). Please modify the comparator circuit so that we can expect an output of one single square pulse at Vout? The square pulse was then fed to the input of Comparator #2, #3, and #4, corresponding to the circuit for digital output, speaker, and LCD display, respectively. Task 3 What is the advantage of the two-stage comparing circuit (comparator #1 as the 1 st stage; comparator #2, #3, and #4 as the 2 nd stage)? 11

12 The output of the comparator #2 is directly connected to the digital output. The output pulse from the comparator #3 is fed to the base of a triode (Q4) and finally to the IC LM555. Task 4 Please illustrate how the square pulse signal in the base of Q4 is converted to a negative trigger in terminal 2 of the LM 555 IC? Here the LM555 functions as a trigger to create a more significant output, which produces an audible click and flashes the LED. The output of the comparator #4 is connected to the microcontroller. It computes the radiation level (CPS) which is displayed on a LCD. SAFETY PRECAUTIONS a. Soldering 1. Only work in an environment that is well lit and ventilated. 2. Always unplug the soldering iron when it is unattended. 3. Be careful to keep clothes, hair, power cables and skin away from the soldering iron tip and the metal shaft. 4. Be careful when returning the iron to its stand, make sure it is secure and does not fall off. 5. Always handle the iron by the plastic handle. 6. Point the circuit away from yourself and others whilst trimming down component legs, and be careful of any sharp bits of metal whilst handling the circuit or components. b. Geiger counter construction Exercise extreme caution when working on the high voltage section. While the Geiger counter is powered, the bare PCB contains exposed high voltage components and pads that can cause an 12

13 electric shock. Even when the circuit is turned off the capacitors may still hold a high voltage for several minutes. CIRCUIT CONSTRUCTION (LCD display) Caution: Double check the polarity of the transistors and diodes before soldering. Be sure to align the diodes with the silkscreen on the PCB. Construction 1. Mount and solder the 5-pin resistor R1 (10 kω). Pin5 should be inserted into the top hole of R1. 2. Insert and solder the 20 kω potentiometer R2, capacitors C1 and C2 (0.1 µf), C3 (10 µf). 3. Solder the diode D1 (1N4007). Be sure to align the marking on the diode with the silkscreened outline on the PCB. 4. Solder the voltage regulator 78L05 at U1. 5. Next mount and solder the power and backlight switch, S1 and S2. 6. The two 8-pin headers should be mounted and soldered to the underside of the PCB at D2, as shown in Fig 11. Place the LCD down onto the headers and solder into place. Fig 11 Backside of the display PCB board 7. Solder the 18-pin IC socket at U2. 13

14 8. Insert the PIC16F88 microcontroller into the IC socket. Be sure to line up the notch in the chip with the notch in the socket. REPORT REQUIREMENTS 1. Complete all the four tasks in the lab sheet. 2. Submit your report by Moodle in two weeks. 14