DISGN AND FABRICATION OF HIGH VOLTAGE POWER SUPPLY FOR RADIATION DETECTOR

Application of ionizing radiation is remaining widely in the daily lives of people such as scientific research, industry , medicine, military, and agriculture. Moreover , safely in work place is very important especially when it involves radioative materials. There are four major types of ionizing radiation from radioactive materials: alpha, beta, gamma and Xray,a part of the electromagnetic spectrum and ionizing radiation, specifically at its high frequency and short wavelengths.

VIETNAM NATIONAL UNIVERSITY, HANOI

VNU UNIVERSITY OF SCIENCE FACULTY OF PHYSICS

DUONG DINH QUY

DESIGN AND FABRICATION OF HIGH VOLTAGE POWER SUPPLY FOR RADIATION DETECTOR

Submitted in partial fulfillment of the requirements for the degree of

Bachelor of Science in Nuclear Technology

(Advanced Program)

VIETNAM NATIONAL UNIVERSITY, HANOI

VNU UNIVERSITY OF SCIENCE FACULTY OF PHYSICS

DUONG DINH QUY

DESIGN AND FABRICATION OF HIGH VOLTAGE POWER SUPPLY FOR RADIATION DETECTOR

Submitted in partial fulfillment of the requirements for the degree of

Bachelor of Science in Nuclear Technology

(Advanced Program )

Supervisor: Dr NGUYEN THE NGHIA

Hanoi - 2017

ACKNOWLEDGEMENT

I have completed my thesis which is a partial fulfillment of requirement for the degree of Bachelor in Nuclear Technology

On this opportunity, I would like to express my gratitude to the Department

of Nuclear Physics, Faculty of Physics, VNU-University of Science generally and especially to my supervisor Dr Nguyen The Nghia for his help, advice and guidance throughout the process of collecting information, analyzing, experiment and completing the thesis

I would also like to thank my teacher Bui Thi Hoa for her assistance and kind attitude in the process of doing this thesis

To my family, I would like to express millions of thanks to them for their support and always understanding in everything Hence, I would like to thanks to all my friends for their great cooperation and supports either direct or indirect

Student,

Duong Dinh Quy

ACKNOWLEDGEMENT i

ACRONYMS iv

LIST OF FIGURES v

LIST OF TABLE vii

PREFACE viii

CHAPTER 1: INTRODUCTION TO HIGH VOLTAGE POWER SUPPLY 1

1.1 Principle of radiation detector 1

1.2 High voltage power supply 3

CHAPTER 2: STRUCTURE OF HIGH VOLTAGE POWER SUPPLY 5

2.1 Oscillator 5

2.1.1 Requirement for common Oscillator 5

2.1.2.Phase-Shifting Oscillators 8

2.2 Inverse phase amplifier 9

2.3 Push-pull amplifier 12

2.4 Ferrite core transformer 15

2.5 Voltage multiplier 16

2.6 Stability voltage 17

2.7 Low voltage power 18

CHAPTER 3: CONSTRUCTION OF REALITYS HIGH VOLTAGE CIRCUIT BOARD 20

3.1 Low voltage DC power circuit 20

3.2 Calculation for Oscillator 20

3.3 Biasing for transistor in phase splitter stage 23

3.4 Design for step-up transformer 24

3.5 Voltage divider and stability voltage stage 25

3.6 Reality high voltage power supply 26

CHAPTER 4: RESULTS AND DISCUSSIONS 30

CONCLUSIONS 34

REFERENCES 35

ACRONYMS List Acronym Definition

LIST OF FIGURES

Figure 1.1.Nuclear instrument for radiation detection and measurement 1

Figure 1.2.Gas-filled detector 2

Figure 1.3.Scintillation detector 3

Figure 1.4.High voltage level apply for gas-filled detector 4

Figure 2.1: Block diagram of the typical radiation detector 5

Figure 2.2.Conventional form of oscillator with feedback 6

Figure 2.3.Phase-shifting oscillator’s principle 8

Figure 2.4 All-pass network filter 9

Figure 2.5.The phase angle of all-pass network 9

Figure 2.6.Common emitter configuration 10

Figure 2.7.Common base configuration 10

Figure 2.8.Common collector configuration 11

Figure 2.9.Transistor with internal resistor 11

Figure 2.10.Class B output characteristics curves 13

Figure 2.11.Push-pull power amplifier 14

Figure 2.12.Ferrite core transformer 15

Figure 2.13.The typical Cockcroft-Walton Multiplier circuit 17

Figure 2.14.Configuration for stability high voltage 18

Figure 2.15.Low voltage DC source 19

Figure 2.16.AC current and DC current 19

Figure 2.17.Half-wave Rectifier RC-Filter 19

Figure 3.1.DC power ±15V 20

Figure 3.2.All-pass filter for sinusoidal oscillator 21

Figure 3.3.Circuit sinusoidal oscillator 21

Figure 3.4.Simulation of sine wave generator by Proteus 8.5 22

Figure 3.5.Phase splitter stage 23

Figure 3.6.Stability voltage stage 25

Figure 3.7.Schematic diagram circuit for high voltage power supply 26

Figure 3.8.PCB in simulation 27

Figure 3.9.Printed circuit board in reality 27

Figure 3.10.High voltage power supply circuit board 28

Figure 3.11.High voltage power supply has been finally approved 28

Figure 3.12.Source ±15V DC 29

Figure 4.1.Operating of High Voltage circuit at 2000V 32

LIST OF TABLES

Table 2.1.Comparision of three configuration operating BJT 12

Table 4.1.Determination of high voltage value at 2kV 30

Table 4.2.Measurement for four level voltage 31

Table 4.3 The dependence of the high voltage on the frequency of oscillator 32

PREFACE

High Voltage Power Supply is essential for nuclear radiation detection or counting system A high voltage power supply unit that is suitable to use with Geiger Muller counter, scintillation detector and semi-conductor detector which are constructed with locally available components The constructed high voltage unit is based on push-pull topology driving high frequency transformer The desired voltage is rectified and multiplied by voltage multiplier unit provides 2 kV-DC at approximately 1mA maximum output current

In this study, I will describe how to make a High Voltage Power Supply from fundamental of theory to manufacture diagram schematic circuit and reality production I constructed a high voltage power supply based on the design, simulation and step by step fabrication stage of High Voltage Supply for G-M counter and Scintillation detector

The content of the thesis is divided into four main chapters:

Chapter 1: Introduction to High Voltage Power Supply

Chapter 2: Structure of High Voltage Power Supply

Chapter 3: Construction of High Voltage Circuit Board

Chapter 4: Results and Discussions

CHAPTER 1 INTRODUCTION TO HIGH VOLTAGE POWER SUPPLY

1.1 Principle of radiation detector

Application of ionizing radiation is remaining widely in the daily lives of people such as scientific research, industry, medicine, military, and agriculture Moreover, safety in work place is very important especially when it involves radioactive materials There are four major types of ionizing radiation from radioactive material: alpha, beta, gamma and X-ray, are part of the electromagnetic spectrum and ionizing radiation, specifically at its high frequency and short wavelengths These ionizing radiations are hazard and cannot be seen, felt or sensed

by the human body in any way Radiation measuring instruments such as detector and dosimeters are needed in order to detect the presence of such radiations and avoid excessive exposure

Figure 1.1.Nuclear instrument for radiation detection and measurement [1]

The use of appropriate and efficient radiation instruments enables exposures

to be controlled and the doses received to be kept as low as reasonable achievable.The ability to identify sources of radiation, specific radioisotopes, and measure

quantities of radiation is crucial to environmental monitoring, radiation protection, and development of security programs Generally the device is able to detect some

or all of the four major types of ionizing radiation by Geiger Muller counter, scintillation detector or semi-conductor detector

A Geiger counter is a metal cylinder filled with low pressure gas sealed in by

a plastic or ceramic window at one end Running down the center of the tube there's

a thin metal wire made of tungsten The wire is connected to a high, positive voltage

so there's a strong electric field between it and the outside tube

When radiation enters the tube, it causes ionization, splitting gas molecules into ions and electrons The electrons, being negatively charged, are instantly attracted by the high voltage positive wire and as they zoom through the tube collide with more gas molecules and produce further ionization

Figure 1.2.Gas-filled detector [2]

The result is that lots of electrons suddenly arrive at the wire, producing a pulse of electricity that can be measured on a meter The ions and electrons are quickly absorbed among the billions of gas molecules in the tube so the counter effectively resets itself in a fraction of a second, ready to detect more radiation Geiger-Muller counters can detect alpha, beta, and gamma radiation

The second most common type of radiation detecting instrument is the scintillation detector The basic principle behind this instrument is the use of a special material which glows or “scintillates” when radiation interacts with it

Figure 1.3.Scintillation detector[2]

The light produced from the scintillation process is reflected through a clear window where it interacts with device called a photomultiplier tube The photocathode has the unique characteristic of producing electrons when light strikes its surface These electrons are then pulled towards a series of plates called dynodes through the application of a positive high voltage When electrons from the photocathode hit the first dynode, several electrons are produced for each initial electron hitting its surface This bunch of electrons is then pulled towards the next dynode, where more electron multiplication occurs The sequence continues until the last dynode is reached, where the electron pulse is now millions of times larger, then it was at the beginning of the tube At this point the electrons are collected by

an anode at the end of the tube forming an electronic pulse The pulse is then detected and displayed by a special instrument Scintillation detectors are very sensitive radiation instruments and are used for special environmental surveys and

as laboratory instruments

1.2 High voltage power supply

Regardless of the type of detector, the most important component is also high voltage power supply It is essential for nuclear radiation detection or counting system When a high voltage is applied an electric field is created by the potential difference between the electrodes Electrons and positively charged gas atom of each ion pair accelerate to anode and cathode, respectively, resulting in an electric signal in the circuit that can be correlated to radiation exposure and displayed as a value

The voltages can vary widely depending upon the detector geometry and the gas type, pressure and type of detector

Figure 1.4.High voltage level apply for gas-filled detector[3]

For example, the different voltage regions are indicated schematically

in Figure 1.4 There are six main practical operating regions of gas-filled detector, where three are useful to detect ionizing radiation: Ionization chamber, proportion and Geiger Muller region The voltage is applied about hundreds of volts

For the high voltage power supply of the photoelectric multiplier tube, the output voltage depending on the different types of photomultiplier tubes to hundreds

of volts to three kilovolts and adjustable The output current is approximately a few

mA In order to meet the detector output and count rate into a good linear relationship, reduce fluctuation error, so the stability of the high voltage power demand is very high

In my graduation paper, I focus on the designed high voltage power supply capable of converting 15V into 2kV The output high voltage can be adjusted at any suitable value by adjusting the mechanical control The basic principle behind this instrument is the use of DC-DC converter based on a push-pull topology utilizing power bipolar junction transistor switches, a center-tapped ferrite core transformer, and an analog feedback control system

Specially, this design has an oscillator capable of adjusting frequency operating circuit so that the efficiency of high voltage power supply is rather high

CHAPTER 2 STRUCTURE OF HIGH VOLTAGE POWER SUPPLY

The high voltage power supply of the system is realized by the DC-DC converter using transistor The main unit of the method is an oscillator, low voltage

DC to the oscillator power supply, the AC output of the oscillator is boosted by a step-up transformer, and after the rectifier, the filter is used to obtain the DC high voltage.The block diagram of the typical radiation detector is as shown in Figure 2.1

Figure 2.1: Block diagram of the typical radiation detector

2.1 Oscillator

There is a need of an oscillator in the inverter circuit for producing stable oscillating AC output Harmonic oscillator of adjustable frequency is used in DC-

DC power inverter The frequency of oscillation of the required AC output is

determined by the values of capacitor and resistors, this is all of purpose in this part 2.1.1 Requirement for Common Oscillator

Oscillator are circuits that produce specific, periodic waveforms such as square, triangular, saw-tooth, and sinusoidal They generally use some form of active device, operational amplifier (Op-amp), transistor or crystal surrounded by passive devices such as resistors, capacitors, and inductors, to generate the output

There are two main classes of oscillator: sinusoidal and relaxation Sinusoidal oscillators consist of amplifiers with RC or LC circuits that have

adjustable oscillation frequencies, or crystals that have a fixed oscillation frequency Relaxation oscillators generate triangular, saw-tooth, square, pulse, or exponential waveforms The focus here is on sinusoidal oscillators, created using operational amplifiers op-amps[5]

Sinusoidal oscillators are used as references or test waveforms by many circuits Thus, a sine wave may be the input to a device or circuit In this case, sinusoidal oscillator is input of amplify stage Op-amp sine wave oscillators operate without an externally applied input signal Instead, some combination of positive and negative feedback is used to drive the op-amps into an unstable state, causing the output to cycle back and forth between the supply rails at a continuous rate The frequency and amplitude of oscillation are set by the arrangement of passive and active components around a center op-amps The basic characteristic of harmonic oscillator is frequency operating circuit, output amplitude, stableness and efficiency[5]

Depending on the purpose of use, the design may pay particular attention to some parameters or lower the requirements for other parameters, so depending on the requirements of use, considering and determining parameters for reasonable

Regardless of the type of oscillator in general, the principle of operation of the oscillators are similar

The principle of a general oscillator is show in figure 2.2 The oscillator consist of two components: amplifier and feedback The gain of them is a complex number In order for the circuit to oscillate, the amplitude and phase conditions must be met

Figure 2.2.Conventional form of oscillator with feedback[28]

Amplifier have a rude amplify signal input, its amplify gain is .ei k

K  K  , and the feedback gain is ei fb

or written another way

( ) . i k fb 1

Requirement of amplitude: K K fb 1

(2)

Requirement of phase:    fb  k  2 n  with n= 0, ±1, ±2… (3)

 is total phase shifting angle of amplifier and feedback, it show that oscillator circuit is only oscillate when gain’s amplifier can be compensate waste of feedback circuit Simultaneous, output of feedback circuit has the same phase with input amplifier signal[28]

There are three case of oscillator circuit will appear in operation:

− If K K. fb  1, amplitude will descend as an exponential function, oscillation will attenuation

− If K K. fb  1, output signal is sine-wave with frequency fo and constant amplitude

− If 0 K K. fb  1, amplitude will progressive as an exponential function

In conclusion, oscillator is only operating when in the first instance initial we have K K. fb  1 for amplitude progressive to saturation mode of circuit, after that amplify gain descend to behavior K K. fb  1 In that time, oscillation is not sine wave, therefore we always need control amplify gain to satisfy K K. fb  1 and establish this state[28]

2.1.2.Phase-Shifting Oscillators

This graduation paper will describes a phase shift oscillator based on phase shifting networks called all-pass networks It also uses a basic automatic gain control circuit to stabilize the amplitude of the output At first, the principle of phase-shifting oscillator is shown in figure 2.3[6]:

Figure 2.3.Phase-shifting oscillator’s principle[6]

The circuit is based on two 90o phase-shifting networks, followed by an inverter stage, giving a total loop phase shift of 360o

The phase-shifting network is in fact a first order all-pass filter, the transfer

function of which is defined by F(s) o

o

s s

 

   , where o is the corner frequency This function has a constant magnitude equal to 1 at all frequencies, while the phase shift varies from 0o to 180o The phase shift attains 90o at the corner frequencyo,

this will thus be the oscillation frequency

A circuit that provides unity gain with variable phase shift, called an all-pass network, is shown in Figure 2.4

Figure 2.4 All-pass network filter[6]

The magnitude of thus, always one and the phase angle is given by

Figure 2.5.The phase angle of all-pass network[6]

The oscillation frequency can be adjusted by varying Ro or Co Since there are two all-pass networks used in the oscillator circuit, a two-ganged element will

be required to adjust the frequency[6]

2.2 Inverse phase amplifier

In order to properly drive a push-pull output stage of an amplifier we need two signals of equal voltage and opposite phase In this part, I will introduce three operating configuration of bipolar junction transistor (BJT)

Normally whatever signals we want to amplify will be of the order millivolts

or less If we directly input these signals to the amplifier they will not get amplified

as transistor needs voltages greater than cut in voltages for it to be in active region Only in active region of operation transistor acts as amplifier So we can establish

appropriate DC voltages and currents through BJT by external sources so that BJT operates in active region and superimpose the AC signals to be amplified The DC voltage and current are so chosen that the transistor remains in active region for entire AC signal excursion All the input AC signals variations happen around quiescent point[7]

The bipolar junction transistor (BJT) has three terminals, so can be used in three different configurations with one terminal common to both input and output signal The circuits shown here are drawn without biasing and power supplies for clarity

Figure 2.6.Common emitter configuration[8]

The common emitter configuration has the emitter terminal common to both the input and output signal The arrangement is the same for a PNP transistor, except that the power supplies will have the opposite polarity Used in this way the transistor has the advantages of medium input impedance, medium output impedance, high voltage gain and high current gain

Figure 2.7.Common base configuration[9]

When the base is used as the common terminal, the transistor will have a low input impedance, high output impedance, unity or less current gain and high voltage gain This configuration also realizes the best high frequency performance

Figure 2.8.Common collector configuration[8]

This last configuration is also commonly known as the emitter follower This

is because the input signal is applied to the base and passes out at the emitter with little loss Stage properties are high input impedance, very low output impedance, a unity (slightly less) voltage gain and high current gain The circuit is also used extensively as a "buffer" converting impedance's or for feeding or driving long cables or low impedance loads

From figure 2.9, we can determine some characteristics type of configuration transistor, emitter common, collector common and base common

Figure 2.9.Transistor with internal resistor[8]

Table 2.1.Comparision of three configuration operating BJT[8]

AMPLIFIER TYPE COMMON

BASE

COMMON EMITTER

COMMON COLLECTOR

From the characteristic of three configurations, This paper selected two configuration for this stage: emitter common and collector common Therefore, two signals of equal voltage and opposite phase by voltage divider biasing for transistor The calculation will illustrate in the next chapter

2.3 Push-pull amplifier

Amplifier circuits form the basis of most electronic systems, many of which are required to produce high power to drive some output device There are many different types of power amplifiers – class A, class B, class C and class AB This thesis mainly deals with class B power amplifiers In Class B amplifier, the positive and negative halves of the signal are dealt with by different parts of the circuit The output devices continually turn on and off Class B operation has the following

Figure 2.10.Class B output characteristics curves[11]

The advantage of the class B push-pull amplifier design over the class A design is greater output power capability With a class A design, the transistor dissipates a lot of energy in the form of heat because it never stops conducting current At all points in the wave cycle it is in the active mode, conducting substantial current and dropping substantial voltage This means there is substantial power dissipated by the transistor throughout the cycle In a class B design, each transistor spends half the time in cutoff mode, where it dissipates zero power (zero current equal to zero power dissipation) This gives each transistor a time to rest and cool while the other transistor carries the burden of the load Class A amplifiers are simpler in design, but tend to be limited to low power signal applications for the simple reason of transistor heat dissipation[10]

Push-pull amplifier is mostly used in situations where low distortion, high power and higher efficiency is required The basic operation of a push pull amplifier

is as follows: The signal to be amplified is first split into two identical signals 180oout of phase Generally the splitting is done using an inverse phase amplifier in before part