122 - Week 2 Electronics and Gas Detectors

Tues and Thurs lecture questions

What is valence?

• Where electrons “live” in a normal state 

• The outer-most shell of the electron 

What is conduction?

Where electrons must be to move freely in order to conduct electricity

What is a conductor?

Material that easily allows electrons to flow through it

What are examples of conductors?

Most metals – copper, silver, and aluminum

What are insulators?

• Material that does not allow electrons to move easily 

• Has a gap that makes it hard for electron to travel the larger that distance

What are examples of insulators?

• Rubber 

• Glass 

• Plastic 

What is the importance of an insulator in terms of safety?

Necessary to protect us from handling the electrical parts so we aren’t shocked

What is a semiconductor?

• A material that sometimes conducts electricity 

• Has a small band gap that will sometimes allow electrons if they’re excited enough or in the right conditions

What are semiconductors a hybrid of?

Hybrid of an insulator and a conductor

What are examples of semiconductors?

• Silicon

• Germanium

Where would you commonly find semiconductors?

Seen in solid-state detectors – our newer cameras

What 3 things make up the electron band theory?

• Valence band 

• Conduction band 

• Forbidden gap 

What are valence bands and conduction bands in the electron band theory?

Energy levels that electrons can occupy – specifically solids

What is a valence band?

• Outermost electron shell where the electrons ‘live’ 

• Electrons that get involved with forming chemical bonds

What is a conduction band?

• Band that’s a higher energy level where electrons can move for freely as they’re not as bound to the nucleus 

• Contributes to electrical conductivity

What is the forbidden gap?

Gap between the valence band and the conduction band

Which factor of the electron band theory influences the electrical properties of the atom?

The forbidden gap – explains why some materials will conduct electricity and others won’t

What is covalent bonding?

Interaction between electron orbitals of two individual atoms

Are covalent bonds relevant to us in nuclear medicine?

No, as we aren’t trying to form covalent bonds as we want to free the electrons so that we can measure them

What is delocalized bonding?

Interaction of electron orbitals of many atoms

A valence bond is ____ to ______

Analogous to bonding orbital

A conduction bond is ____ to _____

Analogous to antibonding orbital

What is a capacitor?

Is the basic premise of all of the gas detectors – but is also found in other instruments

What is a capacitor constructed of?

• Two conducting plates separated by a non-conducting material (gas)

• Power supply that gives a positive charge to one and a negative charge to the other

What type of electric field is made by a capacitor?

Uniform electric field between the two plates with little to no field outside of it

What is electricity in terms of nuclear medicine?

Flow of electrons that carry the electric charge

What happens when electrons flow through a conductor?

Becomes an electric current

What is voltage?

• A measure of electrical potential difference, often described as the “push” that drives electrons through a circuit 

• Force or pressure that pushes electrons through a circuit 

• Like water pressure in a hose – more pressure = stronger push

Why is voltage important in nuclear medicine?

All of our instruments have some sort of high voltage supply that pushes the electrons to be collected

What is current?

The flow of electrical charge, typically carried by electrons

What is resistance?

• A material’s opposition to the flow of current

• How much a material resists the flow of current

• Like a kink in a hose that slows down the water

What happens when there’s too much resistance in our instruments in nuclear medicine?

The more resistance there is in our instruments, the more likely they won’t work as well

What 2 things increase the resistance?

• The length of our cords 

• Temperature as heat will cause more resistance – overheating 

What is Coulomb force in terms of the behavior of electric charges?

Like charges will repel and opposite charges will attract

What is the electric field?

• Accompanies any object with an electrical charge 

• Any thing that has a charge will have its own electrical charge along with its own magnetic field

What forms a dipole?

A positive charge and a negative charge

What is Coulomb?

Basic unit of electric charge (Q)

What is current?

• Amount of charge moving past a reference point per unit of time (I)

• Like water flowing through the pipe – the more flow = higher current

What is potential?

Source of electrical “pressure” to cause current (V)

What is resistance?

Opposition to electron movement (R)

What is capacitance?

Amount of charge stored on either plate of a capacitor (C)

What is current measured in?

Measured in amperes (A)

What is the importance of measuring our instruments in amperes?

• Important with our hybrid cameras that have CT components attached to it 

• How we create the X-rays in CT

What is voltage measured in?

Measured in volts (V)

What is resistance measured in?

Measured in ohms (Ω)

What is the importance of Ohm’s Law?

Important in electronics because it helps you understand how circuits behave as nuclear medicine uses a lot of circuits

What is the equation for Ohm’s Law?

V = I × R – Voltage = Current × Resistance

What are circuits?

Complete path that an electric current can travel through

What are circuits composed of?

Composed of individual electronic components like resistors, transistors, and capacitors

What are circuits connected together by? (explain how this material helps the circuit)

Connected by conductive wires where electrons will flow from the power source through the components that will perform the work and back again

What are open circuits?

Incomplete or broken meaning that there’s no current flowing

What are closed circuits?

Complete and allows the current to flow

What are short circuits?

Low resistance path that bypasses the circuit and won’t cause excessive flow

What are parallel circuits?

Components that are connected across multiple paths

What are series circuits?

All components are connected and to end in a single path

What is the difference between parallel and series circuits?

• Parallel: All of the lights will turn-on at the same time

• Series: Lights will turn-on in turns 

What are combination circuits?

Mix between series and parallel

What are AC circuits?

• Alternating and can change directions 

• Plug into the wall

What are DC circuits?

• More direct and current will only flow in one direction

• Battery operated 

Explain how a closed-loop circuit works. (think flashlight)

• Battery supplies voltage to move the electrons 

• Filament provides resistance, electrons in filaments will generate the light photons 

• Switch determines whether current flows 

What are 3 uses of RC circuits in radiation detectors?

• Conversion of electric charge to voltage signal 

• Pulse shaping 

• Source of voltage (acts like a battery during discharge) 

For RC circuits, radiation detectors convert electric charges to a voltage signal. Explain a little of how it works for this circuit. (think how charge impacts gas detectors)

Will be seen in our gas detectors where the free electrons are collected at the anode – electric charge is converted into a voltage signal

For RC circuits, radiation detectors have pulse shaping. Explain a little on how it works for this circuit. (think scintillation detectors)

Our scintillation detectors create a pulse  and we can then manipulate that pulse – how we’re able to see the energy levels of our photons

What are p-type semiconductors?

• Contains the impurity with one less electron than its needed for the molecular structure – creates holes – can migrate through the semiconductor molecule 

• Has a negative charge

What are n-type semiconductors?

• Has an impurity with an extra electron (also mobile) 

• Has a positive charge 

What is the purpose of a logic circuit?

Found in our scintillation detectors and are used to help filter out what’s contributing to the image and what’s not

What is the function of the amplifier?

Enhances a small signal so that it can be passed through other electronic modules

What type of gases (2) are found in a survey meter?

• Xenon 

• Argon

When is a free electron made in a survey meter?

When the ionizing radiation enters through the window and interacts with the gas – ionizing it – and creates a free electron

What part of the survey meter is the cathode? The anode?

• Cathode: Outside of the probe – attracts the positive ions 

• Anode: Wire on the inside of the probe – attracts the electrons

What normally is needed before the amplifier?

A pre-amplifier as both will work to enhance the small electric signal and preps it to go through the circuit board

What is a photodiode?

• Converts light to an electronic signal (similar to a photomultiplier tube) 

• Like a digital counting pad – when the photon hits the diode, it will immediately get converted into an electronic signal 

What is the purpose of the power supply?

• Used to energize the gas found inside – helps push the electrons to the anode (gas detectors) 

• Or is crucial to certain components of the instruments so that they can operate (scintillation detectors and the PM tubes)

What is a rate meter?

Provides a rough visual indicator of the counting rate

What is a cathode ray tube?

• Focuses electron beam on phosphor-coated screen 

• Seen in older cameras 

When the signal goes through the circuits of our nuclear medicine instruments it undergoes 3 things before it becomes an image…

• Amplified to make the signal stronger 

• Shaped so we can tell where and how strong it was – more easier to manipulate or distinguish between 

• Digitized so the computers can turn it into an image