Physics of Radiography Electricity X-ray tube and circuitry

Physics of RadiographyElectricity X-ray tube and circuitry

What we know so far…

A photon is…………?

A negatively charged electron is attracted to what kind of charge (positive or negative)?

To calculate energy you can multiply plancks constant (h) by what?Radio waves have lower frequency than x-rays. True or false?

What is a vacuum?

The Joule and the electron Volt are both measures of what?

By the end of the session you should be able to:

1. Draw a detailed labelled diagram of an X-ray tube2. Demonstrate understanding of where the electron beam originates

from3. Explain how electrons are accelerated within an x-ray tube4. Understand the importance of the glass envelope5. Calculate maximum kinetic (movement) energy for a given

accelerating voltage6. Have an awareness of what alternating current is7. Understand the principles of electromagnetic induction8. Describe how step up and step down transformers work9. Know why we need rectifiers and what they do10.Understand Dose and Dose limitation11.Understand the inverse square law of attenuation of energy

Dosimetry key terms:

Radiation absorbed dose (D)Energy absorbed per unit mass of tissue

Equivalent dose (H)Dose taking into account a weighting factor due to properties of

ionizing radiation

Effective dose (E) Dose taking into account a weighting factor due to different

sensitivities to radiation of different body parts

Collective doseTotal effective dose on a population

Dose rateDose per unit time

Radiation absorbed dose (D)

Energy absorbed per unit mass of tissue

- The amount of ENERGY absorbed from a beam of radiation per unit of mass tissue

- Units: Gray (Gy) measured in Joules/kg- mGy (milligray is 1000x smaller)

This used to be measured in rad where: 1 Gray = 100 rads

Equivalent dose (H)Dose taking into account a weighting factor due to properties of

specific ionizing radiation

It is a quantity that expresses the probability that exposure to ionizing radiation will cause biological effects.

There are different types of ionizing radiation which ‘lose’ energy in different ways, e.g. alpha particles would only penetrate a few millimetres and be totally absorbed, x-rays are only partially absorbed so the biological effect would be far less severe for x-rays than alpha particles.

To work out equivalent dose you multiply the dose by the weighting factor of the radiation

Equivalent dose (H) = Radiation absorbed dose (D) x Weighting factor (Wr)

Xrays, gamma rays and beta particles; Wr = 1Alpha particles Wr = 10

It is measured in sieverts (Sv) but used to be rems where 1Sv = 100 rems

Effective dose (E) Dose taking into account a weighting factor due to different sensitivities to radiation of different body parts

This is what is normally being referred to when the word ‘Dose’ is used. It is calculated by multiplying the sum of the equivalent dose by the tissue weighting factor

Effective dose (E) = Sum of Equivalent dose (H) x Wt

Units are also Sieverts (Sv) or millisieverts mSv

Organs

Tissue weighting factors (Wt)

ICRP1032007

Gonads 0.08

Red Bone Marrow 0.12

Colon 0.12

Lung 0.12

Stomach 0.12

Breasts 0.12

Bladder 0.04

Liver 0.04

Oesophagus 0.04

Thyroid 0.04

Skin 0.01

Bone surface 0.01

Salivary glands 0.01

Brain 0.01

Remainder of body 0.12

Total 1.00

Collective doseTotal effective dose on a population from a particular source of

radiation.

Collective dose = effective dose (E) x population

(Units: man-sievert (man-Sv))

Dose rateDose per unit time e.g. dose per hour often

Units measured in microsieverts per hour

Sources of radiation

54%

Natural sources:- Cosmic radiation from earths

atmosphere- Gamma radiation from rocks

and soil- Radiation from certain foods

e.g. potassium 40- Radon (gas produced when

uranium in granite decays)

Artificial sources:- Nuclear fallout- Radioactive waste- Medical/dental- Occupational

Limiting dose to general public e.g. people in a waiting room

Consider where x-ray the beam is aimed, e.g. into waiting room/corridorsConsider thickness of walls and what they are made of – are they good at attenuating x-rays?

Limiting dose to radiation workers

Radiation dose sources: Primary beam, scattered radiation, radiation leakage

Ways to limit: Stay out of the primary beam! Move further away from the source (recall how intensity decreases by the square of the distance between you and the source)

The further you go away from an energy source, the more the energy dissipates.The energy spreads in all directions and the surface area of the imaginary sphere on which energy acts increases the further away you go.Energy which is experienced at a distance 2xr away will be one quarter that experienced at r.Energy which is experienced at a distance 3xr away will be one ninth that experienced at r.

The decrease in energy is said to follow an inverse square law where the energy at a distance d is 1/d2

(Where d2 is dxd)

- Check understanding: What fraction of initial energy at r be experiencedAt a distance of 4xr

What fraction of initial energy at r be experiencedAt a distance of 5xr

Electric Current & Static Electricity

Where static electricity continues to build up in one place, it may result in a spark or an electric shock when electrons eventually move.

An electric current is the flow of electric charge (carried by electrons) around a circuit.

Protons play an important part in electrostatics, as they provide the positive charge in an atom. However, protons are tightly bound within the nucleus, so they are not free to move around.

In certain circumstances, there can be a build up of electric charge which is not free to move - this is called static electricity. An excess of electrons causes a negative charge; a shortage of electrons creates a positive charge.

Charge & Conductors

When the current is switched on, the electrons move along the material and are carried around the circuit.

Press ‘No Current’ and ‘Current’ to see how the electrons behave in a conductor...

The flow of electrons around the circuit is called the current.

When no current is flowing, the free electrons in a conductor such as a metal, drift around in random directions.

The size of the current depends upon the number of electrons passing per second.

Charge & InsulatorsIn an insulator such as plastic, electric charge cannot flow because the electrons are all tightly held to the atoms - they are not free to move around.

Insulators are useful in electric circuits, because they stop the flow of current, e.g. insulation around wires.

However in some situations, insulators can develop a static electrical charge.

Charge is a property of certain particles. A particle with charge will experience a force in an electric field (or in a magnetic field if the charge is moving).Charge is either positive or negative. Objects with a similar charge will repel. Objects with opposite charges will attract.Charge is measured in coulombs, C. The amount of charge on an object can be found using a coulomb meter.

Current is the rate of flow of charge; it is the amount of charge flowing per second through a conductor.How can you get the Charge to Flow?A conductor is needed for charge to flow through; then you need to attract or repel the charged particles to make them move. The amount of attracting or repelling you do is measured in volts and is called the voltage or the potential difference (p.d. for short).Work is being done on these charged particles to make them move, so the voltage is a measure of the amount of energy that is provided per coulomb of charge.1 volt = 1 joule per coulomb.

Direct current

Uses ionic solutions to create flow of charge (electrons).

Always involves electrons moving one way round the circuit.

Electrical Generators

Method 1: when the conductor moves and cuts across magnetic field lines, a voltage is induced.

The method by which an electric guitar works islargely based upon induced currents and electrical circuits.

A generator makes electricity from movement - this is known as electromagnetic induction.

Generators can work in one of two different ways:

Electrical Generators

Method 2: the magnet can move in and out of the coil of wire.The moving magnetic force cuts across wires creating the current.

A generator makes electricity from movement - this is known as electromagnetic induction.

Generators can work in one of two different ways:

The direction of the induced current can be reversed by:

reversing the poles of the magnet.

reversing the direction of movement.

Today the modern electrical generator (also known as an alternator) is designed in a similar way to the electric motor.Run the animation to see how the direction of magnetism and motion affect the electricity produced.

Electrical Generator

In the generator, the electricity is taken out by the metal split rings and the carbon brushes.

Increasing the Induced Voltage and Current

1. The speed of movement The faster the speed of movement, the greater the induced voltage, and hence current.

2. The strength of the magnetThe stronger the magnet, the greater the induced voltage, and hence current.

3. The number of turns in the coil of wireThe greater the number of coils in the wire, the greater the induced voltage, and hence current.

4. The cross-sectional area of the coilThe larger the cross-sectional area, the greater the induced voltage, and hence current.

http://www.youtube.com/watch?v=Q8t_12NQpZY

http://www.youtube.com/watch?v=Q8t_12NQpZY

To summarise we can say that:

“the size of the induced voltage across the ends of a coilof wire is directly proportional to the rate at whichthe magnetic lines of flux are being cut”

This was proposed by Michael Faraday in his Law of Electromagnetic Induction, which states that:

the faster the lines are cut, the larger the induced voltage and current.

a voltage and a current are produced when a conducting wire cuts through magnetic field lines.

Increasing the Induced Voltage and Current

The Electrical GeneratorRun the animation to see the display of the electrical output for the generator...

Generators produce an alternating current (AC).

Mains electricity is Alternating current, batteries are direct current with a fixed positive end and a fixed negative end. If you want to use alternating current to continually e.g. attract electric charges you have to change the source to allow only positive charges to be used. For this you use a rectifier.

Ac/DC

Circuit diagram symbols

- The CATHODE (negative charge) heated filament of tungsten providing electrons by thermionic emission

Thermionic emission:

Thermionic emission is the release of electrons from a heated metal.The electrons in the metal gain kinetic energy from heat. Electrons that gain sufficiently high kinetic energy will be able to escape from the surface of metal.


- The ANODE (positive charge) tungsten target in copper block (allowing efficient removal of excess heat) sometimes oil or water are used to facilitate cooling

99% of electrons energy goes into heating rather than x-ray production



- A FOCUSSING DEVICE used to direct the electrons to a focal point on the target




- GLASS ENVELOPE encases the x-ray tube in a vacuum such that the electrons are not impeded by air molecules and number of electrons and speed of flow can be controlled

Glass housing has the purpose of providing an envelope within which a vacuum can be maintained. The vacuum permits independent control of both the number of electrons that constitutes an electron beam and the speed of flow of the electrons. The vacuum eliminates the possibility of collisions between molecules of air and accelerated electrons. In addition, removal of air prevents deterioration of the filament by oxidation.





- Between cathode and anode a high-voltage is connected (kV)





- Between cathode and anode a high-voltage is connected (kV)

- A current (mA) determining the amount of electrons being accelerated flows from the cathode to the anode

For x-ray production we need:• High filament current using a low voltage supply – this gives more

electrons and a higher intensity beam• High tube voltage so the electrons emitted at the filament are

accelerated towards the target Higher electron kinetic energy = greater quality/penetrating power

Step down transformer

Step up transformer

The relationship between the voltages across each of the coils is shown by the following equation:

This can be summarised as:

VP = NP VS NS

input voltage = number of turns on primary coil output voltage number of turns on secondary coil

Transformers - Calculations

VP = NP VS NS

Transformers - CalculationsExample 1:

A transformer has 50 turns on its primary coil and 300 on itssecondary coil. If an alternating voltage of 2 volts is applied across the primary coil, what is the voltage across the secondary coil?

2V = 50 VS 300

VS = 12V

VS = 300 x 2V 50

This is a step-up transformer, and the output voltage is 12V.

Transformers - CalculationsExample 2:

Calculate the output voltage from a transformer when the input voltage is 230V, the number of turns on the primary coil is 500, and the number of turns on the secondary coil is 50.

VP = NP VS NS

230V = 500 VS 50

VS = 23V

VS = 50 x 230V 500

This is a step-down transformer, and the output voltage is 23V.

For an input voltage of 230V with 10 turns on the primary, if the secondary coil had 5 turns, what would the output voltage be?





Write 10 good questions that you think you might be asked on your assessment, add answers on the back.

Now swap with someone on your table, see how they do!

Write an acrostic poem using ‘x-ray tube’

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Physics of Radiography Electricity X-ray tube and circuitry