Ionization of Air by X-rays and Gamma Rays

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Questions and Answers

What is the primary consequence of x-rays or gamma rays passing through air?

Ionisation of air and creation of a charged volume (correct)

Creation of a magnetic field in the air

Heating of the air molecules

Creation of a vacuum in the air

What is the energy required to cause a single ionisation of air at STP?

50.97eV

33.97eV (correct)

40.97eV

20.97eV

What is the relationship between the amount of charge produced and the energy absorbed from x-rays?

The amount of charge produced is directly proportional to the energy absorbed (correct)

The amount of charge produced is independent of the energy absorbed

The amount of charge produced is inversely proportional to the energy absorbed

The amount of charge produced is proportional to the square of the energy absorbed

What is the difference between collision kerma and radiative kerma?

Collision kerma is energy absorbed locally, while radiative kerma is energy taken away Signup and view all the answers

What is the unit of measurement for absorbed dose?

Gy/Kg Signup and view all the answers

What is the process referred to when a photoelectron released from direct ionisation goes on to ionise another atom?

Indirect ionisation Signup and view all the answers

What is the main aim of diagnostic imaging?

To produce images of the highest diagnostic quality at the lowest radiation dose Signup and view all the answers

What is the purpose of ARPANSA codes of practice?

To establish regulatory requirements for ionising radiation in medicine Signup and view all the answers

What is the role of the Responsible Person in radiation protection?

To have overall management responsibility of the radioactive source, radiation-producing equipment or medical practice Signup and view all the answers

What is the purpose of Dose Reference Levels (DRLs) in radiographic imaging?

To reflect practice at local or national level Signup and view all the answers

What is the concept of Justification in radiation protection principles?

To ensure that a practice involving exposure produces sufficient benefit to the exposed individual or society Signup and view all the answers

What is the primary consideration when determining the availability of equipment for a patient?

The patient's clinical condition Signup and view all the answers

What is the benefit of non-ionising modalities in alternative examinations?

They provide similar information to ionising modalities Signup and view all the answers

What is the purpose of Facility Reference Levels (FRLs) in radiographic imaging?

To define local facility doses for procedures Signup and view all the answers

What is the main risk associated with medical exposures to ionising radiation?

Radiation-induced secondary cancers Signup and view all the answers

What is the desirable characteristic of radiation detectors that depends on physical density and thickness?

Absorption efficiency Signup and view all the answers

What is the effect of high dead time on a detector's temporal response?

It misses a lot of radiation Signup and view all the answers

What is the purpose of risk models in radiation protection?

To predict the probability of cancer induction following exposure to radiation Signup and view all the answers

What is the concept of Optimisation in radiation protection principles?

To select equipment and methods that ensure radiation doses administered to a patient are not greater than necessary Signup and view all the answers

What is the unit of measurement for Detective Quantum Efficiency (DQE)?

Percentage Signup and view all the answers

What is the purpose of a line pair phantom in spatial resolution?

To find the upper limit of spatial resolution Signup and view all the answers

What is the main benefit of medical exposures to ionising radiation?

Successful diagnosis and treatment of diseases Signup and view all the answers

What is the unit of measurement for equivalent dose?

Sv Signup and view all the answers

What is the primary function of an electrometer in a gas-filled detector?

To supply voltage and measure charge Signup and view all the answers

What is the advantage of using thin transmission ion chambers?

They can be used to control exposure in different anatomies Signup and view all the answers

Why are tissue weighting factors used in the calculation of effective dose?

To account for the difference in sensitivities of different organs to ionizing radiation Signup and view all the answers

What is the purpose of personal monitoring for radiation workers?

To monitor the dose received by the worker and prevent biological damage Signup and view all the answers

What is the characteristic of xenon gas detectors that makes them suitable for use in old CT scanners?

Fast response time Signup and view all the answers

What is the primary function of scintillators in radiation detection?

To detect x-rays and generate a signal Signup and view all the answers

What is the purpose of the filter system in a TLD holder?

To record the dose due to beta, gamma, or x-rays Signup and view all the answers

What is the advantage of using neutron radiation in radiation therapy?

It has a higher RBE than x-rays Signup and view all the answers

What is the purpose of area survey monitoring?

To monitor the effectiveness of shielding Signup and view all the answers

Why are heavy charged particles or ions more destructive to biological tissue?

Because they have a higher LET Signup and view all the answers

What is the unit of measurement for the calibration coefficient of an ionization chamber?

nC/Gy Signup and view all the answers

Why are thermoluminescent dosimeters (TLDs) useful for personal monitoring?

Because they can store energy and release it as light when heated Signup and view all the answers

What is the purpose of exposing an ionization chamber to an accurately known exposure?

To determine the calibration coefficient of the ionization chamber Signup and view all the answers

What is the primary function of a photocathode in a scintillation detector?

To convert light into very small electrical signals Signup and view all the answers

What is the main advantage of using diodes instead of photomultiplier tubes (PMTs)?

Smaller size and lower cost Signup and view all the answers

In computed radiography (CR), what is the purpose of the red laser light?

To give trapped electrons enough energy to escape the forbidden band Signup and view all the answers

What is the difference between indirect and direct digital radiography (DR)?

INDIRECT DR uses a scintillator, while direct DR uses a semiconductor Signup and view all the answers

What is the purpose of the forbidden band in electronic band theory?

To separate the conduction and valence bands Signup and view all the answers

What is the advantage of using thin film transistor (TFT) technology in digital radiography?

Ability to construct large active pixel arrays Signup and view all the answers

In digital fluoroscopy, what is the purpose of pulsing the x-ray source?

To synchronize the x-ray source with the detector readout Signup and view all the answers

What is the main difference between structured and non-structured scintillator crystals?

Structured crystals have better light directionality Signup and view all the answers

In computed radiography (CR), what is the purpose of the phosphor screen?

To detect x-ray interactions and create electron-hole pairs Signup and view all the answers

What is the primary function of an electron-hole pair in radiography?

To create an electrical charge pair Signup and view all the answers

What is the main problem with an AC x-ray tube?

There is no attractive force across the tube Signup and view all the answers

What is the purpose of rectification in an x-ray tube?

To convert AC to DC Signup and view all the answers

What is the limitation of using a single diode for rectification?

It only allows current to flow 50% of the time Signup and view all the answers

What is the advantage of full-wave rectification over half-wave rectification?

It reduces the exposure time Signup and view all the answers

What is the problem with the voltage in a full-wave rectified circuit?

It varies from 0 to maximum Signup and view all the answers

Why is full-wave rectification not suitable for imaging?

It produces a variable output Signup and view all the answers

What is the main problem with the output of an AC x-ray tube?

It is pulsed Signup and view all the answers

What is the ideal output for an x-ray tube?

A steady output Signup and view all the answers

What is the purpose of a capacitor in an electrical circuit?

To store and release charge, allowing for smoothing Signup and view all the answers

What is the main advantage of three-phase power supplies?

They are the most efficient way to produce, transmit, and consume electricity Signup and view all the answers

What is the purpose of the filament circuit in an X-ray tube?

To control the temperature of the filament Signup and view all the answers

What is the purpose of the autotransformer in the filament circuit?

To provide a precise voltage to the filament circuit Signup and view all the answers

What is the purpose of the step-down transformer in the filament circuit?

To decrease the voltage and increase the current in the secondary side of the filament transformer Signup and view all the answers

What is the purpose of the Mas timer in an X-ray machine?

To terminate the exposure when the desired quantity of charge has passed from cathode to anode Signup and view all the answers

What is the purpose of the AEC timer in an X-ray machine?

To terminate the exposure when the required amount of radiation has been incident on the IR Signup and view all the answers

What is the relationship between the filament temperature and the rate of thermionic emission?

As the filament temperature increases, the rate of thermionic emission increases Signup and view all the answers

What is the primary function of the x-ray tube?

To generate electromagnetic energy from electrical energy Signup and view all the answers

What is the unit of energy required to move a single electron through a potential difference of 1V?

eV Signup and view all the answers

What is the relationship between the length of the cables and the current squared in electric power transmission?

The loss is proportional to the length of the cables and the current squared Signup and view all the answers

What is the purpose of using transformers in the national grid?

To increase the voltage and decrease the current of AC electricity Signup and view all the answers

How does a transformer work?

The primary coil induces a current in the secondary coil through a changing magnetic field Signup and view all the answers

What is the problem with using AC electricity to generate x-rays in an x-ray tube?

The AC electricity cannot be controlled precisely enough Signup and view all the answers

What is the purpose of the iron core in a transformer?

To provide a path for the magnetic field to induce a current in the secondary coil Signup and view all the answers

What is the relationship between the number of turns on the primary and secondary coils of a transformer?

The relationship between the number of turns on the primary and secondary coils determines whether the voltage is stepped up or down Signup and view all the answers

What is the advantage of using AC electricity in the national grid?

AC electricity can be transformed to higher or lower voltages more easily than DC electricity Signup and view all the answers

What is the purpose of the step-down transformer in a substation?

To decrease the voltage of the AC electricity for household use Signup and view all the answers

Study Notes

Ionisation of Air

Air is normally a good electrical insulator because it has no conducting electrons
X-rays and gamma rays can cause ionisation of atoms and molecules in air, resulting in positively charged ions and free electrons
Direct ionisation: small amount of interaction in low-density gas
Indirect ionisation: electrons from direct ionisation can go on to ionise another atom

KERMA (Kinetic Energy Released per Unit Mass)

Measures energy transferred from photons to charged particles
Measured in J/Kg
Two types:
- Collision kerma (energy absorbed locally)
- Radiative kerma (energy taken away from original x-ray interaction site)

Absorbed Dose

Energy transferred from charged particles to the medium
Measured in J/Kg (Gy)
Can be measured using an ionisation chamber

Equivalent Dose and Effective Dose

Equivalent dose: physical quantity that measures biological effect of different types of ionising radiation to human tissues
- Different radiations have different biological effects at the same absorbed dose
- Units are in Sv
Effective dose: takes into account different organs' sensitivities to ionising radiation
- Tissue weighting factor used to account for this
- If multiple organs are exposed, effective dose is the sum of effective doses to all organs

Linear Energy Transfer (LET) and Relative Biological Effectiveness (RBE)

LET: average energy deposited per unit length
- Different ionising particles have different LETs
- Heavy charged particles or ions are densely ionising, giving them a high LET
RBE: quantifies radiobiological effects of different types of radiation
- Can be used to compare biological effects of different radiations

Radiation Protection and Safety

ALARA principle: aim to reduce exposure to ionising radiation to as low as reasonably achievable
Radiation protection principles:
- Justification: ensure benefits outweigh risks
- Optimisation: minimise radiation exposure while achieving desired outcome
- Dose limitation: ensure exposure is within prescribed limits
Codes of practice and legislation:
- ARPANSA sets codes of practice for radiation protection in medicine
- Radiation Safety Act 1999 and Regulations 2010

Personal Monitoring and Dosimetry

Importance of personal monitoring for radiation workers
Types of personal dosimeters:
- Film badges
- Thermoluminescent dosimeters (TLDs)
TLDs:
- Store energy when irradiated
- Release light when heated, proportional to x-ray energy deposited
- Can be reused after annealing

Radiation Protection Principles and Legislation

ARPANSA codes of practice for radiation protection in medicine
Radiation Safety Act 1999 and Regulations 2010
Classes of exposure:
- Occupational
- Medical
- Public
Dose limits for occupational and public exposure

Risks and Benefits of Medical Exposures

Risks:
- Radiation-induced secondary cancers
- Other health effects
Benefits:
- Successful diagnosis and treatment
Risk models:
- Absolute risk model
- Relative risk model
Justification and optimisation: balancing risks and benefits

Radiation Detection and Image Formation

Characteristics of radiation detectors:
- Absorption efficiency
- Conversion efficiency
- Capture efficiency
- Dose efficiency
- Temporal response
- Timing of phosphorescence or afterglow
- Wide dynamic range
- High reproducibility and stability
Detective Quantum Efficiency (DQE)
Noise and spatial frequency
Different types of detectors:
- Gas-filled detectors
- Xenon gas detectors
- Scintillators and photomultiplier tubes
- Image intensifiers### Radiography Basics
Indirect, direct, computed, and digital radiography are types of radiography.
Indirect radiography: x-rays converted to light, then to an electrical signal.
Direct radiography: x-rays to an electrical signal in the detector (single stage).

Electronic Band Theory of Solids

Conduction band: free electrons.
Forbidden band (band gap): no electrons allowed.
Valence band: outer shell bound to an atom.

Computed Radiography (CR)

x-rays interact with a phosphor screen, creating electron-hole pairs.
Electrons have enough energy to be raised to the conduction band energy level.
Trapped in the forbidden band, they de-excite to the valence level, emitting blue light.
Red laser light gives them enough energy to escape the forbidden band.
Blue light intensity is proportional to x-ray intensity.
Typical CR resolutions: 100-200 um.
Readout times: a few seconds.

Indirect Digital Radiography (DR)

x-rays interact with scintillation crystals, creating electron-hole pairs.
No traps, defects, or impurities like CR.
Immediate readout display.

Phosphors and Scintillators

Non-structured scintillator crystals: Gadolinium oxysulfide.
Structured scintillator crystals: crystals well aligned, more costly, and higher conversion efficiency.

Scintillators and Thin Film Transistor (TFT) Technology

TFT allows turning circuits on and off.
Voltage applied to TFT, and charge can be read out.
Can construct large active pixel arrays with photodiodes.

Scintillators and Charge-Coupled Device (CCD) Technology

CCD produces a signal directly from light to charged signal to image.
Pixelated array with a large number of CCD pixels.
Light strikes the surface of the CCD, knocking electrons out of orbit.
Voltage applied across the CCD pixel, causing electrons to travel to the region where they can be collected.

Direct Digital Radiography (DR)

x-rays interact with atoms in an amorphous selenium or silicon semiconductor layer.
Electron-hole pairs are created, proportionally to x-ray quantity and quality.
High voltage applied across electrodes, directing pairs to collect at their respective electrodes.
Prevents recombination of electron-hole pairs.

Digital Fluoroscopic System

Acquires many sequential radiographs, played back as a movie.
Slowly being replaced by flat panel technologies.
Pulsed x-ray tubes enabled significant dose reduction.

Fluoroscopic Flat Panel Detector

Need very fast readout to obtain 30 frames per second.
Low exposure per frame required.
Flat panel detectors have a poor signal-to-noise ratio compared to image intensifiers.
Pulsed sources can significantly reduce dose.

Electron-Hole Pair

Electron and cation pair created from ionization.
Called a hole because when an electron is ionized, the cation gets a "hole" in the valence shell.

X-ray Tube Circuits

Current, voltage, and timing circuits control the x-ray tube.
Operator chooses appropriate kVp, mA, and s for which the current flows.

Energy (eV)

Energy required (work to be done) to move a single electron through a potential difference of 1V.
1000 eV = 1 keV, and 1,000,000 eV = 1 MeV.

Direct Current (DC)

Simple circuit with a battery supplying electrical potential.
Current flows through the circuit (I = V/R, P = I × V).
Power is dissipated in the resistor.

Alternating Current (AC)

Easiest to generate electricity using AC systems.
Conversion of mechanical work into electrical energy using a turbine.
AC current induced in a coil.

Transformers

Devices for increasing or decreasing AC voltage.
Used in the national grid to step up or step down voltage.
Made of an iron core with coils of wire wrapped on either side.

Rectification and Smoothing

Need to convert AC to DC in an x-ray tube using rectification.
Diodes allow current to flow in one direction.
Full wave rectification uses four diodes to get voltage and current for the entire cycle.
Smoothing can be achieved with capacitors to store and release charge.
Three-phase power supplies are the most efficient way to generate, transmit, and consume electricity.

Filament Circuit

Controls x-ray tube current by controlling temperature.
Resistors control current in the primary side of the filament transformer.
Filament temperature controls the rate of thermionic emission and hence tube current.

Timers

Time selector: operator chooses.
Mas timer: monitors tube current and terminates the exposure when the desired quantity of charge has passed.
AEC (autotimer): terminates exposure once the required amount of radiation has been incident on the IR.

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Description

Learn about the process of ionization of air by x-rays and gamma rays, including direct and indirect ionization mechanisms. Understand how ionization occurs and the role of electrons in this process.