Week 2 - Radiation Equipment and Dosimetry

Explain the x-ray tube enclosure and what it can be made from.

Made of either glass or metal and encases the x-ray tube in a vacuum to prevent any interactions with air that can disrupt the x-rays.

Explain which of the 2 materials of x-ray enclosures is preferable and why.

Metal enclosures are preferred over glass enclosures as the heat produced within the glass enclosure over a long period of time results in the tungsten target evaporating, forming a thin metallic field on the interior walls of the enclosure. This metallic film can cause damage to the tube through arcing when electrons are produced and also attenuates the x-rays produced.

Name the 2 main internal components of an x-ray tube

• Filament - negative cathode

• Target - positive anode

Explain the structure of the filament and measurements.

Made of thin tungsten wire that forms a vertical spiral. (0.2mm wire coiled into 0.2cm diameter and 1cm length coil)

Explain why the tungsten wire is so thin.

The very small diameter increases the resistance of current flow, increasing the amount of heat produced and therefore the amount of electrons produced via thermoionic emission.

Explain thermoionic emission and how it occurs.

Connecting wires supply low voltage (10V) and high current (3-5 amp) to the filament in which the metal atoms absorb the heat and emit electrons.

Explain the Edison Effect

After electrons are emitted they form a cloud around the filament as they are repulsed by the negative chargers of each other and the cathode.

Explain the role of the copper focusing cup.

Horse-shoe shaped cup, attached to the cathode. It is negatively charged on either end, forcing the electrons to align into a narrow beam, focusing them towards the tungsten target.

Explain x-ray tube current and give units.

The number of electrons flowing from the cathode (filament) to the anode (tungsten target) per second (mA).

Break down the units of current

1C/s = 1A = 1000mA

Name the 2 types of anodes

• Stationary

• Rotating

Explain stationary anodes

Made of copper with a small square or rectangular tungsten plate attached. It is angles downwards to accommodate for the electron beam.

Explain rotating anode

Tungsten discs rotates at ~3k rpm to distribute the large amounts of heat from the electron beam that would otherwise puncture the target if it was stationary.

Explain how the tungsten target rotates

Stator coils surrounding the neck of the x-ray tube produce a rotating magnetic field that induces rotation in the copper rotor attached to the anode disc via a molybdenum stem.

Give 3 reasons why tungsten is used as the anode target

• High atomic number (Z=74), therefore a large number of electrons for efficient electron-electron interactions that produce x-rays

• High melting point withstands the heat from the electron beam

• Can absorb heat

Explain the role of the high voltage power supply in an x-ray tube

Supplies between 30-120 kV between the cathode and anode, accelerating electrons from the filament towards the target at a very high speed.

Name the 2 types of x-rays produced in an x-ray tube

• Characteristic x-rays

• Bremsstrahlung x-rays

Explain how characteristic x-rays are produced

1. A high speed electron from the cathode filament hits a K-shell electron that absorbs its energy (BE ~70keV) and ejecting it from the atom.

2. An L-shell electron moves into a lower energy state by moving into the K-shell to fill its vacancy, releasing its excess energy as a characteristic photon

Explain how to calculate the energy of the characteristic photon.

E = BE of new orbital - BE of original orbital

Explain how the outer shell electron that fills a vacancy is named

Letter is based on the orbital that had a vacancy and the subscript based on its original orbital, eg. An L-shell electron moving into a K-shell vacancy is named K_a

Explain how Bremmstrahlung x-rays are produced

High speed electrons from the cathode filament pass the electron cloud of tungsten atoms and interact with the nucleus.

Explain why there are varying energies of Bremmstrahlung x-rays.

• An incident electron can hit the nucleus and be stopped, producing a maximum energy x-ray

• An incident electron can come close to the nucleus and deflect, producing an x-ray with less energy

• The further away the electron deflects from the nucleus, the less energy the x-ray produced will have

Name the 2 factors that affect an x-ray emission spectrum

• Changing x-ray tube current

• Changing voltage

Indicate the parts of the x-ray emission spectrum

A - Bremmstrahlung x-rays

B - Characteristic x-rays

C - low energy x-rays that are absorbed by a filter

Explain how increasing tube current will affect the x-ray emission spectrum.

• Increasing current will increase the number of electrons targeting the anode and therefore the number of x-rays produced

• This will increase the amplitude of the spectrum, however shape of the spectrum and peak position always remain the same.

Explain how increasing voltage will affect the x-ray emission spectrum

• Increasing voltage increases the number of electrons targeting the anode and therefore the number of x-rays produced

• The will increase the amplitude of x-ray emission spectrum as well as shifting the peak of the graph towards the right

Explain what occurs at the electron gun of a Linac

This is the cathode made of tungsten wire in a flat spiral (~1cm diameter) which is heated to produce electrons via thermoionic emission. The electrons are then accelerated through a perforated anode to the waveguide.

Explain what occurs at the RF power source of the Linac

RF waves, 3MHz microwaves with 10cm wavelength are produced by either a magnetron or klystron and travel to the waveguide.

Explain what occurs at the waveguide of the Linac

The waveguide is a hollow tube containing cavities that fit the microwaves from the RF source. As microwaves travel through the wave guide, they accelerate electrons from the electron gun to very high speeds (almost the speed of light) towards the treatment head.

Explain what occurs in the treatment head of the Linac

• The electron beam from the waveguide is directed towards a bending magnet that redirects the electron beam, bending it 90 or 270 degrees, towards the tungsten target.

• High speed electrons hit the target and produce x-rays

Explain the process of collimation in a Linac.

• Primary collimators - define maximum possible field size and reduce x-ray beam to 0.1% of its original intensity

• Flattening filter - absorbs low-energy x-rays to ensure uniform beam intensity

• Secondary collimators - moveable (can rotate), further reducing beam size to square/rectangular fields.

• Multileaf collimators - produce non-geometric shapes, shaping the beam to the exact size and shape of the patients tumour

Explain the types of x-rays produced by the Linac.

Both characteristic and Bremmstrahlung x-rays are produced by the linac, however the characteristic x-rays are negligible as the energy is so low (70keV), therefore they are absorbed by the flattening filter.

What is absorbed dose and its units.

• A measurement of the amount of energy from ionising radiation being deposited into any medium.

• Measured in Gray (Gy), where 1Gy = 1J/Kg

Give the formula and units for equivalent dose

Equivalent dose = absorbed dose x W_r (Sv)

Where,

• W_r = radiation weighting factor

• 1Sv = 1Gy = 1J/Kg

Give the formula and units for effective dose

Effective dose = absorbed dose x W_r x W_t (Sv)

Where,

• W_t = tissue weighting factor

What is the tissue weighting factor of the entire body?

1

Explain the differences between external and internal Dosimetry.

• External Dosimetry involves a simple measurement of radiation produced from a source outside of the body.

• Whereas internal Dosimetry is a much more complicated measurement of radiation produced from within the body.

Explain personal Dosimetry and name the 2 types

The use of a personal dosimeter that measures an individuals radiation dose from external exposures. There are active and passive personal dosimeters.

Explain the difference between active and passive dosimeters and give examples.

• Passive dosimeters absorb external radiation and produce a radiation-induced signal that is stored and analysed after a certain period of time, eg. OSL and TLD

• Whereas active dosimeters absorb external radiation and produce a radiation-induced signal that displays real-time radiation exposure, eg. Electronic Personal Dosimeters (EPD)

Explain how an OSL works.

1. An OSL or optically stimulated luminescence dosimeter is made of beryllium oxide ceramic whose electrons are excited and trapped in a higher energy state when exposed to radiation.

2. When the dose is being measured after a period of time, the dosimeter is exposed to a certain wavelength of light that returns the trapped electrons to their ground state, releasing their stored energy as light.

3. The light output from dosimeters is measured by PMTs that converts light to electric signals to display the radiation dose

Explain how a TLD works

1. A TLD or thermoluminescent dosimeter contain calcium or lithium fluoride whose electrons are excited and move to a higher energy state when exposed to radiation.

2. When the dose is being measured after a period of time, the dosimeters are exposed to heat (300-400 degrees C) that return the trapped electrons to their ground state, releasing their stored energy as light.

3. The light output from the dosimeters is measured by PMTs that convert this light into electric signals used to display the radiation dose.

Explain environmental Dosimetry and examples

Environment producing a significant radiation dose, eg. Radiation incidents, nuclear reactor sites, radioactive mines etc.

What are the ARPANSA recommendations dose in households and workplaces.

• Households = 200 Bq/m³

• Workplaces = 1000 Bq/m³

What are the dose limits for students, clinical practitioners, pregnant practitioners and patients.

• Students - 1mSv per year

• Clinical Practitioners - 20mSV per year

• Pregnant Practitioners - 1mSv per year

• Patients - no limit, however is monitored