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X-rays

Like all forms of ionising radiation, X-rays produce electrons and ions when they pass through materials.

Who discovered X-rays?

X-rays were discovered in 1895 by W. C. Roentgen, who called them X-rays because their nature was at first unknown. X-ray line spectra were used by H. G. J. Moseley in his important work on atomic numbers (1913) and also provided further confirmation of the quantum theory of atomic structure. Also important historically is the discovery of X-ray diffraction by Max von Laue (1912) and its subsequent application by W. H. and W. L. Bragg to the study of crystal structure.

What are the properties of X-rays?

X-rays are a form of electromagnetic radiation similar to radio waves, microwaves, visible light and gamma rays. X-ray photons are highly energetic and have enough energy to break up molecules and hence damage living cells. When x-rays hit a material some are absorbed and others pass through. Generally, the higher the energy the more x-rays will pass through. (Table 1) This is what gives x-rays the power to “see inside” things. X-rays cannot be steered by electric and magnetic fields like alpha, beta and other charged particles.

How are X-rays produced?

X-rays are commonly produced by accelerating electrons through a potential difference (a voltage drop) and directing them onto a target material (the metal tungsten is a typical example).

Image showing X-ray tube. Heated filament emits electrons by thermionic emission, electrons are accelerated by high voltage; x-rays produced when high speed electrons hit the metal target

See photographs of historic X-ray tubes and other instruments.

The incoming electrons release x-rays as they slow down in the target (braking radiation or bremsstrahlung). The x-ray photons produced in this manner range in energy from near zero up to the energy of the electrons. An incoming electron may also collide with an atom in the target, kicking out an electron and leaving a vacancy in one of the atom’s electron shells. Another electron may fill the vacancy and in so doing release an x-ray photon of a specific energy (a characteristic x-ray). The x-ray spectrum shown in the picture is a plot of the number of photons against the photon energy.

X-ray spectrum

The accelerating voltage and the target material used to produce x-rays vary depending on the particular application (Table 2).

Table 1

Energy carried by each photon Frequency of Electromagnetic Wave Wavelength (1pm = 10 -12m) Thickness of Material to Halve Number of Photons
in electron-volts in joules Concrete Lead Human Tissue Aluminium
1keV 1.602 X 10 -16 J 2.418 X 10 17 Hz 1240 pm 0.87 µm 0.117 µm 1.76 µm 2.17 µm
10keV 1.602 X 10 -15 J 2.418 X 10 18 Hz 124 pm 147 µm 4.68 µm 1,220 µm 97.9 µm
100keV 1.602 X 10 -14 J 2.418 X 10 19 Hz 12.4 pm 17.3 mm 0.110 mm 38.6 mm 15.1 mm
1MeV 1.602 X 10 -13 J 2.418 X 10 20 Hz 1.24 pm 46.4 mm 8.60 mm 93.3 mm 41.8 mm
10MeV 1.602 X 10 -12 J 2.418 X 10 21 Hz 0.124 pm 132 mm 12.3 mm 298 mm 111 mm

 

Table 2

Use Accelerating potential Target Source type Average photon energy
X-ray crystallography 40 kV
60 kV
Copper Molybdenum Tube 8 keV - 17 keV
Dianostic X-rays Mammography 26 - 30 kV Rhodium Molybdenum Tube 20 keV
Dental 60 kV Tungsten Tube 30 keV
General 50 - 140 kV Tungsten Tube 40 keV
CT 80 - 140 kV Tungsten Tube 60 keV
Baggage screening Carry-on/Checked bags 80 - 160kV Tungsten Tube 80keV
Container screening 450kV - 20MV Tungsten Tube/Linear accelerator 150keV - 9MeV
Structural analysis 150 - 450 kV Tungsten Tube 100keV
X-ray therapy 10 - 25 MV Tungsten/High Z material Linear accelerator 3 - 10 MeV

 

A computed tomography (CT) scanner is a particular type of x-ray machine in which the x-ray tube produces a beam in the shape of a fan and moves around the patient in a circle. The x-rays are detected electronically and a computer uses the information to reconstruct an image of the region of the body exposed.

X-rays can also be produced by a synchrotron. A synchrotron is a device that accelerates electrons in an evacuated ring (often several tens of metres in diameter), steering them with magnets. Manipulating the electron beam in a controlled way with the magnets can produce intense beams of x-rays. Synchrotron facilities are used in materials and protein research - http://www.synchrotron.org.au/

How can X-rays be detected and measured?

Perhaps the most obvious x-ray detector is a sheet of x-ray film. Improved efficiency can be obtained by photographing the light produced when the x-rays strike a fluorescent screen. The screen is incorporated in a light-tight cassette; the film is loaded into the cassette in a dark room and the cassette is then positioned behind the object or body part to be imaged.

Like all forms of ionising radiation, x-rays produce electrons and ions when they pass through materials. An ion chamber collects and measures the amount of charge produced when x-rays pass through a volume of air. The amount of charge collected is proportional to the energy absorbed. A typical diagnostic x-ray machine would produce a reading of 10-100 mGy on such an instrument. Typical ion chambers can measure absorbed energies as low as 10 nGy from a short burst of x-rays and the rate of energy absorption from a continuous beam as low as 100 nGy/min.

In a CT scanner the x-rays enter crystal scintillators and are converted to flashes of light. The flashes of light are detected and processed electronically. A “single slice” CT has a row of these detectors positioned opposite the x-ray tube and arranged to intercept the fan of x-rays produced by the tube. In some cases the detector row rotates with the tube, in others there is a complete ring of stationary detectors. A “multi-slice” (or multi-detector) CT has several rows of these small scintillator detectors.

What is thermoluminescence and how is it used to detect and measure X-rays?

The passage of x-rays deposits energy in a material (see absorbed dose) and in some cases this energy triggers other processes that can be measured at a later time. Indeed the production of ions and electrons mentioned above can be regarded as one of these processes. An example is radiothermoluminescence which is often truncated to thermoluminescence, (TLD). In this process the absorbed energy moves electrons in the material from their normal energy states and they can become trapped at higher energy states. When the material is heated at a later date the electrons move back to their normal positions and light is given off. The light can be measured and the amount of light is related to the amount of x-ray energy absorbed. TLD can measure absorbed doses as low as 10 µGy and will accumulate the dose received until readout. Thermoluminescent dosimeters are widely used to measure occupational doses for radiation workers (i.e. radiographers, uranium miners, nuclear medicine technologists, etc.).

How can I be exposed to X-rays?

Diagnostic medical x-rays are the most likely way you will encounter x-rays. Data from the Health Insurance Commission shows that each year there are over 12 million Medicare claims for exams on x-ray machines and also over 2 million claims for computed tomography (CT) exams. Diagnostic x-rays only expose part of the body to radiation. The quantity effective dose is used as a way of comparing the risk of a partial body exposure to that due to a whole body exposure (such as that due to background radiation in the environment). On average, each Australian receives an effective dose of about 1.7 mSv per year from medical procedures, including about 1.1 mSv from CT scans. This is similar to the dose everyone receives from ionising radiation that is and always has been in our environment.

Typical values of effective dose for various medical X-rays

How can my risk from X-ray exposure be minimised?

As with all types of radiation the protection principles are time, distance and shielding .

A diagnostic x-ray should be performed to provide information that helps medical staff treat a patient’s condition appropriately. In general, this information is much more important to a person’s health than the small estimated rise (typically less than 0.01%) in the chance of developing cancer perhaps a decade or more later. As lead is a very good attenuator of x-rays (see Table 1), a garment impregnated with a small amount of lead can be used to cover sensitive parts of the body. Modern x-ray equipment has many features that if used properly can limit the area irradiated and the dose delivered to the minimum necessary for obtaining the diagnostic information sought. However, in some cases (particularly for x-rays of the abdomen and pelvis) exposure of sensitive organs is unavoidable. In some cases an alternative type of imaging (ultrasound or magnetic resonance imaging) may be able to provide the information sought and therefore may be used instead of an x-ray.

What about the radiographer?

When a patient is x-rayed, about 30% of the x-rays bounce off (scatter) and may irradiate anyone standing nearby. Therefore it is normal practice for a radiographer to position the equipment and patient and then move to a control panel some distance away and behind a screen to trigger the x-ray. While this may be disconcerting to the patient, it is important to limit the exposure of the radiographer since s/he will be performing the procedure hundreds or thousands of times a year. Sometimes it is necessary to x-ray a child or some other person who may require the support of another human being for emotional or positioning reasons. This support person should wear a lead impregnated apron and shouldn’t be in the way of the main x-ray beam.

 

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