8. The properties, uses and dangers of X-ray radiation

The sources and properties of X-ray radiation

X-rays are produced when a metal target is bombarded with high energy electrons.

The target atoms are excited to a high electronic energy state - electrons are promoted to a higher energy level (shell).

The electrons of the excited atoms drop down to lower more stable electronic energy levels by losing energy in the form X-ray FM radiation.

When EM X-ray radiation is absorbed, some of the wave energy is converted into heat, BUT X-rays can cause ionisation - the process of knocking off outer electrons of atoms to create positive ions (see Part 10 ionisation for more details) - so X-rays are an ionising radiation.

 

The uses of X-ray radiation

Medical uses of X-rays:

X-rays have a very short wavelength and cause ionisation ...

X-rays are absorbed by metal and bone - but some X-rays pass through less dense material,

X-rays are mostly transmitted by less dense healthy soft tissue, though there can be slight, but significantly different absorptions between different tissues e.g. muscles and organs.

X-rays affect a photographic film or photoelectric screen (as in a digital camera) in the same way as light, so an image can be built up based on absorption or non-absorption of X-rays.

The wavelength of X-rays is of the same order of magnitude as the diameter of an atom - means you can produce high resolution images.

This means X-rays can be used to diagnose and treat some medical conditions because they can be used to examine the internal structure of the body.

 

Investigating molecular or ionic structures

X-ray diffraction techniques produce images from which you can work out the 3D positions of atoms in a molecular structure, including DNA, or ions in a crystal structure like sodium chloride.

 

X-rays and CT scans: You use X-rays to produce high resolution 2D and 3D images of hard tissues in the body - you can detect cancer tumours and bone fractures by this technique - X-ray photographs.

These images are called computerised tomography (CT) scans and are of a much higher resolution than those using medical images from ultrasound.

X-rays CT scans can detect bone fractures detection, dental problems and cancer cell growths.

Using charge-coupled devices (CCDs) allows images to be formed electronically, rather than with the 'old fashioned' photographic plate.

Be aware of precautions to be taken when X-ray machines and CT scanners are in use, radiographers must be particularly careful in their work.

More on X-raying your body to investigate bone structure - we are definitely pentadactyl! When having an X-ray, the dose should be as low as possible to minimise the risk of side-effects.

X-ray radiation is passed through the object onto a detection screen and the image recorded. This was originally a photographic plate, but now it is like a digital camera screen producing a image file for storage and analysis.

The more dense the bone, or any other tissue, the more X-rays absorbed, hence the differentiation in the image. Trained radiographers in hospitals will take X-ray images to help doctors diagnose broken bones which show up against lesser absorbing surrounding tissue. Any crack in the bone will show up because more X-ray radiation will pass through the crack.

Note: You produce a negative image where the brighter parts of the picture are where fewer X-rays get through e.g. you see the dense bone clearly against the background of the soft less dense, less X-ray absorbing tissue.

X-rays can also be used to investigate internal organs e.g. to produce a mammogram when screening for breast cancer. Here you are exposed to harmful radiation, but the scan might save your life. Most people would accept a very low risk of harm from X-rays compared to the risk of undiagnosed cancers.

In a similar fashion X-rays were used in airport body/luggage scanners for security reasons.

However, these are banned in some countries because of the potential harmful effects of X-rays.

Only a very low dose is used, but is the benefit of preventing a terrorist incidence worth the risk of cancer?

You can now use high-frequency radio wave scanners which are much safer.

 

The scientific technique of X-ray crystallography is used to determine the internal structure of crystals to see how the atoms, ions or molecules are arranged.

When X-rays pass through crystals they create diffraction patterns that can be detected by photographic techniques.

From the pattern you can work out the position of the atoms in the crystal.

That's how we know the crystal arrangement of ions in sodium chloride and how the double helix structure of DNA was worked out.

 

The dangers of X-ray radiation

Doses of radiation risk are measured in sieverts

The quantity of radiation you are exposed to is called the absorbed radiation dose and depends on where you live and whether at work, you are likely to be exposed to harmful radiation (e.g. radiographer, nuclear plant worker etc.).

The sievert dose unit (1 Sv = 1 J kg^-1) is based on the dose equivalent of ionising radiation. 1 sievert is quite a large dose of radiation, so doses often quoted in mill-sieverts (1 Sv = 1000 mSv).

Radiation dose is not a measure of the total amount of radiation your body absorbs, but it is a measure of the risk of harm due to your body absorbing that amount of radiation. The risk depends on the total amount of radiation you absorb and how harmful that type of radiation is.

 

X-rays (and gamma rays) are the most dangerous of the ionising radiations ...

... and easily cause tissue cell damage and interfere with the function of cells e.g. can cause mutations - leading to cancer. X-rays have a very high energy and are quite deeply penetrating in their energy transfer to the absorbing material - which might be the deeper tissues and organs of the body.

The risk of harm from an X-ray scan is very low, but the risk of not diagnosing your injuries quickly and accurately after an accident is much greater.

The risk (radiation dose) from a CT scan can vary from depending on which part of the body is scanned e.g. there is a much greater risk from a chest scan compared to a head scan.

To minimise the chance of harm from X-rays radiographers wear a lead apron and 'press the button' from behind a protective  lead screen. Without these precautions they would be exposed to a large dose of radiation over time.

See section 10. for more general comments on the dangers of ionising radiation