PART 1.2 - CORE CONCEPTS
What are X-rays?
X-rays are a form of electromagnetic energy called photons. Through science and technology we have the ability to change the strength and rate of these photons being produced, allowing them to be used in a wide variety of ways. The electromagnetic spectrum (Figure 1, below) illustrates the use of photons with different strengths (amplitudes) and rates (frequencies) for use as radio waves, light and X-rays.
There are different types of radiation. Some radiation can be harmful to living organisms, whilst others are not. X-ray radiation (along with gamma rays) are ionising, meaning that they can strip off electrons from atoms and cause them to behave in an unstable manner. In short, ionising radiation may make changes at a cellular level within living tissue and can lead to cell death or mutations (i.e. cancer).
All photons can be thought of as a bundle of energy but contain no mass which travel at the speed of light (186,000 miles or 300,000km per second). They have electric and magnetic fields that are constantly changing, hence the name electro-magnetic spectrum. X-ray photons can have different amplitude, frequency and wavelength dependent upon their production. Furthermore, because X-rays have a very short wavelength they are the 'right' size to interact with atoms and subatomic particles. The combined concept of bundles of energy interacting with atoms leads us to describe the passage of X-rays as particles, although they also exhibit characteristics of waves (something called the wave-particle duality).
Within radiography (and therefore paleoradiography) this is important because:
X-rays are small enough to pass through matter
X-rays production can be manipulated, allowing stronger X-rays for specimens of higher density or thickness
We are able to predict the travel of X-rays and make inferences from the resultant imaging
At this point you are wondering why you have to know about photons in such detail. The important point to focus upon here is that we can predict how X-rays travel and why they interact with matter the way they do.
X-rays are not the same as sound waves, which may bounce off surfaces and echo around a room. In fact, sound is not part of the electromagnetic spectrum as it is the motion of particles rather than changes in electric and magnetic fields.
A quick explanation on how X-rays travel and interact with matter is therefore needed.
A quick explanation:
Even though X-rays are small enough to pass through matter they can still hit or come under the influence of atoms during their journey. X-rays may be attenuated as they pass through matter, making them stop altogether or change direction with a substantial reduction in energy and speed (called X-ray scatter). Objects of greater density have more atoms in closer proximity, therefore the chances of X-rays interacting with them are likewise greater.
Figure 2 shows several items; bone (left), a metal object (upper-middle) and a sandbag (right). Within this example the objects of greater density (metal / sandbag) have stopped more X-rays than the bone. Objects that stop X-rays are described as radiopaque, whilst objects that allow the passage of X-rays are described as radiolucent. Both of these terms are important within paleoradiographic research. The item seen bottom-middle is a stepwedge - a tool with graduating thicknesses of aluminium used for quality control and estimating density (covered in a later part of this course).
Incidentally, X-ray images can be inverted with a white background and black specimens, although the image you see here is considered standard practice.
Figure 2. X-ray image of a bone (left), high density metal object (upper-middle) and medium density sandbag (right). More x-rays are stopped by high density objects, making the corresponding part of the image 'white'. The item seen bottom-middle is a stepwedge of aluminium which is used in density estimation and quality control. (Click image to view in full screen)
The last concept to mention is that X-rays travel in much the same way as light, with a decrease in intensity the greater the distance from the source. An analogy can be made with light bulbs, except that X-rays are produced by x-ray tubes of course!
Whether it is a light bulb or X-ray tube the photons are emitted in all directions. As these photons travel further from their source they inherently move further apart from one another, as shown in the diagram below (Figure 3). This is called inverse square law where intensity is inversely proportional to the square of the distance from the source.
In working practice we wouldn't want X-rays to be emitted from all angles, as such the X-ray tube is surrounded by shielding to prevent unwanted X-rays from reaching the operator. This shall be expanded upon in Part 2.
Why does this matter in paleoradiography?
The further the X-ray tube is away from the specimen (and therefore X-ray detector/film) the lower the intensity of the X-ray beam.
Low intensity beams may not be able to pass through the specimen sufficiently to produce a meaningful image.