Wilhem Röntgen discovered X-rays in 1895 (for which he was awarded the Nobel Prize in 1901), and thus opened a new window on the world. On January 16, 1896, the New York Times published a radiography of Mrs Röntgen’s hand. Very soon people were marvelling at photographs showing human bone structure.

As early as February 1896, X-rays were used in medicine to observe broken bones: the first such observation, of a fracture in a patient's arm, was carried out by two physicians at Dartmouth College in the USA. Quite soon, other applications of X-rays were found, including luggage scanning by custom officers.

Images: left - observation of a fracture in February 1896: Doctors G. and E. Frost (pictured) were the first to use X-rays for medical purposes, and right - Luggage scan by French custom officers, 1897.

X-rays attracted the attention of many physicians who then employed these rays to diagnose fractures and foreign bodies inside tissues. At the end of the 19th century, medicine was paying particular attention to the pathology of the individual organs. Continuous improvements allowed physicians to take X-ray photographs of all internal organs by means of “contrast media” - opaque substances which showed up in X-rays. X-rays were also used in therapy, for example in the treatment of skin diseases. However, it was soon realised that radiation of higher energy was required for this type of therapy.

X-rays really came into their own during the First World War. Marie Curie together with her daughter Irene set up a network of medical radiological centres to help improve the diagnosis of fractures and lung diseases among soldiers. In addition, many specialized vehicles with X-rays apparatuses (called “Les Petites Curies”) were covering the battlefields. Marie Curie authored a book about this: “Radiology in War” (1921).

X-rays are a type of electromagnetic radiation. They are artificially produced in a cathode tube, where a hot cathode emits electrons that are then accelerated by a high voltage to hit a metal target at high velocities, and produce some “invisible rays”.

These rays are partly a result of the fluorescence excited in the atoms of the metal, and partly of the so-called Bremsstrahlung radiation effect. The latter appears as the result of rapid changes of direction of the electros in the vicinity of the atomic nuclei of the metal.
Another way in which X-rays can be produced is when the initial high energy electron knocks out one of the electrons from the inner electron shell. Then one of the higher energy electrons will drop down on the inner shell to fill the gap and will emit its excess energy in the form of an X-ray.

In an X-ray tube, electrons are accelerated to an energy of 30 to 150 keV, to hit a tungsten target producing X radiation with energies from 1 eV to 150 keV with a maximum intensity in-between and two peaks at 59 and 67 keV (these are the electronic transitions in the tungsten atoms). For medical applications, lower X-ray energis are used for safety reasons. While 20 keV is a typical energy for soft tissue X-rays, for example mammograms, higher energies (around 150 keV) are used for hard tissues, for example bone.

One is often concerned about the radiation exposure during medical X-ray examinations. However, everyone is exposed to sources of natural radiation throughout their life, from cosmic rays, radon in the atmosphere, soil and rocks, and even from food and water.

It is worth comparing these contributions:

Source	Equivalent dose
Chest X-ray	100 μSv
Living in a stone, brick or concrete building for one year	70 μSv
Flight from London to New York	40 μSv
Average daily natural background dose	10 μSv
Dental X-ray	5 μSv
Eating one banana	0.1 μSv

As you see, even eating a banana exposes one to a radiation dose. The main source of this is the potassium, which in nature contains 0.0117% of the unstable isotope ⁴⁰K.

Every form of electromagnetic radiation is characterised by a wavelength (λ) or equivalently by an energy (E = hc/2λπ, where h is the Planck constant (h = 6.626×10^-27 erg/s), c is the speed of light in vacuum (c = 2.9979·10¹⁰ cm/s).  

The X-rays wavelength range is between 0.01 nm to 10 nm, corresponding to a range of energies of 1 keV (103 eV) to 150 keV. Compare this with the wavelength range of visible sunlight which spans from 400 nm to 750 nm.

The only known type of electromagnetic radiation with more energy than X-rays are the gamma (γ) rays. Gamma rays are used in nuclear medicine, where physicians use γ radiation with energies between 60 and 510 keV.

The electromagnetic spectrum, from radio waves to γ rays (Image credits: ESA).