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It's a Question of Physics: How does an x-ray see my bones?


Radioactivity is a process in which atoms with unstable nuclei spontaneously decay emitting subatomic particles and energy as they reconfigure into more stable forms.

Radiation refers to the subatomic particles and energy emitted by unstable, and therefore radioactive, atoms. There are two types of radiation: non-ionizing and ionizing. The electromagnetic waves of non-ionizing radiation are essential to life and span a wide spectrum of frequencies and energies, from low frequency and low energy radio waves, to infrared light, visible light, and ultraviolet light. Ionizing radiation includes electromagnetic waves, such as gamma rays and x-rays, and particulate radiation, the alpha and beta rays. Ionizing radiation impacts tissues with higher energy and therefore is more likely to present a health hazard.

All materials emit some radiation. Mutations are alterations of the DNA sequences produced by high-energy radiation and play an important role in the evolution of species through natural selection.

(click for larger image)

Marie Curie's Demonstration
The diagram above is based on Mme Sklodowska Curie's doctoral dissertation, Recherches sur les Substances Radioactives. It shows the behaviors of three types of radiation. A through C represents a photographic plate. P is an open lead container holding Radium (R). The alpha (α), beta (β), and gamma (γ) rays cannot penetrate the lead shielding on the sides, and radiate out from the top in three distinctive patterns in which plate area B through C catches the gamma rays.


X-rays are a form of electromagnetic radiation with a wavelength ranging from 10 to 0.01 nanometers (billionths of a meter) and energies between 120 and 120,000 eV (electron-volts). Electromagnetic radiation is absorbed by different types of materials at different rates. The varying rates can be captured on to create a radiograph or x-ray. Soft tissue absorbs few rays, but the calcium in bones absorbs more, so it shows up white in x-rays. Lungs are full of air, which does not absorb the rays at all, so they appear black.

As the nucleus of a radioactive atom decays, three different types of radiation are produced: alpha, beta and gamma. Alpha radiation consists of highly energetic helium atoms. Beta radiation is a stream of fast-moving electrons. Gamma radiation is made up of high-energy photons similar to x-rays.

Cathode rays are electrons that are accelerated inside an evacuated tube. They were commonly used to analyze and reproduce images in television sets and other visual display devices. Today these cathode ray tubes have been largely replaced by liquid crystal displays (LCD) and plasma screens.
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