Image Processing Reference
Next comes the final frontier of the electromagnetic spectrum: the gamma-ray
waveband. The term “gamma-ray” denotes the highest energy waveband of the
electromagnetic spectrum, essentially x rays in or above the MeV (million electron
volt) energy range. Gamma rays are generated by a number of mechanisms,
including annihilation ofantimatterand matter, acceleration of charged particles
by strong magnetic fields, and radioactive decay of the nucleus of an atom.
Gamma-ray imaging has its roots in experiments done in the 1890s. The
scientist Henri Becquerel discovered the phenomenon of radioactive decay in
a serendipitous experiment performed in 1896. He had recently learned of
Roentgen's discovery, and intended to see if fluorescent materials emitted x
rays. One of the fluorescent materials in his laboratory was a salt of uranium
called uranyl potassium sulfate, and he found that the salt caused a photographic
plate wrapped in black paper to darken after several hours of sunlight exposure.
Becquerel mistakenly thought that the sunlight caused the salt to emit x rays.
When he tried to repeat his experiment, the sky in Paris was overcast, so he put
the package away in a drawer for about a week. He then decided to develop the
film anyway, expecting to find the images very weak. But, as he observed: “On the
contrary, the silhouettes appeared with great intensity. I thought at once that the
action might be able to go on in the dark.” 8 This was a clear demonstration of the
phenomenon of radioactive decay that produced penetrating radiation without any
external stimulus. The film was fogged by gamma rays passing through the paper
Later work by the physicist Ernest Rutherford and P.V. Villard established that
radioactive decay gives rise to three distinct types of radiation now named alpha,
beta and gamma radiation after the first three letters of the Greek alphabet. The
first two types are helium nuclei and electrons, respectively, while gamma rays
are electromagnetic waves of a very high energy, so high that it was not known
for some time that these very penetrating rays were in fact electromagnetic waves.
It is quite difficult to observe the wave-like properties of gamma rays since their
wavelength is extremely short, and conclusive proof of their wave-like properties
came many years after their discovery.
Gamma rays are very useful in certain imaging applications. Their highly
penetrating properties make them one of the only means of optically imaging
the interior of particularly thick or dense objects, and they can produce images in
situations where x-ray imaging cannot. Examples of hard-to-image objects include
steel shipping containers or marble statues, where even very hard x rays fail to
produce a usable image. Radioactive gamma-ray emitters such as Cobalt-60 are
used to produce gamma rays at energies of approximately 1 MeV (1 million
electron-volts). The gamma-ray source can be very compact and does not require
external power, making it particularly convenient in applications where access is
difficult, such as inspection of welded joints in pipelines. It is very inconvenient to
8 Emilio Segre, From X Rays to Quarks, W.H. Freeman (1980).