Wednesday, February 12, 2014

Excerpts on Gamma Radiation

Taken from the book "Defining Moments in Science". Article by Kate Oliver

Key Discovery : 1900

While studying the properties of beta radiation, the French chemist Paul Villard made an intriguing observation. He noticed that in experiments where a beam of beta rays was refracted (passed through a medium of different density), there were often traces of another, unrefracted beam in the results. 

Villard set up another instrument, using the newly discovered element radium as a source. He focused a beam of radiation from the radium through a series of glass plates and a magnetic field, to be recorded finally on photographic film. The unrefracted beam appeared again. It did not seem to respond to any external magnetic or electric fields, and would even show up on the photographic film when it was placed behind 0.2 millimeters of lead.

Villard suggested that the radiation he had found was a new type of more penetrating X-ray. He concluded that the three distinct types of radium beams - easily absorbed rays, a dividable stream of charged electrons, and his new super-penetrating X-rays - were analogous to the three types of radiation emitted by cathode ray tubes. With this observation, Villard correctly generalized radiation into the three types we now know as alpha, beta, and gamma. There was, however, very little interest in his discovery or theory, perhaps because it was outside the current scientific paradigm.

In 1903, Ernest Rutherford, having studied the penetrative power of the beams, named them gamma rays and his term soon fell into common usage. Villard, however, remains pretty much forgotten.*


Gamma Rays is a form of electromagnetic radiation. Of all forms of electromagnetic radiation, they have the shortest wavelengths and the greatest energy. 
It can be produced either as a result of a nuclear reaction or by the annihilation of matter by antimatter. Nuclear reactions that result in the emission of gamma rays include some types of radioactive decay and the fission (splitting) of a nucleus.

Gamma rays are very penetrating; even a thick sheet of a dense material such as lead will not block them entirely. When these pass through matter, they eject electrons from the atoms they strike. This process, called ionization, is harmful to living cells. A living thing exposed to intense or prolonged gamma radiation can become seriously ill and die.

Gamma rays are used in industry to inspect castings and welds. The gamma rays are passed through the object being inspected onto photographic film. The image formed on the film can reveal defects that are invisible to the eye or hidden from direct observation. In medicine, gamma rays are used to destroy certain types of cancer. Cobalt 60 is a substance that is commonly used in hospitals as a source of gamma rays for this purpose.

Artificial satellites have revealed that a variety of astronomical objects, including the sun, clouds of interstellar matter, and remnants of supernovae, are sources of gamma rays. They have also detected strong, random bursts of gamma rays from unknown distant sources.**

Source: *Defining Moments in Science: Over a Century of the Greatest Discoveries, Experiments, Inventions, People, Publications and Events that Rocked the World. Page 14.

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