Gamma rays

It seems that several readers have been searching the site for information about gamma rays. So, here’s a potted summary adapted from Wikipedia.

Gamma rays, or gamma radiation is electromagnetic energy of high frequency (very short wavelength) and so high energy. For electromagnetic radiation wavelength is inversely proportional to frequency, the proportionality constant being the speed of light. Similarly, energy and frequency are proportional in this case the relationship is governed by Planck’s constant). Gamma ray frequencies are usually above 10 exahertz (or more than 10 to the 19 Hz), and so their energies are above 100 kilo electronvolts (keV). Their wavelengths are less than 10 picometres, less than the diameter of an atom.

Gamma rays are produced naturally on Earth and elsewhere in the universe by radioactive decay of high-energy states of atomic nuclei. This form of ionising radiation is also generated when certain high-energy subatomic particles interact or through interactions of cosmic rays.

High-energy nuclear reactions are used to product gamma rays artificially for medical and research applications.  Electron-positron annihilation, neutral pion decay, fusion, and induced fission can be used to generate gamma rays.

Gamma rays are also generated by certain astronomical events in which very high-energy electrons are produced. Such electrons produce secondary gamma rays by the mechanisms of bremsstrahlung, inverse Compton scattering and synchrotron radiation.

The distinction between X-rays and gamma rays is arbitrary. Originally, the electromagnetic radiation emitted by X-ray tubes had a longer wavelength than the radiation emitted by radioactive nuclei (gamma rays). The older scientific literature distinguished between X-rays and gamma rays on the basis of wavelength. Today, gamma rays are now usually distinguished on the basis of their origin rather than an arbitary wavelength cut off: X-rays are emitted by definition by electrons outside the nucleus, while gamma rays are emitted by the nucleus.

More on gamma rays here.


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Nitrogen-fixing aliens

Scientists hope that Titan, a moon of Saturn, with its nitrogen-rich atmosphere, could act as a model system for terrestrial chemistry before life began on our planet. Now, another step towards that goal has emerged as researchers at the University of Arizona have incorporated atmospheric nitrogen into organic macromolecules under conditions resembling those on Titan.

“Titan is so interesting because its nitrogen-dominated atmosphere and organic chemistry might give us a clue to the origin of life on our Earth,” explains Hiroshi Imanaka, who is an assistant research scientist in the UA’s Lunar and Planetary Laboratory. “Nitrogen is an essential element of life.” Titan looks orange through a telescope because its atmosphere is a rich smog of organic molecules. Particles in the smog could settle on the surface and be exposed to conditions that might eventually create life, said Imanaka.

Saturn's A and F rings, the small moon Epimetheus and the smog-enshrouded Titan, Saturn’s largest moon. (Credit: NASA/JPL/Space Science Institute)
Saturn's A and F rings, the small moon Epimetheus and the smog-enshrouded Titan, Saturn’s largest moon. (Credit: NASA/JPL/Space Science Institute)

Of course, nitrogen alone is not enough, nitrogen molecules must be converted to a chemically active form that can drive the necessary biochemical reactions that underpin biological systems.

Imanaka and Mark Smith converted a nitrogen-methane gas mixture similar to Titan’s atmosphere into a collection of nitrogen-containing organic molecules by irradiating the gas with high-energy ultraviolet light. The laboratory set-up was designed to mimic how solar radiation affects Titan’s atmosphere.

Most of the nitrogen simply formed solid compounds directly, rather than gaseous ones, explains Smith, whereas previous theories suggested that nitrogen would move from gaseous compounds to solid ones in stepwise process. But, those settling particles may not contain nitrogen at all. If some of the particles are the same nitrogen-containing organic molecules created by the UA team in the laboratory then it would suggest that conditions conducive to life might just exist on Titan, Smith says.

These and other laboratory observations help scientists planning future space missions to decide on what to look for on other worlds that might hint at life and what instruments should be developed to help in the search.


Proc Natl Acad Sci, 2010, online
Mark A. Smith homepage
UA lunar and planetary laboratory

Yet another supernova

Just when you’d given up hope of another starburst, a third type comes along unannounced! This third class of previously unidentified supernova could help explain some anomalous observations in the night sky and even how our bodies come to contain so much calcium.

Until recently, astronomers had assumed there were just two types of supernovae. The first two types of supernova are either hot, young giants that explode on to the scene violently as they collapse under their own weight, or old, dense white dwarves (type a1) that undergo a thermonuclear explosion to briefly add their light to the night sky.

However, a third class appeared in telescope images in early January, 2005 and scientists, seeing that it had recently begun the process of exploding, started collecting and combining data from different telescope sites around the world, measuring both the amount of material thrown off in the explosion and its chemical composition.

Avishay Gal-Yam and colleagues at the Weizmann Institute in Israel and teams in Canada, Chile, Italy, UK, and USA, soon realised that the new supernova was neither old and dense nor young and hot.

There was too little material being ejected by the 2005 supernova for it to be an exploding giant, but its remote location from stellar nurseries suggested it was old. Moreover, its chemical makeup did not match the second type of supernova. The scientists turned to a computer simulation to see if they could figure out what kind of stellar processes could give rise to this anomalous kind of starburst.

Type Ia supernovae are primarily composed of carbon and oxygen as seen in their spectra, but the newly discovered supernova has unusually high levels of calcium and titanium which derive from nuclear reactions of helium not carbon and oxygen. However, the astronomers were initially at a loss to explain the source of the helium. Their simulations suggested that a pair of white dwarves might have been involved, with one assimilating helium from the other. When the thief star’s helium load rises past a certain point, the explosion occurs. “The donor star is probably completely destroyed in the process, but we’re not quite sure about the fate of the thief star,” says Gal-Yam.

Helium theft may have led to a third class of supernova that gives rise to the calcium in your bones and the titanium in a replacement hip! (Credit: Gal-Yam, Weizmann Institute of Science.

These new supernovae are relatively dim, so may not be as rare as they at first seem. This might explain why calcium is so prevalent in the universe and so in life on earth. The existence of radioactive titanium from these supernovae might also preclude the need for exotic explanations, such as invoking dark matter, of positrons at the heart of our galaxy. “Dark matter may or may not exist,” says Gal-Yam, “but these positrons are perhaps just as easily accounted for by the third type of supernova.”


Avishay Gal-Yam homepage