Gamma rays are the highest-energy form of electromagnetic radiation. The light we can see with our eyes is just a tiny portion of the radiation along the electromagnetic spectrum. What makes it a spectrum is that it spans a wide range of energy, from very low-energy radio waves to very high-energy gamma rays. The bands of the spectrum, in order of increasing energy, are: radio, microwaves, infrared, visible, ultraviolet, X rays, and gamma rays.
In particle physics, when matter and antimatter meet, there is a release of energy. Each particle of matter has an antimatter partner. For example, the antiparticle to the electron is the positron. It has the same characteristics as an electron but its charge is positive. When the two meet, their changes cancel out, they annihilate each other, and high-energy radiation (like a gamma-ray photon) gets released.
The reverse can happen, too. A high-energy photon (gamma ray) can break down into a matter-antimatter pair. This actually happens during thunderstorms, and NASA’s Fermi Gamma-ray Telescope has “seen” it as it occurred. Electric fields at the top of thunderstorms (high in the atmosphere) create an upward moving channel of electrons. As the electrons meet charged particles in the atmosphere, some of them emit gamma rays. When gamma-ray photons collide with electrons, they accelerate to nearly the speed of light. Then, when these very fast photons pass close to the nucleus of an atom, the gamma ray can transform into a matter-antimatter pair: an electron and its antimatter partner, a positron. Interactions like these set up a “beam” of matter-antimatter that extends outward through the atmosphere along magnetic field lines – to where the Fermi Telescope picks up the signal.
Electrical activity in thunderstorms lead to gamma-ray flashes, and these bursts create matter-antimatter particle pairs. The Fermi Telescope detected the antimatter because, when the positrons hit the spacecraft and interacted with its sensors, that interaction annihilated the pair and released a gamma ray – exactly the type of radiation Fermi was designed to detect!
Classroom Activity: Gamma-Ray Flash Map
Students review and discuss an animated map of terrestrial gamma ray flashes:
(click on 29 second animation below video frame option)
Compare the gamma-ray flash map to a map of the likelihood of lightning striking:
(based on NASA data)
NARRATOR: At any given moment about 1,800 thunderstorms are in progress somewhere on the globe. New observations by NASA's Fermi Gamma-ray Space Telescope show that thunderstorms make antimatter. The process starts with a terrestrial gamma-ray flash, or TGF; an intense pulse of gamma rays originating from thunderstorms.
These dots mark TGF's observed by Fermi's Gamma-ray Burst Monitor during the spacecraft's first eight months of operations. Researchers estimate that there may be as many as five hundred TGF's each day.
On December 14, 2009, as Fermi passed over Egypt, it spotted a TGF produced by a thunderstorm in Zambia The TGF was over the spacecraft's horizon where Fermi couldn't see it. So how could FermiI have detected it?
Scientists believe that the TGF process begins with thunderstorm's intense electrical field. Electrons within this field become accelerated upward above the storm where the air is thin, the electrons can ramp up to speeds nearly as fast as the speed of light.
When these ultra-fast electrons encounter an atom, they emit gamma rays. Very rarely, one of these gamma-ray photons grazes an atom and transforms into a pair of particles. One, an electron, is normal matter; the other is antimatter, the electron's opposite, called a positron.
The gamma rays travel in straight lines, but the charged particles spiral along lines of Earth's magnetic field. And that was the route to Fermi. The particles created by the TGF rode upwards on magentic field lines and then struck the spacecraft. The positrons annihilated when they struck electrons in Fermi creating a flash of gamma rays.
For an instant Fermi became a gamma-ray source and set off its own detectors. A fraction of a second later, some of the particles were bounced back along the same magnetic field line. They again passed through Fermi and again produced gamma rays.
The spacecraft has observed this phenomenon in at least four other occasion. So the next time lightning flashes and thunder roars remember-you may be witnessing antimatter in the making.
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