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How To Detect Elusive Gamma-Rays
One way gamma-ray astronomers use to build telescopes is based on the
famous Einstein equation E = mc2 -- energy can be changed into matter.
Gamma-rays, like all other forms of light, are pure energy. A gamma-ray,
however, has enough energy that it can be converted into two
particles, an electron and its antiparticle, a positron. A gamma-ray
telescope can measure the energy and arrival direction of these
particles, and from those measurements determine information about the
gamma-ray. This means that a gamma-ray telescope is really a particle
detector like those used in high-energy physics laboratories like CERN,
Fermilab, and SLAC. Gamma-ray astronomers often work with high-energy
physicists to develop new telescopes.
In this cut-away picture of EGRET, you can see the different parts that
make up this gamma-ray detector. The striped structure is many layers of
a high-density material. When a gamma-ray hits EGRET, it has a certain
probability of being converted into a positron and an electron as it
travels through this material. If it does that, the trail of the
positron and the electron can be measured, along with their energies, to
figure out the energy and direction of the incoming gamma-ray that made
them. There is about a 1% chance that the gamma-ray will be converted as
it passes through each layer. There are 30 layers in all, so the total
chance that an incident gamma-ray will turn into a positron-electron
pair is about 30%.
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