Imagine the Universe!

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.

photo of EGRET cross-section

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%.

Button Do you want to return to the Instrument Design Group?

Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

The Imagine Team
Acting Project Leader: Dr. Barbara Mattson
All material on this site has been created and updated between 1997-2012.

DVD Table of Contents
Educator's Index