Energetic Gamma Ray Experiment Telescope

The basic design of EGRET was basically a chamber filled with a special type of metal, a sensor at the bottom of the chamber to capture and record gamma rays, and finally a protective covering over the entire instrument. The chamber would manipulate the gamma ray into a way that it could be recorded. The sensor would capture and record the characteristics of the gamma ray. Finally, the protective covering would block out any unwanted energy rays.[1]

With the purpose of detecting individual gamma rays ranging from 30 MeV to 30 GeV, EGRET was equipped with a plastic scintillator anti-coincidence dome, spark chamber, and calorimeter. Starting from the outside of the telescope, scientists covered EGRET with a plastic scintillator anti-coincidence dome. The dome acted as a shield, blocking any unwanted energy waves from entering the telescope and skewing the data. To actually create recordable, usable data, scientists used a process called electron-positron pair production, which is creating an electron and positron simultaneously near a nucleus or subatomic particle. In order to induce this process, scientists assembled a multilevel thin-plate spark chamber within the telescope. A spark chamber is basically a chamber with many plates of metal and gases such as helium or neon. Finally, to record the data from the electron or positron about the gamma ray, scientists equipped EGRET with a thallium-activated sodium iodide (NaI(Tl)) calorimeter at its base. The calorimeter captured the resolution of the gamma rays that entered EGRET.

Since NASA scientists wanted only certain types of gamma rays to be processed and recorded, they set up EGRET with many systems of checks to filter out any unwanted information. The most basic type of filter EGRET had was only allowing gamma rays entering the telescope from certain angles to be let into the spark chamber. As the gamma ray travelled through the spark chamber, it struck one of the metal plates within the spark chamber. Once the gamma ray came in contact with a plate of metal, it initiated the process of electron-positron pair production and created an electron and positron. Once both the electron and positron were created, if one of these particles was still moving down throughout the telescope and a signal from the anticoincidence scintillator was not fired, the particle was imaged and its energy level recorded. With each gamma ray having to pass all of these systems of checks, the results of EGRET were supported to be the most valuable out of the other CGRO instruments.[2][3]

Throughout EGRET's active life span, which went from 1991 to 2000, all of the gamma rays it collected and recorded were done one at a time. From each individual gamma ray that entered EGRET, scientists were able to create a detailed map of the “entire high-energy gamma-ray sky.” From its findings and mapping of the universe, scientists were able to reaffirm many long holding theories about gamma rays and their origins. NASA scientists also discovered that pulsars, which are “rotating neutron stars that emit a beam of electromagnetic radiation,” are the best sources of gamma rays. Scientists have also been able to detect and characterize the properties of 4 pulsars. EGRET's results also pointed out to scientists that the Earth's Moon is particularly brighter than the Sun the majority of the time. EGRET provided scientists with information that allowed them into a new understanding of the universe.[4][5]

• The EGRET home page at the Compton Observatory Science Center
• The Third EGRET Catalog