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Gamma-ray Detectors

How Do You Detect a Gamma-Ray?

Gamma-rays, like visible light, are made up of packets of energy called photons. However, in the case of gamma-rays, the photons have millions of times more energy than in visible light. Gamma-rays are detected by observing the effects they have on matter. A gamma-ray can do a few basic things inside matter. It can collide with an electron and bounce off it like a billiard ball (Compton scatter) or it can push an electron to a higher energy level (photoelectric ionization). Also since gamma-rays have so much energy, part of this energy can be transformed into matter directly by creating an electron and another particle called a positron (pair production).

All of these interactions cause electrons to move in some way, which basically means that an electric current has been created. These currents can then be amplified and measured to estimate the energy and direction original gamma-ray.

Scintillator Image

Can You Focus Gamma-Rays?

Visible light can be focused by using mirrors or lenses to bend the paths of the photons and concentrate them in one place. This creates a sharper, brighter image. Mirrors and lenses don't work with gamma-rays very well. When a gamma-ray hits matter (like in a mirror or lens), it will interact with the material in such a way as to destroy the gamma-ray or change its energy by a large amount. Focusing means to bend the photon's path without changing its energy too much, and that is not easy to do with gamma-rays. This means that images we have from the gamma-ray region are not as sharp (that is, they have poorer angular resolution) than images taken in the visible or most other wavelengths.

Sources of cosmic gamma-rays are extremely weak (that is, they produce relatively few gamma-ray photons for us to detect in the vicinity of Earth) and require long observations, sometimes several weeks, to get a significant detection or accurate measurement of a source. Focusing helps to increase our ability to detect the "signal" from the cosmic source.

Basic Types of Gamma-ray Detectors

Compton Image
Diagram of a
Compton Scatter Telescope
Gamma-ray detectors can be placed in two broad classes. The first are what would typically be called spectrometers or photometers in optical astronomy. These are instruments which are "light buckets" and focus on a region of the sky containing the object of interest collecting as many photons as possible. These types of detectors typically use scintillators or solid-state detectors to transform the gamma-ray into optical or electronic signals which are then recorded. The second class are detectors which perform the difficult task of gamma-ray imaging. Detectors of this type either rely on the nature of the gamma-ray interaction process such as pair production or Compton scattering to calculate the arrival direction of the incoming photon, or use a device such as a coded-mask to allow an image to be reconstructed.

So you may ask, "Why look at gamma-rays?". The answer is that some very important events in the Universe, like supernova or matter falling into black holes, radiate large amounts of energy in the gamma region. In addition, gamma-ray detectors have revealed completely new discoveries like gamma-ray bursts. Simply put, without gamma-ray detectors some important questions about our Universe just could not be answered.

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

The Imagine Team
Project Leader: Dr. Jim Lochner
Curator:Meredith Bene
Responsible NASA Official:Phil Newman
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