Sky & Telescope October 1985 [antikvár]

Gamma-Ray Astronomy Comes of AgeGiovanni F. Bignami, Institute of Cosmic Physics. Milan, ItalyWHY should astronomers bother to observe gamma rays when they already have their hands full at other wavelengths? The first reasons came in the 1950's from particle physics, following the discovery that pions (unstable subatomic particles produced in collisions between protons) decay by emitting gamma rays. This realization suggested that high-energy gamma rays ought to be produced when cosmic rays collide with atoms in interstellar gas clouds. Their detection would greatly advance our understanding of cosmic-ray physics and also the magnetic and plasma processes in our Milky Way galaxy.In 1958 Philip Morrison went further, and provided a detailed list of the sites and circumstances of gamma-ray production. He gave four explicit examples and, over a quarter of a century later, we now know that he was right in three cases and only marginally wrong in the fourth.Morrison also made a very interesting comparison between detection limits at visible and gamma-ray wavelengths. The faintest object detectable at that time with the 200-inch Palomar telescope was about 23rd magnitude. The flux from such a source is about 0.006 photon per square centimeter per second. A similar flux of gamma rays, Morrison said, should be easily recorded. In fact, his prediction was drastically wrong by a factor of several thousand. Current gamma-ray instruments see sources equivalent to 30th magnitude or fainter!A typical gamma ray has a wavelength 100 million times less than visible light and a corresponding energy 100 million times greater. When imagining particles whose energies are measured in lO's or lOO's of millions of electron volts (MeV), you can almost sense their weight. It is tempting to think that detectors floating in space should recoil visibly when struck by a gamma ray. Nevertheless, their energies, though high by particle-physics standards, are small compared to everyday objects. A dust grain blowing in a gentle breeze could have as much kinetic energy as a 100 MeV photon.Not only do gamma-ray astronomers have to detect very small numbers of these photons, but they also have to contend with a huge background flux of cosmic rays. These energetic charged particles outnumber gamma rays by about 100,000 to one. Add to all this the fact that, like ultraviolet and X-rays, most celestial gamma rays (see the table on page 302) can only be detected from space. Luckily, our atmosphere is opaque to them! Nevertheless, it is evident why this high-energy radiation is so difficult to observe.If this situation were not bad enough, astronomers have an even worse problem. The angular resolution of gamma-ray telescopes is so bad that only in exceptional circumstances have sources been located to better than 1°. The reason, unfortunately, is directly related to the detection process itselfLeft: A gamma ray strikes one of the plates in a spark chamber and creates an electron-positron pair dial passes through die apparatus and the triggering detectors, leaving an ionization trail. Righl: The triggering detectors register the passage of the electron e" and positron e> and then charge alternate plates to a very high voltage. The strong electric field created causes die plates to discharge along the paUis of die charged particles. The resultant trail of sparks can be recorded photographically or electronically and subse-quenUy transmitted lo Earth for analysis.In the energy range above a few tens of MeV, gamma-ray photons interact with matter by creating an electron-positron pair. Yet momentum and energy can only be conserved if this interaction happens in the electric field of an atom. The result is that the paths of the electron and positron are not uniquely related to that of the incoming gamma ray, because when struck the atom's recoil carries away an indeterminate amount of momentum. Therefore, since only the electron and positron are detectable, we can never be entirely sure of the direction from which the gamma ray came.Focusing gamma rays seems out of the question since their wavelengths (less than 0.01 angstrom) are smaller than the distance between atoms in solids. Thus, they must be detected individually using techniques borrowed from elementary-particle physics.DETECTING GAMMA-RAYS Gamma rays with energies less than 10 MeV or so cannot cause electron-positron pair production. They must be detected by the changes they induce in the electrical properties of special semiconductors or by the pulses of light they cause in certain plastic and crystalline materials called scintillators. However, most astronomical'l , 1II 111In August, 1975, the spark chamber on the COS-B satellite detected its first gamma ray. The elevation and plan views (top and bottom, respectively) of this event were reconstructed from data telemetered by the satellite. They show the typical in-verted-V pattem caused when an electron-positron pair is created in one of the plates.All illustrations by the author.