Gamma rays can only be detected by sensors that are under a 1.8-meter layer of concrete and are made of very dense metals. Cosmic gamma rays are high-energy photons generated by some of the most violent events in the universe.
Photons of light are massless particles that are essentially packets of energy. Due to a quantum mechanical phenomenon known as wave-particle duality, photons can behave both as particles and as photons. Photons have a wavelength, and the amplitude of their wavelength determines where they are in the electromagnetic spectrum. Radio and microwave photons are at the low energy and longer wavelength end of the spectrum, while ultraviolet and X-ray photons are at the shorter and higher energy end of the spectrum, as well as the most energetic of them with the shortest wavelength: gamma ray photons.
Gamma rays have a wavelength shorter than 10-eleven meters and frequencies above 30 * 10eighteen hertz. The European Space Agency notes that gamma-ray photons have energies in excess of 100,000 electron volts (eV). We can compare this to X-rays, which NASA describes as having energies between 100 and 100,000 eV, and optical photons, which we can see with our own eyes, are around 1 eV.
On Earth, gamma rays are produced by radioactive decay of elements and lightning flashes, while in space they are produced by strong sources of high energy such as solar flares, quasars, black holes tearing stars apart, accretion disks of black holes. , exploding stars, and the strong gravitational environment of neutron stars.
How were cosmic gamma rays discovered?
At the turn of the twentieth century, two forms of radiation emitted by decaying atoms were known, namely alpha particles (representing helium nuclei) and beta particles (representing electrons and positrons).
However, when the French chemist Paul Villard began experimenting with the radioactive element radium, which had been discovered two years earlier by Marie and Pierre Curie, he noticed that the ionizing radiation produced by the decay of radium had a stronger impact than the alpha or beta radiation of the particle. .
This radiation got its name – gamma rays – simply because gamma is the third letter of the Greek alphabet after alpha and beta. In the early 1900s, Villard and his associates did not know that the key difference between gamma rays and alpha/beta particles is that gamma rays are a form of light, while alpha and beta particles are made of matter. .
How to stop gamma rays?
Blocking gamma rays requires a dense material, and the thickness of this material depends on the substance. To reduce the strength of incoming gamma rays by a billion parts, you need 4.2 meters of water, almost 2 meters of concrete, or 0.39 meters of lead.
This poses a problem for gamma-ray telescopes such as the Fermi Space Telescope (NASA). Conventional telescopes, such as the Hubble Space Telescope, use mirrors and lenses to collect and focus light, but gamma rays simply pass through a conventional telescope. Instead, gamma-ray telescopes must use other instruments.
According to NASA, on the Fermi Space Telescope, a gamma-ray photon will pass through a device called a Coincidence Detector, which blocks cosmic rays that could give a false signal. The gamma radiation is then absorbed by one of 16 sheets of tungsten, a material dense enough to stop the gamma radiation.
Interacting with tungsten, the gamma ray is converted into an electron and a positron, the paths of which are read by a tracker, which is a module of silicon strips intertwined with tungsten foil, which can determine the direction from which the gamma ray came from, based on the trajectory of the electron and positron.
Finally, the energy of the electron, and then the positron, is measured by a calorimeter – a device that measures the energy of a particle by absorbing it – made of cesium iodide, which determines the energy of gamma rays.
Are cosmic gamma rays dangerous?
Because of their high energy, gamma rays are ionizing, which means they can knock electrons out of atoms, eventually damaging living cells and creating a health hazard. However, as with any radiation, it depends on the dose you receive.
In small doses, very carefully chosen to limit exposure, they can be used safely as a medical diagnostic tool or even to kill cancer cells (ironically, since exposure to radiation, including gamma rays, can cause cancer). In particular, one of the tools used by doctors is the “gamma knife”, which is an ultra-precise device with which a beam of gamma rays cuts off diseased brain cells and can even penetrate deep into the brain without damaging the outer lobes.
Fortunately, Earth’s atmosphere is capable of blocking gamma rays. However, for astronomers, this is unfortunate, because it means that in order to conduct gamma-ray studies, observatories must either be built on mountaintops, where the atmosphere is thinner, or sent into space.
The first space-based gamma-ray telescope was launched in 1961, but it wasn’t until the late 1960s and early 1970s that it really started when an important discovery was made, and this happened even without the use of astronomical telescopes.
Over the years, many observatories have been created, both on Earth and in space, designed to observe cosmic gamma radiation. In 1990, NASA launched the Compton Gamma Observatory as a gamma-ray counterpart to the Hubble Space Telescope. The Compton Gamma Ray Observatory explored space from 1991 to 2000. The BeppoSAX instrument was a joint Italian-Dutch mission that operated between 1996 and 2003, while NASA launched HETE-2 (HETE-1 had previously failed), which tracked many GRBs between 2000 and 2008 .
Currently, as of the end of 2022, several satellites, observatories and telescopes continue to conduct gamma-ray observations both on Earth and in space. The Swift satellite (NASA), launched in 2004, combines X-ray and gamma-ray observations, as did the Italian satellite AGILE, launched in 2007. In 2002, the European Space Agency launched INTEGRAL, and in 2008, the complex Fermi Gamma Ray Space Telescope was launched.
Meanwhile, there are several gamma ray observatories on Earth, including VERITAS at the Fred Lawrence Whipple Observatory in Arizona and HESS in Namibia.
In 1963, the Soviet Union, Great Britain and the United States signed a nuclear test ban treaty that prevented the world’s superpowers from testing any nuclear device in the atmosphere or in space. However, the US suspected that the Soviet Union would not join the treaty, so they launched a series of Vela satellites to monitor for any gamma ray pulses that could come from secret nuclear explosions. As it happened, cosmic gamma rays were detected, but from space: random bursts of powerful gamma ray energy that seemed to come from all over the Earth. But how far away were these gamma-ray bursts?
If these gamma-ray bursts, GRB for short, were coming from our galaxy, then astronomers would find them mostly in the plane of the Milky Way. Instead, they were scattered across the sky, which could only mean one thing. Either they were very close, within our solar system, or they were very far away, outside of our galaxy. There was even a special debate in 1995 that discussed the size of our galaxy based on the distribution of globular clusters.
In a 1995 debate chaired by Martin Rees, astronomer Bogdan Paczynski of Princeton University argued that gamma-ray bursts came from very distant parts of the universe, while Donald Lamb of the University of Chicago believed that gamma-ray bursts must have occurred somewhere nearby. , because the energy needed for them would be billions of eV at a distance of several light years, which would be contrary to the laws of physics.
Just two years later, astronomers got the answer when the BeppoSAX satellite detected a gamma-ray burst, which the William Herschel telescope in the Canary Islands was able to quickly follow, in the process of detecting the faint afterglow of some kind of explosion that generated the gamma-ray burst. Measurements of the redshift of the afterglow light showed that it came from a distance of six billion light years. Bogdan Pachinsky was right!
There are two main types of GRB. One type is called short gamma-ray bursts, which last only a fraction of a second, while the other type is known as long gamma-ray bursts, which can last anywhere from a few seconds to an hour. Short gamma-ray bursts are emitted when two neutron stars merge, and long gamma-ray bursts are the death cries of rare massive stars.
Physicists Andrew Macfadyen and Stan Woosley at the University of California at Santa Cruz have developed a model that explains how stars can explode and produce long gamma-ray bursts without violating the laws of physics. When a massive star, 50 to 100 times the mass of the Sun, reaches the end of its life, it begins to collapse and, if the star is spinning fast enough, the energy in the collapsing layers is reflected off the core and ejected in two jets moving at nearly the speed of light. The charged particles inside these jets spin around powerful magnetic fields and produce something called synchrotron radiation, in the form of the gamma rays we see. Since gamma rays are emitted only in the direction of the jets, and not in all directions at once, the total energy released does not violate the laws of physics.