In the early 1900s, scientists thought that radiation from the earth's crust was responsible for the presence of ions in the atmosphere. They reasoned that as the distance from the earth's radioactive sources increased there would be fewer ions, so the number of ions must decrease with altitude. In 1912, Viktor Hess tested this hypothesis by detecting ions at different altitudes in a hot air balloon. Hess used an electroscope to detect the presence of ions. An electroscope contains metal wires that separate when they become charged because like charges repel. Hess's experiment showed that ions actually increase with altitude, so he concluded that radiation comes from outer space.
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| Figure 1. A primary cosmic ray strikes the atmosphere, creating a shower of secondary cosmic rays. |
We refer to these external sources of radiation as cosmic rays. The term refers to any of the various types of particles that head towards the earth at relativistic speeds. The cosmic rays that strike nuclei in the earth's atmosphere are called primaries. Approximately 85% of primary cosmic rays are the nuclei of hydrogen atoms and 12% are helium nuclei. The other primary cosmic rays are electrons and nuclei of other elements.
When primary cosmic rays enter the earth's atmosphere they collide with air nuclei to create secondary cosmic rays (figure 1). Most secondary cosmic rays are pions or muons, although many other types of particles have been found in cosmic rays. The pions and muons formed usually decay into neutrinos before reaching the earth.
An amazing characteristic common to all types of cosmic rays is their high energy. The energy of a particle is measured in electron volts (eV). An electron volt is the energy required to move an electron through one volt of potential. A single air molecule, for example, has .05 eV in thermal energy. Primary cosmic-ray particles typically have more than a billion electron volts in energy. In fact, the most energetic cosmic rays are billions of times more energetic than the particles accelerated in the most powerful particle accelerators.
Although scientists have been studying the origin of cosmic rays since their discovery, the sources of the more energetic cosmic rays are still unknown. The origin of cosmic rays is difficult to pinpoint because they are deflected by Earth's magnetic field. The deflection makes the actual direction a cosmic ray came from hard to identify.
Despite numerous obstacles, scientists have found much out about cosmic rays. Cosmic rays originate from both in and outside of the Galaxy. Cosmic-ray particles with energies below 1017 eV are generally from within the Galaxy. The Sun accelerates many of these low-energy cosmic rays. Shock waves expanding from supernovae also accelerate many of the low-energy particles that reach the earth.
The more energetic cosmic rays come from outside the Galaxy and make up the majority of cosmic rays that reach the earth. The sources of these highly energetic cosmic rays are not entirely understood. In fact, some of the cosmic rays detected are so tremendously energetic that known objects can't account for their energy. Particles traveling though the universe are slowed down by the cosmic microwave background, a source of background radiation created during the Big Bang that fills the universe. The cosmic microwave background forces particles to lose energy until they have less than 5 x 1019 eV at what is known as the Greisen-Zatsepin-Kuzmin cutoff, yet cosmic rays have been detected with energies as high as 3 x 1020 eV. Either a Greisen-Zatsepin-Kuzmin cutoff doesn't actually exist, which is only possible if special relativity doesn't apply at high energies, or the cosmic rays are accelerated by unknown, abnormal astronomical objects.
As a result of the high energies they contain, cosmic rays have helped scientists understand the nature of many subatomic particles. Scientist study cosmic rays with instruments containing photographic emulsions that track the path of cosmic-ray particles. These instruments have helped scientists discover and confirm the existence of several subatomic particles. In one case in 1935, Hideki Yukawa hypothesized that a particle called a pion was responsible for the nuclear force. That particle was found in cosmic rays two years later.
More recently, cosmic rays were used to discover an unexpected characteristic of neutrinos. Neutrinos are produced in cosmic rays through the decay of pions. There are three types, or flavors, of neutrinos corresponding to the particle they produce while interacting with a nucleus: the types are the electron-neutrino, muon-neutrino, and tau-neutrino. These neutrinos fill space, with billions of them traveling through every square centimeter of matter each second. Despite being so abundant, neutrinos are difficult to detect because they rarely interact with matter and can pass through lead and other materials totally unscathed.
In experiments designed to detect proton decay, detectors were built underground in order to avoid interfering signals from neutrinos. Scientist found that regardless of how far underground the detectors were built, unwanted signals from the neutrinos could not be avoided. The detectors failed to identify proton decay, but they found something far more surprising from the unwanted signals caused by neutrinos.
The detectors measured the ratio of muon-neutrinos to electron-neutrinos. The ratio was expected to be 2:1 because pions, which are generated in cosmic rays, ultimately decay into two muon-neutrinos and one electron-neutrino. The ratio of muon-neutrinos to electron-neutrinos found in the underground detectors was 1.3:1. This led scientist to wonder what had happened to the missing muon-neutrinos.
After the unexpected ratio of neutrinos was found, another test was made. In this experiment, both upward and downward traveling cosmic rays were observed. The only difference between cosmic rays that travel up from those traveling down is that their particles have existed longer. The ratio of neutrinos traveling up to neutrinos traveling down was expected to be exactly one for each flavor. As expected, the ratio of electron-neutrinos traveling up to those traveling down was one. However, only half as many muon-neutrinos were found traveling up as traveling down.
The disappearance of muon-neutrinos in both experiments can only be explained by a change in the muon-neutrino's flavor. Since the number of electron-neutrinos traveling up in the second experiment isn't greater than the number traveling down, the muon-neutrinos must change flavor to tau-neutrinos. This was not obvious from the first experiment because tau-neutrinos cannot be identified in detectors.
Quantum physics only allows this kind of metamorphosis in neutrinos if they have mass. Although neutrinos had been considered to be massless for the past 70 years, the existence of mass in neutrinos doesn't violate the Standard Model. In addition, the new expected mass of neutrinos is extremely small so it is understandable why they were thought to be massless.
The discovery that neutrinos have mass has important significance in cosmology. Although the mass of a neutrino is very small, the combined mass of the all neutrinos in the universe is more than the combined mass of all the stars. Scientists have indirectly measured the mass of the universe by measuring the expansion rate and orbital motion of galaxies. When we compare that mass to the mass directly accounted for, we find that there is a lot of "missing" mass. The newly discovered fact that neutrinos have mass helps account for some of the missing mass, but there is still much left to be accounted for.
Cosmic rays are tied to humanity directly through evolution, as well as indirectly through aspects such as the universe's mass. In addition to radiation from gamma and beta rays, cosmic rays create the background radiation present on Earth. The background radiation occasionally affects the reproductive cells of humans (and other species), causing genetic mutations in their offspring. Although the majority of mutations are detrimental, some mutations actually give the subject advantages over other members of his species. Through natural selection, detrimental genetic mutations are removed from the species while the beneficial ones are spread throughout it. Cosmic rays have aided evolution through this process and have helped improve every living species over the long term.
Created by Dan Corbett, Kate Stafford, and Patrick Wright for ThinkQuest.