Cosmic ray

The cosmic radiation ( cosmic rays English ) is a high-energy particle radiation from space that comes from the sun, the Milky Way and distant galaxies. It consists mainly of protons, electrons, and next from the fully ionized atoms. On the outer atmosphere about 1000 particles per square meter and second hit. By interaction with the gas molecules, only a small part of particle showers created with a high number of secondary particles of which, however, reaches the earth's surface.

The cosmic radiation is not to be confused with the cosmic background radiation.

The secondary radiation ( with up to 10 ¹ ¹ particles per primary ) is detected on the ground or by balloon probes. The air showers are some square kilometers, but only a few meters vertically. They indicate the type and energy of the primary cosmic, the curve of the front to the incident direction.

Victor F. Hess postulated in 1912 a so-called cosmic rays to explain the observed in a hot air balloon higher electrical conductivity of the atmosphere and also the increase in gamma radiation at high altitude.

The term radiation has survived to this day, although the (electromagnetic ) Gamma radiation is not expected to cosmic radiation. To distinguish them from the cosmic ( particles ) radiation it is called cosmic gamma radiation.

• 6.1 " secondary " cosmic rays

Classification and origin

Depending on the origin of which divides the cosmic rays in solar radiation (german solar cosmic ray SCR), Galactic (English galactic cosmic ray, GCR ) and extragalactic radiation:

• Solar wind. Its characteristics are: Teilchenstromdichten by 107 / (cm ² · s ), low energy, mainly protons and alpha particles. Particle density to 5/cm ³; Cause of the aurora;
• Solar flares, CME. Features: increase the particle flux within hours and days to 108 / (cm ² · s ) to 1010 / (cm ² · s ), high energy 10 MeV, particle density to 50/cm ³.
• Sometimes called the Van Allen Belt, although its radiation does not reach the earth.
• Anomalous cosmic rays ( engl. anomalous cosmic rays, ACR) is probably formed by interaction of the solar wind with the local interstellar medium ( LISM ) in the region of the termination shock near the heliopause. Features: lower in energy than GCR, less hydrogen and carbon ions than hydrogen and carbon in the LISM.
• Galactic cosmic rays (GCR). Features: low Teilchenflussdichten, very high energies ( 1 GeV and above), proportion of heavy ions. With increasing energy, the deflection is reduced by magnetic fields and the anisotropy of the radiation increases.
• Extragalactic particles with maximum energies up to 1020 eV. The flux densities are below 10-20 particles per second and square meter.

The sources of galactic cosmic rays could be identified only in the past few years. Candidates for this include shock fronts of supernova explosions, cosmic jets of black holes or pulsars. For particle energies below 1018 eV (= 1 EeV ) an origin within the Milky Way, it is assumed, while for larger energies other galaxies or quasars are likely. Cosmic magnetic fields deflect the particles. They seem therefore to radiate isotropically on the earth. However, since many sources emit particles and gamma radiation in addition, their origin can thus be traced back.

Composition

The galactic cosmic radiation is about 87% of protons ( hydrogen nuclei ), 12% alpha particles ( helium nuclei ) and 1% heavier nuclei. The frequency distribution of the nuclei is about the frequency of the solar element. Notable exceptions include lithium ( Li), beryllium (Be) and boron ( B), which are up to 500,000 times more common in cosmic rays as a result of spallation reactions in traversing interstellar matter than in solar matter. By interaction with the atmosphere can be observed on the earth is not the original source, but the reaction products from the interaction with the atmosphere, in particular nitrogen, oxygen and carbon. The proportion of elements heavier than iron and nickel is not precisely known, traces of bismuth are detected.

The distribution of the particles per time, N (E) as a function of energy E as a power law:

With:

In 2008, measurements made seem to confirm the GZK cutoff above 5.1019 electron volts. After that limit interactions with the cosmic background radiation, the particle energy at 1020-1021 eV, where the mean free path is crossed by 160 million light years.

History of Exploration

Victor Franz Hess discovered in 1912 the cosmic radiation using balloon rides in the Earth's atmosphere. He published the discovery in the journal Physics 13 (1912 ), 1084.

In 1949 Enrico Fermi postulated a possible acceleration mechanism, which involves a statistical acceleration in magnetized plasma ( " magnetic clouds" ) or planar shock fronts. A shock front can be given for example by a in comparison to the environment very quickly propagating gas. Shock fronts occur mainly by supernova explosions in the cast-off shell of the supernova. In this statistical acceleration energy of the gas is transferred to the particles for a long time by means of " bumps ". This produces a power spectrum but with a γ deviating from the measurement data spectral index.

Scott E. Forbush dismissed in 1946 after that in solar flares produce particles up to the GeV range.

Walther Bothe and Werner Kolhörster tried to prove that the cosmic radiation is a high-energy gamma radiation. They used a measuring arrangement which consisted essentially of two Geiger- Müller counter tubes, between which one could bring different thick absorber in the form of iron or lead plates for their experiments. They assumed that a gamma quantum can only be detected with a Geiger- Müller counter, when it first strikes an electron from a neutral atom. This electron would then be detected by the counting tube. The secondary electron wanted Bothe and Kolhörster examine, and for this they used the counting tubes. In fact, they discovered very soon coincidences, that means events that took place in two counter tubes at the same time. This suggested that an electron that has been struck by a gamma ray from an atom must have crossed both counter tubes in rapid flight.

Next, they determined the energy of these electrons alleged by they brought always becomes thicker absorber in the form of metal plates between the two counter tubes until no coincidences would occur. Bothe and Kolhörster found, to their surprise, that 75 % of the coincidences were not even to be prevented by a four- inch-thick gold bars.

In fact, the particles, by which the Geiger- Müller counter tubes were triggered were just as pervasive as the cosmic radiation itself has led to the conclusion that the cosmic radiation itself, contrary to the general assumption that no gamma radiation, but at least a part consists of charged particles of very high penetrating power. They were able to show that the secondary radiation produced by the primary cosmic radiation in the interaction with our atmosphere, consists of electrically charged particles.

Interaction with matter

Cosmic radiation triggers when penetrating matter spallation reactions. By measuring the abundances of spallation products in meteorites can be as for example the length of stay will be determined in space (radiation age). Also could be so determined that the average intensity of galactic cosmic rays has changed at most by a factor of two for at least 100 million years.

Interaction with the Earth's atmosphere

Particle showers

On entering the Earth's atmosphere at an altitude of 20 km above the surface creates the cosmic radiation particle showers. For a proton energy of 1015 eV arise over a million offspring. Only a small part of them reaches the earth's surface.

By spallation of nitrogen and oxygen atoms arise neutrons, protons, charged ( π , π - ), and neutral ( π0 ) pions. The neutral pions are annihilated, the charged decay into muons:

The muons are also unstable and decay into electrons and neutrinos:

A chill has

• A soft electromagnetic component, among other things, by the decay of π0 and the annihilation of positron -electron pairs
• A hard muonic and
• A hadronic component, which mainly contains protons and neutrons.

The components can be independently register on the earth, and serve as proof of the cosmic radiation.

The cosmic radiation and its decay products contribute to the emergence of a number of cosmogenic radionuclides in the earth's atmosphere and crust. Often the production by cosmic radiation, the largest natural source of these radionuclides, which brings a number of applications for isotopic analysis. These cosmogenic radionuclides include, for example 14C, 3H, 10Be, 26Al and 36Cl.

The possibility of radiocarbon dating is based on the cosmic radiation, which releases neutrons, which in turn can convert nitrogen in the radioisotope 14C. This is bound only in these during the metabolism of living plants, but decays with a half -life of 5730 years, so that by the end of the metabolism decreases and its content can be inferred from its proportion to the age of organic substances.

Potential climate impact

A correlation between the formation of clouds and galactic cosmic rays (GCR) has been postulated in the U.S. since the 1970s. Since the 1990s in particular was the Danish physicist and climate scientist Henrik Svensmark to the spread of the thesis. A surveillance study of several international research institutions from 2006 looked at the impact of a dynamic heliosphere to the Earth's climate when considering very long periods of time as probable. About the effect associated with the formation of clouds, there are several hypotheses. Research projects on the mechanism of a connection between cosmic rays and cloud formation are currently underway at CERN ( project CLOUD Cosmics Leaving OUtdoor droplets ) and the National Space Institute DTU. The project at CERN is regarded as the first climate experiment on a particle accelerator.

In millions comprehensive timescales Shaviv suggests the paradox of the faint young sun as well as the overall course of the climate history of the earth as part of an overall model. Here, the interaction of the solar wind, star formation rate and cosmic radiation, in addition to a postulated effect of greenhouse gases on the climate. During the first three billion years Earth's history, a strong solar wind have largely shielded the cooling effect of the cosmic radiation, then fell regularly scheduled global cold periods together with equally regular Spiralarmdurchgängen the heliosphere, which hints at a significant impact of the global cosmic radiation. One published in The Astrophysical Journal Letters in 2009 study tested the hypothesis using a more accurate, based on CO data approach and found no indication that the postulated by Shaviv et al context. 2010 was alleged to have Svensmark theses on the influence of cosmic rays on global warming refuted completely. A research team led by Frank Arnold from the Max Planck Institute for Nuclear Physics found no correlation between cloud cover and ion concentration in studies of six prominent Forbush events.

Astrophysicist Nir Shaviv explains the absence of a current global warming with the heat storage capacity of the oceans and maintains the cosmic radiation for better-suited to explain this in interaction as greenhouse gases alone.

A controversy sparked the thesis according to a joint publication Shavivs with the Leibniz Prize winner Jan Veizer in GSA Today from. This imputed Stefan Rahmstorf and others in an article published in Eos comment Shaviv and Veizer serious methodological and substantive weaknesses. Rahmstorf's argument, it lacked a recognized physical mechanism was incorporated in the IPCC reports. Veizer and Shaviv reported Rahmstorf's allegations as politically motivated character assassination.

In a 2012 published by the Royal Astronomical Society study Svensmark posited a clear link between biodiversity, plate tectonics, in particular their impact on the extent of coastal areas and the number of supernovae in the Earth environment over the last 500 million years. Basically, the biodiversity of the sea from the sea and the derived from the occurrence of Supernovaerate cosmic radiation rate CGR was dependent. The Primärbioproduktivität of the sea, the net growth of photosynthetic bacteria can be explained only with the CGR there. In addition, is to be found an inverse relationship between increased Supernovaeerscheinungen and the carbon dioxide content of the atmosphere, the Svensmark leads back to increased ocean in colder areas bioproductivity.

Intensity and detection

For the detection of cosmic rays, different approaches are being pursued. While the larger particle flux at low energies can still be detected directly with balloon and satellite detectors that allow occurring at higher energies extensive air showers from the observation from the ground. Sprawling detector arrays with high time resolution allow the reconstruction of the particle energy and the direction of incidence. Be detected while the charged particles of air showers with scintillation detectors (eg KASCADE - Grande) or with Cherenkov detectors, the photons of the shower ( bremsstrahlung or air - Cherenkov radiation) or even across emitted to chill fluorescent light from nitrogen molecules. With fluorescence telescopes ( the Fly's - Eye in Utah, USA) was 1991, the highest measured particle energy observed, specifically mentioned in an " Oh- My- God particle " proton, which has an energy of 3.2 x 1020 had eV (compared to the LHC at CERN will accelerate protons to about 7.1012 eV, ie about the 45.000.000sten part of this energy ). A recent experiment to observe high energy cosmic rays is the Pierre Auger Observatory, which covers an area of ​​3000 km ². This experiment follows a hybrid approach with simultaneous detection by Cherenkov detectors and fluorescence telescopes.

Apart from the long-term stability, there are short-term periodic and non-periodic variations in the intensity of cosmic rays. To vary the intensity depending on the eleven-year sunspot cycle; The more sunspots there are, the lower the intensity of galactic cosmic rays. There is also a 27 -day variation, which is associated with rotation of the sun. From ground-based detectors and weak full and half- day fluctuations are observed. Solar flares and other solar activity can also cause sudden temporary intensity wastes, which are referred to by their discoverer, Scott E. Forbush as Forbush events. More rarely, a sudden increase in intensity is observed.

" Secondary " cosmic radiation

The cosmic rays produced in interactions with the atmosphere offspring. At sea level, these are mainly positive and negative muons in the ratio with a flux density of 100 m -2s -1. These muons manifest themselves as disturbing "underground" in particle detectors and because of their high energies by ordinary means hardly be screened. For measurements of very weak Teilchenflussdichten, in particular, for example, to neutrinos, we, therefore, differs ( Laboratori Nazionali del Gran eg Sasso ) made in old mines or tunnels in low-lying underground laboratories.

Cosmic radiation and air transport

High-energy radiation from space occurs at high altitude significantly more visible than at sea level. Therefore, the radiation exposure of air travelers is increased. As early as 1990 identified the International Commission on Radiological Protection ( ICRP) estimates from that flight personnel is exposed to the natural cosmic radiation doses that are comparable to or even higher than that of people who deal with artificial radiation in medicine and technology. Therefore, presented the ICRP recommendations on dose limits, which were endorsed by the EU and 2001 in the German Radiation Protection Ordinance 1996.

The introduction of dose limits requires that the current radiation doses can also be determined. Therefore, provided a number of European institutions as a result of the ICRP recommendations on research programs, whose goal was the theoretical and experimental detection of natural radiation exposure in aircraft. At the University of Siegen and at the GSF - National Research Center for Environment and Health, the program EPCARD was developed. With its help it is possible to calculate on any flight paths and profiles, the dose from all natural components of the penetrating cosmic radiation.

With the dose calculation in the Internet is smaller airlines also offers a simple way to determine whether their pilots reach the conditions specified in the Radiation Protection Ordinance limit of 1 mSv per year ever from a dose notification must be made regularly to the Luftfahrt -Bundesamt.

Importance in the history of science

Before the development powerful particle accelerator was the cosmic radiation as a source of high energy particles for particle physics, and many elementary particles were first detected in this, the physicist went on to mountain peaks and balloon experiments ran.

Since we could demonstrate only small traces of antimatter in cosmic rays that are supposedly completely emerged in interactions of charged particle radiation with interstellar gas, which is regarded as decisive indication that in our universe is not a natural anti-matter exists and therefore it is one from the Big Bang to has come asymmetry of matter and antimatter.