Is there photon matter in the universe


Semester paper as part of the advanced physics course




Cosmic rays as a window to the universe


Christoph Boden


January 2004

Figure 1 The electromagnetic spectrum of the Milky Way at different wavelengths

Pasteur Oberschule Berlin Pankow

Introduction .. 3

2. Historical development .. 3

2.1 Excursus: The magnetosphere and the Van Allen radiation belt. 7th

2.2 The cosmic background radiation: 10

3. cosmic rays .. 14

3.1 The spectrum and its special features .. 14

3.2 Excursus: Explanatory models for high-energy particles (Extreme High Energy CR) 18

3.3 Secondary cosmic rays / Airshower: 20

3.4 Air fluorescence .. 24

3.5 Cherenkov radiation .. 24

4. Measurement method .. 27

4.1 direct measurements: balloon / satellite experiments. 28

4.2 Coincidence counting (Auger, Kaskade) 29

4.3 Cascade. 30th

4.4 Fly’s Eye / HiRES. 31

4.5 Auger Upper Vatorium ... 33

4.6 Čerenkov detectors .. 34

5. Acceleration mechanisms: 36

5.1 Fermi acceleration .. 37

5.2 Fermi mechanism of different orders .. 38

5.3.1 Second order Fermi mechanism .. 40

5.3.2 Fermi mechanism of the first order .. 40

6. Potential sources of cosmic rays: 41

6.1 Shock waves from supernova remnants. 41

6.2 Pulsars. 44

6.3 binary star systems. 48

6.4 active galactic nuclei (AGN) 49

7. Gamma-ray and neutrino experiments for "taking evidence". 49

7.1 Production processes for gamma rays .. 50

7.2 A possible evidence for supernova remnants as accelerators for protons? 52

7.3 The H.E.S.S. Project .. 53

7.4 Digression: Gamma Ray Bursts. 56

8. Summary and Outlook .. 59

Appendix A: Sources .. 61

A.1 Sources of the figures .. 61

A.2 Literature: 62

A.3 Internet sources: 63

A.4 Magazines & Conference Publications. 63

Appendix B Glossary ... 63

Appendix C Selected References .. 64





I wrote this semester paper as part of the advanced physics course of the upper level of the state of Berlin.

Inspired by a lecture by Mr., Prof. Dr. T. Lohse of the Humboldt University Berlin during the November 2002 lectures at the HU-Berlin on the subject of "The Riddle of Particle Radiation from Space", I worked deeper and deeper into the subject in the course of 2003 and made interesting insights into areas not directly involved gained in physics, which ultimately led to the decision to choose this topic for the term paper.

"H.E.S.S" was already presented in the lecture by Prof. Lohse as a telescope system for measuring cosmic gamma rays. This is a joint project of several research institutions and universities, in which the Humboldt University of Berlin is also involved. Even if this project is only of minor importance here, it was an interesting guide to get an overview of the current state of knowledge and to be able to understand future experiments. The observatory is now fully operational with the start of operation of the fourth telescope on December 10, 2003.

The interesting thing about cosmic radiation is that many theories are well thought out, but - if at all - are “built” on very sparse measurement data, so they are still waiting to be experimentally verified. Therefore, this research is absolutely topical and subject to all "fluctuations". This leaves a lot of space for your own considerations, which are really not to be expected in the context of a student's work. This work is limited to giving an overview of the existing publications on the topic.

At this point I would like to thank Prof. Dr. Thank you Lohse, who not only aroused interest in the topic with his lecture, but was also ready to answer questions on my part during a visit to Adlershof, as well as to take a close look at this work.

Not last through his remarks, I became aware that just like all the "findings" presented here in the field of cosmic radiation are only to be regarded as theories and that many assumptions must first be confirmed by further measurements. All the results and assumptions presented here are therefore only to be regarded as the best explanations that have existed up to the present day and are in no way proven beyond doubt. If this is not expressed clearly enough at one point or another, it should now be said for the entire work.


Towards the end of the 19th century the electroscope was predominantly used to demonstrate the presence of electrical charge. By 1890, scientists studying the conductivity of gases with gold leaf electroscopes observed that the electroscope discharged steadily, albeit very slowly, no matter how well they shielded the test arrangement. 1901 investigated J. Elster and H. Geitel in Germany as well as CTR Wilson in England this phenomenon and came to the conclusion that there must be a previously unknown source of ionizing radiation. At the time, the explanation of the phenomenon seemed obvious - radioactive radiation in the atmosphere and from the earth, which ionizes the air around the electroscope. This explanation became popular due to the discovery of X-rays by Conrad Röntgen, as well as the knowledge from Bequerell's discovery of radioactivity in a uranium compound in 1896 that radioactive radiation can come from virtually any material on earth.

The Dutch physicist Th. Wulf developed a highly stable electroscope with an ionization chamber in 1907, which he brought to the Eiffel Tower to test this hypothesis. If the ionizing radiation, which causes the steady discharge of an electroscope, actually came from the earth or the near-earth environment, the intensity would have to decrease with increasing altitude. According to Wulf's measurements, the intensity was only 64% of the earth's level, which initially seemed to confirm this, but the drop was far weaker than he had previously calculated.


Figure 2 Victor Hess

The 28 year old Austrian Victor Hess followed these attempts and decided to get to the bottom of the matter himself. In 1912 he undertook a balloon flight with a specially manipulated electroscope of the Wulf design, on which he noted the ionization at different heights. Amazingly, it initially decreased up to a height of 500m, but then rose sharply with increasing distance from the ground

Figure 3 Measurement curves of the balloon experiments of Victor Hess (intensity-height)

Kolhörster [R 2] was later able to determine ten times the intensity in 9km. Hess saw no other explanation for this effect [R 1] than that high-energy radiation enters the atmosphere “from above” and is able to induce ionization there even in the enclosed ionization chambers that he carried with him. This knowledge about the so-called “cosmic radiation” let him participate in a Nobel Prize in 1936.

Because of the First World War, research into cosmic radiation in Europe initially came to a standstill, but the American physicist R.A. Millikan began to be interested in this phenomenon, which he called "cosmic radiation" for the first time, and postulated that it was "ultra-γ-radiation", a particularly high-energy form of γ-radiation (200MeV according to Milikan's calculations). Not only had he made his own balloon measurements with Bownen himself, but also in the middle of snow-covered mountain lakes could not find any significant reduction in the charge decrease of a charged electroscope. The latter is particularly heavy, as the water, snow and ice of a mountain lake contain almost no naturally radioactive substances.

However, his theory disappeared when the pairing process became known, which severely limits the range of the γ-rays of very high energies. The Dutchman J. Clay was able to show in 1927 that the intensity of cosmic rays depends on the geometric latitude. He drove with his ionization chambers on a sailing ship that sailed between the latitudes. He observed an increase in intensity at higher latitudes.

Figure 4 The breadth effect

This made it clear that the “cosmic radiation” had to consist at least partially of charged particles, as this effect could only be explained by deflecting them in the earth's magnetic field. This meant the final end for Milikan's photon theory, as they have no electrical charge and thus cannot experience any deflection. The primary cosmic radiation can penetrate at the poles of the earth parallel to the magnetic field, while at the equator perpendicular to the earth’s magnetic field it feels the full component of the Lorentz force. Therefore, particles of a certain energy can only be detected up to a latitude that depends on their energy on earth. So there are “forbidden” areas in the earth's magnetosphere, in which particles of a certain energy will never be found. Figure 5 illustrates these “forbidden areas” for particles of selected energies.

Figure 5 "Forbidden" and "Allowed" zones for charged particles

Curiously, the intensity of the radiation increased measured from the equator, but no further increase in radiation was measurable from the middle latitudes (> 35 °). This could be explained by the fact that the particles of cosmic radiation were next to the earth's magnetic field also have to overcome the atmosphere in which they suffer a loss of energy due to ionization. Because of this loss of energy, particles of lower energy cannot reach the earth's surface.

In 1930 the Italian physicist Bruno B. Rossi noticed that if the cosmic rays are predominantly negatively or predominantly positively charged, an “east-west effect” should be observed. If the cosmic particles were predominantly positively charged, the majority would have to come from the west, with a negative charge correspondingly from the east.

Figure 6 The east-west effect

He postulated this based on the orbits of charged particles in the earth's magnetic field calculated by Störmer in 1930. Störmer, who originally investigated the phenomenon of the northern lights, had also become aware in subsequent experiments that particles up to a certain energy can also be captured by the earth's magnetic field.


The Earth's magnetic field is deformed into a cometary shape by the sun's magnetic field, the heliosphere, which is expanded by the sun's wind. Electrically charged particles are deflected in different directions at the magnetopause, which leads to the east-west effect. The lower the energy of the particle, the stronger the deflection. Therefore, particles of low energy cannot advance to lower latitudes.

Figure 7 The magnetosphere

Various radiation belts form within the magnetosphere. In these areas, the earth's magnetic field acts as a “magnetic bottle” and keeps charged particles of certain energies trapped on helical orbits. These radiation belts were detected in 1958 by the American researcher James Van Allen using the Explorer 1 satellite and named after him. A distinction is made between two essential areas: the inner and the outer radiation belt. The inner belt extends 1 to 3 earth radii away and consists mainly of protons, which come from decayed neurons, which in turn are desanded by particles of cosmic rays interacting in the atmosphere. The proton belts are stable over periods of up to 11 years (one solar cycle) and are subject to only minor variations.
The outer radiation belt is further away from the earth's surface and consists of electrons and ions in the lower energy range (20-100 keV). This phenomenon is also known as "ring current". In the same spatial area, however, there are also electrons with energies in the MeV range, which cannot possibly come from the solar wind. This “outer radiation belt” is subject to very strong fluctuations. The spatial expansion as well as the density and intensity can fluctuate 1000 times within minutes. The sources of these electrons are still not fully understood. It is assumed that they are accelerated within the magnetic sphere, more precisely in the magnetotail. The energy of the solar shock waves is used in a process that is not yet fully understood.

In 1929, Bothe and Kolhöster developed a special method to display the discharge of two or more separate Geiger-Müller counter tubes. This new method of “coincidence counting” made it possible to follow the path of a charged particle through the counter tubes. An arrangement of counters that are set up in such a way that they only show a discharge when a particle passes through them in a straight line is called counter tube telescopes. With these it was now also possible to determine the direction from which the charged particles come. In fact, the particles prefer to hit the earth perpendicularly, but the intensity of the incidence gradually drops to zero when the apparatus is tilted towards the horizon. This also seems logical, since the particles that do not fall perpendicularly have to penetrate a much thicker layer of air. With the thickness of the air layer, the frequency of the particles naturally drops rapidly - since only particularly energetic particles “get through”.

With the help of this instrument, the American physicist Thomas H. Johnson was able to show in 1935 that the ionization rate increased from the eastern to the western observation angle, which proved that the cosmic particles carry a predominantly positive charge. He had thus observed the East-West effect theoretically assumed by Rossi. Today we know that the majority of cosmic rays are protons, but that the positively charged particles at earth level are positively charged muons () represent.

The coincidence counters were able to reveal much more about cosmic rays. Geiger counters, for example, had sometimes been observed that were set up in such a way that they could never have been discharged at the same time by a single linearly incident particle, which nevertheless registered a coincidence discharge. As a logical conclusion one had to assume that the particles do not appear individually, but in some cases obviously in much larger quantities. D. Skobelzyn accidentally made the first observations of these “cosmic rays” during experiments in Leningrad. When photographing the cloud chamber traces of β-particles in a strong magnetic field (0.15 T), he noticed traces of perpendicularly falling particles on some of his recordings.

Figure 9 Pierre Auger

In 1938 the French physicist Pierre Auger carried out measurements with movable counter tube telescopes in the Alps, which finally revealed the full meaning of these “showers”. He had observed that the two detectors still registered the arrival of particles at exactly the same time even at distances of up to 75 m. Auger had thus proven the existence of extensive air showers (EAS), the extent of which would later turn out to be much larger.

Rossi had already gained important insights into this in 1933. He had mounted lead plates of various thicknesses in front of coincidence counter tubes that were not in a straight line. He found that incident particles were not simply absorbed, but rather caused a greater count rate at a thickness of 1-2 cm than without lead shielding. Obviously the showers were not absorbed but rather intensified by the fact that the shower in the lead layer develops rapidly and so many more particles can escape behind the lead layer than originally entered.

In abstract terms, it was found that the cosmic radiation has its origin in the depths of space, but at least some of the registered particles are of terrestrial origin. In addition to the primary cosmic radiation coming from the cosmos, there is also a secondary radiation that arises from the interaction of the primary particles in the atmosphere and in other materials.


Figure 10 The trace of one of the first observed positrons

Up until around this point in time, only the electron, the proton and the photon were known as elementary particles. When the American physicist Carl D. Anderson investigated the curvature of the lines left by cosmic rays in a cloud chamber with a strong magnetic field in 1932, he found that the cosmic particles flew through the chamber far too quickly to determine any curvature to be able to. He used a layer of lead to slow them down. At this point he observed a track that would have exactly matched an electron had it not been bent in the wrong direction. Anderson had discovered the Postitron, the antiparticle of the electron assumed by Dirac in 1928, and thus, together with Chadwick, who discovered the neutron in 1932, ushered in a new age of elementary particle physics, from which astroparticle physics would later grow.

2.2 The cosmic background radiation:

Figure 11 Penzias and Wilson in front of the Bell radio antenna

In 1965, Arno A. Penzias and Robert W. Wilson were working on an extremely sensitive radio antenna at Bell Telephone Laboratories in Murray Hill, New Jersey.They had to struggle with permanent background noise. When looking for sources of interference, they found pigeons that had nested in the antenna while checking the system. But even after the uninvited guests had been relocated, there was a constant background noise. They pointed the antenna directly at New York City and then in a rather uninhabited direction without noticing a difference. The interference signals were not caused by civilization and therefore did not come from earthly sources.

At the same time, scientists at Princeton University under the direction of Robert Dicke were looking for the cosmic background radiation predicted by the Big Bang theory. This relic of the early universe would have to be in the microwave range today and is evidence of a state only a few 100,000 years after the Big Bang. When they heard about the data from the Bell laboratories, it was immediately clear to them what had been discovered there. Thus, more or less by chance, the Bell researchers provided the most powerful evidence to date for the big bang theory of an expanding universe, which was established by George Gamov in 1948. [R 3] For this, Penzias and Wilson shared the 1978 Nobel Prize in Physics.

According to Gamov, the early universe represented an extremely hot and dense “plasma” in thermal equilibrium and therefore emitted thermal radiation. When this mass had cooled down to a few thousand degrees Kelvin about 300,000 years after the Big Bang, simple hydrogen atoms were formed and “recombination” began. The photons, which previously interacted with the free electrons, moved from now on almost undisturbed because they could no longer interact with the electrons.

Figure 12 The evolution of the universe

The universe was now transparent so that the photons could move freely and undisturbed. Due to the expanding universe and the Doppler effect, this radiation is now shifted to such an extent that its maximum is in the microwave range.


Figure 13 The intensity spectrum of the cosmic background radiation as a function of the wavelength of the photons measured by the Cosmic Background Explorer (COBE)

The cosmic background radiation represents an almost perfect black body spectrum at a temperature of 2,735 Kelvin. This was proven in 1992 with NASA's Cosmic Background Explorer (COBE). It also seems isotropically distributed across the sky. However, precise measurements with COBE succeeded in demonstrating anisotropies were recently examined more closely by NASA's “Wilkinson Microwave Anisotropy Probe” (WMAP). These slight fluctuations are due to fluctuations in the density of the early universe. Fluctuations from which galaxies may later emerge. Investigating them will probably open up a crucial part of the history of our universe and the Big Bang theory. The photons of the cosmic background radiation give us the broadest view back to the beginnings of the universe: almost 13 billion years.

Figure 14 The cosmic background radiation measured by COBE and WMAP

From a trivial point of view, the fundamental development of our universe is a continuous struggle between gravity and expansion. Since the strength of gravity is ultimately determined by the average density of matter, it follows that the development of our universe is already predetermined. If a certain “critical density” is exceeded, the universe will one day stop expanding and collapse again. If this is not the case, it is an "open" universe that is expanding forever. With the help of WMAP it could now be confirmed again that the basic parameters of our universe are based on an open universe ( ) and we in all probability live in a space-time “flat” universe. Through precise investigations of the angle-dependent intensity fluctuations of the background radiation, it was also possible to determine fundamental quantities of the universe such as the Hubble constant with increased accuracy. (Note from Prof. Lohse: The big surprise is that the majority (70%) of Ω can be explained by an unknown “dark energy” (cf. cosmological constant), which acts like a kind of antigravity and drives our universe even accelerates apart.)

Figure 15 The 3 possible forms of the universe: closed, open or flat universe

Figure 16 Relative size of the respective universe depending on its age. Ωm symbolizes the relationship between the density of the universe and critical density. If the density of the universe corresponds exactly to the critical density, then Ω = 1.


3.1 The spectrum and its special features

The cosmic radiation accelerated in the sources is usually referred to as “primary cosmic radiation”, while the particles resulting from this primary radiation through interaction are referred to as secondary cosmic rays. As for the charged component, cosmic rays exist in energies as such, 98% of atomic nuclei and 2% of electrons. 87% of the nuclei are protons, 12% helium nuclei (α-particles) and only 1% heavier nuclei. This distribution seems to be shifted to the limits of the spectrum, i.e. for very small and very large energies. The uncharged components are primarily photons and neutrinos. So far, no noteworthy evidence of antimatter in cosmic rays has been discovered, which also makes the presence of antimatter in the cosmos appear increasingly improbable or at least insignificant. Although individual antiprotons have been measured by balloon experiments, these come with a probability bordering on certainty from secondary interactions.

If one looks at the element abundance of primary cosmic rays in comparison to the element abundance in our solar system, one notices a greater frequency for the elements lithium, beryllium and boron, as well as for the elements “below” iron (Z <26). This can only be explained by fragmentation or “spallation” of the heavier nuclei O, C and N when they collide with matter in interstellar space. Likewise, the "decay" (only in the event of collisions) of the relatively common element iron leads to an accumulation of the elements below iron. The generally decreasing frequency with higher atomic numbers can be explained with the help of nuclear physics, according to which smaller nuclei are more stable and certain proton / neutron configurations seem to be energetically preferred. Nuclei with an even number of protons and neutrons prove to be particularly stable, while nuclei with an odd number of nucleons are the least stable.

Figure 17 Frequencies of the elements in cosmic rays compared to the abundance of elements in the solar system (dashed)

Much more interesting than the abundance of elements, however, is the energy spectrum of cosmic rays. The total energy of the particles falling on the earth is comparable to the energy of the entire starlight which reaches us. In doing so, however, individual particles can carry almost unimaginably large amounts of energy. The most energetic particles that have been observed so far had an energy of the order of magnitude to eV. (One night in October 1991, a “Fly's Eye” detector in Utha, USA detected a particle with measured - that corresponds approximately to an energy of 50 joules! ) The maximum energy of the spectrum is still completely open, apart from the GZK cutoff, which is explained below. The flow (this corresponds to the particles per area, per unit of time, per angle) of the primary cosmic radiation can be described in parts by a power law.

The following applies: , where N is the number of incoming particles and E is their energy.

Figure 18 The energy spectrum of cosmic rays on earth (flow of radiation depending on the energy of the particle)

The first range extends from the lowest energies to the so-called "knee" of the spectrum (E = eV). Here γ applies2.7. Supernova explosions are the main sources for particles of these energies. Since these occur approximately every 30 years in our galaxy, the “sufficient supply” with these particles would be adequately clarified. The cosmic particle radiation is predominantly of galactic origin. However, the magnetic field of our galaxy can only "hold on" to particles, their energy eV is, since otherwise the gyro radii are larger than the radius of the Milky Way itself. For the gyro radius by equating the radial and Lorentz forces, at and :

The knee itself can be interpreted to mean that ateV the particles begin to leave the Milky Way, as well as the fact that supernova explosions can only accelerate particles to these energies and therefore another acceleration mechanism must be responsible for particles of higher energy. At the knee of the spectrum there is a somewhat steeper falling area up to the “ankle” at E = eV. The following applies here: γ 3. The sources for this are in all probability active galactic nuclei and accretion disks around "smaller" black holes. Then the spectrum flattens out to γ ​​again 2.7 from. Particles of this energy must be of extragalactic origin, it is commonly believed. Potential sources for this would be accretion disks around supermassive black holes in the center of galaxies, so-called active galactic nuclei (AGN).

Interestingly, however, E eV the so-called "Greisen-Zatsepin-Kuzmin Cut-off" [R 4] occur, as protons with these energies interact with the 2.7K photons of the cosmic background radiation and generate pions, which leads to a loss of energy.

The threshold energy can be calculated with today's measured values ​​using the cosmic background radiation by:

, at T = 2.728K.

This would have to represent the absolute upper limit in the spectrum. But even with energies of eV, the mean free path of the protons is so small that they can only reach us from sources within our Milky Way.

This theoretically assumed cutoff causes some difficulties, since the data are contradictory and it does not seem to exist in practice. The some air shower telescopes "event fair" with powers up to eV regestrierten thus poses new riddles. At a


With an average density of the photons of the cosmic background radiation of: the result is a free path, i.e. an average path length without a collision, of 6 MPc. With an energy loss of around 20% per collision process, a maximum distance of around 160 MPc results for protons of eV. So far, however, no objects are known within this distance that can accelerate particles to such high energies. The highest particle energy observed so far was 3.2 and, as already mentioned, was measured in October 1991 by the Fly’s Eye telescope.

3.2 Excursus: Explanatory models for high-energy particles (Extreme High Energy CR)

Figure 19 The flow of high-energy cosmic rays

The origin of the observed cosmic rays with extremely high energy (EHE), i.e. those with energies is one of the essential, unsolved problems of modern astrophysics. About 20 of these events have been published in the literature so far, the highest, as mentioned above, from the Fly’s Eye telescope. Since the energy of the EHE cosmic rays can only be measured indirectly via extensive air showers (EAS), the nature of these EHE particles is largely unknown. In addition, events of this magnitude are extremely rare, which makes research difficult. (1 particle / km² / century at eV) In all probability these are nuclei, but photons cannot be excluded at this point in time either
There are two major problems associated with explaining the occurrence of such MARRIAGE events. On the one hand, it is extremely difficult to accelerate particles to these energies; on the other hand, there are not enough source candidates in the “immediate vicinity” (100 Mpc) that could be considered for the process. Sources that are further away are virtually excluded because of the GZK cutoff. Photons are also subject to a distance limitation due to the Pairing process, which starts much earlier, which is why photons can actually be excluded.
Because of these explanatory difficulties within conventional physics, theories about a “new physics” outside of the standard model of elementary particles emerge. These suggestions mostly fall into two classes: One tries to avoid the GZK cutoff distance or to bypass the GZK cutoff effect in the energy spectrum by introducing a “new physics”. In the other one simply assumes that EHE particles are produced during the decay of supermassive particles (mass> eV), which originate from fundamental processes of the early universe ("relics"), arise. This is usually called the “top-down scenario” in contrast to the “bottom-up scenario” in which the particle is accelerated from low energies to high energies.
Examples would be, on the one hand, the postulation of supersymmetric particles with a hadronic character, which would have a higher GZK cutoff limit value. Or on the other hand the assumption that neutrinos have a mass, what the neutrinos of the "cosmic thermal relic neutriono Background" and the neutrinos with a mass of eV excites the Z boson resonance. These Z bosons decay through intermediate stages into electrons, neutrinos, protons and, above all, photons. It has now been suggested that the photons and nucleons generated in this way would only be produced within the GZK distance limit (100 Mpc) from the earth, which would solve this part of the problem. In this so-called “Z-Burst” scenario, the majority of the generated particles would be photons - as is generally also the case in “top-down” models.
Another proposal includes so-called metastable super-heavy relict particles (MSRP) with a mass> GeV and an average lifespan that is greater than the age of our universe. A particle with these properties would lie outside the Standard Model and would probably originate from so-called "topological defects" in the universe (e.g. magnetic monopoles or cosmic strings). In this case, the GZK cutoff would be omitted entirely.

Figure 20 The orbits of charged and uncharged particles of cosmic rays

While the uncharged part of the cosmic radiation spreads almost in a straight line in the cosmos, the electrically charged part of the radiation is deflected by intergalactic magnetic fields of a homogeneous and irregular nature and therefore loses all directional information on the journey through space. Logically, this influence also occurs in the immediate vicinity of the earth. As already described at the beginning, particles of low energies are deflected by the earth's magnetic field towards the poles or even trapped in the radiation belt in “magnetic bottles”. Because of all these chaotic influences on the orbits of charged particles, it is not surprising that the charged component of cosmic rays can reach energies of E = eV approximately isotropic, evenly distributed over the sky, while the uncharged component always falls clearly from the direction of its sources, thus pointing to them. At higher energies, the direction-changing influences of the magnetic fields are lower, which leads to minimal anisotropies. When entering the solar system, cosmic rays interact with the interplanetary magnetic field scattered by the sun and the solar wind (the heliosphere). Therefore, the intensity of the radiation changes with two periods: a 27-day cycle due to solar rotation and an 11-year cycle due to solar activity. Furthermore, the heliophone has a shielding effect on charged particles of low energy of extragalactic origin. The particles of the solar wind themselves have far lower energies than cosmic rays and are therefore not taken into account when considering high-energy radiation. For the radiation belt of the earth, however, the particles of the solar wind are particularly relevant. Because of all of this it quickly becomes clear that only the uncharged component of cosmic rays or the EHE particles of the charged component are of interest for the observation of the sources.

When looking at the frequency with which the very energetic particles of cosmic rays quickly become clear that a direct observation of the primary particles is practically impossible. Most of the primary particles of the cosmic radiation do not even reach the earth's surface. Only the particle showers, which were caused by the interactions of the primary particle in the atmosphere, can be measured on the earth's surface.

3.3 Secondary cosmic rays / Airshower:

Figure 21 artistic impression of an air shower

The primary cosmic radiation is heavily modified by interactions on the last part of the journey with the atomic nuclei of the atmospheric air, which leads to so-called air showers, also known as “particle cascades”.The original particle may be lost during the interaction, but the number and energy of the secondary particles produced make it possible to reconstruct relatively precisely what energy the particle had and what type of particle it is.

Figure 22 Particle cascade of a primary particle of cosmic rays

First of all, the cascade triggered by charged particles should be considered. This is for an incident nucleon with the mass number A (Since nuclei larger than iron practically do not occur), the first interaction, which usually occurs in the upper atmosphere at altitudes of about 20 km, is hadronic. A nucleus A with the energy E is approximately equivalent to the superposition of A independent nuclei with the energy.
The penetration depth X into the atmosphere is usually given in g / cm². This quotient of length and density is material-specific but independent of other variables such as physical state, temperature and pressure. The mass occupancy of the atmosphere normally corresponds to 1000 g / cm², which corresponds to an air pressure of 1000 hPa.
The radiation length for photons and electrons in air () and the interaction length for hadrons () correspond to only a fraction of the atmosphere, which is why practically nothing of the primary cosmic radiation reaches the earth.

In addition to the particles involved in the interaction, the primary interactions produce a large number of secondary particles, especially pions and kaons, which in turn decay or interact again in the air, which keeps the hadronic cascade going. Neutral pions decay (except for very high energies of the primary particles in the EeV range) into photons before they can interact again ( ), so that at each stage of the hadronic cascade 1/3 of the energy is transferred in photons, which creates the electromagnetic component of the shower. The photons produce Couples. These, in turn, high-energy electrons emit photons as bremsstrahlung in the Coulomb fields of the atmospheric atoms. Anihilating Pairs also generate photons. A small fraction of the electromagnetic component is fed back into the hadronic component, which is why showers injected by primary photons can develop a hadronic component under certain circumstances.

The hadronic cascade (except for a few isolated nuclei) ends with a decay of the charged pions into muons and neutrinos (at medium altitudes around 6 km with a large scatter). (, ) Most of the muons reach the ground before they disintegrate without any significant loss of energy. Muons have a very small cross-section, which is why they have a very good penetration capacity, but are relatively easy to detect due to their charge, which is why they are also called the “penetrating component” of cosmic rays.
The electromagnetic cascade continues up to energies below 1 MeV, at which electrons are further decelerated without emitting bremsstrahlung. Except for very steep showers, this is not yet the case at sea level. The electromagnetic cascade never reaches sea level, they have a shower maximum at about 10 km height. The neutrinos produced when the charged pions decay have such a small interaction cross-section that they propagate unhindered through the atmosphere and a large part of the earth, with low energies even far beyond. Because of this, these can only be proven with great difficulty.

At higher energies, the number of charged particles in a shower can be expressed by the Gaisser-Hillas function [R 5] as a function of the depth of the shower X:


In which represents the depth at which the first interaction took place and the interaction length depends on the nature of the primary particle. is the depth of the maximum of . increases linearly with the energy of the primary particle, whereas varies linearly to the logarithm of this energy.

If the primary particle should be a photon or electron, the first steps of the cascade are only electromagnetic and the decrease in energy is less than in your hadronic cascade - therefore larger in a proton or nucleon shower

The physical processes in a particle cascade usually deliver moderate transverse impulses, regardless of the energy. Most of the high-energy particles can be found around the original shower axis, they form the "core" of the shower. Photons and electrons, as well as muons of lower energy, are distributed much further from the Schauer axis. This “electro-organic glow” has a detectable density even a few kilometers away. The electromagnetic part of this "halo" increases with increasing penetration depth as long as the "core" is active, then reaches ata maximum and then decreases rapidly. At he is as good as disappeared. Most muons “travel” further than the electromagnetic cascade and thus form a “muonic tail” with increasing spread.

At sea level, muons clearly form the dominant component of the charged particles in a particle shower. As these mainly originate from pion decays, as described above, the muon spectrum at sea level can be derived directly from the pion source spectrum with a few modifications. With different energies, especially in the edge areas, the decay length of the pions changes, whereby pions> 100 GeV initially generate tertiary pions, which decay into muons, but then emit muons of lower energy. In addition, the intensity of the muons depends on the zenith angle. If all these aspects are taken into account, conclusions can be drawn about the primary particle based on the muons measured on the ground. The direction of the primary particle can be seen almost directly from the shower. Determine mass and energy only indirectly from the measurement data. For example, using knowledge gained in particle accelerators about the interactions between the particles, showers of a specific particle can be simulated on the computer and then the measured values ​​can be compared with them. An example of this is the air shower simulation program CORSIKA (COsmic Ray Simulations for KAscade), which shows, among other things, that high-energy hadrons are concentrated in a relatively narrow radius (~ 30m) around the shower axis, but the photons and muons can sometimes fall up to 100m away .

With a fixed total energy, the number of muons increases only slightly with the mass of the primary particle and therefore allows an initial estimate of its energy. The number of electrons and, in particular, the number of hadrons observed on the ground, on the other hand, decreases as the mass of the primary particle increases. Heavier atomic nuclei have a smaller interaction length in air, this leads to an earlier development of the shower and thus to a stronger absorption of the electromagnetic and hadronic components of the shower in the atmosphere. The ratio of the number of electrons or hadrons relative to the number of muons thus enables the mass of the primary particle to be estimated. Additional measured variables are the forms of the lateral distributions of the respective particle types, the height of the shower maximum, the reconstructed muon production heights, the structure of the hadronic shower core and the time profile of the shower front.

With the interaction lengths given above and a mass occupancy of the atmosphere of 1000 g / cm², 80% photons, 18% electrons and positrons, 1.7% muons and 0.3% hadrons are obtained at sea level. At medium energies of the primary particles, however, the flow of cosmic rays is so great that there is a more or less uniform flow of secondary particles, which makes it impossible to reconstruct individual showers at these energies.

Figure 23 The electromagnetic cascade

For an incident photon, only electromagnetic interactions are initially relevant. The processes bremsstrahlung, ionization, Compton effect, pair generation and photo effect play a role here. Because these processes are always reciprocal Couples that generate electrons and photons make it virtually impossible to determine whether an electron or a photon initiated the shower. From energies of 10 TeV, these showers occasionally reach the surface of the earth (e.g. on mountains), the lateral spread is rather small. At high energies the pairing process clearly predominates, at low energies the photo effect and ionization.

Compton effect:

Photo effect:

Couple clothing:



All processes are of course also conceivable with positrons!

3.4 Air fluorescence

As a side effect, the nitrogen present in the atmosphere, which makes up 78% of the air, is stimulated to glow by the shower particles and emits a blue-ultraviolet fluorescent light. For energies greater than eV, this diffuse radiation is intense enough against the background of the starlight to be distinguishable at sea level in the wavelength range of 290 nm - 440 nm. Since this “light” is isotropic, a shower can be observed from any direction.

3.5 Cherenkov radiation

Figure 24 Model representation of the optical shock wave of a particle propagating at "faster than the speed of light"

When charged particles propagate through a medium at a speed greater than the speed of light in this medium, so-called Čerenkov radiation is emitted. [R 6]

, so is: .                                                                      (3.5.1)

The charged particles polarize the molecules of the medium, which immediately return to their basic state and spontaneously emit photons during this jump. The particle what with flies through the atmosphere emits Čerenkov light continuously at speed c. But since n is very close to 1, is and thus every Čerenkov photon moves just as fast as the particle itself. The Čerenkov photons then all arrive at the observer at about the same time, even if the emission was long, which means that the Čerenkov light is really only a very short flash of light. The generation of Čerenkov radiation in an optical shock wave is the optical analogue of the shock waves that occur at supersonic speeds. The light is almost always emitted at a certain angle.

Figure 25 Model representation of the Cherenkove effect

It applies , thats why  .                                    (3.5.2)

The threshold energy for the process, which is derived from the threshold condition for the speed:

,                                                                                              (3.5.3)

there : .                                                                           (3.5.4)

The number of Čerenkov photons that are emitted per path length dx can be described for the refractive index n (λ)> 1 by:

,                                                                         (3.5.5)

where Z describes the charge of the particle, α the fine structure constant and λ the wavelength. In the spectral range of visible light these would be (assumed an n independent of λ):

 , at .

From this it is easy to see that this is a relatively strong signal (with a distance of a few km), but that it spreads over a circle of around 100m radius, which is why only a few photons can be captured with telescopes. If we put the differential spectrum of the Čerenkov radiation, which is described according to (3.5.5) by:

,                                                                           (3.5.6)

a maxium can be seen at about 330 nm. This shows that the majority of the Čerenkov photons must be in the ultraviolet range, since applies.

Figure 26 The diferrential Čerenkov spectrum at an altitude of 10 km (dashed) and 2 km (solid).

But even in completely darkened liquid detectors one has to struggle with a low "quantum efficiency" (only about 20% of the Čerenkov photons) and an error that arises because it depends on λ, which increases the number of detectable photons again decreased. No Čerenkov photons are emitted in the X-ray range, because here applies.
For the discovery and explanation of the effect, Pavel Alekseyevich Čerenkov received 1/3 of the Nobel Prize in Physics in 1958.

Figure 27 The energy spectrum of cosmic rays on earth and the corresponding measuring methods

The particle flux of cosmic rays depends on the energy via a power law. (). Therefore the flow becomes very small at higher energies. This means that very different measurement methods are suitable for different energies.

For small energies up to eV, direct measurements can be carried out by balloon or satellite experiments. In order to be able to measure the charged cosmic radiation, the ionization chambers mentioned in the historical introduction were initially used as a detection device. Today, however, complicated equipment is used here. The altitude must be guaranteed by satellites or balloons, which can rise up to 40 km, since the primary particles, as just described, already interact in the atmosphere in the upper layers of the air and initiate particle showers. With a particle energy of eV the flux is only 1 particle per m² per year. Such a low flow can no longer be detected directly with satellite or balloon detectors, since the collector surfaces that can be implemented in these devices are much too small for this. Thus there is a natural limit to direct measurements.

4.1 direct measurements: balloon / satellite experiments


Figure 28 The ISOMAX as an example of a (relatively) current balloon experiment.


In experiments designed for the direct determination of cosmic rays, various measuring devices specialized for the respective types of particles are used. Typically, magnetic spectrographs (mass spectrographs) are used for mass and pulse determination, scintillation counters, airgel Cherenkov counters and flight timers to determine the charge and speed.

Figure 29 Schematic structure of the ISOMAX experiment: In the middle the two magnetic field coils of the mass spectrograph, the two Ĉerenkov counters in dark yellow and the flight timers (ToF = Time of Flight) in light blue

While balloon flights enable a very inexpensive version of direct measurements at low energies, satellites must be used as carriers for the measuring equipment at higher energies, since the particle flow is already so low that not enough particles can be measured within the possible flight times of the balloons.

Even if the charged particles in the lower range of the spectrum unfortunately do not provide any directional information, valuable properties of cosmic rays can be determined here. An exact determination of the type of particle is possible here. The above-mentioned composition of the charged components and the measurements of the element abundances are based on direct measurements, for example in the ALICE balloon experiment.

Furthermore, experiments based on direct measurements for the measurement of antimatter in cosmic rays, for the detailed determination of the energy spectrum and for the determination of the isotope ratio are carried out. The IMAX balloon experiment demonstrated antiprotons in cosmic rays.

4.2 Coincidence Counting (Auger, Cascade)

Since direct measurement methods have been available for some time and also provide very specific information, the spectral range is up to eV relatively well "measured" and analyzed, even if experiments are still carried out today. At higher energies, the measurement, but above all the determination of the mass and type of the primary particle, as already mentioned in the explanation of the particle shower, is much more difficult. The number of measurement data available today is correspondingly clear.

4.3 Cascade

Figure 30 The Kascade Experiment in Karlsruhe

Due to the low particle flux at higher energies, detectors can only be used to detect the particle showers of a primary particle through wide-area fields. Scintillation counters can be used for this. An example of this would be the "Karlsruhe Shower Core and Array Detector" (Kascade), a 200x200 m² field of 252 scintillation counters that are positioned at a distance of 13 m and measure the photons, electrons and muons of a particle cascade can. The system is supplemented by a central detector, which is located in the middle of the array and the main component is a 16 x 20 m2 includes large hadron calorimeter. The multi-detector array Kascade was recently supplemented by a 37 scintillation counter, 700 x 700 m² "Grande" array. This extended experiment is called "Kascade Grande" and allows comprehensive measurements of cosmic rays in the energy range of 0.1 PeV - 1.0 EeV, which enables a complete investigation of the "knee" part of the spectrum.
One tries to observe as many air showers as possible in order to be able to better solve the three-part problem of determining primary energy, determining the mass of the primary particle and understanding the hadronic interaction mechanisms in the atmosphere. In addition, you can test the validity of the hadron interaction models that are used in the CORSIKA Monte Carlo simulations of air showers. To do this, parameters such as the angle of incidence, the position of the shower core axis and the total number of charged particles are measured.
The dominant particle type in the knee region is currently being investigated. It seems as if there is a transition from protons to iron nuclei as the main component of the charged component.

Figure 31 Cross section of one of the 252 detector stations of the KASCADE project


4.4 Fly’s Eye / HiRES

Figure 32 The Fly's Eye Telescope in Utah, USA

As early as 1967 A. Bunner suggested using diffuse nitrogen fluorescence light for measurements of cosmic particle radiation. [R 7] This experiment was carried out in the Fly’s Eye Detector in Dugway, Utah in the USA. The detector consists of two arrays of parabolic mirrors, each 1.5 m in diameter and equipped with a matrix of 12 or 14 photomultipliers. The mirrors are all directed differently in order to be able to cover the entire night sky. Each array records the sky, with a phototube representing one pixel on the sky map.
By recording the atmospheric fluorescent light as a function of the time during which the particle shower passes over the detector field, the shower path can be reconstructed. Once the distance and orientation of the line source is known, the signal from each photomultiplier can be converted into energy that was emitted in this section of the path.
In this way, the direction of incidence, the energy and the type of cosmic ray particle can be determined. Shower tracks near the detector cannot be analyzed because the Čerenkov light, which is in the air at an angle is emitted from the shower axis, which covers the very weak isotropic fluorescent radiation. Therefore, this apparatus only works on very clear, moonless nights, since the scintillation light can otherwise no longer be seen in the background. The Fly’s Eye Team aptly describes it with the comparison of measuring the photons of a blue 5W light bulb that flies through the atmosphere at a distance of several kilometers.

Figure 33 One of the mirrors of the Fly's Eye telescope

The Fly’s Eye project, which ended in 1991, found a worthy successor in HiRES, which is another, larger array of fluorescence detectors. A special goal is the search for particles of cosmic rays which have energies beyond the GZK cutoff. As early as 1991 there was evidence of a light particle, presumably a proton, with an energy of 3.5 x 1020eV (or 56J) succeeded.

4.5 Auger Upper Vatorium

Figure 34 One of the 1600 detector stations at the Auger Observatory

The Pierre Auger Observatory is an example of the measurement of extensive air showers in the Argentine pampas. This is an array of several 100 of the planned 1600 detector stations at a distance of 1.5 km each, in which the Čerenkov light from a shower particle is detected in a water tank with photomultipliers. In addition, 2 of the 4 planned fluorescent light detector stations (each with 6 fluorescent telescopes) with an angular resolution of 1.4 ° are already integrated into the system in order to be able to measure additional shower data. Upon completion, the detector will cover an area of ​​3000 m² and it is hoped that around 30 showers of energies> eV per year to be able to measure. In order to be able to cover the northern sky, a similar system is to be built in the northern hemisphere as part of the Auger project.

Figure 35 One of the central fluorescence detectors of the Auger telescope

Figure 36 Drawing of a shower of particles above the eye telescope

4.6 Čerenkov detectors

Another method of measurement is to observe the Čerenkov radiation, which is emitted by the shower particles. This technique is mainly used to measure gamma quanta and neutrinos of cosmic origin. With neutrinos it is often the muon into which a muon neutrino () transformed upon interaction, which is detected in a detector

Figure 37 Panorama view of the AMANDA-II area

An example of a detector for cosmic neutrinos is AMANDA II. A project in the Antarctic, in which the Čerenkov radiation of the neutrinos in the ice is measured by approximately 677 spherical "optical units" with 8 photomultipliers each, which are embedded in 19 lines in the ice. In the arctic ice they form at a depth of 1500m to 2000m, where the ice is crystal clear and absolutely dark. an almost cylindrical detector with a radius of 100m.

Figure 38 One of the optical units of the AMANDA detector

The entire earth is used as a shield for solar and terrestrial neutrinos (e.g. from air showers) in order to be able to observe the neutrino events in the northern sky. Since the earth becomes opaque for neutrinos with an energy> 1 PeV, one tries to observe neutrinos that come from above (southern sky), because with such energies the background is small (and falls sharply) due to atmospheric neutrinos. To date it has not been possible to measure even one cosmic neutrino event without any doubt.

Figure 39 Map of the neutrino events registered by AMANDA in 2000

By 2008, 4800 photomultipliers are to be installed on 80 vertical lines around AMANDA II as part of "IceCube", which will then cover a detector volume of 1 km³. With this experiment, neutrinos with energies of up to eV can be measured, an energy range in which the universe is opaque to gamma radiation and where the observation of neutrinos can therefore play an important role. (Note V. Prof Lohse: However, the attenuation of the gamma ray signal is a direct method of measuring the intergalactic infrared radiation field. This is a very important cosmological variable.)

From the beginning, the most exciting question about cosmic rays was in which objects in our universe particles can be accelerated to such high energies. After all, particle accelerators must be at work here that are around times as large final energies as earthly systems. First of all, some proposed acceleration mechanisms will be explained in rudiments in order to then show in which areas of the universe they are assumed.