Cosmic Rays and Air Showers Marcus Hohlmann Dept. of Physics & Space Sciences, Fl. Tech Irish Science Challenge 2006

A: The elements that make up your body and the Earth are star dust… 1 - Have you ever wondered what you are made off? 2 - Where did the elements come from that make up your body? 3 - The elements that make up your body are the same elements found on the Earth. Where did those Earth elements come from? A: The elements that make up your body and the Earth are star dust… … and they still keep coming in the form of cosmic rays

History of Cosmic Rays But not: Where are they coming from ? 1912 - Victor Hess reaches 5350 m altitude in a balloon and shows conclusively that the rate of charged particles increases significantly with height: There is an extraterrestrial source of radiation ! 1930 – Pierre Auger discovers particle showers. 1936 - Hess gets Nobel Prize for discovery of cosmic rays. History of Cosmic Rays Questions that have been answered by now: - What are these particles made of ? - What different kinds are there ? But not: Where are they coming from ? How do they get their momentum ?

Some Cosmic Ray Facts 1- High energy charged particles. 2- Travel at nearly the speed of light. 3- Strike the Earth from all directions.

Cosmic Rays continually bombard the Earth. The highest energy cosmic rays measured to date have energy of about 1020 eV, equivalent to the kinetic energy of a baseball traveling at about 100 mph! Cosmic Rays continually bombard the Earth. In fact, about 100,000 cosmic rays (muons) will pass through a person, e.g. YOU, every hour!

Q: What’s an electron-volt (eV) ? (A: An energy unit) Giga = 1 Billion or 109 +++++++++++++++++++++++ - Tera = 1 Trillion or or 1000 Billion or 1012 Energy = 1.5 eV Electron accelerates in electric field 1.5 V AA battery - - electron - - - - - - - - - - - - - - - - - - - - - 1 GeV = 1 Giga eV = 109 eV 1 TeV = 1 Tera eV = 1012 eV

Primary Cosmic Ray Composition They include essentially all of the elements in the periodic table. About 89% of Cosmic rays are hydrogen (single protons), 9% Helium (He) nuclei, and about 1% heavier nuclei - in fact, all of the elements in the periodic table . Cosmic rays for the most part are fully ionized atoms, i.e. bare nuclei.

A more detailed view: Relative Abundances of the elements in cosmic rays observed at the top of the Earth’s atmosphere compared with the Solar System abundances, both given relative to silicon (=100%) (Simpson 1983) Solid line: Cosmic ray abundances [%] Dashed line: Solar system abundances

Cosmic ray energy spectrum Fairly similar shapes Max. around a few 100 MeV/nucleon Steeply falling spectra

Where are Cosmic Rays Coming from? 1- Galactic Cosmic Rays (GCRs) These Cosmic Rays originate in sources outside the solar system but inside the Milky Way Galaxy. Most GCRs are probably accelerated in the blast waves of supernovae remnants. This doesn’t mean that a supernova explosion itself gets the particles up to these speeds, but the remnants of the explosions, i.e. expanding clouds of gas and associated magnetic fields can last for thousands of years, and can accelerate Cosmic Rays. Most GCRs have energies between 100 MeV (0.43 c) and 10 GeV (0.996 c). Bouncing back and fourth in the magnetic field of the remnant lets particles gain energy and become Cosmic Rays. Eventually they build up enough speed to escape to the Galaxy.

Cosmic Ray Particles coming from outside galaxies passing through Where are Cosmic Rays Coming from? 2 - Extra Galactic Cosmic Rays Cosmic Ray Particles coming from outside galaxies passing through the Milky Way Galaxy.

Where are Cosmic Rays Coming from? 3 - Solar Energetic Particles (SEPs, “solar wind”) The sun is one of the sources of Cosmic Rays. Nuclei and electrons are accelerated by shock waves traveling through the Corona and by magnetic energy released in Solar flares. The Solar Wind contains roughly equal numbers of electrons and protons along with heavier ions and blows continuously from the Sun at an average speed of 400 km/sec. This leads to a mass loss of 10 million tons of material from the sun every year. Typical energies are less than 10 MeV/nucleon.

What happens when a cosmic rays strikes the earth ? Atmospheric nucleus Muon

“Pi Mesons” or “Pions” (Meson: from the Greek “meso”, meaning “middle”) IF the nucleus of an atom contains many charged protons they should repel each other due to their positive charges. What is the nature of the force that holds the nucleus together ? Mesons exchange between nucleons in the nucleus produces a strong force that overcome Coulomb repulsion, but its range is only about 10-15 m. Pi meson () or simply pion is an example of a Meson (bound quark-antiquark state). Pion comes in three varieties corresponding to three charged states: +, - and 0 Pions are not stable particles, but decay: Mπ = 139.6 MeV/C2 Muons decay, too:

Muons Muons are the most numerous energetic particles at sea level. The other shower particles mostly get absorbed in the atmosphere. A Muon interacts very little with matter except by ionization. Because of this, Muons can travel large distances and commonly reach the ground. BUT: The lifetime of a muon is 2.2 μs = 2,200 ns. As they travel at the speed of light (which as you all know is ~1 ft/ns), they typically would travel 2,200 ft or 700m. How do they get to the ground ? Special Relativity to the rescue !

Relativistic Effects on Muons At relativistic speeds the lifetime of the Muons is much longer, where the rest mass = 0.1 GeV

Energy Loss & Flux of Cosmic Ray Muons: Muons lose energy at a fairly constant rate of about 2 MeV per g/cm2. The vertical depth of the atmosphere ~ 1000 g/cm2 This means that Muons will lose ~ 2 GeV to ionization before reaching the ground. However, the mean energy of Muons at sea level is still 4 GeV. Muons arrive at sea level with an average flux of about 1 muon per square centimeter per minute.

Scintillation detector An Instrument for detecting cosmic ray muons in the lab Scintillation detector 1- Plastic Scintillator Material that emits visible photons when traversed by a high-energy charged particles. This Scintillator is made of organic materials (typically aromatic compounds). These low energy photons are subsequently collected by photomultiplier tubes (“PMTs”).

Light YOU’LL SEE SOON ! 2- Photomultiplier tube (PMTs) It consists of a photocathode and a series of dynodes in an evacuated glass enclosure. Photons that strikes the photo emissive cathode emits electros due to the photoelectric effect. Instead of collecting these few electrons these are accelerated towards a series of additional electrodes called dynodes Light The dynodes are each maintained at an increasingly positive potential. The cascading effect creates 105 to 107 electrons for each photon hitting the first cathode. The amplified signal is finally collected at the anode where it can be measured. YOU’LL SEE SOON !

Summary of the Trip There are many sources for Cosmic Rays. Cosmic rays are affected by many external factors during their trip from source to the Solar system. Muons are produced in air showers initiated at the top of the atmosphere and can reach the ground where we can detect them (fairly) easily. The study of Cosmic rays is extremely useful in many areas of physics and astronomy. Cosmic Ray study is a clue to understanding the composition of the Universe.

Thank you ! Questions ?!

Electromagnetic Processes in Cosmic Ray Air Showers Collisions of cosmic ray nuclei and atmospheric nuclei produces also gamma rays e+e- Pair Production: Creation of two electrons, one negative and the other positive (positron), from a pulse of electromagnetic energy (gamma ray) traveling through matter. This is one of the principal ways in which high energy gamma rays are absorbed in matter. For the pair production to occur the photon energy must be at least equivalent to the mass of two electrons, i.e. 1.02 MeV because the mass of a single electron is 0.51 MeV.

Supernova Explosions A supernova can release more energy in ten seconds than our Sun can produce in 10,000,000,000 years. One of the most energetic events known in the Galaxy. It occurs at the end of a star’s lifetime, when its nuclear fuel is exhausted and it is no longer supported by the release of nuclear energy. When the star is particularly massive, then its core will collapse and in so doing will release a huge amount of energy. This will cause a blast wave that ejects the star’s envelop into interstellar space. Many supernovae have been seen in nearby galaxies, they are relatively rare events in our own Milky Way Galaxy.

Solar Flares Tremendous explosion on the surface of the Sun It occurs when magnetic energy that has built up in the solar atmosphere is suddenly released Radiation are emitted from Radio waves at the long wavelength end through optical emission to x –rays and gamma rays at the short wavelength end. As the magnetic energy is being released, particles, including electrons, protons and heavy nuclei are heated and accelerated in the solar atmosphere. The energy released is on the order 1027 erg /sec or more. -> Joules ! This amount of energy is equivalent to 100 megaton hydrogen bombs exploding at the same time or ten million times greater than energy released from a volcanic explosion.

The maximum energy reached in such an event is 10 to 100 MeV; occasionally energies reach 1 GeV (once a year) and 10 GeV once a decade. Solar flares have a direct effect on Earth’s atmosphere, as the intense radiation can reach Earth in 8 minutes and the Earth’s upper atmosphere becomes more ionized. This can disturb long distance radio signals and Satellite electronic components. From a satellite