Marcus Hohlmann

with help from Ashraf Farahat

Cosmic Rays and Air Showers

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…

•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.

But not: Where are they coming from ? How do they get their momentum ?

Questions that have been answered by now:

- What are these particles made of ?

- What different kinds are there ?

History of Cosmic Rays

Some Cosmic Ray Facts

Cosmic Rays continually bombard the Earth.

In fact, about 100,000 cosmic rays will passthrough a person every hour!

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!

Primary Cosmic Ray Composition

0

20

40

60

80

100

Protons 89% Helium Nuclei10%

Heavy Elements1%

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)

Dashed line: Solar system abundances

Solid line: Cosmic ray 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 have energies between 100 MeV (0.43 c) and 10 GeV (0.996 c).

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.

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.

Where are Cosmic Rays Coming from?

2- 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.

Where are Cosmic Rays Coming from?

3- Anomalous Cosmic Rays (ACR)

They are produced by neutral atoms in the interstellar medium which leak into the heliosphere and become ionized by either solar UV –radiation or charge exchange with the solar wind. They are then carried back by the solar wind to the outer heliosphere.

Then these particles are accelerated by the solar wind termination shock and drift back to the inner heliosphere as Cosmic Rays.

Where are Cosmic Rays Coming from?

4- Extra Galactic Cosmic Rays

Cosmic Ray Particles coming from outside galaxies passing through

the Milky Way Galaxy.

Cosmic Ray

What happens when a cosmic rays strikes the earth ?

Atmospheric nucleus

1- Pi Mesons (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

Mπ = 139.6 MeV/C2Pions decay:

Muons decay, too:

Electromagnetic Processes in Cosmic Ray Air Showers

e+e- Pair Production: Creation of two electrons, one negative and the other positive (positron), from a pulse of electromagnetic energy (photon) 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.

Muons are the most numerous energetic particles at sea level. A

charged particle cannot avoid losing energy by ionization.

A Muon interacts very little with matter except by ionization. Because of this, Muons can travel large distances and commonly reach the ground.

The lifetime of 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 !

Energy Lose: 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.

Relativistic Effects on Muons

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

Scintillation detector

1- Plastic Scintillator Material that emits low energy 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).

An Instrument for detecting cosmic ray muons in the lab

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

These electrodes are each maintained at a more positive potential. This 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.

•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.

•The study of Cosmic rays is extremely useful in many areas of physics and astronomy.

•Cosmic Ray study is the clue to understand the composition of the Universe.

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.

Tremendous explosion on the surface of the Sun

It occurs when magnetic energy that has built up in the solar atmosphere is suddenly released

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 energy released is on the order 1027 erg /sec or more. -> Joules !

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 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 ??? satellite