Transcript Introductory Presentation on Cosmic Rays

Cosmic Rays and Air Showers
Marcus Hohlmann
Dept. of Physics & Space Sciences, Fl. Tech
Irish Science Challenge 2006
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
•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.
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
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
+++++++++++++++++++++++
Energy = 1.5 eV
-
Tera = 1 Trillion or
or 1000 Billion or 1012
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 .
100
Cosmic rays for the most
part are fully ionized
atoms, i.e. bare nuclei.
80
60
40
20
0
Protons 89% Helium Nuclei Heavy Elements
10%
1%
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.
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 ?
Cosmic Ray
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:
Muons decay, too:
Mπ = 139.6 MeV/C2
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.
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”).
2- Photomultiplier tube (PMTs)
It consists of a
photocathode and a
series of dynodes in
Light
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
The dynodes are each maintained at an increasingly
dynodes
positive potential. The cascading effect creates 105 to
107 electrons for each photon hitting the first cathode.
YOU’LL SEE The amplified signal is finally collected at the anode
where it can be measured.
SOON !
• 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.
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.
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
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