Introduction to Cosmic Rays and
Cosmic Air Shower Experiments
• Cosmic Rays in a Nutshell:
• High energy particles traveling throughout
our galaxy at close to the speed of light.
• Consist of elementary particles such as
electrons (and positrons) and
nuclei of atoms, mostly single
protons (a few are much heavier).
Cosmic Rays continually bombard the Earth.
In fact, about 100,000 cosmic rays will pass
through a person every hour!
Cosmic rays-a long story
•
•
•
C.T.R Wilson discovered in 1900
the continuous atmospheric
ionization. It was believed to be
due to the natural radiation of the
Earth. In other words, from the
ground up.
Wilson noticed the reappearance
of drops of condensation in
expanded dust free gas, the first
cloud chamber.
Wilson suspected the
tracks might be condensation on
nuclei - ions that were the cause
of the “residue” conductivity of the
atmosphere.
Where did the ions come from?
• At the beginning of the 20th century
scientists were puzzled by the fact that
more radiation existed in the environment
than could be explained by natural
background radiation
• The debate was solved on a balloon flight
in 1912 from the University of Vienna.
Victor Hess
• In 1912 a Victor Hess, a German
scientist, took a radiation counter (a
simple gold leaf electroscope) on a
balloon flight.
• He rose to 17, 500 feet (without
oxygen) and measured the amount
of radiation increase as the balloon
climbed.
• Victor discovered that up to about
700 m the ionization rate decreased
but then increased with altitude
showing an outer space origin for
ionization.
Not from the Sun
• During subsequent flights Hess
determined that the ionizing radiation was
not of solar origin since it was similar for
day and night.
• It was initially believed that the radiation
consisted of gamma rays only.
• But there was still a dispute as to whether
the radiation was coming from above or
from below.
Birth Cries of the Atoms
• In 1925 Robert
Millikan of Caltech
introduced the term
“cosmic rays” after
concluding that the
particles came from
above not below a
cloud chamber.
• He used elaborate
electroscopes.
Cosmic Rays are Charged
Particles!
• In 1929 a Russian scientists, D.
Skobelzyn, discovered ghostly tracks
made by cosmic rays in a cloud chamber.
• Also in 1929 Bothe and Kolhorster verified
that the cloud chamber tracks were curved
by the magnetic field. Thus the cosmic
radiation was charged particles.
Carl Anderson discovers antimatter
• Milliken became President of
Caltech and was instrumental
in the building of a high
magnetic field cloud chamber.
• Carl Anderson and Milliken
made numerous photographs
of both positive and negative
particles tracks.
• 1933 While watching the
tracks of cosmic rays passing
through his cloud chamber,
Carl Anderson discovered
antimatter in the form of the
anti-electron, later called the
positron.
“Who ordered that?” I. Rabi
1937
• Seth Neddermeyer and Carl
Anderson discovered the
elementary subatomic particle
called the muon in cosmic
rays.
• The positron and the muon
were the first of series of
subatomic particles discovered
using cosmic rays—discovered
using cosmic rays, discoveries
that gave birth to the science
of elementary particles
physics.
• Particle physicists used cosmic
rays for their research until the
advent of particle accelerators
in the 1950's.
Carl Anderson at LBNL
1937
Extensive air showers
1938
•
•
Pierre Auger, who had positioned
particle detectors high in the Alps,
noticed that two detectors located
many meters apart both signaled
the arrival of particles at exactly
the same time. Auger had
discovered "extensive air
showers," showers of secondary
subatomic particles caused by the
collision of primary high-energy
particles with air molecules.
On the basis of his
measurements, Auger concluded
that he had observed showers
with energies of 1015 eV—ten
million times higher than any
known before.
•Movie
An Extensive Air Shower
• Cosmic rays enter the
earth’s upper
atmosphere and
interact with nuclei.
• Secondary particles
result that also
interact.
• The shower grows
with time.
• Certain particles
never reach the
surface.
• Some particles, such
as muons, do reach
the surface and can
be detected.
• It is these that we
wish to detect.
composition of primaries
• 90% protons (not anti-protons)
• The remainder mostly follow
solar system abundances (eg
meteorites and solar
photosphere)
– Spallation of O and C nuclei,
for example, create more Li,
Be, B than is typical of solar
system
cosmic rays
solar system
Cosmic rays at earth’s surface
(secondaries)
• Primaries interact at z15 km, producing a shower
of (mostly) short-lived particles.
– e.g. pion () lifetime is 2.610-8 s
• The long-lived secondaries are:
– e, photons: mostly absorbed
– neutrinos (): practically invisible
– muons (): =lifetime is 2.210-6 s
• Without time dilation, muons would travel
dc=660 m, with a survival fraction e-0.66/15 10-10
• Instead, for a 10 GeV muon, =10/0,1=100, then
mean distance is 66 km. (OK)
• Detectable (vertical) flux is 1/cm2/min
• Simulated event: start...end
• Movie
• Sim tool
Present Cosmic Ray Studies
• Cosmic Ray studies continue in spite of
the development of high energy particle
accelerators.
• The energy of the highest energy cosmic
rays still cannot be duplicated in
accelerators.
• The field is still very active as indicated by
the presentation of over 300 papers at the
most recent international conference on
cosmic rays.
What’s been learned from the research
What are cosmic rays?
• Primaries are
particles with
energies from 109 eV
to 1021 eV.
• An eV is a unit of
energy. A 40 W
reading light uses
about 1034 eV of
energy in one hour.
(from James Pinfoli,
Pinfold@phys.ualberta.ca)
Cosmic rays within the
range of 1012 eV to 1015
eV have been determined
to be:
50% protons
25% alpha particles
13% C, N, and O nuclei
<1% electrons
<0.1% gammas
The Energy Spectrum
• Existing models for the
production of cosmic
rays only work to 1015
eV.
• CR in excess of 1019 eV
are believed to come
from sources relatively
close to our Galaxy, but
the sources are
unknown.
– The highest
energies!
– (www.phys.washington.edu)
The Oh My God Particle
• In 1991 at the Fly’s Eye
CR observatory in Utah
a primary particle of 3 x
1020 eV was recorded.
This is the equivalent of
51 joules
• At present particle
accelerators can reach
energies of 1012 eV.
• The Fly Eye
•
(from www.physics.adelaide.edu)
The AGASMA EVENT
In Japan, in 1993, the worlds largest array
recorded a large air shower believed to be
the result of a primary particle measured
at 1021 eV. These particles have energies
six times higher than present theories
allow.
The mystery is, of course, what is the source
of the high energy particles including these
ultrahigh energy particles.
Where do they come from?
• Low energy rays (less than 10 GeV) come
from the sun.
• Supernovae may be the source of
particles up to 1015 eV.
• The sources for ultrahigh cosmic rays are
probably, active galactic nuclei and
gamma ray bursts.
(www.phys.washington.edu)
Supernovas
1949
• Enrico Fermi put forth an explanation
for the acceleration of cosmic rays. In
Fermi's cosmic ray "shock"
accelerator, protons speed up by
bouncing off moving magnetic clouds
in space. Exploding stars
(supernovae) are believed to act as
such cosmic accelerators, but they
alone cannot account for the highest
energy cosmic rays.
• Nuclei receive energy from the shock
wave of the supernova explosion.
• The energy spectrum indicates that
most of the supernova particles have
less than 1015 eV
• (image from:
www.drjoshuadavidstone.com/
astro/supernova.jpg
SN1006 = Crab Nebula
• The Crab Nebula in visible
light…
…and in cosmic rays
(radiation from electrons in
the supernova remnant),
showing the shell of the
supernova remnant still
expanding into space
How do we know SNs make
galactic cosmic rays?
• One clue: the abundances of different nuclei in galactic cosmic rays
(GCR) is almost the same as the abundances in a mature star like
the Sun
•
Differences between solar and GCR abundances in the graph above are almost
perfectly explained by nuclei fragmenting as they travel through interstellar space
and strike occasional bits of matter
•
The average GCR spends several million years wandering around our Galaxy before
reaching Earth (we deduce this from abundances of radioactive elements)
The ultra high particles?
• Without going into great detail the problem with
the source of the UHECR is that to achieve the
high energies they must originate in a very large
extragalactic field or from a process that doesn’t
require such distance.
• Suggestions abound but there is not a
agreement as to the origin. Maybe there isn’t a
single source.
• One suggestions is that UHE CR’s originate
from the decay of more primary particles
resulting from the big bang.
What’s been learned from the research
Summary -Energy Density of CR
• Lower energy, < 1016
eV:
– Direct observation
possible, 85% are
protons.
– Most likely source are
supernova shock wave
acceleration.
– These are particles
below the knee in the
energy spectrum.
• Ultra High energy, >
1016 eV.
– Only indirect EAR
shower information is
available.
– Source of the particles
with > 1016 eV is
unknown.
High School Based Detectors
Numerous detector arrays using high
schools as sites for individual detectors
have been built or are in the process of
development.
The projects range from arrays using
hundreds of detectors covering thousands
of km2 to small arrays involving only a few
detectors in an area only a few hundred
meters square.
Why put cosmic ray detectors in
schools?
• Important open questions about extremely high energy (UHE)
cosmic rays:
– Where do cosmic rays with E>1020 eV come from?
– How can they be produced and accelerated?
– How can they reach us through intergalactic space?
• UHE-CR research requires simple detectors, spread over a large
area, with accurate time synchronization
• High cost for conventional physics experiment: new equipment,
land use, data networks, and site support
– Example: Auger Project: > US$ 108
• Solution: Physicists provide surplus HEP equipment, schools
provide sites and Internet port
School-network approach
• Pioneered by U. of Alberta in Canada (ALTA) and U. of
Nebraska in USA (CROP)
• University joins secondary schools to build a very large
detector array at low cost, using existing resources in
the community, and surplus equipment
• Use schools’ existing Internet access to link the sites
• Students and teachers participate in forefront research
 More than a one-time field trip or term paper
 Doing, not watching
 Research is ongoing, in the school every day
 Students help monitor detectors and analyze data
 Long-term relationship between school and
University
CHICOS (California high school cosmic ray
observatory)
• Operated by Caltech,
CHICOS is an active
research array with a
goal to study CR is the
range of 1018 to 1021 eV
using refurbished
detectors from a neutrino
experiment and 1 m2
scintillators
• Currently 51 sites are
setup and working.
• Image from
www.chicos.caltech.edu
ALTA (University of Alberta Large Time
Coincidence Array)
• The stated purpose of the
ALTA project is to search
for time correlations
between EAS’s.
• At present 16 high
schools are involved.
• The project is part of the
Canadian learning
standards with students
receiving credit.
•
(image from www.physics.ubs.ca)
ALTA MAP
ALTA DETECTOR MAP
Hinton
Size of planned
Auger detector
Fort McMurray
EDMONTON
CROP (Cosmic Ray Observatory Project,
University of Nebraska)
• A project to study EAS from
particles > 1018 eV.
• Thirty operating schools
covering 75000 sq miles is the
goal of the project.
• Detectors are 1 m2
scintillators donated by the
Chicago Air Shower Array.
• Image from Marion High
School.
Http://marian.creighton.edu
SALTA (Snowmass Area Largescale Time-coincidence Array)
• A project to set up detectors
in Colorado.
• Linking high schools via
Internet connecting to form a
large array.
• A modern hot-air balloon
flight in 2001 reenacted
Hess’s 1912 flight. Image
from:
http://faculty.washington.edu/
~wilkes
WALTA (Washington Large Area Time Array)
• A project of the
University of
Washington.
• As of late 2006
eighteen high schools
around Seattle are
participating. See
image. (from
www.phys.washington.edu )
The Pitt/UMSL Projects
A project of the
University of Pitt and
University of Mo at St.
Louis.
The project involves
high school teachers
building and using
scintillator type
detectors aimed at
muon detection.
The QuarkNet Detector
Cosmic Ray E-lab
Example
Student
Poster
Example
Student
Plot