Cosmic Rays

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Cosmic Rays
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History
1785 Charles Coulomb, 1900 Elster and Geitel
Charged body in air becomes discharged
there are ions in the atmosphere
1902 Rutherford, McLennan, Burton air
is traversed by extremely penetrating radiation
(g rays excluded later)
1912 Victor Hess Discovery of Cosmic
Radiation in 5350m balloon flight, 1936 Nobel
Prize
1933 Anderson Discovery of the positron
in CRs shared 1936 Nobel Prize with Hess
1933 Sir Arthur Compton Radiation
intensity depends on magnetic latitude
1937 Street and Stevenson Discovery of
the muon in CRs (207 times heavier than electron)
1938 Pierre Auger and Roland Maze Rays
in detectors separated by 20 m (later 200m)
arrive simultaneously
1985 Sekido and Elliot
very energetic ions impinging on top of
atmosphere
First correct explanation
Open question today where do they come from ?
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Victor Hess, return from hisdecisive flight
1912 (reached 5350 m !)radiation increase gt 2500m
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Satellite observations of primaries
Primaries energetic ions of all stable isotopes
85 protons, 12 a particles
Similar to solar elemental abundance distribution
but differences due to spallation during
travel through space (smoothed pattern)
Li, Be, or B
Cosmic Ray p or a
C,N, or O(He in early universe)
Major source of 6Li, 9Be, 10B in the Universe
(some 7Li, 11B)
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NSCL Experiment for Li, Be, and B production by
aa collisions
Mercer et al. PRC 63 (2000) 065805
170-600 MeV
Identify and count Li,Be,B particles
Measure cross section how many nuclei are made
per incident a particle
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Reminder Physics Particle Zoo
Leptons
electron muon tauon
1 e m t
-1 e- m- t-
Quarks
Q
2/3 up charm top
-1/3 down strange bottom
Baryons 3 quarks
Meson quark antiquark
Force carriers (bosons) Strong g (8), Weak
Z0,W,W-, Electromagnetic g
Ordinary matter
proton uudneutron ddu
Nuclei combinations of protons, neutrons, and
electrons
26 ns lifetime decay into m and nm
p ud
Pions
26 ns lifetime decay into m- and nm
p- du
p0 uu dd
1e-17 s lifetime decay into gg
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Ground based observations
Space
Cosmic Ray (Ion, for example proton)
Atmospheric Nucleus
Earths atmosphere
(about 50 secondaries after first collision)
p
p-
po
po
g
g
p
p-
e
e-
m (4 GeV, 150/s/cm2)
nm
e-
g
Electromagnetic Shower
Plus someNeutrons14C (1965 Libby)
Hadronic Shower
(on earth mainly muons and neutrinos)
(mainly g-rays)
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Cosmic ray muons on earth
Lifetime 2.2 ms then decay into electron and
neutrino
Travel time from production in atmosphere (15
km) 50 mswhy do we see them ?
Average energy 4 GeV (remember 1 eV 1.6e-19
J)
Typical intensity 150 per square meter and second
Modulation of intensity with sun activity and
atmosphericpressure 0.1
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Ground based observations
Advantage Can build larger detectors
can therefore see rarer cosmic rays
Disadvantage Difficult to learn about primary
Observation methods
1) Particle detectors on earth surface Large
area arrays to detect all particles in shower
2) Use Air as detector (Nitrogen fluorescence ?
UV light)
Observe fluorescence with telescopes
Particles detectable across 6 kmIntensity drops
by factor of 10 500m away from core
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Particle detector arrays
Largest so far AGASA (Japan) 111
scintillation detectors over 100 km2
Other example Casa Mia, Utah
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Air Scintillation detector
1981 1992 Flys Eye, Utah1999 -
HiRes, same site

• 2 detector systems for stereo view
• 42 and 22 mirrors a 2m diameter
• each mirror reflects light into 256
  photomultipliers
• sees showers up to 20-30 km height

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Flys eye
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Flys Eye
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Flys eye principle
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Pierre Auger Project
Combination of both techniques Site Argentina
?. Construction started, 18 nations
involvedLargest detector ever 3000 km2, 1600
detectors
40 out of 1600 particle detectors setup (30
run)2 out of 26 fluorescence telescopes run
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Other planned next generation observatories
Idea observe fluorescence from space to use
larger detector volume
OWL (NASA)(Orbiting Wide Angle Light Collectors)
EUSO (ESA for ISS)(Extreme Universe Space
Observatory)
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Energies of primary cosmic rays
E-2.7
Observable bysatellite
E-3.0
E-3.3
E-2.7
Lower energiesdo not reach earth(but might get
collected)
UHECRs
40 events gt 4e19 eV7 events gt 1e20 eVRecord
October 15, 1991Flys Eye 3e20 eV
Man made accelerators
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Origin of cosmic rays with E lt 1018 eV
Direction cannot be determined because of
deflection in galactic magnetic field
M83 spiral galaxy
Galactic magnetic field
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Precollapse structure of massive star
Iron core collapses and triggers supernova
explosion
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Supernova 1987A by Hubble Space Telescope Jan 1997
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Supernova 1987A seen by Chandra X-ray
observatory, 2000
Shock wave hits inner ring of material and
creates intense X-ray radiation
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Cosmic ray acceleration in supernova shockfronts
No direct evidence but model works up to 1018 eV

• acceleration up to 1015 eV in one explosion,
  1018eV multiple remnants
• correct spectral index, knee can be explained by
  leakage of light particles out of Galaxy (but
  hint of index discrepancy for H,He ???)
• some evidence that acceleration takes place from
  radio and X-ray observations
• explains galactic origin that is observed (less
  cosmic rays in SMC)

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Ultra high energy cosmic rays (UHECR) E gt 5 x
1019 eV
Record event 3 x 1020 eV 1991 with Flys
eyeAbout 14 events with E gt 1020 knownSpectrum
seems to continue limited by event rate, no
energy cutoff Good news sufficiently energetic
so that source direction can be reconstructed
(true ?)
Isotropic, not correlated with mass of galaxy or
local super cluster
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The Mystery

• Isotropy implies UHECRs come from very far
  away
• But UHECRs cannot come from far away because
  collisions with the cosmic microwave
  background radiation would slow down or destroy
  them (most should come from closer than 20 MPc
  or so otherwise cutoff at 1020 eV
• Other problem we dont know of any place in the
  cosmos that could accelerate particles to such
  energies (means no working model)

Speculations include

• Colliding Galaxies
• Rapidly spinning giant black holes
• Highly magnetized, spinning neutron stars
• New, unknown particles that do not interact with
  cosmic microwave background
• Related to gamma ray bursts ?

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Possible Solutions to the Puzzle
1. Maybe the non-observation of the GZK cutoff is
an artefact ?
AGASA Data
HIRES Data
cutoff seen ?
problem with systematic errors in energy
determination ?
2. Maybe intergalactic magnetic fields as high as
micro Gauss
then even UHECR from nearby galaxies would appear
isotropic