Transcript Document

High energy Astrophysics
Mat Page
Mullard Space Science Lab, UCL
12. Cosmic rays
Slide 2
12. Cosmic rays
• This lecture:
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What are they?
Discovery of cosmic rays
Interaction with the atmosphere
Interaction with the interstellar medium
Origin of the cosmic rays
Slide 3
What are cosmic rays?
• Despite the name, cosmic rays are not
electromagnetic radiation.
• They are high energy particles with an
extra-terrestrial origin.
• They are protons, atomic nuclei, electrons
and the anti-particle equivalents because
these are all stable particles.
Slide 4
Discovery
• Story begins at the beginning of the 20th Century.
• Rontgen had discovered X-rays (1895)
• Radioactivity (a,b,g –rays) had been discovered
by Becquerel (1896)
• About 1900 it was discovered that electroscopes
discharged over time even if they were kept in the
dark away from radioactive materials.
• Rutherford determined that most of the ionisation
was due to natural radioactivity in rocks or
contaminated equipment.
Slide 5
• But in 1910 Wulf took an electroscope to the top of
the Eiffel Tower
– Ionisation should have decreased by factor of 16 if the
terrestrial radioactivity was the cause – in fact ionisation
decreased by only a factor of 2.
• 1912 - 1914 Hess and Kolhorster flew balloons to
altitudes up to 9 km.
– Above 1.5 km the level of ionization increased with respect
to that observed on the ground
• ‘Cosmic radiation coming from space’
• 1929 Bothe and Kolhorster used a pair of GeigerMuller detectors with different sized slabs of gold
and lead in between to determine that the cosmic
rays were not g-rays but corpuscular with energies
109-1010eV.
Slide 6
Victor Hess after
one of his first
successful balloon
flights.
Slide 7
Interaction with the atmosphere
• Most of the ‘cosmic rays’ observed at the
earth’s surface are secondary events from
interactions between primary cosmic rays
and the Earth’s atmosphere.
• When high energy particles interact with
material they do 3 things:
– Ionize atoms
– Destroy crystal structures and molecular chains
– Interact with the nuclei of atoms
Slide 8
• Ionization is similar to what happens in a
hot gas – the cosmic ray interacts with an
atom or ion and as a result an electron is
ejected.
– If the cosmic ray is a proton or more massive,
with an energy of MeV or greater, it will produce
many ionizations before being stopped.
– If it is an electron it will lose energy much more
rapidly because it will transfer a larger
proportion of its momentum in each interaction.
Slide 9
Breaking of crystal structures and molecular bonds
• Chemical bonds can be broken if the electrons are
given energy.
– Plastics and other polymers make good particle
detectors – tracks corresponding to the paths of
cosmic rays form, the lengths of the tracks depend
on the mass of the cosmic rays.
• Cosmic rays can affect molecules in our body and
cause us to mutate!
• Tracks are formed in meteorites.
– We can use meteorites of different ages to work out
the history of cosmic rays over the lifetime of the
solar system.
– We can use cosmic ray tracks to work out the ages
of meteorites
Slide 10
Nuclear reactions
• Cosmic ray kinetic energies exceed the rest
mass of a proton (~ 1 GeV)
– Cosmic rays can interact with atomic nuclei,
basically by smashing individual nucleons to
pieces.
– Results in a shower of pions, which decay to grays and muons and end up as electrons and
positrons, neutrinos and electromagnetic
radiation.
– Particle physics was based on cosmic ray
interactions before particle accelerators.
Slide 11
• Decay products may also have enough
energy to interact with further nucleons in
the nucleus.
• Fragments of the nucleus are ejected. The
nucleus and fragments will interact with
other nuclei or decay if they are not stable.
• Elemental abundances are actually
changed by cosmic rays.
• Ejected neutrons are absorbed by 14N to
make radioactive 14C – this is the basis
of radio carbon dating.
Slide 12
Slide 13
Extensive air showers
• The highest energy cosmic rays can initiate
large air showers:
– nuclear interactions initiated by the highest
energy cosmic rays can produce 106 relativistic
particles arriving at the ground.
– These can be detected by detectors with wide
separations (eg 300 metres).
– The atmosphere is like the first stage of a
cosmic ray detector!
Slide 14
Small detectors
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Scintillation (crystal) detectors (really old…)
Proportional or Geiger counters
Cloud or bubble chambers
Nuclear emulsions
– Crystals in the emulsion are activated by
electrons
Slide 15
Larger area detectors
• Ultraviolet fluorescence imagers
– Nitrogen excited and fluoresces after
interactions with secondary particles
• Multi-mirror reflectors
– detect Cerenkov radiation in the atmosphere
• Water tanks to detect secondary particles.
– Cerenkov radiation emitted by secondary
particles in the water and detected by imagers
Slide 16
At the top of the atmosphere
• So now we can get to the astronomical
interpretation of cosmic rays…
• The spectrum as measured at the top of the
atmosphere, either by satellite experiments, or
inferred by the products produced by interactions
with the atmosphere, is approximately a broken
power law with the break at 1015 eV.
• Chemical abundances in cosmic rays similar to
solar, but with more light elements (B, Be, Li)
• Below about 1 GeV, the spectrum is strongly
affected by the Solar wind
Slide 17
Cosmic ray spectrum at top of atmosphere
Slide 18
• At > 1019 eV only expect 1 particle per
km2 per Century!
• Need vast detectors to detect these
cosmic rays.
• Basically, detectors have to cover large
areas on the ground or watch large
areas of the sky.
• Before Pierre Auger Observatory,
largest experiments were AGASSA
(Japan) and HIRES (US)
• Each covers 10s of square kilometres.
Slide 19
Pierre Auger observatory
• Largest
observatory
so far
constructed in
Argentina,
covering 3000
km2.
Slide 20
Water tanks for Auger
Slide 21
Fluorescence imagers for Auger
Interaction with interstellar medium and
microwave background
Slide 22
• Of course cosmic rays will interact with the
interstellar medium just as they interact with our
atmosphere.
– Strong g-ray Galactic background.
• Cosmic rays also interact with the photon field this results in a limit to how far cosmic rays can
travel before their energy is given up.
– Electrons lose energy faster than protons so travel
smaller distance.
– Cosmic rays with E> 1020 eV should not be able to
travel further than about 30 Mpc before losing their
energy through interactions with CMB.
Slide 23
So where are cosmic rays coming from?
• Shocks in supernovae are known to be a
good source of cosmic rays – we see them
in radio by synchrotron radiation.
– Supernovae are probably the source of most of
the cosmic rays we receive
– However, its not thought possible to give
particles > 1015 eV in a supernova shock.
• The origins of cosmic rays with > 1015 eV
are less certain.
Slide 24
More extravagant possibilities:
• Wimpzillas
– Massive weakly interacting massive particles that decay to
produce cosmic rays
– ‘dark matter particles’ in Galaxy dark matter halo
• Z bursts
– Collisions between high energy neutrinos
– Possible if there are populations of high energy neutrinos
• Topological defects
– Distortions in spacetime
– E.g. Magnetic monopoles and cosmic strings
– These distortions are not found in microwave background
Slide 25
Less extravagant possibilities:
• Massive black holes
– We know massive black holes accelerate particles to
high Lorentz factors
• AGN radio lobes
– Shock fronts in lobes may make cosmic rays
• Magnetars (highly magnetised neutron stars)
– If magnetars spin rapidly enough they can do the
business
– Could be many of these sources in star-forming
galaxies
• The directions of cosmic ray sources should
correspond to these sources – we should try to
resolve the cosmic ray background.
Slide 26
• Or g-ray bursts
– We know they accelerate particles to really high
energy
– Large distances, spectrum should be cut off at
5x1019 eV.
• Results 2007 from Auger Observatory:
Correlation between high energy cosmic ray
directions and nearby AGN!
• But… without any fanfare or press release, the Auger
team now say that the significance has gone down
with more data rather than up, i.e. this result is not
yet verified (and may be wrong!).
Slide 27
Some key points:
• Cosmic rays are protons, electrons and atomic
nuclei with high energy.
• They interact with the atmosphere, by ionising it
and by nuclear interactions with atomic nuclei,
revealing subatomic particles.
• They also interact with the interstellar medium.
• The highest energy cosmic rays cannot come
from outside the local Supercluster.
• Cosmic rays up to 1015 eV are predominantly from
supernovae.
• AGN might be in the frame as responsible for
some or all of the the highest energy cosmic rays.