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Cosmic rays
What is a Cosmic Ray?
• Cosmic ray is not a ray, but a particle.
• Most cosmic rays are ionised atoms, ranging from the proton up to the iron
nucleus and even beyond to heavier nuclei.
• Cosmic rays originate from space,
Being produced by different sources,
• the Sun, other stars, neutron stars and black holes, as well as active galactic
nuclei and radio galaxies.
• Most cosmic ray particles are travelling very close to the speed of light. The
most energetic CR particle ever observed had an energy of about 150 Joules.
Kinds of cosmic rays
1. Galactic cosmic rays (GCR)
• Originate far outside of our solar system.
• They are the most energetic CR particles with the energy extending up to 1021 eV.
Composition is mostly protons with ≈ 7 − 10% of He and ≈ 1%of heavier elements.
• The source of the very energetic GCR is not exactly known.
• The GCR flux in the solar system is modulated by solar activity
2. Solar cosmic rays (SCR), also called solar energetic particles.
•
•
•
•
Originate mostly from solar flares.
SCR particles have energies typically up to several hundred MeV/nucleon,
SCR composition is roughly similar to GCR: mostly protons, ≈ 10% of He, < 1% of
heavier elements.
During strong solar flares, the flux of CR at the Earth can increase by a few hundred
percent for hours/days because of the increase of SCR. This is called a Solar Particle
Event.
3. Anomalous cosmic rays (ACR)
• Originate from the interstellar space beyond the heliopause.
• The composition of ACR is quite different from GCR and SCR,
• including, e.g., more helium than protons, and much more oxygen than carbon.
(Heliopause is the region where the Sun's influence wanes and the beginning of interstellar
space can be sensed)
Galactic Cosmic Rays
1.
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•
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Composition
About 90% of the cosmic ray nuclei are hydrogen nuclei (protons),
Helium nuclei (α-particles).
All other elements make up only about 1% and very rare elements and isotopes.
These species require large detectors in order to collect enough particles to
observe their “fingerprint”.
• All galactic cosmic ray particles are fully ionised, i.e., consist of nuclei only
2. Energy spectrum
• The energy spectrum of GCR is based on measurements from different
instruments covering the energy range from ≤ 109 to ≥ 1020 eV.
• The highest energy of a CR particle detected so far was 1021 eV.
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•
•
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This figure shows the CR flux at different energies.
(Number of cosmic ray particles passing through a unit area surface in a unit time from a unit space angle per energy unit)
particles per cm2 s sr GeV .
The spectrum of GCR as measured in the vicinity of the Earth can be divided into the following parts
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•
•
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Particles with energy below about 20-50 GeV are subject to solar modulation.
Within the range 1010 − 1015 eV,
Around 1015 eV, the spectrum changes and becomes steeper.
Within the range of 1015 − 1020 eV,
The spectrum becomes flatter (harder) again at around 10 20 eV.
Origin of Cosmic Rays
1.
•
•
•
Ultra-High Energy Cosmic Rays
2.
High Energy Cosmic Rays
The origin of the very (ultra) high energy cosmic rays above the knee is still a mystery.
Because of low statistics at such high energies it is hard
Cosmic ray particles with energies greater than 1019 eV hit the Earth at a rate of one particle
per square kilometre per century
The particles below the ankle are produced in our Galaxy.
3.
Lifetime of GCR in the Galaxy
• Because of the magnetic field, cosmic rays are trapped in it for a long time.
• During their travel inside the Galaxy (they spend most of the time in the halo), they can gain
more energy or lose energy, and they can collide with other particles.
How long are the cosmic rays trapped in the Galaxy?
• Collisions of cosmic ray nuclei with the interstellar matter (or with each other) can
produce lighter nuclear fragments, including radioactive isotopes, which has a halflife
time
of
1.6
million
years.
CR particles are lost in collisions.
• One can estimate the distance traversed by the particle before absorption to be of
the order of 1024 cm, which corresponds to the time of about 1014 sec (or several
millions of years) for relativistic particles. The corresponding time for the Halo,
where the density is 10−26 g/cm2 , is of the order of 108 years.
THE EFFECTS OF MAGNETIC FIELDS
• The most energetic cosmic rays are dangerous because they are ionising radiation.
• On Earth we have two very effective lines of defence: the Earth’s magnetic field
and its atmosphere.
• The charged particles in cosmic rays are deflected by the magnetic field and many are
prevented from hitting the atmosphere directly.
• Some are trapped into two concentric doughnut-like bands around the Earth called
the Van Allen Belts.
• Cosmic rays, are deflected by the magnetic field to the North and South Poles.
• When they hit the atmosphere they interact with the upper atmosphere. Then, when
the atoms relax they give out light (neon lights).
• The lack of a magnetic field allowed cosmic rays to hit the atmosphere directly,
leading to any gas molecules being effectively
• Any particles that make it through the Earth’s magnetic field still have to face our
atmosphere. The atmosphere acts a bit like a bullet-proof vest.
•
A cosmic ray hitting the upper atmosphere gives up its energy to create a shower of
secondary particles that rain down on the Earth below.
Solar Cosmic Rays (SCR), also called Solar Energetic particles
(SEP)
•
CR detectors have occasionally seen sudden increases in CR intensity,
sometimes as large as several hundred per cent, associated with an outburst
(mostly flares) on the Sun.
• Compared to GCR’s, SCR’s have relatively low energies, generally below 1
GeV and only rarely around 10 GeV.
• That is why such events are often missed by cosmic ray detectors near the
equator where the lowest energies are excluded by the Earth’s magnetic
field.
• The best detectors for observing solar particles are therefore those at highlatitude regions which are more sensitive to the lowest CR energies.
• The flux varies greatly with solar activity.
• The CR intensity returns to normal within tens of minutes to days, as the
acceleration process ends and accelerated ions disperse throughout the
interplanetary space. These short increases of cosmic ray count rates associated
with SEP are called GLEs (Ground Level Events).
Solar Cosmic Rays (SCR), also called Solar Energetic particles
(SEP) (importance)
• The flux during SEP events is high enough to be dangerous for astronauts
and also for the crews of high-altitude airplanes over polar regions.
• SEPs are an important factor in the new concept of Space Weather which, e.g.,
tries to predict the short-term solar activity.
• SCR particles are primary cosmic rays, i.e., their characteristics (energy
spectrum, time profile of intensity, direction of arrival, pitch angle distribution,
etc.) are not very much disturbed during their propagation through the
interplanetary space.
Energy Composition of SCR
• SCR consist of three main components:
• Proton-Nucleon component & Electron-Positron component & Electromagnetic
component.
• The electromagnetic component, although is not a “cosmic ray” , is closely
connected with the other two components.
• The electron-positron component is also accelerated to relativistic energies but it
is almost absent in SCR observed at Earth because it has large energy losses.
 The energy losses can be separated into nuclear and radiative losses.
• The nuclear losses (collisions with other nuclei) depend on the amount of matter
traversed and can be neglected in a typical acceleration process of a solar flare.
• The radiative energy losses dominate the high-energy part of the electron-positron
component so that almost all accelerated electrons lose most of their energy in the
solar corona, producing the X-and γ−ray emission of typical solar flares.
• The difference in the chemical composition between SCR and GCR is mainly due to
the different amounts of matter passed by the two groups of particles
• The SCR composition depends on particle’s energy.
Energy Spectrum of SCR
• The energy spectrum of SCR decreases with particle’s energy. This is the only
similarity with the GCR energy spectrum.
• The first difference is the maximum energy. Solar protons can be accelerated up to
some 20 GeV only. This is in dramatic difference with the maximum observed GCR
energy of about 1021 eV.
• The other dramatic difference is that while the GCR flux is roughly constant and
exists permanently, SCRs appear rather rarely and very irregularly in time and the
SCR flux levels vary accordingly.
• There are two components in the SCR spectrum, the so called prompt and delayed
components with different spectral and temporal characteristics.
• The SCR spectrum is softer than that of GCR.
Solar neutrons
• Another important component in solar cosmic rays is solar neutrons. Since neutrons
are neutral, they cannot be accelerated by electric fields or on magnetic structures and
therefore are not “primary” cosmic rays.
• Solar neutrons carry unique information about the conditions at the flare site.
• They move on straight lines preserving their kinetic energy without being disturbed by
solar.
• Unfortunately we cannot detect galactic or extragalactic neutrons which could locate
the remote sources of GCR acceleration because a free neutron is unstable.
Solar flares
• Solar flares are sudden, huge explosions on the surface of the Sun.
• Flares are very fast processes, with the smallest time scales of only a few minutes.
• Usually they occur near sunspots, along the dividing line (neutral line) between the
areas of oppositely directed magnetic fields where the magnetic field structures get
twisted and sheared, releasing energy after magnetic reconnection.
• Flares are usually characterized and classified by their brightness in Xray radiation.
The biggest flares are called X-class flares.
Anomalous Cosmic Rays
• Anomalous Cosmic Rays (ACRs) are the third primary component of cosmic rays
(along with GCRs and SCRs).
• ACRs were discovered as a ”bump” in the spectra of certain elements (He, N, O, Ne)
at energies of about 10 MeV/nucleon. By now, ACRs have also been observed in H,
Ar, and C.
• ACRs arise primarily from neutral interstellar atoms which are swept into the solar
magnetic field dominated space (called the heliosphere) by the motion of the Sun
through the interstellar medium.
Detection of Cosmic Rays
• Cosmic rays are measured at very different locations:
from underground and underwater detectors up to the far edge of the Solar
system.
• CR particles can be observed by the following types of interactions:
1. Inelastic scattering caused by the Coulomb force between the CR particle and
the orbital electrons of the detector material.
2. Elastic scattering of CR particle from nuclei of the detector by the
electromagnetic or strong force.
3. Emission of Cherenkov radiation by the CR particle moving faster than light in
matter.
4. Emission of transition radiation. Transition radiation is produced when a
charged particle passes through media of different dielectric properties.
5. Nuclear reactions (inelastic scattering by the strong force) between the CR particle
and the detector nuclei.
6. Bremsstrahlung caused by the CR particle in the detector material.
Detection of Cosmic Rays
1. Space-borne detectors
• One group is located onboard satellites which have a fixed orbit
near the Earth.
• The other group of space-borne experiments is located on space
probes which explore different parts of the heliosphere.
1. This detector is actually a combination of many types of detectors.
2. The main part is a tracker which consists of 6 orthogonal silicon strip planes (< 2
mm wide, more than 34,000 channels.
3. for each plane) in a permanent magnetic field of about 2 T produced by a
superconductive magnet with He cooling.
4. The tracker is able to reconstruct the trajectory of a particle in the magnetic field
which allows to determine the particle’s rigidity, mass and the incoming direction.
Detection of Cosmic Rays
Space-borne detectors
• Additional devices are:
1. Synchrotron radiation detector (SRD) measures the synchrotron
2.
3.
4.
5.
6.
radiation and is primarily devoted to detect electrons.
Transition radiation detector (TRD) measures the transition radiation
of particles.
Time of flight (TOF) system measures the time that the particle needs
to fly through the detector. This allows to estimate the velocity of the
particle and to reject fake events.
Veto counter is a simple electronic counter used as a trigger to reject
particles whose trajectories pass through the sides of the detector.
Ring Cherenkov detector (RICH) measures the Cherenkov emission
Electromagnetic calorimeter (Ecal) also measures the energy of
particles.
Detection of Cosmic Rays
Space-borne detectors
• This combination allows to reconstruct the arrival direction, energy, charge
and mass of the CR particle.
Detection of Cosmic Rays
Balloon detectors
• Modern balloons allow to lift detectors to the altitude of 40-70
km.
• At these high altitudes, the atmosphere above the balloon is
negligible for CR, and therefore the balloon-borne detectors
can measure primary CR particles.
• The main disadvantage of balloon-borne experiments is that
they are campaign-like experiments, operating only for a short
time interval.
Detection of Cosmic Rays
Ground-based detectors
• Ground based cosmic ray experiments can be divided into
different subgroups according to the component of the
atmospheric cascade.
1. Neutron monitor
• The nucleonic component of the atmospheric cascade is measured by
neutron monitors (also called cosmic ray stations).
• Extensive Air Shower arrays
• The paraffin layer surrounding the tubes is used as a pre-moderator
decelerating atmospheric neutrons. The lead layer decelerates neutrons
further and produces still more neutrons from the atmospheric neutrons
and protons. There is also a plastic layer around the tubes as the final
moderator, making particles almost thermal
Detection of Cosmic Rays
Ground-based detectors
• Cherenkov detectors
1.
Relativistic electrons and positrons, produced in the atmospheric cascade, generate
Cherenkov emission in the visible light range when propagating through the air.
The Cherenkov array collects such light pulses from a large volume (thousand
cubic kilometers).
2.
A similar technique is also used to study neutrinos but then the Cherenkov light
pulses are produced and detected in deep water or ice.
• Underground muon experiments
• By underground experiments one can study the high-energy part of the
muon component. Such experiments use the good penetration capability of
muons in matter which allows to easily separate them from the other CR
components (except for neutrinos). The underground muon detector may
be either a single detector or a small array
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