The 73th Compton Lecture Series
99 Years of Discovery: What is our current picture of
Cosmic Rays?
Lecture 2: How can we see Cosmic Rays?
Nahee Park
April 9, 2011
In our previous lecture, we talked about the definition of cosmic rays and about
their properties. Also we talked about how comic rays were discovered. Below is
the summary of previous lecture and short introduction of this lecture.
Cosmic rays are often defined as charged particles, originating in outer space,
that reach the Earth. Victor Hess discovered cosmic rays in 1912 through his
balloon experiment. His measurement showed that the ionization of the air
strongly increased with the altitude. This result made him convinced that the only
explanation was the existence of ‘a radiation of very high penetrating power enters the
atmosphere from above’, which marked the discovery of cosmic rays.
Cosmic rays cover wide range of energy from about 10 8 eV 1 up to 10 21 eV. Fluxes
of cosmic rays are strongly correlated to their energies. There is far more low
energy cosmic rays (~1 particle per m 2 -second for ~ 10 11 eV) compared to highenergy cosmic rays (~1 particle per km 2 -year for ~10 18 eV). We also talked about
characteristic kinks in the fluxes, composition and directional information of
cosmic rays. (Please find detailed information from previous handout.)
In this lecture, we will talk about interaction of cosmic rays in the matter. (This
part has been discussed during the previous Compton lectures. For this lecture, I
will briefly summarize the processes that are important for charged particles.)
After reviewing the interactions, we will follow the trip of cosmic ray particles
from the moment when it comes into the Earth until it disappears. Interactions of
cosmic rays in the Earth will give us an idea about the best method for hunting the
cosmic rays of interest. At last, I will introduce the basic design of earlier detectors
that were built to act like our eyes to see cosmic rays. I will also roughly compare
the earlier detector to the current detector system.
Electron volt (eV) is the unit of energy favored in Cosmic Rays. By definition, one eV is
the energy that an electron (more generally, a singly charged particle) gets, if it is
accelerated in a potential difference of on volt.) One electron volt is equal to
1.60217653(14)×10−19 J.
1
1. Interactions of charged particles with matter
When a particle goes through matter - say, when cosmic rays go through
atmosphere, there will be several interactions between cosmic ray particles and
atoms or molecules inside the atmosphere. Details of the interaction will vary
based on the species, energies and status of the particles. Below is the list of
processes that will occur when charged particles go through matter in general.
(Details of interactions related to photon will be discussed in later lecture.)
* Coulomb scattering
Coulomb force is the force between charged particles. Due to the Coulomb force
between injected charged particle and nucleus (or electron), the direction of
injected charged particle can be changed when it moves through the matter.
Rutherford’s alpha particle scattering experiment 2 showed the result of Coulomb
scattering. (Sometimes this scattering is called as Rutherford’s scattering.) Amount
of deflection angle depends on the atomic number (Z) of injected charged particle,
atomic number of atoms in target material and the energy of injected charged
particle.
* Ionization loss
Charged particle moving through matter loses its energy on excitation 3 and
ionization4 of atoms of matter. This is the main process of energy loss for low
energy charged particle. Generally low energy particle losses its energy more by
this process. The amount of energy loss of charged particle depends on atomic
number of injected charged particle and atomic number of target material.
* Cherenkov light
Cherenkov light is emitted when a charged particle moves with a velocity faster
than the velocity of light in the matter. (Velocity of light in the matter is slower
than that of in vacuum. For example, the speed of light in water is only 75% of
velocity of light in vacuum (0.75c).) The amount of energy loss of charged particle
depends on speed and atomic number of injected charged particle.
Rutherford found most of alpha particles penetrated through the thin gold foil
when he shot alpha particles on it. Also he found small amount of alpha particle
that returned back. Based on this result, he developed his own physical model of
subatomic structure.
3
Excited state of a system is where the system has a higher energy than the
minimum energy state (ground state). Excitation is an elevation in energy level of
the system - from ground state to higher level or certain level to higher level.
4
Ionization is a process that converts an atom or molecule into an ion by adding or
removing charged particles.
2
* Bremsstrahlung
Bremsstrahlung is the radiation associated with the deceleration of electrons in
the electromagnetic fields of nuclei of atoms. Energy loss due to Bremsstrahlung
radiation depends on the atomic number and charge number of target material.
There are processes with certain conditions or for particular particles.
* Synchrotron radiation: When there is magnetic field, charged particle losses its
energy by emitting synchrotron radiation.
* Transition radiation: When relativistic charged particle crosses the interface of
two media with different electrical properties, it losses its energy by emitting
transition radiation.
* Inverse Compton scattering: When an electron interacts with a photon from an
ambient photon, it loses energy and boosts the photon.
* Annihilation: When particle meets its respective anti-particle, it will be
disintegrated - generally producing two photons.
* Decays: Process refers either the transformation of an elementary particle into
other elementary particle (particle decay) or change of unstable atom into other
atom by emitting ionizing particles (radioactive decay).
2. Strategy for cosmic ray hunting
Low energy charged particle will lose its energy quickly by ionization loss while
its direction will be changed quite a lot by multiple numbers of scatterings. If
energy of charged particle becomes higher, then it will lose its energy through
chains of interactions.
High-energy electron will lose its energy by generating bremsstrahlung
radiation. If the energy of bremsstrahlung radiation is high enough, it can create
electron-position pair. To distinguish this second generation of electron from the
original electron, particles created from interactions are often called as secondary
particles. If energy of secondary electron is small, then it will quickly lose its
energy like other low energy charged particle through ionization loss. But, if the
energy of secondary electron (and position) is high enough, then it will lose
energy by bremsstrahlung process that may be followed by creation of another
electron-positron pair. This continuous chain of interaction will create secondary
particles until there is not enough energy to create new particles. Then all
secondary particles will eventually lose their energy by ionization loss. This chain
of reaction is named as electromagnetic cascade.
High-energy charged particle larger and heavier than electron, such as proton or
nuclei will make a more or less direct hit on the nucleus of matter rather than
going through the bremsstrahlung process. As result of this violent collision,
various kinds of secondary particles will be created. Some of these secondary
particles are still energetic enough to continue the chain of interaction.
Propagation will continue until all secondary particles lose their energy by
different interactions. But, details of the processes are quite different from
electromagnetic cascade. There are different kinds of secondary particles involved
in the processes that are much heavier than electron or positron. So processes for
the secondary particles to lose their energy are quite different from
electromagnetic cascade. This chain of interaction is called as hadronic cascade.
When cosmic rays go through the atmosphere, these cascades will continue until
there are not enough energy make new particles. Most cosmic rays are energetic
enough to create these cascades in the atmosphere. But, because the atmosphere
surrounding the Earth is thick, practically not a single particle of original primary
cosmic rays arrives at sea level. Already at altitudes of 15~20 km primary cosmic
rays interact with atomic nuclei of the air and initiate - depending on energy and
particle species - electromagnetic and/or hadronic cascades.
So here goes the strategy part. If you are interested in the detailed information of
cosmic rays such as composition, then you should go to top of the atmosphere to
minimize the material above you. But, then you will be limited by realistic size of
detector that can stay at high altitude. If you want to have as much as data, then it
would be better to stay at ground where you can build as large detector as your
budget allows you. Then you will need to think how to evaluate the information of
original particle by looking at the remaining secondary particles.
3. Examples of cosmic ray detector
Up to now, we were talking about different kind of interactions cosmic rays
would go through when they meet matter. But, most of them - may be except for
Cherenkov radiation - are invisible to human eyes. So still we need to figure out
how to see - or rather how to measure the cosmic rays. But, cosmic rays are not the
only thing that cannot be seen by human eyes. There are properties that we are
quite familiar to ‘see’ even if they are invisible. For example, we know how to
measure the temperature even if human eyes cannot see the heat. But, we can read
the temperature by looking at the change in temperature probe. Even if we cannot
see the direction and strength of wind, we know how to convert the properties of
the wind into numbers with universal units by using the proper instruments.
Particle detector is the instrument designed to be sensitive to the particle including the cosmic rays. Detector itself has been improved as our knowledge on
the interaction of particles in the matter has been improved. Here are few
examples of early detectors.
Electroscope was the first instrument, which Hess used to measure the ionization
rate of the air. It was also the first electrical measuring instrument. It was used to
detect the presence and magnitude of electric charge on a body. It consists of
vertical metal rods that hang two parallel strips of thin flexible gold leaf at the
end. To protect the golden leaves from moving by air, they are enclosed in a glass
bottle. When a charged object approaches metal terminal, the golden leaves spread
apart making a ‘V’ shape. Charges of the opposite polarity to the charged object
are attracted to the terminal and charges with the same polarity will be attracted
to leaves. Because the same polarity charge is attracted to two leaves, they will be
spread. If the electroscope terminal is grounded while the charged object is
nearby, the same polarity charges in the leaves drain away to ground leaving the
leaves to be closed. When the charged object is moving away, the charge at the
terminal spreads into the leaves, causing them to spread apart again. After this
procedure, the electroscope will contain this charge. When there is ionizing
particle passing through them, leakage current associate with ionization will
discharge the golden leaves. By looking at the closing rate of leaves, people could
measure the rate of ionization.
Cloud chamber was invented by Charles Thomson Rees Wilson in 1911. When
ionizing particles such as alpha 5 or beta particle 6 interacts with the sealed
environment containing a supersaturated vapor of water of alcohol, they will
ionize the mixture. Around the resulting ions, a mist will form due to
supersaturated environment in the chamber. Tracks formed by the mist show
distinctive shapes depend on the ionizing particle. With the help of magnetic field
around cloud chamber, one can clearly see the difference between positive
charged particle and negative charged particle. Also the bending trajectory will
show the difference depends on the mass of particle. Cloud chamber was used for
the discovery of positron7, muons, kaons and lambda baryons. Wilson received
Nobel Prize on 1927 with Arthur Compton for his work on the cloud chamber.
With cloud chamber, one can tell the differences between ionizing particles. As the
technique was used for more detailed study, particle decay, annihilation, electronpositron pair effect were observed and used for discovery of new particles.
Photographic emulsions are developed by Cecil F. Powell who won the Nobel
Prize in 1950 for his work on this method. Powell developed special nuclear
emulsion that was sufficiently sensitive to register the tracks of protons, electrons
and all other classes of charged particle. By stacking layers of emulsion that can be
separated and developed, observer can get a three-dimensional picture of the
interactions taking place in the emulsion. Pions and their decay have been
discovered by using this method.
Bubble chamber is similar to the cloud chamber. It is made by filling a large
cylinder with a liquid heated to just below its boiling point. When particle enters
the chamber, a piston decreases its pressure making the liquid enter into a super
heated phases. As charged particle create an ionization track, microscopic bubbles
will be formed. Bubbles grow in size as the chamber expands. So one can change
the size of bubble to see or to photograph in certain degree. Like cloud chamber,
Helium nucleus
Electron
7
Anti-matter of electron. It has same mass with electron, but with opposite charge.
5
6
bubble chamber can be put inside the magnetic field. By attaching camera around
the chamber, it can generate three-dimensional image. First signature of xi baryon
was reported by bubble chamber.
Modern detectors are more complicated then earlier detectors. Usually detector
has a lot of individual channels reading out the electrical change of detection
material from the ionization losses. Due to relatively smaller size signals,
techniques like pre-amplifiers, noise filters are added in the electronics. As a
result, more precise measurement became possible. To control over thousands
channels of electronics, digital logic has been implemented for easier handling of
the system. Not only the readout and controls, but also detector design itself has
been improved. Now the tracking system can separate two particles apart by ~ 10-6
meter, which will be hard to be distinguished in human eyes. Also likely there are
more sub-detectors to compose the whole detector system to measure the
information of particles. Usually sub-detectors are using different interaction
processes to get the signals from cosmic rays. So combining all the information
from the sub-detectors gives complementary information reducing the potential
systematic errors. We can generate much higher magnetic field, which allow us to
measure the bending of charged particles with higher energies. With the
improvement in computational power and better understanding on the interaction
models, detector response can be studied by using the simulation model. Now we
are living in the era where one can see the traces of cosmic rays much more
accurately, and can design the detector in detail based on the estimated response
from particles of interest.
Next week
Lecture #3: What can Cosmic Rays tell us about Universe?
Next lecture, we will talk about the connection between the information we got
from cosmic rays to our knowledge of the universe. To understand possible bias in
the cosmic rays measured in the Earth, we will also think about our environmental
conditions.
Useful tips
Energy unit (eV, electron volt)’s different measures and their name
Unit
Value
Unit
Value
keV (killo-)
103 eV
MeV (mega-)
106 eV
GeV (giga-)
109 eV
TeV (tera-)
1012 eV
PeV (peta-)
1015 eV
EeV (eta-)
1018 eV