The 73th Compton Lecture Series
99 Years of Discovery : What is our current picture of
Cosmic Rays?
Lecture 1: What is Cosmic Rays?
Nahee Park
April 2, 2011
Starting the lecture series...
Before I decided to continue my study as ph.D. student, I visited several laboratories
to decide which field in the Physics (if any) I would study more. I eventually settled
down in the astroparticle physics laboratory because people from the laboratory told
me about their sophisticated detectors designed to measure particles coming from the
universe. At that time, it sounds very interesting for me to measure the detailed
properties of something coming from outside of the Earth that you cannot see.
For 99 years from the discovery of cosmic rays, there have been continuous efforts to
find out detailed properties. Our picture of cosmic rays has been improved as new
results coming from experiments and theories. And the picture will change
continuously. Motivation of this series is to share current knowledge about cosmic rays.
I also would like to show the connection between cosmic rays to other messengers from
the universe such as radio, x-ray and gamma-ray.
1. History of Cosmic Rays
Cosmic Rays are discovered by Victor Hess in 1912.
After the discovery of radioactivity it was noticed that the air is being ionized at a
relatively high rate. Natural radioactivity of the Earth was main suspect for the reason.
Hess flew the several series of balloon to measure the change of ionization of the air to
see the change of ionization with altitude. If the hypothesis is correct, as distance from
possible natural radioactivity of the Earth increases, the ionization rate would decrease.
But, instead of decreasing, the ionization of the air strongly increases with altitude. He
believed that the only explanation of his measurement was ‘a radiation of very high
penetrating power enters the atmosphere from above’. This marked the discovery of
cosmic rays (1912) for which Hess received the Nobel prize in 1936. Although current
name of the field ‘cosmic rays’ was created by Millikan to describe the radiations that
were measured.
From the 1930s to the early 1950s, Cosmic Rays provided a tool to study the elementary
particle physics until accelerator took over the role.
Because cosmic rays contain very high energy particles which were energetic enough
to penetrate into the nucleus, it became a very useful tool to discover new particles. By
using cosmic rays, numbers of new particles were discovered which includes positron
(1933), muon (1947), kaons (1947), lambda (1947), pion (1947), Xi baryon (1952) and
Sigma baryon (1953) from the 1930s to the early 1950s. After 1953, the future of high
energy physics lay in the accelerator laboratory rather than in the use of cosmic rays as
the accelerator technology had developed to the point where it can provide high energy
particles.
2. Properties of Cosmic Rays
Cosmic rays are often defined as charged particles originated in outer space that reach
the Earth. The definition describes correctly the majority of the cosmic ray particles
which do consist of fully charged nuclei. Neutral particles are not cosmic ray
component. But, neutrino and gamma-ray are included in the coverage the from time to
time.
This is one of the most famous plot about the cosmic rays. It show the fluxes of all
particles of cosmic rays.
Cosmic rays cover very wide range of energy - from ~108 eV1 up to ~1021eV. Note that
the highest energy of cosmic rays is higher than human’s best accelerator LHC (Large
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
Hadronic Collider) can generate. Fluxes show remarkably smooth curve over this wide
range. But, there are at least two kinks where slope of the fluxes changes. One is
referred as ‘knee’ where it stays around the middle of this plot at ~ 1015eV. The other is
referred as ‘ankle’ at ~1018eV. Fluxes of cosmic rays are very strongly correlated to the
energy of cosmic ray. As energy goes higher and higher, fluxes become smaller and
smaller.
Composition of cosmic rays changes with energy. At 109eV, cosmic rays are composed
of ~ 85% proton, ~12 % helium, ~1% heavy nuclei and 2% electron. There are very
small amount of other component like anti-matter such as positron and anti-proton.
Arrival direction of cosmic rays are isotropic. There are several different magnetic
field from the origin of cosmic rays to Earth - magnetic field of Earth, solar system and
the galaxy. Because cosmic rays are highly charged particles, they cannot reach to Earth
without losing their original direction. So unlikely neutral particles such as gamma-ray,
cosmic rays cannot point to the source.
All these features provide us clues to study the details of cosmic rays which will
eventually lead us to answer essential questions like ‘where are these come from?’, ‘how
they gain this much energy?’,‘Can our knowledge of the universe provide sufficient
mechanisms to explain what we’ve measured at the Earth?’.
99 years of experimental and theoretical efforts have made lots of progress in
understanding of cosmic rays. But, there are still unknown things. Here I list few items .
- Flux : There are two kinks as mentioned earlier. But, the reason why there should be
this flux changing points in certain energy is not clear.
- Composition : Composition of cosmic rays are known to be depended on the energy.
Then how does it change? There is a strong suggestion that the change in composition
will be measured at knee and above. But, it is still part of on-going effort.
Difference between composition of cosmic rays and solar system provides additional
clue to understand the origin of cosmic rays. Whether all these information from the
composition can provide feasible and agreeable picture of cosmic rays or not is an
interesting question.
- Direction : Although most of cosmic rays will lose their directional information when
they get to the Earth, it is possible for the highest energy of cosmic rays may still
provide the directional information. Because of its high energy, it will not bend
significantly like lower energy particle. This will point us the location of universe
where the most highest energy of particle - particle with energy higher than human
can generate. There has been recent measurement on this issue which we will revisit
later part of the lecture series.
Most hardship on the measurements of cosmic rays, especially high energy cosmic
rays, are due to their low fluxes. It will take a year to measure one particle around knee
energy range with 1 m2 area of detector. For the particles with energy of ankle region, it
will take a year to detector one particle with 1 km2 area of detector.
Next week
Lecture #2 : How can we see the Cosmic Rays?
We will talk about how to detect cosmic rays. Also we will go through the list of
interesting experiments related to cosmic rays.
References
Claus Grupen, Astroparticle Physics (2005)
Malcolm S. Longair, High Energy Astrophysics, 2nd ed. (1992)
Todor Stanev, High Energy Cosmic Rays, 2nd ed. (2003)
Credits for figure
Fluxes of Cosmic Rays : Compiled by Simon Swordy, Scientific America, 1997