Most cosmic rays are not detected directly at the surface
of the earth. On arriving at earth, they collide with particles on the earth’s
atmosphere, sparking a chain of reaction resulting in a large number of
particles known as cosmic ray air shower. The collisions result in the
production of many pions and kaons (unstable mesons which quickly decay
into muons). The particles emit a beam of radiation known as Cherenkov radiation.
Since muons do not interact
strongly with the atmosphere, many of these muons are able to reach the surface
of the Earth. Muons are ionizing radiation, and may easily be detected by many
types of particle detector such as cloud chambers or bubble chambers or scintillation
detectors.The modern detectors come in two foms. One seeks to
detect the air shower particles themselves at ground level. And the other
detects the radiation emitted by the particles.
Cosmic rays impact other bodies in the solar system which
are made of elements heavier than hydrogen and helium. This can be detected
indirectly by observing high energy gamma ray emissions from these bodies using
a gamma-ray telescope.
According to the Australian Physicist, Victor Franz Hess, the increase in
ionization rate at an altitude of 9km, can cause a radiation of a very great
penetrating power to enter our atmosphere from above. This radiation turned out
to be cosmic rays. He opined that cosmic rays can be detected directly when they pass through particle
detector flown aboard satellites or in high altitude balloons (Hillas, 1972).
Nevertheless, cosmic rays can be detected using the following instrument:
Ionization Chambers,
Geiger Muller Counting tubes, and
Cloud Chambers
These
can be briefly explained as follows:
Ionization Chamber
An
ionization chamber (or ion chamber) is often portable. An ion chamber is used
to measure the rate of radiation exposure (how much radiation exposure is being
received in a specified period of time). The ability of the ion chamber to
measure the exposure rate from a radionuclide is based upon the ability of the
emission to reach the active portion of the meter and the energy of the
emission. Ion chambers are used where there is measurable exposure to or
potential for measurable exposure to x and gamma rays (Vanessa, 2008).
The
ionization chamber works on the principle that charged particles passing
through matter will pop electrons off of atoms as they tear through, a process
called ``ionization.'' If voltage is applied across the material that is
ionized, the electrons will drift to one side and the leftover positively
charged ions will drift to the other. By having a conductor placed at the
appropriate side one can gather the electrons and measure the total charge they
carry. This can be readily converted to a number which measures how much
ionization took place. Then we have information on the total number, energy,
and type of particles that passed through the material. Since ionization
basically counts a total weighted by these factors, we don't get exact details
unless we know some of this information before. (For example, if we know what
kind of particles and how many, we can infer the energy of them. All three
combinations are possible and appropriate for different cases.) (Jim, 2000)
Geiger Muller Counting Tubes
It
is an instrument which is able to count single events of an ionizing ray
hitting the counter’s tube (Geiger,
1940). It is made up of a wire counter which is the basis for another
important development in cosmic-ray studies, the coincidence method, which was
for the first time described by Bothe and Kohlhörster (1929).
This method makes use of the Geiger counter by connecting at least two of them.
Observing the results of both counters one finds many events that are not
related, but several of them coincide and so give a good clue that those hits
have been caused by strong penetrating rays, which are able to hit both tubes
without losing all of their energy in between. So, after the idea of cosmic
rays had started to settle in scientists’ understanding of the natural world,
these inventions helped to widen the range of research. The fundamental work
that had been done up to around 1932 and the knowledge that had been gained
through it was summed up by Geiger (1940). What had
been acknowledged by then as a fact was the cosmic origin of the radiation. In
addition, it had become clear that the intensity of the rays was independent
from the time of day that they were measured as well as from the direction they
appeared to come from. With about one pair of ions per second, per cubic
centimeter, ionization at sea level was barely measurable; though the
radiation, for lack of a better definition up to then simply called
“ultra-radiation”, was extremely penetrating, even in water or solid rock.
Cloud Chamber
Cloud Chamber is an apparatus that detects the path of high-energy particles
passing through a supersaturated vapor; each particle ionizes molecules along
its path and small droplets condense on them to produce a visible track. It is a device
used by physicists to observe particle trails. It only works with ionizing
radiation. One of the cloud chamber's greatest claims to fame is that it was used
to discover the positron, the first observed form of antimatter (Rochester,
1989).
The
way the cloud chamber
works is based on the principle of condensation nuclei. In a supersaturated
water or alcohol vapor, even a tiny charged particle ionizes molecules in the
medium as it moves through it, causing the water to condense around it and
making observable moisture trails. A supersaturated solution means that the air
is holding as much of the fluid (usually water or alcohol) as it possibly can.
The air is pumped with vapor until it starts pooling on the bottom of the chamber, indicating that the medium is supersaturated.
Small perturbations in the air cause the formation of liquid which then drops
to the bottom (Saxon, 1998).
The
cloud chamber is made up of the tracks of particles visible by means of condensing
water vapor. These tracks could be photographed and analyzed, which led to the
aforementioned discovery of tracks from cosmic rays .It is an important
device for making atoms and their tracks visible (Wilson,
1897).