COSMIC RAY DETECTION | WHAT ARE THE INSTRUMENTS?

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

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).