Primary cosmic rays are high-energy particles that originate from sources outside of our solar system and travel through space at close to the speed of light. These particles include protons, electrons, and atomic nuclei such as helium and carbon.

Primary cosmic rays are thought to originate from various sources, including supernova explosions and other energetic events in our galaxy, as well as from distant galaxies. As they travel through space, these particles can interact with magnetic fields and other matter, which can alter their trajectory and energy.

When primary cosmic rays enter Earth's atmosphere, they collide with atoms in the atmosphere, producing secondary cosmic rays and creating a cascade of particle interactions. These secondary cosmic rays can include muons, electrons, and other particles, and can be detected by scientific instruments on the ground and in space.

The study of primary cosmic rays is an important field of research in astronomy and astrophysics, as it can provide insights into the origins and properties of the universe.

The spectrum of primary cosmic rays refers to the distribution of their energies. The spectrum is generally measured by detecting the secondary particles that are produced when the primary cosmic rays collide with atoms in the Earth's atmosphere.

The spectrum of primary cosmic rays is a power-law distribution, which means that the number of cosmic rays with a given energy is proportional to the energy raised to a certain power. This power-law distribution is observed over a wide range of energies, from about 10^9 to 10^20 electron volts (eV).

The highest-energy cosmic rays observed have energies greater than 10^20 eV, which is about ten million times more energetic than particles produced in the most powerful particle accelerators on Earth. The origin of these ultra-high-energy cosmic rays is still a mystery, and their study is an active area of research in astrophysics.

The spectrum of primary cosmic rays is important to determine the energy entering from the primary cosmic rays in the earth's atmosphere.

The intensity of primary cosmic rays is measured by making use of balloons, rockets and satellite. It is found that the intensity of cosmic rays ( unit of particles m-² steradian^ -1 sec^-1) is highly influenced by the solar activity.

The graph of integral flux versus rigidity (in BV = 10^9 V) is shown in figure.

The following conclusions can be drawn from the graph

1. Each particle has a maximum flux, below which flux rapidly declines.

2. The value of the maximum flux changes for particles with higher mass at higher energies.

3. The particles intensity is almost parallel beyond the rigidity 2 BV.

4. The particle intensity is decreased by solar activity.

Protons with an energy of 15 MeV and other nuclei with an energy of 7 MeV/nucleon can enter the atmosphere of the earth. The cosmic rays particles having higher energy beyond 10 ^13 eV is very low. So they cannot be observed. But those high energy particles bombarded atmospheric nuclei and form the secondary cosmic rays. The secondary cosmic rays are the result of those high energy particles bombarding atmospheric nuclei. These secondary cosmic rays intensities are first determined and connected to the primary cosmic rays' intensities. The particles intensity and energy are related as

where k is constant

E= 5 x 10^10 in Bev.

The following features have been obtained for different energy particles.

1. The particles energy 10^10 - 10^15 eV may be expressed as

N(>E) = (2.08x 10^4) E^-1.67 particles m-²ster^-1sec^-1

2. For the energy spectrum 6x10^15 to 5x10^17 ev, the exponent of energy is about 2.2 .

3. For the energy beyond 5x10^17 eV, the exponent is 1.7.

This note is a part of the Physics Repository.

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