What are Cosmic Rays?

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This backgrounder has information about both cosmic & neutron radiation.

What are cosmic rays?

Cosmic radiation, or cosmic rays, consists of very high-energy particles. These particles come from outer space (the ‘cosmos’) and from our own solar system. Scientists first called these particles “rays” because they thought they were a form of electromagnetic radiation. But they are not rays at all - they’re particles! 

Primary cosmic rays particles can come from a variety of sources, including supernovas
Primary cosmic rays particles can come from a variety of sources, including supernovas (© 2019 Let’s Talk Science)

What are primary cosmic rays?

About 99% of all primary cosmic ray particles are the nuclei of atoms. The rest are free electrons, similar to beta particles. The majority of the nuclei are protons (hydrogen nuclei). Many others are helium nuclei (similar to alpha particles). The nuclei of elements heavier than hydrogen and helium, but lighter than iron, make up the remaining cosmic rays. These high-mass, high-charged particles are known as HZE ions.

Primary cosmic rays come from a variety of sources. For example, they might come from solar flares or from explosions on the Sun. The particles emitted from the sun are often referred to as solar energetic particles. Particles can also come from stellar explosions such as novas and supernovas, which are mostly from within our galaxy. 

But they can also come from other galaxies as well. Particles from outside of our solar system are called Galactic Cosmic Rays (GCRs). Galactic cosmic rays begin as particles propelled out of the expanding cloud of gases and magnetic field. These are caused by a stellar explosion. GCRs tend to bounce around in the magnetic field. Eventually, some gain enough energy to become cosmic rays and escape into the galaxy. As these particles continue to accelerate, some can travel close to the speed of light.

Cosmic ray particles are very dangerous to people and machines. That’s because cosmic ray particles travel at high speeds and have high energy. On Earth,we are protected from primary cosmic rays by Earth's magnetosphere (magnetic field) and atmosphere. To some degree, astronauts and machines are also protected by these things in low-Earth orbit. But in the past, astronauts have travelled to the Moon. In the future, astronauts may travel to Mars or asteroids. And as astronauts travel away from Earth, they are no longer protected. That means they are directly exposed to cosmic rays.

HZE ions are especially dangerous due to their high charges and high energies. These particles can penetrate through thick layers of shielding and body tissue. They can break the strands of DNA molecules, damage genes and kill cells.

When primary cosmic ray particles enter the atmosphere they can become a variety of secondary cosmic ray particles
When primary cosmic ray particles enter the atmosphere they can become a variety of secondary cosmic ray particles (© 2019 Let’s Talk Science)

What are secondary cosmic rays?

Sometimes, primary cosmic rays collide with other particles. For example, they might collide with particles that make up spacecraft or the International Space Station. They might collide with the particles in our atmosphere. When primary cosmic rays collide with other particles, they can split the molecules and form secondary cosmic ray particles.

For example, when primary cosmic rays enter the Earth's atmosphere, they collide with molecules of gas, mainly oxygen and nitrogen.  This shatters the nuclei of the gases into smaller pieces. This process is called spallation. This shattering results in a cascade of ionized particles and electromagnetic radiation in the direction in which the primary particles were travelling. This cascade is known as an air shower.

Typical secondary cosmic ray particles include protons, neutrons, positive and negative pions, and positive and negative kaons. Some of the pions and kaons decay into  muons and neutrinos. Muons are elementary particles similar to electrons. Neutrinos are electrically neutral elementary particles. Other pions decay into gamma ray photons, a form of electromagnetic radiation.

Gamma ray photons can then go on to produce electrons and positrons. Positrons are the antimatter counterpart of electrons. These may in turn go on to release more gamma ray photons, and so on.

Annual outdoor dose from cosmic radiation for North America (in microsieverts)
Annual outdoor dose from cosmic radiation for North America (nGy/hr) (Source: USGS. The exposure was calculated using equations published by Boltneva, Nazarov, and Fridman (1974).

Many of the secondary cosmic ray particles initially produced go on to split more nuclei and decay into more particles. This means that the number of particles increases rapidly as the shower of particles moves downward through the atmosphere. But with each interaction, the particles lose energy. Eventually, they are not able to create new particles. This means that only a small fraction of secondary cosmic ray particles reach the Earth’s surface. 

The energy of the particles and altitude can have an effect on whether or not humans encounter them. For example, flight crews are exposed to more secondary cosmic rays than people at ground level. People on mountains have greater exposure to secondary cosmic rays than people at sea level. In fact, secondary cosmic rays rarely make it down to Earth's surface at all.


Free neutrons do not interact with the electron clouds of atoms but can interact with any nuclei in their paths
Free neutrons do not interact with the electron clouds of atoms but can interact with any nuclei in their paths (© 2019 Let’s Talk Science)

What is neutron radiation?

Neutrons are one type of secondary cosmic ray particles produced when primary cosmic rays interact with matter. Neutrons are particles found in the nucleus of atoms. Unlike protons and electrons, neutrons have no net electric charge. Neutron radiation is a type of indirect ionizing radiation that consists of free neutrons. Free neutrons are neutrons that are released from atoms. Free neutrons are unstable. If they do not interact with matter, they will disintegrate in about 10.6 minutes by beta minus decay.

When free neutrons do come into contact with matter, they do not interact with the electrons the way charged particles do. Instead, they interact only with the nuclei of atoms. When this happens, several results are possible. The result depends on the energy of the neutron and the mass of the nucleus. However, all interactions are governed by the laws of conservation of momentum and energy. Neutrons mainly interact with the small atomic nuclei rather than the atomic electrons. Because of this, they can penetrate very deeply into matter.

How do neutrons interact with other particles?

When a neutron strikes a hydrogen nucleus through an elastic collision, it loses half of its kinetic energy and slows down a lot
Elastic neutron collision (© 2019 Let’s Talk Science)

How does elastic scattering of neutrons work?

Elastic collisions are billiard ball-like collisions that result in the target nucleus and impacting neutron sharing kinetic energy. A collision is said to be elastic if the sum of the kinetic energies of the neutron and nucleus after collision is equal to the sum of these quantities before collision. This means that kinetic energy is conserved.

Maximum energy transfer is when about half of the total energy is transferred. It occurs when the neutron collides with a nucleus of equal mass, namely the hydrogen atom. 

When a neutron strikes a hydrogen nucleus, the protons themselves become ionizing. That’s because their energy level and charge enables them to interact with the electrons in matter. Neutrons tend to bounce and get slowed down by light nuclei due to elastic scattering. This is why hydrogen-rich materials like water (H2O), polyethylene (C2H4)n and concrete make good shielding against neutron radiation.

When a neutron strikes a larger nucleus through an inelastic collision, the energy gained by the nucleus is released as Gamma ray photons.
Inelastic neutron scattering (© 2019 Let’s Talk Science).

How does inelastic scattering of neutrons work?

When a neutron collides with a heavier nucleus, it can ricochet off. When this happens, the neutron can transfer some of its energy to the nuclei. In turn, it can lose some energy itself. A collision is inelastic when part of the kinetic energy is converted into excitation energy of the struck nucleus. That’s because kinetic energy is not conserved. The additional energy acquired by the nuclei is released as gamma ray photons.

How does neutron capture work?

Slower neutrons can interact directly with a nucleus in a process called neutron capture. In this case, the nucleus ‘captures’ the neutron and a new nucleus is produced. This is called nucleosynthesis. The new, heavier nucleus enters an excited state, thus becoming a radioactive isotope. It then emits an alpha particle and a gamma ray photon. The resulting nucleus itself may also be unstable and decay. If so, it will emit various types of ionizing radiation.

Neutron capture can be used in medicine. For example, Boron Neutron Capture Therapy uses neutron capture to kill cancer cells in the head and neck. Boron is used because it can strongly absorb neutrons to produce ionizing radiation.

When Boron-10 captures a neutron, it decays to form Lithium-7, gamma ray photons and an alpha particle
Boron Neutron Capture (© 2019 Let’s Talk Science)


Encyclopaedia Britannica. (2018). Neutron capture.

Friedlander, M. W. (2011, December 27). Cosmic ray. Encyclopaedia Britannica.

Kliewer, S. (n.d.). Primary cosmic rays. Berkeley Lab.

Mirion Technologies. (2015, April 3). Types of ionizing radiation.

Nuclear Power for Everybody. (n.d.). Neutron elastic scattering.

Nuclear Power for Everybody. (n.d.). Neutron inelastic scattering.

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