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PART 1 : COSMIC RADIATION
Cosmic Radiation, also called Cosmic Rays, consists of very high-energy particles which come from outer space (the ‘cosmos’) as well as from our own Solar System. The term ‘ray’ was initially given to these particles because they were believed to be a form of electromagnetic radiation (see Backgrounder: Radiation & Human Space Exploration), but they are actually particles.
PRIMARY COSMIC RAYS
Almost all primary cosmic ray particles (about 99%) are the nuclei (particles in the nucleus) of atoms, with the rest being free electrons (similar to beta particles - see Backgrounder: Radiation & Human Space Exploration). Most of the nuclei are protons (i.e., hydrogen nuclei) as well as helium nuclei (similar to alpha particles - see Backgrounder: Radiation & Human Space Exploration). The nuclei of other heavier elements (elements heavier than hydrogen and helium) also make up cosmic rays. These high-mass, high-charged particles are known as HZE ions.
Primary cosmic rays originate from a variety of sources, including solar flares and explosions on our own Sun (these are often referred to as Solar Energetic Particles) and stellar explosions such as novas and supernovas which are mostly from within our galaxy, but can also come from other galaxies as well (these are referred to as Galactic Cosmic Rays or GCR). Galactic cosmic rays begin as particles propelled out of the expanding cloud of gases and magnetic field caused by a stellar explosion. They tend to bounce around in the magnetic field and some eventually 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.
The high speed and high energy of primary cosmic ray particles make them very dangerous to people and machines. On Earth, and to some degree in low-Earth orbit, we are protected from primary cosmic rays by Earth's magnetosphere (magnetic field) and atmosphere. However, as astronauts travel away from Earth (to the Moon, Mars, or asteroids for example), they are no longer under this protection and hence would be directly exposed to these particles. 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, breaking the strands of DNA molecules, damaging genes and killing cells (see Backgrounder: Radiation Effects on Cells and DNA).
SECONDARY COSMIC RAYS
When primary cosmic rays collide with particles of other things, such as a spacecraft, the International Space Station, or molecules in our atmosphere, they can split molecules, which results in the formation of Secondary Cosmic Ray particles (see Diagram). For example, when primary cosmic rays enter the Earth's atmosphere, they collide with molecules of gas, mainly oxygen and nitrogen, which shatters the nuclei of the gases into smaller pieces (a process called spallation). This shattering results in a cascade of ionized particles and electromagnetic radiation, known as an air shower, in the direction in which the primary particles were travelling.
Typical secondary cosmic ray particles include protons, neutrons, positive and negative pions (short for pi mesons – a type of subatomic particle), and positive and negative kaons (short for K mesons – another type of subatomic particle) (see Figure 1). Some of the pions and kaons decay into muons (elementary particles similar to electrons) and neutrinos (electrically neutral elementary particles), while other pions decay to form gamma ray photons, a form of electromagnetic radiation.
Gamma ray photons can then go on to produce electrons and positrons (antimatter counterpart of electrons), which may in turn go on to release more gamma ray photons, and so on.
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 downwards in the atmosphere. However, with each interaction the particles lose energy and eventually are not able to create new particles. This means that only a small fraction of the secondary cosmic ray particles reaches the Earth’s surface. How many actually come down depends on the energy of the particles and altitude - the higher up you are, the more particles you are likely to encounter. This is why flight crews are exposed to more secondary cosmic rays than people at ground level, and why people on mountains have greater exposure to secondary cosmic rays than people at sea level. In fact, from most primary cosmic rays, no secondary rays make it down to the Earth’s surface at all.
PART 2: NEUTRON RADIATION
Neutrons are one type of secondary cosmic ray particle produced when primary cosmic rays interact with matter. Neutrons are particles found in the nucleus of atoms and, unlike protons and electrons, neutrons have no net electric charge and a mass slightly greater than that of a proton. Neutron radiation is a type of indirectly ionizing radiation that consists of free neutrons (neutrons that are released from atoms). Free neutrons are unstable and will disintegrate in about 10.6 minutes by beta minus decay to a proton and electron if they do not interact with matter.
When free neutrons do come into contact with matter, they do not interact with the electrons as do charged particles, but instead interact only with the nuclei of atoms. When this happens, several results are possible depending 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. Since neutrons interact primarily with the small atomic nuclei rather than the atomic electrons, they can penetrate very deeply into matter.
1. Elastic Scattering
Elastic collisions are billiard ball-like collisions which result in a sharing of kinetic energy between the target nucleus and the impacting neutron. If the sum of the kinetic energies of the neutron and nucleus following collision is equal to the sum of these quantities before collision, the collision is said to be elastic (i.e., kinetic energy is conserved). Maximum energy transfer (about half of the total energy) 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 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, which is why hydrogen-rich materials, such as water, polyethylene and concrete, make good shielding against neutron radiation.
2. Inelastic Scattering
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 and in turn lose some energy itself. When part of the kinetic energy is converted into excitation energy of the struck nucleus, the collision is said to be inelastic (i.e., kinetic energy is not conserved). The additional energy acquired by the nuclei is released as gamma ray photons.
3. Neutron Capture
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 (nucleosynthesis). The new, heavier nucleus enters an excited state (becomes a radioactive isotope) and emits a particle and electromagnetic radiation (gamma ray photon). The resulting nucleus itself may also be unstable and decay, emitting various type of ionizing radiation. Boron Neutron Capture Therapy is an example of the use of neutron capture to kill cancer cells in the head and neck (see below). Boron is used because it can strongly absorb neutrons to produce ionizing radiation.