Neutron
Neutrons carry no electric charge, so they do not interact via electric forces. As a result, neutrons cannot directly cause ionization (where atoms acquire a charge) when passing through a material. Instead, they lose energy through elastic or inelastic nuclear reactions (or in rare cases, magnetic interactions with unpaired electrons). In simple terms, neutrons, being electrically neutral, transfer energy only through direct collisions with particles. Due to this limited interaction mode, neutrons exhibit very high penetration capabilities. Neutrons commonly found on Earth can induce SEE (Single Event Effects), but they generally do not possess enough energy to cause TID (Total Ionizing Dose) or DD (Displacement Damage).
Proton
Protons, which have nearly the same mass as neutrons, carry a positive charge. Thus, they behave differently in materials. Protons not only initiate various nuclear reactions like neutrons but also cause ionization due to their charge. Protons attract electrons and are repelled by positively charged atomic nuclei. Protons with less than 50 MeV of kinetic energy are typically deflected before reaching atomic nuclei due to electric repulsion. However, protons with more than 50 MeV can penetrate the electric barrier and reach the nucleus, enabling nuclear reactions similar to neutrons. Protons, commonly present in space, are major contributors to SEE and can also induce TID and DD.
Elastic Reaction
An elastic reaction occurs when a neutron collides with a target nucleus, transferring part of its kinetic energy and bouncing off with reduced energy. The nucleus recoils (recoil nucleus), and both momentum and energy conservation laws apply. If a neutron with sufficient energy (usually above 100 keV) hits the nucleus effectively, the nucleus is displaced from its original position. These recoiled nuclei are known as recoil nuclei. Such neutron-induced defects can cause local changes in electrical characteristics within semiconductor devices. Repeated collisions of neutrons or protons lead to accumulated damage, resulting in Displacement Damage. Moreover, recoil nuclei generated by neutron collisions are considered heavy ions and can cause significant ionization while moving through the material. Each recoil nucleus may induce a Single Event Effect (SEE).
Inelastic Nuclear Reaction
In inelastic reactions, the neutron is absorbed by the nucleus, and its mass and energy are converted into an excited state of the nucleus. The manner in which this excess energy is released depends on the type of target nucleus and the neutron’s kinetic energy. For neutrons in the several MeV to tens of MeV range, the captured energy is generally shared among all nucleons. The nucleus then breaks apart, often releasing one or more light particles (nucleons or light ions) and forming a heavier recoil nucleus, with gamma rays emitted in the process.
These released particles possess MeV-level energy and can induce ionization. Such secondary radiation is a primary cause of SEE due to neutrons. Some nuclear reactions involve nuclei splitting into two nearly equal-mass fragments and emitting one or more neutrons—this is the principle behind nuclear fission.
However, semiconductor devices contain such heavy elements only in trace amounts (parts per billion), so fission is not a major contributor to TID or SEE. When thermal neutrons (below tens of keV) are absorbed by nuclei, excess energy is typically released as gamma rays.
Spallation
When a neutron’s energy exceeds 100 MeV, its wavelength becomes short enough to interact with individual nucleons (protons or neutrons) inside the nucleus instead of the entire nucleus. This phenomenon is called spallation. In spallation, the neutron transfers energy to a single nucleon inside the nucleus. The excited nucleus then ejects this nucleon and possibly initiates further nuclear reactions as it travels through the material.
The image below summarizes four outcomes after an inelastic nuclear reaction caused by a neutron:
Four phenomena observed in an excited nucleus after inelastic nuclear reaction
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