Why was the neutrons the last subatomic particle to be discovered?


Because they are not encountered under normal earth conditions; they are produced in cosmic rays and during scattering processes

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Particle physics is a branch of physics which studies the nature of particles that are the constituents of what is usually referred to as matter and radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics. Although the word "particle" can be used in reference to many objects (e.g. a proton, a gas particle, or even household dust), the term "particle physics" usually refers to the study of the fundamental objects of the universe – fields that must be defined in order to explain the observed particles, and that cannot be defined by a combination of other fundamental fields. The current set of fundamental fields and their dynamics are summarized in a theory called the Standard Model, therefore particle physics is largely the study of the Standard Model's particle content and its possible extensions.

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Cosmic ray

Cosmic rays are very high-energy particles, mainly originating outside the Solar System. They may produce showers of secondary particles that penetrate and impact the Earth's atmosphere and sometimes even reach the surface. Composed primarily of high-energy protons and atomic nuclei, they are of mysterious origin. Data from the Fermi space telescope (2013) have been interpreted as evidence that a significant fraction of primary cosmic rays originate from the supernovae of massive stars. However, this is not thought to be their only source. Active galactic nuclei probably also produce cosmic rays.

The term ray is a historical accident, as cosmic rays were at first, and wrongly, thought to be mostly electromagnetic radiation. In modern common usage high-energy particles with intrinsic mass are known as "cosmic" rays, and photons, which are quanta of electromagnetic radiation (and so have no intrinsic mass) are known by their common names, such as "gamma rays" or "X-rays", depending on their frequencies.

In the physical sciences, subatomic particles are the particles smaller than an atom. (although some subatomic particles have mass greater than some atoms). There are two types of subatomic particles: elementary particles, which according to current theories are not made of other particles; and composite particles. Particle physics and nuclear physics study these particles and how they interact.

The elementary particles of the Standard Model include:

Neutron radiation

Neutron radiation is a kind of ionizing radiation which consists of free neutrons. A result of nuclear fission or nuclear fusion, it consists of the release of free neutrons from atoms, and these free neutrons react with nuclei of other atoms to form new isotopes, which, in turn, may produce radiation.

Neutrons may be emitted from nuclear fusion or nuclear fission, or from any number of different nuclear reactions such as from radioactive decay or reactions from particle interactions (such as from cosmic rays or particle accelerators). Large neutron sources are rare, and are usually limited to large-sized devices like nuclear reactors or particle accelerators (such as the Spallation Neutron Source).

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Nuclear physics

Nuclear physics is the field of physics that studies the constituents and interactions of atomic nuclei. The most commonly known applications of nuclear physics are nuclear power generation and nuclear weapons technology, but the research has provided application in many fields, including those in nuclear medicine and magnetic resonance imaging, ion implantation in materials engineering, and radiocarbon dating in geology and archaeology.

The field of particle physics evolved out of nuclear physics and is typically taught in close association with nuclear physics.

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