Nucleon

A nucleon is either a proton or a neutron, the two types of particle that make up ordinary atomic nuclei. The term applies to these particles whether they are bound within a nucleus or exist freely. Protons and neutrons have similar masses and closely related strong-interaction properties, but differ in electric charge, quark composition, and stability when isolated. Neither is elementary: both contain quarks and gluons. (energy.gov)

Basic properties

The proton has positive electric charge equal to one elementary charge, $+e$; the neutron has zero net electric charge. Their approximate rest masses are:

Here, $u$ denotes an up quark and $d$ a down quark; MeV is a million electronvolts, and $c$ is the speed of light. The neutron is about $1.293\ \mathrm{MeV}/c^2$, or 0.14%, heavier than the proton. (pdgweb.lbl.gov)

Both nucleons have spin quantum number $1/2$. They belong to the class of fermions, whose quantum statistics constrain the arrangement of particles in matter. In particle classification, they are hadrons—particles composed of quarks and gluons—and, more specifically, baryons. The designation “nucleon” is narrower than either of these categories: not every hadron or baryon is a proton or neutron. (olps.co.za)

Internal structure and the origin of mass

A proton has two up valence quarks and one down valence quark, whereas a neutron has one up and two down valence quarks. This specifies their net quark content, not a complete inventory of their interiors. Nucleons also contain gluon fields and a sea of quark–antiquark contributions. Their structure is therefore a dynamic quantum system, rather than three small objects held in a fixed geometric arrangement. (energy.gov)

The relevant theory is quantum chromodynamics (QCD), which describes the strong interaction between quarks and gluons. Most of a nucleon’s mass arises from the dynamics of this interacting system, not from adding together the small masses of its valence quarks. Through mass–energy equivalence, energy associated with the quark and gluon fields contributes to the rest mass of the composite particle. (jlab.org)

Nucleon spin likewise cannot be explained simply by adding the spins of three valence quarks. Quark and gluon spin contributions, together with orbital angular momentum, enter the description. Determining their contributions is a central problem in nucleon-structure research. (jlab.org)

Protons and neutrons as an isospin pair

The close relationship between protons and neutrons is expressed through isospin, a quantum number associated with an approximate symmetry of the strong interaction. In this description, the two particles form an isospin doublet with total isospin $I=1/2$. By convention, the proton has projection $I_3=+1/2$, and the neutron has $I_3=-1/2$. They can thus be treated as two states of a common nucleon field. (indico.cern.ch)

Isospin is not ordinary spatial spin, despite its similar mathematical structure. Nor does it make the proton and neutron identical in every respect. The symmetry is approximate because the up and down quarks have different masses and electromagnetic properties. It is especially useful when organizing strong-interaction processes in which these differences are relatively small. (indico.cern.ch)

Nucleons in atomic nuclei

For a nucleus containing $Z$ protons and $N$ neutrons, the total number of nucleons is its mass number:

The proton count $Z$ is the atomic number and determines the chemical element. Nuclei with the same proton count but different neutron counts are isotopes of that element. The mass number is an integer count of particles, not the precise measured mass of the nucleus. (energy.gov)

The simplest nucleus, that of hydrogen-1, consists of a single proton. Deuterium, another isotope of hydrogen, has a nucleus containing one proton and one neutron. In nuclei containing multiple nucleons, the strong interaction provides binding that can overcome the electromagnetic repulsion between positively charged protons. (openstax.org)

This interaction between nucleons is commonly called the nuclear force or residual strong interaction. It must be distinguished from the quark–gluon interaction within a nucleon, although both ultimately arise from QCD. Treating nuclei as systems of nucleons is therefore an effective description at a different scale from treating nucleons as systems of quarks and gluons. (energy.gov)

Binding energy

A bound nucleus has less rest mass than its constituent protons and neutrons would have if separated. Its nuclear binding energy is

where $m_p$ and $m_n$ are the free proton and neutron masses. This is the energy required to separate the nucleus completely into free nucleons. The binding energy per nucleon, $B/A$, allows comparisons between nuclei of different sizes. Differences in binding energy account for the energy released in suitable nuclear fusion and nuclear fission reactions. (energy.gov)

Stability and conversion

No spontaneous proton decay has been observed. A free neutron, by contrast, undergoes beta decay, with an average lifetime of approximately 15 minutes:

The products are a proton, an electron, and an electron antineutrino, the antiparticle of an electron neutrino. The quoted lifetime is a mean lifetime, not a half-life. (olps.co.za)

This conversion occurs through the weak interaction; at the quark level, a down quark is converted into an up quark. A neutron bound in a nucleus need not decay as a free neutron does, because the energetic possibility of decay depends on the initial and final nuclear states. Many nuclei containing neutrons are stable. (ncnr.nist.gov)

Historical development

James Chadwick’s discovery of the neutron in 1932 established a neutral nuclear constituent with approximately the proton’s mass. He received the Nobel Prize in Physics in 1935 for this discovery. The proton–neutron description provided the basis for treating nuclei as collections of nucleons. (nobelprize.org)

Later electron-scattering experiments demonstrated that nucleons possess internal structure. Investigations of deep inelastic scattering revealed point-like constituents within them and supplied crucial evidence for the quark model. Jerome Friedman, Henry Kendall, and Richard Taylor received the 1990 Nobel Prize in Physics for this experimental work. (nobelprize.org)

Research and limits of the nucleon description

Nucleons connect nuclear structure with the underlying physics of quarks and gluons. Experiments probe them through scattering, while lattice QCD calculates aspects of their structure from the fundamental theory. These approaches investigate how momentum, spin, and mass are distributed within protons and neutrons. (jlab.org)

The picture of a nucleus as a collection of unchanged, independent nucleons is incomplete. Deep-inelastic scattering shows that quark momentum distributions in bound nucleons differ from those measured in free nucleons, a phenomenon known as the EMC effect. The nucleon remains a useful nuclear constituent, but its internal structure and its nuclear environment cannot always be treated separately. (energy.gov)

Nucleon interactions also underpin nucleosynthesis, the formation of atomic nuclei in the early universe, stars, and other astrophysical environments. Understanding how protons and neutrons combine and transform is essential to explaining the origin and abundance of the chemical elements. (energy.gov)

References

1. DOE Explains...Protonsenergy.gov
2. Physical Constantspdgweb.lbl.gov
3. The neutron properties are well tuned to study materialsnist.gov
4. Nucleon Structure and QCDmisportal.jlab.org
5. Toward Understanding the Emergence of Hadron Massjlab.org
6. The Spin Structure of the Nucleonjlab.org
7. The nucleon spin and momentum decomposition using lattice QCD simulationsarxiv.org
8. Flavour Physics (of quarks)indico.cern.ch
9. DOE Explains...Nucleienergy.gov
10. DOE-HDBK-1122-99; Radiological Control Technician Trainingenergy.gov
11. 3 Substructure of the Nucleusopenstax.org