What Are Protons?

Protons are positively charged subatomic particles found in the nucleus of every atom. Each proton carries an elementary charge of +1 (approximately 1.602 × 10^-19 coulombs) and a rest mass of 1.6726 × 10^-27 kilograms, roughly 1,836 times the mass of an electron. The number of protons in an atom's nucleus defines its atomic number and, in turn, its chemical identity: a nucleus with six protons is always carbon, one with 79 protons is always gold. Unlike neutrons, free protons are stable particles that do not spontaneously decay, making them the only composite subatomic particle with indefinite lifetime under normal conditions.

The physics of protons sits at the intersection of nuclear physics, quantum chromodynamics (QCD), and high-energy particle physics. Although protons were long treated as fundamental, experiments through the 1970s established that they are composite objects built from quarks and gluons. Understanding their internal structure, charge radius, and spin composition remains an active area of research at facilities including CERN, Brookhaven National Laboratory, and Jefferson Lab.

Quark and Gluon Structure

Each proton contains three valence quarks: two up quarks (each carrying a charge of +2/3) and one down quark (carrying a charge of -1/3), which sum to the proton's net charge of +1. These quarks are bound by the strong nuclear force, mediated by gluons, within the framework of QCD. Notably, the rest masses of the three valence quarks account for less than 2% of the proton's total mass; the remainder arises from the kinetic energy of the quarks and the energy stored in gluon fields. The U.S. Department of Energy's Office of Science supports research into proton structure, including measurements of the proton's charge radius and studies of its spin content, which is not fully explained by the valence quark model alone.

Protons in the Space Environment

Protons populate the space environment across a wide energy range and are a primary concern in spacecraft radiation design. Trapped protons in the inner Van Allen radiation belt reach energies of several hundred MeV and are difficult to shield with conventional aluminum structures. Solar energetic particle events can inject intense bursts of 10 to 100 MeV protons into near-Earth space over hours. Galactic cosmic rays include protons at energies up to 10^20 eV. These populations produce both total ionizing dose and displacement damage in semiconductor devices, as documented across decades of experimental data compiled in NASA technical reports on proton effects on spacecraft electronics. High-energy protons are also a component of cosmic ray showers studied by ground-based and space-based observatories.

Protons in Accelerators and Research

Proton accelerators exploit the proton's charge and mass to accelerate beams to high energies for collider experiments, fixed-target nuclear physics, isotope production, and cancer treatment. The CERN Proton Synchrotron, first operated in 1959, feeds the Large Hadron Collider, where proton-proton collisions at 13.6 TeV center-of-mass energy produced data leading to the discovery of the Higgs boson in 2012. In medicine, proton beams in the 70 to 250 MeV range deliver the characteristic Bragg peak dose distribution used in proton therapy, allowing precise tumor irradiation with reduced damage to surrounding tissue.

Applications

Protons have applications in a wide range of disciplines, including:

• Nuclear medicine, as the projectile in cyclotron-based radionuclide production for PET imaging
• Cancer treatment via proton beam therapy at clinical accelerator facilities
• Fundamental particle physics, providing the colliding beams at the Large Hadron Collider and other research accelerators
• Space system radiation qualification, through proton irradiation testing of candidate spacecraft electronics
• Neutron production by spallation, supporting materials science and neutron scattering research