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Beta decay is a weak interaction process changing the proton-to-neutron composition of an atomic nucleus while preserving its mass number. In beta-minus decay, a neutron converts to a proton with an emitted electron and electron antineutrino; in beta-plus decay, a proton converts to a neutron with an emitted Positron and electron neutrino, when the complete energy balance permits it.^[ref]

The emitted beta particle is created in the transition rather than being an ordinary orbital electron expelled from the atom. Its kinetic energy varies because the available Q value is shared with the neutrino and daughter recoil. A magnetic electron spectrometer can separate different emitted momenta; a detector then records a continuous distribution rather than one universal electron energy.

Conservation of electric charge, energy and angular momentum constrains the final state. Nuclear binding matters: a free proton cannot undergo ordinary beta-plus decay, although an energetically allowed proton-to-neutron transition can occur within an appropriate atomic nucleus.

A beta particle is an electron or Positron emitted in beta decay. It has the same intrinsic properties as an electron or Positron produced by another process; the name identifies its origin rather than a new particle species.

Unlike a specified two-body alpha transition, ordinary beta emission shares available energy with a neutrino and daughter recoil. The emitted beta particles therefore have a continuous range of kinetic energies. In an applied magnetic field, electrons and positrons curve in opposite senses for the same incident direction. Detector response, energy loss in the sample and collection geometry modify the measured distribution.

An iodine radionuclide with approximately eight-day half-life, emitting beta particles and gamma rays. Its uptake and retention depend strongly on the carrier and tissue physiology.

Reduce time, increase distance and use radiation-appropriate shielding. At fixed conditions, accumulated absorbed dose is proportional to exposure time. For an approximately isotropic point source, the inverse-square law reduces the uncollided radiation field as distance increases. Confinement of radioactive material prevents contamination; it addresses a different pathway from attenuation of an external field.

For alpha particles, short stopping ranges make light barriers sufficient against external particles, but confinement remains important. For beta particles, a low-atomic number shield such as plastic limits both penetration and secondary bremsstrahlung; high-atomic number material can then attenuate residual photons. For penetrating gamma rays, dense material such as lead, or a thicker bulk shield such as concrete, provides attenuation. One shield material is not equally appropriate for every radiation type.^[ref][ref]

The idealized external photon estimate is approximately $0.557\mathrm{mSv}{\mathrm{h}}^{-1}$ at one metre. First apply radioactive decay to the certified radioactive activity:Treat the source as isotropic and small compared with the observation distance. Neglect photon self-absorption, capsule attenuation and attenuation in air. The metal and capsule stop the primary beta particles, so the direct external contribution is taken to be the two gamma rays; secondary bremsstrahlung is neglected. Their total energy per transformation isThe incident photon energy flow per area at distance $r$ is thereforeInterpret the given $3{\mathrm{m}}^{-1}$ as the linear energy-absorption coefficient ${\mu }_{\mathrm{en}}$, rather than a total photon-removal coefficient. For an absorbing medium of mass density $\rho$, a thin volume $Sdx$ absorbs energy per time ${\Psi }_{E}S{\mu }_{\mathrm{en}}dx$ and has mass $\rho Sdx$. Its absorbed dose rate is consequentlyFor the estimate assume a water-like $\rho =10^3\mathrm{kg}{\mathrm{m}}^{-3}$ and local deposition of the secondary charged-particle energy. At $r=1\mathrm{m}$,For gamma rays, the radiation weighting factor is one, giving $\dot{H}=0.556815\mathrm{mSv}{\mathrm{h}}^{-1}$ under these assumptions.^[ref]

The phrase “equivalent absorbed dose” mixes two quantity names: absorbed dose is measured in gray, while equivalent dose is measured in sievert. Their rates are numerically equal here because the weighting factor is one. The estimate is a physical energy-deposition calculation; the assumed mass density is essential, and the stated inverse-length coefficient must not be used as if it already had area-per-mass units.

A diagnostic example is technetium-99m methylene diphosphonate, which accumulates at sites of bone turnover for bone scintigraphy. A therapeutic example is sodium iodide iodine-131, taken up by thyroid tissue and used for selected thyroid disease including differentiated thyroid cancer.

For diagnosis, an escaping gamma ray or a Positron followed by annihilation photons allows external detection, with an appropriate short half-life and limited deposited energy. For molecular radiotherapy, short-range charged emissions such as beta particles, alpha particles or Auger electrons deposit substantial local energy in the targeted tissue. Iodine-131 emits therapeutic beta particles as well as imageable gamma rays, so these categories are functional rather than mutually exclusive isotope classes.

Radiation transports energy away from a source through propagating fields or emitted particles. Electromagnetic radiation includes radio waves, visible light, X-rays and gamma rays; alpha particles, beta particles and neutrons provide particle-radiation examples. The word alone does not specify an energy, a hazard, or a detection method.

An experiment must distinguish the incident field from its interaction with the detector. A photodetector measures a light-induced response, while an ionization chamber collects charge created in a gas. Neither instrument responds equally to every kind of radiation. Ionizing radiation is classified by its ability to remove bound electrons under the relevant interaction conditions, rather than simply by whether the source is radioactive.