Beta particle

Beta radiation refers to a form of ionising radiation consisting of high-energy, high-speed electrons or positrons emitted during the radioactive decay of unstable atomic nuclei. It occupies an intermediate position in terms of both penetrating power and ionising capability when compared with alpha and gamma radiation. While alpha particles are readily stopped by paper and gamma rays require dense shielding such as lead or concrete, beta particles are typically halted by a few millimetres of aluminium or similar low-density materials.
Beta radiation plays an important role in nuclear physics, radiation protection, medicine, and industrial quality control. Its behaviour, biological effects, and applications are strongly influenced by its charged-particle nature and its interactions with matter.

Nature and Characteristics of Beta Radiation

Beta particles are energetic electrons (β⁻) or positrons (β⁺) released from nuclei undergoing beta decay. Their energies vary widely across different radionuclides, but typical energies range near or above 0.5 MeV. Such a particle usually travels about one metre in air, though the exact range depends on the energy, air density, and composition.
Beta particles carry an electric charge, making them more ionising than gamma radiation but less ionising than alpha radiation. The degree of ionisation affects both biological damage and penetration depth. Higher ionisation correlates with greater tissue damage but shorter mean free paths. Because electrons and positrons interact electromagnetically within matter, they lose energy gradually through ionisation, excitation, and bremsstrahlung emission. These interactions lead to the production of X-ray photons, especially when high-energy beta particles pass through materials with high atomic numbers.
In water, beta particles from many fission products exceed the phase velocity of light in that medium, producing distinctive blue Cherenkov radiation. This phenomenon is commonly visible in reactor pools where water acts as both coolant and shielding.

Modes of Beta Decay

β⁻ Decay (Electron Emission)

Negative beta decay occurs in neutron-rich nuclei. A neutron converts into a proton, an electron, and an electron antineutrino. At the quark level, the weak interaction mediates the process through the emission of a virtual W⁻ boson, converting a down quark into an up quark and increasing the atomic number by one.
Free neutrons also undergo β⁻ decay, and neutron-rich fission products contribute significantly to beta radiation emitted from nuclear fuel rods. The emitted electron carries a portion of the decay energy, typically averaging around 0.5 MeV for many isotopes, with the remaining energy carried by the almost undetectable antineutrino.
Phosphorus-32 provides a well-known example of a β⁻ emitter. With a half-life of approximately 14.29 days, it decays into sulphur-32 while releasing about 1.709 MeV of energy. Its moderate beta energy makes it suitable for certain medical and research applications, and it is readily shielded by a few millimetres of acrylic or similar material.

β⁺ Decay (Positron Emission)

Positive beta decay occurs in proton-rich nuclei. A proton transforms into a neutron, a positron, and an electron neutrino. This process requires that the daughter nucleus has a lower binding-energy state than the parent. Positron emission does not occur in free protons; it can only occur within a nucleus where the energy conditions permit it.
Positrons produced through β⁺ decay annihilate with electrons after slowing down, producing two gamma photons. This annihilation forms the basis of positron emission tomography (PET), where β⁺-emitting radiotracers enable high-resolution medical imaging.

Interaction with Matter

Beta particles interact strongly with surrounding materials because of their charge. Their path is marked by repeated collisions with electrons in matter, resulting in ionisation and excitation. Key interaction features include:

• Moderate penetration: Most beta particles are halted by a few millimetres of aluminium or plastic.
• Bremsstrahlung emission: Deceleration in the electric fields of nuclei leads to X-ray production, particularly in materials with high atomic numbers.
• Secondary gamma radiation: As betas slow down, secondary photons may be produced, affecting shielding requirements.
• Cherenkov radiation: When travelling faster than the local speed of light in a medium such as water, beta particles generate visible blue light.

Although thin aluminium sheets may stop beta particles themselves, the bremsstrahlung produced when they decelerate must also be considered. Lower-atomic-weight materials typically produce lower-energy, more easily absorbed bremsstrahlung and are thus preferred for shielding.

Detection and Measurement

The ionising and excitation effects of beta radiation form the basis of common detection technologies:

• Ionisation chambers and Geiger counters measure the ionisation of gas caused by beta particles.
• Scintillation counters detect beta particles through light emitted by excited scintillator materials.
• Beta spectroscopy involves analysing the energy distribution of emitted beta particles, often by deflecting them in a magnetic field to determine their momenta.

Radiation quantities used in measurement include the gray (Gy) for absorbed dose and the sievert (Sv) for equivalent dose. For beta particles, the radiation-weighting factor is 1, so absorbed dose and equivalent dose are numerically identical. Historically, units such as the rad and rem were used, though they are now deprecated.

Applications of Beta Radiation

Beta radiation has diverse applications across science, medicine, and industry.

• Medical treatment: Beta emitters are used in therapies for eye tumours and certain bone conditions, providing controlled local irradiation.
• Radiotracers: Many β⁺-emitting isotopes serve as tracers in PET imaging.
• Industrial thickness gauging: Beta sources measure the thickness of materials such as paper or plastic sheets based on the proportion of beta particles absorbed.
• Illumination devices: Tritium-based beta lights use phosphors that emit photons when struck by beta particles. These devices require no external power and function for many years, with brightness decreasing according to tritium’s 12.32-year half-life.

Strontium-90 is commonly employed in industrial and scientific applications due to its strong beta emission and suitable half-life.

Historical Development

The study of beta radiation emerged alongside the early discoveries of radioactivity. Henri Becquerel’s experiments in the late nineteenth century revealed unknown rays emitted by uranium salts, capable of penetrating paper-wrapped photographic plates. Ernest Rutherford later distinguished between different types of radiation and demonstrated that beta particles were significantly more penetrating than alpha particles.
In 1900, Becquerel determined the mass-to-charge ratio of beta particles using techniques similar to those employed by J. J. Thomson in studying cathode rays. He found that beta particles possessed the same mass-to-charge ratio as electrons, confirming that beta radiation consisted of high-energy electrons rather than a novel form of matter.

Biological and Health Considerations

Beta radiation is moderately penetrating in living tissue and capable of causing ionisation that may result in DNA damage or mutation. Because electrons and positrons deposit energy over short distances, beta sources can be particularly harmful if ingested or inhaled. Nonetheless, controlled beta radiation plays a valuable role in radiotherapy, where targeted doses destroy cancer cells while minimising exposure to surrounding tissues.
Beta radiation continues to be central in nuclear medicine, radiation physics, and industrial measurement systems. Its unique characteristics, including charged-particle interactions, variable penetration, and widespread occurrence in natural and artificial radionuclides, make it a significant subject of scientific and technological importance.