RADBAR / ENCYCLOPEDIA
Ionizing Radiation Encyclopedia
Rigorous, source-cited explanations of ionizing radiation: what it is, how it is measured, its health effects, the hazards of lead, dose limits and how shielding works.
Sourced
Every article lists its references: ICRP, UNSCEAR, WHO, NIST and national standards.
Conditional
Values are given with their conditions, never compared out of context.
Reviewed
Content reviewed 2026-09-24.
Radiation basics
What ionizing radiation is, where it comes from and how it interacts with matter.
What is ionizing radiation?
Ionizing radiation carries enough energy to remove electrons from atoms or molecules, creating ions. It includes high-energy photons — X-rays and gamma rays — and particle radiation such as alpha particles, beta particles and neutrons. Radio waves, microwaves and visible light are non-ionizing.
Read the articleTypes of radiation and how far they penetrate
Alpha particles are stopped by a sheet of paper or the outer layer of skin; beta particles by a few millimetres of plastic or aluminium; X-rays and gamma rays have no fixed range and are reduced exponentially by dense, high-atomic-number materials; neutrons are slowed by hydrogen-rich materials such as water, polyethylene and concrete.
Read the articleHow X-rays are produced
In an X-ray tube, electrons from a heated cathode are accelerated by a high voltage (kV) into a metal target. Most of their energy becomes heat; a small fraction becomes X-rays, as a continuous bremsstrahlung spectrum whose maximum photon energy in keV equals the peak tube voltage in kVp, plus characteristic lines of the target element.
Read the articleHow materials attenuate X-rays and gamma rays
Photons are removed from a beam by photoelectric absorption, Compton scattering and, above 1.022 MeV, pair production. For a narrow beam, intensity falls exponentially, I = I₀·e^(−μx). The half-value layer (HVL = 0.693/μ) halves the intensity; ten-fold reduction takes about 3.3 HVLs. Absorption edges make every element’s attenuation energy-dependent.
Read the articleIonizing radiation versus everyday electromagnetic fields
Everyday electromagnetic fields — from power lines, induction cookers, Wi-Fi and mobile phones — are non-ionizing: their photon energy is far too low to ionize atoms. X-rays and gamma rays are ionizing. The two need different measuring instruments and different shielding, and evidence for one says nothing about the other.
Read the articleRadioactivity and half-life
A radionuclide decays at a fixed rate described by its half-life: after one half-life, half of the atoms remain; after ten, about 0.1%. Half-lives range from hours (technetium-99m, 6.01 h) to billions of years (uranium-238). Gamma energies are also fixed for each nuclide, which is why gamma shielding is specified for the named radionuclide.
Read the articleDose and standards
Units, everyday doses and the limits set by standards.
Radiation dose quantities and units
Activity (becquerel, Bq) counts decays per second. Absorbed dose (gray, Gy) is energy deposited per kilogram. Equivalent dose and effective dose (sievert, Sv) weight the absorbed dose for radiation type and tissue sensitivity, so that different exposures can be compared for protection purposes. 1 Sv = 1,000 mSv = 1,000,000 μSv.
Read the articleNatural and man-made radiation in everyday life
Everyone receives natural background radiation. UNSCEAR gives a world average of about 2.4 mSv per year, more than half of it from inhaled radon; the 2023 national report gives about 3.1 mSv for China. Medical exposure is the largest man-made source. A chest X-ray is about 0.1 mSv and a CT examination about 2–8 mSv.
Read the articleDose limits and the standards behind them
China’s basic standard GB 18871-2002 limits public exposure to 1 mSv effective dose per year and occupational exposure to 20 mSv per year averaged over five years (no more than 50 mSv in any year). Dose limits do not apply to patients’ medical exposure, which is controlled by justification and optimization instead.
Read the articleHealth and safety
Biological effects of radiation and the toxicity of lead.
Health effects of ionizing radiation
Radiation damages cells mainly by breaking DNA. High doses cause deterministic effects — skin injury, cataract, blood-forming damage — above a threshold, with severity rising with dose. At any dose, stochastic effects such as cancer and heritable effects become more probable, so radiation protection assumes there is no safe threshold and keeps every unnecessary dose as low as reasonably achievable.
Read the articleThe hazards of lead
Lead is a cumulative toxicant that affects the nervous system, blood, kidneys and cardiovascular system. The WHO states that there is no level of lead exposure known to be without harmful effects, and young children are most vulnerable. Lead sheets and lead rubber remain effective shielding materials, but they need careful handling, inspection and disposal.
Read the articleShielding and protection
Protection principles, shielding materials and staff protection.
Principles of radiation protection
Radiation protection rests on three principles: justification (an exposure must do more good than harm), optimization (keep doses as low as reasonably achievable, ALARA) and dose limitation. For external exposure, the practical tools are time, distance and shielding; from a point source, doubling the distance reduces the dose rate to one quarter.
Read the articleAn overview of shielding materials
Photon shielding relies on dense, high-atomic-number materials: lead, tungsten, bismuth, tin, antimony, barium and rare-earth elements, often in polymer composites; concrete and steel for buildings; lead glass or lead acrylic where viewing is needed. Neutrons need hydrogen-rich and boron-containing materials. The right choice depends on energy, required attenuation, weight, form and environment.
Read the articleProtecting staff in medical X-ray work
Staff dose in fluoroscopy and interventional procedures comes mainly from radiation scattered by the patient. It is reduced by limiting beam-on time, stepping back from the patient, using ceiling-suspended and table-mounted shields, and wearing protective garments, thyroid collars and eye protection, with personal dosimetry to verify the result.
Read the articleQuestions worth asking.
What is the difference between electromagnetic and ionizing radiation?
Everyday electromagnetic fields from power lines, radios and phones are non-ionizing: they cannot ionize atoms. X-rays and gamma rays are ionizing photon radiation with far higher energy. Shielding for one is not evidence of shielding for the other.
Can an electromagnetic field meter measure ionizing radiation?
No. Ionizing radiation needs a dosimeter or survey meter designed for X-ray or gamma measurement. Reliable shielding assessment requires calibrated instruments and a defined test method.
Do X-ray and gamma shielding materials stop cosmic rays?
Not effectively. Primary cosmic radiation consists mainly of protons, and materials designed for X-rays and gamma rays offer little protection against it.
How do I read mSv and μSv?
Sievert expresses effective dose; 1 mSv = 1,000 μSv. Survey meters usually show a dose rate such as μSv/h. Only if that rate persisted continuously would multiplying by 24 × 365 give an annual dose.
Why does a scanner curtain matter if the machine is shielded?
Tunnel curtains are part of the machine’s shielding. If strips are damaged, missing or cannot return, for example when bags block them, shielding integrity is reduced. Inspect curtains and replace worn strips.
Does lead-free mean completely harmless?
No. Lead-free describes composition. Chemical safety, intended use and market requirements still need their own evidence for the supplied model.
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