Radiation Dose Calculator

Absorbed dose measures physics, equivalent dose measures biology. One gray is one joule of ionising energy deposited per kilogram of tissue, and it says nothing about how that energy was distributed at the microscopic scale. Radiation that lays down its energy densely along a short track breaks both strands of a DNA helix far more often than radiation that deposits the same joule sparsely, so the International Commission on Radiological Protection assigns each radiation type a dimensionless weighting factor wR and defines equivalent dose in sievert as absorbed dose times wR. In ICRP Publication 103 (2007) photons and electrons of all energies take wR equal to 1, protons 2, alpha particles and heavy ions 20, and neutrons follow a continuous energy-dependent curve that peaks near 20 around 1 MeV and falls to roughly 5 at thermal energies. Because wR is 1 for gamma and beta, a milligray and a millisievert are numerically identical in those cases, which is a frequent source of confusion. The older units survive in some literature: 1 sievert equals 100 rem and 1 gray equals 100 rad. Useful reference magnitudes make results interpretable. Global average natural background is about 2.4 mSv per year, a chest radiograph roughly 0.02 mSv, a typical abdominal CT examination 5 to 10 mSv, and a transatlantic flight about 0.08 mSv. ICRP recommends 1 mSv per year for members of the public above background and 20 mSv per year averaged over five consecutive years for occupationally exposed workers, with no single year above 50 mSv. Acute whole-body doses near 1,000 mSv produce prodromal radiation sickness, and around 4,000 mSv without treatment is lethal in roughly half of cases. The main limitation of this calculation is that equivalent dose applies to a single organ or tissue; whole-body risk requires effective dose, which additionally weights each organ by tissue factors summing to 1. Dose rate estimates from a point source using the inverse square law also ignore attenuation, scatter buildup, and source geometry, and the gamma constant differs for every nuclide.

Radiation injury was documented within months of Wilhelm Roentgen announcing X-rays in 1895, but there was no agreed way to quantify exposure. The International Commission on Radiological Units and Measurements adopted the roentgen in 1928 as a measure of ionisation produced in air, an exposure unit rather than a tissue dose. The same year the body that became the International Commission on Radiological Protection was founded at the Second International Congress of Radiology in Stockholm, chaired for decades by the Swedish medical physicist Rolf Sievert, who had built precision ionisation instruments and pressed for measurement of dose actually delivered to the patient. The absorbed dose concept was developed through the 1940s and 1950s, and the rad, defined as 100 erg per gram, was introduced in 1953. The recognition that biological effect depends on the type of radiation led to relative biological effectiveness and to the quality factor used for protection purposes from the 1950s onward. The General Conference on Weights and Measures adopted the gray as the SI unit of absorbed dose in 1975, naming it for the British physicist Louis Harold Gray, whose work on cavity ionisation theory in the late 1920s underpinned tissue dosimetry, and adopted the sievert for dose equivalent in 1979. ICRP Publication 26 in 1977 introduced the effective dose framework, Publication 60 in 1991 replaced quality factors with the modern radiation weighting factors and tightened the occupational limit to 20 mSv per year, and Publication 103 in 2007 revised the neutron weighting curve and the tissue weighting factors while retaining the linear no-threshold model for protection planning.