HKNIC - More about Radiation

More about Radiation

Nuclear radiation was a new subject at the beginning of the 20th century and pioneering work in the field led to many fundamental discoveries. Among the pioneers, the Curie family has made perhaps the most notable contribution, earning a record number of Nobel prizes. Husband and wife Pierre Curie and Maria Sklodowska-Curie were awarded a Nobel Prize in Physics in 1903, together with Henri Becquerel, for their study of radiation. Maria was later awarded a Nobel Prize in Chemistry in 1911 for her discovery and study of radium and polonium. One of their daughters, Irene Joliot-Curie was awarded a Nobel Prize in Chemistry jointly with her husband Frederic Joliot-Curie in 1935 for their work with radiation on aluminium.

The amount of radioactivity of a substance is measured in becquerels. A becquerel is defined as an atomic transformation per second out of the instability of an atom. It is a very small unit so it is common to apply prefixes to becquerels, so that a million becquerels is one megabecquerel, and a million million becquerels is a terabecquerel. As we saw earlier, nearly all common materials are radioactive. One kilogramme of coffee has about 1,000 becquerels, about the same as a kilogramme of granite. One adult human has about 7000 becquerels. One kilogramme of sea water has about 10 becquerels. "Radioactive substances" are naturally far more radioactive. One kilogramme of high grade natural uranium ore has 25 megabecquerels, which is much more radioactive than 1 kilogramme of Low Level Radioactive Waste (1 megabecquerel). The radioactivity in a kilogramme of vitrified High Level Radioactive Waste is much higher still at 10 terabecquerels. Yet this is only as radioactive as one-tenth of a typical radiation source used for medical therapy in a hospital.

The biological effect of a radiation dose, or the equivalent dose, is measured in sieverts. The equivalent dose allows for the level of sensitivity of certain body organs to the radiation absorbed by body tissue by using a weighting factor for each organ. A sievert itself is a large unit, so that prefixes are often applied and a millisievert is a thousandth of a sievert. Someone who smokes a pack of cigarettes a day will be subjected to increased radiation dose of about 50 millisieverts a year, mainly from the radioactive substances absorbed from the ground and stored in the leaves of a tobacco plant. A passenger taking a return flight from Hong Kong to London will receive about 0.025 millisievert per journey, largely from cosmic radiation from outer space. Public radiation dose limits are usually set at 1 millisievert per year above the background level. A “radiation worker” in a nuclear power station will typically receive no more than a few millisieverts a year, and in most countries, the maximum permissible dose for radiation workers is 20 millisieverts a year averaged over 5 years, with a maximum of 50 millisieverts in any one year.