Истории и востоковедения
particle is emitted into a different element. Its mass number decreases by
жүктеу/скачать 1,14 Mb. Pdf көрінісі
|
- Бұл бет үшін навигация:
- Gamma decay
- Half-lives and probability
particle is emitted into a different element. Its mass number decreases by four and its atomic number by two. For example, uranium-238 will decay to thorium-234. Sometimes one of these daughter nuclides will also be radioactive, usually decaying further by one of the other processes described below. Beta decay Beta decay itself comes in two kinds: β+ and β-. As for β- emission, it occurs by the transformation of one of the nucleus’s neutrons into a proton, an electron and an antineutrino. Byproducts of fission from nuclear reactors often undergo β- decay as they are likely to have an excess of neutrons. β+ decays is a similar process, but involves a proton changing into a neutron, a positron and a neutrino. Gamma decay After a nucleus undergoes alpha or beta decay, it is often left in an excited state with excess energy. Just as an electron can move to a lower energy state by emitting a photon somewhere in the ultraviolet to infrared 95 range, an atomic nucleus loses energy by emitting a gamma ray. Gamma radiation is the most penetrating of the three and will travel through several centimetres of lead. Beta particles will be absorbed by a few millimetres of aluminium, while alpha particles will be stopped in their tracks by a few centimetres of air, or a sheet of paper. Half-lives and probability Radioactive decay is determined by quantum mechanics – which is inherently probabilistic. So, it’s impossible to work out when any particular atom will decay, but we can make predictions based on the statistical behavior of large numbers of atoms. The half-life of a radioactive isotope is the time after which, on average, half of the original material will have decayed. After two half-lives, half of that will have decayed again and a quarter of the original material will remain, and so on. Uranium and plutonium are only weakly radioactive but have very long half-lives – in the case of uranium-238, around four billion years, roughly the same as the current age of the Earth, or the estimated remaining lifetime of the Sun. The half of the uranium-238 around now will still be here when the Sun dies. Iodine-131 has a half-life of eight days, so, once fission has stopped, less than 1% of iodine-131 produced in a nuclear reactor will remain after about eight weeks. Other radioisotopes of iodine are even shorter-lived. Caesium-137, however, sticks around for longer. It has a half- life of around 30 years, and, because of this and because it decays via the more hazardous beta process, is thought to be the greatest health risk if leaked into the environment. Although some radioactive materials are produced artificially, many occur naturally and result in there being a certain amount of radiation in our environment all the time – the «background radiation». жүктеу/скачать 1,14 Mb. Достарыңызбен бөлісу:
|