Учебное пособие Казань-2016 Сайфуллина М. Н., Хабирова Н. М. English for Physicists


particle is emitted into a different element. Its mass number decreases by



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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». 


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