What is radioactivity?| Explore | physics.org
What is radioactivity?
With radioactivity making news headlines, we look at what it is and what the risks are.
Radioactive decay occurs in unstable atomic nuclei – that  is, ones that don’t have enough binding energy to hold the nucleus  together due to an excess of either protons or neutrons.
It comes in three main types – named alpha, beta and gamma for the first three letters of the Greek alphabet.
Alpha decay
An alpha particle is identical to a helium nucleus, being made up of two protons and two neutrons bound together.
It  initially escapes from the nucleus of its parent atom, invariably one  of the heaviest elements, by quantum mechanical processes and is  repelled further from it by electromagnetism, as both the alpha particle  and the nucleus are positively charged.
The process changes the original atom from which the alpha 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-.
β- emission 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 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 be a few centimetres of  air, or a sheet of paper – although this type of radiation does the most  damage to materials it hits.
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 behaviour 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. So 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”.
In the background
There is a natural level of radiation all around us, which comes from several sources.
Some  gamma radiation comes from space as cosmic rays. Other radiation comes  from sources in the atmosphere, such as radon gas and some of its decay  products.
There are also natural radioactive materials in the  ground – and as well as the obvious elements such as uranium there are  also radioactive isotopes of common substances such as potassium and  carbon.
To understand how much background radiation is around, it  helps to distinguish between effects on normal matter and on the human  body.
The amount of radiation absorbed by non-biological matter is  measured in grays, a unit equivalent to a joule of energy per kilogram  of mass. For biological tissue, a dose equivalent is measured in  sieverts (Sv) depending on the type of radiation involved and how much  damage that radiation does to the particular cells affected.
The  dose equivalent in sieverts is the dose in grays multiplied by some “quality factor” for the type of tissue irradiated and for the type of  radiation – for electrons or gamma-rays, 1; for alpha particles such as  those given off by the radioactive decay of uranium, 20.
The  average amount of radiation received from background sources in the UK  is around 2–2.5 mSv per year. Because of the preponderance of granite,  which contains higher than average levels of uranium, in areas such as  Cornwall or Aberdeenshire it can be twice this level – not high enough  to cause any concern, but high enough that nuclear facilities can’t be  built there as the background level already exceeds the maximum allowed  radiation limit. In some parts of the world, such as northern Iran, the  background radiation is as high as 50 mSv per year.
There are a variety of other natural and routine artificial causes of low doses of radiation.
A dental x-ray will give you a dose of under 1 mSv; a full-body CT scan, 10 mSv.
As  fewer cosmic rays are stopped by the atmosphere the higher you go, the  crew of a passenger jet flying between the US and Japan once a week for a  year would receive an additional a dose of around 9 mSv.
Under normal conditions, the dose limit for workers in the nuclear industry is 50 mSv per year.
The effects on human health
There  are two main health effects caused by radiation, which act over the  short- and long-term and also at shorter and greater distances.
Radiation  causes health problems by killing cells in the body, and the amount and  type of damage done depends on the dose of radiation received and the  time over which the dose is spread out.
The dose limits for  emergency workers in the event of a nuclear accident are 100 mSv if  protecting property or 250 mSv in a life-saving operation.
Between  that upper limit and 1 Sv received within a single day, exposure is  likely to cause some symptoms of radiation poisoning, such as nausea and  damage to organs including bone marrow and the lymph nodes. Up to 3 Sv  these same effects are more serious with a likelihood of acquiring  infections due to a reduced number of white blood cells in the body – with treatment, survival is probable but not guaranteed.
Larger  doses will, in addition to those symptoms above, cause haemorrhaging,  sterility and skin to peel off; an untreated dose of more than 3.5 Sv  will be fatal, and death is expected even with treatment for doses of  more than 6 Sv.
The radiation level decreases with the square of  the distance from its source, so someone twice as far away from an  external source will receive a quarter of the radiation.
Receiving  a high dose in a shorter time usually causes more acute damage, as  greater doses kill more cells, while the body can have had time to  repair some damage with more time having elapsed between doses.
However  radioactive material that is spread to a wider area can cause  longer-term health effects via prolonged exposure, particularly if they  enter the food chain or are inhaled or ingested directly.
Taking  radioactive materials into the body also presents the greatest danger  from atoms that undergo alpha-decay, as alpha particles are not very  penetrative and are easily absorbed by a few centimetres of air. It was  alpha-emitting polonium-210 that was used to murder Alexander Litvinenko  in 2006.
Radioactive isotopes of iodine, which undergo  beta-decay, can build up in the thyroid gland and can cause thyroid  cancer. Attempts to prevent this involve distributing pills that include  nonradioactive iodine-127 and which flood the thyroid, preventing  uptake of radioactive iodine.
For one-off doses, such as those  from medical scans, the risk of later developing cancer is estimated at  around 1 in 20 000 per mSv received.
Absorbing an accumulated dose of 1 Sv over a longer period of time is estimated to eventually cause cancer in 5% of people.
However  there is disagreement over whether very small doses comparable to the  level of background radiation actually contribute to health effects.
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