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'Radioactivity' is a term coined by Pierre and Marie Curie1, which is used to describe a weird phenomenon that happens with certain substances. At the turn of the 19th Century, when first advances on the elucidation of the atomic structure were being made, strange 'new kinds' of radiation were being discovered (like cathode rays and x-rays). The physics behind them remained obscure and mysterious. The materials investigated by the Curies showed a radiation similar to the recently discovered x-rays but generated by a then unknown process. (X-Rays were generated by brute force by shooting electrons against metal plates, whereas this strange x-Ray-like radiation did not need to be generated, it simply was found emanating from certain materials.) Substances emitting this weird radiation were simply called 'Radio-active'. 'Radioactivity' is used to denote the presence of this radiation.
Today, radioactivity is a collective term comprising some kinds of radiation (namely alpha-, beta-, gamma- and neutron-radiations) associated with unstable nuclear matter.
Atomic Nuclei - the Source of Radioactivity
Atomic nuclei are mainly made out of neutrons and protons in a proportion between 1:1 and 1.5:1 (excluding the Hydrogen - or 1H - nucleus which consists of only one proton). Heavy nuclei and nuclei with neutrons and protons out of these proportions are found to be unstable. They will undergo nuclear reactions forming other nuclei until they are transformed into a stable nucleus. The radioactivity is detected during these reactions, and the element is said to be radioactive. There are basically four ways an unstable nucleus can react:
It can expel two protons and two neutrons (a Helium nucleus) leaving a doubly negative charged ion. This process is called alpha decay.
It can convert one neutron into one proton and emit one electron (and one anti-neutrino), or convert one proton in one neutron and emit a positron (and one neutrino). The positron (anti-matter) immediately reacts with the electrons of the surroundings thus generating two gamma-photons. These processes are called beta-minus and beta-plus decay, respectively.
It can disaggregate into lighter nuclei emitting a load of neutrons. This is a process called nuclear fission. When these neutrons are fast enough they can trigger other nuclei to disaggregate in a chain reaction. This is the process taking place in a nuclear reactor or in an atomic bomb.
It can capture one of its own electrons and transform a proton into a neutron (almost like in the beta decay, but the other way round). This rather uncommon nuclear reaction is called electron capture.
These nuclear reactions emit four different forms of radiation:
Alpha radiation, which consists of Helium-nuclei stemming from the alpha decay.
Beta radiation, which consists of electrons stemming from the beta decay.
Gamma radiation, which consists of high energy photons, a by-product of all nuclear reactions.
Neutron radiation, stemming from the nuclear fission process.
Radioactive elements are the elements that undergo spontaneous nuclear reactions. The rate at which those elements undergo nuclear reactions is described by the half-life, which is the time in which half the atoms have reacted. Isotopes with a long half-life, like Plutonium-239 (10000 years), Uranium-238 (some million years) or Potassium-40 (million years), are not very radioactive, and relatively safe to handle. (Plutonium is particularly hazardous because of its toxicity. That doesn't mean, though, that the hazards due to its radioactivity are negligible, they are comparable to the hazards from similar radioactive nuclides.) Isotopes with short half-lives like Thorium-234 (24 days) or Protactinium-234 (1.14 minutes) exhibit a higher radioactivity. On the other hand a short half-life means that a contamination by this material will soon be over. A long half-life means that a contamination will take longer to decay. The big problem of nuclear waste is that it is basically a mixed salad of all kinds of radioactive stuff; some taking ages to vanish, others radiating like hell.
All elements have at least one radioactive Isotope (eg, Tritium alias 3H, Helium-3, Carbon-14 - the one used for the carbon dating method), many of which do not occur naturally, and are obtained artificially (eg, Technetium-97 or Californium-247). Most commonly people refer to radioactive elements when they mean Uranium, Plutonium, Polonium or Radium and other heavy nuclides. For this reason most people are unaware of the lighter radioactive elements which can be found anywhere - the source of natural radioactivity. For more information on all kinds of isotopes consult this external page: nucleardata
The Effect of Radiation
The Alpha radiation per se is the most dangerous form of radiation, since the alpha-particles (Helium nuclei) are very fast (5-10% of lightspeed) and are relatively massive compared to electrons and photons. Alpha radiation is responsible for most of the damage of DNA in living cells, the main reason for the cancer cases and occurrence of mutations after fallout. Luckily it only acts on a very short range (in the order of microns) and can be blocked by a sheet of paper. Incorporation of alpha emitters though eg, from fallout dust, will cause a long-term exposure to alpha radiation, which can cause serious harm. Sadly, many decay processes yield Radon, a gaseous alpha-emitter (about four days half-life), that can be inhaled. Radon decays to Polonium (a solid metal) also an alpha-emitter that will stay in the lungs.
Beta radiation penetrates things to a depth of about a centimetre. It is therefore more difficult to block2, but it has not such a high ionisation potential (compared to alpha radiation).
Gamma Radiation has little ionisation potential3 compared to the alpha and beta radiations but it can penetrate many metres of matter. For that reason it cannot be readily blocked, which is the most dangerous thing about gamma radiation. Exposition to gamma radiation is not necessarily tied to the presence of radioactive elements.
Neutron radiation is particularly dangerous since it combines long range penetration and high ionisation potentials.
Uses of Radioactivity
Radioactivity has beneficial and negative effects on our health. Historically the beneficial effects were discovered first, partly because the long-term effects were only discovered when it was too late. Radioactive elements can be used to kill overactive cells like cancer cells. For this reason cancer radiotherapy is still a valuable tool in medicine. Small doses of radioactive elements can be used to mark tissue or body fluids, a tool for certain forms of medical diagnosis. Radioactive elements were used as gamma-ray emitters in medical radiography. Nowadays the gamma-rays are generated in other safer ways.
The radioactive emission of certain elements can be used as a highly precise timer in atomic clocks. In science, radioactivity is still a widely used source of radiation. It is used for mutagenesis and molecular marking in biotechnology (eg, in DNA-fingerprinting), for the synthesis of artificial elements to gain fundamental understanding, in archaeology in the famous radiocarbon dating method and many more.
The big problem of the use of radioactive elements is the waste, a common and well-known problem in the context of nuclear power generation. For that reason methods are being developed to replace technology depending on radioactive elements. Many people are still unaware of the waste coming from the aforementioned 'other' uses of radioactive elements.