The effects of radiation on genes, including the effect of cancer risk, were recognized much later. In , Hermann Joseph Muller published research showing genetic effects and, in , was awarded the Nobel Prize in Physiology or Medicine for his findings.
The committee met in , and After World War II , the increased range and quantity of radioactive substances being handled as a result of military and civil nuclear programmes led to large groups of occupational workers and the public being potentially exposed to harmful levels of ionising radiation.
Units of radioactivity[ edit ] Graphic showing relationships between radioactivity and detected ionizing radiation The International System of Units SI unit of radioactive activity is the becquerel Bq , named in honor of the scientist Henri Becquerel. One Bq is defined as one transformation or decay or disintegration per second. An older unit of radioactivity is the curie , Ci, which was originally defined as "the quantity or mass of radium emanation in equilibrium with one gram of radium element ".
For radiological protection purposes, although the United States Nuclear Regulatory Commission permits the use of the unit curie alongside SI units,  the European Union European units of measurement directives required that its use for "public health Types of decay[ edit ] Alpha particles may be completely stopped by a sheet of paper, beta particles by aluminium shielding.
Gamma rays can only be reduced by much more substantial mass, such as a very thick layer of lead. Nuclear drip line , Gamma decay , Internal conversion , Electron capture , Alpha decay , Nuclear fission , Neutron emission , and Cluster emission Early researchers found that an electric or magnetic field could split radioactive emissions into three types of beams. The rays were given the names alpha , beta , and gamma , in increasing order of their ability to penetrate matter.
Alpha decay is observed only in heavier elements of atomic number 52 tellurium and greater, with the exception of beryllium-8 which decays to two alpha particles. The other two types of decay are produced by all of the elements. Lead, atomic number 82, is the heaviest element to have any isotopes stable to the limit of measurement to radioactive decay.
Radioactive decay is seen in all isotopes of all elements of atomic number 83 bismuth or greater. Bismuth, however, is only very slightly radioactive, with a half-life greater than the age of the universe; radioisotopes with extremely long half-lives are considered effectively stable for practical purposes.
Types of radioactive decay related to N and Z numbers In analysing the nature of the decay products, it was obvious from the direction of the electromagnetic forces applied to the radiations by external magnetic and electric fields that alpha particles carried a positive charge, beta particles carried a negative charge, and gamma rays were neutral.
From the magnitude of deflection, it was clear that alpha particles were much more massive than beta particles. Passing alpha particles through a very thin glass window and trapping them in a discharge tube allowed researchers to study the emission spectrum of the captured particles, and ultimately proved that alpha particles are helium nuclei. Other experiments showed beta radiation, resulting from decay and cathode rays , were high-speed electrons. Likewise, gamma radiation and X-rays were found to be high-energy electromagnetic radiation.
The relationship between the types of decays also began to be examined: For example, gamma decay was almost always found to be associated with other types of decay, and occurred at about the same time, or afterwards. Gamma decay as a separate phenomenon, with its own half-life now termed isomeric transition , was found in natural radioactivity to be a result of the gamma decay of excited metastable nuclear isomers , which were in turn created from other types of decay.
Although alpha, beta, and gamma radiations were most commonly found, other types of emission were eventually discovered. Shortly after the discovery of the positron in cosmic ray products, it was realized that the same process that operates in classical beta decay can also produce positrons positron emission , along with neutrinos classical beta decay produces antineutrinos.
In a more common analogous process, called electron capture , some proton-rich nuclides were found to capture their own atomic electrons instead of emitting positrons, and subsequently these nuclides emit only a neutrino and a gamma ray from the excited nucleus and often also Auger electrons and characteristic X-rays , as a result of the re-ordering of electrons to fill the place of the missing captured electron.
These types of decay involve the nuclear capture of electrons or emission of electrons or positrons, and thus acts to move a nucleus toward the ratio of neutrons to protons that has the least energy for a given total number of nucleons. This consequently produces a more stable lower energy nucleus. A theoretical process of positron capture , analogous to electron capture, is possible in antimatter atoms, but has not been observed, as complex antimatter atoms beyond antihelium are not experimentally available.
Shortly after the discovery of the neutron in , Enrico Fermi realized that certain rare beta-decay reactions immediately yield neutrons as a decay particle neutron emission.
Isolated proton emission was eventually observed in some elements. It was also found that some heavy elements may undergo spontaneous fission into products that vary in composition. In a phenomenon called cluster decay , specific combinations of neutrons and protons other than alpha particles helium nuclei were found to be spontaneously emitted from atoms. Other types of radioactive decay were found to emit previously-seen particles, but via different mechanisms.
An example is internal conversion , which results in an initial electron emission, and then often further characteristic X-rays and Auger electrons emissions, although the internal conversion process involves neither beta nor gamma decay.
A neutrino is not emitted, and none of the electron s and photon s emitted originate in the nucleus, even though the energy to emit all of them does originate there. Internal conversion decay, like isomeric transition gamma decay and neutron emission, involves the release of energy by an excited nuclide, without the transmutation of one element into another.
Rare events that involve a combination of two beta-decay type events happening simultaneously are known see below. Any decay process that does not violate the conservation of energy or momentum laws and perhaps other particle conservation laws is permitted to happen, although not all have been detected.
An interesting example discussed in a final section, is bound state beta decay of rhenium In this process, beta electron-decay of the parent nuclide is not accompanied by beta electron emission, because the beta particle has been captured into the K-shell of the emitting atom.
An antineutrino is emitted, as in all negative beta decays. Radionuclides can undergo a number of different reactions.
These are summarized in the following table. A nucleus with mass number A and atomic number Z is represented as A, Z. The column "Daughter nucleus" indicates the difference between the new nucleus and the original nucleus.
If energy circumstances are favorable, a given radionuclide may undergo many competing types of decay, with some atoms decaying by one route, and others decaying by another.
An example is copper , which has 29 protons, and 35 neutrons, which decays with a half-life of about This isotope has one unpaired proton and one unpaired neutron, so either the proton or the neutron can decay to the opposite particle. The excited energy states resulting from these decays which fail to end in a ground energy state, also produce later internal conversion and gamma decay in almost 0. More common in heavy nuclides is competition between alpha and beta decay.
The daughter nuclides will then normally decay through beta or alpha, respectively, to end up in the same place.