Embedded Files
In 1896, Henri Becquerel was working with compounds containing the element uranium. To his surprise, he found that photographic plates used to keep out light became fogged when these uranium compounds were anywhere near the plates. This fogging suggested that some kind of ray had passed through the plates. Several materials other than uranium were also found to emit these penetrating rays. Materials that emit this kind of radiation are said to be radioactive and to undergo radioactive decay.
In 1899, Ernest Rutherford discovered that uranium compounds produce three different kinds of radiation. He separated the radiations according to their penetrating abilities and named them alpha (α), beta (β), and gamma (γ) radiation representing the first three letters of the Greek alphabet.
Alpha radiation emits alpha particles and can be stopped by a simple sheet of paper. An alpha particle is simply the nucleus of a Helium atom, 4He. Beta particles are high speed electrons and need around six millimeters of aluminum to be stopped. Gamma rays are high energy photons and need several millimeters of lead to be stopped. Alpha and gamma emissions can vary from radioactive isotope to isotope. Beta radiation always emits beta particles with a continuous range of energies from zero up until the maximum for a particular isotope.
Radioactive nuclei are energetically unstable and want to reach a more stable state by releasing energy through decay. Heavy nuclei typically release most of their energy through alpha decay to reduce the number of nucleons they are holding on to.
α decay
The emission of an α particle, or 4He nucleus, is a process called α decay. A 4He nucleus consists of 2 neutrons and 2 protons. So when an atom goes through alpha decay it must give up 2 neutrons and 2 protons from the nucleus to emit an α particle. The mass number, A, of the decaying nucleus is reduced by four as both the Z and N values have decreased. The change in proton number, Z, indicates a change in type of element or transmutation. This can only be achieved through radioactive decay and nuclear reactions. The decay of an isotope of the element seaborgium, 263Sg shows the atomic number, Z, changing from 106 to 104 as the parent isotope decays into rutherfordium with an atomic mass of 259.
α decay typically occurs in heavy nuclei where the electrostatic repulsion between protons is large. The process of alpha decay releases energy as well. Careful measurements show that the sum of the masses of the daughter nucleus and the α particle is a bit less than the mass of the parent isotope. The missing mass, a form of energy, is simply converted to kinetic energy shared amongst the products of decay. This conversion is governed by Einstein's famous equation, E=mc2, which relates the amount of stored energy contained in mass.
β decay
Beta decay is the most common type of radioactivity. The process of beta decay converts a neutron to a proton and results in the emission of an electron. The mass of an electron is a fraction of the value of the atomic mass unit (AMU) meaning a nucleus that undergoes beta decay is changed by a negligible amount. So unlike alpha decay, the atomic number, A, will not change.
Beta decay helps nuclei achieve a more stable energy level by changing the neutron to proton ratio in a nucleus. An unstable atomic nucleus will emit a beta particle, or an electron, as well as an anti neutrino. This anti neutrino has very little mass but still carries with it a good amount of momentum and energy.
We know that in every reaction, energy and nuclear charge must be conserved. Even though the reaction spits out an electron and anti-neutrino, energy is still conserved. Mass is a form of energy, so the difference in mass from the parent isotope 14C to the daughter isotope 14N is converted into the kinetic energy and mass energy of the electron and anti-neutrino products. The example of 14C can also be used to explain electrical charge. The nucleus of our carbon atom has 6 protons or a charge of +6. After undergoing beta decay, one of the neutrons is converted into a proton so the resulting nucleus, 14N, has a charge of +7 and this higher number is balanced out by the product electron with a charge of -1.
This still does not seem to explain why the process of Beta decay spits out an anti neutrino. In addition to energy and nuclear charge, we must conserve lepton number. Leptons are a group of elementary particles that include electrons, each lepton has its own lepton number. The product electron has a +1 lepton number but the initial 14C isotope is an atom without its own lepton number. So to balance out the +1 from the electron, the reaction spits out an antineutrino which has -1 lepton number. So three qualities are conserved: energy, electrical charge, and lepton number.
As mentioned before, decay is not spontaneously happening to every random nucleus. A stable nucleus will not experience beta decay, instead we see beta decay in nuclei with nuclei with an excess of neutrons and free neutrons not bound to a nucleus. In the same way a heavy element with a lot of protons is likely to undergo alpha decay, a nuclei with an excess of neutrons will undergo Beta decay to lower the number of neutrons.
Beta decay does not always bring each nuclei to its lowest energy state. With the example of gamma decay drawn out below, Cesium 137 first undergoes beta decay then emits out the daughter isotope Barium 137 in an excited state. Isotopes that undergo beta decay and remain high energy nuclei will continue to decay through beta or gamma decay until they ultimately reach the lowest state of potential energy.
γ decay
Gamma decay gives off gamma rays, a type of electromagnetic radiation resulting from a redistribution of electric charge within the nucleus. Complex nuclei have many different arrangements of neutrons and protons and they can shift from arrangement to arrangement to release energy in the form of gamma rays. Neither the mass number nor the atomic number is changed when a nucleus emits a gamma ray in a reaction.
A γ ray is a high energy photon. The gamma ray has a much shorter wavelength than visible photons emitted by a light bulb.
Gamma ray emission often goes hand in hand with other nuclear processes as nuclei that have just gone through beta or alpha decay might still be in a high energy state. These nuclei rearrange themselves to reach a favorable energy state by releasing energy in the form of a gamma ray. Although there is no change of isotope, this process is considered decay because the nucleus shifts from a state of higher energy to lower energy.
Half-life
The time required for half of the atoms in any given quantity of a radioactive isotope to decay is the half-life of that isotope. Each particular isotope has its own half-life. For example, the half-life of 238U is 4.5 billion years. Meaning in 4.5 billion years half of the existing 238U on Earth will have decayed. About 1/4th of the original material will remain on Earth after 9 billion years. Nuclear half-lives range from tiny fractions of a second to many, many times the age of the universe.
Half-life has real world applications like carbon dating. The half-life of 14C is 5730 years and using this as a reference we are able to date archaeological material. For more information on half-life and isotopes, please refer to the Isotope Browser.
Page updated
Report abuse