The decay energy is the

energy Energy () is the physical quantity, quantitative physical property, property that is transferred to a physical body, body or to a physical system, recognizable in the performance of Work (thermodynamics), work and in the form of heat and l ...

change of a nucleus having undergone a

radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...

. Radioactive decay is the process in which an unstable

atomic nucleus The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford at the Department_of_Physics_and_Astronomy,_University_of_Manchester , University of Manchester ...

loses energy by emitting ionizing particles and

radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or a material medium. This includes: * ''electromagnetic radiation'' consisting of photons, such as radio waves, microwaves, infr ...

. This decay, or loss of energy, results in an atom of one type (called the parent

nuclide Nuclides (or nucleides, from nucleus, also known as nuclear species) are a class of atoms characterized by their number of protons, ''Z'', their number of neutrons, ''N'', and their nuclear energy state. The word ''nuclide'' was coined by the A ...

) transforming to an atom of a different type (called the daughter nuclide).

Decay calculation

The energy difference of the reactants is often written as ''Q'': :

Q = \left( \text \right)_\text - \left( \text \right)_\text,

Q = \left(\text \right)_ c^2 - \left( \text \right )_\text c^2 .

Decay energy is usually quoted in terms of the energy units MeV (million

electronvolt In physics, an electronvolt (symbol eV), also written electron-volt and electron volt, is the measure of an amount of kinetic energy gained by a single electron accelerating through an Voltage, electric potential difference of one volt in vacuum ...

s) or keV (thousand electronvolts): :

Q \text = -931.5 \Delta M \text,~~(\text\Delta M = \Sigma M_\text - \Sigma M_\text).

Types of radioactive decay include *

gamma ray A gamma ray, also known as gamma radiation (symbol ), is a penetrating form of electromagnetic radiation arising from high energy interactions like the radioactive decay of atomic nuclei or astronomical events like solar flares. It consists o ...

beta decay In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which an atomic nucleus emits a beta particle (fast energetic electron or positron), transforming into an isobar of that nuclide. For example, beta decay of a neutron ...

(decay energy is divided between the emitted

electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...

and the

neutrino A neutrino ( ; denoted by the Greek letter ) is an elementary particle that interacts via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass is so small ('' -ino'') that i ...

which is emitted at the same time) *

alpha decay Alpha decay or α-decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus). The parent nucleus transforms or "decays" into a daughter product, with a mass number that is reduced by four and an a ...

The decay energy is the mass difference ''Δm'' between the parent and the daughter atom and particles. It is equal to the energy of radiation ''E''. If ''A'' is the radioactive activity, i.e. the number of transforming atoms per time, ''M'' the molar mass, then the radiation power ''P'' is: :

P = \Delta \left( \frac \right).

or :

P = E \left( \frac \right).

or :

P = Q A.

Example: ⁶⁰Co decays into ⁶⁰Ni. The mass difference ''Δm'' is 0.003 u. The radiated energy is approximately 2.8MeV. The molar weight is 59.93. The half life ''T'' of 5.27 year corresponds to the activity , where N is the number of atoms per mol, and T is the half-life. Taking care of the units the radiation power for ⁶⁰Co is 17.9W/g Radiation power in ''W/g'' for several isotopes: : ⁶⁰Co: 17.9 : ²³⁸Pu: 0.57 : ¹³⁷Cs: 0.6 : ²⁴¹Am: 0.1 : ²¹⁰Po: 140 (T = 136d) : ⁹⁰Sr: 0.9 : ²²⁶Ra: 0.02 For use in radioisotope thermoelectric generators (RTGs) high decay energy combined with a long half life is desirable. To reduce the cost and weight of radiation shielding, sources that do not emit strong gamma radiation are preferred. This table gives an indication why - despite its enormous cost - with its roughly eighty year half life and low gamma emissions has become the RTG nuclide of choice. performs worse than on almost all measures, being shorter lived, a beta emitter rather than an easily shielded alpha emitter and releasing significant gamma radiation when its daughter nuclide decays, but as it is a high yield product of nuclear fission and easy to chemically extract from other fission products, Strontium titanate based RTGs were in widespread use for remote locations during much of the 20th century. Cobalt-60 while widely used for purposes such as food irradiation is not a practicable RTG isotope as most of its decay energy is released by gamma rays, requiring substantial shielding. Furthermore, its five-year half life is too short for many applications.

References

Radioactivity Radionuclides Radiation by Joseph Magill and Jean Galy, Springer Verlag, 2005
{{Authority control Nuclear physics

Decay calculation

See also

References