Editing Decay energy

{{Short description|Energy change of a nucleus after radioactive decay}}
{{more footnotes needed|date=September 2018}}
{{Nuclear physics}}

The '''decay energy''' is the [[energy]] change of a nucleus having undergone a [[radioactive decay]]. Radioactive decay is the process in which an unstable [[atomic nucleus]] loses energy by emitting ionizing particles and [[radiation]]. This decay, or loss of energy, results in an atom of one type (called the parent [[nuclide]]) transforming to an atom of a different type (called the [[decay product|daughter nuclide]]).

==Decay calculation==
The energy difference of the [[reactant]]s is often written as ''Q'':

:<math>Q = \left( \text{Kinetic energy} \right)_\text{after} - \left( \text{Kinetic energy} \right)_\text{before},</math>
:<math>Q = \left(\text{Rest mass} \right)_{\text{before}} c^2 - \left( \text{Rest mass} \right )_\text{after} c^2 .</math><ref>{{Cite web|title=Alpha Decay|url=http://www.personal.soton.ac.uk/ab1u06/teaching/phys3002/course/07_alpha.pdf|url-status=live|access-date=31 March 2021|website=Soton|archive-url=https://web.archive.org/web/20160508160447/http://www.personal.soton.ac.uk:80/ab1u06/teaching/phys3002/course/07_alpha.pdf |archive-date=2016-05-08 }}</ref>

Decay energy is usually quoted in terms of the energy units MeV (million [[electronvolt]]s) or keV (thousand electronvolts):

: <math>Q \text{ [MeV]} = -931.5 \Delta M \text{ [Da]},~~(\text{where }\Delta M = \Sigma M_\text{products} - \Sigma M_\text{reactants}).</math><ref>{{Cite book|last=Choppin|first=Gregory R.|url=https://www.worldcat.org/oclc/182729523|title=Radiochemistry and nuclear chemistry|date=2002|publisher=Butterworth-Heinemann|others=Gregory R. Choppin, Jan-Olov Liljenzin, Jan Rydberg|isbn=978-0-08-051566-3|edition=3rd|location=Woburn, MA|pages=62|oclc=182729523}}</ref>

Types of radioactive decay include
* [[gamma ray]]
* [[beta decay]] (decay energy is divided between the emitted [[electron]] and the [[neutrino]] which is emitted at the same time)
* [[alpha decay]]

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 decay#Rates|radioactive activity]], i.e. the number of transforming atoms per time, ''M'' the molar mass, then the radiation power ''P'' is:
:<math>P = \Delta{m} \left( \frac{A}{M} \right).</math>
or
:<math>P = E \left( \frac{A}{M} \right).</math>
or
:<math>P = Q A.</math>

Example: [[Cobalt-60|<sup>60</sup>Co]] decays into <sup>60</sup>Ni. The mass difference ''Δm'' is 0.003{{nbsp}}[[atomic mass unit|u]]. The radiated energy is approximately 2.8{{nbsp}}MeV. The molar weight is 59.93. The half life ''T'' of 5.27 year corresponds to the activity {{nobr|A {{=}} N [ ln(2) / T ]}}, where N is the number of atoms per mol, and T is the half-life. Taking care of the units the radiation power for <sup>60</sup>Co is 17.9{{nbsp}}W/g

Radiation power in ''W/g'' for several isotopes:
: <sup>60</sup>Co: 17.9
: <sup>238</sup>Pu: 0.57
: <sup>137</sup>Cs: 0.6
: <sup>241</sup>Am: 0.1
: <sup>210</sup>Po: 140 (T = 136{{nbsp}}d)
: <sup>90</sup>Sr: 0.9
: <sup>226</sup>Ra: 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 - {{chem|238|Pu| link=Plutonium-238}} with its roughly eighty year half life and low gamma emissions has become the RTG nuclide of choice. {{Chem|90|Sr|link= Strontium-90}} performs worse than {{chem|238|Pu}} 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 {{chem|90|Y|link=Yttrium-90}} decays, but as it is a [[fission product yield|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.

==See also==
* [[Q value (nuclear science)]]

==References==
{{Reflist}}
* [http://www.nucleonica.net/nuclTxtbook.aspx Radioactivity Radionuclides Radiation by Joseph Magill and Jean Galy, Springer Verlag, 2005]

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[[Category:Nuclear physics]]