Editing Alpha decay (section)

=== Quantum tunneling ===
Quantum mechanics, however, allows the alpha particle to escape via quantum tunneling. The quantum tunneling theory of alpha decay, independently developed by George Gamow<ref>
{{cite journal
 |author=G. Gamow
 |year=1928
 |title=Zur Quantentheorie des Atomkernes (On the quantum theory of the atomic nucleus)
 |journal=[[Zeitschrift für Physik]]
 |volume=51
 |issue=3
 |pages=204–212
 |doi=10.1007/BF01343196
 |bibcode = 1928ZPhy...51..204G |s2cid=120684789
 }}</ref> and by [[Ronald Wilfred Gurney]] and [[Edward Condon]] in 1928,<ref name="gurney-Condon">
{{cite journal
 |author=Ronald W. Gurney & Edw. U. Condon
 |year=1928
 |title=Wave Mechanics and Radioactive Disintegration
 |journal=[[Nature (journal)|Nature]]
 |volume=122
 |issue=3073
 |page=439
 |doi=10.1038/122439a0
 |bibcode = 1928Natur.122..439G |doi-access=free
 }}</ref> was hailed as a very striking confirmation of quantum theory. Essentially, the alpha particle escapes from the nucleus not by acquiring enough energy to pass over the wall confining it, but by tunneling through the wall. Gurney and Condon made the following observation in their paper on it:
<blockquote>It has hitherto been necessary to postulate some special arbitrary 'instability' of the nucleus, but in the following note, it is pointed out that disintegration is a natural consequence of the laws of quantum mechanics without any special hypothesis... Much has been written of the explosive violence with which the α-particle is hurled from its place in the nucleus. But from the process pictured above, one would rather say that the α-particle almost slips away unnoticed.<ref name="gurney-Condon"/></blockquote>

The theory supposes that the alpha particle can be considered an independent particle within a nucleus, that is in constant motion but held within the nucleus by strong interaction. At each collision with the repulsive potential barrier of the electromagnetic force, there is a small non-zero probability that it will tunnel its way out. An alpha particle with a speed of 1.5×10<sup>7</sup>&nbsp;m/s within a nuclear diameter of approximately 10<sup>−14</sup>&nbsp;m will collide with the barrier more than 10<sup>21</sup> times per second. However, if the probability of escape at each collision is very small, the half-life of the radioisotope will be very long, since it is the time required for the total probability of escape to reach 50%. As an extreme example, the half-life of the isotope [[bismuth-209]] is {{val|2.01|e=19|u=years}}.

The isotopes in [[beta-decay stable isobars]] that are also stable with regards to [[double beta decay]] with [[mass number]] ''A''&nbsp;=&nbsp;5, ''A''&nbsp;=&nbsp;8, 143&nbsp;≤&nbsp;''A''&nbsp;≤&nbsp;155, 160&nbsp;≤&nbsp;''A''&nbsp;≤&nbsp;162, and ''A''&nbsp;≥&nbsp;165 are theorized to undergo alpha decay. All other mass numbers ([[isobar (nuclide)|isobar]]s) have exactly one theoretically [[stable nuclide]]. Those with mass 5 decay to helium-4 and a [[proton]] or a [[neutron]], and those with mass 8 decay to two helium-4 nuclei; their half-lives ([[helium-5]], [[lithium-5]], and [[beryllium-8]]) are very short, unlike the half-lives for all other such nuclides with ''A''&nbsp;≤&nbsp;209, which are very long. (Such nuclides with ''A''&nbsp;≤&nbsp;209 are [[primordial nuclide]]s except <sup>146</sup>Sm.)<ref name=bellidecay>{{cite journal |last1=Belli |first1=P. |last2=Bernabei |first2=R. |last3=Danevich |first3=F. A. |last4=Incicchitti |first4=A. |last5=Tretyak |first5=V. I. |display-authors=3 |title=Experimental searches for rare alpha and beta decays |journal=European Physical Journal A |date=2019 |volume=55 |issue=8 |pages=140–1–140–7 |doi=10.1140/epja/i2019-12823-2 |issn=1434-601X |arxiv=1908.11458 |bibcode=2019EPJA...55..140B|s2cid=201664098 }}</ref>

Working out the details of the theory leads to an equation relating the half-life of a radioisotope to the decay energy of its alpha particles, a theoretical derivation of the empirical [[Geiger–Nuttall law]].