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Gamma decay

Gamma decay is the simplest form of nuclear decay—it is the emission of energetic photons by nuclei left in an excited state by some earlier process. Protons and neutrons in an excited nucleus are in higher orbitals, and they fall to lower levels by photon emission (analogous to electrons in excited atoms). Nuclear excited states have lifetimes typically of only about 10 14 size 12{"10" rSup { size 8{ - "14"} } } {} s, an indication of the great strength of the forces pulling the nucleons to lower states. The γ size 12{γ} {} decay equation is simply

Z A X N * Z A X N + γ 1 + γ 2 + ( γ decay ) size 12{"" lSub { size 8{Z} } lSup { size 8{A} } X rSub { size 8{N} } rSup { size 8{*} } rightarrow "" lSub { size 8{Z} } lSup { size 8{A} } X rSub { size 8{N} } +γ rSub { size 8{1} } +γ rSub { size 8{2} } + dotsaxis ``` \( γ`"decay" \) } {}

where the asterisk indicates the nucleus is in an excited state. There may be one or more γ s emitted, depending on how the nuclide de-excites. In radioactive decay, γ emission is common and is preceded by γ or β size 12{β} {} decay. For example, when 60 Co β size 12{β rSup { size 8{ - {}} } } {} decays, it most often leaves the daughter nucleus in an excited state, written 60 Ni* . Then the nickel nucleus quickly γ size 12{γ} {} decays by the emission of two penetrating γ size 12{γ} {} s:

60 Ni* 60 Ni + γ 1 + γ 2 . size 12{"" lSup { size 8{"60"} } "Ni" rSup { size 8{*} } rightarrow "" lSup { size 8{"60"} } "Ni"+γ rSub { size 8{1} } +γ rSub { size 8{2} } } {}

These are called cobalt γ size 12{γ} {} rays, although they come from nickel—they are used for cancer therapy, for example. It is again constructive to verify the conservation laws for gamma decay. Finally, since γ size 12{γ} {} decay does not change the nuclide to another species, it is not prominently featured in charts of decay series, such as that in [link] .

There are other types of nuclear decay, but they occur less commonly than α , β , and γ size 12{γ} {} decay. Spontaneous fission is the most important of the other forms of nuclear decay because of its applications in nuclear power and weapons. It is covered in the next chapter.

Section summary

  • When a parent nucleus decays, it produces a daughter nucleus following rules and conservation laws. There are three major types of nuclear decay, called alpha α , size 12{ left (α right ),} {} beta β , size 12{ left (β right ),} {} and gamma γ size 12{ left (γ right )} {} . The α size 12{α} {} decay equation is
    Z A X N Z 2 A 4 Y N 2 + 2 4 He 2 . size 12{"" lSub { size 8{Z} } lSup { size 8{A} } X rSub { size 8{N} } rightarrow "" lSub { size 8{Z - 2} } lSup { size 8{A - 4} } Y rSub { size 8{N - 2} } +"" lSub { size 8{2} } lSup { size 8{4} } "He" rSub { size 8{2} } } {}
  • Nuclear decay releases an amount of energy E size 12{E} {} related to the mass destroyed Δ m by
    E = ( Δ m ) c 2 . size 12{E= \( Δm \) c rSup { size 8{2} } } {}
  • There are three forms of beta decay. The β size 12{β rSup { size 8{ - {}} } } {} decay equation is
    Z A X N Z + 1 A Y N 1 + β + ν ¯ e .
  • The β + decay equation is
    Z A X N Z 1 A Y N + 1 + β + + ν e .
  • The electron capture equation is
    Z A X N + e Z 1 A Y N + 1 + ν e .
  • β is an electron, β + size 12{β rSup { size 8{+{}} } } {} is an antielectron or positron, ν e size 12{v rSub { size 8{e} } } {} represents an electron’s neutrino, and ν ¯ e size 12{ {overline {ν rSub { size 8{e} } }} } {} is an electron’s antineutrino. In addition to all previously known conservation laws, two new ones arise— conservation of electron family number and conservation of the total number of nucleons. The γ decay equation is
    Z A X N * Z A X N + γ 1 + γ 2 + size 12{"" lSub { size 8{Z} } lSup { size 8{A} } X rSub { size 8{N} } rSup { size 8{*} } rightarrow "" lSub { size 8{Z} } lSup { size 8{A} } X rSub { size 8{N} } +γ rSub { size 8{1} } +γ rSub { size 8{2} } + dotsaxis } {}
    γ size 12{γ} {} is a high-energy photon originating in a nucleus.

Conceptual questions

Star Trek fans have often heard the term “antimatter drive.” Describe how you could use a magnetic field to trap antimatter, such as produced by nuclear decay, and later combine it with matter to produce energy. Be specific about the type of antimatter, the need for vacuum storage, and the fraction of matter converted into energy.

What conservation law requires an electron’s neutrino to be produced in electron capture? Note that the electron no longer exists after it is captured by the nucleus.

Neutrinos are experimentally determined to have an extremely small mass. Huge numbers of neutrinos are created in a supernova at the same time as massive amounts of light are first produced. When the 1987A supernova occurred in the Large Magellanic Cloud, visible primarily in the Southern Hemisphere and some 100,000 light-years away from Earth, neutrinos from the explosion were observed at about the same time as the light from the blast. How could the relative arrival times of neutrinos and light be used to place limits on the mass of neutrinos?

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Source:  OpenStax, Basic physics for medical imaging. OpenStax CNX. Feb 17, 2014 Download for free at http://legacy.cnx.org/content/col11630/1.1
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