alpha decay
DESCRIPTION
Alpha Decay. Because the binding energy of the alpha particle is so large (28.3 MeV), it is often energetically favorable for a heavy nucleus to emit an alpha particle Nuclides with A>150 are unstable against alpha decay Decay alpha particles are monoenergetic E a = Q (1-4/A). Alpha Decay. - PowerPoint PPT PresentationTRANSCRIPT
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Alpha DecayBecause the binding energy of the alpha
particle is so large (28.3 MeV), it is often energetically favorable for a heavy nucleus to emit an alpha particle Nuclides with A>150 are unstable against
alpha decay
Decay alpha particles are monoenergetic E = Q (1-4/A)
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Alpha DecayTypical alpha energies are 4 < E <
8 MeV But half-lives vary from 10-6s to 1017y!
The decay probability is described by the Geiger-Nuttall law log10λ = C – D/√E
λ is the transition probability C, D weakly depend on Z E is the alpha kinetic energy
The Geiger-Nuttall law can be derived using QM to calculate the tunneling probability
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Alpha DecayGeiger-Nuttall law
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Monoenergetic alphas
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Common alpha sources
Since dE/dx is so large for alpha particles the sources are prepared in thin layers
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Beta Decayβ- decay
β- decay β+ decay Electron capture (EC)
β- decay is the most common type of radioactive decay All nuclides not lying in the valley of stability
can β- decay β- decay is a weak interaction
The quark level Feynman diagram for β- decay is shown on a following slide
We call this a semileptonic decay
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Beta Decay
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Beta Decay
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Beta Decay
Because beta decay is a three body decay, the electron energy spectrum is a continuum
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Beta Decay
The Q value in beta decay is effectively shared between the electron and antineutrino The electron endpoint energy is Q
2
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Note these areatomic masses
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Electron CaptureProton rich nuclei can undergo
electron capture in addition to β+
decay e- + p -> n + EC can occur for mass differences < 2mec2
Most often a K or L electron is captured EC will leave the atom in the excited state Thus EC can be accompanied by the
emission of characteristic fluorescent x-rays or Auger electrons e.g. 201Tl ->201Hg x-rays from EC was used in
myocardial perfusion imaging
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Characteristic X-rays
Nuclear de-excitation Gamma ray emission Internal conversion (IC)
Atomic de-excitation x-ray emission Auger electron emission
Assume the K shell electron was ejected L to K transition == K
M to K transition == K
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Characteristic X-rays
Simplified view
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Auger ElectronsEmission of Auger electrons is a
competitive process to x-ray emission For Auger electrons e.g., EKLL = EK – EL1 –
EL2
The Auger effect is more important in low Z (Z < 15) elements because the electrons are more loosely bound
The fluorescent yield is defined as the fraction of characteristic x-rays emitted from a given shell after vacancy
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Characteristic X-rays and Auger Electrons
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Beta SourcesMost beta sources also emit gamma raysLike alpha sources, beta sources must be
thin because of dE/dx losses
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Gamma DecayGammas (photons) are emitted
when a higher energy nuclear state decays to a lower energy one Alpha and beta decays, fission, and
nuclear reactions often leave the nucleus in an excited state
Nuclei in highly excited states most often de-excite by the emission of a neutron or proton
If emission of a nucleon is not energetically possible, gamma emission or internal conversion occurs
Typical gamma ray energies range from 0.1 to 10 MeV
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Conversion ElectronsA competing process to gamma decay is
internal conversion (IC) In IC, the excitation energy of a nucleus is
transferred to one of the electrons in the K, L, or M shells that are subsequently ejected
The electrons are called conversion electrons
IC is more important for heavy nuclei where the EM fields are large and the orbits of inner shell electrons are close to the nucleus
Internal conversion is a competing process to gamma emission
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Conversion ElectronsExamples are seen in the electron
spectra shown in the two figures The first figure is particularly simple and
shows three conversion lines arising from the transfer of 1.4 MeV to electrons in the K, L, and M shells
Note that the conversion electrons are monenergetic
bexe EEE
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Conversion Electrons
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Conversion Electrons
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Conversion CoefficientsGamma emission and IC compete
λtotal = λgamma + λIC
Conversion coefficient α == λIC/λgamma We can break this up according to the
probabilities for ejection of K, L, and M shell electronsα = αK + αL + αM + …
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Conversion Coefficients Increase as Z3
Decrease with increasing transition energy Opposite to gamma emission
Increase with the multipole order May compete with gamma emission at high L
Decrease with atomic shell number as 1/n3
Thus we expect K shell IC to be important for low energy, high multipolarity transitions in heavy nuclei
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Conversion Coefficients
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Conversion ElectronsCommon conversion electron sources
These sources are the only practical way to produce monoenergetic electrons in the keV-MeV range in the laboratory
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Gamma SourcesGamma sources usually begin with beta
decay to put the nucleus in an excited state Encapsulation of the source absorbs the
electron Typical gamma energies are ~1 MeV
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Gamma SourcesThere are also annihilation gammas In β+ decay (e.g. 22Na) the emitted
positron will usually stop and annihilate producing two 0.511 MeV gammas
ee
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Neutron Sources
Nuclei that decay by neutron decay are rarely found in nature Exotic nuclei can be produced in high
energy processes in stars or at heavy ion accelerators
There are no direct neutron sources for the laboratory
Neutron sources can be produced using spontaneous fission or in nuclear reactions
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Neutron SourcesSpontaneous fission
Many of the transuranic nuclides have an appreciable spontaneous fission decay probability
e.g. 252Cf (most widely used since t1/2=2.6 years)
Dominant decay is alpha emission
Spontaneous fission x32 smaller Yield is 2.5x106 n/s per μg of
material
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Neutron Sources (,n) sources
Make a n source using an beam Usually the source consists of an alloy of the alpha
emitter plus target (e.g. PuBe)
There is an accompanying large gamma decay component associated with these sources that make them troublesome
Even though the emitted alpha is monoenergetic, the alpha beam is not due to dE/dx losses Hence the neutrons are not monoenergetic
nBe
nBeBe
nCBe
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Neutron Sources