Download - Alpha Decay - University of Arizona
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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 monoenergeticEα = Q (1-4/A)
α
α42
23793
24195
42
22
+→
+→ −−
NpAm
YX AZ
AZ
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Alpha DecayTypical alpha energies are 4 < Eα < 8 MeV
But half-lives vary from 10-6s to 1017y!The decay probability is described by the Geiger-Nuttall law
log10λ = C – D/√Eλ is the transition probabilityC, D weakly depend on ZE is the alpha kinetic energy
The Geiger-Nuttall law can be derived using QM to calculate the tunneling probability
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Common alpha sourcesSince dE/dx is so large for alpha particles the sources are prepared in thin layers
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Beta Decayβ- decayβ- 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
epen ν−→enep ν+→
enpe ν→−
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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
maxmax
1,1,
21,,:!!
since
1,, isdecay for value
cmZAMZAMQnote
TTQ
keVTTTTTTQ
ZAMZAMQQ
e
e
ZAMeeZAM
−−−=
==
<+≈++=
+−=
+
−
++−
−
++
−
β
νβ
ννβ
β
β Note these areatomic masses
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Electron Capture
Proton rich nuclei can undergo electron capture in addition to β+ decay
e- + p -> n + νEC can occur for mass differences < 2mec2
Most often a K or L electron is capturedEC will leave the atom in the excited stateThus 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-excitationGamma ray emission Internal conversion (IC)
Atomic de-excitationx-ray emission Auger electron emission
Assume the K shell electron was ejectedL to K transition == Kα
M to K transition == Kβ
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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 boundThe fluorescent yield is defined as the fraction of characteristic x-rays emitted from a given shell after vacancy
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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 stateNuclei in highly excited states most often de-excite by the emission of a neutron or protonIf emission of a nucleon is not energetically possible, gamma emission or internal conversion occursTypical 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 ejectedThe electrons are called conversion electronsIC is more important for heavy nuclei where the EM fields are large and the orbits of inner shell electrons are close to the nucleusInternal 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 CoefficientsGamma emission and IC competeλtotal = λgamma + λIC
Conversion coefficient α == λIC/λgammaWe 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 CoefficientsIncrease as Z3
Decrease with increasing transition energyOpposite to gamma emission
Increase with the multipole orderMay 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 ElectronsCommon conversion electron sources
These sources are the only practical way to produce monoenergetic electrons in the keV-MeVrange 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 electronTypical gamma energies are ~1 MeV
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Gamma SourcesThere are also annihilation gammasIn β+ decay (e.g. 22Na) the emitted positron will usually stop and annihilate producing two 0.511 MeV gammas
γγ→−+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 acceleratorsThere 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 probabilitye.g. 252Cf (most widely used since t1/2=2.6 years)Dominant decay is alpha emissionSpontaneous fission x32 smallerYield is 2.5x106 n/s per μg of material
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Neutron Sources(α,n) sources
Make a n source using an α beamUsually 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 troublesomeEven though the emitted alpha is monoenergetic, the alpha beam is not due to dE/dx losses
Hence the neutrons are not monoenergetic
nBenBeBe
nCBe
+→+
++→+
+→+
αα
αα
α
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