Transcript phys586-lec02a

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
A
Z
X
241
95
Y a
A 2
Z 2
4
2
Am  Np  a
237
93
4
2
 Decay alpha particles are monoenergetic

Ea = Q (1-4/A)
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Alpha Decay
Typical alpha energies are 4 < Ea < 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 Decay
 Geiger-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 Decay
 β- decay

n

pe
e
decay

+
p

ne
e
 β decay

 Electron capture (EC) pe  n
e
 β- decay is the most common type of radioactive
decay
 β-

All nuclides not lying in the valley of stability can βdecay
 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
Q value for   decay is
Q   M  A, Z   M  A, Z  1

Note these are
atomic masses
Q   TM  A, Z 1  Te  T  Te  T since TM  A, Z 1  keV

Q   Te max  T max
note!!: Q   M  A, Z   M  A, Z  1  2me c 2
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Electron Capture
Proton 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 == Ka
M to K transition == K
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Characteristic X-rays
Simplified view
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Auger Electrons
Emission 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 Sources
 Most beta sources also emit gamma rays
 Like alpha sources, beta sources must be thin
because of dE/dx losses
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Gamma Decay
Gammas (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 Electrons
 A 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 Electrons
 Examples 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
Ee  Eex  Eb
 Note that the conversion electrons are
monenergetic
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Conversion Electrons
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Conversion Electrons
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Conversion Coefficients
 Gamma 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 electrons
 α = αK + α L + α M + …
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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 Electrons
 Common 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 Sources
 Gamma 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 Sources
 There are also annihilation gammas
 In β+ decay (e.g. 22Na) the emitted positron
will usually stop and annihilate producing two
0.511 MeV gammas
e e  
 
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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 Sources
 Spontaneous fission

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
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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
 (a,n) sources


Make a n source using an a beam
Usually the source consists of an alloy of the alpha emitter
plus target (e.g. PuBe)
a  9Be12C  n
a  9Be8Be  a  n
a  9Be  3a  n


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
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Neutron Sources
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