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© The scientific sentence. 2010
 βdecay
There are three different type of β decay:
β^{}, β^{+}, and electron capture.
The tables give the masses of atoms
(including their electrons), that is the masses of
neutral atoms, rather than masses of bare
atoms, because it is much more diffucult to measure
with high precision the mass of these "nonormal"
elements.
1. Betaminus particle: β^{}
A β^{} is an electron. This process involves
transformation of a neutron into a proton, an electron, and
an antineutrino, in about 15 minutes. β^{} decay
occurs with nuclides with too large ration N/Z.
The speeds of βparticles range from 0.9995 c to c. their motion
is then relativistic. If the recoiling nucleus and the βparticle
are alone in the products of the decay, the speed of the βparticle
would be definite. The βparticle are emitted with a continuous
spectrum of energies. According, however. Therefore, there is a third
particle which is the antineutrino in the products of the
βdeacy. From the conservation of charge, it nust be neutral,
and from the concervation of momentum, it must be of spin 1/2.
The antineutrino is the antiparticle of the neutrino, without mass
as a photon and of symbol is &vu;. Both of them have no charge
and no mass. There are three varieties of neutrinos. the first
provides from βdecay, the second form the tauparticle decay
and the third from the muonparticle decay, each of them has its
proper antineutrino. We write then:
n → p + β^{} + ν_{e}
Here is the β^{} process:
X(A,Z) = X(A,Z+1) + β^{} + ν_{e}
n → p + β^{} + ν_{e}
β^{} binding energy
X(A,Z) → X(A,Z+1) + β^{}
n → p + e^{}
X(A,Z+1) is not neutral because it laks an electron,
that is it contains (Z+1) protons and Z electrons.
But with β^{} which is an electron
e^{}, gives a neutral atom that exist
and we know its atomic mass. We can write:
Mass of neutral X(A,Z+1) = mass of [non neutral X(A,Z+1) + β^{}]
Therefore, the mass defect is:
Δm = mass[neutral X(A,Z)]  mass[ neutral X(A,Z+1)]
Example:
Co(60,27) → X(A, Z+1) + β^{}
X(A, Z+1) = Ni(60,28)
Mass of Co(60,27) = 59.933822 u
Mass of Ni(60,28) = 59.930791 u
The process β^{} is possible
because the mass defect is positive, that is:
Δm = 59.933822 u  59.930791 u = 0.003031u
2. Betaplus particle: β^{+}
β^{+} is a positron e^{+}, the
electron's antiparticle.
β^{+} decay occurs with nuclides
with too small ration N/Z.
In the process of β^{+} decay, we have:
p → n + β^{+} + ν_{e}
&vu;_{e} is the electron neutrino.
Here is the β^{+} process:
X(A,Z) = X(A,Z1) + β^{+}
p → n + β^{+} + ν_{e}
β^{+} binding energy
X(A,Z) → X(A,Z1) + β^{+} + ν_{e}
p → n + e^{+} + &vu;_{e}
X(A,Z1) is not neutral because it contains one more
electron, that is it contains (Z  1) protons and Z
electrons. We subtract un electron to obtain a neutral
atom X(A,Z1).
The mass of β^{+} = mass of electrom,
then:
Mass [non neutral X(A,Z1)] = mass [neutral X(A,Z1)] +
mass [substracted electron] + mass [β^{+}]=
mass [neutral X(A,Z1)] + 2 mass [electron]
Therefore, the mass defect is:
Δm = mass[neutral X(A,Z)]  mass[ neutral X(A,Z1)]  2 mass [electron]
Example:
Co(57,27) → X(A, Z1) + β^{+}
X(A, Z1) = Fe(57,26)
Mass of Co(57,27) = 59.936296 u
Mass of Fe(57,26) = 59.935399 u
The process β^{} is possible
because the mass defect is positive, but the process β^{+}
is not possible because the mass of the original nuclide
Co(57,27) is less that the mass of its daughter Ni(57,26) plus
the mass of two electrons.
Δm = 59.936296 u  59.935399 u  2. me = 0.003031 u < 0.
me = 9.11 x 10^{31 } kg
u = 1/Avogadro's number = 12 g (C)/ 12 x 6.023 x 10^{23}
= 1/ 6.023 x 10^{26} kg
me/u = 9.11 x 10^{31} x 6.023 x 10^{26} = 0.0005487 u
2 me = 0.00109739
59.935399 u + 2. me = 0.003031 = 59.9364929
Δm < 0.
3. Electron capture process:
There are a few nuclides for wich the process β^{+}
is not energetically possible, but they involve the electron
capture process in which un orbital electron (from Kshell generally)
is captured by the nucleus and combines with a proton to form a neutron
that remains in the nucleus and a neutrino which is emitted.
β^{} + p → p + ν_{e}
X(A,Z) → X(A,Z1)
p + e^{} → n + ν_{e}
Electron capture binding energy
The mass defect is:
Δm = mass[neutral X(A,Z)]  mass[ neutral X(A,Z1)]
so, the binding energy is:
Δmc^{2} = mass[neutral X(A,Z)]c^{2}  mass[ neutral X(A,Z1)]c^{2}
Example:
The nuclide Co(57,27) decays by the electroncapture
process:
X(A,Z) → X(A,Z1)
Co(57,27) → Fe(57,26)
Mass of Co(57,27) = 59.936296 u
Mass of Fe(57,26) = 59.935399 u
Δm = 59.936296 u  59.935399 u > 0.

