Slayt Basligi Yok

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NUCLEAR STABILITY
Nagehan Demirci 10-U
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What is nuclear stability and where does this
come from? As we all know , the nucleus is
composed of protons and neutrons and protons are
positively charged particles and neutrons have no
charge From Coulombs law we learned that like
charges repel one another strongly ,
particularly in the nucleus when we consider how
close they must be to each other. At first
glance, the existence of several protons in the
small space of a nucleus is puzzling. Why would
not the protons be strongly repelled by their
like electric charges?
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The existence of stable nuclei with more than one
proton is due to the nuclear force. The nuclear
force is a strong force of attraction between
nucleons (protons and neutrons together called
nucleons ) that acts only at very short
distances. (about 10-15 m.) Beyond nuclear
distances these forces become negligible. Inside
the nucleus however , two protons are close
enough together for the nuclear force between
them to be effective. This force can more than
compensate for the repulsion of electric charges
and thereby gives a stable nucleus. The fact that
some nuclie are unstable (radioactive) ,and
others are stable leads us to consider the
reasons of stabilitiy.
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• FACTORS DETERMINING NUCLEAR STABILITY
• Two important factors determine the nuclear
  stability.
• The mass number (the total number of nucleons in
  the nucleus)
• The neutron to proton ratio
• This is because in a nucleus , the positively
  charged protons repel each other and as the
  number of protons increases in a nucleus the
  forces of repulsion between the protons increases
  drastically.

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Thus a greater proportion of neutrons is required
for a nucleus to remain stable as the atomic
number increases. The Band Of Stability When you
plot each stable nuclide on a graph with the
number of protons (Z) on the horizantal axis and
the number of neutrons (N) on the vertical axis ,
These stable nuclides fall into a certain region,
or band , of the graph. The band of stability is
the region in which nuclides lie in a plot of
number of protons against number of neutrons.
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The zone of stability. The dots in the blue area
represents the nuclides that do not undergo
radioactive decay. Note that as the number
protons in a nuclide increases, the
neutron/proton ratio required for stability also
increases. The area above the blue area
represents an unstable region in which there are
too many neutrons, therefore beta decay. In the
region below the blue area represents an unstable
region in which there are too many protons
therefore spontaneous positron decay
Slayt 14
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A 1 to 1 neutron to proton ratio holds true
for the stable nuclei of the first twenty
elements in the periodic table. This ratio
increases to 1.5 to 1 around atomic number 80.
Elements above atomic number 83 with 209 nucleons
do not exist as stable isotopes. Thus for
polonium , with 84 protons , the repulsive forces
due to the 84 protons are so large that
regardless of the number of neutrons , its
nuclides are unstable. When the neutron to
proton ratio is too large or too small, the
nucleus is unstable , the atom is called a
radionuclide and undergoes radioactive decay. If
a radionuclide has a higher neutron to proton
ratio , that is it has
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An excess of neutrons and therefore a neutron
disintegrates to form a proton with the emission
of a beta particle. 0n1 ? 1p1 -1e0 This
decreases the neutron to proton ratio and may be
repeated until it reaches the stable value and no
further radioactive decay takes place. An example
of beta decay is 93Np239 ?-1e0 94Pu239 If on
the other hand , a radionuclide has a lower
neutron to proton ratio , it has an excess of
protons and therefore a proton is transformed
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to a neutron either by positron emmission or by
electron capture. 15P30 ?14Si30 -1e0
positron emission 18Ar37 -1e0 ?17Cl37
electron capture In both positron emmission and
electron capture by a nucleus , the nucleus
produced hass one less proton . If however , the
number of nucleons exceeds 209, the limit to be a
stable nuclide is over and lies beyond the stable
value , so several decays are required in order
to attain stability.
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MAGIC NUMBERS The protons and neutrons in a
nucleus appear to have energy levels much as the
electrons in an atom have energy levels. The
shell model of the nucleus is a nuclear model in
which protons and neutrons exist in levels, or
shells analogous to the shell structure that
exists for electrons in an atom. Filled shells of
electrons are associated with the special
stability of the noble gases. The total numbers
of electrons for these stable atoms are
2,10,18,36,54,86. Experimentally it is noted that
nuclie with certain numbers of protons or
neutrons appear to be very stable.
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These numbers are called the magic numbers and
associated with specially stable nuclei.
According to this theory, a magic number is the
number of nuclear particles in a completed shell
of protons and neutrons. For protons the magic
numbers are 2, 8,20, 28, 50, 82 . Neutrons have
these same magic numbers , as well as the magic
number 126. Some of the evidence for these magic
numbers, and therefore for the shell model of the
nucleus is as follows. Many radioactive nuclei
decay by emmitting alpha particles or helium
nuclei.
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There appears to be special stability in the
helium nucleus. It contains two protons and two
neutrons , which 2 is a magic number. Another
piece of evidence is seen in the final products
obtained in natural radioactive decay. For
example uranium-238 decays to thorium-234, which
in turn decays to protactinium- 234 and so forth.
Each product is radioactive and decays to another
nucleus until the final product 82Pb206 is
reached. This nucleus is stable. Note that it
cantains 82 protons, a magic number. Other some
radioactive series end with 82Pb208 and note that
it has magic number of neutrons (208-82126).
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A nucleus with a completely filled shell of
either protons or neutrons is said to be magic
because it is relatively more stable than nuclei
with either a larger or a smaller number of
nucleons. Most magic nuclei are spherical in
shape, but some nuclei can lower their energy
somewhat, and hence increase their stability, by
rearranging their protons and neutrons into
deformed shells accommodating a different number
of nucleons. The closing of these deformed shells
leads to deformed magic numbers.
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Another rule that can be useful in predicting the
nuclear stability is Nuclei with even number of
protons and even number of neutrons are more
stable that those with any other combination.
Conversely nuclei with odd numbers of both
protons and neutron are the least stable.
Remember that magic numbers are also even.
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The Odd-Even Rule In the odd-even rule, when the
numbers of neutrons and protons in the nucleus
are both even numbers, the isotopes tends to be
far more stable than when they are both odd. Out
of all the 264 stable isotopes, only 4 have both
odd numbers of both, whereas 168 have even
numbers of both, and the rest have a mixed
number.This has to do with the spins of
nucleons. Both protons and neutrons spin. When
two protons or neutrons have paired spins
(opposite spins), their combined energy is less
than when they are unpaired.
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Table 1The table showing the number of stable
isotopes according to the number of protons and
neutrons being even or odd.
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• Summary of the rules that are useful in
  predicting the nuclear stability
• All nuclides with 83 or more protons are unstable
  with respect to radioactive decay.
• Light nuclides are stable when atomic number
  equals to the number of neutrons that is when
  the neutron to proton ratio is 1. However for
  heavier elements the neutron to proton ratio
  required for stability is more than 1 and
  increases with the increase in the atomic number.
  
• Nuclides with even number of protons and neutrons
  are more stable compared to others.

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• Nuclei that contain a magic number of proton and
  neutron seems to be more stable.