Nuclear Physics

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Nuclear Physics

• Most of the great success of the technology of
  the world around us results from our ability to
  manipulate atoms and their electrons.
• An atom has another important component, the
  nucleus.
• What can we accomplish by manipulating nuclei?

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Nuclear Structure
Slide 30-12
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Structure and Properties of the Nucleus
The Nucleus is the (tiny) central part of an
atom. The Nucleus is made of protons and
neutrons Proton has positive charge
Neutron is electrically neutral
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Structure and Properties of the Nucleus
Neutrons and protons are collectively called
nucleons. The different nuclei are referred to as
nuclides. Number of protons atomic number,
Z Number of nucleons atomic mass number,
A Neutron number N A - Z
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Structure and Properties of the Nucleus
A and Z are sufficient to specify a nuclide.
Nuclides are symbolized as follows
X is the chemical symbol for the element it
contains the same information as Z but in a more
easily recognizable form.
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Structure and Properties of the Nucleus
Nuclei with the same Z so they are the same
element but different N are called isotopes.
For many elements, several different isotopes
exist in nature.
Different isotopes of the same element have the
same atomic number but different mass numbers.
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Checking Understanding

• How many neutrons are in the following isotope?
  (The isotope may be uncommon or unstable.)
• 8
• 7
• 6
• 5
• 4

Slide 30-13
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Answer

• How many neutrons are in the following isotope?
  (The isotope may be uncommon or unstable.)
• 8
• 7
• 6
• 5
• 4

Slide 30-14
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Structure and Properties of the Nucleus
Masses of atoms are measured with reference to
the carbon-12 atom, which is assigned a mass of
exactly 12u. A u is a unified atomic mass unit.
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Structure and Properties of the Nucleus
From the following table, you can see that the
electron is considerably less massive than a
nucleon.
E mc2 or m E/c2
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Binding Energy and Nuclear Forces
The total mass of a stable nucleus is always less
than the sum of the masses of its separate
protons and neutrons. Where has the mass gone?
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Binding Energy and Nuclear Forces
It has become energy, such as radiation or
kinetic energy, released during the formation of
the nucleus.
This difference between the total mass of the
constituents and the mass of the nucleus is
called the total binding energy of the nucleus.
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Binding Energy of a Helium Nucleus
Slide 30-27
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Binding Energy
Slide 30-26
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Binding Energy
Slide 30-26
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Binding Energy and Nuclear Forces
To compare how tightly bound different nuclei
are, we divide the binding energy by A to get the
binding energy per nucleon.
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Curve of Binding Energy

• Light nuclei can become more stable through
  fusion.
• Heavy nuclei can become more stable through
  fission.
• All nuclei larger than a certain size
  spontaneously fission.

Slide 30-28
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Binding Energy and Nuclear Forces
The force that binds the nucleons together is
called the strong nuclear force. It is a very
strong, but short-range, force. It is
essentially zero if the nucleons are more than
about 10-15 m apart. The Coulomb force is
long-range this is why extra neutrons are needed
for stability in high-Z nuclei.
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Nuclear Forces
Slide 30-29
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Binding Energy and Nuclear Forces
The higher the binding energy per nucleon, the
more stable the nucleus. More massive nuclei
require extra neutrons to overcome the Coulomb
repulsion of the protons in order to be stable.
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Binding Energy and Nuclear Forces
Nuclei that are unstable decay many such decays
are governed by another force called the weak
nuclear force.
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Radioactivity
Towards the end of the 19th century, minerals
were found that would darken a photographic plate
even in the absence of light. This phenomenon is
now called radioactivity. Marie and Pierre Curie
isolated two new elements that were highly
radioactive they are now called polonium and
radium.
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Radioactivity
Alpha and beta rays are bent in opposite
directions in a magnetic field, while gamma rays
are not bent at all.
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Radioactivity

• Radioactive rays were observed to be of three
  types
• Alpha rays, which could barely penetrate a piece
  of paper
• Beta rays, which could penetrate 3 mm of
  aluminum
• Gamma rays, which could penetrate several
  centimeters of lead
• We now know that alpha rays are helium nuclei,
  beta rays are electrons, and gamma rays are
  electromagnetic radiation.

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Alpha Decay
Example of alpha decay Radium-226 will
alpha-decay to radon-222
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Beta Decay
Slide 30-34
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Half Life

• Radioactive decay is random. We can only know
  what the probability of decay is.
• Different nuclei have different probabilities of
  decay.
• Half life is the time you would have to wait for
  half the nuclei to decay.

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Raise your Hands

• Put your hands down if your birthday occurs
• In the first 6 months (Jan-June) of the year
• July, August, September
• Between Oct. 1 and Nov. 15
• After Dec. 8
• Nov.15-30
• HAPPY BIRTHDAY!

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Half Life
Slide 30-44
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Half Life
N0 is the initial number of nuclei t1/2 is the
half life If t t1/2 N, the number of nuclei
left, will be ½ N0 If t 2t1/2 N, the number of
nuclei left, will be ¼ N0
Slide 30-44
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Example Problem Decay Times
The Chernobyl nuclear reactor accident in the
Soviet Union in 1986 released a large plume of
radioactive isotopes into the atmosphere. Of
particular health concern was the short-lived
(half life 8.0 days) isotope 131I, which, when
ingested, is concentrated in and damages the
thyroid gland. This isotope was deposited on
plants that were eaten by cows, which then gave
milk with dangerous levels of 131I. This milk
couldnt be used for drinking, but it could be
used to make cheese, which can be stored until
radiation levels have decreased. How long would a
sample of cheese need to be stored until the
number of radioactive atoms decreased to 3 of
the initial value?
Slide 30-46