Radioactivity and Nuclear Reaction Ch

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					RADIOACTIVITY AND
NUCLEAR REACTIONS

      Chapter 18
The Nucleus
  Protons and neutrons are packed tightly in
  the nucleus, where you find the majority of
  the atom’s mass.
The Strong Force
  Since protons repel each other, the strong
  force allows protons and neutrons to be
  attracted to each other. This is 4 x
  stronger than electric force! Unlike
  electricity, the strong force is a short-
  range force. When protons and neutrons
  move away from each other, the force
  weakens.
Strong Force vs. Large Nuclei
  Because there are so many protons and neutrons in
  large nuclei, the electric repulsion of all the protons
  is stronger than then short-range strong force.
  Therefore, protons and neutrons are held together
  less tightly in a large nucleus.

Instability and Radioactivity
  When the strong force is not strong enough to hold
  the nucleus permanently together, the nucleus
  begins to decay and give off matter and energy
  (radioactivity.) All nuclei containing more than 83
  protons are radioactive (although there are smaller
  radioactive nuclei.) Synthetic elements, produced
  in a lab, contain a large number of protons (92+), so
  they decay soon after they are created.
Radioactivity
Isotopes
   You already know that an isotope is an atom with a
   different number of neutrons to protons. Isotopes of
   heavier elements are stable when the ratio of
   neutrons to protons is about 3 to 2. If the ratios are
   different (less or greater), than the nucleus is
   considered unstable. This makes those isotopes
   radioactive.

   You can tell if an atom is radioactive by comparing
   the mass number (p + n) to the atomic number (p):
              C                               C
mass number   12           mass number   14

atomic number 6            atomic number 6



Which atom of carbon is radioactive?
The Discovery of Radioactivity

In 1896, Henri Becquerel left uranium salt in a desk
   drawer with a photographic plate. When he opened
   the plate later, he found an outline of clumps of the
   uranium salt. He realized that the saltss must have
   emitted some unknown invisible rays, or radiation,
   that darkened the film (it was an x-ray of the salt!)

  In 1898, Marie and Pierre Curie discovered 2 new
  elements (polonium and radium), that were also
  radioactive. They were able to obtain .1 g of radium
  for several tons of the mineral pitchblende after
  more than 3 years of experiments.
Nuclear Decay

The 3 types of nuclear radiation
 are alpha, beta and gamma
 radiation. Alpha and beta are
 particles, and gamma radiation
 is the resulting electromagnetic
 wave.
Nuclear (Radioactive)
       Decay
Alpha Particles
An alpha particle is made up of 2 protons and
2 neutrons that are emitted from the decaying nucleus.
It is the same as a He nucleus and has a charge of +2 and
an atomic mass of 4. It actually
becomes 4 He
          2

 Alpha particles are the largest with the most charge.
 They lose energy quickly when reacting with matter.
 When they go through matter, the electrons in the
 matter’s atoms react and are pulled away. This leaves
 behind positively charged ions. Because alpha particles
 quickly lose energy, they are the least penetrating form
 of nuclear radiation. They can be stopped by a sheet of
 paper.
Alpha Particles
Alpha particles can be dangerous inside the human
  body, as a single alpha particle can damage fragile
  biological molecules.

Alpha particles can be used in some smoke detectors.
  The detectors ionize the surrounding air. An
  electric current flows through this air to form a
  circuit, unless smoke particles break the circuit,
  causing the alarm to sound.
Transmutation
An atom loses 2 protons when it emits alpha
  particles, so it forms into a different element.
  Transmutation is the process of changing one
  element to another through nuclear decay.

The new element has 2 fewer protons and decreases
  the mass number by 4. The charge of the original
  nucleus = the sum of the charges of the nucleus
  and the alpha particle that are formed.
Transmutation of
Polonium into Lead
Polonium (Po) loses 2 protons (out of 84) and the
  mass number is decreased by 4:


210Po         206Pb     + 4He
+84            +82        +2
Beta Particles
Sometimes an unstable nucleus in a neutron decays
  into a proton, and then it emits an electron. This
  electron is called a beta particle. Beta decay is
  caused by a weak force.
Now the atom has one more proton than it did before
  the decay. This means that it goes through
  transmutation. However, the mass number doesn’t
  change during beta decay, so the mass number is
  the same as the original element.
Beta Decay: Iodine
Changing into Xenon
 131                                 131
        I                     +         Xe
 +53              -1                 +54

  Beta Particles (with a charge of -1) are faster and
 more penetrating than alpha particles. They can go
 through paper (but not a sheet of aluminum foil.)
 They can also cause damage to biological cells.
Gamma Rays
The most penetrating form of nuclear radiation (EM
  wave with highest frequency, shortest wavelength.)

They are energy (no mass or charge.) They are
  emitted from the nucleus when alpha or beta decay
  occurs. They can penetrate almost all solids
  except for exceptionally dense materials such as
  lead or concrete. However, because they produce
  no charge, they can actually due less damage than
  alpha particles.
Radioactive Half-Life
and Dating
A measure of the time required by the nuclei of an isotope
  to decay is called a half-life. The half-life of a radioactive
  isotope is the amount of time takes for half the nuclei in
  a sample of the isotope to decay.

Carbon Dating: Used to date once-living things. C14 has a
  half-life of 5,730 years, found in molecules of CO2. This is
  found in plants and in plant-eating animals.

Uranium Dating: Some rocks contain uranium isotopes.
  These isotopes decay into lead isotopes. The ratio of the
  uranium isotopes and the daughter nuclei (lead isotopes)
  is measured and the number of half-lives since the rock
  was formed can be calculated.
Detecting Radioactivity
Cloud Chambers: filled with water or ethanol vapor, in
  which a radioactive sample is placed. It gives off
  charged alpha or beta particles, which travel
  through the chamber. The particle knocks
  electrons of the air atoms in the chamber, creating
  ions. The water/ethanol vapor condenses around
  the ions, creating a visible path of droplets along
  the track of the particle (alpha = short, thick trails;
  beta = long, thin trails.)
Bubble Chambers
A bubble chamber holds superheated liquid (pressure
  prevents boiling.) When a moving particle leaves
  ions behind, the liquid boils along the trail.
Electroscopes
The electroscope has leaves at the bottom. If the
  scope gets an electric charge, the leaves repel
  each other. When an positive charge is introduced,
  then the excess negative charge is neutralized.
Nuclear radiation moving through the air can remove
  electrons from some air molecules, causing other
  air molecules to gain electrons. These positive air
  ions come in contact with the electroscope and
  attract the electrons from the leaves, so the leaves
  move back together.
Geiger Counter
A Geiger counter measures the amount of radiation by
   producing an electric current when it detects a
   charged particle.
It has a tube with a positive charge running through
   the center of a negatively charged copper cylinder.
   The tube is filled with gas at a low pressure. When
   radiation enters the tube at one end, It knocks
   electrons from the atoms of the gas. Then causes
   a chain reaction among the gas atoms, creating an
   “electron avalanche”. When a large number of
   electrons reaches the wire, a current is produced.
   The energy is turned into sound (clicking) and
   flashing light. The intensity of these energies
   measures the intensity of the radiation.
Background Radiation
Low-level radiation is naturally emitted by Earth’s
  rocks, soil and atmosphere. Traces can be found in
  building materials, plants and animals.

Most of the background radiation comes from the
  decay of radon gas. High levels can be very
  dangerous when it is trapped in homes. Sometimes
  radiation from cosmic rays can infiltrate the Earth’s
  atmosphere. Living organisms contain C14.
  Background radiation can never be completely
  eliminated.
Nuclear Reactions
Nuclear Fission
• Enrico Fermi thought that, by bombarding nuclei
  with neutrons, the neutrons would be absorbed
  (creating larger nuclei.) Instead, when a neutron
  struck a uranium-235 nucleus, it split apart into
  smaller nuclei. Fission means to divide.
• Only large nuclei can undergo nuclear fission. The
  products of fission includes both smaller nuclei and
  random neutrons. Some of the mass after fission is
  missing, because it turns into extreme energy.
  Einstein was the first to realize that the Law of
  Conservation of Mass and the Law of Conservation
  of Energy are actually tied to each other.
Nuclear Fission
Fission: Mass and
Energy
Einstein’s Theory of Relativity proposed that mass
  can be converted into energy, and energy could be
  converted into mass:
     Energy (J) = mass (kg)   x 300,000 km/s2 (speed of light)
                                 Or
                              E = mc2


If one gram of mass is converted into energy, then
   approximately 100 trillion joules of energy is
   released.
Fission: Chain Reactions
During nuclear fission, the neutrons emitted can strike other
   nuclei and cause them to split, thus releasing more
   neutrons. A series of repeated fission reactions cause
   the release of neutrons during each reaction.
Critical Mass is the amount of material required so that
   each fission reaction produces at least one more fission
   reaction. If there is not enough material, critical mass is
   not reached, and there is no chain reaction.

If the chain reaction isn’t controlled, then an enormous
    amount of energy is released. Chain reactions can be
    controlled by adding materials that absorb neutrons.
    Then the reaction continues at a constant rate.
Nuclear Fusion
Even more energy can be released when 3 nuclei with
  low masses are combined to form one nucleus with
  a larger mass. Fusion fuses atomic nuclei together.

An example would be 2 hydrogen (H) atoms combining
  to form a helium (He) atom with a larger nucleus.
Fusion and Temperature
Nuclear fusion requires positively charged nuclei to
  get close together. If they are moving very fast,
  then they would have the kinetic energy to
  overcome the repulsive electrical force.

Only at temperatures of millions of degrees Celsius
  are nuclei able to get close enough to fuse. The
  example below is the fusing of deuterium and
  tritium:
Nuclear Reactions and
Medicine
When a radioactive atom is introduced into the body,
  it can travel to specific parts and join other
  molecules, where it can be easily found.

These radioactive isotopes (radioisotopes) are called
  tracers. They can be followed through the body
  and show how particular organs are functioning.
  Examples are how problems are detected in the
  thyroid, heart or gall bladder.

Gamma rays can used with radioisotopes to target
  and destroy the fastest growing cells (tumors.)

				
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posted:7/28/2011
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