Transcript Radioactivity - Williamstown Independent Schools

Radioactivity
What is Radiation?
• Radiation - particles or energy released
from a nucleus during radioactive decay.
• Radioactive decay is the spontaneous
disintegration of a nucleus into a smaller
nucleus
Pioneers of Radioactivity
Clockwise from left: Wilhelm Roentgen,
Henri Becquerel, Marie and Pierre
Curie, Hans Geiger, and Enrico Fermi.
The Nucleus
• The nucleus is
composed of nucleons,
the protons and
neutrons.
• Protons - positive
• Neutrons - neutral
Mass Defect
• Mass defect is the difference between the
mass of all the particles in an atom and the
actual mass of the atom.
• This “missing mass” comes about when the
nucleus forms and a small amount of mass
is converted to energy.
• E=mc2
Quarks
• Electrons are categorized as “leptons”
because they cannot be broken down into
small particles.
• Protons and Neutrons consist of smaller
particles called “quarks”
• There are six types of quarks: up, down,
strange, charm, top and bottom.
Quarks
• A proton consists of 2
up quarks and 1 down.
• A neutron consists of 2
down quarks and 1 up.
Nuclear Binding Energy
• The energy released
when a nucleus forms.
• The higher the energy
the more stable the
nucleus.
• This binding energy is
reported per nucleon.
What makes an atom radioactive?
• A nucleus may be
unstable if the ratio
between neutrons and
protons is too high or
too low.
• As the number of
protons increases so
must the number of
neutrons.
Nuclear Shell Model
• The idea that nucleons are arranged in the
nucleus in levels.
• Supported by “magic numbers” that
represent full levels.
• 2,8,20,28,50, 82 and 126
• Having those numbers of nucleons are most
stable.
What happens to a radioactive
nucleus?
• A nuclear reaction is a
change in the nucleus.
• A transmutation is a
type of change where
the identity of the
atoms changes.
• What would change
the identity of an
atom?
Nuclear Reaction Examples
In each example the total mass
number and atomic number is
conserved!
Alpha Decay
• Alpha decay of a
nucleus produces a
lighter nucleus and an
alpha particle.
• Mass # -4
• Atomic # -2
• An alpha particle (α) is
made up of 2 protons
and 2 neutrons.
Beta Decay
• Beta decay of a
nucleus results in a
new element (atomic
number +1) with the
same mass and a beta
particle.
• A beta particle (β) is
the same as an
electron.
Positron emission
• A positron is an
antimatter particle – a
“positive electron”
• Mass unchanged
• Atomic number
decreases by 1
• PET scans – Positron
Emission Tomography
Electron Capture
• Electron captured by
its own nucleus
• Mass unchanged
• Atomic number goes
down by 1
Gamma Decay
• During Gamma (γ)
decay only energy is
released from the
nucleus.
• Gamma decay often
follows other types of
changes.
Rate of Decay
• Half-life is the term used to describe the
time needed for half the atoms in a sample
of radioactive material to change.
• Half-life times can vary greatly:
– Cobalt – 60
– Carbon – 14
– Uranium – 238
10.5 minutes
5715 years
4.5 billion years
Half – life Diagram
Decay Series
Artificial Transmutations
• New man-made
(synthetic) elements
can be made in
particle accelerators.
• Produces the
Transuranium
elements.
Radiation exposure
• Alpha – low energy,
don’t penetrate
• Beta – higher energy,
penetrate, short range
• Gamma – much higher
energy, hard to stop.
Detecting Radiation
• Geiger counters
measure radiation by
detecting gas ionized
(charged) by
radioactive particles.
• Film badges can
measure exposure to
radiation.
Sources of Radiation
• Radiation is emitted by almost everything.
Rocks, soil, the air and living things all
contain radioactive elements. We are
exposed to radiation from the sun, distant
stars and from our own planet.
• In addition, nuclear testing has added
radiation to our environment.
Using Radiation
Radioactive Dating
• Determining the
approximate age of a
material by measuring the
amount of radioactive
material left over and
comparing it to the
amount that has changed.
• This allows us to estimate
the age of rocks, organic
materials and the Earth!
Medicine
• Radioactive tracers are
materials that can be
safely placed in the
body to track
movement of
materials.
• Radiation can also be
used to kill cancerous
cells.
Agriculture
• Tracers can be used to
track movement of
materials in plants.
• Radiation can be used
to kill insects and
bacteria.
Energy
• Nuclear power plants
use heat from nuclear
reactions to generate
electricity.
• Process uses
– Fuel rods
– Control rods
– Moderators
Nuclear Waste
• Waste must be given time
to decay into safer
elements, and that may
take thousands of years.
• Currently waste is
contained at 77 sites
across the country.
• The materials are buried in
specially designed cases.
Fission
• In fission a heavy
nucleus splits to form
lighter nuclei with the
release of energy.
• Fission may start a
chain reaction where
one split starts more.
• Fission is used in
nuclear power plants.
Fusion
• In fusion lighter nuclei
combine to form a heavier
nucleus with the release of
energy.
• Uncontrolled fusion is the
process in more advanced
nuclear weapons.
• Stars uses fusion to
combine hydrogen atoms
into helium. Energy is
released.