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 10.5 minutes– Carbon – 14 5715 years– Uranium – 238 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.