Radioactivity and Nuclear Reactions

Section 1: Radioactivity

• Review– What are atoms composed of?– What is the charge of each of the particles?– Where is each particle located?– What is the atomic number?

The Strong Force

• The strong force causes protons and neutrons to be attracted to each other.

• The strong force is about 100 x greater than the electric force (the force that would cause the protons to repel each other).

• The strong force is a short range force that quickly becomes extremely weak as protons and neutrons get farther apart.

Radioactivity• radioactivity—process of nuclear decay

• All nuclei that contain more than 83 protons are radioactive.

• Almost all elements with more than 92 protons don’t exist naturally on Earth. They have been produced only in laboratories.

Isotopes• isotopes—nuclei having the same number

of protons but different numbers of neutrons

• The atoms of isotopes of an element have the same number of electrons and the same chemical properties.

Nucleus Numbers

Discovery of Radioactivity• Henri Becquerel accidentally discovered radioactivity in

1896.

• Pierre and Marie Curie, shown here in their laboratory in Paris, helped pioneer the study of radioactivity, unaware of the deadly hazards. Even with deteriorating health, the Curies continued their research. Much of today’s nuclear science is based on their work.

Section 2: Nuclear Decay• When an unstable nucleus decays,

particles and energy are emitted from the decaying nucleus.

• These particles and energy are called nuclear radiation.

• The three types of nuclear radiation are alpha, beta, and gamma radiation.

Alpha Particles

• An alpha particle is made of two protons and two neutrons bound together.

• charge: +2

• atomic mass: 4

Transmutation in Alpha Radiation

• When an atom loses an alpha particle, it no longer has the same number of protons, so it no longer is the same element.

• Transmutation is the process of changing one element to another through nuclear decay.

Beta Particles

• Sometimes in an unstable nucleus, a neutron decays into a proton and releases an electron.

• beta particle: the electron emitted from the nucleus during beta decay

• Beta decay is caused by the weak force.

Transmutation in Beta Radiation• During beta decay, the atom now has one more

proton so it becomes the element with an atomic number one greater than that of the original element.

• Since the neutron decays into a proton, the total number of protons and neutrons does not change during beta transmutation, so the atomic mass number stays the same.

Gamma Rays

• Gamma rays are a form of radiation called electromagnetic waves (so they carry energy).

• They have no mass and no charge.• They travel at the speed of light and are usually

released along with alpha and beta particles.• Because gamma rays only transfer energy and

not particles, no transmutation occurs.

Comparison of the Three Types of Nuclear Radiation

• Compared to beta particles and gamma radiation, alpha particles are much more massive.

• Alpha particles also have the most electric charge. +2 compared to -1 compared to no charge

Comparison of the Three Types of Nuclear Radiation—continued

• Alpha particles are the least penetrating form of nuclear radiation.

• Alpha particles cannot even pass through a sheet of paper.

Comparison of the Three Types of Nuclear Radiation—continued

• Beta particles are much faster and more penetrating than alpha particles.

• They can pass through paper but are stopped by a sheet of aluminum foil or plastic..

Comparison of the Three Types of Nuclear Radiation—continued

• Thick blocks of dense material, such as lead and concrete, are required to stop gamma rays.

Comparison of the Three Types of Nuclear Radiation—continued

Radioactive Half-Life

• The half-life of a radioactive isotope is the amount of time it takes for half the nuclei in a sample of the isotope to decay.

• The nucleus left after the isotope decays is called the daughter nucleus.

Radioactive Dating

• Carbon Dating– The half-life of a carbon-14 atom is 5,730

years.– The age of the remains of plants and animals

that lived within the last 50,000 years can be measured using carbon-14.

• Uranium Dating– can be used to measure the age of rocks

Carbon Dating

Section 4: Nuclear Reactions

Nuclear Fission

• nuclear fission—process of splitting a nucleus into two nuclei with smaller masses

• Only large nuclei, such as the nuclei of uranium and plutonium atoms, can undergo fission.

• The products of fission usually include several individual neutrons in addition to the smaller nuclei.

• The small amount of missing mass is converted to a tremendous amount of energy.

Chain Reactions

• A chain reaction is an ongoing series of fission reactions.

• Billions of reactions can occur each second during a chain reaction, resulting in the release of tremendous amounts of energy.

Critical Mass

• The critical mass is the amount of fissionable material required so that each fission reaction produces approximately one more fission reaction.

• Nuclear power plants use control rods made of nonfissionable material that can absorb neutrons.

• The control rods are moved in and out of the fissionable material to control the rate of the chain reaction.

Nuclear Fusion

• In nuclear fusion, two nuclei with low masses are combined to form one nucleus of larger mass.

• Temperature and Fusion

The Sun• Most of the energy given off by the Sun is

produced by a process involving the fusion of hydrogen nuclei.

• Earth receives a small amount of this energy as heat and light.

Using Nuclear Reactions in Medicine

• What is a tracer?

• What can tracers be used for?

• What happens to the cells in a person’s body when they have cancer?

• How can radiation be used to treat cancer?

• How can physicians be sure that only cancer cells will absorb radiation?