P. Sci.

Unit 12Nuclear Radiation

Chapter 10

Essential Questions

1) Identify four types of nuclear radiations and compare and contrast their properties

2) Distinguish between fission and fusion, and provide examples of each

3) Describe the dangers and possible health effects of exposure to nuclear radiations

4) How do you calculate the half-life of radioactive isotopes.

Nuclear Radiation

• Strong Nuclear force – the force that holds protons and neutrons together.

• Remember that like charges repel each other. The strong nuclear force is what holds the positive protons together.

• Very large atoms are unstable because of their size. (uranium and the transuranium elements) the nuclear strong forces have a hard time holding the nucleus together.

• Over time the nucleus will decay (break up) When this happens, particles (protons or neutrons) or energy is emitted by the nucleus.

electron

neutron

proton

Radioactivity• Process of nuclear

decay in which an unstable nucleus emits particles or energy in the form of electromagnetic radiation.

Radiation vs Nuclear Radiation

• Radiation – Transfer of energy by electromagnetic waves. Also a type of heat transfer (conduction, convection, radiation).

• Nuclear Radiation – radiation associated with nuclear changes.

Types of Nuclear Radiation

1) Alpha particles2) Beta particles3) Gamma rays4) Neutron emissions

Alpha Particles• Made of 2 protons and 2 neutrons

(the same as a helium nuclei)• Positively charged• Largest particle• Can be stopped bya sheet of paper.

Beta Particles

• Made of fast moving electrons• Negatively charged• Small – fast moving particles• Stopped by thin metal or wood

Beta cont.

• Occurs when a neutron (no charge) decays to form a proton and an electron. The electron, having very little mass, is then ejected from the nucleus at a high speed.

Gamma Rays• Not made of matter – this is a form of

electromagnetic energy • No charge • High energy• Can penetrate 7cm of lead• Pose a danger to health

NeutronEmission

• Made of a neutron• The atom becomes a different isotope(same # of protons – different # of neutrons)

• A hazard of neutron radiation is neutron activation

+

Neutron Activation• the ability of neutron radiation to

induce radioactivity in most substances it encounters, including the body tissues of people.

• accounts for much of the radioactive material released by the detonation of a nuclear weapon.

Neutron activation cont.• It is also a problem in

nuclear fission and nuclear fusion installations, because it gradually makes the equipment radioactive; eventually the equipment must be replaced and disposed of as low-level radioactive waste.

Half-Life• During radioactive decay – the time it

takes for half the atoms to decay is the “half-life” of that substance.

• The half-life of each radioactive substance is different.

• A half-life can be a millionth of a second to billions of years.

Half-Life progression

First Second Third

Fourth Fifth

Etc.

Half-Life progression of Iodine 131

First Second Third

Fourth Fifth

Etc.

194.4 hr.(8.1 days)

388.8 hr.(16.2 days)

777.6 hr.(32.4 days)

972.0 hr.(40.5 days)

583.2 hr.(24.3 days)

8.1 days later 8.1 days later

8.1 days later

• If you know how much of a particular radioactive isotope was present in an object at the beginning, you can predict how old the object is.

• Carbon 14 is used to date organic (plant or animal) remains.

Fission vs. Fusion• Fission is the process by which a

nucleus splits into two or more smaller fragments, releasing neutrons and energy.

• Fusion is the process in which light nuclei combine to form heavier nuclei and releasing energy.

Energy

• Theory of relativity (Einstein) – matter can be converted into energy and energy into matter. E = m x c2.

• The mass-equivalent energy of 1 kg of matter is more than the chemical energy of 8 million tons of TNT.

Fission

• What occurs in nuclear reactors.• Example: Uranium-235 when hit with

neutrons produce barium-137 and krypton-84 as well as 15 neutrons and energy.

Chain Reaction• A chain reaction refers to a process in

which neutrons released in fission produce an additional fission in at least one further nucleus. This nucleus in turn produces neutrons, and the process repeats. The process may be controlled (nuclear power) or uncontrolled (nuclear weapons).

Critical Mass• The minimum amount of a substance that can sustain a chain reaction.

• It takes very little Uranium-235 to reach critical mass.

Control Rods• In a nuclear reactor the chain reaction

is controlled by rods of materials (like cadmium) that absorb some of the neutrons thus slowing and controlling the chain reaction.

Fusion• What occurs in stars (including our

sun)• Four hydrogen atoms fuse together in

the sun to produce a helium atom.• A large amount of energy is needed to

start the raction.

Fusion Reactors• Fusion reactions produce much more

energy per gram of fuel and produce less radioactive waste than fission.

• So why do we not have fusion reactors?

Answer • Fusion can occur only in the plasma

state of matter (super-heated gas). Once it gets going it generates enough heat to keep going as long as there is fuel. It requires a heat of about 10 million degrees Celsius. Scientist have to find a way of producing and containing that much heat.

Extra Credit

• Explain what “cold fusion” is and what has been done in this area.

Dangers and Benefits• Dangers –

–nuclear waste–Nuclear radiation

• Benefits – –Medical

• Cancer Treatment• Radioactive tracers

–Nuclear Power