Modern Physics

Model of the atomRadioactivity

Introduction - Today we expand our discussion of explaining what happens at the nuclear level atoms.

• Radioactivity (substances that give of invisible radiations) and the types of radiations.

• What happens when an atom undergoes radioactivity decay

• Uses for radiation

• The types of reactions that cause nuclear reactions

• The affects of nuclear reactions on energy

Nucleons

• Are particles occupying the nucleus• Consist of + charged protons and neutral

neutrons• Have almost 2000 time the mass of electrons• Are made of quarks and leptons (building

blocks of matter, elementary particles)

AtomAtomic mass = protons + neutrons

(number of nucleons)

Atomic number = number of protons

Isotopes

• Atoms of the same element with different numbers of neutrons (different masses)

Radioactive Isotopes

• Has an unstable nucleus• Spontaneously emits a particle and decay into

another element.

Marie Curie – Nobel prize winner• The word radioactivity was

first used by Marie Curie in 1898.

• She used the word radioactivity to describe the property of certain substances to give off invisible “radiations” that could be detected by films.

Radioactive Decay• Three different kinds of

radiation given off by radioactive materials:– Alpha rays– Beta rays– Gamma rays

• called “rays” because the radiation carried energy and moved in straight lines, like light rays.

• Radioactivity comes from the nucleus of the atom.

• If the nucleus has too many neutrons, or is unstable the atom undergoes radioactive decay.

• decay - to "break down."

Atomic Decay• Alpha decay: the nucleus ejects two protons and two

neutrons.• Beta decay: a neutron in the nucleus splits into a proton

and an electron.• Gamma decay occurs because the nucleus is at too high

an energy. The nucleus falls down to a lower energy state and, in the process, emits a high energy photon.

• Radioactive decay gives off energy. • The energy comes from the conversion of

mass into energy.• Because the speed of light (c) is such a

large number, a tiny bit of mass generates a huge amount of energy.

• Radioactivity occurs because everything in nature tends to move toward lower energy.

Radiation• The flow of energy through space. • Forms of radiation:

– Light– Radio– Microwaves– X-rays

• Many people mistakenly think of radiation as only associated with nuclear reactions.

X-ray machines

• X-rays are photons• Used to produce

images of bones and teeth on x-ray film.

• X-ray film turns black when exposed to x-rays.

X-Rays Uses• High level therapeutic

x-rays are used to destroy diseased tissue, such as cancer cells.

• The beams are made to overlap at the place where the doctor wants to destroy diseased cells.

CAT scan• Computerized Axial

Tomography• Produced by a computer that

controls an x-ray machine as it takes pictures of the body from different angles.

• Produces three-dimensional images of bones and other structures within the body.

Radiation Detection

The Geiger counter is a type of radiation detector invented to tell when radiation is present and to measure its intensity.

Half-life

• The time it takes for half the mass of a radioactive sample to decay.

• Ranges from a fraction of a second to billions of years.

• Is not affected by external conditions.

Fusion reactions

• Nuclear reaction that combines, or fuses, two smaller nuclei into a larger nucleus.

• It is difficult to make fusion reactions occur because positively charged nuclei repel each other.

Fission reactions

• A fission reaction splits up a large nucleus into smaller pieces.

• A fission reaction typically happens when a neutron hits a nucleus with enough energy to make the nucleus unstable.

Nuclear Reactions and Energy

• A nuclear reaction is any process that changes the nucleus of an atom.

• Radioactive decay is one form of nuclear reaction.

Nuclear Reactions and Energy• If you could take apart a nucleus and separate

all of its protons and neutrons, the separated protons and neutrons would have more mass than the nucleus did.

• The mass of a nucleus is reduced by the energy that is released when the nucleus comes together.

• Nuclear reactions can convert mass into energy.

• Both these nuclear reactions release a small portion of the mass as large amounts of energy.

• Nuclear fusion is what powers a modern nuclear warhead (hydrogen bomb)

• Nuclear fission (less powerful) occurs in an atomic bomb (like the ones used against Japan in WWII), or in a nuclear power plant.

Nuclear Reactions and Energy