Unit 9, Chapter 30 Radioactivity

Vocabulary Terms  radioactive  alpha decay  beta decay  gamma decay  radiation  isotope  radioactive decay  fission reaction  CAT scan  fusion reaction  Geiger counter  nuclear waste  neutron  x-ray  background radiation  fallout  detector  half-life

Radioactivity  The word radioactivity was first used by Marie Curie in  She used the word radioactivity to describe the property of certain substances to give off invisible “radiations” that could be detected by films.

Radioactivity  Scientists quickly learned that there were three different kinds of radiation given off by radioactive materials. —Alpha rays —Beta rays —Gamma rays  The scientists called them “rays” because the radiation carried energy and moved in straight lines, like light rays.

Radioactivity  We now know that radioactivity comes from the nucleus of the atom.  If the nucleus has too many neutrons, or is unstable for any other reason, the atom undergoes radioactive decay.  The word decay means to "break down."

Radioactivity  In alpha decay, the nucleus ejects two protons and two neutrons.  Beta decay occurs when a neutron in the nucleus splits into a proton and an electron.  Gamma decay is not truly a decay reaction in the sense that the nucleus becomes something different.

Radioactivity  Radioactive decay gives off energy.  The energy comes from the conversion of mass into energy.  Radioactivity occurs because everything in nature tends to move toward lower energy.

Radioactivity  Radioactive decay depends on chance.  It is possible to predict the average behavior of lots of atoms, but impossible to predict when any one atom will decay.  One very useful prediction we can make is the half-life.  The half-life is the time it takes for one half of the atoms in any sample to decay.

30.1 Half-life  The half-life of carbon-14 is about 5,700 years.  If you start out with 200 grams of C-14, 5,700 years later only 100 grams will still be C-14.  The rest will have decayed to nitrogen-14.

Half-life  Most radioactive materials decay in a series of reactions.  Radon gas comes from the decay of uranium in the soil.  Uranium (U-238) decays to radon-222 (Ra-222).

Applications of radioactivity  Many satellites use radioactive decay from isotopes with long half-lives for power because energy can be produced for a long time without refueling.  Isotopes with a short half-life give off lots of energy in a short time and are useful in medical imaging, but can be extremely dangerous.  The isotope carbon-14 is used by archeologists to determine age.

Carbon dating  Living things contain a large amount of carbon.  When a living organism dies it stops exchanging carbon with the environment.  As the fixed amount of carbon-14 decays, the ratio of C-14 to C-12 slowly gets smaller with age.

Calculating with isotopes  A sample of 1,000 grams of the isotope C-14 is created.  The half-life of C-14 is 5,700 years.  How much C-14 remains after 28,500 years?

Step 1  Set up a function chart  Column 1 will be for “time” that passes  Column 2 will be for amount  Start Column 1 at zero  Start Column 2 with the amount you have

Time (in this column you add) Amount (in this column you divide by 2) g

Column 1  Add your half life amount  Keep going until you get to the total amount of time that has passed!

Time (in this column you add) Amount (in this column you divide by 2) g

Time (in this column you add) Amount (in this column you divide by 2) g

Time (in this column you add) Amount (in this column you divide by 2) g

Time (in this column you add) Amount (in this column you divide by 2) g

Time (in this column you add) Amount (in this column you divide by 2) g

After you get the time to what the problem gives you – start filling in column 2  Remember DIVIDE by 2

Time (in this column you add) Amount (in this column you divide by 2) g 500 g

Time (in this column you add) Amount (in this column you divide by 2) g 500 g 250 g

Time (in this column you add) Amount (in this column you divide by 2) g 500 g 250 g 125 g

Time (in this column you add) Amount (in this column you divide by 2) g 500 g 250 g 125 g 62.5 g

Time (in this column you add) Amount (in this column you divide by 2) g 500 g 250 g 125 g 62.5 g g (amount remaining)

Radiation Key Question: What are some types and sources of radiation? *

Radiation  The word radiation means the flow of energy through space.  There are many forms of radiation.  Light, radio waves, microwaves, and x-rays are forms of electromagnetic radiation.  Many people mistakenly think of radiation as only associated with nuclear reactions.

Harmful radiation  Radiation becomes harmful when it has enough energy to remove electrons from atoms.  The process of removing an electron from an atom is called ionization.  Visible light is an example of nonionizing radiation.  UV light is an example of ionizing radiation.

Harmful radiation  Ionizing radiation absorbed by people is measured in a unit called the rem.  The total amount of radiation received by a person is called a dose, just like a dose of medicine.  It is wise to limit your exposure to ionizing radiation whenever possible.  Use shielding materials, such as lead, and do your work efficiently and quickly.  Distance also reduces exposure.

Sources of radiation  Ionizing radiation is a natural part of our environment.  There are two chief sources of radiation you will probably be exposed to: —background radiation. —radiation from medical procedures such as x-rays.  Background radiation results in an average dose of 0.3 rem per year for someone living in the United States.

Background radiation  Background radiation levels can vary widely from place to place. —Cosmic rays are high energy particles that come from outside our solar system. —Radioactive material from nuclear weapons is called fallout. —Radioactive radon gas is present in basements and the atmosphere.

X-ray machines  X-rays are photons, like visible light photons only with much more energy.  Diagnostic x-rays are used to produce images of bones and teeth on x-ray film.  Xray film turns black when exposed to x-rays.

X-ray machines  Therapeutic x-rays are used to destroy diseased tissue, such as cancer cells.  Low levels of x-rays do not destroy cells, but high levels do.  The beams are made to overlap at the place where the doctor wants to destroy diseased cells.

CAT scan  The advent of powerful computers has made it possible to produce three-dimensional images of bones and other structures within the body.  To produce a CAT scan, computerized axial tomography, a computer controls an x-ray machine as it takes pictures of the body from different angles.

CAT scan  People who work with radiation use radiation detectors to tell when radiation is present and to measure its intensity.  The Geiger counter is a type of radiation detector invented to measure x-rays and other ionizing radiation, since they are invisible to the naked eye.

Geiger Counter  These make a “clicking” sound when radiation is present…the higher the radiation the faster the clicking.

Nuclear Reactions and Energy Key Question: How do we describe nuclear reactions?

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.

Fusion reactions  A fusion reaction is a 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.

Fusion Reactions  It takes extremely high temperatures to get a fusion reaction to occur:  On the sun, hydrogen atoms are fused to form helium atoms…all the “sunlight” that we get is the energy released in this fusion reaction.

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.

Fission Reactions  To keep a fission reaction going, you have to continue the rate at which the atoms spilt…this is called a “chain” reaction.

Alpha Decay

Fission Reaction involving alpha decay, can you figure out what new element is produced?

Beta Decay

Fission Reaction involving beta decay, can you figure out what new element is produced?

Neutron

Nuclear Reaction involving a neutron, can you figure out what new element is produced?

Positron

Nuclear Reaction involving a positron, can you figure out what new element is produced?

Fission Reactions  These have given us the atomic bomb – ended WWII  They also give us cheap, clean electricity from nuclear power plants.

Application: Nuclear Power