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Un it 9, Ch apter 30

CPO Scien c e

Fou nd at ion s o f Phy s ics



Unit 9: The Atom

30.1 Radioactivity

30.2 Radiation

30.3 Nuclear Reactions and Energy

Chapter 30 Nuclear Reactions and Radiation



Chapter 30 Objectives1. Describe the cause and types of radioactivity.

2. Explain why radioactivity occurs in terms of energy.

3. Use the concept of half-life to predict the decay of aradioactive isotope.

4. Write the equation for a simple nuclear reaction.

5. Describe the processes of fission and fusion.

6. Describe the difference between ionizing andnonionizing radiation.

7. Use the graph of energy versus atomic number to

determine whether a nuclear reaction uses or releases energy.



Chapter 30 Vocabulary Termsradioactivealpha decaybeta decay

gamma decayradiationisotoperadioactive decay

energy barrier intensityinverse square law

shieldingfission reactionCAT scan

ionizingnonionizingultravioletfusion reaction

Geiger counter remnuclear waste

neutronantimatter x-ray

neutrinobackgroundradiationdose

falloutdetector half-life



30.1 Radioactivity Key Question:How do we model radioactivity?

*Students read Section 30.1 AFTER Investigation 30.1



30.1 Radioactivity

The word radioactivity was first

used by Marie Curie in 1898.She used the word radioactivityto describe the property of

certain substances to give off invisible “radiations” that could

be detected by films.



30.1 RadioactivityScientists quickly learned thatthere were three different kindsof radiation given off byradioactive materials.— Alpha rays— Beta rays

— Gamma raysThe scientists called them “ rays ”

because the radiation carriedenergy and moved in straightlines, like light rays.



30.1 RadioactivityWe now know thatradioactivity comes from thenucleus of the atom.

If the nucleus has too manyneutrons, or is unstable for any other reason, the atomundergoes radioactive decay .The word decay means to"break down."



30.1 RadioactivityIn alpha decay , the nucleus ejects two protons and twoneutrons.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 sensethat the nucleus becomes something different.



30.1 RadioactivityRadioactive decay gives off energy.The energy comes from the conversion of mass

into energy.Because the speed of light ( c ) is such a largenumber, a tiny bit of mass generates a hugeamount of energy.

Radioactivity occurs because everything in naturetends to move toward lower energy.



30.1 RadioactivityIf you started with one kilogram of C-14 it would decay into0.999988 kg of N-14.The difference of 0.012 grams is converted directly into

energy via Einstein’s formula E = m c 2.



30.1 RadioactivitySystems move from higher energy to lower energy over time.A ball rolls downhill to the lowest point or a hot cup of

coffee cools down. A radioactive nucleus decays because the neutrons andprotons have lower overall energy in the final nucleus thanthey had in the original nucleus.



30.1 RadioactivityThe radioactive decay of C-14 does not happenimmediately because it takes a small input of energy tostart the transformation from C-14 to N-14.

The energy needed to start the reaction is called an energybarrier.The lower the energy barrier, the more likely the atom is todecay quickly.



30.1 RadioactivityRadioactive decay depends on chance.

It is possible to predict the average behavior of lots of atoms, but impossible to predict when anyone atom will decay.

One very useful prediction we can make is thehalf-life.

The half-life is the time it takes for one half of theatoms in any sample to decay.



30.1 Half-lifeThe half-life of carbon-14is about 5,700 years.

If you start out with 200grams of C-14, 5,700years later only 100

grams will still be C-14.

The rest will havedecayed to nitrogen-14.



30.1 Half-lifeMost radioactive materialsdecay in a series of

reactions.

Radon gas comes fromthe decay of uranium in

the soil.

Uranium (U-238) decaysto radon-222 (Ra-222).



30.1 Applications of radioactivityMany satellites use radioactive decay fromisotopes with long half-lives for power because

energy can be produced for a long time withoutrefueling.

Isotopes with a short half-life give off lots of

energy in a short time and are useful in medicalimaging, but can be extremely dangerous.

The isotope carbon-14 is used by archeologists to

determine age.



30.1 Carbon dating

Living things contain a large amount of carbon.

When a living organism dies it stops exchangingcarbon with the environment.

As the fixed amount of carbon-14 decays, the ratio of

C-14 to C-12 slowly gets smaller with age.



30.1 Calculating with isotopes A sample of 1,000 grams of the isotope C-14 is created.

The half-life of C-14 is5,700 years.

How much C-14 remainsafter 28,500 years?



30.2 Radiation

Key Question:

What are some typesand sources of radiation?

*Students read Section 30.2AFTER Investigation 30.2



30.2 Radiation

The word radiation means the flow of energy

through space.There are many forms of radiation.

Light, radio waves, microwaves, and x-rays areforms of electromagnetic radiation.

Many people mistakenly think of radiation as onlyassociated with nuclear reactions.



30.2 RadiationThe intensity of radiation measures how much power flows per unit of area.When radiation comes from a single point, theintensity decreases inversely as the square of thedistance.This is called the inverse square law and it applies toall forms of radiation.



30.1 IntensityI = P

A

Power (watt)

Area (m 2)

Intensity (W/m 2)

Intensity = 7.96 W/m 2 Intensity = 1.99 W/m 2



30.2 Harmful radiationRadiation becomesharmful when it hasenough energy to remove

electrons from atoms.The process of removingan electron from an atomis called ionization .

Visible light is anexample of nonionizingradiation .UV light is an example of

ionizing radiation .



30.2 Harmful radiationIonizing radiation absorbed by people is measuredin a unit called the rem .

The total amount of radiation received by a personis called a dose, just like a dose of medicine.It is wise to limit your exposure to ionizing radiationwhenever possible.Use shielding materials, such as lead, and do your work efficiently and quickly.Distance also reduces exposure.



30.2 Sources of radiationIonizing radiation is a natural part of our environment.

There are two chief sources of radiation you willprobably 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 UnitedStates.



30.2 Background radiationBackground radiationlevels can vary widelyfrom place to place .— Cosmic rays are high

energy particles that comefrom outside our solar system.

— Radioactive material fromnuclear weapons is calledfallout .

— Radioactive radon gas ispresent in basements andthe atmosphere.



30.2 X-ray machinesTherapeutic x-rays are usedto destroy diseased tissue,such as cancer cells.Low levels of x-rays do notdestroy cells, but high levelsdo.

The beams are made tooverlap at the place wherethe doctor wants to destroydiseased cells.



30.2 CAT scanPeople who work with radiationuse radiation detectors to tell

when radiation is present and tomeasure its intensity.

The Geiger counter is a type of

radiation detector invented tomeasure x-rays and other ionizing radiation, since they areinvisible to the naked eye.




Key Question:

How do we describenuclear reactions?

*Students read Section 30.3AFTER Investigation 30.3




A nuclear reaction is any process that changesthe nucleus of an atom.

Radioactive decay is one form of nuclear reaction.



30.3 Nuclear Reactions and EnergyIf 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 nucleusdid.

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.



30.3 Nuclear Reactions and EnergyWhen separate protons andneutrons come together in a

nucleus, energy is released.The more energy that isreleased, the lower the

energy of the final nucleus.The energy of the nucleusdepends on the mass and

atomic number.



30.3 Fusion reactions A fusion reaction is anuclear reaction that

combines, or fuses, twosmaller nuclei into a larger nucleus.

It is difficult to make fusionreactions occur becausepositively charged nucleirepel each other.



30.3 Fusion reactionsA fusion reaction is a nuclear reaction that combines, or fuses,two smaller nuclei into a larger nucleus.



30.3 Fission reactions A fission reaction splitsup a large nucleus into

smaller pieces. A fission reaction typicallyhappens when a neutron

hits a nucleus withenough energy to makethe nucleus unstable.



30.3 Fission reactionsThe average energy of the nucleus for a combination of molybdenum-99 (Mo-99) and tin-135 (Sn-135) is 25 TJ/kg.The fission of a kilogram of uranium into Mo-99 and Sn-135 releasesthe difference in energies, or 98 trillion joules.



30.3 Rules for nuclear reactionsNuclear reactions obey conservation laws.

Energy stored as mass must be included in order

to apply the law of conservation of energy to anuclear reaction.

Nuclear reactions must conserve electric charge.

The total baryon number before and after thereaction must be the same.

The total lepton number must stay the same

before and after the reaction.



30.3 Conservation LawsThere are conservation laws that apply to the type of particles before and after a nuclear reaction.— Protons and neutrons belong to a family of particles called

baryons .— Electrons come from a family of particles called leptons .



30.3 Calculating nuclear reactions

The nuclear reaction above is proposed for combining two atoms of silver to make an atom of gold.

This reaction cannot actually happen because itbreaks the rules for nuclear reactions.

List two rules that are broken by the reaction.



30.3 Antimatter, neutrinos and othersparticles

The matter you meet in the world ordinarily

contains protons, neutrons, and electrons.

Cosmic rays contain particles called muons andpions .

Thousands of particles called neutrinos from thesun pass through you every second and youcannot feel them.



30.3 Antimatter, neutrinos and othersparticlesEvery particle of matter has an antimatter twin .

Antimatter is the same as regular matter exceptproperties like electric charge are reversed.— An antiproton is just like a normal proton except

it has a negative charge.

— An antielectron (also called a pos i t ron ) is like anordinary electron except that it has positivecharge.



30.3 NeutrinosWhen beta decay was first discovered, physicistswere greatly disturbed to find that the energy of the resulting proton and electron was less thanthe energy of the disintegrating neutron.

The famous Austrian physicist Wolfgang Pauliproposed that there must be a very light,previously undetected neutral particle that wascarrying away the missing energy.

We now know the missing particle is a type of neutrino .



30.3 NeutrinosDespite the difficulty of detection, several carefullyconstructed neutrinoexperiments havedetected neutrinos coming

from nuclear reactions inthe sun.

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