radioactivity notes

7/28/2019 Radioactivity Notes

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7.1 Atomic Theory and Radioactive Decay

Natural background radiation exists all around us. This radiation consists of high energy particles or waves being

emitted from a variety of materials. Radioactivity is the release of high energy particles or waves.

Being exposed to radioactive materials can be beneficial orharmful.

X-rays, radiation therapy and electricity generation arebeneficial.

High energy particles and waves can do damage to DNA inour cells.

When atoms lose high energy particles and waves, ions or evennew atoms can be formed.

High energy waves and particles are called radiation when theyleave the atom.

The Electromagnetic Spectrum

Searching for Invisible Rays

Radiation is everywhere, but can be difficult to detect. Roentgen named X-rays with an X 100 years ago because they

were previously unknown. Becquerel realized uranium emitted seemingly invisible energy

as well.

Marie Curie and her husband Pierre named this energy radioactivity. Early discoveries of radiation relied on photographic equipment

Later, more sophisticated devices such as the Geiger-Mller counterwere developed to more precisely measure radioactivity.


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Radium salts, after being placed on a photographic plate, leave behind the

dark traces of radiation.

Isotopes and Mass Number Isotopes are different atoms of the same element, with the

difference between the two atoms being the number of neutrons in

the nucleus.

Isotopes have the same number of protons - and therefore thesame atomic number - as each other.

By having different numbers of neutrons, isotopes have differentmass numbers.

Isotopes of an element have the same symbol and sameatomic number

Mass number refers to the protons + neutrons in an isotope Atomic mass = proportional average of the mass numbers for

all isotopes of an element.

19.9% of boron atoms have 5 neutrons, 80.1% have 6neutrons

19.9% have a mass number of 10, and 80.1% have a massnumber of 11

(.199 * 10) + (.801*11) = 10.8 = atomic mass of boronRepresenting Isotopes

Isotopes are written using standard atomic notation. Chemical symbol + atomic number + mass number. Potassium has three isotopes,

K K K


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Potassium is found in nature in a certain ratio of isotopes

93.2% is potassium-39, 1.0% is potassium-40, and6.7% is potassium-41

Atomic mass = (.932 x 39) + (.001 x 40) + (.067 x 41) =39.1

Radioactive Decay

Unlike all previously discovered chemical reactions, radioactivitysometimes resulted in the formation of completely new atoms.

Radioactivity results from having an unstable nucleus. When these nuclei lose energy and break apart, decay occurs.

Radioactive decay releases energy from the nucleus asradiation.

Radioactive atoms release energy until they become stable,often as different atoms.

An element may have onlycertain isotopes that areradioactive.

These are called radioisotopes


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Radioisotope uranium-238 decays in several stages until it finally becomes

lead-206

Three Types of Radiation

Rutherford identified three types of radiation using an electric field. Positive alpha particles were attracted to the negative plate. Negative beta particles were attracted to the positive plate.

Neutral gamma particles did not move towards any plate.

Alpha Radiation

Alpha radiation is a stream of alpha particles, . Positively charged, and are the most massive of the radiation

types.

Alpha particles are essentially the same as a helium atom. Alpha particles are represented by the symbols .

Because it has two protons, it has a charge of 2+. The release of alpha particles is called alpha decay.

Alpha particles are slow and penetrate materials much less thanthe other forms of radiation. A sheet of paper will stop an alpha

particle.


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Radium-226 releases an alpha particle and becomes

Radon-222. Radon has two less protons than radium.

Beta Radiation

Beta radiation,, is an electron. negatively charged, and are less massive than alpha radiation. Beta particles are represented by the symbols . Electrons are very tiny, so beta particles are assigned a mass of

0.

Since there is only an electron, a beta particle has a chargeof 1.

Beta decay occurs when a neutron changes into a proton + anelectron.

The proton stays in the nucleus, and the electron isreleased.

It takes a thin sheet of aluminum foil to stop a betaparticle.

Iodine-131 releases a beta particle and becomes Xenon-131. A neutron has

turned into a proton + the released electron.


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Gamma Radiation

Gamma radiation, , is a ray of high energy, short-wavelengthradiation.

Gamma radiation has no charge and no mass, . Gamma radiation is the highest energy form of electromagnetic

radiation.

It takes thick blocks of lead or concrete to stop gamma rays. Gamma decay results from energy being released from a high-

energy nucleus.

Often, other kinds of radioactive decay will also release gammaradiation.

Uranium-238 decays into an alpha particle and also releasesgamma rays.

Radiation and Radioactive Decay Summaries, and Nuclear

equations for radioactive decay


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Nuclear equations are written like chemical equations, butrepresent changes in the nucleus of atoms.

Chemical equations represent changes in the position ofatoms, not changes to the atoms themselves.

1. The sum of the mass numbers should equal.2. The sum of the charges should equal.

7.2 Half-life

It can be difficult to determine the ages of objects by sight alone. Radioactivity provides a method to determine age by

measuring relative amounts of remaining radioactive

material to stable products formed.

Carbon dating measure the ratio of carbon-12 and carbon-14. Stable carbon-12 and radioactive carbon-14 exist naturally in

a constant ratio. In nature, carbon-12 appears 98.9% of the time, while

one carbon-14 atom appears for every 1 trillion normal

atoms.

When an organism dies, carbon-14 stops being created andslowly decays.

Measuring the relative amounts of carbon-12 : carbon-14 is called radiocarbon dating.

Radiocarbon dating only works for organismsless than 50 000 years old

The half-life of carbon-14 is 5730 years.


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Using radiocarbon dating, these cave paintings of horses,

from France, were determined to have been drawn 30 000 years ago.

The Rate of Radioactive Decay

Half-life measure the rate of radioactive decay. Half-life = time required for half of the radioactive sample to decay. The half life for a radioactive element is a constant rate of decay. Strontium-90 has a half-life of 29 years. If you have 10 g of strontium-

90 today, there will be 5 g remaining in 29 years.

Decay curves show the rate of decay for radioactive elements. The curve shows the relationship between half-life and percentage of

original substance remaining.

The decay curve for strontium-90


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Common Isotope Pairs

There are many radioisotopes that can be used for dating.o Parent isotope = the original, radioactive material.o Daughter isotope = the stable product of the radioactive

decay. The rate of decay remains constant, but some elements require one

step to decay, while others decay over many steps before reaching a

stable daughter isotope.

Carbon-14 decays into nitrogen-14 in one step Uranium-235 decays into lead-207 in fifteen steps. Thorium-235 decays into lead-208 in ten steps.

The Potassium-40 Clock Radioisotopes with very long half-lives can help determine the age

of very old things.

The potassium-40/argon-40 clock has a half-life of 1.3 billionyears.

Argon-40 produced by the decay of potassium-40 becomes trappedin rock.

Ratio of potassium-40 : argon-40 shows age of rock.


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7.3 Nuclear Reactions

Nuclear fission and fusion are processes that involve extremelylarge amounts of energy.

Fission = the splitting of nuclei


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Fusion = the joining of nuclei Nuclear power plants can generate large amounts of electricity. In Canada, Ontario, Quebec and New Brunswick currently use

nuclear power.

Canadian-made nuclear reactors are called CANDU reactors.

CANDU reactors are considered safe and effective, and are soldthroughout the world.

Nuclear Fission

o Nuclear energy used to produce power comes from fission.o Nuclear fission is the splitting of one heavy nucleus into two or

more smaller nuclei, as well as some sub-atomic particles and

energy.

o A heavy nucleus is usually unstable, due to many + protons pushingapart.

o When fission occurs:1. Energy is produced.2. More neutrons are produced.

o Nuclear reactions are different than chemical reactions.o In chemical reactions, mass is conserved, energy changes are

relatively small.

o There are no changes to the nuclei in chemical reactionso In nuclear reactions, the actual nucleus of atoms changes.o Protons, neutrons, electrons and/or gamma rays can be lost or

gained.

o Small changes of mass = huge changes in energyNuclear Equations for Induced Nuclear Reactions

Natural radioactive decay consists of the release ofalpha, beta and gamma radiation.

Scientists can also force ( = induce) nuclear reactions by smashingnuclei with alpha, beta and gamma radiation.


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The rules for writing these equations are the same as earliernuclear equations

Mass numbers must equal on both sides of the equation Charges must equal on both sides of the equation

Nuclear Fission of Uranium-235

It is much easier to crash neutral neutron than a positive protoninto a nucleus to release energy.

Most nuclear fission reactors and weapons use this principle. A neutron, , crashes into an atom of stable uranium-235 to

create unstable uranium-236, which then undergoes

radioactive decay.

After several steps, atoms of krypton and barium are formed,along with the release of 3 neutrons and huge quantities of

energy.


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The induced nuclear fission of uranium-235. This nuclear reaction is the

origin of nuclear power and nuclear bombs.

Chain Reactions Once the nuclear fission reaction has started, it can keep going.

The neutrons released in the induced reaction can thentrigger more reactions on other uranium-235 atoms.

This chain reaction can quickly get out of control Fermi realized that materials that could

absorb some neutrons could help

to control the chain reaction.

Nuclear reactors havecomplex systems to ensure the

chain reaction stays at safe levels.

An uncontrolled chain reaction can resultin the release of excess energy of harmful radiation

It is on this concept that nuclear bombs are created.


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CANDU Reactors and Hazardous Wastes

Canadas nuclear research into the safe use of nuclear reactionshas resulted in the creation of CANDU reactors.

CANDU rectors are found in various countries around theworld.

Canada, South Korea, China, India, Argentina, Romaniaand Pakistan

The reactors are known to be safe and easy to shut down inan emergency.

Heat energy produced turns electricity-generatingturbines.

Hazardous wastes produced by nuclear reactions are problematic. Some waste products, like fuel rods, can be re-used Some products are very radioactive, however,

and must be stored away from living things.

Most of this waste is buried underground,or stored in concrete

It takes 20 half-lives (thousands of years)before the material is safe.


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Nuclear Fusion Nuclear fusion = joining of two light nuclei into one heavier

nucleus.

In the core of the Sun, two hydrogen nuclei joinunder tremendous heat and pressure to form a

helium nucleus.

When the helium atom is formed, huge amountsof energy are released.

Scientists cannot yet find a safe, manageable

method to harness the energy of nuclear fusion.

So-called cold fusion would occur attemperatures and pressures that could be

controlled.

The fusion of hydrogen nuclei


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