RadioactivityRadioactivity

Ch 10Ch 10

Radioactivity• is the process in which an

unstable atomic nucleus emits charged particles & energy

• Any atom containing an unstable nucleus is called a radioactive isotope or radioisotope

How is the Atom Unstable?• The nuclear “glue” that holds

the nucleus together sometimes isn’t strong enough.

• Over time the atom “drops” some of it’s subatomic particles …called radioactive decay!

New Way of WritingNew Way of WritingNuclides & IsotopesNuclides & Isotopes

3919 K ; 40

19 K• Superscript is the mass

number• Subscript is the atomic

number• 14

6C ; 126 C

IsotopesIsotopes• have the same number of p+,

different number of no

• Another way to show an isotope is to have the mass number follow the name of the element (Carbon-14 or C-14)

Types of Nuclear RadiationTypes of Nuclear Radiation

1. Alpha 2.Beta 3. Gamma

Alpha DecayAlpha Decay

• Alpha particle—a positively charged particle made up of two p+ & two no

• the least penetrating

• can be stopped by a sheet of paper

Alpha DecayAlpha Decay• An alpha particle looks like a

helium atom (42He)

• massmass reduces by 4

• atomicatomic # reduces by 2

• Examples:238238

9292 U 209209

8484 Po

Beta DecayBeta Decay

• A beta particle is an e- emitted by an unstable nucleus

• can be stopped by a thin sheet of metal such as aluminum

Beta DecayBeta Decay

• A beta particle is written 0-1 e

• mass remains the same & the atomic # increases by one

• Examples:214

82 Pb218

84 Po

Gamma decayGamma decay

• A gamma ray is a penetrating ray of energy emitted by an unstable nucleus

• Gamma rays are energy waves that travel through space at the speed of light

Gamma Gamma decaydecay

• atomic # and mass remain the same, but the energy of nucleus decreases

• Gamma rays can be stopped by several centimeters of lead or by several meters of concrete

Thanks Cambridge Physics Outlet for amazing graphics!

STAR Questions

• U-238 loses 4 total subatomic particles, 2 being protons.– What elements are formed?

• Thorium and Helium

– What type of decay has it undergone?• Alpha decay

• What is the most penetrating decay?• Gamma rays

Background radiation• is nuclear radiation that

occurs naturally in the environment (levels are low enough to be safe)

– Radioisotopes in the air, water, rocks, plants, & animals all contribute

Background RadiationBackground Radiation

– Cosmic rays (streams of Cosmic rays (streams of charged particles) from outer charged particles) from outer space that collide with the space that collide with the EarthEarth’’s atmosphere also s atmosphere also contributecontribute

– When nuclear radiation When nuclear radiation exceeds background levels, exceeds background levels, cells in your body can mutatecells in your body can mutate

Detecting Detecting RadiationRadiation

• Devices used to detect radiation include Geiger counters & film badges

RATES OF NUCLEAR DECAYRATES OF NUCLEAR DECAY

A half-life is the time required for one half of a sample of radioactive sample of a radioisotope to decay

– Unlike chemical reactions, nuclear decay rates are constant regardless of temperature, pressure or surface area

• C-14 has a half life of 5730 years.– What fraction of a sample will be remaining

after 1 half life?• After 2 half lives?

• C-14 has a half life of 5730 years.– If you have a sample of 50 grams. How much of the

sample will be remaining after 1 half life?• After 2 half lives?

RATES OF NUCLEAR DECAYRATES OF NUCLEAR DECAYTransmutation is the conversion of

atoms of one element to atoms of another

Transuranium elements are elements with atomic numbers higher than 92 (Uranium)

A quark is a subatomic particle theorized to be among the basic units of matter

FISSION AND FUSIONFISSION AND FUSION• Fission is the

splitting of an atomic nucleus into two smaller parts

• Fusion is a process in which the nuclei of two atoms combine to form a larger nucleus

FissionFission• A chain reaction is a chain of fission reactions triggered by neutrons released during the fission of a nucleus

About 20% of the electricity in the US comes from fission reactions

FissionFission• A tremendous amount of energy

is produced during a fission reaction

FissionFission• Advantages: is the

lack of air pollution.

• Disadvantages: include the risk of exposure & radioactive waste

Harris plant, near Raleigh

Lake Harris

Not in my backyard…

Then sealed and transported by way of trucks and trains for more permanent storage. Low-level wastes will stay above ground until they become “stable”.

If high-level, the wastes are stored deep underground, where they wait for hundreds to thousands of years to become “stable”.

FusionFusion• release huge

amounts of energy

• occur in the sun and stars (plasma)

FusionFusion

• We do not use fusion reactions for energy b/c of the extremely high temperatures needed to start the reaction & because the plasma would need to be contained.

THREE MILE ISLAND• Three Mile Island power station is near Harrisburg,

Pennsylvania • In 1979 at Three Mile Island nuclear power plant a

cooling malfunction caused part of the core to melt in the # 2 reactor. The TMI-2 reactor was destroyed. 

• Some radioactive gas was released a couple of days after the accident, but not enough to cause any dose above background levels to local residents. 

• There were no injuries or adverse health effects from the Three Mile Island accident. 

CHERNOBYL• The Chernobyl accident in 1986 was the result of a

flawed reactor design that was operated with inadequately trained personnel & without proper regard for safety.

• The resulting steam explosion & fire released at least five percent of the radioactive reactor core into the atmosphere and downwind.

• 28 people died within four months from radiation or thermal burns, 19 have subsequently died, & there have been around nine deaths from thyroid cancer apparently due to the accident: total 56 fatalities as of 2004.

• An authoritative UN report in 2000 concluded that there is no scientific evidence of any significant radiation-related health effects to most people exposed. This was confirmed in a very thorough 2005-06 study.

Examples

• Iodine-131 has a half-life of 8.07 days. What fraction of a sample of iodine-131 is left unchanged after 16.14 days?

Examples

• The radioactive isotope Ni-63 has a half-life of 100 yrs. How much of a 10g sample remains after 300 yrs?

Examples

• How long will it take a sample of Po-194 to decay to 1/8 of its original amount, if Po-194 has a half life of 0.7 seconds?

Examples

• A sample of Cl-38 is observed to decay to 25% of its original amount in 74.4 min. What is the half-life of Cl-38?

• sorry…no time for Star cards this time :o)