Nuclear Decay

• Radioactivity—unstable atomic nucleus emits charged particles and energy

• Radioisotopes are atoms having unstable nuclei and spontaneously change into other isotopes through nuclear decay

• Ex. Uranium-238 decays into thorium-234

Types of Nuclear Radiation

• Nuclear radiation—charged particles and energy emitted from nuclei of radioactive elements

• Alpha, Beta, and Gamma Decay

Alpha

• Positively charged particle made of 2 neutrons and 2 protons; least penetrating (harmful), only travel a few cm, and can be stopped by clothes or a sheet of paper

• Effects: skin damage like a burn, not serious unless eaten or inhaled, like Radon-222 (gas in basements, can cause lung cancer)

Beta

• negatively charged particle made of an electron emitted from an unstable nucleus (neutron decomposes into a proton and electron); more penetrating (harmful), faster, can be stopped by thin sheet of metal

• Effects: can damage tissues within the body, not as severe as gamma

Gamma

• penetrating ray of energy; no mass/charge; waves of energy travelling at speed of light; more penetrating (harmful); can accompany alpha and beta; several cm of lead or m of concrete needed to stop gamma

• Effects—can penetrate deep into body and affect all tissues and organs; used in medical field to see tumors

Effects of Nuclear Radiation

• Background Radiation—naturally occurring, constant, safe

• Nuclear Radiation exceeding safe levels can ionize atoms—bonds holding proteins and DNA together may break and cells may not function properly

Detecting Nuclear RadiationCannot see, hear, or feel radioactivityGeiger Counters are devices that detect nuclear radiation

Rates of Nuclear Decay

Half-life

• Radioisotopes decay at different rates• Half-life is time required for one half of a

sample of a radioisotope to decay• After one half-life, half of the original sample is

left; after two half-lives, half of the remaining half decays, leaving one fourth of the original

• Can vary from fractions of a second to billions of years

Practice Half-Life

• 1 g sample of iridium-182, half-life is 15 minutes. How much is left after 45 minutes?

H-L elapsed = total time of decay = 45 min = 3Half-Life 15 min

½ x ½ x ½ = 1/8 1/8 of 1 gram is 0.125 g (0.875 g decayed into osmium-182)How much is left after one hour? Show your work.

• http://www.authorstream.com/Presentation/grazianir-265381-half-life-entertainment-ppt-powerpoint/

• H-L of element Z is 10 minutes. How much of a 5 gram sample is left after 30 minutes?

• Calculate # of half-lives:• Find your fraction:• Multiply:

Radioactive Dating• Carbon-14 has a h-l of 5730 years.• Radioactive C-14 decays N-14• Radiocarbon Dating—age of object determined

by comparing object’s C-14 levels with C-14 in air.

• Used for objects less than 50,000 years old• Ex—if ratio of C-14 to C-12 in a fossil is half the

ratio in the atmosphere, then the organism lived ~ 5730 years ago.

• Geologists use other isotopes for older objects.

Artificial Transmutation

• Radioactive isotopes are unstable and decay naturally

• Scientists can bombard atomic nuclei with high energy particles and force decay

• Uranium-238 bombarded with neutrons uranium-239, which undergoes beta decay neptunium-239

Americium-241 used in smoke detectors; as it decays, it emits alpha particles which ionize air inside smoke detector to produce electric current. Smoke disrupts the current and the alarm goes off. Transuranium elements are radioactive, greater than 92 (uranium) and generally not found in nature.

Fission and Fusion

• Strong nuclear force—attractive force that binds protons and neutrons together in nucleus; over short distances is stronger than electric forces

• Nuclei with 83+ protons are radioactive

FissionSplitting atomic nucleus into 2+ partsLots of energy from small mass MODIFIED--law of conservation of mass and energy

Chain Reaction

• How fast can a rumor spread???• Nuclear chain reaction—neutrons released

during splitting of original nucleus trigger a series of nuclear fissions

• Uncontrolled—fast, intense release of energy• Controlled—some neutrons absorbed; can

generate electrical energy• Critical mass—smallest mass of “fissionable”

material to sustain a chain reaction

Nuclear energy from fission 20% electricity in US Uranium-235 (controlled fission)PROs and CONs…

NUCLEAR FUSION

How is this different from fission?

Nuclear Fusion

• Nuclei of 2 atoms combine to form a larger nucleus

• Releases huge amounts of energy• Powers sun, stars• Extremely high temperatures where matter

exists as plasma• Possible future source of “green” electricity