Half-Life and Nuclear Reactions

Review

We learned that all radioactive atoms eventually decay into stable isotopes.– We did not talk about how long this takes.

Half-Life

Cannot predict when a radioisotope (RI) will decay.– Can only give probability.

Half-life – Amount of time during which one-half of a radioactive substance will decay.– During a RI’s half-life, every atom has a

50% chance of decaying.– Half-lives can be as short as a fraction of a

second or as long as billions of years.

Half-Lifes N-13 is radioactive.

– It decays into C-13 via a process called electron capture. The half-life of 13N is 10 minutes.

– If you start with 1000 atoms of N-13, approx. how many will remain after 10 minutes?

• 10 minutes = 1 half-life, so there would be about 500 atoms left.

– Approx. how many will remain after 20 minutes?• 20 minutes = 2 half-lives, so there would be about 250

atoms left.– Approx. how many will remain after 30 minutes?

• 30 minutes = 3 half-lives, so there would be about 125 atoms left.

Half-LifesAmount of N-13 Remaining

0

200

400

600

800

1000

1200

0 10 20 30 40 50

Time (minutes)

Ato

ms

of

N-1

3

Half-Lifes

The half-life of 26Al is 710,000 years.– If you start with 28.0 grams of 26Al, how

much will remain after 1,420,000 years?• At start, 28.0 grams remain.• At 710,000 years, 14.0 grams remain.• At 1,420,000 years, 7.0 grams remain.

Half-Lifes

The half-life of 61Fe is 6.0 minutes. How much time must pass before a 600-mg sample decays to 75 mg?– At start, 600 mg remain.– At 6.0 minutes, 300 mg remain.– At 12.0 minutes, 150 mg remain.– At 18.0 minutes, 75 mg remain.

Half-Lifes

In 48 minutes, 12 mg of 212Rn will decay to 3 mg. What is the half-life of 212Rn?– At start, 12 mg remain.– At (1st half-life), 6 mg remain.– At (2nd half-life), 3 mg remain.– It takes 2 half-lives for 12 mg to decay to 3

mg.• 2 half-lives = 48 minutes• 1 half-life = 24 minutes

Nuclear Fission

A heavy nucleus is struck by a neutron.– It becomes unstable and splits into smaller

fragments, releasing energy.– In some cases extra neutrons are also

released.– The neutrons can strike other nuclei and

cause them to split.• Chain reaction.

Nuclear Fission

Energy

235Uno

no

no

no

90Rb

143Cs

Uses for Nuclear Fission

Nuclear reactors use fission to produce energy.

Uses for Nuclear Fission

Weapons: the bombs that destroyed Hiroshima and Nagasaki, Japan, were fission bombs.

Nuclear Fusion

Two light nuclei are joined to produce a larger nucleus.– Energy is released.

Nuclear Fusion

no

2H

3H

Energy

4He

Uses for Nuclear Fusion

Nuclear fusion can be performed at low temperatures, but it does not produce much useable energy.

Fusion only produces significant energy at extremely high temperature and pressure.– The Sun produces energy via nuclear

fusion.

Fusion in the Sun

The Sun is mostly made of hydrogen. Intense heat and pressure at its center

causes nuclear fusion to occur. This releases energy that supports the

mass of the Sun and prevents it from collapsing.

The Sun

Uses for Nuclear Fusion

Weapons: Thermonuclear (hydrogen) bombs.– Use a conventional fission bomb to trigger

nuclear fusion in a lithium hydride shell.– Much more destructive than fission bombs.– Never used in war.