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Radioactive Decay Half Life

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Page 1: Radioactive Decay Half Life. What’s In An Atom? Electrons: Orbit around the nucleus. Negative charge. Nucleus: Has protons of positive charge and neutrons

Radioactive Decay

Half Life

Page 2: Radioactive Decay Half Life. What’s In An Atom? Electrons: Orbit around the nucleus. Negative charge. Nucleus: Has protons of positive charge and neutrons

What’s In An Atom?Electrons: Orbit around the nucleus. Negative charge.

Nucleus: Has protons of positive charge and neutrons of no charge.

A neutron is a proton and an electron stuck together. Net charge is 0.

Page 3: Radioactive Decay Half Life. What’s In An Atom? Electrons: Orbit around the nucleus. Negative charge. Nucleus: Has protons of positive charge and neutrons

Neutrons

• The force holding the proton and electron together in a neutron is relatively weak.

• Sometimes a neutron will break apart, and the proton will be shot out of the nucleus by the repelling force of the other protons.

• Neutron Proton + Electron

+ - + -

Page 4: Radioactive Decay Half Life. What’s In An Atom? Electrons: Orbit around the nucleus. Negative charge. Nucleus: Has protons of positive charge and neutrons

Neutrons• Since these protons are so close to the

nucleus when they are “set free”, they leave the atom with incredible velocity.

• This is one kind of radiation. • When atoms shoot out these protons, they

loose some of their mass (since the protons leaving have mass).

+

Page 5: Radioactive Decay Half Life. What’s In An Atom? Electrons: Orbit around the nucleus. Negative charge. Nucleus: Has protons of positive charge and neutrons

Half Life

• Def: The amount of time it takes a sample of radioactive material to lose half its mass due to radiation.

Page 6: Radioactive Decay Half Life. What’s In An Atom? Electrons: Orbit around the nucleus. Negative charge. Nucleus: Has protons of positive charge and neutrons

Half Life

• Image a 10 kg sample of radioactive “stuff” with a half life of 5 seconds.

Time Elapsed Amount of Material

Half Life

0 s 10 kg 0

5 s 5 kg 1

10 s 2.5 kg 2

15 s 1.25 kg 3

20 s 0. 625 kg 4

Page 7: Radioactive Decay Half Life. What’s In An Atom? Electrons: Orbit around the nucleus. Negative charge. Nucleus: Has protons of positive charge and neutrons

Example

• A 1.0 kg sample of U-238 (Naturally occurring Uranium) has a half life of 1.41 X 1017 s, or about 4.4 billion years. How much will remain after 3 half lives? How long will that take?

Page 8: Radioactive Decay Half Life. What’s In An Atom? Electrons: Orbit around the nucleus. Negative charge. Nucleus: Has protons of positive charge and neutrons

Example

• A 1.0 kg sample of U-238 (Naturally occurring Uranium) has a half life of 1.41 X 1017 s, or about 4.4 billion years. How much will remain after 3 half lives? How long will that take?

• Set up a table:

Elapsed Time Amount of sample Half Life

Page 9: Radioactive Decay Half Life. What’s In An Atom? Electrons: Orbit around the nucleus. Negative charge. Nucleus: Has protons of positive charge and neutrons

Example

• A 1.0 kg sample of U-238 (Naturally occurring Uranium) has a half life of 1.41 X 1017 s, or about 4.4 billion years. How much will remain after 3 half lives? How long will that take?

• Insert your givens:

Elapsed Time Amount of sample Half Life

0 s 1kg 0

Page 10: Radioactive Decay Half Life. What’s In An Atom? Electrons: Orbit around the nucleus. Negative charge. Nucleus: Has protons of positive charge and neutrons

Example

• A 1.0 kg sample of U-238 (Naturally occurring Uranium) has a half life of 1.41 X 1017 s, or about 4.4 billion years. How much will remain after 3 half lives? How long will that take?

• Fill in the table for correct # of half lives.

Elapsed Time Amount of sample Half Life

0 s 1 kg 0

1.41 x 1017 0.5 kg 1

2.82 x 1017 0.25 kg 2

5.64 x 1017 0.125 kg 3

Page 11: Radioactive Decay Half Life. What’s In An Atom? Electrons: Orbit around the nucleus. Negative charge. Nucleus: Has protons of positive charge and neutrons

Another Example

• Plutonium has a half life of 7.75 x 1011 s. How much will remain of a .10 kg sample after 1.21 x 1013 s?

Page 12: Radioactive Decay Half Life. What’s In An Atom? Electrons: Orbit around the nucleus. Negative charge. Nucleus: Has protons of positive charge and neutrons

Another Example

• Plutonium has a half life of 7.75 x 1011 s. How much will remain of a .10 kg sample after 1.21 x 1013 s?

• Table and givens:Elapsed Time Amount of Sample Half Life

0 s 0.10 kg 0

Page 13: Radioactive Decay Half Life. What’s In An Atom? Electrons: Orbit around the nucleus. Negative charge. Nucleus: Has protons of positive charge and neutrons

Another Example

• Plutonium has a half life of 7.75 x 1011 s. How much will remain of a .10 kg sample after 1.21 x 1013 s?

• Table and givens:Elapsed Time Amount of Sample Half Life

0 s 0.10 kg 0

7.75 E 11 s 0.05 kg 1

1.51 E 12 0.025 kg 2

3.03 E 12 0.0125 kg 3

6.05 E 12 0.00625 kg 4

1.21 E 13 0.003125 kg 5

Page 14: Radioactive Decay Half Life. What’s In An Atom? Electrons: Orbit around the nucleus. Negative charge. Nucleus: Has protons of positive charge and neutrons

Further Exploration

• Go to the following website to answer these questions:

http://colorado.edu/physics/2000/index.pl• Science Trek → Isotopes and Radioactivity• Balance the following equations:• 7Be4 => 7Li? + ?e1

• 238U? => 234Th90 + ?He2

What is the half life of Neon-19?• What is the half life of Nitrogen-17?

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