Upload
bernice-cooper
View
216
Download
0
Embed Size (px)
Citation preview
1
Radioactivity and Half-Life
2
Radioactivity•An unstable atomic nucleus emits a form of radiation (alpha, beta, or gamma) to become stable.
•In other words, the nucleus decays into a different atom.
3
Radioactivity•Alpha Particle – Helium nucleus
•Beta Particle – electron•Gamma Ray – high-energy photon
4
Half-Life•Amount of time it takes for one half of a sample of radioactive atoms to decay
http://www.absorblearning.com/media/item.action?quick=185
• Daughter isotope
• Decay curve• Half-life• Parent
isotope• Radiocarbon
dating
• 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.
See pages 302 - 304
• Carbon dating measures the ratio of carbon-12 and carbon-14.– Stable carbon-12 and radioactive carbon-14
exist naturally in a constant ratio.– When an organism dies, carbon-14 stops
being created and slowly decays.• Carbon dating only works for organisms
less than 50 000 years old.
See pages 302 - 304
Using carbon dating, these cave paintings of horses,from France, were drawn 30 000 years ago.
• Half-life measures 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 5.0 g remaining in 29 years.
See pages 305 - 306
• Decay curves show the rate of decay for radioactive elements.– The curve shows the
relationship between half-life and percentage of original substance remaining.
See pages 305 - 306The decay curve for strontium-90
10
Medical Applications of Half-Life
Nuclide Half-Life Area of Body
I–131 8.1 days Thyroid
Fe–59 45.1 days Red Blood Cells
Sr–87 2.8 hours Bones
Tc–99 6.0 hours Heart
Na–24 14.8 hoursCirculatory
System
11
Half-Life Calculation #1•You have 400 mg of a radioisotope with a half-life of 5 minutes. How much will be left after 30 minutes?
12
Half-Life Calculation #2•Suppose you have a 100 mg sample of Au-191, which has a half-life of 3.4 hours. How much will remain after 10.2 hours?
13
Half-Life Calculation # 3•Cobalt-60 is a radioactive
isotope used in cancer treatment. Co-60 has a half-life of 5 years. If a hospital starts with a 1000 mg supply, how many mg will need to be purchased after 10 years to replenish the original supply?
14
Half-Life Calculation # 4•A radioisotope has a half-life of 1 hour. If you began with a 100 g sample of the element at noon, how much remains at 3 PM? At 6 PM? At 10 PM?
15
Half-Life Calculation # 5•How many half-lives have passed if 255 g of Co-60 remain from a sample of 8160 g?
16
Half-Life Calculation # 6•Suppose you have a sample containing 400 nuclei of a radioisotope. If only 25 nuclei remain after one hour, what is the half-life of the isotope?
17
Half-Life Calculation # 7•If a radioactive element has diminished by 7/8 of its original amount in 30 seconds, what is its half-life?
18
Answers to Half-Life Calculations
•Half-Life Calculation #1– 6.25 mg
•Half-Life Calculation #2– 12.5 mg
•Half-Life Calculation #3– 750 mg
19
Answers to Half-Life Calculations
•Half-Life Calculation #4– 12.5 g, 1.5625 g, 0.09765625 g
•Half-Life Calculation #5– 5 half-lives
20
Answers to Half-Life Calculations
•Half-Life Calculation #6– 15 minutes
•Half-Life Calculation #7– 10 seconds