Thursday, October 15th, 2015

The ABG's (or Alpha, Beta, The ABG's (or Alpha, Beta, Gamma) of RadioactivityGamma) of Radioactivity

What do we mean by What do we mean by Radioactivity?Radioactivity?

Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting radiation in the form of particles or electromagnetic waves.

There are numerous types of radioactive decay. The general idea:

An unstable nucleus releases energy to become more stable

Three Common Types of Three Common Types of Radioactive EmissionsRadioactive Emissions

Alpha

Beta

Gamma

Think about it…Think about it…What does positron mean? (Hint: what does

electron mean?)What would be the nuclear symbol of a

positron?What do you think electron capture means?How would an electron capture nuclear

equation look different from a beta decay nuclear equation?

Kinds of RadioactivityKinds of Radioactivity

The three main decays are Alpha, Beta and Gamma

Positron emission – a way to convert protons to electrons by giving off a positron (electron anti-particle)

Electron capture – a way to convert protons to neutrons by gaining an electron.

Band of Stability ActivityBand of Stability Activity

Nuclear stability depends on the proton to neutron ratio in a nucleus.

Nuclear StabilityNuclear Stability1. Nuclei with even numbers of both protons

and neutrons are generally more stable than those with odd numbers of these particles.

2. Nuclei that contain certain specific numbers of protons and neutrons within a nucleus ensure an extra degree of stability. These so-called magic numbers for protons and for neutrons are 2, 8, 20, 28, 50, 82, and 126.

3. All isotopes of elements after bismuth (Z = 83) are radioactive.

What type of decay?What type of decay?1. Nuclei above the band (high neutron-to-

proton ratios): Undergo beta emission because it decreases the number of neutrons and increases the number of protons.

2. Nuclei below the band (low n to p ratios): Positron emission or electron capture because increase n and decrease p

3. Nuclei with atomic numbers higher than 84: Alpha emission because they are so heavy

Radioactive elements are unstable. They decay, and change into different elements over time.

Not all elements are radioactive. Those that are listed below are the most useful for geologic dating of fossils are:

U-238 Half-life = 4.5 Billion YearsK-40 Half-life = 1.25 Billion YearsC-14 Half-life = 5, 730 Years

Radioactive Isotopes

Half Life of Radioactive Atoms

1. The half-life of an element is the time it takes for half of the material you started with to decay.

2. Each element has it’s own half-life

3. Each element decays into a new element

4. The half-life of each element is constant. It’s like a clock keeping perfect time.

Practice ProblemsPractice Problems1. The half life of iodine-131 is 8.1 days. How

long will it take for the sample to reduce to ¼ its original amount?

2. How long would it take iodine to reduce to less than 1% its original amount?

3. Radon-222 is a radioactive gas. How long is its half-life if it takes 65 days to reduce to 1/16th its original amount?

4. Uranium-238 decays very slowly, with a half-life of 4.47 billion years. What percentage of a sample of U-238 would remain after 13.4 billion years?

Practice ProblemsPractice Problems1. The half life of iodine-131 is 8.1 days.

How long will it take for the sample to reduce to ¼ its original amount?

1 ½ ¼ 2 half lives2 x 8.1 = 16.2 days

Practice ProblemsPractice Problems2. How long would it take iodine to reduce to less than 1% its original amount?

100% 50% 25% 12.5% 6.25% 3.125% 1.56% 0.781%

7 half lives 7 x 8.1 days = 56.7 days

Practice ProblemsPractice Problems3. Radon-222 is a radioactive gas. How long is its half-life if it takes 65 days to reduce to 1/16th its original amount?

1 ½ ¼ 1/8 1/16

4 half lives 65 days/4 = 16.25 days

Practice ProblemsPractice Problems4. Uranium-238 decays very slowly, with a half-life of 4.47 billion years. What percentage of a sample of U-238 would remain after 13.4 billion years?

13.4 / 4.47 = 3 half lives

100% 50% 25% 12.5%

ProblemProblem

A sample of 3x107 Radon atoms are trappedin a basement that is sealed. The half-life ofRadon is 3.83 days. How many radon atoms are left after 31 days?

answer:1.2x105 atoms