Unstable Nuclei

Radioactive Decay

• Some atomic nuclei are unstable

• This is because of an imbalance in the forces within the nucleus

  • Forces exist between the particles in the nucleus

• Carbon-14 is an isotope of carbon which is unstable

  • It has two extra neutrons compared to stable carbon-12

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Unstable nuclei decay by emitting high energy particles or waves

​Radioactive decay is a random process

• This means it is not possible to know exactly when a particular nucleus will decay

• As the radiation moves away from the nucleus, it takes some energy with it

  • This reduces the overall energy of the nucleus

  • This makes the nucleus more stable

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Multiple Choice

Isotopes can be unstable because they have too many or too few neutrons

• False

• True

Multiple Choice

Scientists can predict when a nucleus will decay

• True

• False

Multiple Choice

Radiation refers to the particles or waves emitted from a decaying nucleus

• True

• False

Types of radiation

Alpha Particles

• The symbol for alpha is α

• An alpha particle is the same as a helium nucleus

  • This is because they consist of two neutrons and two protons

• Alpha particles have a charge of +2

  • This means they can be affected by an electric field

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Types of radiation

Beta Particles

• The symbol for beta is β

• Beta particles are fast-moving electrons

• They are produced in nuclei when a neutron changes into a proton and an electron

• Beta particles have a charge of -1

  • This means they can be affected by an electric field

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Types of radiation

Gamma Rays

• The symbol for gamma is γ

• Gamma rays are electromagnetic waves

• They have the highest energy of the different types of electromagnetic waves

• Gamma rays have no charge

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Types of radiation

Neutrons

• The symbol for a neutron is n

• Neutrons are one of the two particles found in the nucleus of atoms

• Neutrons are neutral, they have no charge

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Multiple Choice

• alpha beta

• beta gamma

• gamma alpha

Multiple Choice

• beta gamma

• alpha beta

• alpha gamma

Multiple Choice

• alpha

• beta

• gamma

Multiple Choice

• 234 has 2 less neutrons

• 234 has 2 less protons

• 234 has 2 less electrons

Half Life

The time it takes for the number of nuclei of a sample of radioactive isotopes to decrease by half

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Calculating half life

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Multiple Choice

A particular radioactive sample contains 2 million un-decayed atoms. After a year, there is only 500 000 atoms left un-decayed. What is the half-life of this material?

• 6 months

• 3 months

• 4 months

• 1 month

Calculating Radioactive Decay

Method 1: Halving method

• Determine the number of half-lives elapsed

• Divide the number 1 by half for each half-life elapsed

• For example, if 4 half-lives have elapsed:

  1 ÷ ½ ÷ ½ ÷ ½ ÷ ½ = 1 / 16

• This is the same as a ratio of 1 remaining : 16 original nuclei, or 1:16

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Calculating Radioactive Decay

​ Method 2: Raising to a Power

• Determine the number of half-lives elapsed

• Use your calculator to raise ½ to the number of half-lives

• For example, if 4 half-lives have elapsed:

(1/2)⁴ = 1/16

• This is the same as a ratio of 1 remaining : 16 original nuclei, or 1:16

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Multiple Choice

A radioactive sample has a half-life of 3 years. What is the ratio of decayed : original nuclei, after 15 years?

• 1:32

• 16:1

• 4:1

• 6:1

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Background Radiation

​The radiation that exists around us all the time

• There are two types of background radiation:

  • Natural sources

  • Man-made sources

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Determine background radiation

Step 1: Determine the point at which the source radiation stops being detected

Step 2: State the background radiation count 

• The background radiation is the amount of radiation received all the time

• When the source is moved back far enough it is all absorbed by the air before reaching the Geiger-counter

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Multiple Choice

• Roughly 15 counts

• roughly 50 counts

• roughly 30 counts

• roughly 40 counts

Measuring Radiation Dose

• The amount of radiation received by a person is called the dose and is measured in sieverts (Sv)

• One sievert is a very big dose of radiation

  • It would cause acute radiation poisoning

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Multiple Choice

• 13 counts per minute

• 10 counts per minute

• 5 counts per minute

Open Ended

Open Ended

Calculate the value of background radiation that should be accounted for (MEAN)

Open Ended

Name one naturally occurring source of background radiation

Open Ended

Name one man made source of background radiation

Open Ended