Module 8

Module 8—Nuclear Decay, Energy, and the Standard Model of the Atom

 

Reflect and Connect—Three Types of Nuclear Radiation

 

Rutherford classified the three types of naturally occurring nuclear radiation according to penetrating ability. Listed from least to greatest penetrating ability, the three types are alpha, beta, and gamma rays.

 

When americium-241 decays, it releases alpha particles energetic enough to ionize gas molecules. This fact is used in the ionizing smoke detector where ionized gas molecules are used as conductors between two electrodes, establishing a current in the detector. When smoke particles enter the detector they block the alpha particles, which stops the ionization of the air causing the current in the gas to drop, triggering the alarm.

 

You can explore the effect that smoke and other barriers have on radiation using a Geiger counter, a device that measures the number of alpha particles, beta particles, and gamma radiation emitted by an isotope. Either the type of barrier or the radioactive isotope can be manipulated. The responding variable is the amount (per unit time) of each type of radiation reaching the Geiger counter.

 

Open the Geiger Counter simulation. This simulation uses a Geiger counter to measure the number of alpha and beta decay particles emitted from an isotope in a five-second time interval.

 

Module 8: Lesson 1 Assignment

 

Remember to submit your answers to RC 1, RC 2, RC 3, RC 4, RC 5, RC 6, and RC 7 to your teacher as part of your Module 8: Lesson 1 Assignment. You will be using the Geiger Counter simulation for these Reflect and Connect questions.

 

RC 1. On the Geiger Counter simulation, select Americium-241 as the isotope and select “air” as the barrier. Start the count and complete the data table below. How do you know what type of decay is occurring?

 

Alpha
Beta
Gamma

 

 

 

 

 

 

 

RC 2. Switch the barrier to “smoke,” start the counter, and complete the data table below. How is a Geiger counter like a smoke detector?

 

Alpha
Beta
Gamma

 

 

 

 

 

 

 

RC 3. Switch the isotope to “no isotope,” start the counter, and complete the data table. How can you account for any radioactivity when there is no isotope?

 

Alpha
Beta
Gamma

 

 

 

 

 

 

 

RC 4. Select uranium-238 as the isotope and air as the barrier. Start the counter and record the number of alpha particles detected. Move the Geiger counter away from the isotope and start the count again. Record the new activity. Complete the table below by continuing to move the Geiger counter away from the isotope.

 

Distance (mm)	Alpha Count
10
20
50
100
200
300
400

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

RC 5. Graph the recorded count versus distance of separation. Does the intensity of the radiation obey the inverse square rule? Explain how this graph could be used to find the minimum safe distance from a radioactive source.

 

 

RC 6. Experiment with different types of barriers. Which material is the best shield from decay radiation?

 

RC 7.  Repeat RC 4 for cobalt-60. Make a table to organize your data for beta and gamma decays. Graph the information as in RC 5. What relationship do you see?

 

Try This

 

TR 4. Read the description of each particle below, and explain why Rutherford’s ranking of emitted radiation particles by penetrating power makes sense in terms of the structure of each particle.

• alpha (α) particle: a helium nucleus made up of 2 neutrons and 2 protons;  symbol or
  
  

• beta (β) particle: a very high-speed electron; symbol or
  
  

• gamma (γ) particle: a high-energy photon (higher energy than X-rays);  symbol

The Process of Nuclear Decay

 

The decay process must obey the following laws of physics:

1. conservation of charge
   
   

2. conservation of nucleons
   
   

3. conservation of mass-energy

The first two laws will be used to complete and balance nuclear decay equations while the third will be applied later when you investigate the concept of mass defect and binding energy.

 

There are several types of decay that can be simulated with this Nuclear Decay Gizmo. Use the gizmo to study alpha and beta decay. Watch for the concepts of conservation of charge and nucleon number as you complete the following activity. Activate the animation on the lower right of the graphic.

 

Alpha Decay (α)

 

The alpha decay of uranium-238 can be represented with an equation and animation. The default display for the Nuclear Decay Gizmo is for the alpha decay of a uranium-238 nucleus.

 

Module 8: Lesson 1 Assignment

 

Remember to submit your answers to LAB 1 and LAB 2 to your teacher as part of your Module 8: Lesson 1 Assignment. 

 

LAB 1. On the Nuclear Decay Gizmo, select “Show Equation.” Press the “play” button and add the missing information to the following equation:

 

 

LAB 2. Vary the “starting element” on the simulation and observe alpha decay for several other isotopes.

1. Is the atomic mass (number of nucleons) conserved on both sides of the equation? How can you tell?
   
2. Is the atomic number (number of protons or positive charges) conserved on both sides of the equation?
   
3. What particle is always produced in alpha decay?

In LAB 2 you observed uranium-238 as the parent element and thorium-234 as the daughter element.

 

According to the conservation laws, alpha decay can be represented by the following equation.

Quantity Symbol SI Unit X – parent element X -- Y- daughter element Y -- α- alpha particle α

The atomic mass number (A) and the atomic number (Z) are conserved in nuclear decay reactions.

 

Example Problem 1. Use the americium-241 from the smoke detector to answer the following questions.

1. What is the alpha decay equation?
   
   
   
   

2. What is the parent element and what are the daughter elements?
   
   

   The parent element is americium-241. The daughter elements are neptunium-237 and helium-4.
   
   

3. How does this decay equation obey the law of conservation of charge?
   
   

   The law of conservation of charge is obeyed because there are 95 protons before the transmutation and 95 protons (93+2) after the transmutation. Electrons are not involved in this transmutation.
   
   

4. How does this decay equation obey the law of conservation of nucleons?
   
   

   The law of conservation of nucleons is obeyed because there are 241 nucleons before the transmutation and 241 nucleons (237+4) after the transmutation.

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