Module 8

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

 

Explore

 

Probing the Subatomic World

 

When two particles such as protons collide at sufficient energy, they break down into smaller particles, which leave behind tracks as they move away from the collision. The illustration here shows the particle tracks that could occur when two particles collide. These tracks are used to deduce the nature of the subatomic particle that created them. 

 

For example, if the collision occurs in a uniform magnetic field, the direction of the curved tracks reveals the charge of the particle. The radius of curvature can also be measured to give the charge-to-mass ratio of the particle in a way similar to that of a mass spectrometer.

 

Early devices designed to capture particle tracks include the cloud chamber and bubble chamber.

 

Cloud Chamber	Bubble Chamber
A device that contains dust-free supersaturated water or ethanol vapour, which will condense along the path of a particle that moves through it.	A device that contains liquefied gas, such as hydrogen, which boils and forms bubbles along the path of a particle that moves through it.

 

Only charged particles and photons capable of ionizing the material in the chambers will produce tracks. The nature of the charge can be determined with the appropriate hand rule and the charge-to-mass ratio can be calculated based on the radius of the curvature using . To review charge-to-mass ratio, see Module 7: Lesson 1 about cathode rays and Thomson’s experiment.

 

Read

 

Read “Detecting and Measuring Subatomic Particles” on pages 830 to 835 of your physics textbook.

 

Try This

 

TR 1. Complete “Practice Problems” 1 and 2 on page 834 and “Check and Reflect” questions 2 and 5 on page 835 of the textbook.

 

Module 8: Lesson 4 Assignment

 

Remember to submit your answer to A1 to your teacher as part of your Module 8: Lesson 4 Assignment. 

 

A 1. Explain how to use the following diagram of the bubble chamber paths of an alpha particle, beta particle, beta positive particle, and a gamma ray to determine which path corresponds to which particle.

 

 

The amount of energy required to overcome the strong nuclear force and scatter the contents of the nucleus is significant. Consider the energy used in various experiments so far:

• 13.6 eV: ionizes the hydrogen atom in the study of electron energy levels
  
  
• 1.0 × 107 eV: produces Rutherford scattering, revealing the nature of the nucleus

Early particle accelerators were sometimes called atom smashers, since they could develop enough energy to scatter the contents of the nucleus. The strong nuclear force can be overcome in a particle accelerator, causing the contents of a nucleon to scatter.

• 2.0 × 109 eV: produces heavier nuclei and scatters nuclear particles in a collision

Current particle accelerators, such as the Large Hadron Collider, can generate more energy than that needed to overcome the strong nuclear force.

• 1.4 × 1013 eV: maximum energy used in the LHC to expose subatomic particles  

Read

 

Read “Probing the Structure of Matter” on pages 840 and 841 of the textbook.

 

Fundamental Particles

 

Antimatter

 

Prior to the development of quantum theory it was believed that all matter was made of three fundamental particles: electron, proton, and neutron. More recent developments in this theory predict the existence of other subatomic particles, some with very peculiar properties. For example, quantum theory predicts that every kind of particle has a corresponding antiparticle. The antiparticle of an electron is called the positron, which has an identical magnitude charge-to-mass ratio as an electron but with a positive charge. American physicist Carl Anderson identified it in particle tracks in 1932.

 

When a particle and its antiparticle collide, they are both annihilated and produce a pair of high-energy gamma ray photons. The collision of an electron and a positron are part of the nuclear process in stars. This matter-antimatter collision can be described with an equation .

 

Example Question 1. How much energy is released when an electron-positron pair annihilate? 

 

Given

 

 

Required

 

the energy released from the annihilation

 

Analysis and Solution

 

Remember that in the annihilation one electron and one positron’s worth of mass is annihilated.

 

 

Paraphrase

 

One electron positron pair annihilation releases 1.64×10–13 J.

 

Example Question 2. How much energy would be released from the annihilation of 1.00 kg of electrons in an antimatter reaction?

 

Given

 

 

Required

 

the energy released from the annihilation of 1.00 kg of electrons

 

Analysis and Solution

 

Determine the amount of energy released; remember that to annihilate 1.00 kg of electrons takes 1.00 kg of positrons.

 

 

Paraphrase

 

When 1.00 kg of electrons is annihilated it will produce 1.80 × 1017 J of energy.

 

Remember from page 820 in your physics textbook that 1.0 kg of uranium releases 7.10 × 1013 J/kg and that 1.0 kg of gasoline releases 4.4 × 107 J/kg. So, antimatter reactions are extremely energy dense.

 

Module 8: Lesson 4 Assignment

 

Remember to submit your answers to A 2 and A 3 to your teacher as part of your Module 8: Lesson 4 Assignment. 

 

A 2. Compare and contrast matter and antimatter. 

 

A 3. Give an example of a matter–antimatter pair you have already seen in Physics 30, other than electron–positron.

 

Mediating Particles

 

Quantum theory also predicts the existence of particles that produce fundamental forces like gravity and the strong nuclear force. These mediating particles are thought to carry the fundamental forces and exist for such a short time that they are undetectable. The following table summarizes the mediating particles and their relationship to the fundamental forces.

 

Mediating Particle	Fundamental Forces	Particles Observed?
photons	electromagnetic	yes
gluons	strong nuclear	indirectly
gravitrons	gravitational	no
W+.W–,Zo	weak nuclear	yes

 

Read

 

Read “Quantum Theory and the Discovery of New Particles” on pages 836 to 838 of your textbook.

 

The Subatomic Zoo

 

More than 300 more subatomic particles have been discovered using new and more powerful particle accelerators and detectors. These particles have been classified by family.

 

Leptons do not interact via the strong nuclear force and are relatively small.

 

Hadrons do interact via the strong nuclear force and are subdivided based on size (meso is Greek for “middle”; barus is Greek for “heavy”). The particles are also classified by “spin,” which is analogous to describing the rotational momentum of the spinning particle. Boson and fermion are classifications based on the spin of the particle.

 

Read

 

Read “The Subatomic Zoo” on pages 842 to 844 of your physics textbook. Take note of “Table 17.3: An Introduction to the Subatomic Zoo,” which identifies the particles, symbols, mass, and lifetime of many subatomic particles.

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