Probing the Subatomic World

When two particles such as protons collide with sufficient energy, they break into smaller particles which leave behind tracks as they move away from the collision.  The illustration shows the particle tracks that could occur when such 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: 

© CERN 2008. Used with permission.



Cloud 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.



Bubble chamber: 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, go to Lesson 1 in Module 1 of Unit D and read 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
Complete "Practice Problems" 1 and 2 on page 834 and "Check and Reflect" questions 2 and 5 on page 835 of the textbook.

Energy

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 because 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.