Evans SPH 4U1

Physics Grade 12

Unit 10: Radioactivity (Qualitative Only)

Reference: Chapter 13 (Qualitative Only)

Radioactivity:

• A property of the atom. The atom's nucleus makeup changes and forms a radioactive atom that emits radiation as it decays.

• Unlike electricity and magnetism, it is unaffected by heating or cooling (cannot stop decay - even after strongest chemical treatments)

• 3 types - Alpha particle, Beta particle, Gamma rays

Recall - Structure of Atom:

• protons must repel each other with an electric Force, therefore they must be held together by a Strong Nuclear Force. 

• If an atom's nucleus is to be split apart work must be done to break the nucleus apart (binding energy)

Alpha Decay:

• When a radioactive material emits an alpha particle, the nucleus of one of its atoms loses 2 protons and 2 neutrons. This loss changes the atom to a new element (process called transmutation). The nucleus of this new atom is called the daughter nucleus.

• mass of the atomic nucleus is less than the mass of its constituent neutrons and protons. (the energy equivalent of the change in mass is the Binding Energy (work that needs to be done to break the nucleus apart)

Beta Decay:

• Beta particles emitted in radioactive decay are either electrons - NOT an orbital electron (decay) or positrons (decay).

• The beta particles are made in the nucleus

  • for decay a neutron becomes an electron and a proton (atomic # of daughter will become 1 more)

  • fordecay a proton becomes a positron and a neutron (atomic # of daughter will become 1 less)

• At first it appeared that momentum and energy weren't being conserved but they postulated (1956) that there was another particle being released called a neutrino (no charge).

Gamma Rays:

• decay produces photons with 0 mass and no charge. A highly excited nucleus drops to a lower energy level while emitting a photon.

• no transmutation (i.e. no new atom is formed)

• Like Xrays BUT Xrays are produced when high energy electrons interact with matter whereas those produced within the nucleus are gamma rays.

Example:

Shows alpha decay (loses 2 protons and 2 neutrons)

Daughter (thorium) could become very excited and orbital electrons will drop to a lower energy level and release a photon.

Radioactive Dating:

Talk about carbon 14

Accelerators (discovery of particles):

Accelerate negatively charged hydrogen ions and strip off electrons by passing them through a charged foil.

This leaves proton beams.  Now you can fire the proton beams at a target material and analyze all the bits and pieces (new paticles)

We cannot see the bits and pieces that come flying off so we use:

        1) Cloud chambers - supersaturated gas (verge of condensing) so when particles go through, the gas condenses along the trajectory

        2) Bubble chambers

More Particles!:

At one time scientists thought the atom was the elemental particle.  Then Rutherford determined there were protons and neutrons. NOW physicists believe particles can be grouped into 2 families Quarks and Leptons.

Quarks:     3 Quarks = Proton (2 up spin, one down spin)

                3 Quarks = Neutron (2 down spin, one up spin)

Every particle has an antiparticle.

Lately even quarks are not considered to be elemental and there's a theory called the string theory that says everything is made up of strings.

Leptons are electrons and neutrinos.

Forces:

1) Electromagnetic Force - atoms join together sharing electrons. Electrons are attracted to atoms' nuclei. Holds matter together.

2) Strong Force - holds nucleus (protons and neutrons together).

3) Weak Force - explains Beta decay.

4) Gravity

Physicists are attempting to come up with one theory to explain all of the above forces "Grand Unified Theory".  They feel they are getting close with "The String Theory".  The string theory says that everything is made up of strings.