Half-life problems

• Atoms are the building blocks of everything
• Atoms are too small to be seen with a regular microscope
• Scientists use models to explain basic ideas about atomic structure

Charge Essentials

• Electric charge comes in two forms: positive (+) and negative (-)
• Like charges repel each other
• Unlike charges attract each other

Atomic Structure

• An atom has a central nucleus that contains protons and neutrons
• Electrons orbit the nucleus
• The number of these particles determines the type of the atom
• Protons have a positive (+) charge
• Electrons have a negative (-) charge
• Usually, an atom has an equal number of electrons and protons, making it electrically neutral
• Protons and neutrons are collectively called nucleons
• Nucleons are about 1800 times more massive than electrons
• The nucleus accounts for almost all of an atom's mass
• Electrons are held in orbit due to the attraction between opposite charges
• Protons and neutrons are held together in the nucleus by a distinct force called the strong nuclear force

Elements and Atomic Number

• Elements are the 100 basic substances that make up all materials
• An atom is the smallest piece of an element
• Each element has a different number of protons in its atoms
• This number is referred to as the atomic number or proton number
• The atomic number also indicates the number of electrons in the atom
• Hydrogen (H) atomic number 1
• Helium (He) atomic number 2
• Lithium (Li) atomic number 3
• Beryllium (Be) atomic number 4
• Boron (B) atomic number 5
• Carbon (C) atomic number 6
• Nitrogen (N) atomic number 7
• Oxygen (O) atomic number 8
• Radium (Ra) atomic number 88
• Thorium (Th) atomic number 90
• Uranium (U) atomic number 92
• Plutonium (Pu) atomic number 94
• The total number of protons and neutrons in the nucleus is the mass number (nucleon number)

Isotopes and Mass Number

• Isotopes are versions of an element with the same atomic number but different mass numbers due to a varying number of neutrons
• Most elements occur as a mixture of two or more isotopes
• Metal lithium (atomic number 3) is a mix of two isotopes with mass numbers 6 and 7
• Lithium-7 makes over 93 percent of lithium atoms
• A nuclide is a specific type of atom characterized by its number of protons and neutrons

Electron Shells

• Electrons orbit the nucleus at fixed levels called shells
• Shells can only hold a certain number of electrons
• The innermost shell can hold no more than 2 electrons
• The second shell can hold no more than 8 electrons
• Electron arrangement, especially in the outermost shell, is responsible for the chemical properties of an element

Nuclear Radiation

• Radioactive materials emit nuclear radiation as unstable nuclei disintegrate
• Radioactive decay is the disintegration of a nucleus
• Nuclear radiation originates from both natural sources and nuclear power stations

Isotopes

• Isotopes are different versions of the same element with the same number of protons but with different numbers of neutrons

Ionizing Radiation

• Ions are charged atoms or groups of atoms, gaining charge by losing or gaining electrons
• Nuclear radiation, ultraviolet light, and X-rays can remove electrons from atoms and cause ionization
• Ionization of a gas makes it conductive
• Ionization in living organisms can damage and destroy cells

Alpha, Beta, and Gamma Radiation

• The three types of nuclear radiation are alpha particles, beta particles, and gamma rays
• Gamma rays are the most penetrating
• Alpha particles have the least penetration
• Beta particles have penetrating power that falls in between

Radiation Types

• Alpha Particles (α):
  • Composed of 2 protons and 2 neutrons (identical to a helium-4 nucleus)
  • Relative charge of +2
  • High mass compared to betas
  • Low speed (up to 0.1 x speed of light)
  • Strong ionizing effect Stopped by a thick sheet of paper, skin, or few centimeters of air
  • Deflected by magnetic and electric fields
• Beta Particles (β):
  • Each particle is an electron (created when the nucleus decays)
  • Relative charge of -1
  • Low mass
  • High speed (up to 0.9 x speed of light)
  • Weak ionizing effect
  • Stopped by a few millimeters of aluminum or other metal
  • Deflected by magnetic and electric fields
• Gamma Rays (γ):
  • Electromagnetic waves similar to X-rays
  • No charge (0)
  • No mass
  • Travels at the speed of light
  • Very weak ionizing effect
  • Never completely stopped, but reduced by lead and thick concrete
  • Not deflected by magnetic or electric fields
• Alpha particles are more ionizing than beta particles due to greater charge and lower speed
• Gamma rays are least ionizing since they are uncharged
• Alpha and beta particles are deflected by a magnetic field; alpha beams are equivalent to an electric current and are deflected according to Fleming's left-hand rule
• Beta particles are lighter and have a negative charge so they are deflected more and in the opposite direction

Radiation Dangers

• Radiation can damage or destroy living cells, disrupt organ function, cause abnormal cell growth, and even lead to cancer
• The higher the exposure time, the greater the risk
• Radioactive gas and dust are particularly dangerous if inhaled because they are difficult to remove and can cause internal damage
• Alpha radiation poses the highest level of harm from ionizing
• Shielding and distance can minimize radioactive risk
• Beta and gamma rays, can penetrate further, make internal organs more vulnerable

Background Radiation

• Background radiation comes from natural sources like soil, rocks, air, building materials, food and water, and space
• Radon gas (radon-222) from rocks is a major contributor to background radiation, especially in areas with certain types of granite, and houses in those areas might require added ventilation to prevent radon accumulation

Geiger-Müller (GM) Tube

• The GM tube detects alpha, beta, and gamma radiation
• It contains a thin window for alpha particles to pass through
• Gas inside the tube gets ionized by incoming particle
• Then a high-voltage spark/ current pulse causes this spark to be detected to be read on a connected device

GM Tube Connections

• Ratemeter: Displays radiation count reading in counts per second
• Scaler: Tallies the total count of particles or gamma radiation bursts
• Amplifier and Loudspeaker: Emits a 'click' sound for each detected particle or radiation burst
• Background Radiation Subtraction: Measurements from a radioactive source always include background radiation
• Background radiation measurements must be subtracted from the total

Cloud Chamber

• Allows study of alpha particles by making their paths visible via condensation of cold alcohol vapour
• At one time was widely used in nuclear research

Radioactive Decay

• Radioactive decay occurs spontaneously and randomly and is unaffected temperature, pressure, or chemical changes
• Isotopes decay at different rates

Rate of Decay and Half-Life:

• The rate of decay varies among different types of nuclei
• Iodine-131 Example: on average, 1 nucleus disintegrates every second for every 1,000,000 nuclei present
• Definition: Half-life is the time it takes for half of the radioactive nuclei in a sample to decay
• After one half-life, half of the radioactive nuclei have disintegrated
• After each subsequent half-life, the disintegration number is halved

Half-Life Examples

• Boron-12: 0.02 seconds
• Radon-220: 52 seconds
• Iodine-128: 25 minutes
• Radon-222: 3.8 days
• Strontium-90: 28 years
• Radium-226: 1602 years
• Carbon-14: 5730 years
• Plutonium-239: 24,400 years
• Uranium-235: 7.1 x 108 years
• Uranium-238: 4.5 × 109 years
• Short-lived isotopes can arise due to their creation as radioactive daughters of longer-lived parents or through artificial production in nuclear reactors

Activity and Half-Life

• Activity Definition: Average number of disintegrations per second in a radioactive sample
• SI Unit: Becquerel (Bq)
• Half-life also applies to the time it takes for the activity of a sample to decrease to half of its original value

Stability of the Nucleus

• The stability of nucleus depends on proton-to-neutron ratio
• Stable isotopes are along the stability line when the number of neutrons is plotted against the number of protons
• Isotopes above the stability line emit beta (electron) particles
• Doing so reduces the excess number of neutrons
• Isotopes below the stability line emit beta+ (positron) particles
• This will increase the number of neutrons
• Isotopes with proton numbers > 83 decay by alpha emission

Nuclear Essentials

• Atoms of any one element always have the same number of protons
• If the number of protons changes an atom of a completely different element is formed
• Isotopes: different versions of elements with unstable nuclei that emits particles and gamma rays
• Nuclear energy is released during collisions and temperature rises
• Transformed into heat

Nuclear Reaction

• A nuclear reaction is whenever a particle penetrates and changes a nucleus

Fission

• Fission is when a neutron hits a uranium-235 nucleus making it unstable
• The Uranium splits into two lighter nuclei
• Two or three neutrons are released for every collision
• Energy is released
• The condition for a chain reaction to be maintained is that uranium-235 has to be above a certain critical mass
• Otherwise too many nuetrons escape

Fission in a Nuclear Reactor

• Nuclear reactors use a controlled chain reaction
• Thermal energy (heat) is released at a steady rate
• The resulting heat converts water into the steam which will turn the turbines

Moderators

• Moderators slow reaction to keep neutrons in control
• Either Graphite or Water may be used

Control Rods

• Rate of reaction is controlled by control rods containing boron or cadmium
• These materials absorb neutrons

Nuclear Waste Characteristics

• Nuclear waste includes strontium-90, iodine-131 and plutonium-239
• Breathed in as dust, the smallest amount of Plutonium can kill
• Liquid waste is usually liquid, sealed and shielded around it
• Energy mass is important for safe storage

Energy and Mass

• Energy has mass itself, in effect making them interchangeable
• Mass and energy change by the equation: E = mc2
• The value of C squared is enough to effect the size of energy

Fusion Reactors

• Fusion can be used in the center of the proton and nuetrons

Fusion Process

• Fusion makes protons and neutrons regroup making tighter arrangements than before
• Protons and neutrons tend to be in the middleweight, so they have to be tightly held

Nuclear Fusion

• When very light nuclei join together

Hydrogen

• An element that has one proton

Fusion Reactor*

• Achieve fusion by superheating hydrogen to 40 million degrees Celsius
• Superhot gases are held in containment units using a magnetic field
• Fusion reactors can extract fuel from sea water
• Fusion stops if the system fails for safety

Nuclear Fusion

• In stars there is heat output and huge gravitations
• The large amount of hydrogen makes them hot

Fusion in the Suns Core

• The sun has Hydrogen which makes up the nebular
• Gravity helps the star get enough energy and heat to shine

Test your knowledge of half-life with these practice questions. Determine the amount of radioactive substance remaining after a certain period given its half-life. Match the definitions to the correct half-life terms.