Radioactivity• Elements with unstable
nuclei are said to be radioactive
• Eventually they break down and eject energetic particles and emit high-frequency electromagnetic radiation
• Involves the decay of the atomic nucleus, often called radioactive decay
Alpha, Beta and Gamma Rays • All elements with an atomic
number greater than 82 (after Lead) are radioactive
• These elements emit 3 different types of radiation, named α ß γ (alpha, beta and gamma)
• α : carries positive charge• ß : carries negative charge• γ : carries no charge • Can be separated by placing a
magnetic field
Alpha, Beta and Gamma Rays
• The alpha particle is the combination of 2 protons, and 2 neutrons (nucleus of He)
• Large size, easy to stop • Double positive charge (+2)• Do not penetrate through light
materials• Great kinetic energies • Cause significant damage
Alpha, Beta and Gamma Rays• A beta particle is an electron
ejected from a nucleus • The difference from this and other
electrons is that it originates inside the nucleus, from a neutron
• Faster than an alpha particle• Carries only one negative charge
(-1)• Not easy to stop • They can penetrate light materials• Harming to kill living cells
Alpha, Beta and Gamma Rays• Gamma rays are the high-
frequency electromagnetic radiation emitted by radioactive elements
• It is pure energy • Greater than in visible light,
ultraviolet light or even X rays • No mass or electric charge• Can penetrate through almost all
materials • (except Lead)• Cause damage
Sources of radioactivity• Common rocks and
minerals in the environment
• People who live in brick, concrete and stone building are exposed to greater amounts
• Radon-222 (gas arising from Uranium deposits)
• Non natural sources – medical procedures
• Coal and nuclear power industries (wastes)
Radiation dosage
• Commonly measured in rads (radiation absorbed)
• Equals to 0.01 J of radiant energy absorbed per kilogram tissue
• The unit to measure for radiation dosage based on the potential damage is the rem
• Dosage: # rads x factor of effects
• Letal doses →begin at 500 rems
The atomic nucleus and the strong nuclear force
• Strong nuclear force: attraction between neutrons and protons.
• Strong in short distances
• Repulsive electrical interactions (strong even in long distances)
• A small nucleus has more stability
The atomic nucleus and the strong nuclear force
• A nucleus with more than 82 protons are radioactive. There are many repulsive effects due to all the protons interacting together
• The neutrons are like the “nuclear cement” (hold the nucleus together). Attract p+ and nº
• The more p+, the more nº needed to balance the repulsive electrical forces
The atomic nucleus and the strong nuclear force
• In large nucleus more nº are needed • Neutrons are not stable when alone• A lonely neutron is radioactive and
spontaneously transforms to a p+ and e-
• Nº seems to need p+ to avoid this from happening
• When the nucleus`size reaches a certain point, the #nº> #p+→ nº transform into p+
• More p+= stability decreases, repulsive electric force increases, starts radiation
Half life and transmutation• Half life: the rate of decay for a
radioactive isotope. The time it takes for half of an original quantity of an element to decay
• Example: radium-226 (half life of 1620 years), uranium- 238 (half life of 4.5 billion years)
• Half lives are not affected my external conditions, constant
• The shorter the half life, the faster it desintegrates, and the more radioactivity per amount is detected
Half life and transmutation• To determine the half life
is used a radiation detector
• When a radioactive nucleus emits alpha or a beta particle, there is a change in the atomic number, which means that a different element is formed
• This change is called transmutation (Could be natural or artificial)
Natural transmutation• Uranium- 238 (92 protons, 146 neutrons)• Alpha particle is ejected (2 protons and 2 neutrons)• No longer identified as Uranium- 238 but as
Thorium-234 • Energy is released (kinetic energy of the alpha
particle, kinetic energy of the Thorium atom and gamma radiation
Natural transmutation• When an element ejects a beta particle from its
nucleus, the mass of the atom is practically unaffected, there`s no change in the mass number, its atomic number increases in 1.
• Gamma radiation results in no change in either the mass or atomic number
Artificial transmutation• Ernest Rutherford was the
1st to succeed in transmuting a chemical reaction
• He bombarded nitrogen gas with alpha particle from a piece of radioactive element. The impact of an alpha particle on the nitrogen nucleus transmutes Nitrogen into Oxygen
• Other experiments are used to make synthetic elements
Nuclear Fission • Hahn and Strassmann (1938)• Uranium has not enough
nuclear forces• Stretches into an elongated
shape• Electric forces push it into an
even more elongated shape • Electric forces > strong nuclear
forces • The nucleus splits • U-235 released energy (kinetic
energy, ejects a neutron and gamma radiation)
Nuclear FissionChain reaction
Self sustaining reaction in which the products of one reaction even stimulate further reaction events
Nuclear fission reactors • An important amount of energy in the world is
made up by the use of nuclear fission reactors • Boil water to produce steam for a turbine • The fuel is Uranium
Nuclear fission reactors• BENEFITS
Plentiful electricity
Conservation of
fossil fuels
• DISADVANTAGES
Radioactive waste
products
Mass –Energy equivalence E=mc²
• Albert Einstein discovered the mass is actually “congealed” energy
• E= the energy in rest• M= mass• C= speed of light • c²= constant of energy and mass• This relation is the key in
understanding why and how energy is released in nuclear reactions
Mass –Energy equivalence E=mc²
• More energy →greater mass in the particle
• Nucleons outside > inside
• More energy is required to separate nucleons
Nuclear fusion
• Is the opposite to nuclear fission, it is a combination of nuclei
• Energy is released as smaller nuclei fuse. Less mass is obtained
• For a fusion reaction to occur, the nuclei must collide at a very high speed in order to overcome the mutual electric repulsion
• Examples: Sun and other stars
Thermonuclear fusion
• Hydrogen →Hellium and radiation
• Less mass, more energy
• Depends on high temperatures
Atomic bombHiroshima y Nagasaki Case
• Nuclear attacks near the end of World War II against the Empire of Japan by the United States on August 6 and 9, 1945.
• “Little Boy” →Hiroshima (U-235)
• “Fat Man” → Nagasaki (Plutonium-239)
• Many people died due to the radiation poisoning