Understanding Types of Nuclear Decay

Nuclear Decay

• Spontaneous nuclear decay comes from an unstable nucleus, called radioactivity
• Background radiation comes from naturally decaying elements, such as radon, cosmic rays, and the Earth's interior
• Elements with an atomic number less than 30 have a stable proton-to-neutron ratio of approximately 1:1
• Elements with an atomic number greater than 50 need a proton-to-neutron ration of approximately less than 1:1.5 to be stable

Types of Decay

• Alpha decay results in the loss of a Helium nucleus from an atom
• Alpha decay emits a helium nucleus, reducing the atomic mass by 4 and the atomic number by 2
• Alpha decay has the highest atomic mass but is the least penetrative
• Alpha decay has high kenetic energy
• Alpha decay causes damage to surfaces
• Alpha particles typically do not penetrate lightweight materials like paper or clothing
• Alpha particles cause significant damage to a material's surface, especially living tissue, due to high kinetic energy
• An alpha particle picks up electrons to becomes harmless when traveling through air
• Alpha particles are deflected in the presence of magnetic or electric fields
• Beta decay occurs when a neutron breaks down into a proton and an electron
• During beta decay, the electron leaves and the proton stays
• Beta decay causes the atomic number to increase while the atomic mass remains the same
• Beta particles have very little mass so they are more penetrative than alpha particles
• Beta particles can be stopped by several sheets of aluminum foil
• Beta particles penetrate fairly deeply into skin and have the potential for harming or killing living cells
• Beta particles are deflected in the opposite direction to alpha particles in the presence of magnetic and electric fields
• Positron emission is a type of B+ radiation
• Positrons have a positive charge equal to a proton
• During positron emission, a proton breaks into a neutron and emits the positron
• Positron emission occurs when there are more protons than neutrons
• Gamma decay is the loss of energy from the nucleus
• Gamma decay usually accompanies other types of decay, and it is the most penetrative

Transmutation

• Transmutation is the change of one element to a different element
• Natural transmutation occurs when unstable nuclei undergo spontaneous decay
• Artifical transmutation is the bombardment of a nucleus with other particles

Sources of Radiation

• Natural sources of radiation include cosmic radiation, ground radiation, air (radon-222), and human tissues
• Human sources of radiation include smoking, X-rays, nuclear medicine, TV tubes, weapons testing, coal plants and nuclear plants

Radiation Dosage

• Rads measure radiation-absorbed dose
• One rad is equal to 0.01 J of radiant energy absorbed per kilogram of tissue
• REMS measures the radiation dosage based on the potential damage
• To calculate REMS, multiply the number of rads by a factor corresponding to the different health effects of different types of radiation

Half-Life

• Half-life is the time needed to change ½ of a radioactive element into a different element
• For example, if the half-life of K-41 is 20 days, 50g of K-41 would change into 25g of K-41 and 25g of other elements in 20 days
• Half-life is not changed by heat, other radiation or any external force
• A radioactive sample never completely transmutes
• ½ of any number never equals zero

Half-Life Problems

• Starting with 800g of K-41, with a half-life of 20 days, 50g will remain after 80 days
• A 1kg sample of Cr-52 decaying to 125g of Cr-52, with a half-life of 120 hours will take 360 hours to decay
• Starting with 4.0Mg of Phosphorus-32 with a half-life of 14.3 days, 0.25Mg will remain after 57.2 days
• Starting with 2.0Mg of Po-210 with a half-life of 138.4 days, 0.125Mg will remain after 415.2 days
• Starting with 16Mg of Po-218 with a half-life of 3 minutes, it will take 12 minutes before only 1.0Mg remains
• An 840kg sample of Technetium, with a half-life of 18 minutes, will have 5.12kg of Technetium remaining after 2.4 hours

Uses of Radioactivity

• Treatment of diseases
• Use of tracers
  • Diagnosis of Diseases
  • Study chemical reactions or follow chemical pathways
• Energy Source
• Dating
• Carbon-14 gets into the body of all living things
• C-14 dating is done by comparing the percentage of C that is C-14 in a fossil versus the percentage of C-14 in a modern sample of similar structure/substance
• The decay rate of minerals by uranium decaying into lead, is done in non-living things
• Longer the half-life, the less the accuracy

Fission

• Fission is a nuclear split, giving off elements, particles (often neutrons) and energy
• Chain reactions occur when emitted particles from fission cause other nuclei to fission
• Critical mass is the minimum amount that sustains the chain reaction

Fission Bombs

• The "A" bomb uses uncontrolled fission
• Parts of critical mass need to stay separate by using sub-critical masses
• A chemical explosive is used to brine the parts together
• The Hiroshima bomb called "Little Boy" occurred on August 6th
• The Nagasaki bomb called "Fat Man" occurred on August 9th

Fission Reactors

• Controlled fission has neutron-absorbing control rods to regulate the reaction rate
• Energy that's given off, heats water which powers electric generators
• Nuclear waste must be stored until it decays through many half-lives

Breeder Reactors

• U-238 is abundant but a reluctant fissionable material
• Once changed to Pu-239, the nucleus rapidly fissions
• Twice as much fuel is generated when Plutonium is used

Fusion

• Fusion occurs when relatively light nuclei are put together at very high temperatures, known as thermonuclear reactions.
• Happens on the sun as hydrogen nuclei fuse to form helium
• Massive amounts of energy result, but on Earth, it takes far more energy than it makes

Fusion Bomb

• The “H” bomb contains a fission bomb
• It is required to produce needed high temperatures for fusion to occur