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Nuclear Changes. Section 1: What is Radioactivity?. Vocabulary. Radioactivity Nuclear radiation Alpha particle Beta particle Gamma Ray Neutron emission Half-life. Nuclear Radiation. To discuss radiation, we need to define radioactivity
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Nuclear Changes Section 1: What is Radioactivity?
Vocabulary • Radioactivity • Nuclear radiation • Alpha particle • Beta particle • Gamma Ray • Neutron emission • Half-life
Nuclear Radiation • To discuss radiation, we need to define radioactivity • Radioactivity is a PROCESS by which an unstable nucleus emits one or more particles or energy in the form of electromagnetic radiation • This means two things can happen because of radioactivity • The nucleus of an atom gives off some particle • Energy (electromagnetic energy) is given off by the atom
Changes in Nucleus • Radioactive materials have unstable nuclei • These nuclei undergo changes by emitting particles or radiation • After the changes in the nucleus, the element CAN transform into a new element entirely • But doesn’t always • This process of changing the nucleus is called nuclear decay • The released energy and matter from nuclear decay is called nuclear radiation • CAN cause damage to living tissue
Types of Nuclear Radiation • There are four types of nuclear radiation • Alpha particles • Beta particles • Gamma rays • Neutron emission • After a radioactive atom decays, the nuclear radiation leaves the nucleus of the atom • This nuclear radiation then interacts with nearby matter • The result of this interaction depends on the nuclear radiation itself
Alpha Particles, • Alpha particles are positively charged and more massive than any other type of nuclear radiation • Made of 2 protons and 2 neutrons from the unstable nucleus • Is effectively the nucleus of a helium atom • Alpha particle don’t travel far through other materials • Will barely pass through a sheet of paper • This is because it’s so massive • Because they have a positive charge, they remove electrons (ionize) matter as they pass through • As they ionize material they lose energy and slow down even more
Beta Particles, • Beta particles are fast moving electrons • Travels farther through matter than alpha particles • Still comes from the nucleus • How this works • A neutron (neutral) decays to form a proton and an electron • The electron is then ejected from the nucleus • The proton stays in the nucleus (making this atom a new element) • Will penetrate paper, but can be stopped by 3mm of aluminum foil or 10 mm of wood. • Like alpha particles, will also ionize matter they pass through • And as they ionize matter, they lose energy and slow down
Gamma Rays • Discovered by Marie Curie in 1898 • Gamma rays are high energy electromagnetic radiation emitted by a nucleus during radioactive decay. • Much more penetrating than beta particles • Not made of matter like alpha and beta particles • Does not possess a electrical charge • Because of no electrical charge, does not easily ionize matter it passes through • Damages materials due to high energy, not due to ionization • Because they don’t ionize matter, not easily stopped • Can penetrate up to 60 cm of aluminum or 7 cm of lead
Neutron Emission • Neutron emission is the release of high-energy neutrons by some neutron-rich nuclei during radioactive decay • Scientists actually discovered the neutron by neutron emission • Because neutrons have no charge, they do not ionize matter like alpha and beta particles • Since they don’t ionize, they don’t waste their energy ionizing • So neutrons will travel farther through matter than either alpha or beta particles • A block of lead about 15 cm thick is required to stop most fast moving neutrons during radioactive decay.
Nuclear Decay • When an unstable nucleus emits and alpha or beta particle, the number of protons and neutrons changes. • Example: radium-226 changes into radon-222 by emitting an alpha particle
Alpha Decay • A nucleus will give up two protons and two neutrons during alpha decay • We can write the nuclear decay process like a chemical equation • The nucleus before the decay is like a reactant • Placed on the left side of the equation • The nucleus after the decay is like a product • Placed on the right side of the equation • The particle emitted also treated like a product
Example Equation • Radium-226 Radon-222 • Hint on reading these • Number on top is the mass number • Number of protons + neutrons • Number on bottom is the atomic number • Number of protons • Has mass been conserved here? • Yes
Summary of Alpha Decay • During alpha decay • The mass number (top #) goes down by 4 • The atomic number (bottom #) goes down by 2 • Will always have the alpha particle as part of the products.
Beta Decay • During beta decay, a nucleus GAINS a proton and loses a neutron • This means that in total, the mass number (top #) stays the same • Atomic number (bottom #) increases by one
Gamma Ray Decay • When a nucleus undergoes nuclear decay by gamma rays, there is no change to the atomic number • Only the energy of the nucleus changes
Radioactive Decay Rates • It is impossible to predict the moment when any particular nucleus will decay • But it is possible to predict the time it takes for half of the nuclei in a given sample to decay • The time it takes for half of a sample of radioactive nuclei to decay is called the sample’s half-life.
Half-Life • Half-life is just a certain amount of time • Each substance has a unique half-life • After the first half life of a sample has passed, half of the sample will remain unchanged • After the second half life of a sample has passed, half of the half decays • leaving only a ¼ of the original sample unchanged • After 3rd half-life has passed, half of the ¼ remains (or 1/8) and so on and so forth
Half-Life is a Measure of How Quickly a Substance Decays • Different radioactive isotopes have different half-lives • Half-lives can be as small as nanoseconds to billions of years (like Uranium-238) • The length of the half-life depends on the stability of the nucleus • The more stable the nucleus is, the longer the half-life
Use of Half-Life • If you know how much of a particular radioactive isotope has present at the start, we can predict how old the object is • Geologists know the half-life of long-lasting isotopes, like potassium-40 • They use the half-lives of these isotopes to calculate the age of rocks • Potassium-40 decays into argon-40 • So the more argon-40 there is compared to the potassium-40, the older the rock is
Archaeologists us the half-life of carbon-14 to date more recent materials • Remains of animal or fibers from recent clothing • Can only be used to date once-living things • The ratio of carbon-14 (radioactive) to carbon-12 (stable) decreases with time • So the more carbon-14 to carbon-12 there is, the newer the once-living organism is
Nuclear Changes Section 2 – Nuclear Fission and Fusion
Vocabulary • Strong nuclear force • Fission • Nuclear chain reaction • Critical mass • Fusion
Nuclear Forces • Protons and neutrons are tightly packed into the tiny nucleus of an atom • Certain nuclei are unstable, and undergo decay • Elements can have stable and unstable isotopes • Carbon-12 is stable while carbon-14 is not • The stability of an nucleus depends on the nuclear forces holding the nucleus together. • These forces act between the protons and the neutrons
Nuclei are held together by a special force • We know that like charges repel • Took scientists a while to determine how so many positively charged protons fit into an atomic nucleus without flying apart • Answer is the strong nuclear force • The strong nuclear force is the interaction that binds protons and neutrons together in a nucleus • This attraction is MUCH stronger than the repulsion between the protons • But force only occurs over very short distances (about the width of 3 protons)
Neutrons contribute to nuclear stability • Due to the strong nuclear force, neutrons and protons in a nucleus attract other protons and neutrons • Because neutrons have no charge, they don’t repel anything • Whereas the protons, having charge, repel other protons • In a stable nuclei, the attractive forces are stronger than the repulsive forces
Too many neutrons or protons • While neutrons help hold a nucleus together, too many neutrons makes a nucleus unstable • Nuclei with more than 83 protons are always unstable, no matter how many neutrons they have • These nuclei will always decay • Will always release large amounts of energy and nuclear radiation
Nuclear Fission • The process of the production of lighter nuclei from heavier nuclei is called fission • During fission, a nucleus splits into two or more smaller nuclei, releasing neutrons and energy • Note that this fission reaction was started by firing 1 neutron at the Uranium-235 nucleus • Also note we produced more neutrons, energy, and 2 different, lighter nuclei
Energy is released during nuclear fission • Each dividing nucleus releases about 3.2x10-11 J of energy • Compare this with TNT, which releases only 4.8x10-18J per molecule • During fission reactions, a small amount of mass is lost after the reaction • That mass was converted into energy • E = mc2
Neutrons released by fission can start a chain reaction • A nucleus that splits when it is struck by a neutron forms smaller nuclei • Smaller nuclei need fewer neutrons, so excess neutrons are emitted • Excess neutrons can collide with another large nucleus • Triggering another nuclear reaction • Which releases more neutrons, and so on and so on
Generally each nucleus split will generate about 3 neutrons • So each nucleus that fissions will cause 3 other nuclei to fission • This generates a nuclear chain reaction • A series of fission processes in which the neutrons emitted by a dividing nucleus cause the division of other nuclei. • The more nuclei released, the bigger the chain reaction will be
Critical Mass • There has to be enough of the fissionable material in order for a chain reaction to occur • The minimum mass of a fissionable isotope in which a nuclear chain reaction can occur is called the isotope’s critical mass • This is how they make nuclear bombs
Nuclear Fusion • Just like energy is obtained when nuclei break apart, energy can be obtained when very light nuclei are combined to form a heavier nuclei • This type of nuclear process is called fusion • Its how stars make energy • Requires LARGE amounts of energy
