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This article explores the concepts of radioactivity and radioisotopes in nuclear chemistry, including the types of radiation, nuclear equations, and examples of natural radioactive decay. It also discusses the properties and effects of alpha, beta, gamma radiation, and positron emission. Additionally, the article touches on the topic of antimatter and its potential use in propulsion systems.
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Radioactivity • Radioisotopes are isotopes that have an unstable nucleus. They emit radiation to attain more stable atomic configurations in a process called radioactive decay. • Radioactivity is the property by which an atomic nucleus gives off alpha, beta, or gamma radiation. • Marie Curie named the process. • In 1898, Marie & Pierre Curie identified 2 new elements, polonium & radium. • The penetrating rays and particles emitted by a radioactive source are called radiation.
Radioactivity (cont) • The presence of too many or too few neutrons, relative to the number of protons, leads to an unstable nucleus. • The types of radiation are alpha (α), beta (β), or gamma (γ). • An unstable nucleus loses energy by emitting radiation during the process of radioactive decay. • Spontaneous and does not require any input of energy.
The effect of an electric field on α,β, and γ, radiation. The radioactive source in the shielded box emits radiation, which passes between two electrodes. Alpha radiation is deflected toward the negative electrode, β radiation is strongly deflected toward the positive electrode, and γ radiation is undeflected.
Nuclear Equations • For a nuclear reaction to be balanced, the sum of all the atomic numbers and mass numbers on the right must equal the sum of those numbers on the left. • To figure out the unknown isotope, you need to balance the equation.
Natural Radioactive Decay • Why • The nucleus has many positively charged protons that are repelling each other. • The forces that hold the nucleus together can’t do its job and the nucleus breaks apart. • All elements with 84 or more protons are unstable and will eventually undergo nuclear decay. • How • Alpha particle emission • Beta particle emission • Gamma radiation emission • Positron emission (less common) • Electron capture (less common)
Alpha radiation • A type of radiation called alpha radiation consists of helium nuclei that have been emitted from a radioactive source. • These emitted particles, called alpha particles, contain 2 protons and 2 neutrons and have a double positive charge.
Alpha Radiation (cont) • Because of their large mass and charge, alpha particles do not tend to travel very far and are not very penetrating. • They are easily stopped by a piece of paper or the surface of skin. • Radioisotopes that emit alpha particles are dangerous when ingested.
Alpha radiation occurs when an unstable nucleus emits a particle composed of 2 protons and 2 neutrons. The atom giving up the alpha particle has its atomic number reduced by two. Of course, this results in the atom becoming a different element. For example, Rn undergoes alpha decay to Po.
Beta Particles • A beta particle is essentially an electron that’s emitted from the nucleus. • A neutron is converted (decayed) into a proton & electron…so the atomic number increases by 1 and the electron leaves the nucleus. • Isotopes with a high neutron/proton ratio often undergo beta emission, because this decay allows the # of neutrons to be decreased by one & the # of protons to be increased by one, thus lowering the neutron/proton ratio.
Beta radiation occurs when an unstable nucleus emits an electron. As the emission occurs, a neutron turns into a proton.
Positron Emission • A positron is essentially an electron that has a positive charge instead of a negative charge. • A positron is formed when a proton in the nucleus decays into a neutron & a positively charged electron. • It is then emitted from the nucleus. • The positron is a bit of antimatter (seen in Star Trek). When it comes in contact with an electron, both particles are destroyed with the release of energy.
Positron emission occurs when an unstable nucleus emits a positron. As the emission occurs, a proton turns into a neutron.
Positron emission tomography, also called PET imaging or a PET scan, is a diagnostic examination that involves the acquisition of physiologic images based on the detection of radiation from the emission of positrons. Positrons are tiny particles emitted from a radioactive substance administered to the patient.
Antimatter • National Geographic Article • When a particle and its antiparticle meet, they annihilate each other and their entire mass is converted into pure energy. • Compared to conventional chemical propulsion systems, antimatter energy would slash the travel time to Mars and back from roughly two years to a few weeks. • The world's largest maker of antimatter, the Fermi National Accelerator Laboratory in Batavia, Illinois, makes only one billionth of a gram a year at a cost of $80 million.
An artist's concept of a robotic antimatter-powered probe sailing past planets in an imaginary nearby solar system. Credit: Laboratory for Energetic Particle Science at Pennsylvania State University. This artist's concept of an antimatter-powered rocket ship looks like a big space-borne linear accelerator. Credit: Laboratory for Energetic Particle Science at Pennsylvania State University.
Gamma Radiation • Gamma radiation is similar to x-rays – high energy, short wavelength emissions (photons). • The symbol is γ, the Greek letter gamma. • It commonly accompanies alpha and beta emission, but it’s usually not shown in a balanced nuclear reaction. • Some isotopes, such as Cobalt-60, give off large amounts of gamma radiation. • Co-60 is used in the radiation treatment of cancer…the gamma rays focus on the tumor, thus destroying it.
Gamma radiation occurs when an unstable nucleus emits electromagnetic radiation. The radiation has no mass, and so its emission does not change the element. However, gamma radiation often accompanies alpha and beta emission, which do change the element's identity.
Electron Capture • Electron capture is a rare type of nuclear decay in which an electron from the innermost energy level (1s) is captured by the nucleus. • This electron combines with a proton to form a neutron. • The atomic number decreases by one but the mass stays the same. • Electrons drop down to fill the empty space in the 1s orbital, releasing energy.
Decay Series • The nucleus is always seeking stability. • It will continue to emit particles from the nucleus until balance of charge and mass is obtained. • Let’s look at some examples.
Man-Made Radioactive Decay on Earth • Fission: Split Apart • Fusion: Bring Together • Occurs naturally in space • Powers the sun • Supernovas allow atoms to fuse into heavier elements, this is how the other elements came into existence
Fission • Nuclear fission occurs when scientists bombard a large isotope with a neutron. • This collision causes the larger isotope to break apart into two or more elements. • These reactions release a lot of energy. • You can calculate the amount of energy produced during a nuclear reaction using an equation developed by Einstein: E=mc2
Chain Reactions • A chain reaction is a continuing cascade of nuclear fissions. • This chain reaction depends on the release of more neutrons then were used during the nuclear reaction. • Isotopes that produce an excess of neutrons in their fission support a chain reaction - fissionable. • There are only two main fissionable isotopes used during nuclear reactions – uranium-235 & plutonium-239.
Chain Reactions (cont) • The minimum amount of fissionable material needed to ensure that a chain reaction occurs is called the critical mass. • Anything less than this amount is subcritical.
Chain Reaction Figure Does this image show critical or subcritical mass? Why do you think so?
Atomic Bombs • Because of the tremendous amount of energy released in a fission chain reaction, the military implications of nuclear reactions were immediately realized. • The first atomic bomb was dropped on Hiroshima, Japan, on August 6, 1945. • In an atomic bomb, two pieces of a fissionable isotope are kept apart. Each piece by itself is subcritical. • When it’s time for the bomb to explode, conventional explosives force the two pieces together to cause a critical mass. • The chain reaction is uncontrolled, releasing a tremendous amount of energy almost instantaneously.
Nuclear Power Plants • If the neutrons can be controlled, then the energy can be released in a controlled way. Nuclear power plants produce heat through controlled nuclear fission chain reactions. • The fissionable isotope is contained in fuel rods in the reactor core. All the fuel rods together comprise the critical mass. • Control rods, commonly made of boron and cadmium, are in the core, and they act like neutron sponges to control the rate of radioactive decay.
Nuclear Power Plants (cont) • In the U.S., there are approximately 100 nuclear reactors, producing a little more than 20% of the country’s electricity. • Advantages • No fossil fuels are burned. • No combustion products (CO2, SO2, etc) to pollute the air and water. • Disadvantages • Cost - expensive to build and operate. • Limited supply of fissionable Uranium-235. • Accidents (Three Mile Island & Chernobyl) • Disposal of nuclear wastes
A nuclear power plant. Heat produced in the reactor core is transferred by coolant circulating in a closed loop to a steam generator, and the steam then drives a turbine to generate electricity.
Three Mile Island What factors lead to the partial meltdown of unit 2? What was the result of the partial meltdown?
Chernobyl What factors lead to the total meltdown of at Chernobyl? What was the result of the total meltdown?
Nuclear Fusion • Fusion is when lighter nuclei are fused into a heavier nucleus. • Fusion powers the sun. Four isotopes of hydrogen-1 are fused into a helium-4 with the release of a tremendous amount of energy. • On Earth, H-2 (deuterium) & H-3 (tritium) are used.
Plasma • Plasmas are conductive assemblies of charged particles, neutrals and fields that exhibit collective effects. • Further, plasmas carry electrical currents and generate magnetic fields. • Plasmas are the most common form of matter, comprising more than 99% of the visible universe, and permeate the solar system, interstellar and intergalactic environments.
Nuclear Fusion (cont) • The first demonstration of nuclear fusion – the hydrogen bomb – was conducted by the military. • A hydrogen bomb is approximately 1,000 times as powerful as an ordinary atomic bomb. • The goal of scientists has been the controlled release of energy from a fusion reaction. • If the energy can be released slowly, it can be used to produce electricity. • It will provide an unlimited supply of energy that has no wastes to deal with or contaminants to harm the atmosphere. • The 3 problems are: temperature, time, containment
Nuclear Fusion (cont) • Temperature • Hydrogen isotopes must be heated to 40,000,000 K (hotter than the sun). • Electrons are gone…all that’s left is positively charged plasma. • Time • The plasma needs to be held together for about one second at 40,000,000 K. • Containment • Everything is a gas…ceramics vaporize. A magnetic field could be used but the plasma leaks from those as well.
