From atoms to stars, welcome to the WORLD of NUCLEAR CHEMISTRY
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A "Geiger counter" usually contains a metal tube with a thin metal wire along its middle, the space in between them sealed off and filled with a suitable gas, and with the wire at about +1000 volts relative to the tube.
An ion or electron penetrating the tube (or an electron knocked out of the wall by X-rays or gamma rays) tears electrons off atoms in the gas, and because of the high positive voltage of the central wire, those electrons are then attracted to it. In doing so they gain energy, collide with atoms and release more electrons, until the process snowballs into an "avalanche" which produces an easily detectable pulse of current. With a suitable filling gas, the flow of electricity stops by itself, or else the electrical circuitry can help stop it.
The instrument was called a "counter" because every particle passing it produced an identical pulse, allowing particles to be counted (usually electronically) but not telling anything about their identity or energy (except that they must have sufficient energy to penetrate the walls of the counter). Van Allen's counters were made of thin metal, with insulating plugs at the ends.
Smoke Detectors
Smoke Dectetors sense the presence of smoke particles in two-ways: (1) optical detectors and (2) ionization detectors. Alpha particles emitted by Am-241 ionize the air inside the detector chamber, setting up a low-level current between the anode and cathode. When smoke particles enter the chamber, they compete with ions emitted by Am-241, reducing the current and setting off the alarm. Am-241 has a half-life of 432.2 years and is obtained from the product of plutonium.
A radioisotope used to follow the path of chemical process is called a tracer. The fact that it can be detected or “traced” as it decays make it possible to determine its location at all times. Carbon-14 is the most useful tracer isotope, and has been used to study photosynthesis and the pathway followed by carbon in many organic processes.
Radiation Applications
Some radioisotopes are used in the treatment of cancer because they have the ability to kill living tissues. Radium and cobalt release intense radiation which is used to destroy tumor cells.
Radiation can also be used to treat foods and grains. Molds, bacteria, yeasts and insect eggs in food samples can be destroyed by irradiation to prolong their shelf-life
Industrial applications of radiation are now widely used like the tracking of the uniformity of plastic films and paper stock.
Dating application
Since half-life of a radioisotope is not affected of pressure and temperature, it is assumed that it decays at a constant rate. For example, C-14 decays by beta decay.
146C → 147N + 0-1e
The ratio of C-14 to C-12 in the atmosphere has remained constant for thousands of years. Plant use C02 during photosynthesis. Animals which eat the plant will have C-12and C-14 in their tissues. When the plant or animal dies, the amount of C-14 become less and less as it decays. By comparing the amount of C-14 that remains to the amount of C-12 that is present, the amount of C-14 that has decayed can be measured.
The half-life of C-14 is 5,700 years. If a sample of wood show that the ratio of C-14 to C-12 is only one-half of the ratio, it can be deduced that the wood is 5,700 years old.
In dating minerals, the U-238 à Pb-206 present represents the amount of U-238 that decayed. Calculation based on this method indicate that the earth is about 4.5 billions years old.
A nuclear reactor is a device for controlling nuclear reactions so that the energy released by the reaction can be converted to a useful form at a constant rate. The nuclear reactors presently in use are fission and breeder reactors.
Fission Reactors
Fission Reactors produce energy from the splitting of U-235. While these reactors differ in design, there are certain components common to all. These are fuel, moderators, control rods, coolants, and shields.
Fission Reactor
Pressurized Water Reactor
Fuel
Uranium-235 is the only naturally occurring isotope of uranium that undergoes fission. However, only 0.7% of U-235 is fissionable and 99.3% is not fissionable. Therefore, methods of separating, converting, and enriching the isotopes are done to increase the fissional U-235 to 2% or 4%. They are prepared as pellets of uranium (IV) oxide, UO2 and are packed in stainless tubes called fuel rods.
Moderator
The neutrons produced from the fission reactions move at high velocities. These are slowed down by moderators so that they can be captured by U-235 nuclei. Some moderators are used to absorb more of the energy without absorbing the electrons. In modern reactors, the moderator also acts as a coolant. Light-water reactors use water as moderators. In heavy-water reactors, D2O is used.
Control Rods
Control rods such as cadmium and boron regulate the rate of chain reaction by absorbing neutrons. The movable control rods are placed side by side with the fuel rods. By inserting and withdrawing these rods, the supply of neutrons are regulated. In cases of emergency they are shut off completely.
Coolants
The fission reaction produces a large amount of heat. As the coolant flows around the fuel rods, pumps circulate it through coils that transfer its heat to the water reservoir. The steam in the reservoir turns the turbine that runs the generator which in turn produces electric power.
Accidents at nuclear plants cause negative public reactions toward their use to generate power. In 1979, malfunctions of the cooling system at the Three-Mile Island Facility in Pennsylvania led to melting of some fuel. This caused serious DAMAGE to the reactor core and release of radioactive gases to the atmosphere. In 1986, the failure of a cooling system at the Chernobyl plant in Ukraine caused greater melting of fuel and caused the building to explode and release radioactive materials. Many believe that several thousands of people near the accident have developed o will soon develop cancer. Radiation spread across Scandinavia, Northern Europe, and Great Britain. In 1999, another facility in Tokaimura, Japan was temporarily closed due to a leak within the reactor.
Despite the accidents,several countries like France, Canada, The United States, Japan, and almost all European countries continue to use nuclear power for their electric power source.
Breeder Reactors
Nuclear reactors that are designed to produce fissionable fuel are called breeder reactors. These reactors convert the non-fissionable U-238 to Pu-239 which is a fissionable isotope. This is made possible by the transmutation of U-238 to Pu-239 when neutrons strike U-238 nuclei.
23892U + 10n → 23993Pu + 20-1e
Thus, while the reactor is using U-235 for its fuel, it is also producing Pu-239 which can be used as a fuel in a fission reactor.
23994Pu + 10n → 14756Ba + 9038Sn + 310n
Fusion Reactors
Harnessing power from nuclear fusion is a continuous challenge for research scientists. Its fuel, deuterium, is inexpensive and abundant in seawater, and it is a “clean” process. The isotopes produced are stable unlike the radioisotopes produced in fission reactions.
The major disadvantage is the extremely high activation energy that requires 109 degrees Celsius. No conventional material could withstand such high temperatures. The most promising efforts to date involve the enclosure of the plasma within a magnetic field called the tokamak. The tokamak design has a doughnut-shaped container in which the helical magnetic field confines the plasma and prevents it from getting in touch with the walls.
Radioactive Wastes
One of the serious objections against the nuclear industry is the radioactive waste. Radioisotopes produced by nuclear power plants such as 9038Sr and 13755Cs have half-lives of 29 and 30 years, respectively. Radioisotopes with medium and long half-lives are sealed in special containers and stored in underground chambers or in isolated areas. Low-level radioactivity used in medicine can be released into the environment as long as it does not pose health risks.
There are two types of Nuclear reaction: Fission and Fusion. In both reactions, mass I converted to energy. Fission Reactions involves the splitting of a heavy nucleus to produce lighter nuclei. Fusion reactions involve the combining of light nuclei to produce a heavier nucleus. In reactions, the total mass of the product(s) is less than the total mass of the reactant(s). This mass defect is converted to energy.
The Energy derived from nuclear reactions is much greater than that formed in chemical reactions. The amount of energy produced by the conversions of mass can be calculated using Einstein’s equation
E =mc2
Where m is measured in kg, c is measured in m/s, and E in joules. The value of c is 3 x 108 m/s, so c2 is 9 x 1016. Using 1 kg of mass,
E = (1kg) (9 x 1016 m2/s2) = 9 x 1016 kg.m2/s2
= 9 x 1016 Joules
Fission Reactions
-It begins with the capture of a slow-moving neutron by a nucleus of a heavy element such as uranium or plutonium. The resulting nucleus is very unstable and immediately “splits” or undergoes fission. The products of this fission are two nuclei of unequal mass, one or more neutrons, and a large amount of energy.
The equation for the reaction is
10n + 23592U → 9236Kr + 14156Ba + 310n + energy
This large amount of energy released in this equation is from the small loss of mass during the fission reaction.
One important feature of a fission reaction is the production of neutrons. These neutrons can become reactants to be captured by other U-235 nuclei, which in turn undergo fission and release more neutrons. This is known as a chain reaction, in which one reactant is the cause for further similar reactions. When this kind of reaction is not controlled, the energy is released in one explosive burst. This is the principle used in nuclear bomb.
There are methods by which the rate of a chain reaction can be controlled by limiting the neutrons that are allowed to react with U-235 nuclei. These controlled reactions produce a constant release of energy. They are carried out in nuclear reactors.
The fission products of U-235 presented in the figure is not a simple reaction. The fission of U-235 splits in different ways and produces more than 200 isotopes of 35 different elements. Many of these isotopes are unstable and are therefore radioactive. The radiation associated with these “by products” and the handling and disposal of these hazardous wastes are the major disadvantages of the use of nuclear energy.
Fusion Reactions
Nuclear Fusion combines smaller nuclei into a larger nuclei. It is the opposite of nuclear fission. The mass defect in the fusion of light nuclei produces tremendous energy which can be used for electric power. This can duplicate the furnace of the Sun here on Earth. Nuclear fusion holds great promise for inexpensive, safe, and clean nuclear power. The hydrogen isotopes that are needed as fuels are readily available. The reaction produces every little hazardous waste, compared with the products of fission reactions.
Some possible fusion reactions being used in experimental stage include:
21H + 21H → 42He + 01n + E
This is called the deuterium-tritium or DT reaction. Because tritium31H has a very short half-life, this is produced as needed by bombarding deuteride (LiH) with neutrons.
Deuterium-deuterium (DD) reactions require high temperatures (100,000,000 K) and high activation energies to start the reaction because the electrical repulsion between the nuclei of like charges. Because of the high temperature involved, they are called thermonuclear process. This is also the reaction for hydrogen bomb.
Not all nuclear reactions are spontaneous. These reactions occur when stable isotopes are bombarded with particles such as neutrons. This method of inducing a nuclear reaction to proceed is termed artificial radioactivity. These meant new nuclear reactions, which wouldn't have been viewed spontaneously, could now be observed. Since about 1940, a set of new elements with atomic numbers over 92 (the atomic number of the heaviest naturally occurring element, Uranium) have been artificially made. They are called the transuranium elements.
All naturally occurring elements with atomic numbers 83 or greater have no stable isotopes meaning they are radioactive. Other stable isotopes of other elements can also be made radioactive by bombarding stable nuclei with high energy neutrons. Changes brought about by bombarding the nucleus with high-energy particles is called artificial transmutation or nuclear transmutation. Elements in the periodic table with higher atomic numbers above 92 are called transuranic elements, none of which occur in nature and all of them are radioactive. These elements have been synthesized in nuclear reactors and nuclear accelerators, which accelerate the bombarding particles to very high speeds
Examples are:
2713Al + 10n → 2411Na + 42He
The normally stable nuclei of aluminum are bombarded by neutrons, some of the aluminum nuclei are changed to a radioactive isotope of sodium
94Be + 11H → 63Li + 42He
The bombarding of beryllium by protons (hydrogen nuclei) forming radioisotope lithium
Mass Defect and Binding Energy
The mass of a nucleus is always less than the combined masses of the nucleons (protons and neutrons).
Atomic mass of a proton = 1.00728
Atomic mass of a neutron = 1.00867
A helium nucleus contains 2 protons and 2 neutrons. So the expected mass of a 42He nucleus is:
However, the actual mass of a 42He nucleus is 4.00150. Thus, there is a decrease of 0.03040 when 2 protons and 2 neutrons are formed as shown below.
4.03190 – 4.00150 = 0.03040
This difference between the total mass of the free nucleons and their mass when united in a 42He nucleus is known as mass defect, which is due to the strong forces of attraction that bind nucleons together. When protons and neutrons combine in the nucleus, energy is released. The mass defect is equivalent to the energy released as explained in E=mc2.
The amount of energy to reverse the process, to break down a nucleus into free nucleons, is known as the binding energy of the nucleus. It is equal to the energy of the mass defect and is usually expressed in megaelectron volts (MeV).
In nuclear equations, the symbols represent atomic nuclei. The superscript indicates the mass number and superscript indicates atomic number. When balancing nuclear equations, the sum of the mass numbers of the reactant must be equal to the sum of the mass numbers of the product. The same is true of the atomic numbers.
Example:
Write the balanced equation for the following nuclear reactions
a.Ga-70 undergoes beta decay
b.Co-56 undergoes positron and gamma decay.
Solution:
Write the skeleton equation including the mass numbers, atomic numbers, and symbols of all the particles using X for the unknown nuclide.
a.Mass number : 70 =70 +0
7031Ga → 7032X + 0-1β
Atomic Number: 31 = 32 – 1
From the periodic table, the element with atomic number 32 is germanium, so
7031Ga → 7032Ge + 0-1β
b.Mass number: 56 = 56 + 0 + 0
56 = 56
5627Co → 5626X + 01β + 00⅟
Atomic number: 27 = 26 + 1
Again from the periodic table, the element with atomic number 26 is iron, so
Radioactive decay proceeds according to a principal called the half-life. With the use of half life we can possibly predict what fraction of nuclei in a sample will decay within any given time. This rate of decay is constant for each isotope and different from any other. Thus, half life is the amount of time necessary for one-half of the nuclide of a sample to decay.
For example, the radioactive element bismuth (210Bi) can undergo alpha decay to form the element thallium (206Tl) with a reaction half-life equal to five days. If we begin an experiment starting with 100 g of bismuth in a sealed lead container, after five days we will have 50 g of bismuth and 50 g of thallium in the jar. After another five days (ten from the starting point), one-half of the remaining bismuth will decay and we will be left with 25 g of bismuth and 75 g of thallium in the jar. As illustrated, the reaction proceeds in halfs, with half of whatever is left of the radioactive element decaying every half-life period.
If half-life of an isotope is represented by T, then the fraction of a sample that remain after time t is
Fraction Remaining = (1/2 t/T or (1/2)n
Where t/T or n is the number of half-life periods that have occurred.
Example
A 10.0 g sample of 24Na is used to study the rate of blood flow in the circulatory system. If 24Na has a half-life of 15 hours, how much is left after 60 hours.
Solution
Method 1. Reduce the mass by one-half for each successive period until the total time reached.
After 15 hours, one-half of 10.0 g or 5.0 g will remain.
After 30 hours, one-half of 5.0 g or 2.5 g will remain.
After 45 hours, one-half of 2.5 g or 1.25 g will remain.
After 60 hours, one-half of 1.25 g or 0.625 g will remain.
Method 2. Divide the total time by the half-life to find the number of half-life periods, and then use the formula:
Fraction Remaining = (1/2)nwhere n is the number of half-life periods
In this case, n = 60/15 = 4
Fraction Remaining = (½)4= (1/16)
(1/16) x 10.0 g = 0.625 g
Click HERE for the list of Radioactive Elements and their Half-lives
In 1902, Frederick Soddy proposed the theory that "radioactivity is the result of a natural change of an isotope of one element into an isotope of a different element." Nuclear reactions involve changes in particles in an atom's nucleus and thus cause a change in the atom itself. All elements heavier than bismuth (Bi) (and some lighter) exhibit natural radioactivity and thus can "decay" into lighter elements. Unlike normal chemical reactions that form molecules, nuclear reactions result in the transmutation of one element into a different isotope or a different element altogether.
*NOTE! The number of protons in an atom defines the element, so a change in protons results in a change in the atom.
When a nuclide decays, the reactant decaying nuclide is called the parent and the product nuclide is called the daughter. Nuclide can decay in several ways
1.Alpha Decay
263106Sg →259104Rf + 42He
The alpha particle is emitted by certain radioactive elements as they decay to a stable element. It consists of two protons and two neutrons; it is positively charged and is written as 42He or42 a. An atom that emits an alpha particle is called an alpha emitter. The element that undergoes "alpha decay" reduces the atomic number by 2 units and it’s mass by 4 units. Alpha decay occurs when a nucleus has so many protons that the strong nuclear force is unable to counterbalance the strong repulsion of the electrical force between the protons. Because of its mass, the alpha particle travels relatively slowly (less than 10% the speed of light), and it can be stopped by a thin sheet of aluminum foil.
Ernest Rutherford began experimenting to determine the nature of this radiation in 1898. One experiment demonstrated that the radiation actually consisted of 3 different types: a positive particle called "alpha," a negative particle, and a form of electromagnetic radiation that carried high energy.
2.Beta Decay
188O→189F + 0-1e
147N→146F + 0-1e
Beta decay is a form of radioactive decay in which the nucleus of an atom undergoes a change which causes it to emit a beta particle.
Atoms undergo beta decay when they are unstable because they have too many neutrons or too many protons. To stabilize themselves, the excess neutrons or protons are converted, conserving mass and making the nucleus more stable. In the process, the atom also changes into another element, because while the overall number of particles in the nucleus remains the same, the balance of protons and neutrons changes.
In beta minus decay, an excess neutron becomes a proton, and the nucleus emits an electron. The electron is the beta particle symbolized as 0-1e. When a nucleus undergoes beta plus decay, a proton is converted into a neutron, with the nucleus emitting a positron. Beta particles can be electrons or positrons, as illustrated, depending on whether a nucleus goes through beta minus or beta plus decay. Before researchers realized that beta particles were just electrons or positrons, they referred to these particles as “beta rays,” which is why some antiquated texts contain references to beta rays.
A beta particle has more penetrating power than an alpha particle, but less than a gamma particle. Beta particles can be stopped with a thick sheet of metal, a large pocket of air, or several sheets of paper. This makes them relatively safe to work around, as long as safety precautions are observed when people are around elements which undergo beta decay.
3.Gamma Radiation
Gamma rays from radioactive gamma decay are produced alongside other forms of radiation such as alpha or beta, and are produced after the other types of decay occur. The mechanism is that when a nucleus emits an α or β particle, the daughter nucleus is usually left in an excited state. It can then move to a lower energy state by emitting a gamma ray, in much the same way that an atomic electron can jump to a lower energy state by emitting infrared, visible, or ultraviolet light.
23892U → 23490Th + 42He + 200⅟
In gamma decay, a nucleus changes from a higher energy state to a lower energy state through the emission of electromagnetic radiation (photons). The number of protons (and neutrons) in the nucleus does not change in this process, so the parent and daughter atoms are the same chemical element. In the gamma decay of a nucleus, the emitted photon and recoiling nucleus each have a well-defined energy after the decay. The characteristic energy is divided between only two particles.
4.Positron Decay
Positron decay
involves the emission of a positron. A positron, symbolized by 01β,
is the “antiparticle” of an electron. Positron decay occurs when a proton in
the nucleus is converted into a neutron and a proton is released.
11p → 01n + 01β
Positron emission is
a byproduct of a type of radioactive decay known as beta-plus decay. In the
process of beta plus decay, an unstable balance of neutrons and protons in
the nucleus of an atom triggers the conversion of an
excess proton into a neutron. During the conversion process,
several additional particles, including a positron, are emitted.
116C
→ 115B + 01β
5.Electron Capture
Electron capture
occurs when the nucleus of an atom captures an electron from an orbital of the
lowest energy level.
11p
+ 0-1e → 01n
19579Au
+ 0-1e → 19578Pt
The theory of
electron capture was first discussed by Gian-Carlo Wick in a 1934
paper, and then developed by Hideki Yukawa and others. K-electron
capture was first observed by Luis Alvarez, in vanadium-48. He
reported it in a 1937 paper in the Physical Review. Alvarez went on to
study electron capture in gallium-67 and other nuclides
Electron
capture is the primary decay mode for isotopes that
have a relative superabundance of protons in the nucleus, but
there is insufficient energy difference between the isotope and its prospective
daughter with one less positive charge, for the nuclide to decay by simply
emitting a positron. Electron capture also exists as a viable decay mode
for radioactive isotopes that do have enough energy to decay by
positron emission, and in that case, it competes with positron emission. It is
sometimes called inverse beta decay, though this term can also refer to
the capture of a neutrino through a similar process.
