I. Artificial Radioactivity
Mon.
class
BBC
BOOK
Book assignment
book assignment
book assignment
Oct. 4, 2013
Video to watch
In unit II, we studied natural radioactivity, in that it occurs under natural conditions and results in natural transmutation. The products of natural transmutation are radioactive isotopes, which decay finally into a stable isotope of lead. However, elements can be made radioactive by bombarding their nuclei with high energy particles such as protons, neutrons, and alpha particles. When the element nitrogen, for example, is bombarded by alpha particles, the nuclear reaction that takes place is:Alpha Particle + nitrogen = Proton + oxygen
He + N = H + O
As noted from the above reaction, isotopes of two new elements are formed. This is called artificial transmutation. The process includes the bombardment of nuclei by accelerated particles which cause nuclei to become unstable and may result in the formation of radioactive isotopes, or radioactive isotopes of new elements called radioisotopes. Another example follows:
Sum of atomic masses Sum on atomic masses on left =31 on right =31
Al + He = P + n
Sum of atomic numbers Sum of atomic numbers on left =15 on right =15
In the above equations you should note the following:
a. The sum of the atomic masses (the superscripts) on the left must equal the total of atomic masses ( the superscripts) on the right.
b. The sum of the atomic numbers (the superscripts) on the left must equal the total of the atomic numbers (the superscripts) on the right.
c. In an equation depicting artificial radioactivity, there are at least two reactants on the left side of the equation. In natural radioactivity equations, there appears only one reactant on the left side of the equation. The following points out how the above suggestions work:
X + H = Li +He
Problem: In the previous reaction what nucleus is represented by X?
Solution: Noting that the sums of both superscripts of the completed right side of the equation are 10 and 5, respectively, and the sums on the incomplete left side are 1 and 1, respectively, we now can easily ascertain the atomic mass and number of element X.
Since the atomic number identifies the element, this element with an atomic number of 4 is Beryllium, which has an atomic mass of 9. Therefore, the answer is written (Below):
X
Be
Accelerators. Accelerators are used to give charged particles sufficient kinetic energy to overcome electrostatic forces and penetrate the nucleus. Electric and magnetic fields are used to accelerate these charged particles.
II. Nuclear Energy
In nuclear reactions, mass is converted to energy. Nuclear reactions involve energies a million or more times greater than ordinary chemical reactions. The energy changed are due to the changes in binding energy as a result of what is called mass defect. In order to understand these reactions, it is better that we clarify these terms now.
Mass Defect. The mass of a free proton (1.6725 x 10^-24g) and free neutron (1.6748 c 10^-24g) is known. Knowing that the nucleus of elements represents the total number of protons and neutrons, one would think that we can easily predict what the mass of an element will be by adding together the total mass of all neutrons and protons in the nucleus.
In the case of Helium with 2 neutrons and 2 protons in its nucleus, consider the following:
Mass of 2 free neutrons= 1.6748 x 10^-24g x 2 =3.3496 x 10^-24g
Mass of 2 free protons = 1.6725 x 10^-24 x 2 =3.3450 x 10^-24g
Total mass of 2 free protons and neutrons =6.6946 x 10^-24g
However, the sum of the masses of the nucleons in the Helium nucleus was found to be 6.641236 x 10^-24g. If we subtract one total from the other, we find a total mass deficiency of 0.053364 x 10^-24g for every Helium atom.
If we substitute this value for the mass factor in Einstein's equation, E=mc^2, we find, that when nuclear particles merge to make up a nucleus, a great deal of energy is released as this small amount of mass is changed into energy. The amount of energy released is called the Binding Energy and is a measure of the stability of the atom. The greater the amount of energy released in the formation of the nucleus, the greater will be the amount of energy required to separate the nucleus into separate particles.
A. Fission Reaction
A fission reaction results in the "splitting" of heavier nuclei into lighter ones. Fission is brought about by nucleus capturing slow moving neutrons, which results in the nucleus becoming very unstable. The unstable nucleus splits to form fission fragments of elements of lighter weight, liberation of energy, and release of two or more neutrons. The liberation of energy is the result of conversion of mass into energy.
Only unstable elements of high atomic numbers can be fissioned. When a heavy element fission, the new elements formed have more stable configuration because of the greater binding energy per nucleon. In a nuclear reactor,the chain reaction can be controlled with control rods which limit the amount of interacting neutrons. In an atomic bomb, the chain reaction is not controlled.
1.Fuels.Uranium-233, Uranium-235, and plutonium-239 are fissionable. Natural uranium 999.3% uranium-238 and 0.7% uranium-235), enriched uranium (3-4 percent enrichment with uranium-235) are commonly as fuels. Uranium-233produced from thorium-232 and plutonium-239 produced from uranium-238, are obtained as fuels in breeder reactors. Breeder reactors produce more fuel than is consumed. Through a complex process, these fuels are used to make the uranium oxide which is packed in the fuel rods made of stainless steel.
2. Moderators. For efficient nuclear fission, it is necessary to slow down the speed of the neutron. Moderators are materials that have the ability to slow down neutrons quickly with little tendency to absorb them.
Particles of similar mass such as hydrogen and its isotopes deuterium have been found effective as moderators. The neutrons are slowed down most effectively by a head-on collision with a particle of similar mass. Water, heavy water, beryllium, and graphite are commonly used as moderators, which slow down the neutrons without capturing them.
3. Control Rods. The fission process in a reactor can be controlled by adjusting the number of neutrons available. Boron and cadmium are commonly used in control rods because they absorb neutrons very well. They are placed alongside the fuel rods and by withdrawing and inserting these rods the amount of neutrons available for fission is controlled. In event of an emergency, these rods are inserted to completely absorb the neutrons needed for fission. thereby shutting down the fission reaction.
4. Coolants. Coolants are used to keep the temperatures generated by fission at reasonable levels within the reactor and to carry heat to heat exchanges and turbines so that it can be utilized in the production of energy. Water, heavy, air, helium , carbon dioxide, molten sodium and molten lithium are examples of coolants.
In some reactors, the coolants also serve as a help to the moderator in the removal of heat from the reactor core, where the fuel rods are located, and help prevent a core meltdown.
5.Shielding. There are two shields used in nuclear reactors.
The internal shield, made of a steel lining, protects the walls of the reactor from radiation damage.
The external shield, made of high density concrete, acts as a radiation containment vessel in the event of nuclear accident.
B. Fusion Reaction
When the two light nuclei fuse into a heavier nucleus at high temperatures and pressures, an element of more stable configuration (with greater binding energy per nucleon) is formed. The mass of the heavier nucleus formed is less than the sum of the masses of the lighter nuclei. The difference in mass is converted into energy.
Fission is the process of combining two light nuclei to form a heavier one. The energy released in a fusion reaction is much greater than in a fission reaction. Solar energy is probably the result of the fusion of ordinary hydrogen atoms to form helium. The nuclear reaction of a hydrogen bomb utilizes fission as a trigger for fusion.
Nuclear fusion presents the most appealing method of producing great amounts of energy for many reasons. They include:
1.Production. The production process is safe and does not present a threat. That is, it can be much more easily controlled than other forms of energy producing forms and shut downs automatically. Also, the isotopes produced are not radioactive but "clean", stable isotopes, that reduce the pollution threat to life.
2. Fuels. The isotopes of hydrogen, H(deuterium) and H(tritium0, are used as fuels. Heavy water (deuterium oxide) is obtained by concentrating the trace qualities present in water. Tritium is made by the nuclear reaction.
Li + n =H + He
The fuel, deuterium, is abundant and can be obtained cheaply from sea water. Tritium, another isotope of hydrogen is also used.
The following considerations must be addressed concerning fusion reactors:
3. High Energy Requirement. Since each nucleus carries a positive charge, all nuclei repel one another with increasing strength as they are moved closer together. Consequently, for the nuclei to interact, they must have enough kinetic energy to overcome this repulsion.
The magnitude of repulsion increases with charge; therefore, only the nuclei of lowest possible charge can be used. Fusion with ordinary hydrogen, however, is very slow. Fusion reactions involving deuterium, or deuterium and tritium, are useful sources for the release of energy. Of these, the most rapid reaction is between deuterium and tritium. the thermonuclear approach, through the use of very high temperatures, appears to be very promising for controlled fusion. However, this requires temperatures of one billion (10^9) degrees Celsius.
Technical problems with nuclear fusion, such as the requirement extremely high temperatures and their containment, continue to challenge nuclear scientists and engineers. For once begun, the containment of the reaction with high temperatures is extremely difficult.
In order that the reaction on in an area where it does not come in contact with materials which can break down, "magnetic bottles" were designed to contain the reaction. "Magnetic bottles" make use of strong magnetic fields and confine the reaction to an area and where it does not come in contact with any materials except the magnetic field itself.
C. RADIOACTIVE WASTES.
Environmental ecosystem which are finely balanced are threatened by the presence of radioactive wastes. The nuclear energy industry has addressed this problem.
However, grave problems still plague the safe disposal of such products as radioactive strontium -90, which has a half-life of 29 years. Even after 6 half-life periods, 174 years, enough will remain to cause great harm if it is not contained. The problem is that fission products from nuclear reactors are intensely radioactive and cannot be discarded. They must be stored for a long time or disposed of in special ways. Solid and liquid wastes, such as strontium -90 and cesium -137, are encased in special containers for permanent storage underground or in isolated areas.
Low level radioactive wastes may be diluted and released directly into the environment. Gaseous radioactive wastes such as radon -222, krypton -85, and nitrogen -16, are stored at safe levels for decay and then dispersed into the air.
D. USES OF RADIOISOTOPES
1. Based on chemical reactivity. Since radioisotopes are chemically similar to stable isotopes of the same element, they can be used as tracers to follow the course of a reaction without seriously altering the chemical by the use of carbon -14 as a tracer.
2. Based on radioactivity. Radioisotopes are used in medical diagnosis, therapy, for preservation, and as a means of measuring the physical dimensions of many industrial products.
Isotopes with very short half- lives and which will be quickly eliminated from the body are used for diagnostic injections. Technetium -99 is used for pinpointing brain tumors. Iodine -131 is used for diagnosing thyroid disorders. Radium and cobalt -60 are used in Cancer therapy.
Radiation kills bacteria, yeasts, molds, and insect eggs in foods, permitting the food to be stored for a much longer time.
3. Based on half-life. Radioisotopes give a fairly consistent method of dating some geologic events. The ratio of uranium -238 to lead -206 in a mineral can be used to determine the age of the mineral.
Carbon -14 has a half-life of approximately 5,568 years. Part of the CO2 in the air is carbon -14. Plants take in CO2 from the air, consequently fixing a quantity in its protoplasm. A gram of carbon from a living plant will radiate about 15 beta particles per minute. When the plant dies, it no longer takes in carbon -14. Scientists determine the age of old trees by comparing the radiation of beta particles of the old materials with that of present plants. In this manner, they can very accurately determine their age.
Questions X1-2
1. In a nuclear reactor, the purpose of the moderator is to:
(1) absorb neutrons
(2) split neutrons
(3) produce neutrons
(4) slow down neutrons
2. in a nuclear reactor, the radioisotope U-235 serves as a:
(1) shield
(2) coolant
(3) neutron absorber
(4) fissionable material
3. Which particle cannot be accelerated by the eletric or magnetic fields in a particle accelerator?
(1) neutron
(2) proton
(3) alpha particle
(4) beta particle
4. During which process would the ratio of uranium -238 to lead -206 be used?
(1) diagnosing thyroid disorder
(2) dating geologic formations
(3) detecting brain tumors
(4) treating cancer patients
5. Which sun=bstance may be used as both the coolant and moderator in a reactor?
(1) boron
(2) cadmium
(3) heavy water
(4) solid graphite
6. An isotope of which element may be used as a fuel in some fission reactions?
(1) hydrogen
(2) carbon
(3) lithium
(4) plutonium
7. Which is a gaseous radioactive waste produced during some fission reactions?
(1) nitrogen-16
(2) thorium-235
(3)uranium-235
(4) plutonium-239
8. The radioactive isotope carbon - 14 can be used for
(1) determining the age of a sample
(2) determining medical disorders
(3) controlling fission reactions
(4) controlling speeds of neutrons
Self-Help Questions X1
Unit XI - Nuclear Chemisty
I. Artificial radioactivity, mass defect fission reactions, and fission reactors. Fusion reactions.
1. Given the reactions:
13^27 Al + 2^4 He = X = 0^1n
When the equation is cotrrectly balanced, the nucleus represented by X is
(1) 1330Al
(2) 14^30 Si
(3) 15^30 P
(4) 16^30 S
2. Which substance may be used as fuel in the nuclear reactor?
(1) Ra-226
(2) u-235
(3) Fr-220
(4) Pb-204
3. Which pair of nuclei can undergo a fusion reaction?
(1) potassium-40 and cadmium-113
(2) zinc-64 and calcium-44
(3) uranium-238 and lead-208
(4) hydrogen-2 and hydrogen-3
4. Which is the symbol for the deuterium isotope of hydrogen?
(1) 1^1H
(2) 1^2H
(3) 1^3H
(4) 1^1H
5. In the nuclear reaction 4^9Be + X + 3^6Li + 2^4He, the particle is represented by X is
(1) -1^0e
(2) +1^0e
(3) 1^2H
(4) 1^1H
6. Cadmium and boron are commonly used in a nuclear reactor as
(1) External shielding
(2) internal shielding
(3) control rods
(4) moderators
7. Which process occurs in a controlled fusion reaction?
(1) Light nuclei collide to produce heavier nuclei.
(2) Heavy nuclei collide to produce lighter nuclei.
(3) neutron bombardment splits light nuclei.
(4) neutron bombardment slipts heavy nuclei.
8. given the equation: 7^14N + 2^4He = X + 8^17O
When the equation is correctly balanced, the particle represented by the X will be
(1) -1^0e
(2)0^1n
(3) 1^1 H
(4) 1^2 H
9. The atoms of some elements can be made radiactive by
(1) placing them in a magnetic field
(2) bombarding them with high-energy particles
(3) separatin gthem into their isotopes
(4) heating them to a very high tempature
10. In the reaction 1^2H + 1^3H = 2^4He + X, what is the particle represented by X?
(1) a neutron
(2) a positron
(3) an alpha particle
(4) a beta particle
11. Which substance is used in control rods of a nuclear ractor?
(1) boron
(2) helium
(3) carbon dioxide
(4) beryllium
12. Which substance may serve as both a moderator and coolant in some nuclear reactors?
(1) carbon dioxide
(2) boron
(3) graphite
(4) heavy water
13. Which particle can NOT be accelerated by the electric or the magnetic field in a particle accelerator?
(1) electron
(2) neutron
(3) helium nucleus
(4) hydrogen nucleus
II. Radioactive wastes; use of radioisotopes based on reavivity and half-lfie
14. Which is a gaseous radioactive waste produced during some fission reactions?
(1) nitrogen - 16
(2) thorium- 232
(3) uranium-234
(4) plutonium-239
15. The radioactive isotope carbon-14 can be used for
(1) determining the age of a sample
(2) determining madical disorders
(3) controlling fission reactions
(4) controlling speeds of neutrons
16. Which radioisotope is ued for diagnosing thydroid disorders?
(1) cobalt-60
(2) uranium-238
(3) lead-206
(4) idonine-131
17. radiated food can be safely stored for a longer time because radiation
(1) prevents air oxidation
(2) prevents air reduction
(3) kills bacteria
(4) causes bateria to mutate
18. Which two characteristis do radioisotopes have that are ueful in medical diagnosis?
(1) long half-lives and slow elimination
(2) short half-lives and slow elimination from the body
(3) long half-lives and quick elimination from the body
(4) short half-lives and quick elimination from the body
19. Iodine-131 is used for diagnosing thyroid disorders because it is absorbed
(1) has a very short half-life
(2) has a very long half-life
(3) emits alpha radiation
(4) emits gamma radiation
20. A radioactive-dating procedure to determine the age of a mineral compares the mineral's remaining amounts of isotopes ^238U and isotope
(1) ^206 Pb
(2) ^206 Bi
(3) 214 Pb
(4) ^214 Bi
TUESDAY - SEPTEMBER 24 --- WHAT TO EXPECT IN CLASS
Watch the video, The Manhattan Project, about the development of the atomic bomb. For homework, you will be asked to write your response to the video.
HOMEWORK
Watch the video RADIOACTIVITY and read the associated text. (Textbook Sec. 25.1)
Write your response to The Manhattan Project video we watched in class.
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