1. Radioactivity and Nuclear Energy

2. Radioactivity
The nucleus is made of particles called nucleons. The nucleons are protons and neutrons. The atomic number (Z) is the number of protons. The mass number (A) is the protons plus the neutrons. Atoms that have the same number of protons but a different number of neutrons are called isotopes. The general term nuclide is applied to each unique atom, and we represent it as follows:

3. Nuclide Notation

4. Nuclide Notation

5. Examples
Carbon 12, 13 and 14
Hydrogen 1, 2 and 3

6. Radioactive Decay
Many nuclei are radioactive. They spontaneously decompose forming a different nucleus and producing one or more particles that leave the atom.
Over 85% of all nuclides are radioactive and go through
some type of decay.

7. The Nuclear Equation
The nuclear equation is quite different than those for normal chemical reactions. In a balanced chemical equation, the atoms must be conserved.
In a nuclear equation both the atomic number (Z) and the mass number (A) must be conserved.
This means that the sums of the Z values on both sides of the arrow must be equal. The same is true for the A value.

8. Alpha Decay
Alpha Particle (αparticle) - a helium nucleus produced in radioactive decay. Alpha Particle Production – a common mode of decay for radioactive nuclides resulting in a loss of 4 in mass number and a loss in 2 in atomic number.
The mass number is conserved (238 = ).
The atomic number is also conserved(92 = ).

9. Beta Decay
Beta-Particle (βparticle) – an electron produced in radioactive decay. Beta-Particle production- a decay process for radioactive nuclides in which the mass number does not change and the atomic number increases by one.
The beta particle has a mass number (A) of zero and the atomic
Number (Z) is -1, so the atomic number for the new nuclide is
greater by 1 than the atomic number for the original nuclide.
Therefore, the net effect of B-particle production is to change a neutron
to a proton. (8n+6p = 14), and (7n+7p =14)

10. Gamma Decay
Gamma Ray (γray) – a high-energy photon produced in radioactive decay. Gamma Ray Production- a decay process in which an excited nuclide produces gamma rays that results in no change in mass number (A) and no change in atomic number (Z). Gamma ray production accompanies other types of decay such as alpha decay that produces a helium nuclide.
Gamma rays have no mass and no atomic number.

11. Gamma Ray Decay
Beta-particle produced along with a 2 gamma rays produced.

12. Positron Emission
Positron- a particle with the same mass as an electron but the opposite charge. ( positive electron) Positron Production – production of a positron results in no change in the mass number (A), and a decrease of one in the atomic number (Z).
Note that the production of a positron has the effect of changing a proton to a
neutron. (11p + 11n = 22) and (10p + 12n = 22)

13. Electron Capture
Electron Capture- is a process in which one of the inner-orbital electrons is captured by the nucleus. In this process, gamma rays are also produced. This reaction would have been of great interest to alchemists in medieval times as a means of turning mercury to gold. However, this reaction does not occur enough to make this practical.

14. Decay Series
Often a radioactive nucleus cannot achieve a stable (nonradioactive) state through a single decay process. In such a case, a decay series occurs until a stable nuclide is formed. A well known example is the decay series that starts with Uranium 238 and ends with Lead 206

15. Nuclear Transformations
Nuclear Transformation- the change of element into another. In 1919, Lord Rutherford observed this process when bombarding an αparticle with Nitrogen 14 to produce Oxygen 17. Seen on the following slide Other small particles such as Carbon12 and Nitrogen14 can also be used to bombard heavier nuclei and cause transformations. Because these positive bombarding ions are repelled by the positive charge of the target nucleus, the bombarding particle must be moving at a very high speed to penetrate the target. These high speeds are achieved in a particle accelerator.

16. Nuclear Transformations Positive Ion Bombardment
Positive ion bombardment with an alpha-particle.

17. Nuclear Transformation Neutron Bombardment
Neutrons are also employed as bombarding particles to effect nuclear transformation. Because neutrons are uncharged, and thus not repelled by a target nucleus, they are readily absorbed by many nuclei. The most common source of neutrons is in a fission reactor, aka. a “nuclear reactor”. By using positive ion and neutron bombardment, scientists have extended the periodic table and have created elements 93 through 118, the transuranium elements.

18. Neutron Bombardment

19. Neutron Bombardment
Uranium 239 further decays to Neptunium 239 with the release of a
Beta particle (electron).

20. Nuclear Transformation Examples

21. Radioactivity Detection
Geiger-Müller Counter ( Geiger counter)- an instrument that measures the rate of radioactive decay by registering the ions and electrons produced as a radioactive particle passes through a gas- filled chamber. It contains Argon gas. They have no charge, but can be ionized by a rapidly moving particle. The fast moving particles knock electrons off some of the argon atoms. Although uncharged Argon does not conduct a current, the ions-electrons formed by the incoming high energy particles allow current to flow briefly. Every time a surge of high energy particles enters the counter, a pulse of current is generated momentarily, and the counter “counts” each pulse of current.

22. Geiger-Müller Counter
Hans Geiger
Speaker gives
“click” for
each particle
Window
Particle
path
Argon atoms

23. Geiger Counter (-) (+) e- + e- + e- + + e- Ionization of fill gas
takes place along
track of radiation
(-)
Speaker gives
“click” for
each particle
(+)
Metal tube
(negatively
charged) e- + e-
Window + e- + + e-
Radiation cannot be seen, heard, felt, or smelled. Thus warning signs and radiation detection instruments must be used to alert people to the presence of radiation and to monitor its level. The Geiger counter is one such instrument that is widely used.
Other devices used to detect and measure ionizing radiation: scintillation counter, film badge
Free e- are attracted to (+) electrode, completing the circuit and generating a current.
The Geiger counter then translates the current reading into a measure of radioactivity.
Ionizing
radiation
path
Free e- are attracted to
(+) electrode, completing
the circuit and generating a current. The Geiger counter then translates the current reading into a measure of radioactivity.
Atoms or molecules
of fill gas
Central wire electrode
(positively charged)
Wilbraham, Staley, Matta, Waterman, Chemistry, 2002, page 857

24. Scintillation Counter
Scintillation Counter- an instrument that measures the rate of radioactive decay by sensing flashes of light that the radiation produces in a detector. For example, sodium iodide gives off photons of light when the high energy particles strike it. A detector senses the flashes of light and counts the decay events.

25. The Concept of Half Life
Half-Life- is the time required for half of the original sample of radioactive nuclides to decay. For example, if a nuclide sample has 1000 nuclei and 7.5 days later it has only 500 nuclei, then its half life is 7.5 days. It is important to remember that the range of radioactive half-lives is tremendous! For example, Protactinium 234 has a half life of 1.2 minutes, whereas Uranium 238 has a half live of 4.5 billion years. Some half–lives are fractions of a second. The smaller amount of time the half life is, the “hotter” the radioactive element.

26. Half Life Calculations
Manual Calculation Vs. Formula

27. Example Problem
How long will it take for a sample containing 1.00 mol of Ra-223 to reach a point where it contains only 0.25 mol? The half-life of of Ra-223 is 12 days. NOTE: Each radioactive isotope has its own unique half life. NOTE: Manually is easier
2 half–lives or 24 days

28. Half Life Problems
Technetium-99 has been used as a radiographic agent in bone scans because it is readily absorbed by the bones. If Tc-99 has a half life of 6.0 hour, what fraction of an administered dose of 100μg of Tc-99 remains in a patient’s body after 2.0 days? Gold-198, which has a half life of 2.7 days, is used as an implant for cancer therapy. For an implant containing 50μg of Au-198 ,approximately how much remains after one week?
μg or less than 4.0%
Approximately 8.5 μg
After 8.1 weeks 6.25 μg

29. Dating by Radioactivity
Radiocarbon dating (Carbon 14-dating) is a method for dating ancient wood or cloth on the basis of radioactive decay of the carbon-14 nuclide. Carbon-14 decays by beta particle production and is continually produced by neutron transformation from Nitrogen- 14. These opposing processes keep Carbon-14 balanced in the atmosphere. Therefore it remains relatively constant. As long as a plant lives, its carbon-14 content remains the same as in the atmosphere because it takes in CO2 from the atmosphere along with the Carbon-14 isotope.

30. Carbon-14 Dating

31. Carbon-14 Dating
As soon as a tree is cut down, it stops making carbon. There is no more carbon incorporated into the plant to replace the decaying carbon-14. The half-life of carbon-14 is 5,730 years. A wooden bowl that has half the carbon-14 of a living tree, therefore is approximately 5,730 years old.

32. Decay of Carbon-14
Decay of Carbon-14 over 17,190 years or 3 half-lives.

33. Medical Applications of Radioactivity
Radiotracer- a radioactive nuclide introduced into an organism in food or drugs and traced (followed) for diagnostic purposes. Iodine-131has proved to be very useful in the diagnosis and treatment of the thyroid gland. Patients drink a small amount of NaI with Iodine-131 contained. The patient’s thyroid is scanned for iodine absorption. The Iodine-131 destroys abnormal thyroid cells.

34. Radioactive Tracers, Half Lives, and Medical Applications

35. Nuclear Energy
Protons and neutrons in atomic nuclei are bound together with forces that are much greater than the forces that bind atoms together to form molecules. The forces associated with nuclear processes are more than a million times greater than those of chemical reactions. Because of this, the nucleus becomes a very attractive source of energy. There are TWO types of nuclear processes that produce energy: Fusion-the process of combining two light nuclei to form a heavier, more stable nucleus. Fission-the process of splitting a heavy nucleus into two more stable nuclei with smaller mass numbers.

36. Nuclear Fission
Nuclear Fission was discovered in the late 1930’s when U-235 nuclides bombarded with neutrons were observed to split into two lighter elements. This process shown on the next slide, releases 2.1 x 1013 joules of energy per mole of U-235. Compared with what we get from typical fuels, this is a huge amount of energy. For example, the fission of one mol of U-235 produces about 26 million times as much energy as the combustion of one mol of methane gas.

37. Nuclear Fission
The Nuclear Fission of Uranium-235

38. Nuclear Fission
Upon capturing a neutron, the U-235 undergoes fission to produce two lighter nuclides, more neutrons (typically three), and a larger amount of energy. This fission reaction is one of many reactions that U-235 goes through. Over 200 isotopes of 35 different elements are among the fission products of U-235. In addition to the fission products produced, neutrons are made, that can produce more fission reactions. Because each fission event produces neutrons, the process is self sustaining. This is a chain reaction.

39. Chain Reactions & Critical Mass
Chain Reactions – are self sustaining fission processes that are caused by the production of neutrons that proceed to split other nuclei.

40. Chain Reactions & Critical Mass
For the fission process to be self-sustaining, at least one neutron from each fission event
Must go on to split another nucleus. If, on average, less than one neutron causes another
Fission event, the process dies out.

41. Critical Mass
Critical Mass-the mass of fissionable material required to produce a chain reaction. If exactly one neutron from each fission event causes another fission event, the process sustains itself at the same level and is said to be critical. However, it more than one neutron from each fission event, causes another fission event, the process rapidly escalates and the heat build-up causes a violent explosion. To achieve the critical state, a certain mass of fissionable material called critical mass, is needed. If the sample is too small, too many neutrons escape before they have a chance to cause a fission event, and the process stops.

42. Nuclear Reactors
Because of the tremendous energies involved, nuclear reactors, produce electricity, and heat water to produce steam that runs turbine generators.
The reactor core contains 3% U-235 and is housed in the metal cylinders.
A moderator-which surrounds the cylinders slows down the neutrons so that the U-235 fuel can capture them more efficiently.
Control rods, composed of cadmium, absorbs neutrons, and are used to regulate the power level of the reactor.
If a malfunction occurs, the control rods are automatically inserted into the core to absorb neutrons and stop the reaction. Water (coolant) is circulated in the core to absorb the heat generated, and changes water into steam, which runs turbines that run electrical generators.

43. Nuclear Reactors

45. Nuclear Reactors
Although the U-235 contained in reactors is not enough to create an explosion like that of a fission bomb, like the one dropped on Hiroshima and Nagasaki in 1945, a failure of the cooling system can lead to temperatures high enough to melt a reactor core. The housing for core must be designed to contain the core in the event of a containment breech. Accidents such as the one at Three Mile Island in Pennsylvania in 1979 and at Chernobyl in the Soviet Union in 1986 have had led many people to question the wisdom of continuing to build fission-based power plants.

46. Nuclear Fusion
Nuclear Fusion- the process of combining two light nuclei, produces more energy than even nuclear fission. Stars produce their energy through nuclear fusion.
Our own sun, gives off vast quantities of energy from the fusion of protons to form helium.
In order for the process of fusion to be feasible, two protons (positive charges) must get close together and bind to become one atom, such as two hydrogen molecules to form helium. Since like charges repel, the the atoms must be shot at each other with such speed to overcome their repulsions of each other.
The repulsive forces of two hydrogen nuclei are so great that temperatures of 40 million Kelvin are thought to be necessary for the nuclei to combine.

47. Nuclear Fusion
Scientists are currently studying methods for fusion…

48. Effects of Radiation
Radiation Damage is classified as somatic or genetic. Somatic damage is damage to the organism itself, resulting in sickness or death. A massive dose of radiation may cause immediate sickness and death while a milder dose may appear much later as cancer. Genetic damage is damage to the genetic machinery of reproductive cells, creating problems that often afflict the offspring of the organism. REM is a unit of radiation dosage that accounts for both the energy of the dose and its effectiveness in causing biological damage.

49. Factors Determining Biological Effects of Radiation
1.The energy of the radiation. The higher the energy, the more damage it can cause. 2. The penetrating ability of the radiation. Gamma rays are highly penetrating, beta rays penetrate approximately 1cm, and alpha rays are stopped at the skin. 3. The ionizing ability of the radiation. Radiation that removes electrons from the tissues in the body, disturb body functions. Alpha particles cause the most problems, but must be ingested to do so since they are not penetrating. 4. The chemical properties of the radiation source. When a radioactive nuclide is ingested, its ability to cause damage depends on how long it remains in the body. For example, Sr-90 is similar to calcium and can absorb into the bone and cause bone cancer and leukemia.

50. Penetrating Ability of Radiation
Alpha rays are stopped at the skin also known as UVA rays. Beta rays penetrate I cm and are known as UVB rays.
Beta rays are known for developing many skin cancers.
Gamma rays penetrate completely through the body and
unlike UVA and UVB, humans do not have exposure to
this type of radiation through the sun.

51. REM Exposure in the USA

53. Radiation Exposure per Year

54. Radiation Exposure as Reported by the EPA