1. Radioactivity: The Study of Unstable Atoms Chapter 25

2. Wilhelm Conrad Roentgen
On December , this physicist "photographed" his wife's hand, revealing the unmistakable image of her skeleton, complete with wedding ring.
Roentgen's wife had placed her hand in the path of X-rays which Roentgen created by beaming an electron ray energy source onto a cathode tube.
He studied fluoresence and phosphoresence.

3. Antoine Henri Becquerel
Becquerel found that, while the phenomena of fluorescence and phosphorescence had many similarities to each other and to X-rays, they also had important differences.
Fluorescence and X-rays stopped when the initiating energy source was halted
Phosphorescence continued to emit rays some time after the initiating energy source was removed.
However, in all three cases, the energy was derived initially from an outside source.
In March of 1896, during a time of overcast weather, Becquerel found he couldn't use the sun as an initiating energy source for his experiments.
He put his wrapped photographic plates away in a darkened drawer, along with some crystals containing uranium.
Much to his surprise, the plates were exposed during storage by invisible emanations from the uranium.
The emanations did not require the presence of an initiating energy source--the crystals emitted rays on their own!

4. The Curies: Lives Devoted to Research
Working in the Becquerel lab, Marie Curie and her husband, Pierre, began what became a life long study of radioactivity.
Marie Curie wrote, "The subject seemed to us very attractive and all the more so because the question was entirely new and nothing yet had been written upon it."
Becquerel had already noted that uranium emanations could turn air into a conductor of electricity.
Using sensitive instruments invented by Pierre Curie and his brother, Pierre and Marie Curie measured the ability of emanations from various elements to induce conductivity.
On February 17, 1898, the Curies tested an ore of uranium, pitchblende, for its ability to turn air into a conductor of electricity.
The Curies found that the pitchblende produced a current 300 times stronger than that produced by pure uranium.
They tested and recalibrated their instruments, and yet they still found the same puzzling results.
The Curies reasoned that a very active unknown substance in addition to the uranium must exist within the pitchblende.
In the title of a paper describing this hypothesized element (which they named polonium after Marie's native Poland), they introduced the new term: "radio-active."

5. The Curies: Lives Devoted to Research (Cont)
After much grueling work, the Curies were able to extract enough polonium and another radioactive element, radium, to establish the chemical properties of these elements.
Marie Curie, with her husband and continuing after his death, established the first quantitative standards by which the rate of radioactive emission of charged particles from elements could be measured and compared.
In addition, she found that there was a decrease in the rate of radioactive emissions over time and that this decrease could be calculated and predicted.
But perhaps Marie Curie's greatest and most unique achievement was her realization that radiation is an atomic property of matter rather than a separate independent emanation.
Despite the giant step forward which science had now taken in it's understanding of radioactivity, scientists still understood little of the structure of the atom.

6. Radiation: Basic Definitions
Radioactive materials are composed of atoms that are unstable.  An unstable atom gives off excess energy until it becomes stable.  The energy the atom emits is radiation.  The process by which an atom changes from an unstable state to a more stable state by emitting radiation is called radioactivity. 
Radiation can be classified as either non-ionizing (low energy) or ionizing (high energy) radiation.  Types of non-ionizing radiation are ultraviolet light, visible light, infrared radiation, radio frequency radiation and microwaves.  Ionizing radiation  is given off by the sun (cosmic rays), radioactive materials, and high energy electronic devices (X-ray machines).  

7. 4 Types of Ionizing Radiation
Alpha (a) particles
2 protons, 2 neutrons (He)
Beta (b) particles
1 electron
Gamma (g)particles
1 photon
X-rays
N.B. g and X-ray photons are different types of electromagnetic radiation and have different energy levels

8. Alpha Particles
Positively charged particles made up of two neutrons and two protons. 
They are relatively heavy and slower moving than other radioactive emissions. 
Alpha particles can be stopped  by a piece of paper or the dead outer layer of our skin.
Alpha particles are dangerous when ingested or inhaled.

9. Beta Particles Negatively charged particles made up of an electron.
A beta particle is lighter and faster than an alpha particle.
Beta radiation can pass through an inch of water or skin, but not through a thin sheet of aluminum, plywood or steel.

10. Gamma Radiation
Short wavelength electromagnetic radiation emitted in the radioactive decay of an unstable atom.  
Gamma radiation is highly penetrating but carries no mass or charge.
Stopping gamma rays requires one inch of a dense material like lead or concrete.  

11. X-rays
X-rays are similar to gamma rays, but are generally lower in energy and less penetrating. 
X-rays are emitted from processes outside the nucleus, while gamma rays originate inside the nucleus.   
A few milimeters of lead can stop medical x-rays. 

12. Sources of Radiation

13. The Penetrating Power of Radiation

14. Atoms
Atomic Nuclei are held together by the “strong nuclear force” one of four forces in the universe.
Gravity is also one of these four forces.
It is the weakest.
The positively charged protons also repel each other by electromagnetism.
Remember, when an electrically charged particle moves, it creates a magnetic field.
The neutrons provide a buffer between the positive charges in the nucleus.
Remember, neutrons have no net charge.

15. Atoms (Cont)
The larger the nucleus, the larger both types of forces, strong and electromagnetic.
When the electromagnetic force is greater than the strong force, the nucleus will decay.
To balance the forces, the ratio of neutrons:protons must be ~1:1 for atoms with <20 protons, and ~1.5:1 for the largest atoms.
Ratios both above and below this range result in radioactive isotopes.
Radioactivity: During decay, matter and energy are expelled from the nucleus, making it unstable. Decay continues until stability has been reached within the nucleus.

16. Alpha, Beta, and Gamma Decay
Beta decay occurs when the n:p ratio is above the stability curve, as it reduces the # of neutrons in the nucleus
All elements with 83 protons or greater are radioactive and undergo alpha decay.
Alpha decay reduces both the number of neutrons and protons in the nucleus, helping to stabilize these very large nuclei.
Gamma decay tends to occur in both scenarios, but doesn’t affect mass or charge, and is therefore usually omitted when balancing the equation.
Some isotopes of an element are stable, others are radioactive.

17. Positron Emission and Electron Capture
A positron has the same mass as an electron, but opposite charge
When the n:p ratio lies below the stability curve, a positron may be emitted to convert a proton into a neutron
During electron capture, an electron from a low-energy orbital is combined with a proton to form a neutron
This also occurs when the n:p ratio lies below the stability curve
An X-ray photon is emitted during electron capture

18. Transmutation
When the atomic number of an atom changes, its elemental identity changes
All forms of radiation except g and X-ray radiation cause transmutation to occurs.

19. Reading the Periodic Table
Element symbols: C
Top number is atomic mass, lower number is atomic number.
Atomic mass is the total number of protons and neutrons in the nucleus.
Atomic number is the total number of protons in the nucleus. 12 6

20. Calculating Atomic Particles
Carbon-14 is radioactive C
How many neutrons does carbon-14 have? 6 14

21. Balancing Nuclear Equations
Oxygen-15 undergoes positron emission. Show the balanced equation.
Reactant: 15O 8
Product: 15N + 0b

22. Balancing Nuclear Equations
Thorium-231 becomes Protactinium Show the balanced equation and identify the type of radioactive decay.
Reactant: 231Th 90
Product: 231Pa + 0b

23. Radioactive Dating
Half-life: the time necessary for 50% of the element to decay.
The age of an item can be determined by comparing the original amount of radioactive material with the current amount.
Amt Remaining=Initial Amt (1/2)t/T
t=time elapsed, T=period of one half-life

24. Radioactive Decay—the Real Equation
A common example of exponential decay is radioactive decay. Radioactive materials, and some other substances, decompose according to a formula for exponential decay. That is, the amount of radioactive material A present at time t is given by the formula
A=A0ekt
where k < 0.
A radioactive substance is often described in terms of its half-life, which is the time required for half the material to decompose.

25. Problem
After 500 years, a sample of radium-226 has decayed to 80.4% of its original mass. Find the half-life of radium-226.

26. Solution
Let A= the mass of radium present at time t (t=0 corresponds to 500 years ago). We want to know for what time t is A = (1/2)A0. However, we do not even know what k is yet. Once we know what k is, we can set A in the formula for exponential decay to be equal to (1/2) A0, and then solve for t. First we must determine k. We are given that after 500 years, the amount present is 80.4% of its original mass. That is, when t=500, A=0.804 A0. Substituting these values into the formula for exponential decay, we obtain:
0.804 A0=A0ek(500).
Dividing through by A0 gives us
0.804 = e500k
which is an exponential equation.

27. Solving the Equation
To solve this equation, we take natural logs (ie. ln) of both sides.
ln ( 0.804) = ln (e500k)
We know that ln (e500k) = 500k by the cancellation properties of ln and e. So the equation becomes
ln ( 0.804) = 500k
and
k= (ln 0.804)/500.
This is the exact solution; evaluate the natural log with a calculator to get the decimal approximation k = Since we now know k, we can write the formula (function) for the amount of radium present at time t as
A=A0 e t.

28. Finding the half-life
Now, we can finally find the half-life. We set A=1/2 A0 and solve for t.
(1/2)A0=A0 e t
Dividing through by A0 again, we get:
1/2 = e t.
To solve for t, take natural logs:
ln(1/2) = ln[e t].
Then applying the cancellation property for logarithms yields
ln (1/2) = t So
t= ln(1/2) /( )
or t = The half-life is approximately 1590 years.

29. Carbon Dating
The dating of radioactive carbon has helped to define the history of life on this planet.
Any living organism takes in both radioactive and non-radioactive carbon, either through the process of photosynthesis or by eating plants or eating animals that have eaten plants.
When the animal dies, however, uptake of carbon stops.
As a result, radioactive carbon atoms are not replaced as they decay, and the amount of this material decreases over time.
The rate of decrease is predictable and can be described with accuracy, vastly increasing our ability to date the biological events of our planet.

30. Example: Carbon Dating
The half-life of carbon-14 =5,730 years.
If an organism had C atoms during its life, then, in 5,730 years, it will have 50.
Carbon dating is used to date organic samples less than 50,000 years old.

31. Other Radioactive Dating
Uranium: 4,040,000 years
Iodine: 8.04 days
Lead 10.6 hours

32. Applications: Isotopes in Research and Medicine
Scientists can now create radioactive forms of common elements, called isotopes.
Each isotope has a fixed rate of decay which can be characterized by its half-life, or the length of time that it takes half of the radioactive atoms in a sample to decay.
Because each isotope decays at a unique and predictable rate, different isotopes can be used for a variety of purposes.
For example, isotopes play an important role in modern medicine.
They can be ingested and traced in their path through the body, revealing biochemical and metabolic processes with precision.
These isotropic "tracers" are currently used for practical diagnosis of disease as well as in research.

33. Other Uses
PET Scans: Radioactive Fluorine is injected into the bloodstream, allowing doctors to view brain activity.
Radiation therapy: Radiation is targeted to a specific tumor to kill those cells.
Used to trace chemical reactions.

34. Damage from Radiation
All forms of radiation can damage a DNA molecule.
E.g. UV causes lesions in the DNA
E.g. g causes double-strand breaks in the DNA
Any time structural change occurs in the DNA, it has to be repaired. If it is not repaired correctly, mutation occurs.
When damage occurs to an important gene, cancer can result.
What is an important gene?
Any gene encoding a protein involved in regulating the cell cycle, DNA replication, or DNA repair.
Defects in these genes lead to accumulations of mutations, loss of apoptosis and cell death in ineffective cells, and cancer arises because these non-functioning “tumor” cells out-divide functioning cells in the body.

35. Embryos
If the DNA of an embryo is damaged, birth defects, rather than cancer can result.
In fact, researchers study embryonic and fetal development in the hopes of understanding the roots of cancer, as cancer is almost the “reverse” or “second-coming” of normal development.

36. Specific Uses of Certain Elements
Americium -241: Used in many smoke detectors for homes and business...to measure levels of toxic lead in dried paint samples...to ensure uniform thickness in rolling processes like steel and paper production...and to help determine where oil wells should be drilled.
Cadmium -109: Used to analyze metal alloys for checking stock, sorting scrap.
Calcium - 47: Important aid to biomedical researchers studying the cell function and bone formation of mammals.
Californium - 252: Used to inspect airline luggage for hidden explosives...to gauge the moisture content of soil in the road construction and building industries...and to measure the moisture of materials stored in silos.
Carbon - 14: Helps in research to ensure that potential new drugs are metabolized without forming harmful by-products.
Cesium - 137: Used to treat cancers...to measure correct patient dosages of radioactive pharmaceuticals...to measure and control the liquid flow in oil pipelines...to tell researchers whether oil wells are plugged by sand...and to ensure the right fill level for packages of food, drugs and other products. (The products in these packages do not become radioactive.)
Chromium - 51: Used in research in red blood cell survival studies.

37. Specific Uses of Certain Elements (Cont)
Cobalt - 57: Used in nuclear medicine to help physicians interpret diagnosis scans of patients' organs, and to diagnose pernicious anemia.
Cobalt - 60 : Used to sterilize surgical instruments...to improve the safety and reliability of industrial fuel oil burners...and to preserve poultry, fruits, and spices.
Copper - 67: When injected with monoclonal antibodies into a cancer patient, helps the antibodies bind to and destroy the tumor.
Curium - 244: Used in mining to analyze material excavated from pits slurries from drilling operations.
Iodine - 123: Widely used to diagnose thyroid disorders.
Iodine - 129: Used to check some radioactivity counters in vitro diagnostic testing laboratories.
Iodine - 131: Used to diagnose and treat thyroid disorders. (Former President George Bush and Mrs. Bush were both successfully treated for Grave's disease, a thyroid disease, with radioactive iodine.)
Iridium - 192: Used to test the integrity of pipeline welds, boilers and aircraft parts.
Iron - 55: Used to analyze electroplating solutions.

38. Specific Uses of Certain Elements (Cont)
Krypton - 85: Used in indicator lights in appliances like clothes washer and dryers, stereos and coffee makers...to gauge the thickness of thin plastics and sheet metal, rubber, textiles and paper...and to measure dust and pollutant levels.
Nickel - 63: Used to detect explosives...and as voltage regulators and current surge protectors in electronic devices.
Phosphorus - 32: Used in molecular biology and genetics research.
Plutonium - 238: Has safely powered at least 20 NASA spacecraft since 1972.
Polonium - 210: Reduces the static charge in production of photographic film and phonograph records.
Promethium - 147: Used in electric blanket thermostats...and to gauge the thickness of thin plastics, thin sheet metal, rubber, textiles, and paper.
Radium - 226: Makes lightning rods more effective.
Selenium - 75: Used in protein studies in life science research.
Sodium - 24: Used to locate leaks in industrial pipelines...and in oil well studies.
Strontium - 85: Used to study bone formation and metabolism.
Technetium - 99m: The most widely used radioactive isotope for diagnostic studies in nuclear medicine. Different chemical forms are used for brain, bone, liver, spleen and kidney imaging and also for blood flow studies.

39. Specific Uses of Certain Elements (Cont)
Thallium - 204: Measures the dust and pollutant levels on filter paper...and gauges the thickness of plastics, sheet metal, rubber, textiles and paper.
Thoriated tungsten: Used in electric are welding rods in the construction, aircraft, petrochemical and food processing equipment industries. It produces easier starting, greater arc stability and less metal contamination.
Thorium - 229: Helps fluorescent lights to last longer.
Thorium - 230: Provides coloring and fluorescence in colored glazes and glassware.
Tritium: Used for life science and drug metabolism studies to ensure the safety of potential new drugs... for self-luminous aircraft and commercial exit signs... for luminous dials, gauges and wrist watches...and to produce luminous paint.
Uranium - 234: Used in dental fixtures like crowns and dentures to provide a natural color and brightness.
Uranium - 235: Fuel for nuclear power plants and naval nuclear propulsion systems...also used to produce fluorescent glassware, a variety of colored glazes and wall tiles.
Xenon - 133: Used in nuclear medicine for lung ventilation and blood flow studies.