Chemeketa Community College

Chemeketa Community College
Nuclear Chemistry Chapter 18 Larry Emme Chemeketa Community College

Discovery of Radioactivity

In 1890, five years before Röentgen announced his discovery of the rays that made the field of radiology possible, a University of Pennsylvania physics professor and a photographer inadvertently exposed two coins to a photographic plate and produced an X-ray. Not understanding the accident, however, they filed the film, only to recall it and realize what they had done when Röentgen's work became public.

Feb. 22nd , 1890

Wilhelm Röentgen

In 1895 Wilhelm Röentgen discovered X-rays.
Röentgen observed that a vacuum discharge tube enclosed in a thin, black cardboard box had caused a nearby piece of paper coated with the salt barium platinocyanide to phosphorescence.

From this and other experiments he concluded that certain rays, which he called X-rays, were emitted from the discharge tube, penetrated the box, and caused the salt to glow.

Known in Britain by the trade name ‘Pedoscope’
Known in Britain by the trade name ‘Pedoscope’. The machine produced an X-ray of the customer’s foot inside a shoe to ensure shoes fitted accurately, which both increased the wear-time of the shoe and with that, the reputation of the shoe shop. The customer placed their foot over an X-ray tube contained within the wooden base of the Pedoscope. From this, a beam of X-rays passed through the foot and cast an image onto a fluorescent screen above. The screen could be observed via three viewing points – one for the shoe-fitter, one for the customer, and one for a third party (usually the guardian of a child being fitted). The accommodation for three viewing points may seem a little extravagant, but it may be an indication of the popularity of the Pedoscope and the interest the public had in the machine.

Shoe-Fitting Fluoroscope (ca. 1930-1940)
Basic Description The shoe fitting fluoroscope was a common fixture in shoe stores during the 1930s, 1940s and 1950s. A typical unit, like the Adrian machine shown here, consisted of a vertical wooden cabinet with an opening near the bottom into which the feet were placed. When you looked through one of the three viewing ports on the top of the cabinet (e.g., one for the child being fitted, one for the child's parent, and the third for the shoe salesman or saleswoman), you would see a fluorescent image of the bones of the feet and the outline of the shoes.

Antoine Henri Becquerel

Shortly after Röentgen’s discovery, Antoine Henri Becquerel attempted to show a relationship between X-rays and the phosphorescence of uranium salts. Becquerel wrapped a photographic plate in black paper, sprinkled a sample of a uranium salt on it, and exposed it to sunlight.

When Becquerel attempted to repeat the experiment the sunlight was intermittent.
He took the photographic plate wrapped in black paper with the uranium sample on it, and placed the whole setup in a drawer.

Several days later he developed the film and was amazed to find an intense image of the uranium salt on the plate. He repeated the experiment in total darkness with the same result.

Two years later, in 1896, Marie Curie coined the name radioactivity.
This proved that the uranium salt emitted rays that affected the photographic plate, and that these rays were not a result of phosphorescence due to exposure to sunlight. Two years later, in 1896, Marie Curie coined the name radioactivity. Elements having this property are radioactive. Radioactivity is the spontaneous emission of particles and/or rays from the nucleus of an atom.

Ernest Rutherford

In 1899 Rutherford began to investigate the nature of the rays emitted by uranium.
He found two particles in the rays. He called them alpha and beta particles. Rutherford’s nuclear atom description led scientists to attribute the phenomenon of radioactivity to reactions taking place in the nuclei of atoms.

The gamma ray, a third type of emission from radioactive material, was discovered by Paul Villard in 1900.

Definitions

nucleon a proton or a neutron
mass number the total number of nucleons in the nucleus. nucleon a proton or a neutron

nuclide any isotope of any atom
isotope atoms of the same element with different masses

Isotopic Notation

6 protons + 6 neutrons 12 C 6 6 protons A nuclide of carbon

8 protons + 8 neutrons 16 O 8 8 protons A nuclide of oxygen

In August 1932, Carl D. Anderson found evidence for an electron with a positive charge, later called the positron. Anderson discovered the positron while using a cloud chamber to investigate cosmic rays. According to this theory, a positron was a hole in a sea of ordinary electrons. The positron was the antimatter equivalent to the electron.

Symbols for Bombarding & Ejected Particles
Name Nuclide Symbol Particle Symbol Alpha  Beta  Proton p

Name Nuclide Symbol Particle Symbol Deuteron d Tritium t Positron +

Name Nuclide Symbol Particle Symbol Neutron n Gamma Ray

Natural Radioactivity

Radioactive elements continuously undergo radioactive decay or disintegration to form different elements. Radioactivity is a property of an atom’s nucleus. It is not affected by temperature, pressure, chemical change or physical state.

radioactive decay the process by which a radioactive element emits particles or rays and is transformed into another element.

Each radioactive nuclide disintegrates at a specific and constant rate, which is expressed in units of half-life. The half-life (t1/2) is the time required for one-half of a specific amount of a radioactive nuclide to disintegrate.

Willard Libby and his apparatus for carbon-14 dating (1946).

The amount of radioactive carbon-14 in the skeleton diminishes by ½ every 5730 years. The result is that the skeleton contains only a fraction of the carbon-14 it originally had. The red arrows symbolize the relative amounts of carbon-14.

Scientists are able to calculate the age of carbon-containing artifacts, such as wooden tools or skeletons, by measuring their current level of radioactivity. This process, carbon dating, enables one to probe as much as 50,000 years into the past. Beyond that time span, there is too little carbon-14 remaining to permit accurate analysis. The dating of older things is accomplished with radioactive minerals, such as uranium-238 and uranium-235 which decay very slowly.

Carbon-14 dating would be an extremely simple dating method if the amount of radioactive carbon in the atmosphere had been constant over the ages. The fact is, it has not. Fluctuations in the Sun’s magnetic field as well as changes in the strength of Earth’s magnetic field affect cosmic-ray intensities in Earth’s atmosphere, which in turn produce fluctuations of the in the production of C-14. Also, changes in Earth’s climate affect the amount of CO2 in the atmosphere. The oceans are enormous reservoirs of CO2. When the oceans are cold, they release less CO2 into the atmosphere than when they are warm.

The half-life of 131I is 8 days
The half-life of 131I is 8 days. How much 131I from a 32-g sample remains after five half-lives? half-lives number of days amount remaining 32 g 1 8 16 g 5 40 1 g 4 32 2 g 3 24 4 g 2 16 8 g Trace a horizontal line from this point on the plotted line to the y-axis and read the corresponding grams of 131I. Take a perpendicular line from any multiple of 8 days on the x-axis to the line on the graph.

Nuclides are said to be either stable (nonradioactive) or unstable (radioactive).
Elements that have an atomic number greater than 83 are naturally radioactive. Some of the naturally occurring nuclides of elements 81, 82 and 83 are radioactive and some are stable.

Only a few naturally occurring elements that have atomic numbers less than 81 are radioactive.
No stable isotopes of element 43 (technetium) or of element 61 (promethium) are known.

Radioactivity is believed to be a result of an unstable ratio of neutrons to protons in the nucleus.
Stable nuclides of elements up to about atomic number 20 generally have a about a 1:1 neutron-to-proton ratio.

In elements above atomic number 20, the neutron-to-proton ratio in the stable nuclides gradually increases to about 1.5:1 in element number 83 (bismuth). When the neutron to proton ratio is too high or too low, alpha, beta, or other particles are emitted to achieve a more stable nucleus.

Alpha Particles, Beta Particles and Gamma Rays

Marie Curie, in a classic experiment, proved that alpha and beta particles are oppositely charged.
three types of radiation are detected by a photographic plate radiation passes between the poles of an electromagnet Beta rays are strongly deflected to the positive pole. Gamma rays are not deflected by the magnet. Alpha rays are less strongly deflected to the negative pole. a radioactive source was placed inside a lead block

Alpha Particles

 The symbols of an alpha particle are
An alpha particle is a helium nucleus.  It consists of two protons and two neutrons. It has a charge of +2. It has a mass of 4 amu.

Loss of an alpha particle from the nucleus results in loss of 4 in the mass number (A) loss of 2 in the atomic number (Z)

Formation of thorium from the radioactive decay of uranium can be written as
mass number decreases by 4 atomic number decreases by 2 or

To have a balanced nuclear equation
the sum of the mass numbers (superscripts) on both sides of the equation must be equal. the sum of the atomic numbers (subscripts) on both sides of the equation must be equal. sum of mass numbers = 238 sum of atomic numbers = 92

Beta Particles

The symbols of the beta particle are
The beta particle is identical in mass and charge to an electron.  Its charge is -1.

A beta particle and an electron are formed by the decomposition of a neutron.
The beta particle leaves the nucleus and the proton remains in the nucleus.

Loss of a beta particle from the nucleus results in
no change in the mass number an increase of 1 in the atomic number

Gamma Rays

 The symbol of a gamma ray is A gamma ray is a high energy photon.
It is emitted by radioactive nuclei. It has no electrical charge. It has no measurable mass.

Loss of a gamma ray from the nucleus results in
no change in the mass number no change in atomic number

Write an equation for the loss of an alpha particle from the nuclide 194Pt.

Pt 194 78 A nuclide of platinum 78 protons + 116 neutrons 78 protons
Write an equation for the loss of an alpha particle from the nuclide 194Pt. Mass number (sum of protons and neutrons in the nucleus) 78 protons neutrons 194 Pt 78 78 protons Atomic number (number of protons in the nucleus) A nuclide of platinum

Atomic number of new nuclide: Z-2 78 – 2 = 76 or
Write an equation for the loss of an alpha particle from the nuclide 194Pt. Loss of an alpha particle, 4He, results in a decrease of 4 in the mass number and a decrease of 2 in the atomic number. A = mass number Mass of new nuclide: A-4 194 – 4 = 190 or Z = atomic number Atomic number of new nuclide: Z-2 78 – 2 = 76 or Element number 76 is Os, osmium. The equation is

What nuclide is formed when 194Ra loses a beta particle from its nucleus?

Ra 228 88 A nuclide of radium 88 protons + 140 neutrons 88 protons
What nuclide is formed when 194Ra loses a beta particle from its nucleus. Mass number (sum of protons and neutrons in the nucleus) 88 protons neutrons 228 Ra 88 88 protons Atomic number (number of protons in the nucleus) A nuclide of radium

Atomic number of new nuclide: Z-(-1) 88 + 1 = 89 or
What nuclide is formed when 194Ra loses a beta particle from its nucleus. The loss of a beta particle from a 194Ra nucleus means a gain of 1 in the atomic number with no essential change in mass. A = mass number Mass of new nuclide: A-0 228 – 0 = 228 or Z = atomic number Atomic number of new nuclide: Z-(-1) = 89 or The equation is

Balancing Nuclear Equations

Balance the following equations by replacing the “
Balance the following equations by replacing the “?” with the correct isotopic notation.

Penetrating Power of Radiation

The ability of radioactive rays to pass through various objects is in proportion to the speed at which they leave the nucleus. Thin sheet of paper – stops  particles. Thin sheet of aluminum – stops  and  particles. 5-cm lead block – will reduce, but not completely stop  radiation

Radioactive Disintegration Series

The naturally occurring radioactive elements with a higher atomic number than lead fall into three orderly disintegration series. Each series proceeds from one element to the next with the loss of either an alpha or a beta particle, finally ending in a nonradioactive nuclide.

The uranium series starts with 238U and ends with 206Pb.
92 82 The thorium series starts with 232Th and ends with 208Pb. 90 82 The actinium series starts with 235U and ends with 207Pb. 92 82

A fourth series begins with the synthetic element plutonium.
The neptunium series begins with 241Pu and ends with 238Bi. 94 83

The uranium disintegration series
The uranium disintegration series. 238U decays by a series of alpha () and beta () emissions to the stable nuclide 208Pb.

Transmutation of Elements

Transmutation occurs spontaneously in natural radioactive disintegrations.
Transmutation is the conversion of one element into another by either natural or artificial means.

The first artificial transmutation occurred in 1919 when Rutherford succeeded in producing oxygen from nitrogen. Alchemists for hundreds of years attempted to transmute mercury and lead into gold by artificial means. They were never successful.

Gold was finally produced at about this same time
Gold was finally produced at about this same time. Unfortunately one needs to start with Platinum! Alchemists for hundreds of years attempted to transmute mercury and lead into gold by artificial means. They were never successful.

Some of these transmutations are:

Artificial Radioactivity

Irene and Frederick Curie observed that the bombardment of aluminum-27 with alpha particles results in the emission of neutrons and positrons.

This suggested that neutron emission and positron emission were a result of separate reactions.
When alpha particle bombardment is halted neutron emission stops, but positron emission continues.

Further investigation on their part showed that when aluminum-27 is bombarded with alpha particles, phosphorous-30 is produced.

Phosphorous-30 is radioactive, has a 2
Phosphorous-30 is radioactive, has a 2.5 minute half-life, and decays to silicon-30 with the emission of a positron.

The Joliot-Curies received the Nobel Prize in chemistry in 1935 for the discovery of artificial, or induced, radioactivity. The radioactivity of nuclides produced in this manner is known as artificial or induced radioactivity.

Measurement of Radioactivity

Radiation from a radioactive source can be measured by a variety of instruments.
Geiger counters film badges scintillation counters

Radiation knocks off an electron Ions detected by Counter
Geiger Counter Radiation knocks off an electron - An ion Gas in instrument tube Ions detected by Counter

The curie is the unit indicating the rate of decay of a radioactive substance.
One curie (Ci) = 3.7 x 1010 disintegrations per second. Because a curie is so large the millicurie (one thousandth of a curie) and the microcurie (one millionth of a curie are more commonly used). This very high radiation level is the amount of radiation emitted by 1 gram of radium in one second.

Nuclear Fission

Nuclear fission was discovered in 1939 by Lise Meitner (a physicist) and Otto Hahn (a chemist). They were attempting to create heavier elements than uranium. They detected lighter isotopes of Ba, Kr, and La which suggested that the uranium nucleus had split.

Enrico Fermi built the first experimental nuclear reactor, the Atomic Pile at the University of Chicago on December 2, 1942.

Depiction of the setting in the squash court beneath the stands at the University of Chicago’s Stagg Field, where Fermi constructed the first nuclear reactor.

Upon the discovery of fission, by Hahn and Meitner early in 1939, Fermi immediately saw the possibility of emission of secondary neutrons and of a chain reaction. He directed a classical series of experiments which ultimately led to the first controlled nuclear chain reaction. This took place on December 2, on a squash court situated beneath Chicago's stadium.

As the atom splits, it releases energy and two or three other neutrons, each of which can cause another nuclear fission. In nuclear fission, a heavy nuclide splits into two or more intermediate sized fragments when struck in a particular way by a neutron

Characteristics of Nuclear Fission
Upon absorption of a neutron, a heavy nuclide spits into one or more smaller nuclides (fission products). The mass of the nuclides ranges from abut amu. Two or more neutrons are produced from the fission of each atom.

Characteristics of Nuclear Fission
Large quantities of energy are produced as a result of the conversion of a small amount of mass into energy. Many nuclides produced are radioactive and continue to decay until they reach a stable nucleus.

Fission of 235U Each time fission occurs three neutrons and two nuclei are produced.

In a chain reaction the products cause the reaction to continue or magnify.
For a chain reaction to continue, enough fissionable material must be present so that each atomic fission causes, on average, at least one additional fission.

The minimum quantity of an element needed to support a self-sustaining chain reaction is called the critical mass. The critical mass of uranium was calculated By Richard Feynman.

Fission and chain reaction of 235U.

Nuclear Stability

A plot of the number of neutrons (N) versus the number of protons (Z) for stable and radioactive isotopes from hydrogen (Z = 1) to bismuth (Z = 83).

Stable nuclides, if plotted on a graph of number of protons vs
Stable nuclides, if plotted on a graph of number of protons vs. number of neutrons, would all fall in an area enclosed by two curved lines known as the band of stability. The band of stability also includes radionuclides because smooth lines cannot be drawn to exclude them. The band of stability also stops at element 83 because there are no known stable isotopes above it. Elements lying outside the band of stability would be too unstable to justify the time and money for an attempt to make it.

Another thing that is noticed about the band of stability is that as the number of protons increases, the ratio of neutrons to protons increases. This is because more neutrons are needed to compensate for the increasing proton-proton repulsions. Isotopes occurring above and to the left of the band tend to be beta emitters because they want to lose a neutron and gain a proton. Those lying below and to the right of the band tend to be positron emitters because they want to lose a proton and gain a neutron. Isotopes above element 83 tend to be alpha emitters because they have too many nucleons.

In the odd-even rule, when the numbers of neutrons and protons in the nucleus are both even numbers, the isotopes tends to be far more stable than when they are both odd. Out of all the 300 stable isotopes, only 4 have both odd numbers of both ( and whereas 200 have even numbers of both, and the rest (about 120) have a mixed number. This has to do with the spins of nucleons. Both protons and neutrons spin. When two protons or neutrons have paired spins (opposite spins), their combined energy is less than when they are unpaired.

The Magic Numbers Another rule of nuclear stability is that isotopes with certain numbers of protons or neutrons tend to be more stable then the rest. These certain numbers are called the magic numbers, and they are: 2, 8, 20, 28, 50, 82, and 126. When a nucleus has a number of protons and neutrons that are the same magic number, it is very stable. For example: One stable isotope of lead, has 82 protons and 126 neutrons.

Predicting the Type of Decay

Z > 83 α and β emission

neutron and β emission

K-capture and positron emission

Nuclear Power

A nuclear power plant is a thermal power plant in which heat is produced by a nuclear reactor instead of by combustion of fossil fuel. The major components of a nuclear reactor are an arrangement of nuclear fuel, called the reactor core. a control system, which regulates the rate of fission and thereby the rate of heat generation.

Trojan Nuclear Power Plant – Rainier, Oregon

May 21, 2006

Trojan Nuclear Reactor– Rainier, Oregon

Uranium oxide pellet used in nuclear fuel rods.
Uranium is the fuel of the nuclear power plant in the US. However, we can not just dump uranium into the core like we shovel coal into a furnace. The uranium must be processed and formed into fuel pellets, which are about the size of a pencil eraser. The fuel pellets are then stacked inside hollow metal tubes to form fuel rods. Fuel rods are 11 to 25 feet in length. Each UO2 pellet has the energy equivalent to burning 136 gal of oil, 2.5 tons of wood, or 1 ton of coal.

In the United States breeder reactors are used to generate nuclear power.
Breeder reactors use U3O8 that is enriched with scarce fissionable U-235. In a breeder reactor, excess neutrons convert nonfissionable isotopes, such as U-238 or Th-232, to fissionable isotopes, Pu-239 or U-233.

Yucca Mountain in Nevada – site for nuclear depository?

Conceptual Design of Yucca Mountain Disposal Plan
Canisters of waste, sealed in special casks, are shipped to the site by truck or train. Shipping casks are removed, and the inner tube with the waste is placed in a steel, multilayered storage container. An automated system sends storage containers underground to the tunnels. Containers are stored along the tunnels, on their side. More current information:

Pros Department of Energy (DOE)
In a desert location Isolated away from population centers (Las Vegas, the nearest metropolitan area, is 90 miles away) Secured 1,000 feet under the surface In a closed hydrologic basin Surrounded by federal land Protected by natural geologic barriers Protected by robust engineered barriers and a flexible design

Cons: Nevada's Agency for Nuclear Projects
Yucca's location in an active seismic (earthquake) region the presence of numerous earthquake faults (at least 33 in and around the site) and volcanic cinder cones near the site the presence of pathways (numerous interconnecting faults and fractures) that could move groundwater (and any escaping radioactive materials) rapidly through the site to the aquifer beneath and from there to the accessible environment. evidence of hydrothermal activity within the proposed repository block

Putting end to Yucca Mountain project ‘within reach,’ state commission says
Jan. 21, 2013

In 1986, a meltdown occurred at Chernobyl, Ukraine
In 1986, a meltdown occurred at Chernobyl, Ukraine. There was no containment building so large amounts of radioactive isotopes were released into the environment Today 10,000 square kilometers of land remain contaminated with high levels of radiation.

The Atomic Bomb

Albert Einstein

Einstein’s Letter to FDR
Albert Einstein Old Grove Rd Nassau Point Peconic, Long Island August 2nd 1939 F.D. Roosevelt President of the United States White House Washington, D.C.

Sir: Some recent work by E. Fermi and L. Szilard, which has been communicated to me in manuscript, leads me to expect that the element uranium may be turned into a new and important source of energy in the immediate future. Certain aspects of the situation which has arisen seem to call for watchfulness and, if necessary, quick action on the part of the Administration. I believe therefore that it is my duty to bring to your attention the following facts and recommendations:

In the course of the last four months it has been made probable - through the work of Joliot in France as well as Fermi and Szilard in America - that it may become possible to set up a nuclear chain reaction in a large mass of uranium, by which vast amounts of power and large quantities of new radium-like elements would be generated. Now it appears almost certain that this could be achieved in the immediate future.

This new phenomenon would also lead to the construction of bombs, and it is conceivable - though much less certain - that extremely power- ful bombs of a new type may thus be constructed. A single bomb of this type, carried by boat and exploded in a port, might very well destroy the whole port together with some of the surrounding territory. However, such bombs might very well prove to be too heavy for transportation by air. The United States has only very poor ores of uranium in moderate quantities. There is some good ore in Canada and the former Czechoslovakia.

while the most important source of uranium is Belgian Congo
while the most important source of uranium is Belgian Congo. In view of the situation you may think it desirable to have more permanent contact maintained between the Administration and the group of physicists working on chain reactions in America. One possible way of achieving this might be for you to entrust with this task a person who has your confidence and who could perhaps serve in an inofficial capacity.

His task might comprise the following:
a) to approach Government Departments, keep them informed of the further development, and put forward recommendations for Government action, giving particular attention to the problem of securing a supply of uranium ore for the United States; b) to speed up the experimental work, which is at present being carried on within the limits of the budgets of University laboratories, by providing funds, if such funds be required, through his contacts with private persons who are willing to make contributions for this cause, and perhaps also by obtaining the co-operation of industrial laboratories which have the necessary equipment.

I understand that Germany has actually stopped the sale of uranium from the Czechoslovakian mines which she has taken over. That she should have taken such early action might perhaps be understood on the ground that the son of the German Under-Secretary of State, von Weizsäcker, is attached to the Kaiser-Wilhelm-Institut in Berlin where some of the American work on uranium is now being repeated. Yours very truly, (Albert Einstein)

Robert Oppenheimer

The atomic bomb is a fission bomb.
“Wild” or uncontrolled fission occurs in an atom bomb, whereas in a nuclear reactor the fission is carefully controlled. A minimum critical mass of fissionable material is required for a bomb.

Richard P. Feynman

Hans Bethe

Philip Morrison

When a quantity of fissionable material smaller than the critical mass is used, too many neutrons escape and a chain reaction does not occur. The fissionable material of an atomic bomb is stored as two or more subcritical masses and are then brought together to achieve a nuclear detonation.

Uranium-235 and plutonium-239 are the nuclides used to construct an atomic bomb.
99.3% of uranium is nonfissionable uranium-238. Uranium-238 can be transmuted to fissionable plutonium-239.

Nuclear Fusion

Nuclear fusion is the process of uniting two light elements to form one heavier element.

The difference in mass is released as energy.
The masses of the two nuclei that fuse into a single nucleus are greater than the mass of the nucleus formed by their fusion. The difference in mass is released as energy. tritium deuterium amu amu amu amu amu – amu = amu

Fusion reactions require temperatures on the order of tens of millions of degrees for initiation.
Such temperatures are present in the Sun but have been produced only momentarily on earth.

Fusion power will be far more superior to fission power because
Virtually infinite amounts of energy are possible from fusion power. While uranium supplies are limited, deuterium supplies are abundant. It is estimated that the deuterium present in a cubic mile of seawater used as fusion fuel can provide more energy than the petroleum reserves of the entire world.

Fusion power will be far more superior to fission power because
Fusion power is much “cleaner” that fission power. Fusion reactions (unlike uranium and plutonium fission reactions) do not produce large amounts of long-lived and dangerously radioactive isotopes.

(a) An interior view of the Tokamak Fusion Reactor at Princeton
(a) An interior view of the Tokamak Fusion Reactor at Princeton. Magnet fields confine a fast-moving plasma to a circular path. At high enough temperatures, the atomic nuclei in the plasma fuse to produce energy. (b) A cross-sectional view of the ITER planned to be built and operating in France before 2020.

Applications of Radioactivity

Food Irradiation Food irradiation with gamma rays from 60Co or 137Cs sources is commonly used in Europe. Astronaut’s food is irradiated. Foods are pasteurized by irradiation to retard bacteria, molds, and yeasts. Chicken normally has a 3 day refrigerated shelf-live; after irradiation it has a 3 week shelf-live.

Irradiation of Foods Irradiation kills many of the microorganisms that promote spoilage, greatly increasing the shelf life of the food. The gamma radiation used passes through and does not make foods radioactive.

In the US irradiation is only approved for a small number of foods: potatoes, strawberries, grapefruit, fish and shrimp for export.

Radioactive Tracers Radioactive isotopes are used as tracers where a reactant compound consists of both radioactive and stable isotopes. The tracer is feed to an organism and the radioactivity is measured to determine which parts of the organism contain the radioisotope. Geiger counters or other radiation detectors are used.

Radioisotopes Used as Tracers
Half-Life Use 14C 5730 yr CO2 for photosynthesis research 3H 12.33 yr Tag Hydrocarbons 35S 87.2 d Tag pesticides, measure air flow 32P 14.3 d Measure phosphorus uptake by plants

Medical Imaging Radioactive isotopes used in nuclear medicine for diagnosing & therapy. Diagnosis of internal disorders done by imaging. Imaging concentrates a radioisotope at the site of the disorder. The radioisotope emits a radiation which is detected.

Diagnostic Radioisotopes
Half-Life (Hours) Site for Diagnosis 99Tc 6.0 Thyroid 201Tl 72.9 Heart 123I 13.2 67Ga 78.2 Tumors & Abscesses

The thyroid gland absorbs much of the iodine that enters the body through food. Images of the thyroid gland, shown here, can be obtained by giving the patient a small amount of iodine-131. The image is useful in diagnosing metabolic disorders.

Positron Emission Tomography (PET)
Radioisotopes such as 11C, 18F, 13N, or 15O decay forming a positron. These positrons react with electrons to create gamma rays. The gamma rays are recorded by a circular detector when the scan is performed. In about 10 min the region of tissue containing the radioisotope can be imaged with a computer.

PET Scans Normal Alzheimer's

Nature’s Fundamental Particles
174

Quarks Among nature’s fundamental particles are six kinds of quark, of which two are the fundamental building blocks of all nucleons (protons and neutrons). Quarks carry fractional electrical charges. One kind, the up quark, carries +2/3 the proton charge, and another, the down quark, has -1/3 the proton charge. Quarks in the proton are the combination up up down, and in the neutron up down down. The other four quarks bear the whimsical names strange, charm, top, and bottom. No quarks have been isolated and experimentally observed. Most theorists think quarks cannot be isolated.

The End

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