3 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.
6 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.
7 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’. 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. 10
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. 12
15 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.
16 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.
17 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.
18 Radioactivity is the spontaneous emission of particles and/or rays from the nucleus of an atom. 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. Elements having this property are radioactive. Two years later, in 1896, Marie Curie coined the name radioactivity.
20 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.
21 The gamma ray, a third type of emission from radioactive material, was discovered by Paul Villard in 1900.
27 C 6 6 protons 12 6 protons + 6 neutrons A nuclide of carbon
28 8 8 protons 16 8 protons + 8 neutrons O A nuclide of oxygen
29 8 8 protons 17 8 protons + 9 neutrons O A nuclide of oxygen
30 8 8 protons 18 8 protons + 10 neutrons O A nuclide of oxygen
31 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.
32 Symbols for Bombarding & Ejected Particles Name Nuclide SymbolParticle Symbol Alpha Beta Proton p
33 Symbols for Bombarding & Ejected Particles Name Nuclide SymbolParticle Symbol Deuteron d Tritium t Positron +
34 Symbols for Bombarding & Ejected Particles Name Nuclide SymbolParticle Symbol Neutron n Gamma Ray
36 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.
37 radioactive decay the process by which a radioactive element emits particles or rays and is transformed into another element.
38 Each radioactive nuclide disintegrates at a specific and constant rate, which is expressed in units of half-life. The half-life (t 1/2 ) is the time required for one-half of a specific amount of a radioactive nuclide to disintegrate.
41 Willard Libby and his apparatus for carbon-14 dating (1946).
42 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.
43 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.
44 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 CO 2 in the atmosphere. The oceans are enormous reservoirs of CO 2. When the oceans are cold, they release less CO 2 into the atmosphere than when they are warm.
45 The half-life of 131 I is 8 days. How much 131 I from a 32-g sample remains after five half-lives? Take a perpendicular line from any multiple of 8 days on the x-axis to the line on the graph. half-lives number of days amount remaining 0 32 g 1 8 16 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 131 I. 3 24 4 g 4 32 2 g 5 40 1 g
46 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.
47 No stable isotopes of element 43 (technetium) or of element 61 (promethium) are known. Only a few naturally occurring elements that have atomic numbers less than 81 are radioactive.
48 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.
49 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. 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).
50 Alpha Particles, Beta Particles and Gamma Rays
51 Marie Curie, in a classic experiment, proved that alpha and beta particles are oppositely charged. radiation passes between the poles of an electromagnet a radioactive source was placed inside a lead block Alpha rays are less strongly deflected to the negative pole. Gamma rays are not deflected by the magnet. Beta rays are strongly deflected to the positive pole. three types of radiation are detected by a photographic plate
53 It consists of two protons and two neutrons. It has a mass of 4 amu. It has a charge of +2. The symbols of an alpha particle are An alpha particle is a helium nucleus.
54 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)
55 Formation of thorium from the radioactive decay of uranium can be written as or mass number decreases by 4 atomic number decreases by 2
56 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
62 It is emitted by radioactive nuclei. It has no measurable mass. It has no electrical charge. The symbol of a gamma ray is A gamma ray is a high energy photon.
63 Loss of a gamma ray from the nucleus results in –no change in the mass number –no change in atomic number
64 Write an equation for the loss of an alpha particle from the nuclide 194 Pt.
65 78 protons Pt 78 194 78 protons + 116 neutrons A nuclide of platinum Atomic number (number of protons in the nucleus) Mass number (sum of protons and neutrons in the nucleus) Write an equation for the loss of an alpha particle from the nuclide 194 Pt.
66 Write an equation for the loss of an alpha particle from the nuclide 194 Pt. Loss of an alpha particle, 4 He, results in a decrease of 4 in the mass number and a decrease of 2 in the atomic number. Mass of new nuclide:A-4 194 – 4 = 190 or A = mass number Atomic number of new nuclide:Z-278 – 2 = 76or Z = atomic number Element number 76 is Os, osmium. The equation is
67 What nuclide is formed when 194 Ra loses a beta particle from its nucleus?
68 88 protons Ra 88 228 88 protons + 140 neutrons A nuclide of radium Atomic number (number of protons in the nucleus) Mass number (sum of protons and neutrons in the nucleus) What nuclide is formed when 194 Ra loses a beta particle from its nucleus.
69 The loss of a beta particle from a 194 Ra nucleus means a gain of 1 in the atomic number with no essential change in mass. Mass of new nuclide:A-0 228 – 0 = 228 or A = mass number Atomic number of new nuclide: Z-(-1) Z = atomic number 88 + 1 = 89or What nuclide is formed when 194 Ra loses a beta particle from its nucleus. The equation is
73 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 aluminum – stops and particles. Thin sheet of paper – stops particles. 5-cm lead block – will reduce, but not completely stop radiation
75 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.
76 –The uranium series starts with 238 U and ends with 206 Pb. 92 82 –The thorium series starts with 232 Th and ends with 208 Pb. 90 82 –The actinium series starts with 235 U and ends with 207 Pb. 92 82
77 A fourth series begins with the synthetic element plutonium. –The neptunium series begins with 241 Pu and ends with 238 Bi. 94 83
78 The uranium disintegration series. 238 U decays by a series of alpha ( ) and beta ( ) emissions to the stable nuclide 208 Pb.
82 Alchemists for hundreds of years attempted to transmute mercury and lead into gold by artificial means. They were never successful. The first artificial transmutation occurred in 1919 when Rutherford succeeded in producing oxygen from nitrogen.
83 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. Unfortunately one needs to start with Platinum!
86 Irene and Frederick Curie observed that the bombardment of aluminum-27 with alpha particles results in the emission of neutrons and positrons.
87 When alpha particle bombardment is halted neutron emission stops, but positron emission continues. This suggested that neutron emission and positron emission were a result of separate reactions.
88 Further investigation on their part showed that when aluminum-27 is bombarded with alpha particles, phosphorous-30 is produced.
89 Phosphorous-30 is radioactive, has a 2.5 minute half-life, and decays to silicon- 30 with the emission of a positron.
90 The radioactivity of nuclides produced in this manner is known as artificial or induced radioactivity. The Joliot-Curies received the Nobel Prize in chemistry in 1935 for the discovery of artificial, or induced, radioactivity.
95 The curie is the unit indicating the rate of decay of a radioactive substance. One curie (Ci) = 3.7 x 10 10 disintegrations per second. This very high radiation level is the amount of radiation emitted by 1 gram of radium in one 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).
98 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.
99 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.
101 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, 1942 - on a squash court situated beneath Chicago's stadium.
102 In nuclear fission, a heavy nuclide splits into two or more intermediate sized fragments when struck in a particular way by a neutron As the atom splits, it releases energy and two or three other neutrons, each of which can cause another nuclear fission.
103 Characteristics of Nuclear Fission 1.Upon absorption of a neutron, a heavy nuclide spits into one or more smaller nuclides (fission products). 2.The mass of the nuclides ranges from abut 70-160 amu. 3.Two or more neutrons are produced from the fission of each atom.
104 Characteristics of Nuclear Fission 4.Large quantities of energy are produced as a result of the conversion of a small amount of mass into energy. 5.Many nuclides produced are radioactive and continue to decay until they reach a stable nucleus.
105 Each time fission occurs three neutrons and two nuclei are produced. Fission of 235 U
106 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.
107 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.
110 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).
111 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.
112 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.
113 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.
114 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.
120 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.
127 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 UO 2 pellet has the energy equivalent to burning 136 gal of oil, 2.5 tons of wood, or 1 ton of coal. Uranium oxide pellet used in nuclear fuel rods.
130 In the United States breeder reactors are used to generate nuclear power. Breeder reactors use U 3 O 8 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.
131 Yucca Mountain in Nevada – site for nuclear depository?
132 1.Canisters of waste, sealed in special casks, are shipped to the site by truck or train. 2.Shipping casks are removed, and the inner tube with the waste is placed in a steel, multilayered storage container. 3.An automated system sends storage containers underground to the tunnels. 4.Containers are stored along the tunnels, on their side. Conceptual Design of Yucca Mountain Disposal Plan More current information: http://newterra.chemeketa.edu/faculty/lemme/CH%20122/YuccaWasteSite.htm
134 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
135 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 http://www.lasvegassun.com/news/2013/jan/21/putting-end-yucca-mountain-project-within-reach-st/
137 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.
138 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. http://newterra.chemeketa.edu/faculty/lemme/CH%20122/handouts/chernobylincident.htm
141 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.
142 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:
143 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 quant- ities of new radium-like elements would be generated. Now it appears almost certain that this could be achieved in the immediate future.
144 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.
145 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.
146 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.
147 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)
149 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.
153 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.
154 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.
156 Nuclear fusion is the process of uniting two light elements to form one heavier element.
157 The masses of the two nuclei that fuse into a single nucleus are greater than the mass of the nucleus formed by their fusion. tritiumdeuterium3.0150 amu 1.0079 amu 4.0026 amu 4.0229 amu 4.0229 amu – 4.0026 amu = 0.0203 amu The difference in mass is released as energy.
158 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.
159 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.
160 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.
161 (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.
163 Food Irradiation Food irradiation with gamma rays from 60 Co or 137 Cs 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.
164 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.
165 In the US irradiation is only approved for a small number of foods: potatoes, strawberries, grapefruit, fish and shrimp for export.
167 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.
168 IsotopeHalf-LifeUse 14 C5730 yrCO 2 for photosynthesis research 3H3H12.33 yrTag Hydrocarbons 35 S87.2 dTag pesticides, measure air flow 32 P14.3 dMeasure phosphorus uptake by plants Radioisotopes Used as Tracers
169 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.
170 Diagnostic Radioisotopes RadioisotopeHalf-Life (Hours) Site for Diagnosis 99 Tc6.0Thyroid 201 Tl72.9Heart 123 I13.2Thyroid 67 Ga78.2Tumors & Abscesses
171 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.
172 Positron Emission Tomography (PET) Radioisotopes such as 11 C, 18 F, 13 N, or 15 O 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.
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.