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What is one example of indirect evidence that scientists use to study an atom? What is the simplest way of explaining what atoms, elements, compounds and mixtures are? I am not clear on what a molecule is. If water is a molecule, is it also a compound because the hydrogen and oxygen have been chemically... What is the biggest atom? The smallest atom? How do atoms form? Does gravity affect atoms? Can you crush atoms? What are the components of an atom? How much does each atom weigh? Is there an atom that does not have neutrons? What is the modern view of the structure of the atom? How are the protons different from neutrons? What are the differences between protons... What kind of charge does a proton have? And what kind of charge does a neutron have? Why do protons and neutrons stay together in the nucleus? How many times bigger is a proton than an electron? Why are electrons so far away from the nucleus of an atom? How are electrons placed in shells around the nucleus? What holds an electron revolving around the nucleus? Why don't they just go zooming around everywhere? What keeps the electrons revolving around the nucleus of an atom? What is all of the rest of the space left in the atom not counting neutrons, protons and electrons? If atoms are 99.999999999999% empty space then why don't things pass right through them? If all physical things are made of atoms, then why do different physical things have different physical properties? Like why is wood hard... How fast do electrons move? What do we use to measure mass? Is matter everywhere? If everything around us is matter, what about germs? How do you prove air is matter? Why is matter not being created at the present time, nor being destroyed? Why don't atoms weigh anything? What are the exact relative masses of protons, neutrons and electrons? If different types of quarks have different masses, then why are protons and neutrons said to have the same mass, when they have different... What is plasma? How do you know plasma is real if you can't see it? What is the smallest particle of matter known? Could there be anything smaller inside of an electron or inside of a neutron or a proton? How many quarks are in a proton? How many quarks are in a neutron? What kinds of quarks are protons and neutrons made of? What was the old name for the Top and Bottom quark? What is the charge of an up quark and the charge of a down quark? How many quarks make up a proton and a neutron? Are quarks confineable for study? Is vacuum matter? What are ten things that are not matter? What is a vacuum? Is it matter? Elements and the Periodic Table Why is hydrogen's atomic number 1? Will scientists ever find smaller elements or is hydrogen the smallest possible? When studying elements I noticed that there is a boiling point and melting point. What are they? What are the freezing and melting points of krypton? Is krypton dangerous? Why aren't Chlorine-35 and Chlorine-37 two different elements? What makes Hassium an element if it has no uses except basic scientific research? If you have 11 protons and 12 neutrons this is an atom of what element? What's the difference between Hydrogen, Nitrogen, Oxygen, Neon, Copper, Gold and Magnesium? Does sodium have any isotopes? Is it radioactive? Anything with an atomic number greater than 92 (Uranium), is called transuranic. These elements are manmade, but is there a difference in... What atoms make up sugar? After sugar is melted over heat, what is the black substance called? Is carbon found in all organic and inorganic matter? In the chemical equation for methane gas why is there more hydrogen than carbon? How do you separate tungsten from its ore? What would you get if you combined one atom each from all the elements in the periodic table? Electricity and Magnetism What type of charge is produced when an electric field separates positive and negative charges? What is an electric current? How do you explain electrical resistance? Do you know what an electromagnet is? I am creating an electromagnet for my school's science fair project. Does the shape of the iron core make a difference? Is solid copper wire... Why is it (in detail) that the more coils you add to an electromagnet, the stronger its magnetic field is? Which jobs use electromagnets? Why is a non-permanent, but long lasting, magnet called a permanent magnet? Permanent magnets can lose their magnetism if they are dropped or banged on enough to bump their domains out of alignment. Can you turn... What happens when a magnet is cut in two? Does the strength of an electromagnet depend on the number of turns of wire? If you have two... Does the size of the magnet make it stronger? How are magnets made? How does the shape of a magnet effect it's magnetic field? Why does rubbing plastic and wool together create electricity? History of Science Where does the word atom come from and who first used this word? Are Democritus' theories of atoms still relevant today? How do people really know that atoms exist even though they can't see them? If there is no way in the world to see a atom then how do we know that the atom is made of protons, electrons, neutrons, the nucleus and the... When was the nucleus discovered? How do the elements on the element table get their chemical symbols? Why do the electron shells begin being named with K, L, M, N, and not with A, B, C? Was "beauty" a quark name they used in the past? Who invented magnets? What did Thomas Jefferson do as a scientist? Can you explain why the United States uses Fahrenheit instead of Celsius? Scientific Math If the energy could be directly converted to matter, how long would it take a person on a bicycle-type generator to create a single atom? Is there a way to tell how many atoms are in a certain object? How many atoms are in the human head? How many atoms are in the human body? How many atoms are there in the world? How many atoms would it take to create a ton? Could you please explain density? What is it? What is the concept of D = M over V? How do you determine how many protons, neutrons, and electrons are in an atom? What would happen to the atomic number and the electric charge of an atom if two neutrons escaped the nucleus? What are vector quantities and how do they work? Radiation and Radioactivity How is radioactivity measured - in quantity? How does the radioactivity of an atom affect the body? If radioactivity is so dangerous why do we use it to destroy cancers, and could the treatment cause more damage? What are alpha rays? How are they produced? What is an alpha particle? Is it possible for an element to emit more than one kind of radiation? How many neutrons can you add to an atom without it getting unbalanced? Are nitrogen, arsenic, and tantalum radioactive? How long is the life span of an atom? Someone told me that Cherenkov radiation is analogous to breaking the sound barrier. In the latter, an object travels faster than the speed... Do radioactive things glow in the dark? Energy If energy is formed by a generator, how does the generator form the energy? Do rotating magnets create energy? Where can I find more information on this subject? What is the fastest type of energy? Lasers and Light How do lasers work? My science fair project is on how prisms separate light. What is light made of? Can you use light to push light? Is possible to make a real light saber like in the Star Wars movies? Scientific Instruments and Experimentation How does a scientist work? What does a physicist do? What is an accelerator operator? What process do you go through before you start the experiments? Do you use a hypothesis? What is an accelerator? Does an atom smasher really smash atoms? Where and how do you get your electrons for your accelerator? What kind of machine or process did they use to split the first atom all those... Why do you need high energy to look at or observe quarks? Since the accelerator at your facility is linear and can't always get as much... How did you make the detectors work in halls A, B and C? How can you tell if a new type of element has been created, especially if only a few atoms of it were created and if they only existed for a... What kind of equipment do you work on? How did you make so much neat stuff? Miscellaneous Questions What is air made of? Does baking soda lower water temperature? If so, why? Do liquids freeze at the same temperature? Why doesn't oil freeze? In space we feel weightlessness because the earth's gravity has less effect on us. Why do we not see the effect of the gravitational force... What is a lattice vibration? How cold is liquid nitrogen? Is there anything colder than liquid nitrogen? If you jumped into a pool of liquid oxygen, would your body instantly crystallize? What's the melting point of steel? What is a material with a freezing point above 0 degrees Celsius? What is a meniscus? How can I explain the Quantum/Wave theory to my class? What two minerals is the sun made out of? What is the biggest atom? The smallest atom? If by "biggest" and "smallest", you mean mass (which is a measure of how much matter is there), then the smallest is the hydrogen atom with one proton and one electron. Since electrons are about 2000 times less massive than protons (and neutrons), then the mass of an atom is mostly from the protons and neutrons. So the hydrogen atom "weighs" in as ONE. As you add more protons and neutrons, the mass increases. However, for very massive atoms, the force holding them together becomes unstable and they tend to break apart (a phenomenon known as radioactive decay). Very massive atoms such as nobelium and lawrencium have lifetimes of only a few seconds. On the other hand, if you are speaking of size, then atoms are all about the same size whether it's a hydrogen atom (the simplest and least massive with one proton and one electron) or a lead atom (with 82 protons, 82 electrons and 125 neutrons). Atoms are composed of a nucleus (where the positively charged protons and uncharged neutrons reside) surrounded by a cloud of orbiting negatively charged electrons. An atom is about 10-8 centimeters in size (meaning that 100 million of them would fit side-by-side within one centimeter). The tightly packed nucleus is 100,000 times smaller than the electron cloud. You might think that as you add more protons (and thus more positive charge), the electrons would be attracted more strongly to the inner nucleus and hence the atom would shrink. In reality the electrons tend to screen each other somewhat from the inner positive charge and so the size stays about the same. Is there an atom that does not have neutrons? There is only one stable atom that does not have neutrons. It is an isotope of the element hydrogen called protium. Protium, which contains a single proton and a single electron, is the simplest atom. All other stable atoms contain some number of neutrons. How do you determine how many protons, neutrons, and electrons are in an atom? What is an atom's atomic number? The number of protons in the nucleus of an atom determines an element's atomic number. In other words, each element has a unique number that identifies how many protons are in one atom of that element. For example, all hydrogen atoms, and only hydrogen atoms, contain one proton and have an atomic number of 1. All carbon atoms, and only carbon atoms, contain six protons and have an atomic number of 6. Oxygen atoms contain 8 protons and have an atomic number of 8. The atomic number of an element never changes, meaning that the number of protons in the nucleus of every atom in an element is always the same. What is an atom's mass number? All atoms have a mass number which is derived as follows. Review: 1. An element's or isotope's atomic number tells how many protons are in its atoms. 2. An element's or isotope's mass number tells how many protons and neutrons in its atoms. How do atomic particles interact? There are forces within the atom that account for the behavior of the protons, neutrons, and electrons. Without these forces, an atom could not stay together. Recall that protons have a positive charge, electrons a negative charge, and neutrons are neutral. According to the laws of physics, like charges repel each other and unlike charges attract each other. So what makes the protons stay together in an atom? A force called the strong force opposes and overcomes the force of repulsion between the protons and holds the nucleus together. The energy associated with the strong force is called the binding energy. The electrons are kept in orbit around the nucleus because there is an electromagnetic field of attraction between the positive charge of the protons and the negative charge of the electrons. Does the nucleus of an atom ever lose particles? In some atoms, the binding energy is great enough to hold the nucleus together. The nucleus of this kind of atom is said to be stable. In some atoms the binding energy is not strong enough to hold the nucleus together, and the nuclei of these atoms are said to be unstable. Unstable atoms will lose neutrons and protons as they attempt to become stable. Review: 1. Electromagnetic fields cause like charges to repel each other and unlike charges to attract each other. 2. The protons stay together in the nucleus because the strong force opposes and overcomes the forces of repulsion from the electromagnetic field. 3. Binding energy is the energy that is associated with the strong force, and this energy holds the nucleus together. 4. A stable atom is an atom that has enough binding energy to hold the nucleus together permanently. 5. An unstable atom does not have enough binding energy to hold the nucleus together permanently and is called a radioactive atom. What is radioactivity? Atoms with unstable nuclei are constantly changing as a result of the imbalance of energy within the nucleus. When the nucleus loses a neutron, it gives off energy and is said to be radioactive. Radioactivity is the release of energy and matter that results from changes in the nucleus of an atom. What is a radioisotope? On an earlier page covering isotopes is was learned that isotopes are atoms of the same element that have a different number of neutrons. In other words, the atoms have the same number of protons but a different number of neutrons in the nucleus. Because the like charges of the protons repel each other,there are always forces trying to push the atom nucleus apart. The nucleus is held together by something called the binding energy. In most cases, elements like to have an equal number of protons and neutrons because this makes them the most stable. Stable atoms have a binding energy that is strong enough to hold the protons and neutrons together. Even if an atom has an additional neutron or two it may remain stable. However, an additional neutron or two may upset the binding energy and cause the atom to become unstable. In an unstable atom, the nucleus changes by giving off a neutron to get back to a balanced state. As the unstable nucleus changes, it gives off radiation and is said to be radioactive. Radioactive isotopes are often called radioisotopes. All elements with atomic numbers greater than 83 are radioisotopes meaning that these elements have unstable nuclei and are radioactive. Elements with atomic numbers of 83 and less, have isotopes (stable nucleus) and most have at least one radioisotope (unstable nucleus). As a radioisotope tries to stabilize, it may transform into a new element in a process called transmutation. We will talk about transmutation in more detail a little later. Review: 1. Radioactivity is the release of energy and matter due to a change in the nucleus of an atom. 2. Radioisotopes are isotopes that are unstable and release radiation. All isotopes are not radioisotopes. 3. Transmutation occurs when a radioactive element attempts to become stabilized and transforms into a new element. What is radioactive decay? Radioactive decay is the spontaneous breakdown of an atomic nucleus resulting in the release of energy and matter from the nucleus. Remember that a radioisotope has unstable nuclei that does not have enough binding energy to hold the nucleus together. Radioisotopes would like to be stable isotopes so they are constantly changing to try and stabilize. In the process, they will release energy and matter from their nucleus and often transform into a new element. This process, called transmutation, is the change of one element into another as a result of changes within the nucleus. The radioactive decay and transmutation process will continue until a new element is formed that has a stable nucleus and is not radioactive. Transmutation can occur naturally or by artificial means. * Take this link to learn about the two forms of nuclear radiation: o Two Principle Forms of Nuclear Radiation Review: 1. As an unstable atom tries to reach a stable form, energy and matter are released from the nucleus. This spontaneous change in the nucleus is called radioactive decay. 2. When there is a change in the nucleus and one element changes into another, it is called transmutation. What is the difference between chemical reactions and nuclear reactions? Nuclear reactions can be described mathematically in much the same way as chemical reactions. We commonly express these reactions by equations, although there is a unique difference in the nature of the reactions. The principle difference between them lies in how the reaction occurs, specifically how the atom is affected. Chemical reactions involve an atom’s electrons while nuclear reactions involve the atom’s nucleus. Writing a nuclear reaction equation In order to write an equation for a nuclear reaction, we must first establish some basic rules. Each of the elements involved in the reaction is identified by the chemical symbol. Two numbers are attached to the symbol. The number at the upper right is the mass number, also known as the ‘A’ number. The 'A' number describes the atomic weight of the atom and identifies the number of protons and neutrons in the nucleus. The number at the lower left is the atomic number, or ‘Z’ number. The 'Z' number describes the number of protons in the nucleus and determines the type of atom. The symbol for Uranium-238 = 92U238 This shows you that Uranium has a mass number of 238 and an atomic number of 92. Symbols are also utilized to represent alpha and beta particles. * The symbol for an alpha particle = 2He4 The symbol for a beta particle is –1e0. The chemical symbol for a neutron = 0n1 Can you determine the mass number and atomic number of the neutron? Now that we know what these symbols represent, let's see how they can be applied to a nuclear equation. Uranium-238 is an isotope, which undergoes alpha decay to produce Thorium and gamma rays. This is expressed mathematically by the following equation: Note that when the mass numbers on each side of the equation are added together that they are equal. The same principle is true for the atomic numbers, and it shows that none of the atomic particles have been lost. One way to check to see if you have written the proper nuclear equation is to make sure both sides of the equation have the same number or atomic particles represented. Review: 1. A nuclear reaction can be described by an equation, which must be balanced. 2. The symbol for an atom or atomic particle includes the symbol of the element, the mass number, and the atomic number. 3. The mass number, which describes the number of protons and neutrons, is attached at the upper right of the symbol. 4. The atomic number, which describes the number of protons in the nucleus, is attached at the lower left of the symbol. Not all of the atoms of a radioisotope decay at the same time, but they decay at a rate that is characteristic to the isotope. The rate of decay is a fixed rate called a half-life. The half-life of a radioisotope describes how long it takes for half of the atoms in a given mass to decay. Some isotopes decay very rapidly and, therefore, have a high specific activity. Others decay at a much slower rate. How do you measure the decay of radioactive isotopes? Now that we have an idea of how radioactive isotopes decay, let's look at how this is measured and apply the terms we just learned. The basic unit of measure for describing the activity (radioactivity) of a quantity of radioactive material is the curie, named after Marie Curie. A quantity of radioactive material is considered to have an activity of 1 curie or 1 C, when 37 billion of its atoms decay (disintegrate) in one second. In scientific terms, this is expressed by the equation: 1C = 3.7 X 1010 disintegrations/sec. Remember that we said each isotope has its own decay pattern. If the rate of decay is greater than 37 billion atoms in one second, then the source would have an activity greater than one curie, and if that source had fewer than 37 billion atoms decaying in one second, its activity would be less than one curie. * Take this link to learn how to determine radioactive sources in curies: o Comparing Radioactive Activity * Take this link to learn how to assess how much radiation is emitted from a source: o Assessing the Amount of Radiation Coming From a Source Now that you know that the activity of a radioactive source is the measure of the number of atoms that decay each second and that the activity varies as a function of the size of the source, let's see why half-life is important. Review: 1. The term half-life describes how long it will take for half of the atoms to is the fixed rate decay of an isotope. 2. The curie the unit of measure used to describe the radioactivity of radioactive material. (1C = 3.7 X 1010 disintegrations/sec) 3. The disintegration of the atoms from different isotopes can produce different amounts of radiation. As we have mentioned before each radioactive isotope has its own decay pattern. Not only does it decay by giving off energy and matter, but it also decays at a rate that is characteristic to itself. The rate at which a radioactive isotope decays is measured in half-life. The term half-life is defined as the time it takes for one-half of the atoms of a radioactive material to disintegrate. Half-lives for various radioisotopes can range from a few microseconds to billions of years. See the table below for a list of radioisotopes and each of unique their half-lives. Radioisotope Half-life Polonium-215 0.0018 seconds Bismuth-212 60.5 seconds Sodium-24 15 hours Iodine-131 8.07 days Cobalt-60 5.26 years Radium-226 1600 years Uranium-238 4.5 billion years How does the half-life affect an isotope? Let's look closely at how the half-life affects an isotope. Suppose you have 10 grams of Barium-139. It has a half-life of 86 minutes. After 86 minutes, half of the atoms in the sample would have decayed into another element, Lanthanum-139. Therefore, after one half-life, you would have 5 grams of Barium-139, and 5 grams of Lanthanum-139. After another 86 minutes, half of the 5 grams of Barium-139 would decay into Lanthanum-139; you would now have 2.5 grams of Barium-139 and 7.5 grams of Lanthanum-139. How is half-life information used in carbon dating? The half-lives of certain types of radioisotopes are very useful to know. They allow us to determine the ages of very old artifacts. Scientists can use the half-life of Carbon-14 to determine the approximate age of organic objects less than 40,000 years old. By determining how much of the carbon-14 has transmutated, scientist can calculate and estimate the age of a substance. This technique is known as Carbon dating. Isotopes with longer half-lives such as Uranium-238 can be used to date even older objects. You will learn more about carbon dating in the next sub-unit. Uses of the half-life in NDT In the field of nondestructive testing radiographers (people who produce radiographs to inspect objects) also use half-life information. A radiographer who works with radioisotopes needs to know the specific half-life to properly determine how much radiation the source in the camera is producing so that the film can be exposed properly. After one half-life of a given radioisotope, only one half as much of the original number of atoms remains active. Another way to look at this is that if the radiation intensity is cut in half; the source will have only half as many curies as it originally had. It is important to recognize that the intensity or amount of radiation is decreasing due to age but not the penetrating energy of the radiation. The energy of the radiation for a given isotope is considered to be constant for the life of the isotope. Review: 1. The half-life of radioisotopes varies from seconds to billions of years. 2. Carbon-dating uses the half-life of Carbon-14 to find the approximate age of an object that is 40,000 years old or younger. 3. Radiographers use half-life information to make adjustments in the film exposure time due to the changes in radiation intensity that occurs as radioisotopes degrade. As you learned in the previous page, carbon dating uses the half-life of Carbon-14 to find the approximate age of certain objects that are 40,000 years old or younger. In the following section we are going to go more in-depth about carbon dating in order to help you get a better understanding of how it works. What exactly is radiocarbon dating? Radiocarbon dating is a method of estimating the age of organic material. It was developed right after World War II by Willard F. Libby and coworkers, and it has provided a way to determine the ages of different materials in archeology, geology, geophysics, and other branches of science. Some examples of the types of material that radiocarbon can determine the ages of are wood, charcoal, marine and freshwater shell, bone and antler, and peat and organic-bearing sediments. Age determinations can also be obtained from carbonate deposits such as calcite, dissolved carbon dioxide, and carbonates in ocean, lake, and groundwater sources. How is carbon-14 produced? Cosmic rays enter the earth's atmosphere in large numbers every day and when one collides with an atom in the atmosphere, it can create a secondary cosmic ray in the form of an energetic neutron. When these energetic neutrons collide with a nitrogen-14 (seven protons, seven neutrons) atom it turns into a carbon-14 atom (six protons, eight neutrons) and a hydrogen atom (one proton, zero neutrons). Since Nitrogen gas makes up about 78 percent of the Earth's air, by volume, a considerable amount of Carbon-14 is produced. The carbon-14 atoms combine with oxygen to form carbon dioxide, which plants absorb naturally and incorporate into plant fibers by photosynthesis. Animals and people take in carbon-14 by eating the plants. The ratio of normal carbon (carbon-12) to carbon-14 in the air and in all living things at any given time is nearly constant. Maybe one in a trillion carbon atoms are carbon-14. Both Carbon-12 and Carbon-13 are stable, but Carbon-14 decays by very weak beta decay to nitrogen-14 with a half-life of approximately 5,730 years. After the organism dies it stops taking in new carbon. How do scientist use Carbon-14 to determine the age of an artifact? To measure the amount of radiocarbon left in a artifact, scientists burn a small piece to convert it into carbon dioxide gas. Radiation counters are used to detect the electrons given off by decaying Carbon-14 as it turns into nitrogen. In order to date the artifact, the amount of Carbon-14 is compared to the amount of Carbon-12 (the stable form of carbon) to determine how much radiocarbon has decayed. The ratio of carbon-12 to carbon-14 is the same in all living things. However, at the moment of death, the amount of carbon-14 begins to decrease because it is unstable, while the amount of carbon-12 remains constant in the sample. Half of the carbon-14 degrades every 5,730 years as indicated by its half-life. By measuring the ratio of carbon-12 to carbon-14 in the sample and comparing it to the ratio in a living organism, it is possible to determine the age of the artifact. Review: 1. Carbon-14 dating can determine the age of an artifact that is up to 40,000 years old. 2. Living organisms absorb carbon my eating and breathing. 3. After burning a small piece of an artifact, scientists compare the amount of Carbon-14 to the amount of Carbon-12 to determine the age of the object. So far our discussion has been primarily centered around radioactive elements, the structure of the atom, and the phenomenon of radioactivity. As mentioned earlier, another type of radiation commonly utilized is X-radiation. Where as gamma radiation is one of the products of nuclear decay of radioactive elements, X-rays are produced in high voltage electron tubes. You will recall from the history section that W.C. Roentgen discovered X-ray in the late 1800's while working with a cathode tube in his lab. X-rays can be produced in parcels of energy called photons, just like light. How do you generate an x-ray? To generate x-rays, we must have three things. We need to have a source of electrons, a means of accelerating the electrons at high speeds, and a target material to receive the impact of the electrons and interact with them. Why do we need electrons to produce x-rays? X-rays are generated when free electrons give up some of their energy when they interact with the orbital electrons or nucleus of an atom. The energy given up by the electron during this interaction appears as electromagnetic energy known as X-radiation. There are two different atomic processes that can produce x-ray photons. One is called Bremsstrahlung and the other is called K-shell emission. X-rays produced by Bremsstrahlung are the most useful for medical and industrial applications. * Take this link to learn about a phenomenon in the generation of x-rays called Bremsstrahlung: o What is Bremsstrahlung? Review: 1. The three things needed to create x-rays are a source of electrons, a means of accelerating the electrons to high speeds, and a target for the accelerated electron to interact with. 2. X-rays are produced when the free electrons cause energy to be released as they interact with the atomic particles in the target. So far we have learned about the atom, the phenomenon of radioactivity, and we have looked at both nuclear reactions and X-ray generation. We already know that X- and gamma rays differ only in their source of origin or how they are generated. There are, however, very distinct characteristics associated with these energy forms. Two key points to remember about the characteristics of radiation are that X- and gamma rays are not bits of matter, they are electromagnetic wave forms possessing no charge and no mass, and they can be characterized by frequency, wavelength, and velocity. Let's take a closer look at the characteristics of these wave forms so that we may better understand the nature of them. What are Electromagnetic waves? X- and gamma rays are part of what scientists refer to as the electromagnetic spectrum. They are waveforms that are part of a family in which some of the relatives are very familiar to us, such as light rays, infrared heat rays, and radio waves. However, X- and gamma rays cannot been seen, felt, or heard. In other words, our normal senses cannot detect them. Since X- and gamma rays have no mass and no electrical charge, they are not influenced by electrical and magnetic fields and will travel in straight lines. Continued research over the years since Roentgen’s discovery indicated that the radiation possesses a dual character. Acting somewhat like a particle at times and like a wave at other times. The name that has been given to the small "packets" of energy with these characteristics is "photon." It is said that the radiation photon is a wave that is both electric and magnetic in nature. Electromagnetic radiation has also been described in terms of a stream of photons (massless particles) each traveling in a wave-like pattern and moving at the speed of light. This diagram shows the electromagnetic spectrum. Notice the changes in wavelengths of the various wave forms. Every point across the spectrum represents a wave form of differing wavelength. It should be noted that the lines between the groupings are not precise, and that each group phases into the next. Wave forms may be graphically represented as following: Take the following links to learn more! * Take this link to learn about the definition of a wavelength: o Wavelength Defined * Take this link to learn about the frequency of a wave: o Frequency of the Wavelength Defined * Take this link to apply what you have learned about the characteristics of electromagnetic waves: o Check of Understanding * Take this link to learn how to measure radiation: o Measuring Radiation Review: 1. Radiation is an electromagnetic wave that has no charge and no mass. 2. X-rays and gamma-rays can be characterized by frequency, wavelength, and energy. In this section we will discuss how radiation interacts with matter, and conversely how matter affects radiation. We already know that radiation is capable of penetrating matter. We also know that X and gamma radiation varies in energy with respect to wavelength. As mentioned in the introduction, radiography relies on the principles of absorption and transmitted intensity to the film. So now let's consider another factor that determines how the radiation penetrates matter. Let's consider the material being radiographed. How does matter affect radiation? You may not realize it, but the air around you is made up of matter. X and gamma radiation will penetrate air to a considerable depth, but as with any material, air will eventually absorb the radiation. What if we were radiographing the hand of a human being? Would the radiation penetrate the tissue the same as it would with air? The answer is yes, but with less depth of penetration. This is because the human body is more dense than air. If we were radiographing a piece of metal, the depth of penetration would be even less than that of the human tissue for the same reasons. The principle concept here is that radiation will penetrate light materials better than it will heavy (dense) materials. Heavier, more dense materials offer greater resistance to radiation penetration because they absorb more of the energy. This seems logical if you consider the number of atoms that make up air versus the number of atoms that make up steel and that each atom has the potential to absorb some the energy of the radiation. Remember our discussion on the atomic structure. Atoms with more subatomic particles will be harder for the radiation to travel through without interaction with the particles. Think about when you go to the dentist, and you get the x-ray taken of your teeth. What happens to you before they take the picture? Normally, you have a lead apron draped over your chest, this is a protective measure to shield your internal origins from the radiation. Lead is often used as a radiation shielding material because it has a high number of subatomic particles and it is a relatively common element making it affordable to use. Look up lead on the Periodic Table of Elements and you will find that it has a high atomic number (Z number). When radiation penetrates a material what happens? Now we know that in addition to the energy of the radiation, the depth of penetration is also dependent on the density of the material being penetrated. But what happens to the radiation as it penetrates and interacts with the material? Remember, radiation is electromagnetic and composed of energy moving at the speed of light. When the radiation is stopped and absorbed we know that something else must happen. One of the laws of classical physics states that energy can neither be created nor destroyed, only converted from one form to another. Energy is converted in many different ways, but the energy is always there. Therefore, we know that when radiation is absorbed by a material, it must transfer its energy to the material. Review: 1. When radiation encounters a material, some of the energy will be absorbed through interactions subatomic particles. 2. More radiation will be absorbed by materials with high atomic numbers (generally more dense materials) because there are more subatomic particles to interact with the radiation. 3. Energy can never be created or destroyed; therefore, the energy does not disappear but is converted into something other form. What does the process of ionization entail? Radiation is absorbed by the material it penetrates by a process known as ionization. Radiation creates ions in the material that it passes through, and some or all of the radiation energy is lost during this process. An ion is an atom, group of atoms, or a particle with a positive or negative charge. Ionization is any process that changes the electrical balance within an atom. If we remove an electron from a stable atom, the atom becomes electrically incomplete. That is, there are more protons in the nucleus (positive charges) than there are electrons (negative charges). With an electron removed, the atom possesses a plus one charge, therefore it is a positive ion. Consequently, the liberated electron is a negative ion, as long as it exists by itself and does not combine with another atom. Remember from our discussion on atomic structure, that atoms are held together by a binding energy. This means that the electrons are held in their orbital shells by a quantity of binding energy. In order to liberate an electron from its parent atom, it will take energy that is at least equal to the binding energy that holds the electron. When X and gamma ray photons penetrate matter, they interact with the atomic particles in the material and are said to be absorbed by the material. This absorption can result due to ionization or bremsstruhlung processes. The Bremsstruhlung mechanism was covered in the material on X-ray generation and, therefore, will not be re-addressed here. The three mechanism of ionization The amount of ionization that occurs is dependent upon two principle factors, (1) the radiation energy, and (2) the type of material for which the radiation is interacting. For a given material, the level of ionization will vary with varying levels of radiation energy. There are three principle mechanisms of ionization that are of interest in radiography. These include the Photoelectric effect, Compton effect, and Pair Production. * Take the following links to learn more about the three principle mechanisms of ionization: o Photoelectric effect o Compton Effect o Pair-Production * Take this link to learn what happens to liberated electrons: o Subionization Review: 1. The three principle level of ionization are the Photoelectric effect, the Compton Effect, and Pair-Production. 2. This process of radiation absorption is called ionization. How deep will radiation penetrate into a material? Now that we have looked at the interaction that the radiation has with matter, let's consider the radiation ability to penetrate materials. We know that one of the factors affecting ionization is the material type. We also know that radiation has a more difficult time penetrating dense materials, such as metal than it does less dense materials, such as plastic. Radiation photons of the same energy will not penetrate a given material to the same depth. Some of the photons will collide with atoms and lose their energy before others. Some may pass completely through the material with minimal or no interaction. Also, the depth of penetration for a given photon energy is dependent upon material density (atomic structure). The more subatomic particles in a material (higher Z number), the greater the likelihood that interactions will occur and the radiation will lose its energy. Therefore, the more dense a material, the less the depth of radiation penetration will be. When does the absorption of radiation start? The absorption of radiation starts as soon as the radiation enters a material. The process is progressive and continues as the radiation penetrates deeper into the material. Additional energy is absorbed through the various processes of ionization. At some point in the material, there is a level at which the radiation intensity becomes one half that at the surface of the material. This depth is known as the Half Value Layer, (HVL) for that material. Each material has its own specific HVL thickness. Not only is the HVL material dependent, but it is also energy dependent. This means that for a given material, if the radiation energy changes, the point at which the intensity decreases to half its original value will also change. How does radiation energy affect the depth of penetration? If we raise the energy of the radiation interacting with the same material, the HVL will occur deeper in that material. X-rays and gamma rays with shorter wavelengths will have more energy that must be absorbed and, therefore, more energy will make it deeper into the material or through the material. Conversely, if we lower the radiation energy, the HVL will occur shallower in depth. The following table shows some examples of the relationships between radiation energy and material types: Energy (KeV) Lead (mm) Concrete (mm) 50 0.06 4.32 70 0.18 12.70 100 0.27 15.10 150 0.30 22.32 200 0.52 25.00 250 0.88 28.00 300 1.47 31.21 400 2.50 33.00 1000 7.90 44.45 2000 9.98 62.23 -- - - Review: 1. The more subatomic particles in a material the more quickly radiation energy will be absorbed resulting in less depth of penetration. 2. The half-value layer is the depth within a material where half of the radiation energy has been absorbed. The HVL is useful in making material comparisons. 3. Higher energy radiation will penetrate deeper into a material before it is absorbed. As you know, there are a number of sources of radiation, ranging from naturally occurring radioisotopes to X-ray machines, and other forms of particle accelerators. In this section we are going to take a look at the different sources of gamma radiation commonly used today. Radioisotopes Remember from our previous discussion, that radioisotopes are elements that are atomically unstable and radioactive. Radioisotopes stabilize by releasing energy and matter. Natural radioisotopes, which have relatively low radioactive energy, have been largely replaced by artificially produced radioisotopes. Artificially produced radioisotopes are widely utilized as sources of radiation for radiography, gauging, and as tracers for a multitude of measurements that are not easily made by other methods. How are radioisotopes produced? Present day production of radioisotopes includes three principle categories, which are (1) neutron activation (bombardment), (2) fission product separation, and (3) charged particle bombardment. Nuclear bombardment constitutes the major method for obtaining industrially important radioisotope materials. Radioisotopes may exist in any form of matter, with solid materials comprising the largest group. To learn more see below! * Take this link to learn about what the study of radioactive decay led scientists to believe: o The Study of Radioactive Decay * Take this link to learn about neutron activation: o Neutron Activation to Produce Radioisotopes * Take these links to learn about the uses of fission and fusion: o Uses of Fission o Uses of Fusion Review: 1. Artificially produced radioisotopes are primarily used by industry because they can be produced so as to have much more radioactive energy that natural types. 2. The three ways to produce radioisotopes are neutron activation, fission product separation, and charged particle bombardment. 3. Elements that are atomically unstable and radioactive are called radioisotopes. In this section the basic construction of X-ray equipment and some different types of x-ray systems will be introduced. Most standard x-ray systems have three main components which are a x-ray tube, a high voltage power supply, and a control unit. Working together, these components are common to all standard systems. From our introductory discussion on the generation of x-rays you may recall that there were three principle requirements to generate X-radiation. These three requirements include a source of electrons, a means of acceleration, and a target for interaction. You should recognize that electrical power is necessary for X-ray generation. Where do the electrons come from? You already know that matter is made up of atoms, and atoms have electrons that orbit around the nucleus in shells. All we need to do is get the electron free of their orbit. How do we do this? The answer is fairly simple. If we take a piece of conductive wire and pass a current through it, the wire will heat up due to the resistance in the wire. The heat of the wire excites the electrons and they will break away (boil off) from the wire to expend the energy picked up from the heat of the current. When the energy of the electron is expended, it will return to the wire to become heated again. So this heated wire serves as our source of electrons. Why do the electrons need to be accelerated and how is it done? Our second requirement is to get the electrons traveling at high speeds. The reason we need to propel the electrons at high speeds is because the energy that the electron possesses and can transfer is dependent on its velocity. The higher the velocity of the electron when it interacts with an atom, the greater the energy of the radiation that will be produced. Propelling the electron is fairly simple. Since unlike charges (positive and negative) attract, and electrons posses a negative charge, all we need is a positive charge nearby to attract the electron. We can accomplish this by placing a piece of metal (anode) a short distance away from the wire filament (cathode). When we apply a voltage to this anode, we place a high positive charge on it. This high positive charge acts much like a magnet, only it is attracting free electrons. The positive charge will possess a strong attractive force to the negative charge of the electrons that are boiling off of the filament. This attractive force pulls the electrons towards the anode at high speeds. By increasing the voltage applied to the anode we can increase the speed of the electrons. What does the target material do? The third and final requirement is to have a target material for the electrons to interact with. By placing some sort of matter between the electrons (filament) and the positive charge (anode) we meet our need. Also, the anode itself can be used as the target. In high voltage X-ray generators a special target material (Tungsten) is usually embedded into the anode. This gives the electrons a suitable material to interact with and produce x-rays. When the electron hits the target material, several things can happen. The electron can be absorbed by an atom and its energy transferred to the atom, the energy of the electron can cause another electron to be knocked out of its energy shell, or the electron may just slightly interact with other atomic particles. Radiation will be produced in all of these cases, but the energy of the radiation will be different. The following illustration is a basic schematic representing an x-ray tube: Modern x-ray tubes come in many shapes and sizes, normally they are of the glass or metal-ceramic tube (envelope) style. As compared to early gas filled x-ray tubes, modern tubes are of the high vacuum style. The modern techniques of tube design have allowed for smaller tubes, extended tube life, and more efficient and stable operation. The means of acceleration of the electrons is provided by applying a potential difference (voltage) across the tube anode and cathode and is independent of the voltage and current across the filament. The x-ray tube is technically referred to as an envelope. Typical construction may be from blown glass or metal-ceramic styles. Glass envelope tubes are still common today, although they have definite disadvantages to the newer metal-ceramic designs. Due to the tremendous amount of heat generated during x-ray production, glass suffers from thermal and mechanical shock. Metal-ceramic materials do not suffer damage from the excessive heat to the degree that glass does and are rapidly replacing the glass style tube. The Cathode From the above illustration let's look at each of the components separately beginning with the cathode. The cathode is the negative terminal of the tube assembly and includes the filament, which is a small-coiled wire that is commonly made from tungsten. The filament provides the electrons for acceleration to the target (anode). Tungsten is metal with the desired properties for filaments, you have probably seen a tungsten filament in a light bulb before. The filament is normally powered by an alternating current that is supplied to it by a separate transformer. In many of the x-ray tubes, the current supplied to the filament ranges from a few hundred micro-amperes (symbol 109 \f "Symbol" \s 12mA) to several milli-amperes (mA). Filament current may be varied or fixed to maintain a constant tube current. Remember from our earlier discussion that the filament supplies the electrons. Adjustments in current to the filament varies the number of electrons that will boil off the filament. This in turn controls the number of x-rays that the tube is generating. Filament current controls the x-ray intensity. The Anode The positive terminal of an x-ray tube is called the anode, it serves three important functions, (1) it provides a complete circuit for purposes of accelerating the electrons, (2) it houses the target material, and (3) it helps to cool the tube. We already mentioned before that the generation of X-rays generates a tremendous amount of heat. If the heat in a tube was ignored, the target material that is embedded in the anode would be destroyed in a short period of time. The anode is typically made from materials with good thermal properties to dissipate heat. Copper and tungsten are common anode materials. In addition to using thermally conductive materials for the anode, alternate means of cooling that may be employed are gas, oil, water, or air. Does the density of the target material matter? As previously mentioned, the anode also houses the target material. As an integral part of the tube, the target requires special consideration. The target provides the means for electron interaction (bombardment). The target is commonly made from tungsten and other materials like cobalt, iron, or copper. Another important characteristic of the target material is its density. The material must be of high atomic mass for electron interaction. Remember that when the electron interacts with the target atoms the result is the generation of x-rays. Low density materials do not provide sufficient density for interaction. The High Voltage Power Supply A high voltage power supply is an important component of an X-ray generation system. When we say high voltage supply, we need to differentiate from that of commercial electricity. Keep in mind that the filament uses a relatively small voltage supply to cause small currents (mV) in the filament, while the anode of the tube requires a large voltage supply to maintain a high positive charge for acceleration of the electrons. Commercial power is commonly available as 110 volts, 220, or 440 volts. X-ray systems require very high voltages commonly in the range from 5 kilovolts (Kv) to as much as 400 Kv or more. So how can we supply low voltage to the filament, and high voltage to the anode? This is accomplished by using a transformer. A transformer will allow us to supply the proper voltages to the filament and anode. The next question we need to answer would be what is a transformer and how does it work? What are transformers? Transformers are electromagnetic devices that allow a voltage of alternating current to be changed; the voltage may be increased or decreased. Two common types of transformers which are of importance to x-ray generation are step-up and step-down. Transformers are comprised of two sets of windings (coiled conductors) that are electrically isolated from each other. One set of windings is connected to a power supply and is known as the primaries. The other set of windings is connected to a load (in this case the x-ray tube) and is referred to as the secondary windings. The principle operation of a transformer is based on induction. If you have studied electricity, you should know that when you pass current through a conductor, a magnetic field is established in and around the conductor. This magnetic field can be used to induce a voltage and current flows in a conductive material that is placed close by. The Control Unit The third essential component to a standard x-ray system is the control unit. We have discussed the tube design and the power supply, now we need to know how to control the energy and intensity of the radiation being generated. There are three principle controls to a standard x-ray system, which are the ma control, the kV control, and a timer. The first two are the most important in terms of the radiation characteristics. We will briefly describe the timer control. The controls for the system are usually housed in a panel. Current Control The ma control on an x-ray system commonly includes some type of a panel meter or digital display (millimeter) which is a rheostat connected to the circuit that allows adjustment in tube current. Adjusting the current being applied to the filament results in variations in the radiation intensity. Remember that the filament provides the electrons for interaction with the target. When the tube current is varied, the number of electrons being supplied to the anode (target) varies. Voltage Control The kV control on a x-ray system is similar to the ma control in that it includes some type of metered display (kilovolt meter) and a rheostat in the circuit. The voltage being supplied to the anode is referred to as the tube voltage, and is primarily measured in kilovolts. Variations in the tube voltage affects the energy of the radiation; penetrating power varies with the voltage. Increasing the tube voltage increases the speed of the electrons interacting with the target. Remember from our previous discussions that the energy of radiation is a function of the wavelength. Increasing the energy results in a shorter wavelength x-ray photon with greater penetrating power. Time Control The third control feature of an x-ray system is the timer. The timer is no different then one you set when baking cookies. The timer is much like that of a stop watch. It may be an analog or digital display of some sort. The function of the timer is simply to control the duration of the exposure, in other words, how much time the tube is generating radiation. It is, however, connected to the circuits of the system. When the time has elapsed, the system shuts down and no more radiation will be produced until the system is reset. Review: 1. The three main parts to an x-ray generator setup are an x-ray tube, a high voltage power supply, and a control unit. 2. The X-ray generator provides three things that are required to produce X-rays, and they are a source of electrons, a means of acceleration, and a target for interaction. As we mentioned before, there are many different ways that radiation can be generated. We have looked closely at two of the most widely utilized sources, radioisotopes and x-ray generation. We will briefly introduce a few other means of generating radiation. These methods are often referred to as the high energy radiation systems. Some of our greatest scientific developments include high energy particle accelerators. Many of the natural and artificial radioisotopes along with standard X-ray systems are limited in terms of energy. Much of the scientific research dedicated to subatomic nature would not be possible without particle accelerators. * Take the following links to learn about particle accelerators: o The Electrostatic Generator o The Cyclotron o The Betatron o The Linear Accelerator Review: 1. Radioisotopes, x-ray generation, and particle accelerators are different methods that generate radiation. DETECTION AND MEASUREMENT OF RADIOACTIVITY After reading this section you will be able to do the following: * Describe how we detect radioactivity/radiation and name the instrument that is used. * List some safety precautions and explain their importance. Although some forms of electromagnetic energy, such as light and heat, can be detected by the human senses. One of the greatest draw backs to high energy radiation is the inability to detect it. We cannot see, feel, taste, smell, or hear the various forms of ionizing radiation. Fortunately, ionizing radiation interacts with matter which makes detection and measurement possible by utilizing specialized equipment. In this section we want to introduce you to the various ways and means of detecting and measuring ionizing radiation. As mentioned previously, Becquerel discovered radioactivity because it left marks on photographic film as a means of detecting radiation. However, there are more definitive means commonly used by scientists and technicians who study and work with radiation. The equipment utilized for the detection and measurement of radiation commonly employs some type of a substance or material that responds to radiation. Many common methods use either an ionization process or molecular excitation process as a basis. Remember that we stated earlier that radiation interacts with matter. For detection and measurement purposes the process of ionization is the most commonly employed technique, based on the principle of charged particles producing ion pairs by direct interaction. These charged particles may collide with electrons, which removes them from their parent atoms, or transfer energy to an electron by interaction of electric fields. How do you choose a detection device? Important considerations for choosing a particular type of detection device include the application, the type of radiation, the energy of the radiation, and the level of sensitivity needed. Remember from previous discussion that radiation exists as waveforms with varying energies and may be either particulate or electromagnetic in nature. * Take the following links to learn about different detection devices: o The Electroscope o The Cloud Chamber o Other Detection Devices Safety Precautions Some of the principle safety precautions commonly used in working with radioactivity/radiation are time, distance, and shielding. Recall our earlier discussion of the dentist wanting to photograph your teeth. Have you ever wondered why the dentist lays a heavy apron across your chest? The dentist is practicing a means of protection from exposure. In that, they are using distance and shielding from the source of radiation. The concepts of these three principles are fairly simple. The first principle is time. The less time you spend around a radioactive material the less exposure you will receive. The second principle states that the greater the distance away from a radioactive source the lesser your exposure to the radiation. Lastly, if you can protect yourself with some type of material to act as a shielding device you will also reduce your overall exposure. Review: 1. Devices that measure ionization are the most commonly used instruments for detecting radiation. 2. Three important words to help you minimize your exposure to radiation are time, distance, and shielding. USES OF RADIOACTIVITY/RADIATION After reading this section you will be able to do the following: * List and describe uses of radioactivity/radiation. There are many practical applications to the use of radioactivity/radiation. Radioactive sources are used to study living organisms, to diagnose and treat diseases, to sterilize medical instruments and food, to produce energy for heat and electric power, and to monitor various steps in all types of industrial processes. Tracers Tracers are a common application of radioisotopes. A tracer is a radioactive element whose pathway through which a chemical reaction can be followed. Tracers are commonly used in the medical field and in the study of plants and animals. Radioactive Iodine-131 can be used to study the function of the thyroid gland assisting in detecting disease. Nuclear reactors Nuclear reactors are devices that control fission reactions producing new substances from the fission product and energy. Recall our discussion earlier about the fission process in the making of a radioisotope. Nuclear power stations use uranium in fission reactions as a fuel to produce energy. Steam is generated by the heat released during the fission process. It is this steam that turns a turbine to produce electric energy. Other uses of radioactivity Sterilization of medical instruments and food is another common application of radiation. By subjecting the instruments and food to concentrated beams of radiation, we can kill microorganisms that cause contamination and disease. Because this is done with high energy radiation sources using electromagnetic energy, there is no fear of residual radiation. Also, the instruments and food may be handled without fear of radiation poisoning. Radiation sources are extremely important to the manufacturing industries throughout the world. They are commonly employed by nondestructive testing personnel to monitor materials and processes in the making of the products we see and use every day. Trained technicians use radiography to image materials and products much like a dentist uses radiation to x-ray your teeth for cavities. There are many industrial applications that rely on radioactivity to assist in determining if the material or product is internally sound and fit for its application. Review: 1. Radioactivity tracers are commonly used in the medical field and also in the study of plants and animals. 2. Radiation is used and produced in nuclear reactors, which controls fission reactions to produce energy and new substances from the fission products. 3. Radiation is also used to sterilize medical instruments and food. 4. Radiation is used by test personnel who monitor materials and processes by nondestructive methods such as x-rays. The making of a radiograph requires some type of recording mechanism. The most common device is film. A radiograph is actually a photographic recording produced by the passage of radiation through a subject onto a film, producing what is called a latent image of the subject. A latent image is an image that has been created on the film due to the interaction of radiation with the material making up the film. This latent image is not visible to the naked eye until further processing has taken place. To make the latent image visible the film is processed by exposure to chemicals similar to that of photographic film. In the next sub-unit you will learn about the process of developing film. Review: 1. A radiograph is a image that is produced by the passage of radiation through a subject onto a piece of film . This produces an latent image of the subject. To understand how the image on a radiograph is formed, we need to first look at the characteristics of the film itself. There are three important parts to a radiographic film. These include the base, the emulsion, and the protective coating. The base All radiographic film consists of a base for which the other materials are applied. The film base is usually made from a clear, flexible plastic such as cellulose acetate. This plastic is similar to what you might find in a wallet for holding pictures. The principle function of the base is to provide support for the emulsion. It is not sensitive to radiation, nor can it record an image. The clarity or transparency of the film base is an important feature. Radiographic film must be capable of transmitting light. Once a film has been processed chemically, it is subject to interpretation. This is commonly done by using a film illuminating device, which is usually a high intensity light source. The emulsion The film emulsion and protective coating comprise the other two components and are essentially made from the same material. They are applied to the film during manufacturing and usually take on a pale yellow color with a glassy appearance. Although they are made from the same material, they offer two distinct features to the film. These features are separated into the image layer of the emulsion, and the protective layer. The protective layer The protective layer has the important function of protecting the softer emulsion layers below. It is simply a very thin skin of gelatin protecting the film from scratches during handling. It offers very important properties to film manufacturers, which include shrinkage (during drying that forms glassy protective layers) and dissolving in warm water. It will absorb the water and swell if it is dissolved in cold water. The softer layers of the gelatin coating are technically known as the emulsion. An emulsion holds something in suspension. It is this material in suspension that is sensitive to radiation and forms the latent image on the film. During manufacturing of the film, silver bromide is added to the solution of dissolved gelatin. When the gelatin hardens the silver bromide crystals are held in suspension throughout the emulsion. Upon exposure of the film to radiation, the silver bromide crystals become ionized in varying degrees forming the latent image. Each grain or crystal of silver bromide that has become ionized can be reduced or developed to form a grain of black metallic silver. This is what forms the visible image on the radiograph. This visible image is made up of an extremely large number of silver crystals each is individually exposed to radiation but working together as a unit to form the image. Once a film has been exposed to radiation and possesses the latent image, it requires chemical development. The purpose of developing the film is to bring the latent image out so that it can be seen visibly. There are three processing solutions that must be used to convert an exposed film to a useful radiograph. These are the developer, stop bath, and the fixer. Each of these solutions is important in processing the image so that it may be viewed and stored over a period of time. The process of developing film 1. To begin the process of converting the latent image on the radiograph to a useful image we first expose the film to the developer solution. The developer’s purpose is to develop, and to make the latent image visible. A special chemical within the developer solution acts on the film by reducing the exposed silver bromide crystals to black metallic silver. This process of developing is actually a multi-step process. Recall the characteristics of the film manufacturing mentioned earlier, they become important in the development process. Before the developer can change the silver crystals it must penetrate the protective coating of the film. Keep in mind that the protective coating of the film is made of gelatin and is sensitive to temperature and water. The developer solution is comprised of a combination of chemicals, consisting of alkali and metol or hydroquinone mixed with water. The purpose of the alkali is to penetrate the protective coating allowing the metol to reduce the exposed silver bromide to black metallic oxide. 2. The second step in the development process is the stop bath. This bath is comprised of a glacial acetic acid and water. It is important to recognize that alkali’s and acid’s neutralize each other. The function of the stop bath is to quickly neutralize any excessive development of the silver crystals. Over development of the silver crystals results in a radiographic image that is virtually impossible to interpret. 3. The third step in development is the fixer. Its function is to permanently fix the image on the film. This is also a multi-step process. The fixer must first remove any unexposed silver crystals and then harden the remaining crystals in the emulsion. It is this process that is used to preserve the radiographic image over time. 4. Once the film has been properly developed, it is then rinsed in water and dried so that it may be visually examined. Review: 1. The three main part to radiographic film are the base, the emulsion, and the protective coating. 2. Steps in developing film include developing, stopping the developer, fixing, rinsing and drying. http://www.ndt-ed.org/EducationResources/HighSchool/Radiography/highenergyrad.htm http://education.jlab.org/qa/pen_number.html http://www.hss.energy.gov/healthsafety/ohre/roadmap/achre/intro.html http://education.jlab.org/qa/archive_idx.html |