Transformers

Transformers

Transformers are one of the most common and useful applications of inductunce. They can step up or step down an input primary voltage(V1) to a secondary voltage(V2). The relationship is given by V1/V2=n, where n is the ratio of the primary turns to the secondary turns.

The parameter n can be adjusted by editing the transformer’s model.

To properly simulate the transformers, both sides must have a common reference point, which may be ground.

Diode

A diode conducts electric current very easily in one direction and very poorly in the other direction. It is the simplest form of solid-state switch, being either open or closed.

An ideal diode is included in the parts bin. You can specify a real-world diode by changing
its model.

Capacitor

A capacitor stores electric energy in the form of an electrostatic field. It affects AC relative
to capacitance and frequency and DC depending on capacitance alone.

Its capacitance, measured in farads, can be any value from pF to mF.

Resistor

A resistor’s resistance is measured in ohms. It can have any value, from ohm to kiloohm

Zener diode

Zener diodes are special diodes designed to continue operation within the reverse breakdown or Zener region, beyond the peak inverse voltage rating of normal diodes. This reverse breakdown voltage is called the Zener test voltage (Vzt), which can range between 2.4V and 200V.

Zener diodes are used primarily for voltage regulation.

BJTs (Bipolar junction transistors)

BJTs (bipolar junction transistors) are current-based valves used for controlling electronic currents and for amplication and switching applications. They are three layer devices with two junction:base-emitter and base-collector.

BJTs come in two versions, PNP and NPN. They have different power supply polarities and internal current flow directions. The letters refer to the polarities (positive or negative) of the materials that make up the transistors “sandwich.”
Transistors began the solid-state phase of electronics, and they still play an important part. Their small size made “chip” technology possible; even small ICs (integrated circuit) may contain many transistors. Transistors make battery power practical for instruments and communicators, allowing very complex systems to be light and portable.

The parts bin includes two BJTs:

Transformers INDIR

Yorum Yazın 25.12.2008

Radioactivity

Radioactivity
Except for H all nuclei have more than 1 p+. Since like charges repel, how can any nucleus be stable? The electrostatic +ve forces are not the only one’s present in a nucleus. p+ in fact do repel each other but also at work in the nucleus is a “strong force” which acts to overcome the electrostatic force of repulsion within the nucleus, and it binds nucleons into a package. This “strong force” has some of its own peculiar characteristics. It decreases far more rapidly with distance than an electrostatic force. The strong force exerted by one nucleon on another nucleon falls to zero within the nucleus. The strong force between two adjacent nucleons, therefore, does not contribute anything to the binding of the nucleons on the other side of the nucleus. The electrostatic force of repulsion of p+ in the nucleus does not fall off to zero. p+ in one nuclear region repel p+ in all other regions. Such repulsions are toned down by the intervening no because they help separate the p+.

Radioactivity INDIR

Yorum Yazın 25.12.2008

Radioactivity and half-life demonstration

Radioactivity and half-life demonstration

Materials
• Shoebox with lid
• 200 pennies (or any coins of the same denomination)
• Graph paper
• Notebook paper
• Pen or pencil
Procedure
Each student or pair of students should have the materials listed above.
1. Put the 200 coins in the shoebox tails up. Do not overlap them.
2. Place the lid on the box.
3. Hold the box with your thumbs placed on top of the lid.
4. Shake the box with one quick up-and-down motion.
5. Set the box down. Take off the lid and remove all the coins that face heads up.
6. Record the number of coins remaining in the box in the chart below.
7. Repeat steps 2 through 6 until no coins remain in the box.
8. Prepare a graph of your data. Let the x-axis represent the number of trials. Let the y-axis represent the number of coins remaining in the box. Be prepared to interpret the graph and discuss your results. Click here to view a sample chart and graph.

Radioactivity and half-life demonstration

Yorum Yazın 25.12.2008

Equipment INTRODUCTION

Equipment:
Teaching microscope, stage micrometer, objective micrometer, hair sample, rule, plastic.

The Purpose of the Experiment:
1- Measuring the eyepiece magnification and whole system magnification,
2- Measuring the thickness of a sample of hair.

The Methodology of the Experiment:
Magnified images of very small object (1m) are obtained and their accurate measurements are determined with a light optical microscope. Eyepiece, objective, and total magnification are measured by using different eyepiece and objectives. Then the thickness of a hair sample is measured.

INTRODUCTION

Equipment INTRODUCTION

Yorum Yazın 25.12.2008

Medical Uses

Medical Uses
I understand that radiation sources are used for medical purposes. Is that true?
Yes it is. In fact, the ionizing radiation that is emitted from radioactive atoms has many, many applications in the diagnosis and treatment of disease.
Where is radioactivity typically used?
There are traditionally three branches of medicine that use ionizing radiation. The first is called “diagnostic imaging”, which includes diagnostic x-ray and computed tomography. Magnetic resonance imaging and ultrasound are also included in diagnostic imaging, but do not use ionizing radiation.
And the other two branches?
One is “nuclear medicine”, which includes diagnostic nuclear medicine and radionuclide therapy. The other is “radiation therapy”, which includes external beam therapy and brachytherapy.
Why would anyone want to use radiation in medicine since radiation exposure has some risks associated with it?
Good question. Radiation makes the diagnosis of certain diseases and conditions, like broken bones, and the treatment of other diseases, like cancer, somewhat easier. The general goal in using radiation in medicine is to balance the risk of the exposure with the benefit of the diagnosis or treatment. The benefit of the exposure is usually obvious and personally relevant to patients undergoing procedures or their families. Therefore, the risks of these procedures are more readily accepted by the general population than other uses of radioactivity (i.e., industrial uses).
I have heard of an occupation called a Medical Physicist. What does this person do?
Medical physicists are trained professionals who study the physical characteristics of the subject of diagnostic imaging or therapy and the human body. In diagnostic imaging and nuclear medicine, medical physicists work closely with the physician to reduce the purposeful radiation exposure in a way that does not lose the necessary diagnostic information. While any medically-related radiation dose can usually be reduced to lower and lower levels, there is always an accompanying loss of information for the physician. Eventually, the dose gets so low that there is not enough information to accomplish the objective. We call this a “wasted” dose.
Is the same thing true in radiation therapy?
To a great extent, yes. In radiation therapy, the goal is to destroy malignant tissue while sparing healthy tissue. If the dose is minimized too much, there may be only negligible effect on the malignant tissue, again resulting in “wasted” dose.
What kinds of radiation are used in diagnostic imaging?
X-ray machines are most commonly used for this purpose. The x-rays produced in the machines have energies ranging from about 20 kiloelectron volts, or “keV”, to 125 keV. X-ray studies primarily reveal structural changes like broken bones, for example, rather than functional changes like a section of the lung that is not getting a supply of blood. The quality of the image produced is dependent upon the physical characteristics of the organ in question.
What do you mean by physical characteristics?
Diagnostic imaging provides a “contrast” in the image which varies based on the density of the materials that make up the human body. For example, bone is quite dense, while soft tissue is not. Therefore, there is a distinct contrast between bones and tissue that is visible on an x-ray image. On the other hand, other body structures such as kidneys, blood vessels, intestines, etc. may not be visible in an x-ray image because there is not enough of a difference in the absorption of x-rays in these organs (i.e., less dense). However, other forms of diagnostic imaging, like computerized tomography or “CT Scans”, are able to show differences between these less-dense structures.
Is there a way to improve the contrast for materials of similar densities?
Yes. One technique is to fill these structures with contrast material. The contrast material is a stable compound (meaning that it is not radioactive) of greater density than the surrounding structures. As a result it will absorb more radiation and enhance the contrast and visibility of the image.
Can you give me an example, please?
Certainly. A barium compound, serving as a contrast material, is often placed inside the gastrointestinal (GI) tract to image it. When barium is inside the colon, for example, an outline of the colon becomes clearly visible on the x-ray image. For that matter, other portions of the GI tract along with blood vessels, the urinary system of the kidney, the bladder, etc. can also be imaged with contrast material. Contrast material is also used in CT studies.
What is required to obtain good image quality?
For good image quality, the characteristics of the radiation must be coupled with the characteristics of the subjects or organs being imaged.
What do you mean?
Well, imaging geometry is an important factor that affects how structures will appear in the image. For example, a conventional x-ray tube is placed on one side of the patient and the image receptor, usually film, is placed on the other. In this geometry, a three-dimensional object is imaged on a two-dimensional image receptor. This results in many structures being superimposed on top of each other in the image.
That doesn’t sound too good.
Actually, this is not a problem for a large number of routine x-ray studies. But, as you might imagine, there are circumstances where a “static” image would probably would be unacceptable.
You mentioned “image receptor”. What is that?
The image receptor is the material or device designed to capture the image or picture as the x-rays pass from the x-ray tube and through the patient.
What kinds of image receptors are there?
Photographic film is still the most prevalent type of image receptor, but there are actually many others that will likely be in widespread use in the future. Some of these are fluorescent screens, video screens, computer monitors, laser discs, and phosphors which are “stimulated” upon exposure to radiation.
I take it a mammography machine is a type of imaging device, right?
Most definitely. These are machines utilize x-rays for the difficult task of breast imaging. To accomplish this objective, the machine needs some pretty unique design features. There are definitely challenges in obtaining useful static images of this soft tissue.
Let’s move on to fluoroscopy. What is it and what does it accomplish?
Fluoroscopy is a radiographic technique that is used to image organs in “real” time. This means that you can capture the organ while it is moving. These images, called “dynamic images”, are recorded on special film or videotape. And, as you might imagine, fluoroscopy machines, just like other x-ray machine, come in different shapes and sizes.
How does fluoroscopy work?
The patient is placed in a radiographic/fluoroscopic (R/F) x-ray room. The radiographic x-ray tube is positioned above the x-ray table, and the fluoroscopic x-ray tube is usually underneath the table. The image receptor for fluoroscopy, which is colloquially known as “fluoro”, is a fluorescent screen assembly that is attached to the device that the radiologist moves over the patient. The fluoro beam is activated by a foot switch. The radiologist typically stands next to the patient, with one foot on the switch, eyes on the TV monitor, and his hand moving the image receptor and tube. The radiographic x-ray tube is used to make “static” images on an as-needed basis.
I notice that the radiologist wears a lead apron. Why is that?
The radiologist must wear a protective lead apron during the procedure, as must any technologists or other people present. This is necessary because they perform this work day in and day out and, if not protected, could accumulate a significant amount of radiation exposure. In fact, you may also have observed radiologists wearing lead gloves if the procedure calls for him or her to reach into the x-ray beam to assist the patient during the procedure. In addition, you can also see a lead drape hanging from the image receptor assembly to the edge of the table. This drape is used to intercept radiation that “scatters” from the patient before it reaches the radiologist.
So, the medical staff takes radiation safety precautions to limit their exposures. What about the patient?
Every effort is made to keep the radiation exposure to patients as low as possible, while making sure the radiologist gets as much information about the patient as possible. While there are legal dose limits for the medical staff, there are no limits to the amount of radiation exposure patients can receive, except in radiation therapy where higher doses are used to begin with.
Wow. That doesn’t seem quite fair.
Before you jump to that conclusion, let me give you an example. Suppose there was a limit on the amount of radiation exposure you could receive, and you already reached that limit while being treated for a severe case of pneumonia. What if, on your way home from the hospital, someone ran a red light and broadsided your car, breaking some of your ribs or an arm or leg. It would be senseless to not permit the physician to use more x-rays to diagnose and treat you fractures in the best way possible, particularly when the radiation risk from those additional procedures would be small compared to not treating broken bones properly.
So there are no limits at all?
Well, not quite. Some state agencies do have ceilings on the amount of exposure permitted per procedure, but there are no limits on the number of procedures performed. Similarly, regulations issued by the Food and Drug Administration (FDA) limit the entrance exposure rate (EER) to the patient for routine fluoroscopic procedures to ten roentgens per minute (10 R/min), but the duration of fluoro is not limited. For non-routine fluoroscopic procedures, such as angiographic (blood vessel-related) procedures or interventional procedures (such as balloon angioplasty), higher entrance exposure rates may be necessary, and can be used.
Should this concern me?
The whole basis for our standard of medical care is that procedures are performed when medically necessary. If radiological procedures are required, the law states that the equipment must be checked by a physicist and a state inspector, and that all personnel involved in the procedure, like the radiologist (physician) and the technologist, are qualified by training and experience to perform the procedure. As a result, only the necessary amount of radiation will be delivered in order to achieve the medical objective. The risk of radiation exposure to the patient is really minimal when you compare it to the potentially dangerous consequences of not having the procedure. However, patients can help ensure their own radiation safety by asking questions of their physicians, radiologists, and physicists. While patients always have the right to refuse procedures, you should take care in making this request, as it might subject you to far greater risk than the procedure you are trying to avoid.
Okay. I’ll keep that in mind. In the meantime, would you please explain what vascular imaging procedures are?
Vascular imaging procedures are another category of radiographic procedures. The word “vascular” simply refers to images of the blood vessels.
Do you have an example where this is utilized?
Yes. One example is “arteriograms”, where images of arteries are taken. These are semi-surgical procedures that are usually performed in rooms that are dedicated for this purpose.
Like how?
The procedure involves placement of a catheter, or a hollow plastic tube, into the arterial system during fluoroscopy. The catheter has a “radiopaque” tip that allows it to be visible to the radiologist or vascular surgeon on the fluoro image. Once they see on the image that the catheter is correctly positioned in the desired artery, contrast material is automatically injected into the bloodstream. Images are then obtained in rapid sequence and recorded on film, video tape, or some sort of digital media. Special x-ray tubes are usually required for these studies because of the power output and heat loading demands of this type of rapid sequence x-ray production. In many angiographic procedures, not one, but two x-ray machines are used to make exposures simultaneously at 90 degrees from each other.
This is interesting. Give me some other applications of the vascular imaging technique.
You got it. How about a selective renal arteriogram? In these studies, the catheter is introduced into the femoral artery, which is located in the upper thigh and groin area. From there it is “threaded” up through the aorta and into the right renal artery. The contrast material is then injected, and the images are obtained. The technique can be used to identify small blockages in the artery.

Devamını okuyun…»

Yorum Yazın 25.12.2008

Its Everywhere!

Its Everywhere!
Is radioactivity unique?
The earth has always been radioactive. Everyone and everything that has ever lived has been radioactive. In fact, the natural radioactivity in the environment is just about the same today as it was at the beginning of the Neolithic Age, more than 10,000 years ago.
What is radiation?
Radiation is energy in the form of particles or rays given off by atoms as they go from an unstable to a stable state. Some radioactive atoms exist naturally; others are made artificially.
Is there radioactivity in our bodies?
Yes. During our lifetime, our bodies harbor more than 200 billion billion radioactive atoms. About half of the radioactivity in our bodies comes from Potassium-40, a naturally-occurring radioactive form of potassium. Potassium is a vital nutrient and is especially important for the brain and muscles. Most of the rest of our bodies’ radioactivity is from Carbon-14 and tritium, a radioactive form of hydrogen. These naturally-occurring radioactive substances expose our bodies to about 25 “millirem” per year, abbreviated as “mrem/yr”.
Is there radioactivity in food and water?
Yes. Most radioactive substances enter our bodies as part of food, water or air. Our bodies use the radioactive as well as the nonradioactive forms of vital nutrients such as iodine and sodium. Radioactivity can be found at every step of the food chain. It is even in our drinking water. In a few areas of the United States, the naturally-occurring radioactivity in the drinking water can result in a dose of more than 1,000 millirem in one year.
What kinds of radioactivity are in food?
In general, the foods we eat contain varying concentrations of radium-226, thorium-232, potassium-40, carbon-14, and hydrogen-3, also known as tritium.
How much of these radionuclides are in foods?
Well, it depends, of course, on the food item. The U. S. Department of Energy gives the following concentrations as examples: Salad Oil 4,900 pCi/l; Milk 1,400 pCi/l; Whiskey 1,200 pCi/l; Beer 390 pCi/l; Tap Water 20 pCi/l; Brazil Nuts 14.00 pCi/g; Bananas 3.00 pCi/g; Tea 0.40 pCi/g; Flour 0.14 pCi/g; and Peanuts and Peanut butter 0.12 pCi/g.
Is there radiation in outer space?
Yes. Another type of natural radiation is cosmic radiation from the sun and outer space. Because the earth’s atmosphere absorbs some of this radiation, locations at higher altitudes receive a greater exposure than those at lower altitudes. In Ohio, for example, the average resident receives a dose of about 40 millirem in one year from cosmic radiation. In Colorado, it is about 180 millirem in one year. Generally, for each 100-foot increase in altitude, there is an increased dose of one millirem per year.
Flying in an airplane increases our exposure to cosmic radiation. A coast-to-coast round trip gives us a dose of about four millirem.
The rocks and soils around us are radioactive.
In Ohio, radiation in soil and rocks contributes about 60 millirem in one year to our exposure. In Colorado, it is about 105 millirem per year. In Kerala, India, this radioactivity from soil and rocks can be 3,000 millirem per year, and at a beach in Guarapari, Brazil, it is over 5 millirem in a single hour — but only a few residents who use that beach receive doses in excess of 500 millirem per year.
Is there radioactivity in our homes?
As a matter of fact, there is. If you live in a wood house, the natural radioactivity in the building materials gives you a dose of 30 to 50 millirem per year. In a brick house, it is 50 to 100 millirem per year. And, if your home is so tightly sealed that there is little ventilation, natural radioactive gases (radon) can be trapped for a longer period of time and thus increase your dose.
Is is true that we can’t escape from radioactivity?
Yes, its quite true. Each person with whom we spend eight hours a day gives us a dose of about 0.1 millirem in a year.
Using a gas stove can increase the dose by about two millirem per year because of radioactive materials in the natural gas.
A person who smokes two packs of cigarettes a day receives a radiation dose of about 1,300 millrem per year. This is because polonium (a radioactive element) is part of the smoke and when inhaled, it gets trapped in the lungs.
So, its everywhere, right?
Radiation really is everywhere. We are exposed to a constant stream of radiation from the sun and outer space. Radioactivity is in the ground, the air, the buildings we live in, the food we eat, the water we drink, and the products we use. The average person in the United States receives a dose of about 360 millirem per year from these natural sources of radioactivity as well as from typical medical radiation exposures.
To put these radiation doses into perspective, although theoretically the risk increases with increased exposure to radioactivity, no effects have ever been observed at levels below 5,000 millirem delivered over a one year period. In fact, effects seen when humans are exposed to 100,000 millirem over a short time period are temporary and reversible. It takes a short-term dose of more than 500,000 millirem to cause a fatality.
Is it true that we can’t live without it?
Yes, our bodies are radioactive. Its a simple fact of nature. But there is no cause for alarm. These very small but detectable levels of radioactivity are natural . . . as natural as life itself

Yorum Yazın 25.12.2008

Industrial Uses

Industrial Uses
Is it true that radioactivity can be found in industry?
Yes. In fact, there are wide-spread uses of radiation and radioactivity in industrial operations.
I wonder why?
Well, we take advantage of the following four characteristics of radiation sources for industrial uses: that radiation affects materials; that materials affect radiation; that radiation traces materials; and that radiation produces heat and power in a variety of industries.
How does the fact that radiation affects materials make it useful in industry?
You can compare this characteristic to that of receiving a sun tan. Radiation affects, to various degrees, any materials that are exposed to it. As a result, applications such as pasteurization and sterilization of food, polymerization of organic compounds, sterilization of medical supplies, and elimination of static electricity are possible.
What about the fact that materials affect radiation? How is that useful?
Think of this one as being similar to the use of sunglasses, where the intensity of the sun’s rays is reduced by the use of thicker or darker glasses. Likewise, the intensity of nuclear radiation is reduced by thicker or denser materials that are in the path of the radiation. This is the characteristic that is responsible for such applications as radiographs (i.e., taking pictures through objects), locating or controlling hidden levels of solids and liquids, which is especially helpful if the liquid is hot, corrosive or under pressure, and determining the thicknesses of materials.
What do you mean by the fact that radiation “traces” materials?
Radioactive elements and stable elements have identical chemical behaviors. However, radioactive elements are able to “announce” their presence through the radiations that are given off. So, not only do the radioactive elements take part in the same reaction or process as the stable elements, but they continually show their exact location by the “signals” they give off. All that is necessary is some sort of device to detect their presence. Our ability to trace the location of radioactive elements permits us to test wear, to locate leaks, to trace fluid flow, to evaluate detergent efficiency, and a host of other operations.
You also said radiation produces heat and power. I can kind of see how that might be useful.
You’re right. Whenever an energetic particle or ray is slowed down or stopped, heat is given off. We can take advantage of this characteristic by converting the heat produced to electrical or mechanical energy, or simply using it directly. Among the applications that use this characteristic are electrical generators for unmanned weather stations and buoys, power devices for thrusters in the space program, and heat for diving suits.
So, tell me about some specific uses of radioactivity in industry.
Okay. Let’s start first with applications in the metals industries. In blast furnace operations, radioactivity is used to study the residence time and distribution of constituents in the various metallurgical processes. Other tracer studies compare methods of chemically cleaning copper and stainless steel parts, evaluating plating techniques, and adding to our knowledge of the structure of electroplated coatings. Radionuclides have also been used to evaluate the diffusion of gases into metals (causing brittleness), and they have been used to provide valuable information on the rate of tool wear.
That’s tracing, now what about gauging?
Using radioactivity to gauge thicknesses has been well-recognized by industry. It permits us to impose continuous control of the uniformity of the thicknesses of various kinds of sheets and layers to very close tolerances. Furthermore, these types of systems can be completely automated so that the response to thickness changes can be used to actuate rollers, thus providing closer control than would otherwise be possible. But in addition to thickness, we can also use radioactivity to gauge the density of various materials.
Density? How?
The density of a variety of liquid slurries, powders, and granular solids can be measured by having a radiation source and a detector mounted on opposite sides of the material being measured (i.e., like in a hopper or pipe line). If the detected intensity of radiation from the source increases or decreases, we know that the density of the material has decreased or increased, respectively.
What about radiography?
Well, the major advantage of radiography using radiation sources versus x-ray inspections is portability, the absence of electrical wires and connectors, and the ability to make exposures with the source of radiation placed inside a complex shape. The use of radioactive cobalt for flaw detection in masses of metal was one of the earliest applications of radionuclide radiography. Most foundry operations maintain a selection of radiation sources, including radioactive cobalt, iridium, and cesium, among others. While x-ray machines are still used, radiation sources are the preferred methodology where the shape and accessibility of the casting makes x-ray techniques ineffective.
How do they compare, cost-wise?
Although the purchase price of a radioactivity-bearing device can be much less than the price of an x-ray machine, compliance costs for users of radioactivity tend to be much higher than for users of radiation-producing machines.
Okay, that’s the metals industry. What about, say, the electrical industry?
There are uses there too! One example is the use of radioactive krypton gas for leak testing. This procedure involves exposing electronic components to the gas under pressure for some period of time, during which any leaky components are at least partially filled with the gas. After the exposure period, the surfaces of the components are cleaned, and the leaky components are quickly identified by detecting the residual radioactivity. Kind of a clever way to go.
Are there other uses of tracers in the electrical industry?
Yes there are. For example, they are used to study adsorption and desorption of mercury by glass surfaces in mercury switches. In addition, there are studies of corrosion of silver contacts by fused salt, the development of a high- integrity compression seals, evaluation of methods for cleaning metal surfaces prior to electroplating or enameling, wear testing of bearings, determination of lubrication and seal characteristics, and improving the doping of semiconductors by investigating the mechanisms of the diffusion.
Any gauging applications?
Gauging in the electrical industry is limited. We don’t typically see many applications here.
What about radiography?
Yes, there are some uses here. Radiation sources are used to check the integrity of welds on structural components of heavy industrial electrical equipment.
Any thing else in the electrical industry?
Yes. Radiation sources are also used for static elimination, in fire detection equipment, and in luminous dials, gauges and signs. Certain navigational lights also contain radioactivity. In addition, there has been considerable interest in the use of radionuclides to replace batteries and related power sources.
Okay. That’s metals and electrical. What about chemical applications?
The use of radionuclides in this industry is widespread and includes pretty much all conceivable categories. In fact, petroleum refiners were among the first industrial operations to use radionuclides.
Why is that?
Refineries pump a lot of fluids, including raw materials and other in-plant inventory and products. Radiation sources are used as part of the automatic (computerized) control of the flow of these fluids. They also let the operators know if a blockage occurs! However, by far the most extensive use of radionuclides in this and other chemical industries is as a tracer.
How is radioactivity used in chemical processing?
There are many, many types of radiation sources used in this industry . . . too numerous to mention here. For example, radioactive sulfur can be used to determine the efficiency of separation; radioactive gold and iodine can be used to determine the thoroughness of mixing; radioactive sodium and bromine are used for locating leaks; and radioactive cobalt and cesium are used for gauging liquid or solid levels. Other radiation sources might be used to study process stream flow patterns, locate pipe obstructions, study mass balances in refinery streams, measure flow velocities, study catalyst movement, study carbon deposits in fuel research for drug metabolism studies, determine tire wear, study diffusion in glass, eliminate static, and sterilize medical supplies. I could go on.
I also understand radioactivity is involved in things that I, as a consumer, use every day.
Absolutely. Let me just give you a few ways in which radioactivity is used in our consumer product industries. I don’t mean radioactivity that is incorporated into products themselves, which is another subject for another day. I mean how radioactivity is used to improve the products that we use and take for granted.
Okay. Give me a couple of examples.
Radioactivity is sometimes used for determining the rate of wear in floor wax. It can also be used to assess laundering efficiencies of various detergents.
Imagine that! Are there more?
Yes. Radioactivity has been used to determine the firmness of cigarettes, the rate of pesticide removal from surfaces, the metabolism of food additives, biosynthesis, the movement of textile layers, the control of solid and liquid levels of foods and beverages in their containers, sterilization and pasteurization of food, and even the migration of dyes in the printing business.
Wow. I didn’t know there were so many uses of radiation and radioactivity in industry.
You know, the world as we know it today would be a very different place without the use of radioactive elements. A number of the examples I just gave you are seldom, if ever used today, but they certainly served their purpose when they were employed. On the other hand, we are seeing even more applications coming our way. Every day there are new uses of radioactivity in such industries as natural gas production, mining, utilities, agriculture, aerospace, and even environmental uses. In the case of industry, radiation and radioactivity are definitely beneficial!

Yorum Yazın 25.12.2008

Food Irradiation

Food Irradiation
You hear a lot about Food Irradiation these days. What is it?
Food irradiation is the process of exposing food to ionizing radiation. Typical radiation sources are cobalt-60, cesium-137, x-ray machines, or electron accelerators. These emit radiation that is able to penetrate deeply into food, killing insect pests and microorganisms without raising the temperature of the food significantly.
Why in the world would anyone want to irradiate food?
Food is most often irradiated commercially to extend shelf-life, eliminate insect pests, or reduce numbers of pathogenic microorganisms. Food irradiation for these purposes is practiced in many countries, including the United States.
Can you tell me a bit more about how radiation is able to do this without causing a problem in the food?
Certainly. First, the food is packaged in a manner that permits the irradiation facility to deliver a precise radiation dose over the entire food package. The radiation strikes the chemical bonds in the food and forms stable products, much like what happens when you cook food. Many years of research has shown that the chemical by-products in food that has been exposed to high doses of ionizing radiation are the same by-products that appear during conventional cooking or other preservation methods.
Sounds kind of experimental to me. Why do we need another form of food processing other than those we already have available?
Actually, food irradiation is not new, and is definitely not experimental. The Food and Drug Administration, or the FDA, first approved irradiation as a preservation method in 1963. However, at that time, its application was confined to wheat and wheat flour for the purpose of limiting the growth of insects and microorganisms. Since then, it has proven to have other valuable applications as well.
Isn’t food irradiation better suited for food being shipped long distances in ships and barges?
Not really. It is indeed a good application for foodstuffs being shipped and stored for a long time but it has more wide-spread applications than just shipping. For example, beef is consumed in great quantities in the U.S.; more than 8 billion pounds per year. Unfortunately E.coli, a potentially-deadly bacteria, has been found in beef in concentrations that are higher than any of us thought possible.
Is that the bacteria that was linked to the deaths of four children and the illness of hundreds of people in the Pacific Northwest?
Yes. If you recall, in 1997, Hudson Foods voluntarily recalled 25 million pounds of hamburger containing E. coli; the largest recall of meat products in U. S. history.
Wasn’t that just a freak occurrence.
Unfortunately, it wasn’t. Nationally, E.coli is responsible for over 20,000 illnesses and 500 deaths per year (CDC). However, this microorganism has only been linked to illnesses in humans since 1982.
Are there any other illnesses I should be concerned about?
Yes, there are a few others that are troubling. Salmonella, commonly found in poultry, eggs, meat and milk, and is responsible for the illness of over four (4) million people. Salmonella poisoning can be fatal as well. In fact, a report by the President’s staff issued in May of 1997 indicates that millions of Americans are stricken by food borne illness like E.coli and Salmonella each year, and that as many as 9,000, mostly the very young and elderly, die as a result.
What did the government do in response to that report?
In December of 1998, the FDA approved the irradiation of red meat with a measured dose of radiation. This level of radiation exposure is very effective at killing E.coli and other disease- causing microorganisms in meat.
So if irradiation is so useful, does that mean that restaurants and grocery stores won’t have to be as careful about cleanliness and refrigeration?
No. FDA officials are quick to point out that irradiation is a useful tool for reducing the risk of food-borne disease but it does not replace proper food handling practices by producers, processors and consumers.
How much radiation exposure does it take to sterilize food?
The FDA has approved specific doses to be applied to different foodstuffs. In general, a “low” dose (i.e. less than 1,000 grays) is used to delay physiological processes like ripening or sprouting of fresh fruits and vegetables. Low doses are also useful for controlling insects and parasites in food. A “medium” dose (i.e. from 1,000 to 10,000 grays) is used to reduce spoilage and extend the shelf life of many foods. A high dose (i.e. greater than 10,000 grays) is used to sterilize meat, poultry, seafood and other prepared foods.
Can you give me some examples?
Sure. A good example is irradiation of fresh pork to a dose of 300 gray. This fairly low level is able to completely control trichinella spiralis in this product. Here are a few other examples: Fresh foods irradiated to 1,000 gray inhibits growth and maturation; Poultry irradiated to 3,000 gray, refrigerated meat irradiated to 4,500 gray or frozen meat irradiated to 7,000 gray controls pathogens; dry spices and seasonings irradiated to 30,000 gray controls eliminates microbial infection.
What about super-high doses?
Well, there is a National Aeronautical and Space Administration (NASA) requirement that the frozen foods given to the astronauts be completely sterilized. These products receive about 44,000 gray.
These numbers seem awful high. How much radioactivity remains in the food after the irradiation, and how much of it would I actually ingest if I ate irradiated food?
Absolutely none. The radioactive material like cobalt-60 and cesium-137 used in the irradiation facility emits gamma rays. The food itself never comes in contact with the radioactive material. Instead it is simply “struck” by the gamma radiation.
I’m not sure I follow that. Do you mean once the food leaves the vicinity of the radiation source it is not radioactive?
Sort of. It actually never becomes “radioactive”. It is simply “irradiated”. Think of how you cook food on the gas stove. The food is surrounded by the radiant heat from the gas source for enough time to cook your steak the way you like it (i.e., rare, medium). to taste. However, no gas enters the meat, and once the food cools off, all of the gas “energy” is gone.
How do I know that something has been irradiated before I buy it a the grocery store?
The FDA requires a label to be placed on packages that have been irradiated. That label contains the Radura logo. They also require a statement that the food has been irradiated, has been “Treated by irradiation” or has been “Treated with radiation”.
I don’t know. I’m still not convinced. Has anybody else studied the possibility of a problem with the food?
Many national and international committees, organizations and regulatory agencies have reviewed the safety of irradiated foods. These include the World Health Organization (WHO), the Food and Agricultural Organization of the United Nations (FAO), the Codex Alimentarius Commission, and the U.S. FDA. They have all concluded that food irradiation is safe when Good Manufacturing Practices (GMPs) and Good Irradiation Practices are used. For the evaluation of safety, three main areas of concern were addressed: potential toxicity, nutritional adequacy, and potential microbiological risk. Information regarding the chemical structures and the amounts of radiolytic products in particular food types, together with the information obtained from toxicological testing, forms a sound basis for evaluating the toxicological safety of an irradiated food. Scientists have repeatedly concluded that the animal feeding studies have found no toxic effects from irradiated foods. Therefore, irradiation of food does not lead to changes in the composition of the food that, from a toxicological point of view, would have an adverse effect on human health.
Is there any place I can look for additional information on this topic?
Absolutely! We encourage you to visit the “Tool Box” section of the IEM web page. In the category entitled “Bibliography”, we have listed some references that you may wish to review.
How about web sites?
There are lots of those too. For example, the FDA has several papers on the subject. You can read them at http://vm.cfsan.fda.gov/list.html; http://vm.cfsan.fda.gov/~lrd/hhsirrad.html; http://vm.cfsan.fda.gov/~lrd/fr990224.html; http://vm.cfsan.fda.gov/~dms/qa-fdb33.html; and http://vm.cfsan.fda.gov/~dms/opa-fdir.html.
Any more examples?
Absolutely. Here are just a few that are worth a few minutes of your time. You can read about the TTen most commonly-asked questions about food irradiation” at http://www.physics.isu.edu/radinf/food.htm#food1. The “Policy of the American Medical Association (AMA) on Food Irradiation” can be found at http://www.physics.isu.edu/radinf/food.htm#ama. The International Atomic Energy Agency’s “Facts About Food Irradiation” is located at http://www.iaea.or.at/worldatom/inforesource/other/food/index.html. You can take an “Inside Look at Food Irradiation”, courtesy of the Grocery Manufacturers of America, by visiting http://www.iaea.or.at/worldatom/inforesource/other/food/index.html. The Mayo Clinic has posted a write-up entitled “Food Irradiation: The Answer to E.Coli?” at http://www.mayohealth.org/mayo/9709/htm/food_irr.htm. And finally, the USDA has an annotated bibliography of food irradiation references at http://warp.nal.usda.gov/fnic/pubs/bibs/gen/foodirrad.html.

Yorum Yazın 25.12.2008

The Discovery of DNA

The Discovery of DNA
In 1953 James D. Watson and Francis H.C. Crick published a paper in which they proposed a model for the physical and chemical structure of the DNA molecule. According to their model, most DNA consists of two polynucleotide chains wound around each other in a right-handed (clockwise) helix. In generating their model, Watson and Crick used three main pieces of evidence:
1. The DNA molecule was known to be composed of bases, sugars, and phosphate groups linked together as a polynucleotide (deoxyribonucleotide) chain.
2. By chemical treatment Erwin Chargaff had hydrolyzed the DNA of a number of organisms and had quantified the purines and pyrimidines released. His studies showed that in all the (double-stranded) DNAs the amount of the purines was equal to the amount of the pyrimidines. More important, the amount of adenine (A) was equal to the amount of thymine (T), and the amount of guanine (G) was equal to that of cytosine (C). These equivalencies have become known as Chargaff’s rules. In comparisons of DNAs from different organisms, the A/T and G/C ratios are always the same, although the (A+T)/(G+C) ratio (typically presented as %GC) varies.
3. Rosalind Franklin, working with Maurice H.F. Wilkins, studied isolated fibers of DNA by using the X-ray diffraction technique, a procedure in which a beam of parallel X rays is directed on a regular, repeating array of atoms. The beam is diffracted by the atoms in a pattern that is characteristic of the atomic weight and the spatial arrangement of the molecules. The diffracted X rays are recorded on a photographic plate. By analyzing the photograph, Franklin could obtain information about the molecule’s atomic structure. The analysis of X-ray diffraction patterns is extremely complicated. As a result, given diffraction patterns can usually be interpreted in more than one way, and models built of the analyzed molecules may not be accurate. Moreover, since the experiments usually use molecules in a crystalline or fiber formation, the structures deduced may not precisely reflect the form of the molecules in the cell.
The diffraction patterns obtained by directing X-rays along the length of drawn-out fibers of DNA indicated that the molecule is organized in a highly ordered, helical structure. Franklin interpreted these kinds of data to mean that DNA was a helical structure which had two distinctive regularities of 0.34 nm and 3.4 nm along the axis of the molecule. Watson and Crick considered all the evidence just described and began to build three-dimensional models for the structure of DNA. The model they devised, which fit all the known data on the composition of the DNA molecule, is the now-famous double-helix model for DNA. Unquestionably, the determination of the structure of DNA was a momentous occasion in biology, leading directly to many Nobel prize-winning discoveries in molecular biology. The double-helical model of DNA proposed by Watson and Crick has the following main features:

Yorum Yazın 25.12.2008

The Discovery Of Radioactivity

The Discovery Of Radioactivity:
The Dawn of the Nuclear Age
Fran Slowiczek, Ed.D and Pamela M. Peters, Ph.D.

One hundred years ago, a group of scientists unknowingly ushered in the Atomic Age. Driven by curiosity, these men and women explored the nature and functioning of atoms. Their work initiated paths of research which changed our understanding of the building blocks of matter; their discoveries prepared the way for development of new methods and tools used to explore our origins, the functioning of our bodies both in sickness and in health, and much more. How did our conceptions of atomic properties change? How has that change affected our lives and our knowledge of the world?
Atoms and Elements: A Beginning
Elements are the building blocks of matter. The smallest particle of an element that still retains the identity of that element is the atom. All atoms of a given element are identical to one another, but differ from the atoms of other elements. Ancient Greeks first predicted the existence of the atom around 500 BC. They named the predicted particle ‘atomos,’ meaning “indivisible.”
In 1803, John Dalton (1766-1844) proposed a systematic set of postulates to describe the atom. Dalton’s work paved the way for modern day acceptance of the atom. But scientists of his day considered the atom to be merely a subordinate player in chemical reactions, an uninteresting, homogeneous, positively charged “glob” that contained scattered electrons. That premise remained unchallenged until the end of the nineteenth century, when a series of brilliant discoveries opened the door on the atomic science of the twentieth century. Working concurrently and often collaboratively, three pioneering scientists helped release the genie of the atom.
Antoine Henri Becquerel
Becquerel, a French physicist, was the son and grandson of physicists. Becquerel was familiar with the work of Wilhelm Conrad Roentgen on December 22 1895, “photographed” his wife’s hand, revealing the unmistakable image of her skeleton, complete with wedding ring. Roentgen’s wife had placed her hand in the path of X-rays which Roentgen created by beaming an electron ray energy source onto a cathode tube. Roentgen’s discovery of these “mysterious” rays capable of producing an image on a photographic plate excited scientists of his day, including Becquerel. Becquerel chose to study the related phenomena of fluorescence and phosphorescence. In March of 1896, quite by accident, he made a remarkable discovery.
Becquerel found that, while the phenomena of fluorescence and phosphorescence had many similarities to each other and to X-rays, they also had important differences. While fluorescence and X-rays stopped when the initiating energy source was halted, phosphorescence continued to emit rays some time after the initiating energy source was removed. However, in all three cases, the energy was derived initially from an outside source.
In March of 1896, during a time of overcast weather, Becquerel found he couldn’t use the sun as an initiating energy source for his experiments. He put his wrapped photographic plates away in a darkened drawer, along with some crystals containing uranium. Much to his Becquerel’s surprise, the plates were exposed during storage by invisible emanations from the uranium. The emanations did not require the presence of an initiating energy source–the crystals emitted rays on their own! Although Becquerel did not pursue his discovery of radioactivity, others did and, in so doing, changed the face of both modern medicine and modern science.
The Curies: Lives Devoted to Research
Working in the Becquerel lab, Marie Curie and her husband, Pierre, began what became a life long study of radioactivity. It took fresh and open minds, along with much dedicated work, for these scientists to establish the properties of radioactive matter. Marie Curie wrote, “The subject seemed to us very attractive and all the more so because the question was entirely new and nothing yet had been written upon it.”
Becquerel had already noted that uranium emanations could turn air into a conductor of electricity. Using sensitive instruments invented by Pierre Curie and his brother, Pierre and Marie Curie measured the ability of emanations from various elements to induce conductivity. On February 17, 1898, the Curies tested an ore of uranium, pitchblende, for its ability to turn air into a conductor of electricity. The Curies found that the pitchblende produced a current 300 times stronger than that produced by pure uranium. They tested and recalibrated their instruments, and yet they still found the same puzzling results. The Curies reasoned that a very active unknown substance in addition to the uranium must exist within the pitchblende. In the title of a paper describing this hypothesized element (which they named polonium after Marie’s native Poland), they introduced the new term: “radio-active.”
After much grueling work, the Curies were able to extract enough polonium and another radioactive element, radium, to establish the chemical properties of these elements. Marie Curie, with her husband and continuing after his death, established the first quantitative standards by which the rate of radioactive emission of charged particles from elements could be measured and compared. In addition, she found that there was a decrease in the rate of radioactive emissions over time and that this decrease could be calculated and predicted. But perhaps Marie Curie’s greatest and most unique achievement was her realization that radiation is an atomic property of matter rather than a separate independent emanation.
Despite the giant step forward which science had now taken in it’s understanding of radioactivity, scientists still understood little of the structure of the atom. This understanding awaited the work of Ernest Rutherford.
Ernest Rutherford and the Atom
In 1911, Rutherford conducted a series of experiments in which he bombarded a piece of gold foil with positively charged (alpha) particles emitted by radioactive material. Most of the particles passed through the foil undisturbed, suggesting that the foil was made up mostly of empty space rather than of a sheet of solid atoms. Some alpha particles, however, “bounced back,” indicating the presence of solid matter. Atomic particles, Rutherford’s work showed, consisted primarily of empty space surrounding a well-defined central core called a nucleus.
In a long and distinguished career, Rutherford laid the groundwork for the determination of atomic structure. In addition to defining the planetary model of the atom, he showed that radioactive elements undergo a process of decay over time. And, in experiments which involved what newspapers of his day called “splitting the atom,” Rutherford was the first to artificially transmute one element into another–unleashing the incredible power of the atom which would eventually be harnessed for both beneficial and destructive purposes.
Taken together, the work of Becquerel, the Curies, Rutherford and others, made modern medical and scientific research more than a dream. They made it a reality with many applications. A look at the use of isotopes reveals just some of the ways in which the pioneering work of these scientists has been utilized.
Applications: Isotopes in Research and Medicine
Scientists can now create radioactive forms of common elements, called isotopes. Each isotope has a fixed rate of decay which can be characterized by its half-life, or the length of time that it takes half of the radioactive atoms in a sample to decay. Because each isotope decays at a unique and predictable rate, different isotopes can be used for a variety of purposes. For example, isotopes play an important role in modern medicine. They can be ingested and traced in their path through the body, revealing biochemical and metabolic processes with precision. These isotropic “tracers” are currently used for practical diagnosis of disease as well as in research.
The dating of radioactive carbon has helped to define the history of life on this planet. Any living organism takes in both radioactive and non-radioactive carbon, either through the process of photosynthesis or by eating plants or eating animals that have eaten plants. When the animal dies, however, uptake of carbon stops. As a result, radioactive carbon atoms are not replaced as they decay, and the amount of this material decreases over time. The rate of decrease is predictable and can be described with accuracy, vastly increasing our ability to date the biological events of our planet.
Conclusion: The Contradictions of Radioactivity
Radiation is a two edged sword: its usefulness in both medicine and anthropological and archaeological studies is undisputed, yet the same materials can be used for destruction. Human curiosity drove inquiring scientists to harness the power of the atom. Now humankind must accept the responsibility for the appropriate and beneficial uses of this very powerful tool.

Yorum Yazın 25.12.2008