Irradiation and contamination

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Abraham Sparks
6 years ago
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1 Irradiation and contamination Irradiation Irradiation means exposure to radiation. Science uses the word radiation to refer to anything that travels in straight lines (rays) from a source. Therefore, irradiation could mean exposure to ionizing radiation or to nonionizing radiation. In most cases, when the word irradiation is used, it is used to refer to exposure to ionizing radiation. However, most non-scientists do not make this connection. (2,25,26) In my classroom, students predicted that lighted light bulbs would produce more radioactivity that unlighted ones. They would check their idea by using a geiger counter to measure the background radiation in the room (see Extension 20.1, Background radiation). They found a background of about 0.2 counts per second. When they measured the unlighted light bulb, they found a result indistinguishable from the background, which was not a surprise to most of them. When they measured the lighted light bulb, they also found that the result was indistinguishable from background, which really was a surprise to them. In fact, in some cases, the number of counts from the lighted light bulb was below background (not significantly), and those groups were the most surprised of all, although after thought about the random nature of radioactivity, they lost their surprise. Prather and Harrington found a similar result. (25) My students were also surprised that TV screens did not register as radioactive sources. They did find that some plates (orange Fiestaware TM ) were sources of radioactivity and that others were not.

2 Energy, Ch. 20, extension 3 Irradiation and contamination 2 This means that ionizing radiation is emitted in specific cases and not in others. We can detect the radiation with geiger counters (or some other method). Irradiation occurs only when material is brought into the vicinity of or touches a source of radioactive emanations. Contamination Contamination means that radioactive material has been mixed or inserted into nonradioactive material. Prather (Ref. 25) suggested in building on the work of others, particularly R. Millar, (27) that an experiment be done to find a source of radiation and expose material by contact. When students did this in my laboratory, they were again surprised that the material itself did not become radioactive, as measured by the geiger counter. But a material seldom becomes radioactive itself from contact with the emanations of radioactivity. It is possible for material exposed to neutrons (or protons) to become radioactive themselves. This is the case in our world only near particle accelerators and in reactors (see Extension 19.6, Effects of irradiation on materials), or near fissioning material (because of the large number of neutrons produced). Irradiation and contamination are two different things. Irradiation exposes materials to the decay products of radioactive decay, and may change the matter in its path by knocking electrons out of atoms there (ionization, which is why it s called ionizing radiation). This can have deleterious effects, especially if the material is a living cell. A person could be exposed to x rays for medical purposes, for example. Cells could be changed, but the person will not become radioactive as a result.

3 Energy, Ch. 20, extension 3 Irradiation and contamination 3 Contamination involves having radioactive material become part of another material. As an example of contamination, a person could inhale a radioactive dust, or eat a radioactive material. A plant could use strontium-90 in place of calcium. In each case, the thing becomes radioactive by having radioactive material become a part of it. Much contamination occurs as part of medical procedures. The cost in exposure to radioactivity is deemed in these cases to be worth the benefits brought on by the treatment. Examples include tumor suppression, seeds used to treat prostate cancer, treatment of coronary arteries, treatment for rheumatoid arthritis, (28) and radioactive iodine used to treat thyroid tumors. See Refs. 29 and 30 for a review of some medical uses of radioactivity. Food irradiation Millions of Americans, and many more millions the world over, sicken each year because of the food they eat. Irradiation of food kills resident Staphylococcus aureus, Campylobacter jejuni, and the nasty strain of gut bacterium Escheria coli 0157:H7 as well as Salmonella and many other pathogens. Those with long memories may recall the 1993 Jack in the Box deaths from E. coli 0157:H7. The bacteria killed 4 and made over seven hundred people ill. (31) Research over the years has shown how safe and effective irradiation is. (32) The World Health Organization has pointed out that irradiation at exposures as high as 75 kgy results in safe and wholesome food. (32) However, many food markets refuse to try to sell irradiated meats, spices, or vegetables because they are afraid that the public will confuse irradiation with contamination and become alarmed. (32,33) The FDA was asked to allow food that is irradiated to be labeled cold pasteurization, or electronic pasteurization and agreed. (33) Three methods are

4 Energy, Ch. 20, extension 3 Irradiation and contamination 4 currently allowed in food irradiation: gamma rays, x rays, and high energy electron beams. (34-36) Poultry products are especially susceptible to harboring disease-causing agents such as listeria, and irradiation would help preserve health relatively cheaply, not only in America, but also in many parts of the world where the problem of food-borne illness is much worse. (37) Why is there not a greater push, when it is clear that irradiation would save lives? Congress is just as confused as the general public, and so have defined irradiation as a food additive. (32) This forces users to demonstrate safety product by product, and according to Ref. 32, directs FDA to address the wrong question whether irradiation is safe rather than whether food irradiation reduces risks to public health. In addition, the process is clumsy and time consuming. But well-publicized deaths among the public from eating food containing disease pathogens and the resulting backlash against the food industry are leading those companies to give irradiation a second look. Only about 5% of beef was irradiated as of 2003, but the amount is expected to grow swiftly. (34) The SureBeam Company, largest American irradiator, experienced greatly increased business after the Pilgrim Poultry recall, but had to idle a plant because of insufficient business. (34,35,38) SureBeam uses both x rays and electron beams. The company runs a truckload of food through the irradiator and reloads the cargo within 75 minutes of arrival. The treatment adds about 5 cents per pound to the cost. (35)

5 Energy, Ch. 20, extension 3 Irradiation and contamination 5 There may be some small amount of discoloration in some meats as a result of irradiation, but it does not affect the taste. It is not only meat that can be irradiated. Lettuce has been tested. (39) Sunflower oil can be preserved from rancidity. (40) The first use of irradiated fruits may be in allowing tropical fruits into the U. S. while assuring that any living pest is eliminated. Candidate imports include apples, mangos, papayas, rambutans, and mangosteens. (36,41) In one test, irradiated apples fared better than natural apples after 3.5 months storage. (41) Only more knowledge can solve the problem of public incomprehension. Activation In some cases, irradiation can become activation, which occurs when a particle collides with a nucleus and makes it radioactive. In many cases it is a neutron that accomplishes this task because it is neutral and can approach the nucleus without being deflected as a charged particle might. The particle must strike a nucleus and be absorbed for the nucleus to become radioactive, and only some nuclei will become radioactive when another neutron is added. In most cases, activation is accomplished by placing an object within a reactor vessel, which is full of neutrons. Beams of protons or neutrons can also lead to activation. Ordinary products of radioactivity are not able to make a nucleus radioactive.

Fission is the process by which energy is released in the nuclear reactor. Figure 1. Figure 2

Q1.Electricity is generated in a nuclear power station. Fission is the process by which energy is released in the nuclear reactor. (a) Figure 1 shows the first part of the nuclear fission reaction. Complete