Quantum Physics

960 Chapter 29 Nuclear PhysicsAPPLICATIONOccupational RadiationExposure LimitsAPPLICATIONIrradiation of Foodand Medical EquipmentAPPLICATIONRadioactive Tracersin MedicineAPPLICATIONRadioactive Tracers inAgricultural ResearchLow-level radiation from natural sources, such as cosmic rays and radioactiverocks and soil, delivers a dose of about 0.13 rem/year per person. The upper limitof radiation dose recommended by the U.S. government (apart from backgroundradiation and exposure related to medical procedures) is 0.5 rem/year. Many occupationsinvolve higher levels of radiation exposure, and for individuals in theseoccupations, an upper limit of 5 rem/year has been set for whole-body exposure.Higher upper limits are permissible for certain parts of the body, such as thehands and forearms. An acute whole-body dose of 400 to 500 rem results in a mortalityrate of about 50%. The most dangerous form of exposure is ingestion or inhalationof radioactive isotopes, especially those elements the body retains andconcentrates, such as 90 Sr. In some cases, a dose of 1000 rem can result from ingesting1 mCi of radioactive material.Sterilizing objects by exposing them to radiation has been going on for at least25 years, but in recent years the methods used have become safer to use and moreeconomical. Most bacteria, worms, and insects are easily destroyed by exposure togamma radiation from radioactive cobalt. There is no intake of radioactive nuclei byan organism in such sterilizing processes, as there is in the use of radioactive tracers.The process is highly effective in destroying Trichinella worms in pork, salmonellabacteria in chickens, insect eggs in wheat, and surface bacteria on fruits and vegetablesthat can lead to rapid spoilage. Recently, the procedure has been expanded to includethe sterilization of medical equipment while in its protective covering. Surgical gloves,sponges, sutures, and so forth are irradiated while packaged. Also, bone, cartilage, andskin used for grafting is often irradiated to reduce the chance of infection.TracingRadioactive particles can be used to trace chemicals participating in various reactions.One of the most valuable uses of radioactive tracers is in medicine. For example,131 I is an artificially produced isotope of iodine. (The natural, nonradioactiveisotope is 127 I.) Iodine, a necessary nutrient for our bodies, is obtained largelythrough the intake of seafood and iodized salt. The thyroid gland plays a majorrole in the distribution of iodine throughout the body. In order to evaluate theperformance of the thyroid, the patient drinks a small amount of radioactivesodium iodide. Two hours later, the amount of iodine in the thyroid gland is determinedby measuring the radiation intensity in the neck area.A medical application of the use of radioactive tracers occurring in emergencysituations is that of locating a hemorrhage inside the body. Often the location ofthe site cannot easily be determined, but radioactive chromium can identify thelocation with a high degree of precision. Chromium is taken up by red blood cellsand carried uniformly throughout the body. However, the blood will be dumped ata hemorrhage site, and the radioactivity of that region will increase markedly.The tracer technique is also useful in agricultural research. Suppose the bestmethod of fertilizing a plant is to be determined. A certain material in the fertilizer,such as nitrogen, can be tagged with one of its radioactive isotopes. The fertilizeris then sprayed onto one group of plants, sprinkled on the ground for asecond group, and raked into the soil for a third. A Geiger counter is then used totrack the nitrogen through the three types of plants.Tracing techniques are as wide ranging as human ingenuity can devise. Presentapplications range from checking the absorption of fluorine by teeth to checkingcontamination of food-processing equipment by cleansers to monitoring deteriorationinside an automobile engine. In the last case, a radioactive material is usedin the manufacture of the pistons, and the oil is checked for radioactivity to determinethe amount of wear on the pistons.Computed Axial Tomography (CAT) ScansThe normal x-ray of a human body has two primary disadvantages when used as asource of clinical diagnosis. First, it is difficult to distinguish between various typesof tissue in the body because they all have similar x-ray absorption properties.