Chapter 32: Nuclear Physics and Nuclear Radiation
Applications

RADIOTRACERS

A Bright Idea

In the 1920's radiation and radioactivity were a hot topic in physics and chemistry. There is a story about a physics student named Georg von Hevesy who was suspicious of his landlady's culinary practices. The Sunday meal reminded him of the dishes she served during the week. He suspected that the Sunday meatloaf was filled with the weekly leftovers. The landlady indignantly denied any such scheme.

To track this down, he one day sprinkled a large piece of meat with Radium and left it on his plate. The following Sunday, when the meatloaf was put on the table, he pulled out his home made radiation detector and confronted the surprised landlady with scientific proof of his charge.
Von Hevesy just invented one of the most versatile non-invasive scientific and medical diagnostic tool, a radioactive tracer. By tagging the meat with a trace amount of a radioactive substance, he could find the meat again later, even though it had been ground up and buried in the meatloaf.

Von Hevesy applied the idea to his research in plant biology and in 1943 won the Nobel Prize for his pioneering work with radioactive tracers.

Nuclear Medicine

Nuclear Medicine is a medical specialty that works with trace amounts of trace amounts of radioactive materials to investigate the organs' functioning and structure. Radioactive materials are also used in therapy.

Nuclear medicine traces its history to the discovery of x-rays and of radioactivity in 1898 and to the discovery of so called "artificial" radioactivity in 1934. The distinction between "natural" and "artificial" radioactivity is important to nuclear medicine. The term natural radioactivity applies to naturally occurring isotopes of chemical elements that are radioactive. The term artificial radioactivity applies to radioactive isotopes that are created in the laboratory, typically by altering the nucleus of a non-radioactive isotope. Usually this occurs in some type of accelerator.
Nuclear medicine and other biological applications of tracer technology are usually interested in tracking a biologically significant chemical compound as it winds its way though a living system. They accomplish this task by replacing some atoms in the compound by the radioactive isotopes of the same element. Since the element they need may not have a naturally occurring isotope it is important that such isotopes be created in the laboratory. Such radiopharmaceuticals usually have very short half lives (that is why they don't exist in nature.)
Many people consider the Donner lab at Berkeley the birthplace of nuclear medicine . Donner lab was founded by Dr. John Lawrence pictured on the right (courtesy of the Earnest Orlando Lawrence Berkeley National Laboratory.) In 1937 Dr. Lawrence used cyclotron made radioactive phosphate to treat a patient with leukemia, a nuclear medicine first.
Berkeley lab was the place where many radioisotopes were created or discovered. Among them are: iodine-131 with which hyperthyroidism was first discovered and treated in 1941, carbon-14, the indispensable dating tool, and the most commonly used isotope in medicine, technetium-99m, used for brain, bone, liver, spleen and kidney imaging and for blood flow studies..

The Carbon-14 Story

Carbon compounds are the stuff of life. To be able to follow carbon atoms as they wind their way through living tissue would change the work of a biochemist in a fundamental way. The discovery of carbon-14 in 1938 by Martin Kamen was such a fundamental event.

Prof.Kamen, pictured here with an early sample of carbon-14 (courtesy of the Earnest Orlando Lawrence Berkeley National Laboratory), used the Lawrence cyclotron to manufacture carbon-11 to trace biological and chemical processes. Carbon-11 has a short half-life (21 minutes). Kamen searched for the longer lived isotope, carbon-14, whose existence had been predicted since 1934. He succeeded in preparing large enough samples of the isotope to determine its half-life (5700 years) and its other physical properties. Uses trace elements of radioactive carbon Kamen showed studies such fundamental processes as photosynthesis and CO₂ assimilation. During the photosynthesis green plant absorb water and carbon dioxide and give of molecular oxygen (O₂) which is essential for life as we know it. By tagging the carbon Kamen showed that the oxygen plants give off comes from water and not from carbon dioxide as previously thought.

In 1947 Willard Libby suggested that the relative amount of Carbon-14 in old organic objects could be used to estimate their age. He received the Nobel Prize in 1960 for his development of carbon dating. Carbon-14 is an indispensable tool of modern chemists, biochemists, medical researchers, archeologists, geologists and environmental scientists. Carbon-14 dating techniques enable researchers to determine the age of archeological and anthropological finds as old as 60,000 years. Artifact scholars are using carbon-14 to check the authenticity of ancient art objects and relics, such as the Shroud of Turin. (follow Further Study link #13 for the latest on the Shroud of Turin controversy.)

The Versatility of Radioactive Tracers

Radioisotopes are a remarkably versatile tool. Here is a partial list showing the great variety of radioisotope use:

Americium-241 is used in smoke alarms , to determine the level of lead in paints, to regulate the thickness in the production of rolled products such as steel and paper.
Calcium-47 is used in the study of bone formation.

Californium-252 is used to inspect airline luggage for explosives and to gauge the moisture content in road building and the construction industry.
Cesium-137 is used in cancer treatment. It is also used to monitor the flow of oil in pipelines and to monitor the fill level in the packaging of food, drugs and other products.
Two isotopes of iodine are used in the diagnose and treatment of thyroid disorders.
Iridium-192 is used to analyze the welds in pipelines, aircraft parts and other metal products.

Promethium-147 is used in electric blanket thermostats and to control the thickness in the production of thin plastics, sheet metal, textiles and paper.
Technetium-99m is the most widely used radioisotope in medicine. It is used in imaging the brain , bone, liver, spleen and the kidneys.
Tritium, hydrogen-3, is used in the life sciences and as the source of radiation for luminous dials, gauges and luminous paint.

Risks vs. Benefits

Whenever living tissue is exposed to any kind of radiation there is some probability that the tissue will be damaged. The scientific community has developed elaborate procedures to assess the risk from radiation.

We are all subjected to radiation that is present in the environment. Radiation comes form outer space, there are radioactive substances in the soil we come in contact with, in the food and water we drink and in the air we breathe. The amounts of radiation we are exposed during a medical procedure is usually a realtively small fraction of the radiation burden we get from the environment.
We have become more aware of the dangers associated with radiation and we no longer routinely expose ourselves for trivial reasons. Not very long ago shoe stores featured x-ray machines so people could watch their toe bones wiggle in their new shoes.
Properly used, radioactive materials can perform valuable services where the benefits far outweigh the inevitable risks.

Some Further Study Links:

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