Radioisotopes Applications in Nuclear Medicine and Nuclear Energy
IntroductionIn nature there are nearly 300 nuclei, consisting of differentelements and their isotopes.Isotopes are nuclei having the same number of protons butdifferent number of neutrons.Radioactivity is the release of energy and matter that resultsfrom changes in the nucleus of an atom.Radioisotopes A version of a chemical element that has anunstable nucleus and emits radiation during its decay to a stableform.
Where Do Radioisotopes Come From?Radioisotopes come from: Nature, such as radon in the air or radium in the soil. Nuclear reactors by bombarding atoms with high-energy neutrons.
Radioisotopes radiationThree predominant types of radiation are emitted by radioisotopes: 1. alpha particles 2. beta particles 3. gamma rays. The different types of radiation have different penetration powers.
The half-life of radioisotopes A half-life of a radioactive material is the time it takes one-half of the atoms of the radioisotope to decay by emitting radiation. It can vary from a fraction of a second to millions of years. The half-life of a radioisotope has implications about its use and storage and disposal.
The Basic of Nuclear Energy Nuclear power uses the energy created by controlled nuclear reactions to produce electricity or uncontrolled nuclear reaction to be used in nuclear weapons.
Nuclear Fission Nuclear fission is the splitting of an atoms nucleus into parts by capturing a neutron. It is the most commonly used nuclear reaction for power generation. Nuclear fission produces heat (also called an exothermic reaction), and electromagnetic radiation, and it produces large amounts of energy that can be utilized for power.
Nuclear Fission Fission produces neutrons which can then be captured by other atoms to continue the reaction (chain reaction) with more neutrons being produce at each step. If too many neutrons are generated, the reaction can get out of control and an explosion can occur. To prevent this from occurring, control rods that absorb the extra neutrons are interspersed with the fuel rods. Uranium-235 is the most commonly used fuel for fission.
Nuclear Fusion Nuclear fusion is another method to produce nuclear energy. Two light elements, like tritium and deuterium, are forced to fuse and form helium and a neutron. This is the same reaction that fuels the sun and produces the light and heat. Unlike fission, fusion produces less energy, but the components are more abundant and cheaper than uranium.
Nuclear weapons Nuclear weapons, like conventional bombs, are designed to cause damage through an explosion, i.e. the release of a large amount of energy in a short period of time. In conventional bombs the explosion is created by a chemical reaction, which involves the rearrangement of atoms to form new molecules. In nuclear weapons the explosion is created by changing the atoms themselves - they are either split or fused to create new atoms.
Nuclear MaterialsNuclear materials are the key ingredients in nuclear weapons. They include:1. Fissile materials: which are composed of atoms that can be split by neutrons in a self-sustaining chain-reaction to release energy, and include plutonium-239 and uranium-235.
Nuclear Materials2.Fussionable materials: In which the atoms can be fused in order to release energy, and include deuterium and tritium.3.Source materials: Which are used to boost nuclear weapons by providing a source of additional atomic particles for fission. They include tritium, polonium, beryllium, lithium-6 and helium-3.
Uranium When refined, uranium (U) is a silvery white, weakly radioactive metal. Uranium has an atomic number of 92 which means there are 92 protons and 92 electrons in the atomic structure. U-238 has 146 protons in the nucleus, but the number of neutrons can vary from 141 to 146. It is the principle fuel for nuclear reactors, but it also used in the manufacture of nuclear weapons.
Uranium Isotopes Natural uranium consists of three major isotopes: 1.uranium-238 (99.28% natural abundance). 2.uranium-235 (0.71%). 3.and uranium-234 (0.0054%).
Uranium Isotopes All three are radioactive, emitting alpha particles, with the exception that all three of these isotopes have small probabilities of undergoing spontaneous fission, rather than alpha emission. Table : Half-lives of Uranium Isotopes
Enriched uranium Enriched uranium is a kind of uranium in which the percent composition of uranium-235 has been increased through the process of isotope separation. Natural uranium is 99.284% 238U isotope, with 235U only constituting about 0.711% of its weight. 235Uis the only isotope existing in nature (in any appreciable amount) that is fissile with thermal neutrons.
Enriched uranium Enriched uranium is a critical component for both civil nuclear power generation and military nuclear weapons. The 238U remaining after enrichment is known as depleted uranium (DU), and is considerably less radioactive than even natural uranium.
Uranium enrichment gradesSlightly enriched uranium (SEU)Slightly enriched uranium (SEU) has a 235U concentration of 0.9% to 2%. This new grade is being used to replace natural uranium (NU) fuel in some reactorsLow-enriched uranium (LEU)Low-enriched uranium (LEU) has a lower than 20% concentration of 235U.
Uranium enrichment gradesHighly enriched uranium (HEU) Highly enriched uranium (HEU) has a greater than 20% concentration of 235U or 233U. The fissile uranium in nuclear weapons usually contains 85% or more of 235U known as weapon(s)-grade. HEU is also used in fast neutron reactors, whose cores require about 20% or more of fissile material, as well as in naval reactors, where it often contains at least 50% 235U, but typically does not exceed 90%. Significant quantities of HEU are used in the production of medical isotopes, for example molybdenum-99 for technetium-99m generators.
First nuclear weapon in historyTwo major types of atomic bombs were developed by the United States during World War II:1. A uranium-based device (codenamed "Little Boy") whose fissile material was highly enriched uranium.2. A plutonium-based device (codenamed "Fat Man") whose plutonium was derived from uranium-238.
First nuclear weapon in history The uranium-based Little Boy device became the first nuclear weapon used in war when it was detonated over the Japanese city of Hiroshima on 6 August 1945. Exploding with a yield equivalent to 12,500 tonnes of TNT, the blast and thermal wave of the bomb destroyed nearly 50,000 buildings and killed approximately 75,000 people.
Fig: The mushroom cloud over Hiroshima after the dropping of the uranium-based atomic bomb nicknamed Little Boy (1945)
Uranium Nuclear Fission A team led by Enrico Fermi in 1934 observed that bombarding uranium with neutrons produces the emission of beta rays. Uranium-235 was the first isotope that was found to be fissile. Upon bombardment with slow neutrons, its uranium-235 isotope will most of the time divide into two smaller nuclei, releasing nuclear binding energy in the form of warmth and radiation and more neutrons.
Uranium Nuclear Fission If these neutrons are absorbed by other uranium-235 nuclei, a nuclear chain reaction occurs that may be explosive unless the reaction is slowed by a neutron moderator, absorbing them. As little as (7 kg) of uranium-235 can be used to make an atomic bomb.
Fig: An induced nuclear fission event involving uranium-235
What Happens When People Are Exposedto Radiation? Radiation can affect the body in a number of ways, and the adverse health effects of exposure may not be apparent for many years. These adverse health effects can range from mild effects, such as skin reddening, to serious effects such as cancer and death, depending on the amount of radiation absorbed by the body (the dose), the type of radiation, the route of exposure, and the length of time a person was exposed. Exposure to very large doses of radiation may cause death within a few days or months. Exposure to lower doses of radiation may lead to an increased risk of developing cancer or other adverse health effects later in life.
Gulf War syndrome Gulf war syndrome (GWS) or Gulf War illness (GWI) affects veterans and civilians who were near conflicts during or downwind of a chemical weapons depot demolition, after the 1991 Gulf War. Approximately 250,000 of the 697,000 veterans who served in the 1991 Gulf War are afflicted with enduring chronic multi- symptom illness, a condition with serious consequences. Epidemiological evidence is consistent with increased risk of birth defects in the offspring of persons exposed to depleted uranium.
Signs and symptomsA wide range of acute and chronic symptoms have included: fatigue loss of muscle control headaches dizziness and loss of balance memory problems muscle and joint pain indigestion skin problems immune system problems birth defects
Depleted uranium exposure effect on gulf warveterans Depleted uranium (DU) was widely used in tank kinetic energy penetrator and autocannon rounds for the first time in the Gulf War. DU is a dense, weakly radioactive metal. After military personnel began reporting unexplained health problems in the aftermath of the Gulf War, questions were raised about the health effect of exposure to depleted uranium. Depleted uranium aerosol particles, if inhaled, would remain undissolved in the lung for a great length of time and thus could be detected in urine. Uranyl ion contamination has been found on and around depleted uranium targets.
Depleted uranium exposure effect on gulf warveterans Several studies confirmed the presence of DU in the urine of Gulf War veterans. The use of DU in munitions is controversial because of questions about potential long-term health effects. DU has recently been recognized as a neurotoxin. Epidemiological evidence is consistent with increased risk of birth defects in the offspring of persons exposed to DU.
Radiation effects from Fukushima Inuclear accidents The radiation effects from the Fukushima I nuclear accidents are the results of release of radioactive isotopes from the Fukushima Nuclear Power Plant after the 2011 Tōhoku earthquake and tsunami. This occurred due to both deliberate pressure-reducing venting, and through accidental and uncontrolled releases. These conditions resulted in unsafe levels of radioactive contamination in the air, in drinking water, milk and on certain crops in the vicinity of the prefectures closest to the plant.
Fig: The explosion at the Fukushima nuclear plant.
Isotopes of possible concern The isotope iodine-131 is easily absorbed by the thyroid. Persons exposed to releases of I-131 from any source have a higher risk for developing thyroid cancer or thyroid disease, or both. Iodine-131 has a short half-life at approximately 8 days. Children are more vulnerable to I-131 than adults. Increased risk for thyroid neoplasm remains elevated for at least 40 years after exposure.
Isotopes of possible concern Caesium-137 is also a particular threat because it behaves like potassium and is taken-up by the cells throughout the body. Cs-137 can cause acute radiation sickness, and increase the risk for cancer because of exposure to high-energy gamma radiation. Internal exposure to Cs-137, through ingestion or inhalation, allows the radioactive material to be distributed in the soft tissues, especially muscle tissue, exposing these tissues to the beta particles and gamma radiation and increasing cancer risk.
Isotopes of possible concern Strontium-90 behaves like calcium, and tends to deposit in bone and blood-forming tissue (bone marrow). 20–30% of ingested Sr-90 is absorbed and deposited in the bone. Internal exposure to Sr-90 is linked to bone cancer, cancer of the soft tissue near the bone, and leukemia.
Isotopes of possible concern Plutonium is also present in the fuel of the Unit 3 reactor and in spent fuel rods, although there has been no indication that plutonium has been detected outside the reactors. Plutonium-239 is particularly long-lived and toxic with a half- life of 24,000 years, and if it escaped in smoke from a burning reactor and contaminated soil downwind, it would remain hazardous for tens of thousands of years.
Nuclear medicine Nuclear medicine is a special field of medicine in which radioactive materials are used for: 1. Conducting medical research. 2. generating diagnostic information relating to functioning of specific organs. 3. Therapeutic treatment of ailing organs. The radioactive materials used are generally called radionuclides or radioactive tracers, meaning a form of an element that is radioactive (radioisotops). Technetium-99m is a reactor-produced radioisotop that is used in more than 80% of nuclear medicin procedures worldwide.
RadionuclidesRadionuclides are powerful tools for diagnosing medical disorders for three reasons: 1. Many chemical elements tend to concentrate in one part of the body or another. 2. The radioactive form of an element behaves biologically in exactly the same way that a nonradioactive form of the element behaves. 3. Any radioactive material spontaneously decays, breaking down into some other form with the emission of radiation. That radiation can be detected by simple, well-known means.
Important factors to consider when choosing aradioisotope for medical use1. It must emit gamma rays only: Gamma rays pass through the body, which means they can be detected with a gamma camera. Alpha particles would not be able to penetrate through the skin so they could not be detected. Gamma rays do not ionize cells inside the body so no damage is caused. Alpha particles and beta particles would ionize cells, which could lead to the formation of cancer cells.
Important factors to consider when choosing aradioisotope for medical use 2. It must have a short half-life (typically around a few hours): A short half-life ensures that all the radiation inside the patient leaves the body quickly and does not accumulate. 3. It must be able to be easily administered to the patient. Injections and tablets are used.
Technetium Technetium is the chemical element with atomic number 43 and symbol Tc. It is a silvery-gray radioactive metal with an appearance similar to that of platinum. It is commonly obtained as a gray powder. Nearly all technetium is produced synthetically and only minute amounts are found in nature.
Technetium Its short-lived gamma ray-emitting nuclear isomer— technetium-99m—is used in nuclear medicine for a wide variety of diagnostic tests. Technetium-99 is used as a gamma ray-free source of beta particles. Long-lived technetium isotopes produced commercially are by- products of fission of uranium-235 in nuclear reactors and are extracted from nuclear fuel rods.
Technetium-99m Technetium-99m is a major product of the fission of uranium- 235 (235 92U), making it the most common and most readily available Tc isotope.
The technetium-99m decay chain A molybdemum-99 nucleus decays into a technetium-99m nucleus by beta emission. After a period of a few hours or so the technetium-99m emits a gamma ray and changes into technetium-99.
Technetium -99m Production Technetium below Uranium in the periodic element is actually the only element which does not naturally occur. Technetium -99m is produced by bombarding molybdenum 98Mo with neutrons. The resultant 99Mo decays with a half-life of 66 hours to the metastable state of Tc . This process permits the production of 99mTc for medical purposes.
Why is technetium-99m a good tag for medicalimaging?1. its a pure gamma emitter .This means that it doesnt produce more damaging alpha and beta particles and its radioactivity disappears after a few hours. Most substances arent pure gamma emitters.2. The "short" half life of the isotope (in terms of human-activity and metabolism) allows for scanning procedures which collect data rapidly, but keep total patient radiation exposure low.
Common nuclear medicine techniques usingtechnetium-99m Bone scan Myocardial perfusion imaging Cardiac ventriculography Functional brain imaging Blood pool labeling
Exposure, contamination, and elimination Radiation exposure due to diagnostic treatment involving technetium-99m can be kept low. Because technetium-99m has a short half-life and emits primarily a gamma ray, its quick decay into the far-less radioactive technetium-99 results in relatively low total radiation dose to the patient per unit of initial activity after administration, as compared to other radioisotopes. In the form administered in these medical tests (usually pertechnetate), technetium-99m and technetium-99 are eliminated from the body within a few days.