Nucleus, Radioactivity, & Nuclear

MedicineDr. Michael P. Gillespie

Radioactive

Natural Radioactivity

• Radioactivity is the process by which some atoms emit energy and particles.

• The energy and particles are termed radiation.

• Radioactivity is a nuclear event: matter and energy released during this process come from the nucleus.

Radioactive Atim

Types of Radiation

• Three types of radiation are emitted by unstable nuclei:• Alpha particles• Beta particles• Gamma rays

Alpha Particles α

• Alpha particles consists of 2 protons and 2 neutrons.

• They have no electrons and therefore have a +2 charge.

• They have a relatively large mass and are slow moving. Traveling at approximately 5-10% the speed of light.

• They can be stopped by barriers as thin as a few pages of paper.

Alpha Particle Decay

Beta Particles β

• A beta particle is a fast moving electron. Traveling at approximately 90% the speed of light.

• It is formed in the nucleus by the conversion of a neutron into a proton.

• They are more penetrating and are stopped only by more dense materials such as wood, metal, or several layers of clothing.

Beta Particle Decay

Gamma Rays γ

• Gamma rays are the most energetic part of the electromagnetic spectrum and result from nuclear processes.

• Electromagnetic radiation has no protons, neutrons, or electrons. Unlike alpha and beta particles, gamma rays have no matter.

• Gamma radiation is highly energetic and the most penetrating form of nuclear radiation.

• Barriers of lead, concrete, or a combination of the two are required to stop gamma rays.

• Travels at the speed of light.

Gamma Particle Decay

Penetration

Radioactive Decay

Properties of Alpha, Beta, and Gamma

RadiationName and Symbol

Identity Charge Mass (amu)

Velocity Penetration

Alpha α Helium nucleus

+2 4.0026 5-10% speed of light

Low

Beta β Electron -1 0.000549 90% speed of light

Medium

Gamma γ Radiant Energy

0 0 Speed of light

High

Nuclear Structure and Stability

• A measure of nuclear stability is the binding energy of the nucleus. The binding energy is the amount of energy required to break a nucleus up into its component protons and neutrons.

• The binding energy must be very large to overcome the extreme repulsive forces of the positive protons for one another.

Half-Life

• The half-life is the time required for one-half of a given quantity of a substance to undergo change.

• Each isotope has its own characteristic half-life.

• The half-life can be as short as a few millionths of a second or as long as billions of years.

Nuclear Energy Production

Nucular

• George W. Bush would mispronounce the word nuclear as ‘Nucular’

Nuclear Energy Production

• Einstein predicted that when the nucleus breaks apart, the small amount of nuclear mass produces a tremendous amount of energy.

• The heat energy released converts water into steam.

• The steam turns a turbine, which drives an electrical generator, producing electricity.

Nuclear Fission

• Fission (splitting) occurs when a heavy nuclear particle is split into smaller nuclei by a smaller nuclear particle (such as a neutron).

• The splitting of the nuclear particle releases a tremendous amount of energy.

• The fission reaction, once initiated, is self-perpetuating.

• The fission process continues and intensifies. The process of intensification is referred to as a chain reaction.

Energy Transformation in a Fission Reaction

• Nucear energy heat energy mechanical energy electrical energy

Fission Chain Reaction

Nuclear Fission

Nuclear Fission

Nuclear Fusion

• Fusion (joining together) results from the combination of two small nuclei to forma larger nucleus with the concurrent release of large amounts of energy.

• The Sun is a great example of a fusion reactor.

• In fusion, two isotopes of hydrogen (deuterium and tritium) combine to produce helium, a neutron, and energy.

Nuclear Fusion

Nuclear Fusion

Nuclear Fusion

Nuclear Fusion

Nuclear Fusion

Nuclear Fusion

• No commercially successful fusion plant exists because of the containment issues.

• The fusion reaction results in temperatures in the millions of degrees and extremely high pressures. These conditions are necessary to sustain the fusion reaction.

Breeder Reactors

• A breeder reactor is a variation of a fission reactor that literally manufactures its own fuel from abundant starting materials.

• Breeder reactors cost a tremendous amount, have considerable potential to damage the environment, and create a lot of plutonium which can be used for nuclear bombs.

Breeder Reactors

Nuclear Waste Disposal

• Solid waste is difficult enough to dispose of, but nuclear waste poses even more of a challenge.

• We cannot alter the rate at which nuclear waste decays. This is determined by the half-life. Plutonium has a half-life greater than 24,000 years and it takes ten half-lives for radiation to reach background levels.

Nuclear Waste Disposal

• Where can we store hazardous, radioactive material for a quarter of a million years?

• Burial in a stable bed-rock formation seems like the best option right now, but an earthquake could release this.

Nuclear Waste Disposal

Nuclear Waste Disposal

Radiocarbon Dating

• Natural radioactivity can be utilized to establish the approximate age of archaeological, anthropological, or historical objects.

• Radiocarbon dating measures isotopic ratios of carbon to estimate the age of objects.

• Carbon-14 is formed in the upper atmosphere.

Carbon-14 Enters The Food Chain

Radiocarbon Dating

• Carbon-14 (radioactive) and carbon-12 (more abundant) are converted into living plant material through photosynthesis.

• The carbon-14 works its way into the food chain.

Radiocarbon Dating

• When a plant or animal dies, the carbon-14 slowly decreases because it is radioactive and decays to produce nitrogen.

• When an artifact is found, the relative amounts of carbon-14 to carbon-12 are used to approximate its age.

• Carbon-14 dating technique is limited to objects that are less than 50,000 years old.

Carbon Dating

Isotopes Useful In Radioactive Dating

Isotope Half-Life (years)

Upper Limit (years)

Dating Applications

Carbon-14 5730 5X104 Charcoal, organic material, artwork

Tritium 12.3 1X102 Aged wines, artwork

Potassium-40 1.3X109 Age of earth (4x109)

Rocks, planetary materials

Rhenium-187 4.3x1010 Age of earth (4x109)

Meteorites

Uranium-238 4.5x109 Age of earth (4x109)

Rocks, earth’s crust

Cancer Therapy Using Radiation

• When high energy radiation, such as gamma radiation, passes through a cell, it may collide with one of the molecules in the cell and cause it to lose one or more electrons. This leads to the production of ion pairs. Consequently, this form of radiation is referred to as ionizing radiation.

Cancer Therapy Using Radiation

• This ions are highly energetic, can damage biological molecules, produce free radicals, and damage DNA.

• This alters cell function and can even lead to cell death.

Cancer Therapy Using Radiation

• An organ that is cancerous has both healthy cells and malignant cells.

• The tumor cells are undergoing cell division more rapidly and are therefore more susceptible to gamma radiation.

Cancer Therapy Using Radiation

• Carefully targeted high doses of gamma radiation will kill more abnormal cells than normal cells.

• This can destroy the tumor and allow the organ to survive.

• The gamma radiation can also cause cancer in the healthy cells.

Nuclear Medicine

• Medical tracers are small amounts of radioactive substances used as probes to study internal organs.

• Medical techniques that utilize tracers are referred to as nuclear imaging procedures.

Nuclear Medicine

• Certain radioactive isotopes are attracted to particular organs.

• The radioactivity emitted allows us to track the path of the tracer and obtain a picture of the organ of interest.

Magnetic Resonance Imaging (MRI)

• MRI is a noninvasive technique used to study the body.

• It uses no radioactive substances. It is quick, safe, and painless.

Magnetic Resonance Imaging (MRI)

• The patient is placed in a cavity surrounded by a magnetic field.

• An image (based on the extent of radio frequency energy absorption) is generated, stored, and sorted on a computer.

Magnetic Resonance Imaging (MRI)

Biological Effects of Radiation

• Radiation affects biological tissues.

• We must use suitable precautions when working with radiation.

• “Tolerable levels” have been established for radiation exposure.

Radiation Exposure and Safety

• Factors to consider when working with radioactive materials:• The magnitude of the Half-life• Shielding• Distance from the radioactive source• Time of exposure• Types of radiation emitted• Waste disposal

Magnitude of the Half-life

• Short half-life radioisotopes produce a larger amount of radioactivity per unit of time than larger half-life substances.

• Shorter half-life materials can be safer to work with, especially if an accident occurs.

Magnitude of the Half-life

• Radioactive isotopes will eventually decay into background radiation. This will happen faster with a shorter half-life.

• Higher levels of exposure in a short time produce a clearer image.

Shielding

• Alpha and beta particles are low in penetrating power and therefore require low levels of shielding. A lab coat and gloves are usually sufficient.

• Gamma rays have significant penetrating power. Lead, concrete, or both are required for shielding from gamma rays.

• X-rays are also very high energy and require lead and concrete shielding.

Distance from the Radioactive Source

• Radiation intensity varies inversely with the square of the distance from the source.

• Doubling the distance from the source decreases the intensity by a factor of four.

• Robot manipulators can allow us to get a greater distance between the operator and the radioactive source.

Distance from the Radioactive Source

Time of Exposure

• The effects of radiation are cumulative.

• Potential damage is directly proportional to time of exposure.

Types of Radiation Emitted

• Alpha and beta emitters are generally less hazardous than gamma rays due to differences in energy and penetrating power that require less shielding.

• Ingestion or inhalation of an alpha or beta emitter can cause serious damage over time.

Waste Disposal

• Radioactive waste is created from nuclear medicine applications, nuclear power, etc.

• Safe handling and disposal of this waste is a serious problem.

• Temporary solutions are being used, but it is necessary to find more suitable long-term storage solutions.

Waste Disposal

Waste Disposal

Measurement of Radiation

• Radiation is detected using either photographic film to create an image of the location of the radioactive substance or using a counter that measures the intensity of the radiation emitted from a source.

Nuclear Imaging

• Used in nuclear medicine.

• A radioactive isotope is administered to a patient and it concentrates on the organ of interest.

Nuclear Imaging

• Nuclear images are taken at various intervals using a film that is sensitive to the radiation being emitted.

• This creates an image on the film showing the organs uptake of the isotope over time.

Computer Imaging

• Specialized television cameras that are sensitive to the radiation emitted from a radioactive substance are used.

• A CT scanner records the interaction of x-rays with biological tissue.

Geiger Counter

• A Geiger counter is an instrument that detects ionizing radiation.

• The ionizing radiation produces a current flow in a tube filled with ionizable gas.

• The current flow is proportionate to the level of ionizing radiation.

Geiger Counter

Film Badges

Units of Radiation

• Intensity of the emitted radiation:• Curie – measures the amount of

radioactivity. Independent of the nature of radiation and its effect upon biological tissue.

• Roentgen – measures very high energy ionizing radiation (x-ray and gamma).

Units of Radiation

• Biological effects of the emitted radiation:• Rad – Radiation absorbed dosage –

measures the transfer of energy to matter due to radiation.

• Rem – Roentgen equivalent for man – describes the biological damage caused by the absorption of different kinds of radiation.

Radioactive Waste