Radiation in a Radioactive World Nuclear Physics and Engineering By: Douglas Osborn
Do you think of these things as well? • Food • Space • Utilities • Consumer Products • Medicine
RADIOLOGICAL FUNDAMENTALS Atomic Structure Definitions Types of Ionizing Radiation Units of Measure
Atomic Structure • Atomic Structure Particles • Elements & Isotopes • Stable vs. Unstable • Standard Nomenclature • Ions
Nucleus Proton Neutron P+ N Nucleus Electron Atomic StructureParticles Protons (positive) Neutrons (neutral) Electrons (negative) e-
P+ P+ P+ P+ P+ P+ N N N N N N hydrogen helium lithium Elements • The number of protons in the nucleus determines the element • If the number of protons changes, the element changes
P+ P+ P+ N N N Hydrogen (deuterium) Hydrogen (protium) Hydrogen (tritium) Isotopes • Isotopes - atoms of the same element which have the same number of protons, but a different number of neutrons • Isotopes have the same chemical properties; however, the nuclear properties can be quite different
e- e- Hydrogen (protium) Hydrogen (tritium) P+ P+ N N Stable vs. Unstable Atoms If there are too many or too few neutrons for a given number of protons, the nucleus will not be stable STABLE “Non-Radioactive” UNSTABLE “Radioactive”
e- e- e- e- Positive Ions e- e- e- P+ P+ P+ P+ P+ P+ P+ P+ P+ Neutral N N N N N N N N N N N N e- e- Negative Ions Ions are atoms with positive or negative charge:
Definitions • Ionization • Radiation • Ionizing vs. Non-Ionizing • Radioactivity & Radioactive Decay • Radioactive Half-Life • Radioactive Material • Radioactive Contamination
Ionization The process of removing electrons from neutral atoms + AND Free ejected electron
ENERGY RADIATION UNSTABLE ATOM PARTICLE Radiation • Energy released from unstable atoms and some devices in the form of rays or particles • Can be either ionizing or non-ionizing
Ionizing Radiation • Radiation that possesses enough energy to cause ionization in the atoms with which it interacts • Released from unstable atoms and some devices in the form of rays or particles - alpha - beta - gamma/x-ray - neutron a b g 0n1
Non-Ionizing Radiation • Radiation that doesn’t have the amount of energy needed to ionize the atom with which it interacts • Examples: - radar waves - infrared radiation - microwaves - ultraviolet radiation - visible light
N P+ N P+ N P+ P+ N P+ P+ N P+ P+ N N P+ N P+ P+ P+ N P+ N N P+ N N N P+ e- N Excess Energy Released P+ N P+ P+ N P+ N P+ N P+ N P+ P+ P+ N P+ P+ N N N N N N P+ N P+ N P+ P+ P+ P+ P+ N P+ N N P+ P+ P+ N N P+ N N P+ P+ N N P+ P+ N P+ N N P+ N N N N Large, unstable nucleus Radioactivity The process of unstable (or radioactive) atoms becoming stable by emitting radiation. This event over time is called radioactive decay. alpha beta gamma neutron
238 234 234 206 U Th Pa Pb 92 90 91 82 b Decay Chain After 18 decays we arrive at stable:
Ni-60 Ni-60 Ni-60 Co-60 Co-60 Co-60 Co-60 Radioactive Half-Life The time it takes for one half of the radioactive atoms present to decay Example: Co-60 = 5 years 100 atoms today 50 atoms after 5 yrs 25 atoms after 10 yrs 12 atoms after 15 yrs
Radioactive Decay Develop a model for radioactive decay. Call it the radioactive decay law.
How do we describe the rate of de-energization? • Observations in Nature: • Decay / De-energization Occurs • Number of Radioactive Nuclides decreases with time • De-energization of a single nuclide is a statistical process • Let’s perform a simulation
Rules • DON’T OPEN the packages until I give you instructions !! • Need one volunteer from each table group You are the data runner. • Carefully open the package. • Pour the contents onto your desk – carefully. DO NOT EAT THEM! • Determine the total number in the bag. • Report this number to the data runner. • Count those with the “M” UP and return them to the bag. • Report this count to the data runner. • Eliminate (eat?) those not returned to the bag. • Calculate and record total counts • Shake the bag and repeat the above.
Next Question:What have we observed? • Decay / De-energization Occurs • Number of Radioactive Nuclides decreases with time • De-energization of a single nuclide is a statistical process • This being the case, at the beginning of the de-energization process when a lot of radioactive nuclides are present, the statistics are much better • Thus sample counting statistics are much better in the beginning than after most of the nuclides have de-energized • Why is this?
Counting Statistics: Randomness • De-energization events are random • Quantity per unit time depends on the total number of radioactive nuclides present • Thus the quantity decreases with time • Detection events also are random within the counting media depending on random processes associated with the detector • Probability of penetration into the detector • Probability of interaction in the detector • Variability and precision of repeated counts can be described with reasonable rigor based solely on the total number of detected events
Variability refers to the distribution of a number of repeated counts around a true value or a mean value Repeat counts follow a Poisson Distribution, but when a large number of repeat counts are taken, the Normal Distribution is a good approximation The shape of the Normal curve can be described by using only the mean, m, and the standard deviation, s or s The mean is the arithmetic average of all counts In the normal distribution, about 68% of all counts will fall within one standard deviation 95% within 1.96 standard deviations 99% within 2.58 standard deviations A property of the Poisson Distribution is that the Standard Deviation is simply the square root of the mean Counting Statistics – Variability
Precise Example of a Normal Distribution • Note the symmetry • Note how the “counts” are distributed
Counting Statistics – Precision • Precision refers to the repeatability of a single count • How close will a repeated count be to the previous count – or to the next count? • How close will one count be to the “true mean” of many repeated counts? • If we have only one count, we expect the true mean is probably different from our one count • Probability that the true mean lies within specific limits around the count is determined from the shape of the normal error curve, the Normal Distribution • The obtained (measured) count, N, is taken as the mean value, and the standard deviation, s or s, is then the square root of the measured count: • Thus there is a 68% probability that the true mean lies within one standard deviation, or the square root of the measured count • The “error” in a given count is then generally considered to be:
Counting Statistics: Precision Decision • How good is good enough in practice? • Analyzing the %Error formula clearly says that the more counts you are able to obtain, the more precise your measurement will be. • The %Error formula states there is a 68% probability that the true value lies within + one standard deviation of the single measured count • This can also be stated as being within the 68% Confidence Interval • This is a good estimate for general applications • For more precise work, it’s preferred to be within the 95% Confidence Interval • And for critical work, you may need to be within the 99% Confidence Interval
Derivation of the Radioactive Decay Law • Define • Mathematically Where N(t) is the number of radioactive nuclei present at time t • Need a constant of proportionality • Why do we have a minus sign in the formula?
Activity (Continued) Rearrange the terms
Units of Activity • Curie • The traditional unit of activity • 1 Ci = 3.7x1010 disintegrations/second • Based on the disintegration rate of 1 gm of Ra-226 • Becquerel • SI Unit • 1 Bq = 1 dis/sec
Half Life Definition Derivation => initial conditions: Half-life
Mean Lifetime • Half life is the average amount of time for half of a large sample of nuclides to de-energize • Mean lifetime is the average (statistical mean) amount of time a single nucleus exists before de-energizing • It can be shown that this is
Radioactive Decay on aLinear Scale Normalizing has been done for illustration only. It is NOT necessary!!
Radioactive Decay on aSemi-Log Scale Normalizing has been done for illustration only. It is NOT necessary!!
Summary of Concepts Activity Radioactive Decay Law (Two identical expressions) Half Life and the Radioactive Decay Constant
Radioactive Material Radioactive material is any material containing unstable atoms that emit radiation
Radioactive Contamination • Radiation is energy • Radioactive material is the physical material emitting the radiation • Radioactive contamination is radioactive material that is uncontained and in an unwanted place • Exposure to radiation does not result in contamination
Types of Ionizing Radiation • Alpha (a) - particle • Beta (b) - particle • Gamma (g) - ray • Neutron (h) - particle
Characteristics Range Shielding Hazards Sources Alpha Radiation (a) Particle, Large Mass, +2 Charge Very Short 1 - 2” in air Paper Outer layer of skin Internal Plutonium, Uranium, Americium
Characteristics Range Shielding Hazards Sources Beta Radiation (b) Particle, Small Mass, -1 Charge 12ft / MeV in air Plastic, glass, aluminum, wood Internal and the skin and eyes Tritium, Sr-90, Fission products
Characteristics Range Shielding Hazards Sources Gamma Rays (g) and X-Rays No mass, no charge electromagnetic Hundreds of feet in air Lead, Steel Concrete External Source Whole Body Penetrating Co-60, Kr-88, Cs-137
Characteristics Range Shielding Hazards Sources Neutron Radiation (h) Particle with no charge Hundreds of feet in air Hydrogenous material - water, polyethylene External Source Whole Body Penetrating Uranium, Plutonium, Californium
Units of Measure Energy • Radiation Roentgen, RAD, REM • Radioactivity Rate dpm, Curie • Contamination Spread Radioactivity Area or volume
Wilhelm Roentgen 1845 -1923 Discovered X-rays Roentgen (R) • Unit for measuring exposure • Defined only for ionization in air • Applies only to gamma and x-rays • Not related to biological effects