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## Radiation in a Radioactive World

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**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