Radioactivity 29.3

1. Radioactivity 29.3 • Radioactivity is the spontaneous emission of radiation • Experiments suggested that radioactivity was the result of the decay, or disintegration, of unstable nuclei

2. Radioactivity – Types • Three types of radiation can be emitted • Alpha particles • The particles are 4He nuclei • Beta particles • The particles are either electrons or positrons • A positron is the antiparticle of the electron • It is similar to the electron except its charge is +e • Gamma rays • The “rays” are high energy photons

3. Distinguishing Types of Radiation • A radioactive beam is directed into a region with a magnetic field • The gamma particles carry no charge and they are not deflected • The alpha particles are deflected upward • The beta particles are deflected downward • A positron would be deflected upward

4. Penetrating Ability of Particles • Alpha particles • Barely penetrate a piece of paper • Beta particles • Can penetrate a few mm of aluminum • Gamma rays • Can penetrate several cm of lead

5. The Decay Constant 29.4 • The number of particles that decay in a given time is proportional to the total number of particles in a radioactive sample • ΔN = -λ N Δt • λ is called the decay constant and determines the rate at which the material will decay • The decay rate or activity, R, of a sample is defined as the number of decays per second

6. Decay Curve • The decay curve follows the equation • N = No e- λt • The half-life is also a useful parameter • The half-life is defined as the time it takes for half of any given number of radioactive nuclei to decay

7. Units A • The unit of activity, R, is the Curie, Ci • 1 Ci = 3.7 x 1010 decays/second • The SI unit of activity is the Becquerel, Bq • 1 Bq = 1 decay / second • Therefore, 1 Ci = 3.7 x 1010 Bq • The most commonly used units of activity are the mCi and the µCi

8. Alpha Decay • When a nucleus emits an alpha particle it loses two protons and two neutrons • N decreases by 2 • Z decreases by 2 • A decreases by 4 • Symbolically • X is called the parent nucleus • Y is called the daughter nucleus

9. Alpha Decay – Example • Decay of 226 Ra • Half life for this decay is 1600 years • Excess mass is converted into kinetic energy • Momentum of the two particles is equal and opposite

10. Decay – General Rules • When one element changes into another element, the process is called spontaneous decay or transmutation • The sum of the mass numbers, top #, must be the same on both sides of the equation • The sum of the atomic numbers, bottom #, must be the same on both sides of the equation • Conservation of mass-energy and conservation of momentum must hold

11. Beta Decay • During beta decay, the daughter nucleus has the same number of nucleons as the parent, but the atomic number is changed by one • Symbolically

12. Beta Decay, cont • The emission of the electron is from the nucleus • The nucleus contains protons and neutrons • The process occurs when a neutron is transformed into a proton and an electron • Energy must be conserved

13. Beta Decay – Electron Energy • The energy released in the decay process should almost all go to kinetic energy of the electron (KEmax) • Experiments showed that few electrons had this amount of kinetic energy

14. Neutrino • To account for this “missing” energy, in 1930 Pauli proposed the existence of another particle • Enrico Fermi later named this particle the neutrino • Properties of the neutrino • Zero electrical charge • Mass much smaller than the electron, probably not zero • Spin of ½ • Very weak interaction with matter

15. Beta Decay – Completed • Symbolically •  is the symbol for the neutrino • is the symbol for the antineutrino • To summarize, in beta decay, the following pairs of particles are emitted • An electron and an antineutrino • A positron and a neutrino

16. Gamma Decay • Gamma rays are given off when an excited nucleus “falls” to a lower energy state • Similar to the process of electron “jumps” to lower energy states and giving off photons • The photons are called gamma rays, very high energy relative to light • The excited nuclear states result from “jumps” made by a proton or neutron • The excited nuclear states may be the result of violent collision or more likely of an alpha or beta emission

17. Gamma Decay – Example • Example of a decay sequence • The first decay is a beta emission • The second step is a gamma emission • The C* indicates the Carbon nucleus is in an excited state • Gamma emission doesn’t change either A or Z

18. Enrico Fermi • 1901 – 1954 • Produced transuranic elements • Other contributions • Theory of beta decay • Free-electron theory of metals • World’s first fission reactor (1942) • Nobel Prize in 1938

19. Uses of Radioactivity • Carbon Dating • Beta decay of 14C is used to date organic samples • The ratio of 14C to 12C is used • Smoke detectors • Ionization type smoke detectors use a radioactive source to ionize the air in a chamber • A voltage and current are maintained • When smoke enters the chamber, the current is decreased and the alarm sounds

20. More Uses of Radioactivity • Radon pollution • Radon is an inert, gaseous element associated with the decay of radium • It is present in uranium mines and in certain types of rocks, bricks, etc that may be used in home building • May also come from the ground itself

21. Natural Radioactivity 29.5 • Classification of nuclei • Unstable nuclei found in nature • Give rise to natural radioactivity • Nuclei produced in the laboratory through nuclear reactions • Exhibit artificial radioactivity • Three series of natural radioactivity exist • Uranium • Actinium • Thorium • See table 29.2

22. Decay Series of 232Th • Series starts with 232Th • Processes through a series of alpha and beta decays • Ends with a stable isotope of lead, 208Pb

23. Nuclear Reactions 29.6 • Structure of nuclei can be changed by bombarding them with energetic particles • The changes are called nuclear reactions • As with nuclear decays, the atomic numbers and mass numbers must balance on both sides of the equation

24. Nuclear Reactions – Example • Alpha particle colliding with nitrogen: • Balancing the equation allows for the identification of X • So the reaction is

25. Q Values • Energy must also be conserved in nuclear reactions • The energy required to balance a nuclear reaction is called the Q value of the reaction • An exothermic reaction • There is a mass “loss” in the reaction • There is a release of energy • Q is positive • An endothermic reaction • There is a “gain” of mass in the reaction • Energy is needed, in the form of kinetic energy of the incoming particles • Q is negative

26. Threshold Energy • To conserve both momentum and energy, incoming particles must have a minimum amount of kinetic energy, called the threshold energy • m is the mass of the incoming particle • M is the mass of the target particle • If the energy is less than this amount, the reaction cannot occur

27. Radiation Damage in Matter • Radiation absorbed by matter can cause damage • The degree and type of damage depend on many factors • Type and energy of the radiation • Properties of the absorbing matter • Radiation damage in biological organisms is primarily due to ionization effects in cells • Ionization disrupts the normal functioning of the cell

28. Types of Damage • Somatic damage is radiation damage to any cells except reproductive ones • Can lead to cancer at high radiation levels • Can seriously alter the characteristics of specific organisms • Genetic damage affects only reproductive cells • Can lead to defective offspring

29. Units of Radiation Exposure • Roentgen [R] • That amount of ionizing radiation that will produce 2.08 x 109 ion pairs in 1 cm3 of air under standard conditions • That amount of radiation that deposits 8.76 x 10-3 J of energy into 1 kg of air • Rad (Radiation Absorbed Dose) • That amount of radiation that deposits 10-2 J of energy into 1 kg of absorbing material

30. More Units • RBE (Relative Biological Effectiveness) • The number of rad of x-radiation or gamma radiation that produces the same biological damage as 1 rad of the radiation being used • Accounts for type of particle which the rad itself does not • Rem (Roentgen Equivalent in Man) • Defined as the product of the dose in rad and the RBE factor • Dose in rem = dose in rad X RBE

31. Radiation Levels • Natural sources – rocks and soil, cosmic rays • Background radiation • About 0.13 rem/yr • Upper limit suggested by US government • 0.50 rem/yr • Excludes background and medical exposures • Occupational • 5 rem/yr for whole-body radiation • Certain body parts can withstand higher levels • Ingestion or inhalation is most dangerous

32. Applications of Radiation 29.7 • Sterilization • Radiation has been used to sterilize medical equipment • Used to destroy bacteria, worms and insects in food • Bone, cartilage, and skin used in graphs is often irradiated before grafting to reduce the chances of infection

33. Applications of Radiation, cont • Tracing • Radioactive particles can be used to trace chemicals participating in various reactions • Example, 131I to test thyroid action • CAT scans • Computed Axial Tomography • Produces pictures with greater clarity and detail than traditional x-rays

34. Applications of Radiation, final • MRI • Magnetic Resonance Imaging • When a nucleus having a magnetic moment is placed in an external magnetic field, its moment processes about the magnetic field with a frequency that is proportional to the field • Transitions between energy states can be detected electronically

35. Radiation Detectors 29.8 • A Geiger counter is the most common form of device used to detect radiation • It uses the ionization of a medium as the detection process • When a gamma ray or particle enters the thin window, the gas is ionized • The released electrons trigger a current pulse • The current is detected and triggers a counter or speaker

36. Detectors, 2 • Semiconductor Diode Detector • A reverse biased p-n junction • As a particle passes through the junction, a brief pulse of current is created and measured • Scintillation counter • Uses a solid or liquid material whose atoms are easily excited by radiation • The excited atoms emit visible radiation as they return to their ground state • With a photomultiplier, the photons can be converted into an electrical signal

37. Detectors, 3 • Track detectors • Various devices used to view the tracks or paths of charged particles • Photographic emulsion • Simplest track detector • Charged particles ionize the emulsion layer • When the emulsion is developed, the track becomes visible • Cloud chamber • Contains a gas cooled to just below its condensation level • The ions serve as centers for condensation • Particles ionize the gas along their path • Track can be viewed and photographed

38. Detectors, 4 • Track detectors, cont • Bubble Chamber • Contains a liquid near its boiling point • Ions produced by incoming particles leave tracks of bubbles • The tracks can be photographed • Wire Chamber • Contains thousands of closely spaced parallel wires • The wires collect electrons created by the passing ionizing particle • A second grid allows the position of the particle to be determined • Can provide electronic readout to a computer