OBJ 1 – Radioactivity & Radioactive Decay

1. OBJ 1 – Radioactivity & Radioactive Decay

2. Chart of the Nuclides • General Layout • Each nuclide occupies a square in a grid where • Atomic number (Z) is plotted vertically • Number of neutrons (N) is plotted horizontally • Heavily bordered square at left side of each row gives • Name • Chemical symbol • Elemental Mass • Thermal neutron absorptions cross section • Resonance integral

3. Chart of the Nuclides • Nuclides on diagonal running from upper left to lower right have same mass numbers, called isobars • Colors and shading used to indicate in chart squares used to indicate relative magnitude of • Half-lives • Neutron absorption properties • Four different colors used • Blue • Green • Yellow • Orange

4. Chart of the Nuclides • Background color of upper half of square represents T1/2 • Background color of lower half of square represents greater of the thermal neutron cross section or resonance integral • When nuclide is stable and thermal neutron cross section is small or unknown, entire square is shaded grey • Gray shading also used for unstable nuclides having T1/2 sufficiently long (>5E8 yrs) to have survived from the time they were formed

5. Chart of the Nuclides • Some squares, such as 60Co, 115In, and 116In are divided • Occurs when nuclide has one or more isomeric or metastable states • Has same A and Z, but different nuclear and radioactive properties due to different energy states of the same nucleus

6. Chart of the Nuclides • Nuclide Properties Displayed on the Chart • Chemical Element Names and Symbols • Same element names and symbols as used on the Periodic Table of the Elements • Atomic Weights and Abundances • Isotopic masses in AMUs are given for • Stable isotopes • Certain long-lived, naturally occurring radioactive isotopes • Nuclides particle decay becomes a prominent mode (>10%)

7. Chart of the Nuclides • Isotopic Abundance • Values on chart given in atom percent • Specified for 288 nuclides (266 stable and 22 radioactive) • Half-lives • Half-life listed below nuclide symbol and mass number • Units used • ps picoseconds (1E-12 s) • ns nanoseconds (1E-9 s) • µs microseconds (1E-6 s) • ms milliseconds (1E-3 s) • s seconds • m minutes • h hours • d days • a years

8. Chart of the Nuclides • Background Color of Chart Square Upper Half • 1 day to 10 day  orange • >10 days to 100 days  yellow • >100 days to 10 years  green • >10 years to 5E8 years  blue • Background Color of Chart Square Lower Half • Refers to thermal neutron cross section or resonance integral

9. Chart of the Nuclides • Major Modes of Decay and Decay Energies • α alpha particle • β- beta minus (negatron) • β+ beta plus (positron) • γ gamma ray • n neutron • p proton • d deuteron • t triton • εelectron capture • IT isomeric transition • e- conversion electron • β-β- double beta decay • cluster decay • D delayed radiation

10. Chart of the Nuclides • To understand decay schemes and energies, look at chart square for 38Cl • β-energies listed on 1st line in order of abundance • γ energies listed on 2nd line in order of abundance • Particle energies always given in MeV • γ energies always given in keV

11. Chart of the Nuclides • When more than one decay mode possible, modes listed on chart in order of abundance or intensity • Different modes of beta decay (ε, β+, β-) appear on separate lines if intensity of one of the decay modes is <10% absolute intensity • Conversely, appear on same line if intensities of both >10% absolute intensity with most abundant listed first.

12. Chart of the Nuclides • When branching decay occurs by both β- and β+ and/or ε, and each decay is accompanied by γemission, format shown in 146Pm square is used • Metastable (or isomeric) state frequently decays to ground by IT γemission, followed by one or more γ in cascade • Internal conversion is process resulting from interaction between nucleus and extra-nuclear electrons. Nuclear excitation energy xfr’d to orbital electron (usually K shell) and is indicated by e-

13. Chart of the Nuclides • Delayed γemission indicated by symbol D. • When daughter product has too short of a half-life to have its own spot on the chart or half-life is much shorter than that of the parent nuclide, γenergy is listed with the parent

14. Chart of the Nuclides

15. Chart of the Nuclides

16. Radiation Classifications • Introduction • All radiation possesses energy • Inherent — electromagnetic • Kinetic — particulate • Interaction results in some or all of energy being transferred to surrounding medium • Scattering • Absorption

17. Radiation Classifications • Ionizing or Non-Ionizing • Non-Ionizing • Visible light • Radio and TV • Ionizing • Particulate or Photonic • Particulate • α • β • n • Electromagnetic • γ • x

18. Radiation Classifications • Directly or Indirectly Ionizing • Directly Ionizing • Possesses charge • Does not need physical contact • Indirectly Ionizing • Does not have charge • Needs physical contact

19. Radiation Characteristics • Alpha (α) • Charge – +2 • Range – 2-4 in. (5 – 10 cm.) • Shielding • Paper • Dead skin • Hazard – Internal • Target Organ – Anything internal (living tissue)

20. Radiation Characteristics • Beta (β) • Charge • Negatron (β-) – -1 • Positron (β+) – +1 • Range • Average – ≈ 10 ft. • Energy Specific – ≈ 10 – 12 ft./MeV • Shielding • Plastic • Wood • Al, Cu • Hazard – Internal • Target Organ • External – eye (lens) • Living tissue Low Z

21. Radiation Characteristics • Gamma (γ) and X-Ray (x) • Charge – 0 • Range – ≈ Infinite • Shielding • Pb • DU • Hazard – Internal • Target Organ – Living tissue High Z

22. Radiation Characteristics • Neutron (n) • Charge – 0 • Range – ≈ Infinite • Shielding • H20 • Concrete • Plastic • Hazard – Internal • Target Organ – Living tissue Hydrogenous

23. Energy Transfer Mechanisms • Ionization • Removing bound e- from electrically neutral atom or molecule by adding sufficient energy to allow it to overcome its BE • Atom has net positive charge • Creates ion pair consisting of negatively charged electron and positively charged atom or molecule

24. Energy Transfer Mechanisms Ionizing Particle e- e- Negative Ion N P+ P+ Positive Ion N e-

25. Energy Transfer Mechanisms • Excitation • Process that adds sufficient energy to e- such that it occupies higher energy state than lowest bound energy state • Electron remains bound to atom • No ions produced, atom remains neutral • After excitation, excited atom eventually loses excess energy when e- in higher energy shell falls into lower energy vacancy • Excess energy liberated as X-ray, which may escape from the material, but usually undergoes other absorptive processes

26. P+ N N P+ Energy Transfer Mechanisms e- e- N e- + + N N N + + e- N e-

27. Energy Transfer Mechanisms • Bremsstrahlung • Radiative energy loss of moving charged particle as it interacts with matter through which it is moving • Results from interaction of high-speed, charged particle with nucleus of atom via electric force field • With negatively charged electron, attractive force slows it down, deflecting from original path • KE particle loses emitted as x-ray • Production enhanced with high-Z materials (larger coulomb forces) and high-energy e- (more interactions occur before all energy is lost)

28. Energy Transfer Mechanisms e- e- e- e- N + + N N N + + e- N e-

29. Directly Ionizing Radiation • Charged particles don’t need physical contact with atom to interact • Coulombic forces act over a distance to cause ionization and excitation • Strength of these forces depends on: • Particle energy (speed) • Particle charge • Absorber density and atomic number • Coulombic forces significant over distances > atomic dimensions • For all but very low physical density materials, KE loss for e- continuous because of Coulombic force

30. Directly Ionizing Radiation • Alpha Interactions • Mass approximately 8K times > electron • Travels approximately 1/20th speed of light • Because of mass, charge, and speed, has high probability of interaction • Does not require particles touching—just sufficiently close for Coulombic forces to interact • Energy gradually dissipated until α captures two e- and becomes a He atom • α from given nuclide emitted with same energy, consequently will have approximately same range in a given material

31. Directly Ionizing Radiation • Beta Interactions • Interaction between β- or β+ and an orbital e- is interaction between 2 charged particles of similar mass • βs of either charge lose energy in large number of ionization and/or excitation events, similar to α • Due to smaller size/charge, lower probability of interaction in given medium; consequently, range is >>α of comparable energy • Because β’s mass is small compared with that of nucleus • Large deflections can occur, particularly when low-energy βs scattered by high-Z elements (high positive charge on the nucleus) • Consequently, β usually travels tortuous, winding path in an absorbing medium • β may have Bremsstrahlung interaction resulting in X-rays

32. Indirectly Ionizing Radiation • No charge • γand n • No Coulomb force field • Must come sufficiently close for physical dimensions to contact particles to interact

33. Indirectly Ionizing Radiation • Small probability of interacting with matter – Why? • Doesn’t continuously lose energy by constantly interacting with absorber • May move “through” many atoms or molecules before contacting electron or nucleus • Probability of interaction depends on its energy and absorber’s density and atomic number • When interactions occur, produces directly ionizing particles that cause secondary ionizations

34. Indirectly Ionizing Radiation • Gamma absorption • γ and x-rays differ only in origin • Name used to indicate different source • γs originate in nucleus • X-rays are extra-nuclear (electron cloud) • Both have 0 rest mass, 0 net electrical charge, and travel at speed of light • Both lose energy by interacting with matter via one of three major mechanisms

35. Indirectly Ionizing Radiation • Photoelectric Effect • All energy is lost – happens or doesn’t • Photon imparts all its energy to orbital e- • Because pure energy, photon vanishes • Probable only for photon energies < 1 MeV • Energy imparted to orbital e- in form of KE, overcoming attractive force of nucleus, usually causing e- to leave orbit with great velocity • Most photoelectrons are inner-shell e-

36. Indirectly Ionizing Radiation • High-velocity e-, called photoelectron • Directly ionizing particle • Typically has sufficient energy to cause secondary ionizations • Most photoelectrons are inner-shell electrons

37. Indirectly Ionizing Radiation e- Photoelectron Gamma Photon (< 1 MeV) e- e- N + + N N N + + e- N e-

38. Indirectly Ionizing Radiation • Compton Scattering • Partial energy loss for incoming photon • Dominant interaction for most materials for photon energies 200 keV – 5 MeV • Photon continues with less energy in different direction to conserve momentum • Probability of Compton interaction  with distance from nucleus — most Compton electrons are valence electrons • Beam of photons may be randomized in direction and energy, so that scattered radiation may appear around corners and behind shields where there is no direct line of sight to source • Probability of Compton interaction  with distance from nucleus — most Compton electrons are valence electrons

39. Indirectly Ionizing Radiation • Pair Production • Occurs when all photon energy is converted to mass (occurs only in presence of strong electric field, which can be viewed as catalyst) • Strong electric fields found near nucleus and are stronger for high-Z materials • γ disappears in vicinity of nucleus and β-- β+ pair appears • Will not occur unless γ> 1.022 MeV • Any energy > 1.022 MeV shared between the β--β+ pair as KE • Probability < photoelectric and Compton interactions because photon must be close to the nucleus

40. Indirectly Ionizing Radiation Electron e- Gamma Photon (E > 1.022 MeV) e- e- e+ N e- + + Positron N N N + + e- e- N 0.511 MeV Photons e- e-

41. Indirectly Ionizing Radiation • Neutron Interactions • Free, unbound n unstable and disintegrates by β- emission with half-life of ≈ 10.6 minutes • Resultant decay product is p+, which eventually combines with free e- to become H atom • n interactions energy dependent –classified based on KE

42. Indirectly Ionizing Radiation • Classifying according to KE important from two standpoints: • Interaction with the nucleus differs with n energy • Method of detecting and shielding against various classes are different • n detection relatively difficult due to: • Lack of ionization along their paths • Negligible response to externally applied electric, magnetic, or gravitational fields • Interact primarily with atomic nuclei, which are extremely small

43. Indirectly Ionizing Radiation • Slow Neutron Interactions • Radiative Capture • Radiative capture with γ emission most common for slow n • Reaction often results in radioactive nuclei • Process is called neutron activation

44. Indirectly Ionizing Radiation • Charged Particle Emission • Target atom absorbs a slow n, which  its mass and internal energy • Charged particle then emitted to release excess mass and energy • Typical examples include (n,p), (n,d), and (n,α). For example

45. Indirectly Ionizing Radiation • Fission • Typically occurs following slow n absorption by several of the very heavy elements • Nucleus splits into two smaller nuclei, called primary fission products or fission fragments • Fission fragments usually undergo radioactive decay to form secondary fission product nuclei • There are some 30 different ways fission may take place with the production of about 60 primary fission fragments

46. Indirectly Ionizing Radiation • Fast Neutron Interactions • Scattering • Free n continues to be free n following interaction • Dominant process for fast n • Elastic Scattering • Occurs when n strikes nucleus of approx. same mass • Neutron can xfer much of its KE to that, which recoils off with energy lost by n • No γ emitted by nucleus • Recoil nucleus can be knocked away from its e- and, being (+) charged, can cause ionization and excitation

47. Indirectly Ionizing Radiation e- N N P+

48. Indirectly Ionizing Radiation • Inelastic Scattering • Occurs when n strikes large nucleus • n penetrates nucleus for short period of time • Xfers energy to nucleon in nucleus • Exits with small decrease in energy • Nucleus left in excited state, emitting γ radiation, which can cause ionization and/or excitation

49. P+ N N P+ P+ N Indirectly Ionizing Radiation e- e- γ N N e- e-

50. Indirectly Ionizing Radiation • Reactions in Biological Systems • Fast n lose energy in soft tissue largely by repeated scattering interactions with H nuclei • Slow 0n1 captured in soft tissue and release energy in one of two principal mechanisms: and (2.2 MeV) (0.66 MeV)