1. EN250: Lecture 2 Radiation Physics X-ray Production Disclaimer: These slides are a compilation of pictures obtained from the WWW and various books, etc, and none is original. For example the flash demos are obtained from http://learntech.uwe.ac.uk/radscience/xray_prod/production_of_xrays03_files/frame.htm

2. Overview Review of the structure of the atom Electromagnetic Radiation Production of X-rays Interaction of X-ray with matter Properties of X-rays Formation of an X-ray image Potential for 3D?

3. History of the Development of Atomic Models 1. Democritus (400 B.C.)– matter is made of atomos. 2. John Dalton (1800’s) – proof for atoms. 3. J.J. Thomson (1904) – discovered electrons in atoms; his model was of a positive sphere with e- embedded in it. 4. Ernest Rutherford (1911) – discovered nucleus; proposed that e- orbited a nucleus that had almost all the mass. 5. Niels Bohr (1913) – said e- orbited in fixed paths. 6. James Chadwick (1932) – nucleus contained protons & neutrons. 7. Erwin Schrodinger (1926) – electron “cloud” model.

4. Review of the Structure of the Atom Atom is mostly empty space Mass is concentrated in its Nucleus Neutrons, the NucleonsNucleus consists of Protons and Neutrons, the Nucleons Electrons orbiting around the nucleus

6. ELECTRONSPROTONS NEUTRONS ELECTRONSPROTONSNEUTRONS Negative chargePositive chargeZero or neutral charge # of electrons determines the charge of the atom # of protons determines type of element the atom is # of neutrons determines type of isotope the atom is Much much smaller than protons Much much bigger than electron Contributes little to mass or weight of atom- hardly anything Contributes significantly to mass or weight of atom

7. Atomic Number Atomic Number A: Number of Nucleons Mass number Z: number of protons

8. Characterizing Atoms A Nuclide is characterized by A and Z

9. ORBITAL BASICS (1) A shell is sometimes called an orbital or energy level. (2) Shells are areas that surround the center of an atom. (3) The center of the atom is called the nucleus. (4) Electrons live in something called shells.

10. SHELLS ONLY HOLD SOME ELECTRONS Not all shells hold the same number of electrons. For the first 18 elements, there are some rules. –The k-shell only holds two electrons. –The l-shell only holds eight electrons. –The m-shell only holds eight electrons (for the first 18 elements). The m-shell can actually hold up to 18 electrons as you move further along the periodic table. YOU CAN'T KNOW WHERE AN ELECTRON IS Niels Bohr came up with all these ideas in 1913.

13. Valence Shell The outermost shell is the valence shell Valence Shell determines chemical, thermal, optical, and electrical properties of the element X-rays involves inner shells Radioactivity and Gamma rays involve the nucleus Valence shells have at most 8 electrons Metals have 1, 2, or 3, one of which can be easily detached, the “free” electron, responsible for the excellent conduction of heat and electricity

14. Binding Energy An atom is ionized when one of is electrons is completely removed The Binding Energy E is the energy required to remove this electron Usually measured in electronvolts (eV) Example: Tungsten (W; Z=74) –EK = 70, EL = 11, EM = 2, All in Kev Binding Energy EK for various atoms (usually < 100 kev) –WZ = 7470Kev –IZ = 5333Kev –MoZ = 4220Kev –CuZ = 29 9 Kev eV is the amount of energy gained by a single unbound electron when it accelerates through an electrostatic potential difference of one volt

15. Exciting an Atom Atom is called excited when an electron is raised from one shell to another further out, as a result of expenditure of energy When the electron falls back it emits this energy in a packet of energy, a photon of light (visible or ultraviolet) A Photon is an elementary particle, the quantum of the electromagnetic field and the basic unit of light and all other forms of electromagnetic radiation –Developed by Einstein (1905-1917) to explain the non-wave properties of EM waves

16. Electromagnetic Radiation X-rays and Gamma rays are EM waves Quantum aspects: Travel in a straight line, photons: packets or quanta of energy Wave aspects: Electric and Magnetic Fields at right angle to each other and to the direction of wave travel of the wave; field strength sinusoidal: frequency f, period T, wavelength

17. Electromagnetic Specturm Radio Waves and X-Ray/Gamma rays are the only E&M Waves that can penetrate the body

20. Combined View E = h f, h Plancks constant E (in Kev)= 1.24 / Lambda (in nm) –Blue Lightlambda = 400nmE = 3ev –X-raylambda = 0.1 nmE = 140Kev Rays travel from source in all directions A beam is a collimated set of rays Energy flux: Total energy (total number of photons ) per unit area Intensity is inversely proportional to distance from source

21. Discovery of X-ray: Wilhelm Conrad Roentgen (1845-1923) Physical Institute of the University of Wurzburg

22. Roentgen’s early tube design

23. Roentgen’s Laboratory

24. Nov 1895: Penetrating solids

25. First X-Ray Picture: 22 Dec 1895

32. Modern X-ray Tubes

33. Production of X-rays X-Rays are produced when fast moving electrons are suddenly stopped by impact on a metal target The kinetic energy of the electrons is converted into heat (99%) and X-rays (1%) Structure of an X-ray Tube –A Negative Electrode, Cathode, fine Tungsten coil or filament, heated to incandescence (2200 C) which gives off electrons (thermionic emission) –A Positive Electrode, Anode, smooth flat metal target, usually Tungsten, collects these electrons which hit it with about half the speed of light

34. Filament heating voltage about 10V and using about 10A The accelerating voltage in the range 30-150 kV and current of 0.5-1000 mA

35. Processes underlying X-ray formation –Elastic collision with Atoms –Inelastic collision with electrons in the outer shell of an atom: Excitation Ionization –Inelastic collision with electrons in the inner shell of an atom (Characteristic Radiation) –Inelastic collision with the nuclei of the atom (Bremsstrahlung)

36. Elastic collision with target The electron is deflected but looses very little kinetic energy because its mass is negligible, and continues in tortuous path because of successive interactions

37. Interaction between filament electron and outer shell electron (1) Excitation i) This results in an outer shell electron gaining energy & being raised to a higher level. ii) Heat is produced as the electron falls back into its original path. No contribution to x-ray production

38. Interaction between filament electron & outer shell electron (2) Outer shell electron ejected from target atom results in an outer shell electron being completely removed from the target atom. Both the filament & the ejected electrons may interact in either the first or second of these interaction processes with other target atoms Ultimately, this type of interaction may cause the target material to heat up

39. Illustration (2)

40. Inelastic collisions with electrons in the inner shell of an atom (characteristic radiation) The incoming electron transfers sufficient energy to remove an inner shell electron from its atom in the target.The incoming electron transfers sufficient energy to remove an inner shell electron from its atom in the target. In order for this to occur the electron must possess energy at least as great as the binding energy of the inner shell.In order for this to occur the electron must possess energy at least as great as the binding energy of the inner shell. Any surplus energy appears as additional kinetic energy in the ejected electronAny surplus energy appears as additional kinetic energy in the ejected electron The inner shell vacancy is quickly filled by an electron falling inwards from a shell further out from the nucleusThe inner shell vacancy is quickly filled by an electron falling inwards from a shell further out from the nucleus This transition is accompanied by a burst of electromagnetic radiation with energy equal to the difference in binding energies of the two shells.This transition is accompanied by a burst of electromagnetic radiation with energy equal to the difference in binding energies of the two shells. This type of x-ray production is termed characteristic because the exact photon energy is characteristic of the element of which the target is made.This type of x-ray production is termed characteristic because the exact photon energy is characteristic of the element of which the target is made.

41. Illustration (3)

42. Energy levels The difference in the binding energies of the K and L shells in tungsten is 70 keV - 11 keV = 59 keV. So a characteristic photon of 59 keV is emitted. The difference in the binding energies of the M and K shells in tungsten is 70keV - 2 keV = 68 keV. These two electron transitions are the most likely to occur, producing x-ray photons of 68 and 59 keV. Characteristic x-rays contribute less than 10% of an x-ray beam. The majority of x-ray production results from inelastic collisions of incoming electrons with the nuclei of the target atomsCharacteristic x-rays contribute less than 10% of an x-ray beam. The majority of x-ray production results from inelastic collisions of incoming electrons with the nuclei of the target atoms

43. Inelastic collisions with the nuclei of the atom (Bremsstrahlung Radiation) The incoming electron passes very close to the nucleus of a target atom (1).The incoming electron passes very close to the nucleus of a target atom (1). The attraction causes the electron to deviate in its course (2)The attraction causes the electron to deviate in its course (2) The sudden change of direction stimulates the electron to release energy in the form of a photon of electromagnetic radiation (3)The sudden change of direction stimulates the electron to release energy in the form of a photon of electromagnetic radiation (3) The emission of radiation results in a reduction in the electrons kinetic energy causing it to slow down.The emission of radiation results in a reduction in the electrons kinetic energy causing it to slow down. The energy of the radiation depends on the degree of deviation the electron suffers.The energy of the radiation depends on the degree of deviation the electron suffers. In an extreme case the electron may actually be brought to rest. Thus the photon energy can be of any value from zero up to a maximum equal to the initial kinetic energy of the incoming electron.In an extreme case the electron may actually be brought to rest. Thus the photon energy can be of any value from zero up to a maximum equal to the initial kinetic energy of the incoming electron. This gives rise to a continuous spectrum of x-radiation and is known as braking (Bremsstrahlung) radiation.This gives rise to a continuous spectrum of x-radiation and is known as braking (Bremsstrahlung) radiation.

44. Illustration (4)

47. Interaction of X-Ray with matter

50. Example X-ray images

54. End of Lecture 2

55. Additional Slides

58. Collimated vs non-collimated x-ray

59. Dose Limits Occupational Whole Body5,000 mrem per yearExtremity and Skin of Whole Body 50,000 mrem per year Lens of the Eye 15,000 mrem per year Any single organ (e.g., Thyroid) 50,000 mrem per year Fetus of Declared Pregnant Worker500 mrem per term of pregnancyGeneral Public100 mrem per yearFor additional information see Section 6 of the University of Washington Radiation Safety ManualRadiation Safety Manual

60. http://www.amptek.com/xrf.html