原子物理簡介

1. 原子物理簡介

2. Structure of matter---The atoms • All matter is composed of individual entities called elements. Each element is distinguishable from the others by the physical and chemical properties of its basic components --- the atom • Each atom consists of a small central core, the nucleus, where most of the atomic mass is located and a surrounding "cloud" of electrons moving in orbits around the nucleus.

3. Structure of matter---The atoms • The radius of the atom (radius of the electronic orbits) is approximately 10-10 m • The nucleus has a much smaller radius, namely, about 10-14 m • Thus, for a particle of size comparable to nuclear dimensions, it will be quite possible to penetrate several atoms of matter before a collision happens

4. Structure of matter---The atoms • it is important to keep track of those particles that have not interacted with the atoms • --- the primary beam • and those that have suffered collisions • --- the scattered beam

5. Structure of matter---The nucleus • The properties of atoms are derived from the constitution of • their nuclei and • the number and the organization of the orbital electrons • The nucleus contains protons and neutrons. Whereas protons are positively charged, neutrons have no charge

6. Structure of matter---The nucleus • The number of protons in the nucleus is equal to the number of electrons outside the nucleus, thus making the atom electrically neutral • Atomic nomenclature X : atomic symbol A : atomic mass number Z : atomic number ＊ A = Z + N (質量數=質子數+中子數)

7. Terms used to describe a nucleus Ei-Ef

8. 原子的分類 • 1.同位素(isotope)：相同質子數，不同中子數 • 2.同中素(isotone)：相同中子數，不同質子數 • 3.同重素(isobar) ：相同質量數，不同中子數， • 不同質子數 • 4.同質異能素(isomer)：相同核種，不同核子能階

9. Structure of matter---The nucleus • Certain combinations of neutrons and protons result in stable (nonradioactive) nuclides than others • stable elements in the low atomic number • --- N = Z • as Z increases beyond about 20, the N/P ratio for stable nuclei becomes > 1 and increases with Z

10. Structure of matter---The nucleus combinations of neutrons and protons result in stable

11. Atomic mass and energy units • atomic mass unit (amu) • defined as 1/12 of the mass of a nucleus • Thus the nucleus of is arbitrarily assigned the mass equal to 12 amu

12. Masses and charges of the main subatomic particles

13. Atomic mass and energy units • mass defect ( binding energy) • the mass of an atom is not exactly equal to the sum of the masses of constituent particles • a certain mass is destroyed and converted into energy that acts as a "glue" to keep the nucleons together

14. THE NUCLEUS Binding Energy

15. THE NUCLEUS • Binding Energy • Mass defect • The difference between the atomic weight and the sum of the weights of the parts = W - M W = ZmH + ( A-Z )mn M = atomic weight BE = ( W – M )amu × 931 MeV/amu

16. Atomic mass and energy units • The basic unit of energy is the joule (J) • equal to the work done when a force of 1 newton acts through a distance of 1 m • Energy unit in atomic and nuclear physics • electron volt ( eV ), • defined as the kinetic energy acquired by an electron in passing through a potential difference of 1 V

17. Atomic mass and energy units

18. Distribution of orbital electrons • Rutherford’s atomic model (1911)

19. Distribution of orbital electrons • Bohr atomic model (1913)

20. Bohr’s postulates • 1.An electron in an atom moves in a circular orbit about the nucleus under the influence of the Coulomb attraction between the electron and the nucleus, and obeying the law of classical mechanics. • 2.Instead of the infinity of orbits which would be possible in classical mechanics, it is only possible for an electron to move in an orbit for which its orbital angular momentum L is an integral multiple of Planck’s constant h, divided by 2π. (L=nħ)

21. 3.Despite the fact that it is constantly accelerating, an electron moving in such an allowed orbit does not radiate electromagnetic energy. Thus its total energy E remains constant. • 4.Electromagnetic radiation is emitted if an electron, initially moving in an orbit of total energy Ei, discontinuously changes its motion so that it moves in an orbit of total energy Ef. The frequency of the emitted radiation ν is equal to the quantity (Ei - Ef) devided by Planck’s constant h. (hν= Ei - Ef)

22. ATOMIC ENERGY LEVELS • Energy level diagram of the tungsten atom

23. NUCLEAR FORCES • There are four different forces in nature, in the order of their strengths : • strong nuclear force • electromagnetic force • weak nuclear force • gravitational force

24. NUCLEAR FORCES • strong nuclear force • responsible for holding the nucleons together in the nucleus • electromagnetic force • force between charged nucleons is quite strong, but it is repulsive and tends to disrupt the nucleus

25. NUCLEAR FORCES • weak nuclear force • appears in certain types of radioactive decay • gravitational force • in the nucleus is very weak and can be ignored

26. NUCLEAR FORCES • The strong nuclear force is a short-range force that comes into play when the distance between the nucleons becomes smaller than the nuclear diameter

27. NUCLEAR ENERGY LEVELS • The shell model of the nucleus assumes that the nucleons are arranged in shells, representing discrete energy states of the nucleus similar to the atomic energy levels • If energy is imparted to the nucleus, it may be raised to an excited state, and when it returns to a lower energy state , it will give off energy equal to the energy difference of the two states.

28. NUCLEAR ENERGY LEVELS • Sometimes the energy is radiated in steps, corresponding to the intermediate energy states, before the nucleus settles down to the stable or ground state

29. PARTICLE RADIATION • Radiation • emission and propagation of energy through space or a material medium • particle radiation • energy propagated by traveling corpuscles that have a definite rest mass and within limits have a definite momentum and defined position at any instant

30. PARTICLE RADIATION • Besides protons, neutrons, and electrons , many other atomic and subatomic particles have been discovered • Interact with matter and produce varying degrees of energy transfer to the medium

31. ELECTROMAGNETIC RADIATION • Wave Model • in terms of oscillating electric and magnetic fields • the mode of energy propagation for such phenomena as light waves, radio waves, microwaves, ultraviolet rays, g-rays, and x-rays • with the speed of light ( 3 x 108 m/sec in vacuum)

32. Electromagnetic radiation • A. Wave model • λ= C/ν

34. ELECTROMAGNETIC RADIATION • Quantum Model • wavelength becomes very small or the frequency becomes very large, the dominant behavior of electromagnetic radiations can only be explained by considering their particle or quantum nature

35. Electromagnetic radiation • B. Quantum model • E = hν= hc /λ • h : Planck’s constant (6.62×10-34 J‧sec) • c：3×108 m/sec • E (keV) = 1.24 /λ(nm)

36. 放射物理簡介

37. X 射線的產生

38. The mechanism of x–ray production • 1. Characteristic x-rays

39. The mechanism of x–ray production 2. Bremsstrahlung x-rays (braking radiation)

40. Diagnostic x-ray tube (thicktarget)

41. Spatial distribution of therapeutic x-rays (thin target)

42. The efficiency of bremsstrahlungx-ray production • Efficiency is defined as the ratio of output energy emited as x-rays to the iuput energy deposited by electrons. Efficiency = 9 x 10-10 ZV Z︰atomic number of target V︰tubevoltage

43. X-ray energy spectra

44. Operating characteristics

48. 醫用直線加速器之結構

50. NUCLEAR TRAMSFORMATIOMS