Interactions of X-Rays with matter

1. Interactions of X-Rays with matter Recommended Book: Walter Huda, REVIEW OF RADIOLOGIC PHYSICS By: Maisa Alhassoun maisa@inaya.edu.sa

2. Discovery of X‐Rays November 1895, Würzburg Wilhelm Conrad Röntgen Roentgen's Wurzburg Laboratory

3. Crooke's Tube Similar to That Used in Roentgen's Discovery of X‐rays

4. The left picture taken at a lecture in January 1896 by Roentgen. The right by Mr. Haschek and Dr. Lindenthal, in Professor Franz Exner's physicochemical institute in Vienna was obtained by injecting a mixture of lime, cinnabar (mercury) and petroleum in the hand of a cadaver.

5. The photographic use of x‐rays advanced quickly. X‐ray photographs were used as early as February 1896 in the U.S. to diagnose bone fractures.

6. Interactions of X-Rays with matter • X-rays traveling through matter can be transmitted, absorbed, or scattered. Absorption Scattering Transmission Energy deposition

7. Five forms of x‐ray Interactions • Coherent Scattering • Pair production • Photodisintegration • Compton Effect • Photoelectric Effect -Compton scatter and thephotoelectric (PE) effect are the two most important X-Ray interactions in diagnostic radiology.

8. -Coherent scatter is of minor importance in diagnostic radiology. -Pair production and photodisintegration DO NOT occur in a diagnostic radiology environment.

9. Coherent scatter -Coherent scatter occurs when a low-energy x-ray photon excites an atom but then passes through without any net energy transfer to the atom. -Coherent scatter is sometimes referred to as Rayleigh or classical scatter. -Coherent scatter does NOT result in any energy deposition in the patient (i.e., dose).

10. -The scattered x-ray photon is usually emitted from the atom in a forward direction. -Coherent scatter is generally present in diagnostic radiology but is of minimal concern. -Coherent scatter typically accounts for only 5% of all photon interactions at diagnostic energies.

11. Pair production and photodisintegration -Pair production occurs when a high-energy photon interacts with the nucleus of an atom and is converted to matter and antimatter. -In pair production, the photon disappears, and the energy is converted into an electron and a positron.

13. -Pair production has a photon energy threshold of 1.02 MeV, which is the energy required to produce an electron (511 keV) and positron (511 keV) pair. -The positron eventually combines with an electron, producing two 511 keV photons that are emitted at 180 degree to each other (annihilation radiation).

14. -Photodisintegration occurs when a high-energy photon is absorbed by a nucleus, resulting in immediate disintegration of the nucleus. -The energy threshold for photodisintegration is approximately 15 MeV. -Photodisintegration and pair production are important only at the high-photon energies encountered in megavoltage radiotherapy.

15. Photoelectric effect -The PE effect occurs between tightly bound (inner-shell) electrons and incident x-ray photons. -The PE effect occurs when a photon is totally absorbed (PE absorption) by an inner-shell electron and an electron is ejected (photoelectron is emitted). -As a result of the photoelectron emission, a positive atomic ion is formed.

17. - The energy of the emitted photoelectron equals the difference between the incident photon energy and the electron binding energy. The photoelectric effect is an interaction where the incoming photon (energy Eγ = hv) is absorbed by an atom and an electron (energy=Ee) is ejected from the material: Ee= E γ ‐ BE Here BE is the binding energy of the material (typically a few eV). - The photoelectron loses energy by ionizing other atoms in the tissue and contributes to patient dose.

18. Photoelectric absorption (I‐131)

19. - Outer-shell electrons then fill the inner-shell electron vacancies to stabilize the atom, and the excess energy is emitted as characteristic radiation or as Auger electrons. -An Auger electron is an outer-shell electron with a binding energy less than the energy difference of the electron transition. -The alternative to an Auger electron is the emission of a characteristic x-ray.

21. Probability of photoelectric effect -For the PE effect to occur, the incident x-ray must have energy at least equal to or greater than the binding energy of the inner-shell electron. -The absorption of photons increases markedly as the x-ray photon energy is increased from below to above the binding energy of the K-shell electrons (K-edge). -The binding energy of the K-shell electrons (K-edge) in iodine is 33 keV, and a sharp increase in the interaction of photons occurs when the x-ray photon energy exceeds 33 keV.

22. -However, the probability of PE absorption decreases rapidly as the photon energy (E) further increases above the k-edge, and isproportional to 1/E3. -The PE effect predominates at energies just above the k-edge of the absorber. -The probability of PE absorption increases significantly with atomic number and is proportional to Z3.

23. -The more tightly bound an electron is, the greater is the probability of the PE effect, if E is greater than the binding energy. -Photoelectric absorption is thus highest for K-shell electrons, which are most tightly bound in an atom, followed by the L-shell, and so on. -The PE effect is important if the atomic number (Z) is high and the photon energy is just above the K-edge.

24. -Important K-shell binding energies are as follows: oxygen (Z = 8), 0.5 keV calcium (Z = 20), 4 keV iodine (Z = 53), 33 keV barium (Z = 56), 37 keV lead (Z = 82), 88 keV

27. Compton scatter -In Compton scatter, incident photons interact with loosely bound valence (outer-shell) electrons. -A Compton interaction results in a scattered photon that has less energy than that of the incident photon, and that travels in a new direction (compare coherent scatter).

29. -The higher the incident photon energy, the more likely that the direction of scatter will be in a forward direction. -A scattered (ejected or recoil) electron carries the energy lost by the incident photon. -This electron loses energy by excitation and ionization of other atoms in the tissue, thereby contributing to the patient dose.

30. -As a result of the Compton interaction, a positive atomic ion, which has lost an outer-shell electron, remains. -Compton interactions occur most commonly with electrons with a low binding energy. -Outer-shell electrons have binding energies of only a few electron volts, which is negligible compared with the high energy (30 keV) of a typical diagnostic energy x-ray photon.

31. -Compton interactions account for most scattered radiation encountered in diagnostic radiology.

32. Probability of a Compton interaction -The probability of a Compton interaction is proportional to the number of outer-shell electrons available in the medium (electron density). -Compton interactions are inversely proportional to the photon energy (1/E). -Scattered photons may move in any direction, including 180 degrees to the direc-tion of the incident photon (backscattered).

33. -As the angle of deflection increases, the energy retained by the scattered x-ray decreases. -Energy transfer to the electron is maximized when the photon is backscattered, thus.