interaction of xray with matter

interaction of xray with matter

1. INTERACTION OF X-RAY WITH MATTER BY:- DR. PRADEEP PATIL Professor Dept. of Radiodiagnosis D.Y. Patil hospital and medical college

2. Ionizing radiation includes 1. Charged particles (such as alpha and beta particles, and electrons) 2. Uncharged particles include :- A. Electromagnetic radiation or photons (such as X-rays and Gamma rays) B. Neutrons Electromagnetic radiation has dual characteristic, comprises of both Wave Particle INTRODUCTION

3. METHODS OF INTERACTIONS Attenuation : Reduction of intensity. Difference in attenuation gives the radiographic image. Absorbed : completely removed from the x-ray beam & cease to exist. Scattered : Random course. No useful information. No image only darkness. Adds noise to the system. Film quality affected : “film fog” . About 1% of the x rays that strike a patient's body emerge from the body to produce the final image. Remaining 99% of the x-rays --- Scattered / Absorbed.

4. BASIC INTERACTIONS BETWEEN X- RAYS AND MATTER A: PHOTON SCATTERING: COHERENT SCATT E RING COMPTON SCATT E RING B : PHOTON DISAPPEARANCE PHOTOELECTRIC EFFECT PAIR P R ODUCTION - PHOTODISINTEGRATION

5. 1. COHERENT SCATTERING Radiation undergoes Only Change in direction. No change in wavelength thats why sometime called “ unmodified scattering” Coherent scattering of X-rays is an interaction of the wave type in which the X-ray is deflected. Coherent Scattering occurs mainly at low energies. It is of two types Thomson scattering : Single electron involved in the interaction. Rayleigh scattering : Co-operative interaction of all the electrons of the atom .

6. Low energy radiation encounters electrons Electrons are set into vibration Vibrating electron, emits radiation. Atom returns to its undisturbed state Fig : Rayleigh scattering WHAT HAPPENS IN COHERENT SCATTERING?

7. No ionization --- why??? because, no energy transfer. Only change of direction. Only effect is to change direction of incident photon. Less than 5%. Not important in diagnostic radiology. Produces scattered radiation but of negligible quantity.

8. 2. PHOTOELECTRIC EFFECT What happens in Photoelectric effect ? An incident PHOTON encounters a K shell electron and ejects it from the orbit The photon disappears, giving up ( nearly) all its energy to the electron The electron ( now free of its energy debt) flies off into space as a photoelectron carrying the excess energy as kinetic energy. The K shell electron void filled immediately by another electron and hence the excess energy is released as CHARACTERISTIC RADIATION. The atom is ionised.

9. PHOTOELECTRIC EFFECT

10. Thus the Photoelectric effect yields three end products : Characteristic radiation A -ve ion ( photoelectron ) A+ve ion (atom deficient in one electron )

11.  The incident photon energy > binding energy of the electron.  Photon energy similar to electron binding energy has higher probability of causing P.E.E  The probability of a reaction increases sharply as the atomic no. increases Probability of occurrence of P.E.E :- Photoelectric effect  1 (energy) ³ Photoelectric effect  (atomic no.) ³

12. CHARACTERISTIC RADIATION Characteristic radiation generated by the photoelectric effect is exactly the same The only difference in the modality used to eject the inner shell electron. In x ray tube a high speed electron ejects the bound electron, whil e in photoelectric effect an X ray photon does the trick. In both cases the atom is left with an excess of energy = the binding energy of an ejected electron Usually referred to as Secondary Radiation to differentiate It from scatter radiation…… End result is same for both, “A Photon that is deflected from its original path”

13. Characteristic radiation How does this happen ? After the electron has been ejected, the atom is left with a void in the K shell & an excess of energy equivalent to the binding energy. This state of the atom is highly unstable & to achieve a low energy stable state ( as all physical systems seek the lowest possible energy state ) an electron immediately drops in to fill the void. As the electron drops into the K shell, it gives up its excess energy in the form of an x-ray photon. The amount of energy released is characteristic of each element & hence the radiation produced is called Characteristic radiation.

14. Low atomic number : interaction mostly at the K shell. High atomic number : interaction mostly at L and M shell. In summary, Photoelectric reactions are most likely to occur with low energy photons and elements with high atomic numbers provided the photons have sufficient energy to overcome the forces binding the electrons in their cells.

15. Applications in diagnostic radiology : Advantages : Excellent radiographic images No scatter radiation. Enhances natural tissue contrast. Depends on 3 rd power of the atomic no., so it magnifies the difference in tissues composed of different elements, such as bone & soft tissue Disadvantage: Maximum radiation exposure. All the energy is absorbed by the patient whereas in other reactions only part of the incident photon’s energy is absorbed.

16. 3- Compton Scattering (inelastic scattering) Compton scattering is the predominant interaction of x-ray with soft tissue in the energy range approximately from 30 KeV to 24 MeV. • This interaction is most likely to occur between photons and outer ("valence") shell electrons. The electron is ejected from the atom, and the photon is scattered with some reduction in its energy. The energy of the incident photon ( Eo ) is equal to the sum of the energy of the scattered photon (Ese) and the kinetic energy of the ejected electron ( Ee -) The binding energy of the electron that was ejected is very small and can be ignored.

17. ENERGY OF COMPTON SCATTERED PHOTONS The change in wavelength of a scattered photon is calculated as : Δλ = 0.024 ( 1 – cos θ ) , where Δλ = change in wavelength θ = angle of photon deflection

18. Probability of occur r enc e . Compton reaction is independent of the atomic no. of the absorber Energy of the radiation : The no. of reactions gradually diminishes as photon energy increases, so that a high energy photon is more likely to pass through the body than a low energy photon

19. Disadvantages of Compton reaction : Scatter radiation : Almost all the scatter radiation that we encounter in diagnostic Radiology comes from Compton scattering. In the diagnostic energy range, the photon retains most of its original energy. This creates a serious problem, because photons that are scattered at narrow angles have an excellent chance of reaching an x- ray film & producing fog. Exceedingly difficult to remove – cannot be removed by filters because they are too energetic. cannot be removed by grids because of narrow angles of deflection. It is also a major safety hazard . Even after 90˚ deflection most of its original energy is retained. Scatter radiation as energetic as the primary radiation. Safety hazard for the radiologist, personnel and the patient.

20. 4. PAIR PRODUCTION What happens in Pair production ? A high energy photon interacts with the nucleus of an atom. The photon disappears & its energy is converted into matter in the form of two particles  An electron  A positron (particle with same mass as electron, but with +ve charge.) Mass of one electron is 0.51 MeV. 2 electron masses are produced. So the interaction cannot take place with photon energy less than 1.0 2 MeV. No importance in diagnostic radiology.

21. Positron annihilation. What happens to the Positron ? Slowly moving Positron combines with a free electron to produce two photons of radiation. 2 mass units are converted, giving a total energy of 1.022 MeV. To conserve momentum, two photons each with 0.511 MeV energy are ejected in opposite direction.

22. 5. PHOTODISINTEGRATION A photon with extremely high energy ( 7-15 MeV) , interacts directly with the nucleus of an atom. May eject a neutron, proton or on rare occasions even an alpha particle. No diagnostic importance. We rarely use radiation >150 KeV in diagnostic radiology. What happens in Photodisintegration ? A high energy photon encounters the nucleus of an atom. Part of the nucleus which may be a neutron, a proton, an alpha particle or a cluster of particles, is ejected.

23. RELATIVE FREQUENCY OF BASIC INTERACTIONS Coherent scattering : About 5% . Minor role throughout the diagnostic energy range. Compton scattering : D ominant interaction in water. Water is used to represent tissues with low atomic nos. such as air, fat and muscle. Photoelectric reaction : usually seen in the contrast agents because of their high atomic numbers. Bone is intermediate between water & the contrast At low energies, Photoelectric reactions are more common, while at high energies, Compton scattering is dominant.

24. RELATIVE FREQUENCY OF BASIC INTERACTIONS

25. SUMMA R Y Only two interactions are important in diagnostic radiology, the Photoelectric effect & Compton scattering. The Photoelectric effect is the predominant interaction with low energy radiation & high atomic no. absorbers. It generates no significant scatter radiation & produces high contrast in the x-ray image. But, unfortunately it exposes the patient to a great deal of radiation. Compton scattering is the most common interaction at higher diagnostic energies. responsible for almost all scatter radiation. radiographic image contrast is less compared to photoelectric effect. Coherent scattering is numerically unimportant. Pair production & Photodisintegration occur at energies above the useful energy range.