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X-Rays: Wavelengths, Production, and Characteristics

This lecture in Physics 102 covers the wavelengths of x-rays, their production methods, and the characteristics of bremsstrahlung and characteristic x-rays. It also includes examples and practice questions.

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X-Rays: Wavelengths, Production, and Characteristics

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  1. Physics 102: Lecture 26 X-rays • Today’s Lecture will cover Section 27.4 Make sure your grade book entries are correct. Physics 102: Lecture 26, Slide 1

  2. X-Rays Photons with energy in approx range 100eVto100,000eV. This large energy means they go right through you (except for your bones). What are the wavelengths? Physics 102: Lecture 26, Slide 2

  3. X-Rays Photons with energy in approx range 100eVto100,000eV. This large energy means they go right through you (except for your bones). What are the wavelengths? .01 nm to 10 nm Physics 102: Lecture 26, Slide 3

  4. X-Ray Production How do you produce 100 eV photons? • Black Body Radiation • Would require temperature over 10 times hotter than surface of sun • Excitation of outer electrons • Typically have energy around 10 eV • Radioactive Decays • Hard to turn on/off Physics 102: Lecture 26, Slide 4

  5. Recall from Lecture 3: Electron Tubes • Accelerate an electron through a voltage difference to give it some energy... Example An electron is accelerated through a potential difference of 70,000 V. How much energy does it emerge with? KE = U = U of voltage gap becomes K.E. for electron. Physics 102: Lecture 26, Slide 5

  6. Recall from Lecture 3: U = qV KE = U = (1 e-) (70,000 V) = 70,000 eV = 11.2 x 10-14 J = 1.6 x 10-19 C Electron Tubes • Accelerate an electron through a voltage difference to give it some energy... Example An electron is accelerated through a potential difference of 70,000 V. How much energy does it emerge with? U of voltage gap becomes K.E. for electron. Physics 102: Lecture 26, Slide 6

  7. From Electrons to X-Rays • Now take these high energy electrons (up to 100,000 eV) and slam them into heavy atoms - any element. • 2 kinds of X-Rays are produced: • “Bremsstrahlung” • “Characteristic” Physics 102: Lecture 26, Slide 7

  8. intensity  Bremsstrahlung X-Rays • Electron hits atom and slows down, losing kinetic energy. • Energy emitted as photon • Electron hitting atom makes many photons (X-Rays), all with different energy. • Many different wavelengths. • If all of electron’s energy is lost to a single photon, photon has maximum energy (minimum wavelength). • Minimum X-Ray wavelength = lo.

  9. intensity  0 Bremsstrahlung X-Rays • Electron hits atom and slows down, losing kinetic energy. • Energy emitted as photon • Electron hitting atom makes many photons (X-Rays), all with different energy. • Many different wavelengths. • If all of electron’s energy is lost to a single photon, photon has maximum energy (minimum wavelength). • Minimum X-Ray wavelength = lo.

  10. An electron is accelerated through 50,000 volts What is the minimum wavelength photon it can produce when striking a target? intensity  0 Example Bremsstrahlung Practice Physics 102: Lecture 26, Slide 10

  11. An electron is accelerated through 50,000 volts What is the minimum wavelength photon it can produce when striking a target? Minimum wavelengthMaximum energy intensity  0 Example Bremsstrahlung Practice Electron loses ALL of its energy in one collision and emits one photon. Physics 102: Lecture 26, Slide 11

  12. L shell (n=2) e- e- e- K shell (n=1) e- e- e- (high energy electron) Characteristic X-Rays Electron knocks one of the two K shell (ground state) electrons out of an atom. L (n=2) or higher shell electron falls down to K shell (ground state) and x-ray photon is emitted Characteristic x-ray nomenclature n=1 “K shell” n=2 “L shell” n=3 “M shell” Physics 102: Lecture 26, Slide 12

  13. e- ejected electron e- Characteristic X-Rays e- Electron knocks one of the two K shell (ground state) electrons out of an atom. L (n=2) or higher shell electron falls down to K shell (ground state) and x-ray photon is emitted L shell (n=2) e- e- e- Characteristic x-ray nomenclature n=1 “K shell” n=2 “L shell” n=3 “M shell” K shell (n=1) e- e- Physics 102: Lecture 26, Slide 13

  14. e- L shell electron falls down Characteristic X-Rays e- X-Ray photon emitted “K X-ray” (n=2 n=1 transition) Electron knocks one of the two K shell (ground state) electrons out of an atom. L (n=2) or higher shell electron falls down to K shell (ground state) and x-ray photon is emitted L shell (n=2) e- e- e- Characteristic x-ray nomenclature n=1 “K shell” n=2 “L shell” n=3 “M shell” K shell (n=1) e- e- Physics 102: Lecture 26, Slide 14

  15. Careful! the formula n=2 assumed a single electron bound to just a positive nucleus. L n=1 Ka photon K intensity  Example Ka X-Rays Estimate the energy of Ka X-rays from a silver (Ag) target (Z=47). Physics 102: Lecture 26, Slide 15

  16. Careful! the formula n=2 assumed a single electron bound to just a positive nucleus. L n=1 Ka photon K intensity (vs. Expt)  Example Ka X-Rays Estimate the energy of Ka X-rays off of a silver (Ag) target (Z=47). Not bad!

  17. Ka X-rays come from n=2 n=1 transition. What about n=3 n=1 transition? Kb X-Rays are higher energy (lower l) than Ka.(and lower intensity) Ka Kb intensity  Kb X-Rays Not as likely, but possible. Produces Kb X-Rays! Different elements have different Characteristic X-Rays Physics 102: Lecture 26, Slide 17

  18. intensity  Ka Kb intensity  All Together Now... Brehmsstrahlung X-Rays and Characteristic X-Rays both occur at the same time. intensity  0 Physics 102: Lecture 26, Slide 18

  19. Preflight 26.1 K K Kb Kb intensity intensity   0 0 These two plots correspond to X-Ray tubes that: (1) Are operating at different voltages (2) Contain different elements (3) Both (4) Neither Physics 102: Lecture 26, Slide 19

  20. Preflight 26.1 K K Kb Kb intensity intensity   0 0 These two plots correspond to X-Ray tubes that: (1) Are operating at different voltages (2) Contain different elements (3) Both (4) Neither Ka and Kb are the same lo is different Physics 102: Lecture 26, Slide 20

  21. Kb Kb K K intensity  0 intensity  0 ACT: X-Rays I Which graph corresponds to the tube being operated at the higher voltage? 1) Top 2) Bottom Physics 102: Lecture 26, Slide 21

  22. Kb Kb K K intensity Ka and Kb are the same for each! Higher voltage means higher energy deceleration x-ray photon can be produced, or smaller maximum wavelength,l0.  0 intensity  0 ACT: X-Rays I Which graph corresponds to the tube being operated at the higher voltage? 1) Top 2) Bottom Physics 102: Lecture 26, Slide 22

  23. Kb Kb K K intensity 0  intensity 0  ACT: X-Rays II The top spectrum comes from a tube with a silver target (Ag, 47). What is the bottom target? 1) Pd 46 2) Ag 47 3) Cd 48 Physics 102: Lecture 26, Slide 23

  24. Kb Kb K K lo is the same for each! intensity Energy of characteristic X-ray is proportional to (Z-1). Higher energy = higher Z 0  intensity 0  ACT: X-Rays II The top spectrum comes from a tube with a silver target (Ag, 47). What is the bottom target? 1) Pd 46 2) Ag 47 3) Cd 48 Physics 102: Lecture 26, Slide 24

  25. Nuclear Physics A Z Nucleus = Protons+ Neutrons nucleons Z = proton number (atomic number) Gives chemical properties (and name) N = neutron number A = nucleon number (atomic mass number) Gives you mass density of element A=N+Z Periodic_Table Physics 102: Lecture 26, Slide 25

  26. Preflight 27.1 A material is known to be an isotope of lead Based on this information which of the following can you specify? 1) The atomic mass number 2) The neutron number 3) The number of protons Physics 102: Lecture 26, Slide 26

  27. Preflight 27.1 A material is known to be an isotope of lead Based on this information which of the following can you specify? 1) The atomic mass number 2) The neutron number 3) The number of protons Chemical properties (and name) determined by number of protons (Z) Lead Z=82 Physics 102: Lecture 26, Slide 27

  28. Strong Nuclear Force • Acts on Protons and Neutrons • Strong enough to overcome Coulomb repulsion • Acts over very short distances Two atoms don’t feel force Physics 102: Lecture 26, Slide 28

  29. Coulomb force proton electron proton neutron Very strong force Binding energy ofdeuteron=or 2.2Mev!That’s around 200,000 times bigger! Strong Nuclear Force Hydrogen atom:Binding energy=13.6eV (of electron to nucleus) Simplest Nucleus: Deuteron=neutron+proton Physics 102: Lecture 26, Slide 29

  30. Can get 4 nucleons into n=1 state. Energy will favor N=Z # protons = # neutrons Pauli Principle - neutrons and protons have spin like electron, and thus ms= 1/2. But protons repel one another (Coulomb Force) and when Z is large it becomes harder to put more protons into a nucleus without adding even more neutrons to provide more of the Strong Force. For this reason, in heavier nuclei N>Z. 7 Physics 102: Lecture 26, Slide 30

  31. See you next time! • Read Textbook Sections 29.1-4 Physics 102: Lecture 26, Slide 31

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