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

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

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

Physics 102: Lecture 26, Slide 4 X-Ray Production 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 How do you produce 100 eV photons?

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

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

Physics 102: Lecture 26, Slide 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”

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

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

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

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

Physics 102: Lecture 26, Slide 12 Characteristic x-ray nomenclature n=1 “K shell” n=2 “L shell” n=3 “M shell” 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 (high energy electron) e-e- K shell (n=1) L shell (n=2) e-e- e-e- e-e- e-e- e-e-

Physics 102: Lecture 26, Slide 13 Characteristic x-ray nomenclature n=1 “K shell” n=2 “L shell” n=3 “M shell” 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 e-e- e-e- ejected electron e-e- e-e- e-e- e-e- e-e- e-e- K shell (n=1) L shell (n=2)

Physics 102: Lecture 26, Slide 14 Characteristic x-ray nomenclature n=1 “K shell” n=2 “L shell” n=3 “M shell” 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 e-e- e-e- e-e- e-e- e-e- K shell (n=1) L shell (n=2) X-Ray photon emitted L shell electron falls down e-e- e-e- “K  X-ray” (n=2 n=1 transition)

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

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

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

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

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

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

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

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

Physics 102: Lecture 26, Slide 23 The top spectrum comes from a tube with a silver target (Ag, 47). What is the bottom target? 1) Pd 462) Ag 473) Cd 48 ACT: X-Rays II intensity KK KK KK KK 0 0

Physics 102: Lecture 26, Slide 24 The top spectrum comes from a tube with a silver target (Ag, 47). What is the bottom target? 1) Pd 462) Ag 473) Cd 48 ACT: X-Rays II intensity KK KK Energy of characteristic X-ray is proportional to (Z-1). Higher energy = higher Z KK KK 0 0 o is the same for each!

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

Physics 102: Lecture 26, Slide 26 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 Preflight 27.1

Physics 102: Lecture 26, Slide 27 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 LeadZ=82 Preflight 27.1 Chemical properties (and name) determined by number of protons (Z)

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

Physics 102: Lecture 26, Slide 30 Can get 4 nucleons into n=1 state. Energy will favor N=Z Pauli Principle - neutrons and protons have spin like electron, and thus m s =  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. # protons = # neutrons 7

Physics 102: Lecture 26, Slide 31 See you next time! Read Textbook Sections