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The Picture by ~1932

The Picture by ~1932. Electrons were discovered ~1900 by J. J. Thomson Protons being confined in a nucleus was put forth ~1905 Neutrons discovered 1932 by James Chadwick Quantum theory of radiation had become “widely accepted”, although even Einstein had his doubts . HW8 due Wed.

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The Picture by ~1932

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  1. The Picture by ~1932 • Electrons were discovered ~1900 by J. J. Thomson • Protons being confined in a nucleus was put forth ~1905 • Neutrons discovered 1932 by James Chadwick • Quantum theory of radiation had become “widely accepted”, although even Einstein had his doubts HW8 due Wed.

  2. Quick recap on radiation from atoms • Energetic gamma rays come from excited nuclei (Co60, for example)These photons emerge from the nucleus of the atom!!! • They are generally in the gamma ray region of the EM spectrum------------------------------------------------------------------------------ • Ordinary atoms also radiate photons when their atomic electrons “fall” from a higher energy state to a lower one. (The configuration where all the electrons are in their lowest energy state is referred to as the ground state) • The transitions of atomic electrons from a high energy state to alower energy state produces radiation (light)! • The radiation which emerges when electrons make these transitions(ie., quantum transitions) is generally in the visible or X-ray region. Let’s continue on this issue of transitions in atoms…

  3. Before After Radiatedphoton n = Electronin lowest“allowed”energy level (n=1) 5 5 4 4 3 3 2 2 1 1 Electronin excitedstate (n=5) Electron falls to the lowest energy level Allowed Orbits Bohr Atom & Radiation Electrons circle the nucleus due to the Electric force Note: There are many more energy levels beyond n=5, they are omitted for simplicity

  4. Energy Electronin excitedstate(higher PE) Energy Electronin loweststate(lower PE) E5 E5 n = 5 n = 5 E4 E4 n = 4 n = 4 E3 E3 n = 3 n = 3 E2 E2 n = 2 n = 2 E1 E1 n = 1 n = 1 Before After Atomic Radiation It is now “known” that when an electron is in an “excited state”,it spontaneously decays to a lower-energy stable state. E5 > E4 > E3 > E2 > E1 • The difference in energy, DE, is given by:DE = E5 – E1 = hn = Ephotonh = Planck’s constant = 6.6x10-34 [J s] • = frequency of light [hz] The energy of the light is DIRECTLY PROPORTIONAL to the frequency, n.Recall that the frequency, n, is related tothe wavelength by:c = n l (n = c / l)So, higher frequency  higher energy  lower wavelengthThis is why UV radiation browns your skinbut visible light does not ! One example could be:

  5. 5 4 3 2 1 Hydrogen atom energy “levels” Quantum physics provides the tools to compute the values ofE1, E2, E3, etc…The results are: En = -13.6 / n2 These results DO DEPEND ON THE TYPE OF ATOM OR MOLECULE So, the difference in energy between the 3rd and 1st quantum state is: Ediff = E3 – E1 = -1.51 – (-13.6) = 12.09 (eV) When this 3 1 atomic transition occurs, this energy is released in the form of electromagnetic energy.

  6. Example 4 In the preceding example, what is the frequency, wavelength of theemitted photon, and in what part of the EM spectrum is it in? E = 12.1 [eV]. First convert this to [J]. Since E = hn n = E/h, so: n = E/h = 1.94x10-18 [J] / 6.6x10-34 [J s] = 2.9x1015 [1/s] = 2.9x1015 [hz] l = c/n = (3x108 [m/s]) / (2.9x1015 [1/s]) = 1.02x10-7 [m] = 102 [nm] This corresponds to low energy X-rays !

  7. Some Other Quantum Transitions

  8. This completed the picture, or did it… • Electrons were discovered ~1900 by J. J. Thomson • Protons being confined in a nucleus was put forth ~1905 • Neutrons discovered 1932 by James Chadwick • Quantum theory of radiation had become“widely accepted”, although even Einstein had his doubts • Radiation is produced when atomicelectrons fall from a state of highenergy  low energy. Yields photonsin the visible/ X-ray region. • A nucleus can also be excited, andwhen it “de-excites” it also givesoff radiation  Typically gamma rays !

  9. Cosmic Rays • Cosmic Rays are energetic particles that impinge on our atmosphere (could be from sun or other faraway places in the Cosmos) • They come from all directions. • When these high energy particles strike atoms/molecules in our atmosphere, they produce a spray of particles. • Many “exotic” particles can be created.As long as they are not so massive as to violate energy conservation they can becreated. • Some of these particles are unstable and “decay” quickly into other stable particles. • Any of these exotic particles which live longenough to reach the surface of the earth can bedetected !

  10. 1932 : Discovery of the antiparticle of the electron, the positron. Confirmed the existence and prediction that anti-matter does exist!!! Discoveries in Cosmic Rays • 1937 : Discovery of the muon. It’s very much like a “heavy electron”. • 1947 : Discovery of the pion. We’ll touch on these today… and some other things…

  11. Positron Discovery in Cosmic Rays (1932) A “Cloud Chamber” is capable of detecting charged particles as they pass through it. The chamber is surrounded by a magnet.The magnet bends positively charged particles in one direction, and negatively charged particles in the other direction. By examining the curvature above and belowthe lead plate, we can deduce: (a) the particle is traveling upward in this photograph. (b) it’s charge is positive Using other information about how far it traveled, it can be deduced it’s not a proton. Cloud Chamber Photograph Lead plate Larger curvatureof particleabove platemeans it’s moving slower(lost energy as itpassed through) It’s a particle who’s mass is same as electronbut has positive charge  POSITRON ! Positron

  12. If an electron and a positron collide, they ANNIHILATE andform pure energy (EM Radiation).This conversion of matter into energy is a common event in the life of physicists that studies these little rascals… Significance of Positron Discovery The positron discovery was the first evidence for ANTIMATTER.That is, the positron has essentially all the same properties as an electron, except, it’s charge is positive ! Carl Andersonaward Nobelprize for thediscovery of thepositron Carl Anderson1905-1991

  13. e+ e+ e+ e- e- e- E=5 [MeV] e+ e- e+ e- E=5 [MeV] Example: Matter  Energy E=5 [MeV] E=5 [MeV] %*&* An electron and positron, eachwith energy 5 [MeV] collide,and annihilate into pure energyin the form of 2 photons.Each photon carries away ½of the total energy available.

  14. = 1 Example follow-up In the preceding example, what are the wavelengths of thephotons which emerge from this interaction, and from whatpart of the spectrum are they? Since E=hc / l, We can get wavelength using: l = hc/ E First we need to convert the 5 [MeV] to the equivalent number of [J]First note that: 5 [MeV] = 5x106 [eV] l = hc/ E = (6.6x10-34)(3x108) / 8.0x10-13 = 2.5x10-13 [m] You will find that this corresponds to gamma rays ! Very energetic photons !!!

  15. Discovery of the muon • The muon was discovered in 1937 by J. C. Street and E. C. Stevenson in a cloud chamber. • Again, the source is cosmic rays produced in the atmosphere. • The muon behaves identally to an electron, except: It is about 200 times as massive • It’s unstable, and decays in about 2x10-6 [s] = 2 [ms](m e + n + n) More on these n guys later ! • Note that many muons are able to reach the earth from the upper atmosphere because of time dilation ! Because of their large speed, weobserve that their “clocks” run slow  they can live longer !!!

  16. Discovery of the Pion • Cecil Powell and colleagues at Bristol University used alternate types of detection devices to see charged tracks (called “emulsions”) in the upper atmosphere. • In 1947, they annouced the discovery of a particle called the p-meson or pion (p) for short. Cecil Powell1903-19691950 NobelPrize winner Muon (m)comes to resthere, and thendecays: m e + n + n Two more neutrinos are also produced but also escapeundetected. Pion (p)comes to resthere, and thendecays: p m + n + n Two neutrinos are also produces but escapeundetected. m e p

  17. The Plethora of Particles Because one has no control over cosmic rays (energy, types of particles, location, etc), scientists focused their efforts on accelerating particles in the lab and smashing them together. Generically people refer to them as “particle accelerators”. (We’ll come back to the particle accelerators later…) Circa 1950, these particle accelerators began to uncover many newparticles. Most of these particles areunstable and decay very quickly, and hence had not been seen in cosmic rays.Notice the discovery of theproton’s antiparticle, theantiproton, in 1955 !Yes, more antimatter !

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