Outline of Presentations • Wave Particle Duality and the Quantum (Bohr) Atom(Dr. Steven Blusk) • Particle Discoveries in Cosmic Rays and Accelerators(Dr. Tomasz Skwarnicki) • Making Sense of it All - The Standard Model(Dr. Marina Artuso) • The Instruments and Techniques of Discovery: Particle Accelerators and Detectors(Dr. Sheldon Stone) We encourage you to ask questions as we progress..These slides will be posted on the web soon !
Light Waves Until about 1900, the classical wave theory of light describedmost observed phenomenon. • Light waves:Characterized by: • Amplitude (A) • Frequency (n) • Wavelength (l) • Move at speed “c” invacuum. Energy of wave A2
And then there was a problem… In the early 20th century, several effects were observed which could not be understood using the wave theory of light.Two of the more influential observations were:1) The Photo-Electric Effect (~1905) 2) The Compton Effect (1923)
Photoelectric Effect (I) What if we try this ? Vary wavelength, fixed amplitude No No Yes, withlow KE No Yes, withhigh KE No No “Classical” Method Increase energy by increasing amplitude electrons emitted ? electrons emitted ? No electrons were emitteduntil the frequency of the light exceeded a critical frequency, at which point electrons were emitted from the surface! Light behaving like a particle with E 1/l
Photo-Electric Effect (II) “Light particle” Before Collision After Collision • In the latter “quantum-mechanical” picture, the energy of the light particle (photon) must overcome the binding energy of the electron to the nucleus. • If the energy of the photon exceeds the binding energy, the electron is emitted with a KE = Ephoton – Ebinding. • The energy of the photon is given by E = hn = hc/l, where theconstant h = 6.6x10-34 [J s] is Planck’s constant.
Photons • In Quantum theory light is composed of individual quanta (or wave packets) called photons. • According to quantum theory, each photon has an energy given byE = hn = hc/lh = 6.6x10-34 [J s]Planck’sconstant, • 10 photons have an energy equal to ten times a single photon. • The photoelectric effect cannot be understood via a Wave Picture. We must regard light as composed of particles, each carrying energyand momentum.
The Electromagnetic Spectrum Shortest wavelengths (Most energetic photons) E = hn = hc/l Longest wavelengths (Least energetic photons)
The Compton Effect Incident X-raywavelength l1 M A T T E R Scattered X-raywavelength l2 l2 >l1 e Electron comes flying out In 1924, A. H. Compton performed an experiment where X-rays impinged on matter, and he measured the scattered radiation. Problem: According to the wave picture of light, the incident X-ray should give up some of its energy to the electron, and emerge with a lower energy (i.e., the amplitude is lower), but should have l2=l1. It was found that the scattered X-ray did not have the same wavelength ?
Quantum Picture to the Rescue Electroninitially atrest (almost) Scattered X-ray E2 = hc / l2 Incident X-rayE1 = hc / l1 l2 >l1 e e Ee e Compton found that if you treat the photons as if they were particles of zero mass, with energy E=hc/l and momentum p=h/l The collision behaves just as if it were 2 particles colliding !Photon behaves like a particle with energy & momentum as given above!
Photons, Digital Camera & Images ~10,000 photons ~3000 photons ~100,000 photons ~1M photons ~4 M photons ~30 M photons Using a digital camera with manypixels !A given pixel is very, very small gives fine image resolution The individual spots on thisimage and on the previous oneare the actual results of individual photons striking thepixel array.
How do we see ? Light reflects (or photonsscatter) from a surface and reaches our eye.Our eye/brain forms an image of the object.
Wavelength versus Size But what if we want to “see” smaller things, like inside an atom, or inside the nucleus, or even inside a nucleon ??? Even with a visible light microscope, we are limited to beingable to resolve objects which are at least about:10-6 [m] = 1 [mm] = 1000 [nm] in size.This is because visible light, with a wavelength of ~500 [nm]cannotresolve objects whose size is smaller than it’s wavelength.
Matter Waves ? “If light can behave like a particle, might particles act like waves”? Louis de Broglie The short answer is YES. The explanation lies in the realm of the uncertainty principle & quantum mechanics, Particles, like photons, also have a wavelength given by: l = h/p = h / mv That is, the wavelength of a particle depends on its momentum, just like a photon!The main difference is that matter particles have mass, and photons don’t !
Electron Microscope • Theelectron microscopeuses the wave behavior of electrons to make images which are otherwise undiscernable for visible light! This image was taken with a Scanning Electron Microscope (SEM). These devices can resolve features downto about 1 [nm]. This is about 100 times better than can be done with visible light microscopes! IMPORTANT POINT HERE: High energy particles can be used to reveal the structure of matter !
What is Matter - A Sense of Scale <1x10-18[m] ~5x10-6[m] ~2x10-9[m] ~2x10-10[m] ~5x10-15[m] ~1.5x10-15[m] q e But how do weknow any of this ?
Uncovering matter a (42He) p n n p b e g-ray Before ~1900, scientists knew aboutradioactivity.They knew that certain isotopes emittedvarious types of penetrating radiation. Known were:
Scattering Experiments In 1911, Rutherford set out to test this hypothesis. Alphaparticlesource a Around ~1900, the structure of the atom was not known. Common thinking was that it was like a plum-pudding Calculations, based on the known laws of electricity and magnetism showed that the heavy alpha particles should be only slightly deflected by this “plum-pudding” atom… Ernest Rutherford1871-1937 Awarded the Nobel Prize in 1908 The calculations suggestedthat a negligible fraction ofthe alpha particles should be scattered by more than90o.
Au Contraire a a Contrary to expectations, Rutherford found that a significantly large fraction (~1/8000) of the alpha particles “bounced back” in the same direction in which they came…The calculation, based on the plum-pudding model, was that fewer than 1/10,000,000,000 should do this ??? Gold foil In Rutherford’s words…“It was quite the most incredible event that ever happened to me in my life. It was as if you fired a 15-inch naval shell at a piece of tissue paper and the shell came right back and hit you.” Huh ???
The (only) interpretation a a a a a The atom must have a solid core capable of imparting largeelectric forces onto an incoming (charged) particle.
Neils Bohr and the Quantum Atom Circa 1913 • Pointed out serious problems with • Rutherford’s atom • Electrons should radiate as they orbit the nucleus, and in doing so, lose energy, until they spiral into the nucleus. • Atoms only emit quantized amounts of energy (i.e., as observed in Hydrogen spectra) 1885-1962 • He postulated Awarded the Nobel Prize in 1922 • Electric force keeps electrons in orbit • Only certain orbits are stable, and they do not radiate energy Radiation is emitted when an e- jumps from an outer orbit to an inner orbit and the energydifference is given off as a radiation.
Bohr’s Picture of the Atom 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 Electrons circle the nucleus due to the Electric force Note: There are many more energy levels beyond n=5, they are omitted for simplicity
Hydrogen atom energy “levels” 5 4 3 2 1 So, the drop in PE between the 3rd and 1st quantum state is: Ediff = E1 – E3 = -13.6 – (-1.51) = - 12.09 (eV) Quantum physics provides the tools to compute the values of E1, E2, E3, etc…The results are: En = -13.6 / n2 These results DO DEPEND ON THE TYPE OF ATOM OR MOLECULE The energy difference is given off in the form of EM Radiation.That is, a photon.
James Chadwick and the Neutron 42a + 94Be 126 C +10 n Circa 1925-1932 Picked up where Rutherford left off with more scattering experiments… • Performed a series of scattering experiments • with a-particles 1891-1974 Awarded the Nobel Prize in 1935 • Applying energy and momentum conservation he found that the mass of this new object was ~1.15 times that of the proton mass. • Chadwick postulated that the emergent radiation • was from a new, neutral particle, the neutron.
This completed the picture, or did it… Electrons had been know about since ~1900 (J. J. Thomson et al) By ~1932 Collisions of alpha particles with mattergave us the picture that the atom has adense core at it’s center composed ofprotons & neutrons. The fundamental units of matter areprotons, neutrons and electrons. Atomic spectra could be understood fromquantum theory. Photons acting like particles, well OK…