ParticlePhysicsFest Talk1

Outline of Presentations: 

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: 

Light Waves Until about 1900, the classical wave theory of light described most observed phenomenon. Light waves: Characterized by: Amplitude (A) Frequency (n) Wavelength (l) Move at speed “c” in vacuum. Energy of wave  A2

Light: Particle or Wave ?: 

Light: Particle or Wave ? OR

And then there was a problem…: 

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): 

Photoelectric Effect (I) No electrons were emitted until the frequency of the light exceeded a critical frequency, at which point electrons were emitted from the surface! electrons emitted ? Increase energy by increasing amplitude “Classical” Method electrons emitted ? Light behaving like a particle with E  1/l

Photo-Electric Effect (II): 

Photo-Electric Effect (II) 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 the constant h = 6.6x10-34 [J s] is Planck’s constant.

Photons: 

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 by E = hn = hc/l h = 6.6x10-34 [J s] Planck’s constant, 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 energy and momentum.

The Electromagnetic Spectrum: 

The Electromagnetic Spectrum Shortest wavelengths (Most energetic photons) Longest wavelengths (Least energetic photons)

The Compton Effect: 

The Compton Effect 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: 

Quantum Picture to the Rescue 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: 

Photons, Digital Camera & Images Using a digital camera with many pixels ! A given pixel is very, very small  gives fine image resolution The individual spots on this image and on the previous one are the actual results of individual photons striking the pixel array.

How do we see ?: 

How do we see ? Light reflects (or photons scatter) from a surface and reaches our eye. Our eye/brain forms an image of the object.

Wavelength versus Size: 

Wavelength versus Size Even with a visible light microscope, we are limited to being able 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] cannot resolve objects whose size is smaller than it’s wavelength.

Matter Waves ?: 

Matter Waves ? “If light can behave like a particle, might particles act like waves”? 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 ! Louis de Broglie

Electron Microscope: 

Electron Microscope This image was taken with a Scanning Electron Microscope (SEM). These devices can resolve features down to about 1 [nm]. This is about 100 times better than can be done with visible light microscopes! The electron microscope uses the wave behavior of electrons to make images which are otherwise undiscernable for visible light! IMPORTANT POINT HERE: High energy particles can be used to reveal the structure of matter !

What is Matter - A Sense of Scale: 

What is Matter - A Sense of Scale But how do we know any of this ?

Uncovering matter: 

Uncovering matter Before ~1900, scientists knew about radioactivity. They knew that certain isotopes emitted various types of penetrating radiation. Known were:

Scattering Experiments : 

Scattering Experiments Ernest Rutherford 1871-1937 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… Awarded the Nobel Prize in 1908 The calculations suggested that a negligible fraction of the alpha particles should be scattered by more than 90o.

Au Contraire: 

Au Contraire 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 Huh ??? 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.”

The (only) interpretation: 

The (only) interpretation The atom must have a solid core capable of imparting large electric forces onto an incoming (charged) particle.

Neils Bohr and the Quantum Atom: 

Neils Bohr and the Quantum Atom 1885-1962 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) He postulated 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 energy difference is given off as a radiation. Awarded the Nobel Prize in 1922

Bohr’s Picture of the Atom: 

Electrons circle the nucleus due to the Electric force Bohr’s Picture of the Atom Note: There are many more energy levels beyond n=5, they are omitted for simplicity

Hydrogen atom energy “levels”: 

Hydrogen atom energy “levels” Quantum physics provides the tools to compute the values of E1, E2, E3, etc…The results are: En = -13.6 / n2 The energy difference is given off in the form of EM Radiation. That is, a photon. These results DO DEPEND ON THE TYPE OF ATOM OR MOLECULE

Some Other Quantum Transitions: 

Some Other Quantum Transitions UV

James Chadwick and the Neutron: 

James Chadwick and the Neutron Awarded the Nobel Prize in 1935 1891-1974 Circa 1925-1932 Performed a series of scattering experiments with a-particles Applying energy and momentum conservation he found that the mass of this new object was ~1.15 times that of the proton mass. Picked up where Rutherford left off with more scattering experiments… Chadwick postulated that the emergent radiation was from a new, neutral particle, the neutron.

This completed the picture, or did it…: 

This completed the picture, or did it… The fundamental units of matter are protons, neutrons and electrons. Atomic spectra could be understood from quantum theory. Photons acting like particles, well OK… Collisions of alpha particles with matter gave us the picture that the atom has a dense core at it’s center composed of protons & neutrons. Electrons had been know about since ~1900 (J. J. Thomson et al) By ~1932