Outline of Presentations

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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 !
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Light Waves
Until about 1900, the classical wave theory of
light describedmost observed phenomenon.

• Light wavesCharacterized by
• Amplitude (A)
• Frequency (n)
• Wavelength (l)
• Move at speed c invacuum.
• Energy of wave ? A2

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Light Particle or Wave ?
OR
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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 were1) The
Photo-Electric Effect (1905) 2) The Compton
Effect (1923)
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Photoelectric Effect (I)
Classical Method
Increase energy by increasing amplitude
electrons emitted ?
electrons emitted ?
No electrons were emitted until 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
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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 theconstant h 6.6x10-34 J s is
  Plancks constant.

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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 Plancks 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 energyand
  momentum.

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The Electromagnetic Spectrum
Shortest wavelengths (Most energetic photons)
E hn hc/l
Longest wavelengths (Least energetic photons)
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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 l2l1.
It was found that the scattered X-ray did not
have the same wavelength ?
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Quantum Picture to the Rescue
Compton found that if you treat the photons as if
they were particles of zero mass, with energy
Ehc/l and momentum ph/l ? The collision
behaves just as if it were 2 particles colliding
!Photon behaves like a particle with energy
momentum as given above!
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Photons, Digital Camera Images
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.
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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.
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Wavelength versus Size
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
its wavelength.
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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 dont !
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Electron Microscope

• The electron microscope uses 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 !
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What is Matter - A Sense of Scale
But how do weknow any of this ?
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Uncovering matter
Before 1900, scientists knew aboutradioactivity.
They knew that certain isotopes emittedvarious
types of penetrating radiation. Known were
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Scattering Experiments
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.
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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 cameThe calculation,
based on the plum-pudding model, was that fewer
than 1/10,000,000,000 should do this ???
Gold foil
In Rutherfords wordsIt 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 ???
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The (only) interpretation
The atom must have a solid core capable of
imparting largeelectric forces onto an incoming
(charged) particle.
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Neils Bohr and the Quantum Atom
Circa 1913

• Pointed out serious problems with
• Rutherfords 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.
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Bohrs Picture of the Atom
Electrons circle the nucleus due to the Electric
force
Note There are many more energy levels beyond
n5, they are omitted for simplicity
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Hydrogen atom energy levels
Quantum physics provides the tools to compute
the values of E1, E2, E3, etcThe 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.
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Some Other Quantum Transitions
UV
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James Chadwick and the Neutron
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.

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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
its 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