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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)

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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 describedmost observed phenomenon.

  • Light waves:Characterized by:

  • Amplitude (A)

  • Frequency (n)

  • Wavelength (l)

  • Move at speed “c” invacuum.

    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)

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

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

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

The Electromagnetic Spectrum

Shortest wavelengths

(Most energetic photons)

E = hn = hc/l

Longest wavelengths

(Least energetic photons)


The compton effect

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.


Some other quantum transitions

Some Other Quantum Transitions

UV


James chadwick and the neutron

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

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…


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