Atomic Structure and Quantum Mechanics

1
Atomic Structure to X-Rays
2
The Atom
3
Discovery of the Electron

• Cathode Rays
• In 1897 J.J. Thompson proposed the rather
  startling hypothesis that cathode rays were
  streams of negatively charged particles
  electrons.

Red Hot Energy Transfer
-ve cathode
ve anode
4
Discovery of the Nucleus

• Rutherford began studying the scattering of
  a-particles by thin foils of various metals.
• Because the foils consisted of so many layers of
  atoms, it seemed evident that there were no gaps
  between atoms for the a-particles to pass
  through.
• He discovered that occasionally an a-particle
  would be scattered into the backward hemisphere.
• No possible with plum-pudding model

5
The Bohr Atom
6
How do we make Light?

• Need energy
• Need atoms
• Need electrons
• Need energy levels.

7
The Bohr Atom (3)
Difference in energy between two
levels Absorption Emission
8
X-Rays What are they and how do we get them?
9
What are X-rays?

• Electromagnetic photons.
• Originate from electrons energy.
• Do not come from the nucleus - these photons are
  gamma rays.
• No mass - No charge.

10
Wilhelm Rontgen (1845 - 1923)

• November 8th 1895, he noticed a glowing
  fluorescent screen caused by invisible rays
  originating from a cathode ray tube.
• He had discovered X-rays.
• Spent two months investigating the properties of
  x-rays prior to formally reporting the discovery.
• Refused to take out any patents on the discovery.

11
X-Ray Production

• X-rays are produced when highly energetic
  electrons interact with matter, with the
  conversion of the kinetic energy into
  electromagnetic radiation.
• An x-ray system is composed of an electron
  source, an evacuated path to accelerate the
  electrons, a target electrode, and an external
  energy source to accelerate the electrons.
• Production is characterised by two processes 1)
  The Bremmstrahlung process and 2) characteristic
  radiation.

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X-Ray Production
Evacuated Envelope
Electrons
ve charge
-ve Charge
X-Rays
Heated tungsten filament cathode
Copper Anode
Tungsten Target
13
Cathode

• Cathode is the source of electrons in an x-ray
  tube. It is composed of a helical filament made
  of tungsten wire and surrounded by a focusing
  cup.
• A current of 3 - 6 amps is passed through the
  filament. Electrical resistance causes a
  temperature rise.
• Electrons are released from the surface by a
  process called thermionic emission.

14
Anode

• The anode is the target electrode and is held at
  a positive potential difference w.r.t. the
  cathode. Electrons are therefore accelerated to
  the anode.
• Upon collision, x-rays are generated.
• A large amount of heart is generated (99 of
  energy), and this limits the rate of x-ray
  production that can be achieved.
• Tungsten is the most widely used material because
  of its high melting point.

15
The Bremsstrahlung Process.

• The accelerated electrons, on impact with the
  target electrode, convert their KE to other forms
  of energy.
• The majority is unwanted heat (99).
• Occasionally the electron passes close to the
  positively charged nucleus. The electron is
  decelerated, resulting in a loss of KE, which is
  converted to x-rays.
• These photons are called bremsstrahlung or
  braking radiation.

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The Bremsstrahlung Process.

• The amount of energy lost, and therefore the
  energy of the x-ray photon, depends on the angle
  through which the electron is dirverted. This is
  dependent on how close the electron travels to
  the nucleus.
• A bremsstrahlung spectrum depicts the number of
  x-ray photons as a function of energy.

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X-Ray Spectrum
Photon Energy (keV)
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X-Ray Tube
Collimator
X-Ray Beam
Patient
Table
Film
19
Inverse-Square Law Effect
1
Area 4 Exposure 1/4
2
Area 9 Exposure 1/9
3
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Inverse-Square Law Effect

• From a point source, the radiation falls-off as a
  function of the distance squared, i.e. 1/r2
  drop-off.
• Area increases as the square of the distance.
• Product of exposure x area (Dose Area Product)
  remains constant at all point in the beam.

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How much Ionising Radiation is Dangerous?

• 10 Sv Immediate illness and subsequent death
  within a few weeks.
• 1 Sv Temporary illness such as nausea and
  decreased white blood cell count, but not death.
  Probable fatal cancer some years later.
• 0.05 Sv/yr Lowest dose rate where there is any
  evidence of cancer being caused.
• 0.001 - 0.005 Sv/yr Average background radiation
  from natural sources - dependant on location,
  altitude, etc.
• Dose from a Dental X-Ray (0.00001 - 0.00008 Sv)

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Relative risk of 1 in a Million Chance of Dying

• Smoking 1.4 cigarettes (lung cancer)
• Eating 40 tablespoons of peanut butter
• Spending 2 days in New York City (air pollution)
• Driving 40 miles in a car (accident)
• Flying 2500 miles in a jet (accident)
• Canoeing for 6 minutes (drowning)
• Receiving 0.1 mSv of radiation (cancer)