Accelerator Principles and Techniques

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Accelerator Principles and Techniques

• Jason M. Hill
• Funded by Wittenberg University

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Overview

• Why Use Accelerators?
• PIXE, RBS
• Our Accelerator
• Schematics, details
• Work I did
• Vacuum system
• Magnetic steering, focusing, alignment
• Field Trip

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What is an accelerator?

• An accelerator produces a beam of a particular
  type of particle at a specific energy.

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Common Uses

• PIXE Particle Induced X-ray
  Emission
  
• RBS Rutherford Back Scattering

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PIXE

• Used for analysis of small quantities of
  elements.
• Sample bombarded by protons, alpha particles, or
  heavy positive ions.
• Coulomb interaction can remove target electrons,
  ionizing the target.

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PIXE, Contd

• A certain number of K and L shell electrons will
  be removed in the ionization process.
• An outer electron will jump inward to fill the
  vacancy, giving off x-rays at energy specific to
  the type of atom.

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RBS

• Rutherford Back Scattering
• Positive charges directed towards a sample.
• Energy of back scattered particle determines the
  mass of the target nucleus.
• If the target is not of uniform composition, the
  distribution of back scattered energies
  determines relative compositions.

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Accelerator Requires

• Source of charged particles
• hot filament emits electrons
• ionized atoms from ion source
• Accelerating Voltage (100s of kV)
• Focusing/Steering Apparatus
• Beam tends to diverge like natural light
• Target

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Requirements, Contd

• Method for energy selection
• Electromagnet bends beam (Depends on energy)
• Detection and analysis devices
• Scattering Chamber
• Uniform Vacuum
• Heavy ions easily impeded by air and other
  materials

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Vacuum System

• Pressure on the order of 10-5 microns is needed
  to perform experiments.
• Both beam tube and scattering chamber must be
  evacuated.

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Mechanical Pump

• Also known as a backing pump in this setting.
• Rotor rotates within the stator which is driven
  by a motor.
• Gas is drawn in, compressed, and exhausted to the
  atmosphere through a spring loaded exhaust valve.

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Diffusion Pump

• Pump oil is heated and the vaporized fluid rises.
• The vapor is then directed back downward into a
  vapor jet.
• The momentum of the heavy, fast-moving oil
  molecules is imparted to the gas molecules at the
  inlet flange.

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Diffusion Pump

• Gas molecules are compressed by the vapor jet.
• The gas molecules are then removed by the
  mechanical pump.
• Cooling lines help condense the vapor and return
  it to the base of the pump heater.

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Low Pressure Measurement

• Thermocouple Gauge
• Ionization Gauge

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Thermocouple Gauge

• Used for measuring pressures from 10 microns up
  to atmospheric pressure.
• Constant current is run through a small filament.
• The temperature of the filament is dependent on
  the surrounding gas--the less gas pressure, the
  hotter the filament gets.

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Thermocouple Gauge

• The filament temperature is converted to a
  voltage by a thermocouple and read out on an
  analog scale.

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Ionization Gauge

• Used for low pressure measurement (below the
  range of the thermocouple gauge).
• An electron source (hot filament) lies in an
  electric field.
• The electrons are energetic enough to ionize
  surrounding gas particles.
• Collector measures ion current which is
  proportional to pressure.

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Problems

• Bad seals
• Mechanical feeds into scattering chamber
• Packed grease seals

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Momentum Selection

• Magnetic field points downward, causing positive
  charge to bend towards angled end.
• Bending radius under a fixed magnetic field and
  accelerating voltage is related to the charge and
  mass of the accelerated particle by the following

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Momentum Selection

• Since our radius of curvature is fixed, we can
  record the magnetic field needed to direct a
  particle with a given mass and charge to the
  proper radius of curvature.

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Determining the Magnetic Field

• A Hall Probe was inserted in the center of the
  region between the magnetic pole faces and field
  vs.. current was mapped out.

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Determining the Magnetic Field

• In order to calculate the trajectory of ions
  through our field, we must first develop a model
  of the field.
• Due to fringing, the magnetic field is not
  uniform throughout the region.

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Mapping the Field

• This function was divided into three segments.
• Equations for force, acceleration, velocity, and
  position were entered into a spreadsheet to find
  what value of Bmaxgives the appropriate curvature.

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Selecting Momentum

• The appropriate value of Bmax was then documented
  as specific to a particle with a particular (q/m)
  at a specified accelerating voltage.
• Analysis was done for molecular hydrogen, singly
  ionized hydrogen, deuterium, Helium-3 and
  Helium-4.

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Focusing the Beam

• Quadrupole magnets focus the beam.

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Current Status

• We have obtained a beam using the hydrogen
  source.
• Source produces both H and H2.
• The two different species are seen when we bend
  the beam.
• Scattering chamber not in use yet.
• Need to better seal the feed-throughs.

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References

• Chambers, A., et. al. Basic Vacuum
  Technology. New York Adam Hilger, Bristol,
  1989.
• Krane, Kenneth S. Introductory Nuclear Physics.
  New York John Wiley and Sons, 1987.
• Segre, Emilio. Nuclei and Particles, 2nd ed.
  London Benjamin/Cummings, 1977.

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Acknowledgements

• Dr. Elizabeth George
• Dr. Al Frasca
• Dr. Paul Voytas