An Overview of GammaRay Optics

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An Overview of Gamma-Ray Optics

• Dr. Mike McDermott
• Department of Physics and Astronomy
• Hampden-Sydney College

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Gamma-Ray Optics

• Area of research that started in late 1980s
• Originally born out of SDI (Star Wars)
• Ultimate goal is to build a Gamma-Ray Laser
• (GRASER)
• Spectrally very pure (Linewidths 10-9 eV!)
• Applications include Materials, Medical, Defense

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Gamma-Ray LaserSome Problems.

• Currently a Gamma-Ray Laser does not exist
• Gamma-Rays
• Absorption (or possibly lack of it)
• How do you control the direction?
• What about stimulated emission?

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Model for a GRASER

• Use conventional lasers as a guide
• Know we will need the Mössbauer effect
• Know it will be a solid state laser (because of
  the Mössbauer Effect)

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Conventional LaserHe-Ne
Image from http//www.fou.uib.no/fd/1996/h/404001/
kap04.htm
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Conventional LaserHe-Ne

• Active medium
• Produces the excited states
• GRASER -- Radioactive Material
• Stimulated Emission
• Resonant Cavity provided by the optics
• Provides gain

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Stimulated EmissionEmission Profile of a
PhotonConventional Laser

• Emitted photon energy width is given by
  Heisenberg
• DGDt h
• The linewidth has the shape of a Lorentzian Line

Image from http//mathworld.wolfram.com/Lorentzian
Function.html
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Stimulated EmissionAbsorption of Photons

• The absorption profile is also a Lorentzian.
• The probability of absorption occurring can be
  calculated by the overlap of the emission
  linewidth and the absorption linewidth
• Stimulated Emission also depends on this overlap
  (resonance)

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Recoil

• When a nucleus emits a photon, there is a change
  in momentum.
• mNv Eg/c
• This resulting velocity produces a reduction in
  energy by
• DE ½ mNv2

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Mössbauer Effect

• Discovered in 1957 Rudolph Mössbauer
• Gamma Rays do not recoil when emitted or absorbed
  because the entire lattice takes up the recoil
  energy
• Recoilless Absorption and Emission of Gamma
  Radiation
• To get the emitted line to overlap with the
  absorption line, we will need to use the
  Mössbauer Effect. This requires us to use a
  solid state material.

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Time-integrated Mossbauer Spectroscopy Setup

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How do we direct these things?

• Mossbauer will help with resonant absorption.
• What about electronic absorption and attenuation?
• What kind of optics can we use?

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Attenuation of Radiation

• Attenuation is given by
• I I0e-mt
• The attenuation is related to the amplitude of
  the electric field of the photon at the atomic
  site (electron) that is absorbing the photon

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Diffraction of Radiation through a Solid

• If we have a solid we have some form of crystal
  forming.
• If the crystal is perfect (single crystal) then
  the atoms are arranged in a well defined fashion.
• Braggs Law says that at certain angles, the
  radiation will see the atoms as sets of planes
  and will reflect from them.

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Borrmann Effect

• Anomolous Transmission of Radiation through a
  single crystal
• Electric field of wave is zero (or near zero) at
  each atomic site therefore electronic absorption
  is minimized
• Produces a standing wave in the crystal
• Allows transmission of radiation through a sample
  that would otherwise be too thick

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Nuclear Borrmann Effect

• Electric field is near zero at the atomic sites.
• Since photons are electro-magnetic waves, what is
  the amplitude of the magnetic field at each
  atomic site?
• The standing wave solution for the electric field
  being zero at the atomic sites also gives rise to
  the magnetic field being a maximum at the atomic
  sites.

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Canonical Mössbauer Isotope

• 1st excited state in 57Fe
• M2 Transition

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57Fe Mössbauer Effect

• First excited state of 57Fe is a magnetic dipole
  transition.
• Absorption and emission is through magnetic
  radiation instead of electric.
• If the magnetic field of a resonant photon is a
  maximum at the atomic site, there is a very high
  probability for absorption.
• Possibility for Stimulated Emission

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Photon Evolution

• What happens to the photon as it travels through
  the medium?
• Does the characteristics of the photon change as
  it gets absorbed?

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Time Evolution of Gamma Radiation Through a
Resonant Filter

• First published by Lynch, Holland, and Hamermesh
• Showed a speed-up of the emitted radiation
• Explained classically via absorption

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Classical Time-Filtering

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57Fe Decay scheme
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Time-Filtering Experiment

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Results

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Time Filtering Angular Dependence

• Classically time-filtering should only occur in
  the forward direction.
• Quantum Theory says the time filtering dependence
  should be preserved in non-forward directions
  (Hoy).
• This is currently being measured at
  Hampden-Sydney (Jaysen Stokes)

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Phase Problem in Stimulated Emission

• When a photon is absorbed and re-emitted, there
  is a p phase shift.
• Hoy used this to quantum mechanically explain the
  Mössbauer Effect
• If we are trying to stimulate emission, we need
  to solve this problem.

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Step-wise Phase Modulation of Gamma-Radiation

• First Observed by Helisto in 1991
• Source is stepped forward to change phase of
  gamma-ray by p.

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Phase Modulation of Gamma-rays

P. Helisto, I. Tittonen, M. Lippmaa, T. Katila,
Phys. Rev. Lett. 66 2037 (1991)
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Stimulated Emission of Gamma-Rays

• Take Helistos Phase Modulator and replace
  absorber with another radioactive source
• Apparatus is currently built, needs to be tested
• Need Second source

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Putting it all together

• Construct a single crystal of a Mossbauer Isotope
  
• Stimulate it with a phase modulation
• Cant this thing spontaneously go off?

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Controlling absorption of nuclear resonant gamma
rays

• Possible to make the nucleus transparent to
  incoming gamma rays.
• FeCO3 shown to have this property by the Gamma
  Optics Group at IKS in Leuven, Belgium
• Nuclear hyperfine states of 57Fe become
  degenerate because of local fields.

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Hyperfine Splitting

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Degenerate Fields

• Strong Magnetic Fields in FeCO3 cause two of the
  hyperfine levels to overlap at 31.5 K.
• Degeneracy make it difficult for the photon to
  get absorbed into a well defined state so there
  is no absorption
• Electromagnetic Induced Transparency (EIT)

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EIT of Gamma Radiation (First Results)

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So why isnt it work yet?

• Need to find a suitable Isotope
• Key is to make perfect single crystals
• Test the control Mechanism
• Currently under way in Leuven (EIT)
• Currently underway at H-SC (angular dependence)
• Test phase modulated stimulated emission
• To be done at H-SC (when ??????)

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Thanks!

• Questions?????