Title: An Overview of GammaRay Optics
1An Overview of Gamma-Ray Optics
- Dr. Mike McDermott
- Department of Physics and Astronomy
- Hampden-Sydney College
2Gamma-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
3Gamma-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?
4Model 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)
5Conventional LaserHe-Ne
Image from http//www.fou.uib.no/fd/1996/h/404001/
kap04.htm
6Conventional LaserHe-Ne
- Active medium
- Produces the excited states
- GRASER -- Radioactive Material
- Stimulated Emission
- Resonant Cavity provided by the optics
- Provides gain
7Stimulated 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
8Stimulated 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)
9Recoil
- 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
10Mössbauer 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.
11Time-integrated Mossbauer Spectroscopy Setup
12How do we direct these things?
- Mossbauer will help with resonant absorption.
- What about electronic absorption and attenuation?
- What kind of optics can we use?
13Attenuation 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
14Diffraction 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.
15Borrmann 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
16Nuclear 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.
17Canonical Mössbauer Isotope
- 1st excited state in 57Fe
- M2 Transition
1857Fe 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
19Photon Evolution
- What happens to the photon as it travels through
the medium? - Does the characteristics of the photon change as
it gets absorbed?
20Time 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
21Classical Time-Filtering
2257Fe Decay scheme
23Time-Filtering Experiment
24Results
25Time 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)
26Phase 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.
27Step-wise Phase Modulation of Gamma-Radiation
- First Observed by Helisto in 1991
- Source is stepped forward to change phase of
gamma-ray by p.
28Phase Modulation of Gamma-rays
P. Helisto, I. Tittonen, M. Lippmaa, T. Katila,
Phys. Rev. Lett. 66 2037 (1991)
29Stimulated 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
30Putting it all together
- Construct a single crystal of a Mossbauer Isotope
- Stimulate it with a phase modulation
- Cant this thing spontaneously go off?
31Controlling 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.
32Hyperfine Splitting
33Degenerate 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)
34EIT of Gamma Radiation (First Results)
35So 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 ??????)
36 Thanks!