1 Historical Developments and Future Perspectives …
13
electronic scattering, the nuclear Borrmann effect, and the suppression of the
inelastic channel has to be mentioned.
• Phase sensitive optics will allow interference experiments with a large coherence
length.
• Gravitation experiment in tribute of the very difficult and famous red-shift experiment by Pound and Rebka [30].
• Small angle scattering, known as small angle x-ray scattering (SAXS), may be
expanded to the Mössbauer radiation to become sensitive as well to magnetic and
electric domains/distributions.
This is an impressive list of topics, which was updated for the final technical design
report of the ESRF [31]. Remarkable from a present-day perspective is the focus on
investigations of “basic science” effects, spectroscopy is nearly absent. Looking back,
the explanation might be simply the background of the involved people. They were
mainly grown up in nuclear physics.
As we will witness in the Sects. 1.7 and 1.8 the development over the years went
after all mainly in a different direction.
1.2.2 Technical Challenges
The main challenge was the unfavorable signal-to-noise-ratio. SR is thought to be a
“white” radiation source, i.e., the energy band width of the radiation coming from
the bending magnets of the synchrotron spans from literally “zero” to several tens
keV. The low energy part could easily be absorbed, however, still a band width of
let’s say 20 keV was left. On the other side, in case of the 14.4 keV nuclear level in
57 Fe, the nuclear level width is 4.66 neV. That means a signal-to-noise-ratio of about
10
−13 . How to find those resonant quanta in this haystack?
In the following we will discuss various ideas and avenues.
1.2.2.1 Time and Polarization Properties
As discussed above, SR is a pulsed radiation with pulse widths t in the 100 ps regime
and—in principle—selectable repetition rate. That means that one may accordingly
adopt the time structure to the life time τ 0 of the excited nuclear levels. Then it is
possible to excite the nuclear levels instantaneously (t < τ 0 ) and to record their
de-excitation products such as γ -rays and conversion electrons inbetween the synchrotron radiation pulses. Due to the fact that electronic scattering processes are very
fast one should see after very intense “prompt” scattering processes only the “slow”
scattered γ -radiation from the scattering via the nuclear resonances. That means one
would be able to generate out of the SR a pure beam of γ -radiation through time
discrimination. Unfortunately, even with todays fast detector systems, it is vrtually
impossible for the detectors to sustain the extreme count rates beyond 10
9 photons/s.
13
electronic scattering, the nuclear Borrmann effect, and the suppression of the
inelastic channel has to be mentioned.
• Phase sensitive optics will allow interference experiments with a large coherence
length.
• Gravitation experiment in tribute of the very difficult and famous red-shift experiment by Pound and Rebka [30].
• Small angle scattering, known as small angle x-ray scattering (SAXS), may be
expanded to the Mössbauer radiation to become sensitive as well to magnetic and
electric domains/distributions.
This is an impressive list of topics, which was updated for the final technical design
report of the ESRF [31]. Remarkable from a present-day perspective is the focus on
investigations of “basic science” effects, spectroscopy is nearly absent. Looking back,
the explanation might be simply the background of the involved people. They were
mainly grown up in nuclear physics.
As we will witness in the Sects. 1.7 and 1.8 the development over the years went
after all mainly in a different direction.
1.2.2 Technical Challenges
The main challenge was the unfavorable signal-to-noise-ratio. SR is thought to be a
“white” radiation source, i.e., the energy band width of the radiation coming from
the bending magnets of the synchrotron spans from literally “zero” to several tens
keV. The low energy part could easily be absorbed, however, still a band width of
let’s say 20 keV was left. On the other side, in case of the 14.4 keV nuclear level in
57 Fe, the nuclear level width is 4.66 neV. That means a signal-to-noise-ratio of about
10
−13 . How to find those resonant quanta in this haystack?
In the following we will discuss various ideas and avenues.
1.2.2.1 Time and Polarization Properties
As discussed above, SR is a pulsed radiation with pulse widths t in the 100 ps regime
and—in principle—selectable repetition rate. That means that one may accordingly
adopt the time structure to the life time τ 0 of the excited nuclear levels. Then it is
possible to excite the nuclear levels instantaneously (t < τ 0 ) and to record their
de-excitation products such as γ -rays and conversion electrons inbetween the synchrotron radiation pulses. Due to the fact that electronic scattering processes are very
fast one should see after very intense “prompt” scattering processes only the “slow”
scattered γ -radiation from the scattering via the nuclear resonances. That means one
would be able to generate out of the SR a pure beam of γ -radiation through time
discrimination. Unfortunately, even with todays fast detector systems, it is vrtually
impossible for the detectors to sustain the extreme count rates beyond 10
9 photons/s.