1 Historical Developments and Future Perspectives …
15
diffraction is only possible at distinct energies, the Mössbauer transition energies,
and therefore shows a very pronounced and complex resonance character.
The theoretical treatment of nuclear diffraction has independently been developed
by two groups well before any thoughts went to SR. Trammell [40] mentioned first
the possibility; later Kagan et al. [41], Afanas’ev and Kagan [19, 20], and Hannon
and Trammell [17, 18] laid down the theory in detail.
Pure nuclear reflections are named such reflections, which are forbidden due
to crystal symmetry, i.e., the electronic scattering amplitude is zero, but allowed for
nuclear resonance diffraction due to a different symmetry caused by the properties of
the Mössbauer nuclei. Intuitively this might be understood for a hypothetical crystal,
which has alternative layers of
56 Fe- and
57 Fe-atoms with lattice constant d. For
electronic scattering the lattice constant is d, because for both isotopes the number of
electrons is the same, whereas for nuclear resonance scattering via the
57 Fe nuclear
exited level the “Mössbauer lattice” constant is 2d. Choosing an appropriate Bragg
angle the electronic scattering interferes destructively whereas the nuclear resonance
scattering constructively giving rise to the pure nuclear reflection.
Already shortly after the discovery of the Mössbauer effect first diffraction experiments have been conducted with radioactive sources and pure nuclear reflections
reported. Those may be classified in reflections due to accidential extinction and
magnetic, electric, and combined magnetic and electric superstructures, respectively.
Early examples may be found e.g. for an accidental reflection in the [080]-reflection of
K 4 Fe(CN) 6 ·3H 2 O [42], for “magnetic” reflections in α-Fe 2 O 3 [43] and FeBO 3 [44],
for “electric” reflections in the (003)-reflection of Na 2 Fe(CN) 5 NO·2H 2 O [45] and for
tellur [46], and finally for “combined” reflections in KFeF 4 [47], yttrium-iron-garnet
(YIG) [48], and Fe 3 BO 6 [49].
1.2.3 Political Facts
It might be surprising to find here as well a paragraph on this issue, politics. However,
we are very sure, looking back with our todays knowledge and the current scientific
situation, that already at that time “politics” played a major role, in one or another
way.
It was at a time were “impact factors” and “high-impact” journals have not yet
played that important role as today. However, the budgetary situation became more
crucial and more and more big scientific facilities competed with each other and
with other projects. In the beginning, the endeavour of NRS with SR was thought
to be on a short timescale. However, when it took longer and longer the funding
was no longer granted on that time scale and most of the “beginners” had to stop
the endeavour. Eventually, it was only the Hamburg group, which could convince
the funding agencies to invest in “no results” for a long period of nine years, and
eventually, it paid off. Would that still be possbile nowadays?
Another long-term issue was the discussion on dedicated SR sources. It became
very early clear that the common use of those accelerators for high-energy physics
15
diffraction is only possible at distinct energies, the Mössbauer transition energies,
and therefore shows a very pronounced and complex resonance character.
The theoretical treatment of nuclear diffraction has independently been developed
by two groups well before any thoughts went to SR. Trammell [40] mentioned first
the possibility; later Kagan et al. [41], Afanas’ev and Kagan [19, 20], and Hannon
and Trammell [17, 18] laid down the theory in detail.
Pure nuclear reflections are named such reflections, which are forbidden due
to crystal symmetry, i.e., the electronic scattering amplitude is zero, but allowed for
nuclear resonance diffraction due to a different symmetry caused by the properties of
the Mössbauer nuclei. Intuitively this might be understood for a hypothetical crystal,
which has alternative layers of
56 Fe- and
57 Fe-atoms with lattice constant d. For
electronic scattering the lattice constant is d, because for both isotopes the number of
electrons is the same, whereas for nuclear resonance scattering via the
57 Fe nuclear
exited level the “Mössbauer lattice” constant is 2d. Choosing an appropriate Bragg
angle the electronic scattering interferes destructively whereas the nuclear resonance
scattering constructively giving rise to the pure nuclear reflection.
Already shortly after the discovery of the Mössbauer effect first diffraction experiments have been conducted with radioactive sources and pure nuclear reflections
reported. Those may be classified in reflections due to accidential extinction and
magnetic, electric, and combined magnetic and electric superstructures, respectively.
Early examples may be found e.g. for an accidental reflection in the [080]-reflection of
K 4 Fe(CN) 6 ·3H 2 O [42], for “magnetic” reflections in α-Fe 2 O 3 [43] and FeBO 3 [44],
for “electric” reflections in the (003)-reflection of Na 2 Fe(CN) 5 NO·2H 2 O [45] and for
tellur [46], and finally for “combined” reflections in KFeF 4 [47], yttrium-iron-garnet
(YIG) [48], and Fe 3 BO 6 [49].
1.2.3 Political Facts
It might be surprising to find here as well a paragraph on this issue, politics. However,
we are very sure, looking back with our todays knowledge and the current scientific
situation, that already at that time “politics” played a major role, in one or another
way.
It was at a time were “impact factors” and “high-impact” journals have not yet
played that important role as today. However, the budgetary situation became more
crucial and more and more big scientific facilities competed with each other and
with other projects. In the beginning, the endeavour of NRS with SR was thought
to be on a short timescale. However, when it took longer and longer the funding
was no longer granted on that time scale and most of the “beginners” had to stop
the endeavour. Eventually, it was only the Hamburg group, which could convince
the funding agencies to invest in “no results” for a long period of nine years, and
eventually, it paid off. Would that still be possbile nowadays?
Another long-term issue was the discussion on dedicated SR sources. It became
very early clear that the common use of those accelerators for high-energy physics