X-ray
diffraction / topography and spectroscopy, electron microscopy, materials science
X-ray radiation is very high
in energy E = h f (≈ 2 10-15
J = 1,25 104 eV so it’s the same energy an electron would have if it
were accelerated by an electrical force going through a potential of 12,500 V) that’s
why it penetrated skin and flesh easily, bones not quite so easily and have
usage in medicine – is that the main usage???
Who was Conrad Wilhelm
Röntgen, discoverer of X-rays? A medical doctor? A physicist, the very first
Nobel prize winner in Physics?
how did he discover X-rays? 1895, by chance, experimenting with cathode rays
(doing similar things to J.J Thompson) on one end of the laboratory, there was
a sheet of paper that was covered with a phosphor sitting around at the other
end of the laboratory, experimenting in the dark, he noticed that phosphor
lights up when he switches on his cathode ray tube, dragging out electrons and
accelerating them by a potential difference, the cathode ray tube is expected
to be under vacuum, but there was just enough rest gas (air) that electrons got
slowed down by being scattered by the molecules,
today we know: when electrons
are slowed down they radiate off their lost in kinetic energy – and that is
X-rays an electromagnetic wave + a stream of high energy photons traveling at
the speed of light
at the time nobody knows how
the radiation originates and of what kind it was: wave or particles? Röntgen
could in a long series of experiments showed:
- X-rays penetrate all
substances to some extend, elements of low atomic weight are especially
transparent, elements of high atomic number can bye used as shields for
protection against X-rays
- X-rays cause many
substances to fluoresce, i.e. absorbing them and radiating off electron-magnetic
waves of a different wavelength
- X-rays are not deflected in
a magnetic field, so they do not consist of charged particles
- X-rays can effectively be
generated by “cathode rays = accelerated electrons” fall on a metal anode (and
any other material placed in their path
- X-rays ionize the gasses
through which they pass, causing an increase of electrical conductivity
as a matter of fact a new science was enabled which goes by the name materials science/crystallography
but not diffraction which would prove X-rays are to be
described by waves!
reason: then - of course - unknown, the wavelength of X-rays is so small
that there were no manmade diffraction gratings available
how does a diffraction at a
grating differ from that of a double slit arrangement?
a diffraction grating has a
much much greater number of slits
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for the positions of the
maxima
d sin θ = m λ
where m = 0, 1, 2, … are
called the order of the line and d is called the grating period, i.e. quotient
of some length W divided by the number of lines
as the maxima of the lines is
very well defined and narrow, one can measure the wavelength of the incident
(mono- or polychromatic if it is not too many) radiation that produced the
interference/diffraction pattern (for a double slit experiment the bright
fringes arising from different wavelength light would overlap too much to be
distinguished
by 1912, Sommerfeld at the
Physics Department of University of Munich, nominated 80 times for Nobel prize
without ever winning it, died from a car accident had calculated an order of
magnitude for the wavelength of X-rays: 0.1 nm
if that were to be true,
wavelength would be so short that diffraction of 0.1 nm X-rays from a standard
optical grating of period 3 μm were to give diffraction angle of 0.002°
so pessimism, how is somebody
going to prove X-rays are waves or not
a PhD student of Sommerfeld,
Ewald working on solid state theory implications what if crystals were, as some
of the leading mineralogist (e.g. Victor M. Goldsmith at the Mineralogy
Department of U of Munich) expected a three-dimensional ordered arrangement of
atoms with inter-atomic distances of the order of magnitude 0.1 nm – you have
to pause here, the very existence of atoms was not well established at that
time let alone their possible arrangement in crystalline solids,
approached Max Laue (an
assistant professor at Sommerfeld’s chair) at Café “Lutz”, where students and
faculty used to hang out, on an quite unrelated matter
Laue was at that time working
on a treatise on classical optics and heard for the first time both hypotheses,
that the wavelength of X-rays may be as short at 0.1 nm and that crystals may
possess a possible periodic arrangement of the atoms in 3 dimensions with a
spacing of the order of magnitude 0.1 nm
on their way back to the
Physics department, the idea occurred to Laue, well, there should be interferences
if a crystal is exposed by a beam of “sufficiently monochromatic” X-rays
he himself being a theorist,
he talked a postdoc, Friedrich, who had previously worked with Röntgen) and a postgraduate,
Knipping, into trying the experiment, behind the back of the big boss,
Sommerfeld, when Sommerfeld heard about it he was furious especially as they
didn’t get the results Laue expected as they didn’t know where to look for - so
the project came to an halt
Laue finally got permission
to go ahead but the big boss was still convinced it was a stupid experiment as
he calculated, even if there were X-ray interferences, the thermal motion of
the atoms at their respective places in a crystal would smear them out.
little did he know …
and Max von Laue, i.e.
he got knighted, got for the correct interpretation of the intensity maxima and
the development of the so called kinematic theory of X-ray diffraction his 1914
Nobel prize.
what was found in one
experiment not only that X-rays do indeed possess waves properties and that
crystals indeed possess a three dimensional arrangement of atoms, but also nothing less than the discovery
of a method to probe the inner structure of crystalline matter, which we
know today is almost all solids, be they metals, ceramics, semiconductors, …. (not glass, rubber, and certain plastics)
a diffraction grating is
really a one dimensional thing, the naturally occurring crystals are, however, 3 dimensional,
therefore we get a slightly different relation for the distribution on intensity
on a screen of photograph
Bragg’s equation: n λ = 2 d
sin θ where n
is order of reflection
allowing crystals to be used
as kind of a grating for measuring wave length of characteristic X-rays which
arise from electronic transitions of the innermost electrons
and analysis of crystals
structures, solids in general
as a matter of fact a new science was enabled which goes by the name materials science/crystallography
new sciences always start out
as a parepistemes (R.W. Cahn)
from the ancient Greek:
episteme – meaning a domain of knowledge
par - means
amongst other things subsidiary
i.e. a subsidiary subject
within a science is something right at the fringes of attention and funding that
does not primarily aim for solving any practical problem, people only do it for
the fun of it, because they find it interesting. Chan described it as: “looking at the forest, not just selecting a tree
for logging …”
later on, electron, and neutron
diffraction, which are complementary to X-ray diffraction – the single most
important tools of materials science
a science in its own right? It’s
a science when it has its own journals, conferences, societies,
institutionalized departments, degrees, graduate students, … when it has
something unique that distinguishes it from the science that gave birth
to it (which is probably always Physics)
what is unique about
materials science/crystallography?
R.W Chan: “… the materials
scientist has to work at several levels of organization, each of which is under-pinned
by the next level. Here, again, he is brother under the skin of the biologist,
who does just the same: staring with the cell wall, say, the goes on to study
the morphology and economy of the cell as a whole, then the isolated organ
(made of cells), then the organism as a whole. … the concept of microstructure is the most
important single definition theme of Materials Science and Engineering.”
perhaps something is a
science when it has spun off an engineering discipline, what do you think?
perhaps something is a science when consistently Nobel
prizes have been awarded in that field, what do you think?
Nobel prizes Physics:
1914 Max von
Laue, X-rays are an electromagnetic wave of about 0.1 nm wavelength, atoms in
crystals are arranged in a regular 3D lattice with spacing in the 0.1 nm range,
there is X-ray diffraction from crystals, the possibility of analyzing the
arrangements of matter in crystals, (Laue equations), 1912
1915 Sir William
Henry Bragg and William Lawrence Bragg (son of the former), X-ray
crystallography, W.L. Bragg equation, X-ray crystal spectrometer, 1913
1937 Clinton
Davidson and Sir George P. Thompson (J.J. Thompson’s son who first showed
electrons are particles), electron diffraction by crystals, in other words
electrons are waves, 1927
1986 Ernst
Ruska, inventing the electron microscope where one operation mode is electron
diffraction (1931) together with Gerd Binning
and Heinrich Rohrer, scanning tunneling microscope, (completely unrelated)
1994 Bertram
N. Brockhouse and Clifford G. Schull, pioneering neutron diffraction and
spectroscopy
Nobel Prizes Chemistry
1936 Peter J. Debye
X-ray and electron diffraction analyses of molecules
X-ray and electron diffraction analyses of molecules
1962 Max F. Perutz, John C.
Kendrew
X-ray diffraction analysis
of haemoglobin und myoglobin
1964 Dame Dorothy Crowfoot
Hodgkin
X-ray diffraction analysis
of biochemical macromolecules
1976 William N. Lipscomb
X-ray diffraction analysis
of borhydrids
1982 Aaron Klug
X-ray diffraction analysis
of nucleic acids and protein complexes
1985 Herbert A. Hauptmann, Jerome Karle
direct methods of X-ray
crystallography
1988 Johann Deisenhofer, Robert Huber, Hartmut
Michel
structure and function of
protein-chromophor-complex
Nobel Prize Medicine
1962 James
D. Watson, Francis H.C. Crick, Maurice H.F. Wilkins, X-ray diffraction analysis of DNA and its importance for genetic code
note there has not been any Nobel prize for medical
X-rays, so probably that was much less important to the development of mankind as the
physical and crystallographic use of X-rays
so in a sense: Materials
Science/Crystallography is the real legacy of Konrad Willhem Röntgen, what
happened to him, he got the first ever Nobel prize in Physics in 1901, but
refused to accept the money,
during the hyperinflation
following world war I, he died a very poor man from malnutrition, exhaustion,
and poor health due to his experiments with X-rays
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