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Re: Nuclear spectroscopy



I think there are two definitions of what spectroscopy is. Without
thinking too much as to whether these are the right words to use, allow
me to distinguish these as "fundamental" versus "applied."

The fundamental meaning is analysis of photon-emission/absorption in
order to determine energy levels, spin states, parity, etc. of the
quantized energy levels in the system. This is what I typically think
of when I think of nuclear spectroscopy because the grad-school group
of which I was a part was the nuclear spectroscopy group at the
Michigan State University Cyclotron. Although I did not do this
myself, most of my fellow grad students were doing a decay scheme for
their Ph.D. theses. Each picked an interesting nuclide and did all
sorts of gamma-ray spectroscopy to determine the quantum structure of
that nuclide. This typically included gamma-ray energy studies,
angular correlation studies of the gamma rays, and coincidence studies
of the gamma rays. This typically required 4-6 years of hard work.

The applied meaning of spectroscopy is using the spectral properties of
some system as a tool to learn something other than the quantum
structure. The given example of neutron activation is this type. In
neutron activation the goal is to determine the composition of the
material being analyzed by determining the energies and intensities of
the gamma rays and matching them to known gamma rays from specific
nuclides. This requires that some of the work mentioned under
"fundamental" has already been done so that a library of energies and
intensities exists and contains the nuclides/elements of interest.

A couple direct analogies of neutron activation would be (1) the use of
infrared photons in organic chemistry to identify organic compounds,
(2) the use of x-ray florescence to identify elements, (3) spectral
analysis in astronomy to identify elements/compounds in space. In all
these we are making use of the known quantum behavior of the nucleus,
atom, or molecule.

Another applied type of spectroscopy has to do with quantitative
analysis via absorption/emission of photons. This is a "Beer's Law"
type of thing. I would say this is not exactly spectroscopy because
once you have the correct wavelength/energy chosen, all you do is find
the intensity of photons emitted or the intensity absorbed.
Nonetheless, chemists and physicists widely call this spectroscopy.
Typical examples include (1) atomic absorption spectroscopy [to
measure the presence and amount of an element... often a heavy metal
such as iron, lead, cadmium], (2) visible/UV light spectroscopy to
determine the amount of a substance or to follow the progress of a
reaction.

I would have to agree with the physicians that NMR is indeed a
spectroscopic technique, although today it is more of the applied type
rather than the fundamental type. In the beginning, NMR was more
fundamental as physicists and physical chemists were verifying that a
proton in a magnetic field had two quantized spin states. But later,
when we discovered the actual magnetic field experienced by the proton
was slightly different than the external applied field (because the
neighboring atoms perturbed the field) we found we could determine some
properties of the neighboring atoms by determining how much we had to
vary the external field (or the radio frequency) to get the protons to
absorb the photons. Organic chemists use proton NMR spectroscopy as a
powerful tool to determine the structure of an organic molecule.

Later, we developed the tools to do this type of spectroscopy with
other nuclei with net spin, i.e. the odd-mass nuclei. Later yet we
discovered that when a group of nuclei have been brought into
"resonance" and are absorbing/emitting photons, that when the
conditions establishing the resonance are suddenly changed, how fast
the nuclei switch to the new conditions is also determined by the
properties of the neighboring atoms. This is heavily used in the NMR
technique used by physicians under the new name "magnetic resonance
imaging" (MRI).

Therefore I would claim that NMR used by chemists and physicians (MRI)
is indeed spectroscopy in the applied sense. The interaction between
photons and matter is being used to determine some of the properties of
that matter... not necessarily to determine information about the
quantized levels... but something else, (1) perhaps the identity of the
species that has the quantized levels, (2) perhaps how many of that
species are present, (3) perhaps how those quantized levels are
affected by neighboring atoms/molecules.


Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817



-----Original Message-----
From: K. Lee Lerner [SMTP:lerndesk@SPRYNET.COM]
Sent: Wednesday, August 04, 1999 4:10 PM
To: PHYS-L@LISTS.NAU.EDU
Subject: Nuclear spectroscopy

Dear Colleagues,

What procedures beside neutron activation analysis and positron
annihilation
analysis would you consider to fall under the old term "nuclear
spectroscopy" -- a simple reference to procedures would be of great
help.
Pointers to web resources would also be most welcome.

I was overwhelmed by a group of physicians at a seminar today when I
claimed
that NMR was not truly a spectroscopic technique. Fatigued... out of
my
area of expertise... and probably off tack..

Best Regards.
K.