Jacob Keller wrote:
> Wow, neutrons are pretty cool! No radiation damage--and time
> resolution? I guess this is since they have much higher energy, and
> are measurable individually? What are the numbers for fluxes
> (neutrons/sec)? Are the neutrons all at one energy, or is there a
> bandwidth?
The energy of neutrons is even lower when compared to X-rays.
A neutron with a wavelength of 1.8A has an energy of about 25 meV.
The flux at neutron sources compared to synchrotrons is unfortunately low:
Diffractometer "LADI III" reactor ILL/France:
3 x 10^7 neutrons/sec/cm^2 (quasi-Laue, delta L / L = 20%)
Diffractometer "BioDiff" reactor FRM II / Germany:
1 x 10^7 neutrons/sec/cm^2 (monochromatic, delta L / L = 2.5%)
Diffractometer "BIX4" reactor JRR3M / Japan:
4 x 10^6 neutrons/sec/cm^2 (monochromatic, delta L / L = 2.0%)
BUT you can detect hydrogen atoms even at a moderate resolution of
about 2A ! With neutrons the scattering power of hydrogen/deuterium
is "comparable" to the scattering power of carbon. You can even distinguish
between isotopes. Since the nucleus is a point scatterer the "form factor"
-for neutrons called scattering length- is not scattering angle depended.
A typical measurement time is about 2-3 weeks for a crystal of 1 mm^3.
I know...of course not every protein can be crystallized up to 1 mm^3 but
if you have such a system and you are interested in the protonation states
of amino acids in the active centre for example, than neutrons are worth
a try
for sure! If you fully deuterate your protein (which gets more and more
routine
work for example at the D-LAB at ILL/EMBL) you can even work with smaller
crystals.
Because of the relative low flux most reactor based neutron
diffractometers for
proteins uses large cylindrical neutron image plate detector, which
cover a solid angle
of about 2 Pi. At spallation sources (which are pulsed neutron sources)
detectors
with time resolution are used. This instruments (PCS in Los Alamos; iBIX
in Japan
and MANDI in Oak Ridge) are time of flight instruments. They uses the
fact that
neutrons with different energy/wavelength show different velocities ( a
1.8A neutron
has a velocity of about 2200 m/s). They measure different wavelength
neutrons at
different time at the detector.
Hope to see some of you as new "neutron users" in the future,
cheers,
Andreas
--
Dr. Andreas Ostermann
Technische Universität München
Research reactor FRM II
Instrument "BioDiff"
Lichtenbergstr. 1
D-85747 Garching
Tel.: +49-89-289-14702
Fax.: +49-89-289-14666
Email: [email protected]
Web: http://www.frm2.tum.de/en/science/index.html
