On March 1 of this year, the steady stream of experimentation on the 4 meter normal
incidence monochromator (NIM) was abruptly interrupted. A problem with the scan mechanism
was quickly traced to the precision lead screw. After the scan assembly was disassembled,
the precision self aligning nut of the lead screw proved to be broken at the flexural
The lead screw of the 4 meter NIM is a special high precision lead screw. It has a
specified tolerance of only 5 microns error in 1/3 meter of travel. This high precision is
necessary because the wavelength of the instrument is measured by monitoring lead screw
rotation, so errors in the lead screw will translate into errors in wavelength. To achieve
these specifications, the nut must be lapped to fit the individual lead screw.
The heavy demand for the monochromator made it desirable to keep it operating in some
form while the repairs were being made. Tom Nelson, SRC head machinist and part-time
miracle worker, quickly constructed a less precise but workable lead screw from a length
of threaded rod, and a substitute Everdur nut. Mary Severson and Roger Hansen installed
these in the monochromator, along with a hand crank on the end of the emergency lead screw
in place of the stepping motor and rotary encoder. A scale was added to give a rough
indication of the wavelength.
Although the monochromator performance at any given wavelength was not affected,
wavelength scanning could only be done manually. Ron Kelley, Christoph Quittmann and
Marshall Onellion are credited with continuing to work with the monochromator in this
configuration. They became quite adept at using the monochromator in this fashion although
they never fully mastered CIS scans.
On April 20, the Universal Thread Grinding Company of Fairfield, Connecticut returned
the lead screw with a new precision nut. This was installed in the beamline on April 21.
After some testing and adjustment, the beamline was again ready for users. Awny Al-Omari
and Ruben Reininger installed their instrument and recorded data for the rest of the
quantum. We are happy to report that the 4 meter scan mechanism has been successfully
repaired and the monochromator is back in service.
Slit-size restrictions recommended
In another area of interest to NIM users, some problems have been recognized due to
degradation of the 4 meter gratings over time. It was initially believed that a grating
would be used until it became contaminated and the beam intensity decayed significantly.
Instead, it was found that the resolution degrades and the flux remains constant for the
life of the gratings. According to Roger Hansen, NIM beamline manager, the degradation of
the resolution occurs because of figure errors produced by exposure to light. "These
changes are more severe for the 3600 line per mm gold grating than for the 1200 line per
mm Aluminum MgF2 grating," Roger elaborated. "This has led us to believe that
the figure degradation is caused by light transmitted through the gold layer degrading the
epoxy layer of the replica." Gold has an appreciable transmittance for light between
100 and 200 eV, and the 4 meter beamline has no reflections that block this light.
To minimize the effect of this problem several steps are being taken. First, a grating
with a thicker gold layer has been ordered. This should decrease the flux penetrating the
gold layer by a factor of ten and slow down the degradation of the epoxy layer and the
distortion of the grating figure. Second, the new beam time schedule has been arranged so
that users requiring the highest resolution are scheduled to run immediately after the
installation of the new grating. A maximum slit size "speed limit" will be
enforced on the 4 meter NIM until all high resolution experiments have been completed.
These changes should result in improved performance for everyone.