BT9 triple-axis neutron spectrometer
The instrument on beam tube 9 in the thermal neutron confinement building
at the NCNR is a conventional triple-axis spectrometer featuring PG(002)
or Ge(311) monochromators. It receives flux from a 6 inch diameter
tangential beam tube, which helps to reduce fast neutron background,
and exits at the southwest corner of the reactor face. The main shutter
control for BT9 is mounted on the south outside wall and can only
be operated by contacting reactor operations (x6292). The user controlled
local shutter control is mounted on the monochromator drum shielding
to the left side of the beam exit from the monochromator drum.
CRITICAL SAFETY NOTE: The main beam is a HIGH-RADIATION
AREA with exposures exceding 1 R, so that when the beam is ON (beam
indicator at the top of the drum says ON) it is extremely important
that all persons must remain out of the main beam path area. Infrared
sensor alarms will activate in the case that anyone approaches the
beam area with the beam ON. Any malfunction of these alarms requires
that the beam must be immediately closed and the malfunction fixed.
It is a requirement for the instrument to operate that the HIGH-RADIATION
AREA extend to no more than 30 cm from the main beam line in any direction.
This means that radiation levels may not excede 100 mRem beyond that
distance. It is the user's responsibility to ensure that these radiation
levels are not exceded.
SAFETY NOTE: Be aware of the Emergency Control Procedures
posted at the instrument. ECP
name |
office |
extension |
e-mail@nist.gov |
home phone |
Ross Erwin |
A120 |
x6245 |
ross.erwin |
(301) 208-1674 |
Jeff Lynn |
A126 |
x6246 |
jeff.lynn |
(301) 424-3353 |
- June 1998
replaced monochromator plug with multiple crystal elevator.
- January 2002
replaced secondary spectrometer with H-7 from BNL.
- March 2003
installed Ge(311) monochromator crystal.
- December 2003
new motor controllers.
Data collection is handled by the ICP program, ICP
If you have a vrml plug-in for your browser vrml browsers ,
you can manipulate a model of a triple-axis spectrometer at VRML 3-axis spectrometer.
Directions for this model are at VRML 3-axis spectrometer description
BT9 is equipped with a standard Huber double arc (+-23 degrees on
each arc) goniometer which can handle a large selection of sample
environment equipment. If you need to disconnect the cables to the
motors that drive the goniometer arcs or translations, make sure that
the motors are powered down first (e.g. ICP command to disable motor
7: md 7 ;and to enable motor 7: me 7) See ancilliary equipment
- BT9-reactor beam collimation
The locally controlled instrument shutter provides two Söller slit
collimation selections (along with the closed beam position) which
are nominally 10 arc minutes (1/6 degree) and 40 arc minutes (2/3
degree). There is an LED display mounted on top of the instrument
which indicates the reactor beam status as either BEAM ON(red lamp)
or BEAM OFF(green lamp). The BEAM OFF lamp can only be active if the
rotating collimator motor is off and the closed beam position limit
switch is activated. There is also a red lamp indicator for the collimation
position when the beam is ON.
SAFETY NOTE: Any malfunction of the beam indicators
or local beam shutter operation must be immediately reported to your
instrument contact or reactor operations (x6292).
- monochromator to sample collimator
The Söller slit collimator at the exit from the monochromator drum
has removable 30 inch or 20 inch cadmium coated steel blades for adjustable
collimation. The slot to slot spacing is 0.105 inches and the slot
width is 0.0115 inches. This gives approximately 0.2 degrees (12 arc
minutes) per channel between blades. The blade height is 1.75 inches.
The calculated FWHM = (N*0.105 - 0.0115)/Lblade gives:
N |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
30'' blades |
10.7' |
22.7' |
34.8' |
46.8' |
58.9' |
70.9' |
83' |
95' |
20'' blades |
16.1' |
34.1' |
52.2' |
70.2' |
88.3' |
106.4' |
124.5' |
142.6' |
SAFETY NOTE: Be aware that cadmium becomes a strong
isotropic source of prompt gamma radiation when put in a neutron beam,
and this can produce a prohibited HIGH-RADIATION area. The prompt
gamma radiation from cadmium dies away in a few seconds after removing
the neutron source, and then the cadmium coated blades are safe to
be handled. Whenever possible Li-6 shielding is preferable to cadmium
because there are no prompt gammas from Li-6.
- sample to analyzer collimation
The sample to analyzer collimation position uses removable Söller
slit units that can be either 12 inches or 16 inches long. They are
positioned on an X-rail base by two steel pins that match holes in
the bottom of the collimation units. The Söller slit units have a
2 inch x 2 inch cross-section so that most of them can be mounted
as horizontal or vertical collimators. The collimation values are
stamped on the units and have values 10', 20', 40', 60' and 80'. Once
the unit is locked into place it is covered with a black boron carbide
epoxy snout shield. At the front end of the collimation unit there
is a slot to hold a cadmium mask. There is a large selection of cadmium
masks in a brown wooden box. There is an additional black boron carbide
shielding section to cover the cadmium mask section of the collimator.
SAFETY NOTE: When using high magnetic fields be sure
to use a cadmium mask to prevent the steel collimation blades from
being pulled out of the collimator by the magnetic field. The use
of the collimators with aluminum housings is also recommended in this
case.
- analyzer to detector collimation
The 12 inch long Söller slit units described in the sample to analyzer
section can be used here. Remove the two boron carbide top shields
from the detector housing and carefully lay the collimator into the
channel. At the front of this channel there is a holder for a removable
cadmium mask.
- reactor beam filters
There are two filters which can be placed in the beam before the monochromator
via remotely controlled elevators (The elevator controls are mounted
on the shielding walls near the beam shutter controls). A 4 cm PG(pyrolytic
gaphite) filter, which has relatively sharp peaks in the transmission
as a function of wavelength, is useful as a higher order filter. As
can be seen from the following link, this filter provides a lambda/2
rejection factor of about 100 at 14.7 meV when properly tuned. It
also helps to remove fast neutron background from the beam, and thus
makes an excellent filter for inelastic scattering experiments with
fixed incident energy. PG transmissionThe
second filter is c-axis oriented sapphire sapphire transmission,
which is useful for removing fast neutron background when the PG filter
cannot be used (e.g. variable incident energy). The beam monitor rates
are also plotted versus angle or incident energy. These plots can
be used to gauge the relative sample beam flux when using these filters.
PG monitor rate vs. twothetaM,
PG monitor rate vs. incident energy,
Ge311 monitor rate vs. twothetaM,
Ge311 monitor rate vs. incident energy.
- exit beam filters
A 1.75'' PG filter is available for placement in the exit beam from
the monochromator drum when running the instrument in fixed-incident
energy mode. This provides a lambda/2 rejection factor of about 150
at 14.7 meV. It can be used in addition to the pre-monochromator PG
filter when very high levels of lambda/2 rejection are required. It
is on an X-rail mount and should be stored on the wall mounted X-rail
section when not in the beam. A 2'' PG filter is available for placement
in the beam just before the detector shielding cavity. It is spring
loaded and fits between two aluminum plates. This can be used for
fixed final energy inelastic scattering experiments.
Motor controlled horizontal and vertical slits can be placed on the
X-rail before the sample position.
Last modified 18-February-2005