Transcript J. Lynn - Physics & Astronomy

A New High Intensity Cold Neutron Spectrometer at NIST
J. A. Rodriguez1,3, P. Brand3, C. Broholm2,3, J.C. Cook3, Z. Huang3, P. Hundertmark3, J. Lynn3, N.C. Maliszewsky3, J. Moyer3,
J. Orndorf2, D. Pierce3, T. Pike3, G. Scharfstein2 and S. Smee2
1University of Maryland, Department of Materials Science and Engineering, CollegePark, MD 20742-2115,USA
2The Johns Hopkins University, Department of Physics & Astronomy, 3400 N. Charles Street, Baltimore, MD 21218,USA
3National Institute of Standards and Technology, 100 Bureau Drive, MS 8562, Gaithersburg, MD 20899-8562, USA
A helium filled cask contains all items
associated with converting a divergent white
neutron beam to a monochromatic beam that
converges on the sample.
A novel cold neutron spectrometer is under development at the NIST Center for
Neutron Research, optimized for wave vector resolved spectroscopy below 20
meV with a resolution of 0.05 meV to 2.8 meV. The versatile monochromating
system, which includes radial collimator, three filters, and a variable beam
aperture offers considerable flexibility in optimizing the choice of Q-resolution,
energy resolution and intensity. Vertical focusing of the DXAL allows for smaller
detectors for enhanced signal to noise. Post Sample collimators and filters
options provide flexibility in the choice of scattered beam energy resolution
MACS flux on sample versus energy. the
sample cross section was a rectangle 2 cm
wide and 4 cm tall. The neutron flux for E<5
meV were measured with a Be filter placed
after the source. Measurements for E>5 meV
were taken without the Be filter and were λ/2
corrected. The solid lines are Monte Carlo
Simulations. All beam line components from
source to the sample position were simulated,
excluding the filters.
Image of the MACS doubly
focusing
monochromator mounted with mirrors for
optical testing. The device contains 357 pieces
of 1o mosaic PG(002) for a total area of 1428
cm2. Each of the 21 vertical aluminum blades
rotates independently by individual stepping
motors thus providing full flexibility for
horizontal focusing. Vertical focusing is
achieved by bending aluminum backing plates
under compression. These plates are profiled
so the front surface assumes a circular shape
when bent.
The sample
positioning
system contains
no magnetic
materials within
75 cm of the
sample and can
tilt sample
environment
systems
weighing up to
400 kg.
Isometric view of the
MACS cold neutron
spectrometer.
40 channel detection system
Sample position
Aperture
Attenuator
Monitor
Shielding
Focusing supermirror
Helium
Cold Source
Wave vector resolution is controlled by a
variable beam aperture. It defines the envelope
of the neutron beam. The fully open aperture
subtends 4.10-3 Sr to the source. There are two
degrees of freedom: Beam Width and beam
height. The absorbing apertures are 10 cm thick
and made from high density poly-ethylene
surrounded by Hot Pressed B4C. Radiation
hardened motors and resolvers are used.
Energy resolution is
controlled by a
segmented radial
collimating system.
The blades of the
device converge at
the center of the cold
source. Two in-line
segments A and B are
available. Segment A
provides 60’
divergence of
radiation incident on a
volume element of the
monochromator.
Segment B alone
reduces this
divergence to 40’.
Segments A and B
combined provide 24’
divergence.
Cooled filters
Shutter
Variable Aperture
Focusing monochromator
on translation stage
One of the 20 vertically
focusing double crystal
analyzers for the MACS
detector system. A
single stepping motor
operates each device
from below. The range
of analyzer scattering
angles is 40o to 140o.
The PG(002) graphite
has an average mosaic
of 1.4o. The area of a
single vertically focusing
blade is 108 cm2. The
total solid angle
subtended by the
detection system is
approximately 0.15 Sr.
Radial Collimators
6.2 m
Top view of MACS. Horizontal monochromatic focusing requires that the
monochromator is tangent to the dashed Rowland circle.
The detection system consists of 20 identical channels separated by 8o for a total
of 152o coverage in one setting. Each channel contains: (a) Cooled Be, BeO, or
PG filter (b) 36’-60’-90’ or open collimation. (c) Double Crystal analyzer (see full
description to the right) (d) Diffraction detector (e) Spectroscopic detector
The performance of a vertically focusing double
crystal analyzer was compared to a
conventional analyzer on SPINS at NIST. Both
the energy resolution and the overall
transmission are comparable. The DXAL offers
lower background and a smaller foot-print.
Multiple Bragg scattering effects are more
pronounced for the DXAL because neutron
passage involves two reflections