A High Performance Hybrid Spectrometer for the Single Crystal Studies at the Pulsed SNS

1
A High Performance Hybrid Spectrometer for the
Single Crystal Studies at the Pulsed SNS
Igor Zaliznyak
Outline

• Scientific case, design objectives, and
  background for the proposed hybrid spectrometer
  for the SNS
• General layout of the proposed spectrometer and
  its place in the SNS instrument suite
• Comparison of different spectrometer concepts
  and evolution of our thinking
• Analysis of the instrument performance, current
  questions, plans

2
Scientific case for a proposed instrument for the
SNS
Neutron spectrometer for studies of the coherent
low-energy states in single crystals.

• Coherent collective excitations in single
  crystals
• lattice dynamics (phonons)
• spin dynamics (magnons, critical scattering)
• Structure and dynamics of partially ordered and
  glassy states
• spin glasses
• charge glasses
• correlated amorphous phases
• Study of the microscopic physical properties of
  samples in a variety of extreme environments
• magnetic field
• pressure
• temperature
• Characterization of spin-dependent
  cross-sections by means of polarization analysis

3
What are the typical samples we want to study?
CuGeO3 sample used by M. Arai group for detailed
measurement of the excitation dispersion on MAPS
in 2000
CuGeO3 sample used by L.-P. Regnault for the
original measurement in 1993
4
General requirements for a single crystal neutron
spectrometer

• Transmission of both primary (monochromator)
  and secondary (analyzer) spectrometers should
  be close to 1 within the resolution acceptance
  range, and vary smoothly over a substantial
  energy interval, typically from 2.5 meV to 60
  meV.
• Both spectral (energy resolution) and angular
  ( wavevector resolution) acceptances of the
  monochromator and analyzer should be flexible
  and easily adjustable, typical resolutions are 1
  to 5.
• Scattering volume seen by a detector should
  be well defined and easily adjustable depending
  on the sample size to minimize the background.
• Efficient use of the large incident neutron
  beam by focusing it on the sample is very
  important, and should be previewed.

5
What are our main design goals?
Aiming at great achievements, step on the
shoulders of giants 40 years of experience in
3-axis spectroscopy, honored by the Nobel prize

• Use most of the monochromatic neutron intensity
  produced by the source, while providing the
  highest possible signal-to-background ratio in
  the detector for good and moderate energy and
  wavevector resolution
• Use the pulsed time structure of the neutron beam
  at the SNS
• Avoid the direct view of the moderator and the
  source by the sample
• Make solid angle accepted by the secondary
  spectrometer as large as necessary
• Make range of the scattering angles accessible to
  the secondary spectrometer as large as possible
• Preview an easy setup of the polarized beam
  option
• Trade-off of the resolution for intensity should
  not degrade the instrument performance

6
How does the instrument we propose fit SNS
inelastic instruments suite?

• High energy transfer
• 10-1000 meV Fermi Chopper Spectrometer
• E 10 - 1000 meV
• Q 0.1 22 Å-1

• High intensity at moderate resolution and medium
  energy transfer polarized beam
• Crystal Monochromator Hybrid Spectrometer
• E 2.5 - 60 meV
• Q 0.1 8 Å-1

• High resolution and low energy transfer
• 10-100 meV Multichopper Spectrometer
• E 2 - 20 meV
• Q 0.1 - 4 Å-1

7
FOCUS, hybrid TOF direct deometry spectrometer at
SINQ_at_PSI. Off-the-shelf system?
Monotonous limited brains converge.
Layout of the SINQ experimental hall
FOCUS spectrometer
8
FOCUS concept mode of operation and technical
details.
Concept so attractive, they even chop a
CONTINUOUS beam!
9
FOCUS technical characteristics and first results.
S. Janssen et al. Physica B 283 (2000)
Monochromator Bragg angle Ei meV Reselast
meV QmaxÅ PG002 17.5 20.0 1.00
5.6 PG002 36.6 5.1 0.14 2.8 PG002 63.4 2.3
0.04 1.9 PG004 36.6 20.0 0.50 5.6 Mica 002
65.4 0.3 0.01 0.6
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So what is the difference?
We will do better!

• FOCUS
• large segmented monochromator with independently
  variable vertical and non-monochromatic(!)
  horizontal focusing
• designed for cold neutrons, PG crystals with
  rather large isotropic mosaic 50 mica
• long, 3 m guide-to-monochromator and 1.5 m - 3 m
  monochromator-to-sample distances
• 2.5 m secondary flightpath

• Hybrid Spectrometer for the SNS (HYSPEC?)
• monochromator immediately at the end of the
  guide, lt1.8 m from the sample
• thick PG crystals with anisotropic mosaic
• Heusler crystals for polarized beam
• guide tube before sample
• 4-5 m secondary flightpath

Most important pulsed beam allows using 1/4 to
gt1/2 of the total amount of neutrons produced gt
gain factor 100
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Choice of the moderator
Benchmark the instrument performance at Ei 15
meV
Ei 10 meV
Ei 20 meV
Intensity
Intensity
decoupled H2
decoupled H2
coupled H2
coupled H2
t
t
Figure of merit is the integral flux within 30-50
?s time window, defined by the length of the
secondary flight-path
Coupled supercritical H2 moderator
12
Schematics of the proposed primary spectrometer.
13
Schematic layout of the proposed Hybrid
Spectrometer
Tentative placement beamline 14.

• Monochromator shielding beam stop
• Guide and analyzer shieldings

Important but not shown
14
One of the possibilities for T2 chopper
counter-rotating drum pair.
The closer is T2 chopper to the sample, the
better is analyzer resolution. Could we put it
after the monochromator?
Advantages

• Light and compact.
• No gyroscopic forces from horizontal
  displacement.
• No restriction on beam height.
• Fast to speed up, slow down, re-phase, similar
  to modern Fermi choppers.

Problems

• High rotation rate, up to 900 Hz is necessary
  for 50?s pulse.
• Nested vertical magnetic bearings.

15
How does the instrument we propose fit SNS floor
layout?
Needs rather short, 15 to 25 m primary flight
path, but large, 6-8 m radius floor area for
the moving analyzer bank and the sample table.
Beamline 14?

• Use the space following a shorter instrument?
• Occupy a tangential position with no other
  instruments on one side?

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Pros and Cons of a Crystal Monochromator

• Advantages of a crystal monochromator
• beam may be compressed on the sample using
  focusing Bragg optics, beam height is not limited
  
• sample and its environment are exposed to
  reflected monochromatic beam only reduced
  background
• works for wide beams
• resolution is easily adjusted by putting the
  collimators
• (Heusler) may provide a polarized beam for
  energies up to 60 meV
• Disadvantages of a crystal monochromator
• lower than one and energy-dependent reflectivity
  of the crystal, is not well-suited for providing
  high-energy incident neutrons
• energy and wavevector resolutions are coupled
  and depend on the instrument position
• sample table and secondary spectrometer need to
  be moved when incident neutrons energy is changed

17
Secondary spectrometer (TOF analyzer) and its
operation in the polarized beam mode.
All scattering angles of interest in the
polarized mode are filled in by appropriately
moving the detector bank!

• Ei5-60 meV incident polarized beam is produced
  by reflection from VF Heusler crystal
  monochromator
• Set of 5-10 bender-type polarizers provide for
  the polarization analysis for Eflt15 meV.

• Scattering from the BULKY SAMPLE ENVIRONMENT
  (aluminium, etc.) is outside analyzer acceptance.

18
Secondary flight path and analyzer performance
Uncertainty of the flight time in the analyzer
gives largest contribution to the energy
resolution.
LSD ?t/t ?E/E Ef5.0 meV 4 m 0.0098 2.0 5
m 0.0078 1.6 Ef14.7 meV 4 m 0.0168 3.3 5
m 0.0134 2.6 Ef60.0 meV 4 m 0.0339 6.8 5
m 0.0271 5.4
Analyzer resolution for the length of the
secondary flight path LSD 4 m and LSD 5 m and
for the t40 ?s burst width at the sample (FWHM)
kf/ki0.5
60 - 90 coverage of the scattering angle by the
detector array gives simultaneous access to large
enough interval in Q for 0.5ltkf/kilt1
kf/ki1.0
Moving the analyzer is cost-effective!
19
Performance of the guide.
Impact of the guide curvature (relative to the
similar straight guide)
Impact of the guide coating (relative to 3qc
supermirror guide)

• L 20 m curved 3qc supermirror guide at a
  reasonable offset of 8 cm 2 times the width of
  the guide provides 75 transmission at 60 meV
• Shorter (L 15 m?) but narrower (3 cm?) guide
  may still be O.K.

20
Evolution of our concept from multi-chopper TOF
to hybrid spectrometer
We choose to optimize the instrument performance
at Ei 15 meV
Optimized layout of the direct geometry disc
chopper spectrometer curved guide loss 6,
tapered section gain lt1.4
Optimized layout of the direct geometry hybrid
spectrometer with vertically focused crystal
monochromator expected gain from vertically
focusing the beam gt 2
21
Comparative layout of the vertically focused
crystal and conventional disk-chopper TOF primary
spectrometers.
The side view of the guide cross-section is shown
schematically.
22
Neutron intensity for 2 cm tall sample obtained
from MC simulation using NISP package.
Focusing monochromator yields a gain of factor 2
or more for Eigt15meV!

• NISP algorithm seems to under-estimate gain from
  focusing gt actual gain may be even bigger!
• Decrease in intensity for hybrid spectrometer at
  Eilt10 meV is due to the improved energy
  resolution.

23
Hybrid spectrometer where the gain comes from?
Taller guide!
Relative performance of the hybrid instrument at
lower energies will improve further if more
realistic m2 guide coating is implied.
24
Hybrid spectrometer some more MC results.
High-E tail is nicely cut by the monochromator.
Especially PG(004).
Spectral distribution of neutrons incident on the
sample
Time profile of the neutron pulse on the sample
Although it nicely illustrates the trend, this
preliminary calculation contained an error, which
underestimated the total intensity for the hybrid
model.
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Summary of the technical features of a proposed
direct geometry hybrid instrument for the SNS
Tall neutron guide efficient vertical focusing
provide reasonable data collection rates even for
very small samples !

• Spectrometer is optimized for
• energy resolution dE/E0.03-0.05
• wavevector resolution dQ/Q0.01-0.03
• High flexibility
• continuous variation of the incident energy from
  2.5 to 60 meV with no higher-order contamination
• full range of energy transfers available at each
  Q
• easily and broadly variable energy and Q
  resolution
• possibility of time focusing/defocusing
• easily adaptable for polarization analysis
• Very low background
• sample is 1.8 m away from the direct beam
• scattering volume is well defined by collimators
• High sensitivity to small scattering
  cross-sections (signal to BG ratio)
• Can be operated at small scattering angles (near
  the forward direction)
• User friendly allows for easy on-line data
  monitoring and analysis and experiment planning

26
Principal features of the proposed hybrid
spectrometer.
Bonus monochromator shapes resolution function,
cutting ugly high-E tail.

• High flux on sample at Ei 5 - 60 meV tall
  neutron guide efficient vertical focusing by
  curved crystal monochromator.
• Polarized beam option polarized incident
  neutron beam for Ei 5 - 60 meV, polarization
  analysis of the scattered beam for Ef lt 15 meV.
• Low background even with bulky sample
  environments collimator(s)slit(s) define
  scattering volume seen by detector(s) and
  restrict analyzer acceptance to scattering from
  sample only. Scattering from cryogenic
  environment, magnet, pressure cell, etc., is
  rejected.
• Continuous wavevector coverage no blind
  spots caused by spaces in the detector array.
• Flexibility both energy and wavevector
  resolutions are variable and easily adjustable,
  typical resolutions are 1 to 5. In addition,
  different crystal reflections may be user on
  monochromator.

27
Punchline current questions and future plans.

• Questions
• Placement beamline 14? Is there enough floor
  space for the instrument large footprint?
• Main beam stop can it be incorporated in the
  monochromator shielding? How significant
  background source would it be?
• Beamline shielding how thick is enough and what
  restrictions result?
• T2 chopper can it be mobile and placed after
  the monochromator?

• Plans
• Workshop on the proposed Hybrid Spectrometer at
  BNL, October 12-13.
• Assemble Instrument Development Team and submit
  formal Letter of Intent to the SNS.
• Put together a comprehensive proposal and search
  for funding.
• Find answers to the above questions.
• Work on further optimization of the instrument
  setup.