peter gehring nist center for neutron researchpeter gehring nist center for neutron research ....
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Choosing the Right Spectrometer
Peter Gehring NIST Center for Neutron Research
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Review: Main Messages of the Week
(1) Neutron scattering experiments measure the flux of neutrons scattered by a sample into a detector as a function of the change in neutron wave vector (Q) and energy (hω).
(2) The expressions for the scattered neutron flux Φ involve the positions and motions of atomic nuclei or unpaired electron spins.
Energy Momentum
hQ = hki - hkf hω = hωi - hωf
hωn = h2kn2/2m hkn = h(2π/λn)
Φ provides information about all of these quantities! Φ = fri(t), rj(t), Si(t), Sj(t)
Φ(Q,hω) = neutrons sec–cm2
2θ
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(3) The scattered neutron flux Φ(Q,hω) is proportional to the space (r) and time (t) Fourier transform of the probability G(r,t) of finding one or two atoms separated by a particular distance at a particular time.
dtrdtrGe trQi 3)(
2
),(∫∫ −⋅∝∂Ω∂
∂∝Φ ω
ωσ
Time space
ω 2π/T
ω-space
d
Real space
Q 2π/d
Q-space
~1/ξ ~1/Γ
T
Review: Main Messages of the Week
time
Φ Φ
ξ Γ
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Question: Can one measure elastic scattering from a liquid?
Why? Why not? Hint: What is the correlation in time of one atom in a liquid with another atom a distance r away?
Pop Quiz!
ω
Φ
ω=0
?
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A source
A method to specify the incident wave vector ki
A method to specify the final wave vector kf
A well chosen sample + good idea
Detector(s) (Ei,ki)
(Ef,kf)
Basics Elements of a Neutron Inelastic Scattering Experiment
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25 K
300 K
2000 K
Φ ~ v3e (-mv2/2kBT)
1
2
3
4
5
6
5 10 15 Neutron velocity v (km/sec)
0 0
Liquid Hydrogen
Heavy Water (D2O)
Hot Graphite
Maxwellian Distribution
“Fast” neutrons: v = 20,000 km/sec
Neutron Source: Moderation
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BT7, SPINS, HFBS
1. Bragg Diffraction
2. Time-of-Flight (TOF)
DCS, HFBS
3. Larmor Precession
NSE
nλ=2dSinθ
B
Methods of Specifying and Measuring ki and kf
d
L
v = L/t
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Why So Many Different Spectrometers?
Thermal Neutrons
Cold Neutrons
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Because neutron scattering is an intensity-limited technique. Thus detector coverage and resolution MUST be tailored to the science.
Uncertainties in the neutron wavelength and direction imply Q and ω can only be defined with a finite precision.
Courtesy of R. Pynn
The total signal in a scattering experiment is proportional to the resolution volume better resolution leads to lower count rates! Choose carefully …
∝
Why So Many Different Spectrometers?
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Neutron X-ray
Neutron
-0.050 -0.025 0.025 0.050 0.000 q (A-1)
X-ray
-0.050 -0.025 0.025 0.050 0.000
q (A-1)
Q-Resolution Matters!
The “right” resolution depends on what you want to study.
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0
2
4
6
8
0 1 2 3
Q = (0,K,0.5)
hω (m
eV)
0
400
800
1200 In
tens
ity
hω-Resolution Matters!
0
400
800
1200
0 2 4 6 hω (meV)
Inte
nsity
0
400
800
1200
Inte
nsity
Focusing Analyzer
9 Blades
Focusing Analyzer
5 Blades
Focusing Analyzer
Flat
Gap
SPINS
Another example …
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How do I Choose the Right Spectrometer?
Two basic considerations:
2. What are the length scales (Q) of interest?
1. What are the time scales (hω) of interest?
Two additional considerations:
2. What momentum resolution (∆Q) is required?
1. What energy resolution (∆hω) is required?
(Some spectrometers overlap the choice may boil down to one of resolution)
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Different Spectrometers Cover Different Regions of Phase Space
Do you see a pattern here?
Larger “objects” tend to exhibit slower motions.
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Rules of Thumb
ω > 10-20 meV - use a thermal triple-axis spectrometer like BT7.
1. What are the energies (hω), i.e. time scales (∆t ~1/ω), of interest?
REMEMBER - Qmin and Qmax are inversely proportional to the incident neutron wavelength
ω < 20-30 µeV - use HFBS or NSE
In between - use DCS or a cold neutron TAS spectrometer.
2. Make sure that the length scales L of the relevant motions lie within the range of the spectrometer. For example, consider the HFBS. (Q ~ 2π/L)
Qmin = 0.25 Å-1 Lmax~ 25 Å Qmax = 1.75 Å-1 Lmin~ 3.5 Å
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Is your sample polycrystalline or amorphous?
Is the expected Q-dependence of the scattering weak?
YES? Consider instruments with large analyzer areas.
More Rules of Thumb
NO? Consider using a triple-axis spectrometer like BT7 or SPINS.
MACS DCS HFBS BT7
Does ONLY the magnitude (not the direction) of Q matter?
This often means that you want to look at a large region of Q-ω space, or that you can sum the data over a large region of Q-ω space.
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Incoherent scattering Broad surveys in Q-ω
For Example: DCS versus BT7
Rules of Thumb: (think carefully before violating)
DCS – systems requiring resolution < 100 µeV BT7 – single crystals
Coherent scattering Limited regions in Q-ω DCS BT7
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Quantities varied • wavelength λ • chopper slot widths W
I(E) ∆E
Things to Consider When Choosing DCS
Remember – Intensity Resolution
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Triple axis spectrometers are typically used when either - (1) the direction of Q is important or (2) the interesting region of Q-ω space is of limited extent.
Collimation(') λ rel. signal FWHM . 55-80-80-80 4 Å 1.00 0.28 meV 55-40-40-40 4 Å 0.24 0.17 meV 69-80-80-80 5 Å 0.26 0.13 meV 84-80-80-80 6.1 Å 0.03 0.05 meV
Remember – Intensity Resolution
ki kf
One data point at a time …
0 1 2 3 0
1
2
3
Things to Consider When Choosing BT7
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Do the features of interest lie within this hω-range?
Do you really require such good energy resolution δE ~ 1 µeV (or perhaps even
better resolution)?
Do the length scales of interest lie within this Q-range? 0.25 A-1 < Q < 1.75 A-1 o o
Can you live with such coarse Q-resolution? δQ < 0.1 - 0.2 A-1 o
Things to Consider When Choosing HFBS
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If the hω-resolution of backscattering is “not good enough,” or if you are only interested in a “limited” region of Q-space (typically small Q) …
0.1
2
46
1
2
46
10
I/cm
-1
2 3 4 5 6 7 8 90.1
2 3 4
Q/Å-1
AOT micelles AOT/D2O/C10D22
These cases typically involve coherent scattering, which tends to peak near
Remember – slower motions usually imply longer length scales.
Q ~ 2π relevant length scale
… then use NSE (low Q, long times)
Many atoms moving together Coherent scattering
Things to Consider When Choosing NSE
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Know as much about your sample as possible!! (Beamtime costs ~ $5000/day!!)
General Sample “Design”
Magnetization vs T
Specific heat vs T
Muon spin relaxation
Raman spectroscopy
X-ray data
Other considerations: What’s the structure (in a general sense)? Are there any phase transitions (or a glass transition)? What isotopes are present? Supplementary data from other measurements …
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Try to avoid isotopes that are strongly absorbing.
6Li 10B 113Cd 157Gd
http://www.ncnr.nist.gov/resources/n-lengths
For a complete listing go to
General Sample “Design”
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Single crystals yield the most information.
Increase the intensity by increasing the amount of sample.
ki kf
Almost all experiments on triple-axis spectrometers involve coherent scattering sample should be deuterated (if it contains H at all).
Sample “Design” for Triple-Axis Spectrometers
If you have a powder, use a cylindrical container (rather than flat plate).
Annular may be the best sample geometry.
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The maximum beam size is usually given in the instrument description: DCS: 3 cm x 10 cm (or 1.5 cm x 10 cm) Backscattering: 3 cm x 3 cm
I/Io = exp –(nσΤD)
ki kf
Sample “Design” for DCS and HFBS
If possible, use cylindrical samples (rather than flat plate) Remember - for incoherent, quasielastic scattering the transmission of the beam should be ~90%.
Often annular is the best sample geometry
Increase the intensity by increasing the amount of sample Fill the beam with sample
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Does the sample contain H?
Create a sample where - the “interesting” portions are hydrogenated and the “uninteresting” portions are deuterated.
Remember: Neutrons LOVE H!!
Sample “Design” for DCS and HFBS
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Create a sample where the “interesting” portions of the sample have a different SLD than the “uninteresting” portions
D2O (deuterated)
C6D14 (deuterated)
AOT (hydrogenated) SLD core 6.4×10-6 Å-2
SLD shell 10.0×10-6 Å-2
SLD solvent 6.1×10-6 Å-2
http://www.ncnr.nist.gov/resources/sldcalc.html
Sample “Design” for NSE
Typically this means deuterating the major phase in order to reduce the incoherent background
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Typically use flat plate samples (at small angles)
ki kf
I/Io = exp –(nσΤD)
Increase the intensity by increasing the amount of sample Fill the beam with sample
Rule of thumb - the transmission should be ~70%
Sample “Design” for NSE
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39%
22%
12%4%
2%
21%
MagnetismMaterials ScienceSmall MoleculesPolymersComplex FluidsBiology
6%
8%
40%
23%
16%7%
NSE
Typical Distributions of Science by Instrument
0% 17%
50%
21%
0%
12%
HFBS
DCS
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Access to the neutron scattering instruments that you’ve used over the past week is merit-based. Open to all qualified users,
but subject to an anonymous peer-review of proposals.
Calls for proposals are issued about twice/yr.
http://www.ncnr.nist.gov/programs/CHRNS/CHRNS_prop.html
Further information on submitting proposals :
Applying for Beam Time
Next deadline for new proposals ~ December 2013.
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Some Summer School Success Stories
2003
1997 1999
William Ratcliff NCNR Staff Physicist
2001
Jae-Ho Chung University Prof.
Vicky Garcia-Sakai ISIS Staff Scientist
Rob Dimeo NCNR Director (Management)
Ok, so you can’t win them all …
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Organizers – Bulent Akgun and Yamali Hernandez
Administrative staff Experiment teams Invited speakers
Scatter Well!
Acknowledgements