Neutron facilities and generation
Rob McQueeney, Ames Laboratory and Iowa State University
September 12, 2018 19-Sep-18 Physics 502 2 Neutrons compared to other probes of matter
Bulk probe Interacts with nucleus Interacts with magnetic dipole moment
2 Neutron interaction with matter Properties of the neutron -27 Mass mn =1.675 x 10 kg Charge = 0
Spin-1/2, magnetic moment mn = -1.913 mN
Neutrons interact with… Nucleus Crystal structure/excitations (eg. phonons) Unpaired e- via dipole scattering Magnetic structure/excitations (eg. spin waves)
Nuclear scattering Magnetic dipole scattering
3 Neutron and x-ray comparison Neutron X-ray Source intensity ~108 neutrons s-1 ~1013 photons s-1 Beam size cm mm mm 10 nm Energy at l = 1 Å 81.8 meV 12.4 keV Spectroscopic range neV eV meV keV Beam penetration ~cm ~10 mm Magnetic/structural ~1 ~10-5 Unique contrast Isotopic (H/D) Chemical/valence
4 Cross-sections
Abundance Cross- Absorption (%) section (bn) (bn) Neutrons Gd --- 180 49700 Random with Z 152Gd 0.2 13 735 Depends on isotope 154Gd 2.1 13 85 155Gd 14.8 66 61100 Depends on nuclear spin 156Gd 20.6 5 1.5 Absorption can be problem 157Gd 15.7 1044 259000 158Gd 24.8 10 2.2 160Gd 21.8 10.52 0.77
5 Neutron scattering can cover decades of length and time scales
6 Time and length scales
Shorter length scales Faster processes Faster
7 Producing neutrons Fission Spallation Nuclear reactor Particle accelerator
neutrons
Moderators →Cold-Thermal Moderators →Cold-Epithermal
8 Reactor sources 120,000,000,000,000 neutrons per cm2 per second Swimming pool reactor
Fission reaction by neutron capture
REACTOR
Neutrons slow down by colliding with water molecules Use 235U enriched Uranium
HZB Neutron School, A. Tennant 9 Neutrons by reactor fission
High flux isotope reactor - ORNL
NIST
10 HFIR instrument suite
11 Spallation Sources
Production of neutrons: “spallation source”
a) Nuclear reaction: spallation, b) slowing down to useful energies: fission, radioactivity… E ~ MeV epithermal (~ eV), hot (~ 300 meV) thermal (~ 40 meV) cold (~ 5 meV)
Example of a spallation source: Lujan Center
steel partially coupled rings water moderator Be
partially coupled LH2 moderator
12 Neutrons by pulsed spallation
Spallation Neutron Source (ORNL)
13 SNS facility components
Section of SNS linac
SNS Hg target
14 Target-moderator system
STS STS c-H2 HFIR FTS FTS dc-H2 STS
HFIR FTS
FTS dc-H2O (high res)
15 SNS instrument suite
16 SNS and HFIR instruments by technique Development Imaging 9% 3% Reflectometry 6% Diffuse Powder 25% 25%
Diffraction Engineering SANS 37% Single-crystal 17% 12% 33%
Inelastic scattering 33%
• Diffraction - crystal and magnetic structures Texture and strain Disordered and amorphous materials • SANS – large scale structures • Inelastic scattering - dynamics • Reflectometry – layers and surfaces • Imaging – radiography and tomography
17 Powder diffraction Determine the crystal structure
D20 (ILL)
18 Monochromators/analyzers Selects the incident/final wavevector
ki or kf
q
Q(hkl)=2ki sin q Mono d(hkl) uses PG(002) 3.353 General
Be(002) 1.790 High ki Si(111) 3.135 No l/2
19 Detectors • Gas Detector – n + 3He 3H + p + 0.764 MeV – Ionization of gas – e- drift to high voltage anode – High efficiency counting of scattered neutrons • Beam monitor – Low efficiency measure of incident flux Monitor
Detector 20 TOF powder diffraction
POWGEN @ SNS Scattering angle Scattering Time-of-flight t = L/v = lmL/h = 2mLdsinq/h Time, wavelength, or d-spacing
21 Guides • Transport beam over long distances • Background reduction • Total external reflection – Ni coated glass – Ni/Ti multilayers (supermirror)
22 Single-crystal diffraction Single-crystal: more detail than powders Wide angle diffraction: Get an overview of everything
Incommensurate magnetism
23 Position sensitive detectors • 3He tubes (1-4 meters) • Charge division • Position resolution ~ cm • Time resolution ~ 10 ns
SEQUOIA detector bank
24 TOF single-xtal diffraction
25 Single crystal diffraction Triple-axis diffraction: focus in on specific points of interest
HB-1A 3-axis spectrometer
Orbital ordering in YVO3
26 Sample environment
• Temperature, field, pressure • Heavy duty for large sample environment
– CCR HB3-HFIR – He cryostats – SC magnets IN14-ILL – …
27 Places to go
AMES NSLS-II APS SSRL NIST LCLS MURR HFIR LANSCE SNS Europe
28 Further references
• General neutron scattering – G. Squires, “Intro to theory of thermal neutron scattering”, Dover, 1978. – S. Lovesey, “Theory of neutron scattering from condensed matter”, Oxford, 1984. – R. Pynn, http://www.mrl.ucsb.edu/~pynn/. • Structural refinements – GSAS http://www.ncnr.nist.gov/xtal/software/gsas.html – FullProf http://www.ill.eu/sites/fullprof/ • How to get beam time – Talk to one of us at Ames about your experiment – We can identify a suitable instrument – Talk to instrument scientist – Write a beamtime request
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