facilities and generation

Rob McQueeney, Ames Laboratory and Iowa State University

September 12, 2018 19-Sep-18 Physics 502 2 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 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

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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