Neutron facilities and generation. Rob McQueeney, Ames Laboratory and Iowa State University

Transcription

1 Neutron facilities and generation Rob McQueeney, Ames Laboratory and Iowa State University September 12, 2018

2 19-Sep-18 Physics Neutrons compared to other probes of matter Bulk probe Interacts with nucleus Interacts with magnetic dipole moment 2

3 Neutron interaction with matter Properties of the neutron Mass m n =1.675 x kg Charge = 0 Spin-1/2, magnetic moment m n = m N 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

4 Neutron and x-ray comparison Neutron X-ray Source intensity ~10 8 neutrons s -1 ~10 13 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

5 Cross-sections Neutrons Random with Z Depends on isotope Depends on nuclear spin Absorption can be problem Abundance (%) Crosssection (bn) Absorption (bn) Gd Gd Gd Gd Gd Gd Gd Gd

6 Neutron scattering can cover decades of length and time scales 6

7 Faster processes Time and length scales Shorter length scales 7

8 Producing neutrons Fission Nuclear reactor Spallation Particle accelerator neutrons Moderators Cold-Thermal Moderators Cold-Epithermal 8

9 Reactor sources 120,000,000,000,000 neutrons per cm 2 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

10 Neutrons by reactor fission High flux isotope reactor - ORNL NIST 10

11 HFIR instrument suite 11

12 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 partially coupled water moderator Be steel rings partially coupled LH 2 moderator 12

13 Neutrons by pulsed spallation Spallation Neutron Source (ORNL) 13

14 SNS facility components Section of SNS linac SNS Hg target 14

15 Target-moderator system STS FTS HFIR STS c-h 2 FTS dc-h 2 FTS dc-h 2 O (high res) HFIR FTS STS 15

16 SNS instrument suite 16

17 Reflectometry 6% Imaging 3% SNS and HFIR instruments by technique Development 9% Diffuse 25% Powder 25% SANS 12% Diffraction 37% Engineering 17% Single-crystal 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

18 Powder diffraction Determine the crystal structure D20 (ILL) 18

19 Monochromators/analyzers Selects the incident/final wavevector k i or k f q Q(hkl)=2k i sin q Mono d(hkl) uses PG(002) General Be(002) High k i Si(111) No l/2 19

20 Detectors Gas Detector n + 3 He 3 H + p 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

21 Scattering angle TOF powder diffraction SNS Time-of-flight t = L/v = lml/h = 2mLdsinq/h Time, wavelength, or d-spacing 21

22 Guides Transport beam over long distances Background reduction Total external reflection Ni coated glass Ni/Ti multilayers (supermirror) 22

23 Single-crystal diffraction Single-crystal: more detail than powders Wide angle diffraction: Get an overview of everything Incommensurate magnetism 23

24 Position sensitive detectors 3 He tubes (1-4 meters) Charge division Position resolution ~ cm Time resolution ~ 10 ns SEQUOIA detector bank 24

25 TOF single-xtal diffraction 25

26 Single crystal diffraction Triple-axis diffraction: focus in on specific points of interest HB-1A 3-axis spectrometer Orbital ordering in YVO 3 26

27 Sample environment Temperature, field, pressure Heavy duty for large sample environment CCR He cryostats SC magnets IN14-ILL HB3-HFIR 27

28 Places to go AMES SSRL LCLS LANSCE MURR APS HFIR SNS NIST NSLS-II Europe 28

29 Further references General neutron scattering G. Squires, Intro to theory of thermal neutron scattering, Dover, S. Lovesey, Theory of neutron scattering from condensed matter, Oxford, R. Pynn, Structural refinements GSAS 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 29