Department of Materials Science and Engineering The University of Sheffield

THERAMIN Summer School12th-14th June 2019

Professor Neil C. Hyatt

n.c.hyatt@Sheffield.ac.uk

@ISL_Sheffield

2019 © The University of Sheffield

The views expressed in this talk are the personal opinion of the speaker and do not necessarily reflect those of sponsors or funding agencies

Synchrotron radiation and X-ray Absorption Spectroscopy

Need for Synchrotron Radiation

Synchrotron radiation is electromagnetic radiation emitted by a charged particle moving at relativistic speed in a curved trajectory

Laboratory experiments typically use divergent radiation sources: trade off of intensity vs. resolution

Key characteristics of synchrotron radiation:

• High brightness: ph s-1 mm-2 mrad-1 (BW0.1%)-1

• Highly collimated

• Broad band emission

This means, for example:

• Precision in eV or Å

• Lower concentrations are accessible

• Exploration of extreme conditions

• Time resolved experiments

Image credit: DLS.

The synchrotron radiation source

7

1. Electron gun – generates electrons

2. LINAC – linear accelerator

3. Booster ring – accelerates electrons

4. Storage ring – electron bunches

5. Beamline – various optics

6. End station – your experiment

7. RF source – boost energy of bunches

Image credit: Australian Synchrotron.

Main components of the storage ring

ESRF: 32 straight sectionsImage credit: ESRF.

Generating synchrotron radiation

Bending magnets

Function is to bend electron trajectory between straight sections of synchrotron by application of magnetic field. SR is emitted tangentially as a fan of radiation but with high vertical collimation. Emission is a broadband spectrum of moderately high brightness.

Insertion devices

Function is to generate very high brightness SR emission in straight sections of synchrotron. An array of alternating magnetic dipoles causes deflection of electron beam – SR emitted at each deflection. Result is highly collimated beam of SR.

Wigglers: very high brightness broadband spectrum

Undulators: extremely high brightness in narrow energy range – tuned using magnetic field

Insertion device

Bending magnet

Image credit: ESRF.

Insertion device - undulator

Image credit: APS.

Comparison of synchrotron sources

Image credit: APS.

Fundamental basis of X-ray Absorption Spectroscopy

Provides information on absorber element:

• Oxidation state

• Number and type of nearest neighbours

• Static / dynamic disorder of neighbours

Involves excitation of an electron from a core shell into an unoccupied state

XANES region: qualitative or comparative analysis (reference compounds), calculation difficult

EXAFS region: quantitative analysis using scattering theory

Advantages: any element in any material at measurable concentration; any state; non-crystalline

Disadvantage: bulk average information with limited information content – uniqueness of models

XAS Beamline

Image credit: B. Ravel, BNL.

Synchrotron radiation incident on double single crystal monochromator (M)

Select appropriate wavelength by tilting monochromator in accordance with Bragg Law: l = 2d sin q

Measure the incident beam intensity (I0) in first ion-chamber (A)

Measure the transmitted beam intensity (It) through the sample (S) in second ion-chamber (B)

X-rays ionise gas molecules in ion chambers – filled with mixture of inert gas

Ionisation proportional to intensity

Tune wavelength by tilting monochromator to correct angle

q

q

Dl l

M

I0 It

A BSB

A

S

Beamline 16.5 at the SRS.

EXAFS Equation

iii

k

i

i

iikkRe

kR

kFNk ))(2s in(

)()(

222

2 Amplitude term

Phase term

Determined from semi-classical analysis of the scattering process. Think of it as analogous

Where k = 2p / l is the electron wave vector

The components of this expression are:

Ni: number of scattering atoms (neighbours) of type i

Fi(k): scattering amplitude at atom i – determined by no. of electrons, i.e. atomic number

Ri(k): distance from absorber atom to neighbouring atoms of type i

i(k): phase shift of the photoelectron as a result of the scattering process

e-22k2: The Debye Waller factor which accounts for dynamic and static disorder in the solid

The EXAFS oscillations therefore contain information about:

Number of neighbouring atoms (Ni)

Type of neighbouring atoms (Fi(k), di(k))

Distance of neighbouring atoms (e-2s2k2)

B18 – Diamond Light Source

Image credit: DLS

1: Thermal treatment of PCM wastes: Ce L3 XANES

Plutonium contaminated materials – PCM, 20,000m3 on Sellafield site

Packaged in 200 L drums; some waste requires additional treatment

Benchs-cale demonstration of thermal treatment approach

Ce used as Pu surrogate

N.C. Hyatt et al., J. Nucl. Mater., 444, 186-199, 2014.

1: Thermal treatment of PCM wastes: Ce L3 XANES

Thermal treatment produces a slag-like wasteform – crystallised glass

Partitioning of Ce between glass and ceramic phase 99:1

Solubility limit in aluminasilicate glass depends on speciation (1500oC):

• Ce3+ / Pu3+ = 4-6 mol%

• Ce4+ / Pu4+ = <1 mol%

Ce L3 XANES shows Ce reduced from Ce4+

to Ce3+

Ce incorporated as 1.2 mol% Ce3+ - well below solubility limit

Waste loading could be increase further; but 98% volume reduction already approaches criticality limit

Very low dissolution rate in sat. Ca(OH)2

at 50oC (anoxic)

2: Iron phosphate glasses: Fe K-XANES

137Cs loaded glass pencils manufactured for medical irradiation – will also form final wasteform

Formulation Cs2O-Fe2O3-P2O5 glass: processed at 900oC, incorporates 1200 Ci = 44 TBq 137Cs

How does BaO addition impact glass structure?

Consideration of simple BaO-Fe2O3-P2O5 glasses, with BaO addition to 60 P2O5 – 40 Fe2O3

Investigation by Fe K-edge XANES and Raman spectroscopy

K. Joseph et al., J. Nucl. Mater., 494, 342-353, 2017.

2: Iron phosphate glasses: Fe K-XANES

2: Iron phosphate glasses: Fe K-EXAFS

2: Iron phosphate glasses: Fe K-EXAFS

No significant change in Fe2+ / Fe3+ ratio

Fe co-ordinated as FeO5 species on

average – verified by 57Fe Mossbauer

3: Radiation damaged ceramics

Alpha recoil damage in actinide ceramics drives a crystalline to amorphous phase transition

Some models suggest that the amorphous material is a severely disordered crystalline material

A good problem for XAS as a probe of local co-ordination

Use ion beam implantation to induce surface amorphisation

Then use XAS in grazing angle configuration to probe only damage depth with fluoresencedetection

D.P. Reid et al., Nucl. Inst. Meth. Phys. Res., B268, 1847-1852, 2010

3: Radiation damaged ceramics

3: Radiation damaged ceramics

3: Radiation damaged ceramics

5: Brannerite ceramics for MOX residues

Decision to close Sellafield MOX fuel fabrication plant in 2011

SMP used short binderless route: attrition milling to blend UO2 and PuO2

MOX fuel pellets sintered at 1650oC (capability for 1750-1880oC)

Consideration of ceramic wasteform for residues from future post operative clean out

Suggest solid solution based on brannerite UTi2O6 – Pu counterpart is also known

e.g. U0.72Gd0.1Ca0.1Ce0.08Ti2O6

Both U and Pu (Ce) could be redox active and play a role in charge compensation

Bailey et al., RSC Advances, 8, 2092-2099, 2018.Image credit: NDA.

5: Brannerite ceramics for MOX residues

Bailey et al., RSC Advances, 8, 2092-2099, 2018.

• Turnkey laboratory XAS/XES system

• 4 – 18 keV energy range (100W air cool tube)

• Concentrated to moderately dilute absorbers

easyXAFS XES 100 Extended – first in UK

Jahrman et al., Review of Scientific Instruments 90, 024106 (2019); https://doi.org/10.1063/1.5049383