Mineral Physics Group

1
Mineral Physics Group University of Cambridge

• The Degree and Nature of Radiation Damage in
  Zircon
• Paradigm shifts in understanding radiation damage
  in ceramic materials
• Ian Farnan
• Kostya Trachenko, Martin T Dove, Ekhard KH Salje,
  
• Susana Rios, Greg R Lumpkin
• Cambridge Centre for Ceramic Immobilisation
• Department of Earth Sciences, University of
  Cambridge, UK
• Herman Cho, William J. Weber
• PNNL, Richland, WA

2
a-damage process
Mineral Physics Group University of Cambridge
Flight of a-particle causes ionisations and
stopping leads to 100-200 atomic
displacements.Recoil of heavy nucleus causes
most damage 1000-2000 atomic displacements (in
metals).
3
Mineral Physics Group University of Cambridge

• Radiation resistance measured via ion beams
• Initial damage recovery (epitaxial
  recrystallisation)
• Critical amorphisation temperatures
• Possible influence of surface in recovery
  mechanism
• Total energy not deposited in sample, channelling

• The Degree and Nature of Radiation Damage in
  Zircon

4
Mineral Physics Group University of Cambridge

• Measuring internal radiation damage
• Natural samples e.g. zircon containing 238U or
  232Th
• Accelerated radiation damage using 238Pu or 244Cm

• X-ray diffraction
• Density measurements
• Separate crystalline and amorphous fractions, fa,
  fc

5
Mineral Physics Group University of Cambridge
Amorphisation as a function of a-dose
Non-linear growth of amorphous -fraction -
multiple overlap models
6
Mineral Physics Group University of Cambridge
Previous work on quantifying radiation damage in
zircon based on volume/density considerations.
Non-linear growth of amorphous -fraction -
multiple overlap models
7
Mineral Physics Group University of Cambridge
Previous work on quantifying radiation damage in
zircon based on volume/density considerations.
Non-linear growth of amorphous -fraction -
multiple overlap models
8
Mineral Physics Group University of Cambridge
Previous work on quantifying radiation damage in
zircon based on volume/density considerations.
Non-linear growth of amorphous -fraction -
multiple overlap models
9
Mineral Physics Group University of Cambridge
Previous work on quantifying radiation damage in
zircon based on volume/density considerations.
Non-linear growth of amorphous -fraction -
multiple overlap models
10
Mineral Physics Group University of Cambridge
Previous work on quantifying radiation damage in
zircon based on volume/density considerations.
Non-linear growth of amorphous -fraction -
multiple overlap models
11
Mineral Physics Group University of Cambridge
29Si MASNMR (p/12 pulse every 300 s)
Spin-counting NMR records signals from atoms with
magnetic nuclei. Does not depend on long range
order and gives damaged fraction in number of
atoms not amorphous volume fraction as from
volume/density measurements.
12
Mineral Physics Group University of Cambridge
First 29Si MASNMR data on radiation damaged
natural zircon

• Si in Amorphous phase

13
Mineral Physics Group University of Cambridge
Magnetic resonance data
Density/volume data
14
Mineral Physics Group University of Cambridge

• Damage accumulates but volume of amorphous phase
  is constrained to be same as zircon crystal
• Damaged areas join up, macroscopic swelling
  begins - percolation transition.
• Second percolation point crystal islands in
  amorphous matrix no increase in macroscopic
  volume.
• Complete amorphisation

15
Mineral Physics Group University of Cambridge

• Si NMR data gives the amorphous fraction based on
  the number of atoms displaced.

• Can fit to direct model
• 811 60 Si atoms (permanently) displaced per
  alpha decay.
• Assuming other 5 atoms in the formula unit
  (ZrSiO4) will be displaced with the Si
• 4866 360 atoms displaced per a
• Radius of sphere of 4866 atoms ZrSiO4 23.3 Å
  (rcryst 4.7 g cm-3)
• cf TEM observation 50 Å dia features low damage
  samples

16
Mineral Physics Group University of Cambridge
300,000 atoms, 30 keV 4000 atoms displaced
MD simulation of damage caused by heavy nucleus
recoil (Kostya Trachenko, Martin Dove)
17
Mineral Physics Group University of Cambridge
TRIM/SRIM Calculations on Zircon
Zr 79 eV Si 23 eV O 47 eV
Displacement energies
Density 4.7 g cm-3 Particle 234U, 86 keV
18
Mineral Physics Group University of Cambridge
TRIM/SRIM Calculations on Zircon
Zr 79 eV Si 23 eV O 47 eV
Displacement energies
Density 4.7 g cm-3 Particle 234U, 86 keV
792 atoms displaced in a highly branched
cascade
19
Mineral Physics Group University of Cambridge
TRIM/SRIM Calculations on Zircon
792 atoms displaced in a highly branched
cascade
20
Mineral Physics Group University of Cambridge
NMR chemical shift calculations

• NMR chemical shift shielding of the 29Si due
  to the electronic current induced by the external
  magnetic field
• Pseudo-potentials description of the valence
  electrons
• GIPAW (Pickard and Mauri, 2001) reconstruction
  of the all-electron electronic structure
  (code PARATEC, cutoff 70 Ry)
• accurate prediction of relative NMR chemical
  shifts for periodic systems.
• reference gt absolute scale zircon at zero
  pressure

21
Mineral Physics Group University of Cambridge
Changes in local structure dose dependent.
Macroscopic volume no longer changes

• Centre of gravity of amorphous phase becomes more
  negative with dose up to second percolation
  point.
• Increasing connectivity in amorphous phase
• Crystal structure changes until first
  percolation point

22
Mineral Physics Group University of Cambridge
Simulation of secondary events
Damaged region receives a second hit (head on)
Increase in connectivity of SiO4
tetrahedra Consistent with spectroscopic evidence
23
Mineral Physics Group University of Cambridge
Changes in local structure dose dependent.
Macroscopic volume no longer changes

• Centre of gravity of amorphous phase becomes more
  negative with dose up to second percolation
  point.
• Increasing connectivity in amorphous phase
• Crystal structure changes until first
  percolation point

24
Mineral Physics Group University of Cambridge
Shocked zircon sample (52 GPa, Fiske et al.)
experiments
- 91.1 ppm
Reidite (scheelite structure)
12 GPa (23 GPa at RT) Theory 11 GPa
- 81.6 ppm
Zircon
25
Mineral Physics Group University of Cambridge
Theoretical 29Si NMR shifts in zircon and
reiditeunder pressure

• Not simply related to swelling
• Not pressure induced
• Role of point defects ?

26
Mineral Physics Group University of Cambridge
Densification associated with a-recoil
200 reflection of zircon

• main peak -gt low 2q lattice expansion
• Extra intensity to high 2q, compression

27
Mineral Physics Group University of Cambridge
Simulation of secondary events
Damaged region receives a second hit glancing
28
Mineral Physics Group University of Cambridge

• Summary
• Amorphisation by internal radioactive decay
  (with density of damaged areas constrained) may
  differ significantly from exterior bombardment by
  heavy ions.
• Structure of amorphous phase is dose dependent.
• Densification accompanies a-decay events
• Scattering from densified rims can cause rate of
  amorphisation to increase

29
Mineral Physics Group University of Cambridge

• Perspective
• Need to work on bulk samples.
• Implement active experiments to quantify bulk
  radiation damage.

30
Mineral Physics Group University of Cambridge

• Perspective
• Need to work on bulk samples.
• Implement active experiments to quantify bulk
  radiation damage.
• Collaboration with PNNL - active MASNMR
  239Pu/238Pu

31
Mineral Physics Group University of Cambridge
29Si MASNMR Pu containing zircons
32
Mineral Physics Group University of Cambridge
29Si MASNMR Pu containing zircons
10 wt 238Pu
238 Pu sample gt 40 GBq/g
10 wt 239Pu
33
Mineral Physics Group University of Cambridge

• Perspective
• Need to work on bulk samples.
• Implement active experiments to quantify bulk
  radiation damage.
• Collaboration with PNNL - active MASNMR
  239Pu/238Pu
• Actinet Euratom FP6 network - Pu doped
  pyrochlores

34
Mineral Physics Group University of Cambridge

• Perspective
• Need to work on bulk samples.
• Implement active experiments to quantify bulk
  radiation damage.
• Collaboration with PNNL - active MASNMR
  239Pu/238Pu
• Actinet Euratom FP6 network - Pu doped
  pyrochlores
• RWIN Network?
• Can we develop access routes to UK active
  facilities?