1. Wir schaffen Wissen – heute für morgen
15.10.2015
Vladimir Krsjak, Yong Dai
17th International Conference on Fusion Reactor Materials,
October 11th - 16th, 2015, Aachen, Germany

2. V. Krsjak, ICFRM—17, Aachen, Germany, 15th October 2015 2

3. V. Krsjak, ICFRM—17, Aachen, Germany, 15th October 2015 3

4. V. Krsjak, ICFRM—17, Aachen, Germany, 15th October 2015
We have only a limited spa1al resolu5on and sensi5vity (to helium)
with most experimental methods (TEM).
1) requirement of reliable accelerator-driven systems (ADS) for an effec1ve
transmuta1on of long-lived radioisotopes in nuclear fuel;
2) requirement of affordable neutron sources for neutron scaIering and imaging
techniques;
3) requirement of irradia5on facili5es that enable greatly accelerated irradia1on
studies with fusion-relevant transmuta1on rates.
Neutron produc1on in spalla5on reac5ons is always accompanied
by the emission of light charged par5cles. Small atoms like H or
He have a profound effect on material proper1es
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5. V. Krsjak, ICFRM—17, Aachen, Germany, 15th October 2015
C. Vieh, PhD Thesis, 2015, EPFL, Lausanne H. Ch. Andersen, The Princess and the pea, 1835
STEM/EELS: Only a weak fingerprint of helium can be seen in the
EELS spectrum. Moreover, this approach is limited by the TEM
resolu1on to rela1vely large He bubbles (>2nm).
5

6. V. Krsjak, ICFRM—17, Aachen, Germany, 15th October 2015
C. Vieh, PhD Thesis, 2015, EPFL, Lausanne H. Ch. Andersen, The Princess and the pea, 1835
STEM/EELS: Only a weak fingerprint of helium can be seen in the
EELS spectrum. Moreover, this approach is limited by the TEM
resolu1on to rela1vely large He bubbles (>2nm).
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7. V. Krsjak, ICFRM—17, Aachen, Germany, 15th October 2015
Severe displacement damage, large He bubbles; He
bubbles can be well characterized by TEM and
STEM/EELS; Lot of experimental data – good level of
understanding
~20 dpa
~10 dpa
Intermediate damage, small helium bubbles;
quan1ta1ve characteriza1on by TEM; limited
informa1on on helium – good level of understanding
Early stage displacement
damage, He-V clusters;
almost no experimental
d a t a f ro m el ect ro n
microscopy techniques
~5 dpa
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8. V. Krsjak, ICFRM—17, Aachen, Germany, 15th October 2015
PAS
1mm
Positron annihila1on spectroscopy provides
nanoscale informa1on from a macroscopic region
Resolu1on/sensi1vity
0.1nm
vacancies
1nm
vacancy clusters
100nm
precipitates
Size of the inves1gated region
1nm 10nm 100nm 1000nm
HR TEM
Atom Probe
TEM
10nm
cavi1es
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9. V. Krsjak, ICFRM—17, Aachen, Germany, 15th October 2015
PAS
1mm
Positron annihila1on spectroscopy provides
nanoscale informa1on from a macroscopic region
Size of the inves1gated region
100nm 1000nm
TEM
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10. V. Krsjak, ICFRM—17, Aachen, Germany, 15th October 2015 10

11. V. Krsjak, ICFRM—17, Aachen, Germany, 15th October 2015
sample
Photons
(XRD, EDS)
Electrons
(SEM,
TEM)
Positrons
(PALS,
CDBS)
Neutrons
(SANS.
Diffr.)
Protons
(PIXE)
Muon
(mSR)
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12. V. Krsjak, ICFRM—17, Aachen, Germany, 15th October 2015
Self-Seeking
(positrons diffuse typically ~100nm in metals and seek sites
with higher positron affinity than bulk i.e. it
is aIracted by certain type of defects!)
Defect type
Sensitivity range (detection limit vs.
saturated trapping)
neutral vacancies 5×1021... 1025 m-3
dislocations 1012 ... 5×1015 m-2
precipitates (r=2 nm) 1020 ... 1023 m-3
grain boundaries 5 µm ... 200 nm (particle size)
microvoids (>50 atoms) 1020 ... 5×1023 m-3
Sensi1ve to H and He.
H and He presence in defects affect positron life1me and
changes the electron momentum distribu1on
Macroscopic samples
Informa1on on sub-nm scale features from a large volume
(few mm3)
Sensi1ve
12

13. Coincidence
Doppler
Broadening
Spectroscopy
Energy
E0 = 511keV
Positrons annihilate mainly with the electrons of the
outermost shell due to the repulsion of the nucleus.
Such annihila1on results in Eγ ≅ 511keV. But the
annihila1on occurs also with core electrons (electrons
with higher momentum). Such annihila1on leads to a
devia5on in the energy of annihila1on gamma, which
is propor5onal to the momentum of the electron-
positron pair.
Aeer thermaliza1on (~ 3ps), positrons diffuse through
the lafce un1l trapping / annihila1on. Diffusion 5me
and trapping rate are a func5on of the microstructure
and they can be measured.
Two techniques of positron annihila1on
spectroscopy, based on different physical
principles, have been widely established
in material research.
Positron
Annihila5on
Life5me
Spectroscopy

14. Doppler Broadening
(gamma) Spectrometer
Life5me spectrometer

15. V. Krsjak, ICFRM—17, Aachen, Germany, 15th October 2015
Positrons are obtained for laboratory setups usually by the β+ decay of isotopes 22Na, 64Cu
or 58Co
22Na → 22Ne + β+ + νe + γ(1.27MeV)
- 22Na positron source - continuous spectra
0 – 545 keV
- correspondent depth in bcc iron 0 – 130 µm
545 keV
↓
3+
2.602 y
2+
1274
γ 1274
keV 0
3.7 ps
stable 0
22Ne
22Na
β- (0,1%)
E.C. (9,5%)
β+ (90,4%)
Physical Half-Life: 2.602 Years
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16. V. Krsjak, ICFRM—17, Aachen, Germany, 15th October 2015 16

17. V. Krsjak, ICFRM—17, Aachen, Germany, 15th October 2015
T91 irradiated in BR2 reactor
at the SCK CEN, Belgium
T91 irradiated at SINQ
22Na was found in all spalla1on
samples of ferri1c/martensi1c
steels but its ac5vity is too low to
enable reasonable experiments.
There is however another suitable
positron source in all spalla1on
steel samples.
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18. V. Krsjak, ICFRM—17, Aachen, Germany, 15th October 2015
44Ti isotope is a product of
56Fe(p, x) reaction and it is
present in all STIP samples of
steels Gamma emiIed
together with
positron
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19. V. Krsjak, ICFRM—17, Aachen, Germany, 15th October 2015 19

20. V. Krsjak, ICFRM—17, Aachen, Germany, 15th October 2015
Benefits of using internal 44Ti/44Sc source:
- The 44Ti/44Sc source has a much longer half-life (59.6 y)
which ensures a long-term stable produc1on of positrons.
- Homogeneous probing of the whole bulk
- No surface treatment required
- There are prac1cally no limita1ons as regards the sample
size and shape.
- Methodology is suitable for very ac1ve samples (60Co
ac1vity is usually equal or lower than 44Ti ac1vity).
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21. V. Krsjak, ICFRM—17, Aachen, Germany, 15th October 2015
Benefits of using internal 44Ti/44Sc source:
- The 44Ti/44Sc source has a much longer half-life (59.6 y)
which ensures a long-term stable produc1on of positrons.
- Homogeneous probing of the whole bulk
- No surface treatment required
- There are prac1cally no limita1ons as regards the sample
size and shape.
- Methodology is suitable for very ac1ve samples (60Co
ac1vity is usually equal or lower than 44Ti ac1vity).
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22. V. Krsjak, ICFRM—17, Aachen, Germany, 15th October 2015 22

23. Materials: (EM10, CLAM, Eurofer 97, F82H, Op1fer,
MA956, Eurofer, ODS Eurofer, T91 …)
Irradia1on doses: (5 – 21dpa)
Irradia1on temperatures: (100 – 600°C)
Annealing temperatures: (200 – 800°C)

24. V. Sabelova et al. J. Nucl.
Mater. 450 (2014) 54–58
Helium introduces a dis1nct peak
in the momentum spectra around
7×10-3 m0c. In the conven1onal
approach with the W parameter
set to 15-25 m0c, He effect is
prac1cally invisible.
The dis1nct peak appears in the
experimental spectra at the stage
when we expect a maximum
concentra1on of He in vacancy
clusters (He/V = max.)
V. Sabelova et al. J. Nucl. Mater. 458 (2015) 350–354
Simula1on
Experiment

25. CDB ra1o curve of the SINQ-
irradiated F82H steel annealed at
400°C to the electron-irradiated F82H
steel.
K. Sato et al. J. Nucl. Mater. 2015, in
press, corrected proof
Positrons get trapped at the
vacancy clusters containing helium

26. V. Krsjak et al, J. Nucl. Mater 456 (2015) 382-388
A good correla1on between PALS and CDBS results were observed in all samples
Optifer IX – STIP II

27. Large defects
Small defects
Lifetime spectra decomposition to 1 or 2
defect components.
(Two defect components of STIP II Optifer IX)

28. Saturated positron lifetime for an
empty vacancy cluster
He/V = 0.5
He/V = 1
He/V = 2
Troev et al. 2009
Large defects (lifetime ~ 450ps )
Since we have observed positron lifetimes above 400ps (large vacancy clusters) in all materials,
there must be only a small amount of helium in these defects. As we have not seen vacancy
clusters by TEM, they must not be much larger than ~ 30 vacancies
Saturated positron lifetimes for large vacancy
clusters with helium
(calculations of J. Kuriplach, 2014)
TEM visible

29. Small defects (lifetime ~ 200ps )
- Small defects (lifetime ~ 200ps):
Troev et al. 2009
- empty di-vacancy
- 6 vacancy + 2-3 He
- 12 vacancy + 12 He
Isolated He atom has an
extremely low migration energy;
He mobility drastically reduced in
the vicinity of defects
Most of He is expected to be
accommodated by defects
(at 6dpa ~ 450appm He)

30. Small defects (lifetime ~ 200ps )
- Small defects (lifetime ~ 200ps):
Troev et al. 2009
- empty di-vacancy
- 6 vacancy + 2-3 He
- 12 vacancy + 12 He
Isolated He atom has an
extremely low migration energy;
He mobility drastically reduced in
the vicinity of defects
Most of He is expected to be
accommodated by defects
(at 6dpa ~ 450appm He)

31. In a simplified model, where all helium is trapped in small VC (no “free” helium in the
matrix, negligible amount of helium in large coalesced bubbles) and where the small
VC are represented by an average cluster of 12 vacancies.

32. Empirical determination of positron trapping coefficient at nano-scale
“helium bubbles”
DD Nµκ =
TEM
1-1.5 nm
∼ 5-10×1023 m-3
PALS
∼ 1nm
∼ 5×109 s-1
First comparison of TEM and PALS data predicts the value of positron trapping
coefficient for small helium bubble in f/m steels to be 1.8 ± 1.1 x 10-14 m3s-1

35. The deriva1ve of the positron mean life1me with
respect to the temperature. As measured on various
f/m steels irradiated in STIP targets.
This satura1on is due to an enhanced coalescence of
helium bubbles, a decrease in their number density
and the related increase of positron mean free path
(i.e. decrease of positron trapping rate).

36. V. Krsjak, ICFRM—17, Aachen, Germany, 15th October 2015 36

37. V. Krsjak, ICFRM—17, Aachen, Germany, 15th October 2015
• More than 100 samples of various nuclear materials were
inves1gated by means of positron annihila1on
spectroscopy. At this 1me PSI has perhaps the largest
database of PAS data on irradiated f/m steels worldwide.
• The presence of helium was for the first 1me successfully
detected in the radia1on-induced vacancy clusters by
means of PAS.
• The correla1on of TEM and PALS data enabled the
positron trapping coefficient for nm-sized He-V clusters in
iron (steels) to be experimentally determined
• Experimental PALS data on the He-V ra1os in Fe-based
materials are in good agreement with theore1cal
modeling data
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38. V. Krsjak, ICFRM—17, Aachen, Germany, 15th October 2015
• A Positron life1me spectrometer and Doppler broadening
(gamma) spectrometer were built at PSI hotlab for
various experiments on nuclear and other materials.
• A new methodology of positron annihila1on spectroscopy
based on internal transmuta1on positron source was
developed for f/m steels irradiated in spalla1on neutron
targets. A similar approach is feasible for various
aluminum and 1tanium alloys as well as SiC/SiC materials
irradiated by high-energy protons.
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