1. CEA DSM Dapnia Antimatter in materials research: defect spectroscopy and study of porous systems using positrons Laszlo Liszkay DAPNIA/SACM/LEDA

2. CEA DSM Dapnia Outline condensed matter – positron interaction methods of positron annihilation spectroscopy positrons in materials research (defect spectroscopy, porosity) the SOPHI/SELMA project and its possible applications

3. CEA DSM Dapnia Positron-electron annihilation e+e+ from  + decay or pair production e-e- from a condensed matter positronium (Ps) e + -e - atom annihilation 2  photons (511 keV) 25 % paraPs (singlet) 125 ps lifetime 2  photons (511 keV) 75 % orthoPs (triplet) 142 ns lifetime 3  photons (0...511 keV)

4. CEA DSM Dapnia Conventional positron annihilation spectroscopy 0-540 keV e + ( 22 Na) slowing down ~ ps diffusion (E~kT) diff length L~100 nm “bulk” annihilation from Bloch state  b ~100..300 ps higher momentum “trapping” in a vacancy  v >  b lower momentum positronium (Ps) formation in voids 1-2 ns Bloch statemonovacancydivacancy Si  1.28 MeV 100  m

5. CEA DSM Dapnia positron moderator principle thin (~  m) W (Ni, Pt) foil (negative e + work function), solid Ne (Kr) 200 keV e + annihilation fast e + thermalization, diffusion slow (eV) e + Ps efficiency: 10 -4 (W) 10 -2 (Ne)

6. CEA DSM Dapnia Positron spectroscopy with “slow” (keV) positrons e+ ~1-10 keV diffusion to surface annih. in crystal (see before) e + emission (~ eV, negative work function) positronium (Ps) emission (oPs 142 ns, pPs 125 ps) thermal (3/2kT) or fast (few eV) surface state (450 ps) Makhovian profile Mean impl. range

7. CEA DSM Dapnia Detectables: lifetime e+e+ pulse or “start”  photon sample 511 keV annihilation  photon I i intensities – proportional with vacancy concentration  i lifetimes – characteristic value for each vacancy type typically 100-300 ps (bulk solid, vacancies) 1-2 ns (large voids, positronium) Schema of the pulsed positron beam in Munich

8. CEA DSM Dapnia Detectables: gamma energy distribution (Doppler spectroscopy) High purity Ge detector measurement of the Doppler broadening of the annihilation radiation due to the Doppler shift where p L is the longitudinal momentum component of the electron-positron pair proportional with electron momentum (e + thermalized) two lineshape parameters: S (low momentum) : valence electrons W (high momentum): core electrons  chemical information S-W plot: identification of the defect Sample e+e+ the 511 keV annihilation peak

9. CEA DSM Dapnia energy-dependent positron spectrum surface bulk implantation induced defects defect-free crystal defect profile

10. CEA DSM Dapnia defect spectroscopy with positrons sensitive to defects with free volume (vacancy, vacancy complex, voids) sensitivity up to 10 -7 (lifetime changes below 1 ps are reliably observed) open volume defects: important role in mechanical failure (metals), dopant compensation (compound semiconductors), radiation damage (reactor pressure vessel steels, implantation) non-destructive probe, in most cases does not require special sample treatment

11. CEA DSM Dapnia sensitivity range depth range: surface, ~10 nm – ~5  m sensitivity: defect dependent e.g. in silicon:

12. CEA DSM Dapnia Lifetime spectroscopy with positrons: identification of defects in semiconductors thin Mg doped GaN layers (2  m) (slow positron beam only) problem: electrical compensation of dopant (Mg) that limits p type doping shallow positron traps + vacancy defects (S parameter measurements) vacancies + vacancy clusters (lifetime measurements) identification of V N -Mg Ga complex with 180 ps characteristic lifetime(lifetime + Doppler coincidence measurement) 15 keV e + Doppler e + lifetime Doppler coincidence Hautakangas et al, Physical Reviews Letters 90, 137402 (2003)

13. CEA DSM Dapnia Defect spectroscopy using slow positrons: implantation-induced defects trapping in vacancies saturated trapping in vacancies trapping in larger defects (500 ps) 250 MeV Kr and 710 MeV Bi in sapphire (Al 2 O 3 ) homogeneous defect concentration in the positron range vacancies and larger defects can be identified

14. CEA DSM Dapnia positronium used in antimatter research: search for an efficient positron – slow orthopositronium converter positron + antiproton  antihydrogen more efficient to use positronium in the reaction + further reaction creates positive antihydrogen ion configuration: orthopositronium “cloud” as a target for antiproton beam p + Ps  H + e - H + Ps  H + + e - H + deceleration + cooling H + at μK

15. CEA DSM Dapnia Scheme of antihydrogen production (Patrice Perez, SPP) aim: maximize the effective orthopositronium density during the antiproton pulse e + beam Ps antiproton beam Ps e + /positronium converter 13 keV, 20 ns every 20 min oPs cloud from neutral e - - e + plasma trap 10 11 -10 12 e + in ~ 10 ns (transmission or reflection configuration)

16. CEA DSM Dapnia Study of nanoporous systems using positrons: detection of open porosity with orthopositronium time-of-flight e+e+ oPs self-organized porous SiO 2 system, 400  m layer potential use in filtration, sensor technology similar layers used as low-k dielectrics in semiconductor technology porosity detected by positron Doppler or lifetime method open porosity (permeability) detected by oPs TOF method TOF 3  annihilation

17. CEA DSM Dapnia Positron microcope: positron spectroscopy with pulsed microbeam Vacancy clusters close to a fatigue crack in Cu spot diameter: ~ 2  m Vacancies at a crack tip in GaAs David et al, Phys. Rev. Letters 87, 067402 (2001) Egger et al, Applied Surface Science 194, 214 (2002)

18. CEA DSM Dapnia III. Potential use of the SELMA/SOPHI system in materials research potential development of the SELMA/SOPHI system why do we need intense positron sources for positron spectroscopy? the positron source in international comparison

19. CEA DSM Dapnia Positron source project in Saclay: Linac-based intense positron source Linac (SELMA) e + /e - separation (SOPHI) e+ moderator W target electrons at 6 MeV 300 Hz 0.2 mA positrons 1 MeV 10 11 1/s positrons 3 eV 10 7- 10 9 1/s e + -positronium converter Trap(s) cold orthopositronium antiprotons 10 10 -10 12 e + pulse 10 ns Trap continuous beam Chopper/buncher/accelerator Target Lifetime spectrometer  Materials research electrons + positrons 100 ps pulses; 0-30 keV (200 keV?) energy Patrice Perez (SPP), Jean-Michel Rey Catherine Corbel (LSI) Aline Curtony, Olivier Delferrière...

20. CEA DSM Dapnia SOPHI geometry W target Dipole (e - -e + selection) ~ 0.2 T transport field Moderator (next phase) Coils expected performance: > 10 11 e + /s with 1 MeV peak energy 6 MeV electron beam (Linac)

21. CEA DSM Dapnia Why do we need intense positron sources? conventional 22 Na-based sources with up to 100 mCi (4x10 9 Bq) activity  10 6 -10 7 cps slow positron yield  ~ 1-2 energy-dependent positron measurement in a day  temperature-dependent, electric field-dependent measurement, annealing is very time consuming positron microscopy: losses during focussing  even longer measuring time / pixel  imaging with positrons is hardly feasible measurements requiring longer specimen-detector distance (e.g. at very high temperature or angular correlation) are very time consuming effects on shorter (s, min) timescale are not detectable with long measurements experiments with Bose-Einstein condensed positronium requires high positronium density antihydrogen production

22. CEA DSM Dapnia Performance of SOPHI/SELMA: comparison with other intense source projects conventional 22 Na source: max. 4x10 9 fast, ~10 6 -10 7 moderated e + Intense positron source projects in Europe: NEPOMUC (Garching near Munich, Germany) based on a 20 MW research reactor (FRM II, 8x10 14 cm -2 s -1 thermal neutron flux)  5x10 8 moderated e + /s (functional) EPOS (Rossendorf, near Dresden, Germany) based on a 40 MeV (0.4 mA avg. max. current) Linac 8x10 8 moderated e + /s expected (1/4 max Linac current) (not yet functional) POSH (Delft, The Netherlands) based on a research reactor about 7x10 7 moderated e + /s (2001), max. 4x10 8 e + /s (with fresh moderator) SELMA/SOPHI project in Saclay 10 11 fast (unmoderated) e + /s ( ~ 20 strong 22 Na source)  10 7 -10 9 moderated e + /s can be expected advantages: no permanent radioactivity, source can be switched off, easy to maintain (no neutron production) dedicated Linac, no maintenance needed, no shared resource  available nearly 100% of the time

23. CEA DSM Dapnia Summary SOPHI/SELMA project: possibility of a stable, reliable, independent positron source for materials research with competitive slow positron yield