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

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

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 ( keV)

CEA DSM Dapnia Conventional positron annihilation spectroscopy keV e + ( 22 Na) slowing down ~ ps diffusion (E~kT) diff length L~100 nm “bulk” annihilation from Bloch state  b ~ 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

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: (W) (Ne)

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

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 ps (bulk solid, vacancies) 1-2 ns (large voids, positronium) Schema of the pulsed positron beam in Munich

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

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

CEA DSM Dapnia defect spectroscopy with positrons sensitive to defects with free volume (vacancy, vacancy complex, voids) sensitivity up to (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

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

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, (2003)

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

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

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 e + in ~ 10 ns (transmission or reflection configuration)

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

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, (2001) Egger et al, Applied Surface Science 194, 214 (2002)

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

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 /s positrons 3 eV /s e + -positronium converter Trap(s) cold orthopositronium antiprotons 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...

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

CEA DSM Dapnia Why do we need intense positron sources? conventional 22 Na-based sources with up to 100 mCi (4x10 9 Bq) activity  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

CEA DSM Dapnia Performance of SOPHI/SELMA: comparison with other intense source projects conventional 22 Na source: max. 4x10 9 fast, ~ 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 fast (unmoderated) e + /s ( ~ 20 strong 22 Na source)  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

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