Positron-Annihilation Lifetime Spectroscopy at a Superconducting Electron Accelerator

Positron-Annihilation Lifetime Spectroscopy at a Superconducting Electron Accelerator Andreas Wagner Nuclear Physics Division Institute of Radiation Physics

The team Helmholtz-Zentrum Dresden-Rossendorf W. Anwand, M. Butterling #, T. E. Cowan*, J. Ehrler *, M.O. Liedke, K. Potzger, T.T. Trinh *, A.W. * also at Technische Universität Dresden # now at TU Delft Apply for beam time: next deadline end of October Martin-Luther-Univ. Halle-Wittenberg R. Krause-Rehberg, A. G. Attalah, M. Elsayed, M. John, M. Jungmann, A. Müller, I. Smirnov and many collaborators S.V. Stepanov, D.S.Zvezhinskiy (ITEP, MEPhI) H 2 O C. Hugenschmidt, M. Reiner (TU Munich) MnSi F. Selim (U Bowling Green) ZnO M. Kraatz, E. Zschech (Fraunhofer IKTS) low-k A. Uedono (Univ. Tsukuba) low-k R. Brusa, L. Ravelli (Univ. Trento) nano-si D. Enke, G. Dornberg (Univ. Leipzig) porous glasses Funding provided by BMBF (05K2013) and the Helmholtz Energy Materials Characterization Platform Page 2

Outline Motivation Courtesy: R. Krause-Rehberg / M. Butterling Accelerator-based positron production and annihilation studies at a superconducting electron LINAC: What marks the difference to reactors, radio-isotope sources, warm LINACs? Applying pulsed beams: positron annihilation lifetime spectroscopy for thin films, bulk materials, fluids, and gases MePS the Monoenergetic Positron Source GiPS the Gamma-induced Positron Source Some apps porous glasses w/ D. Enke Si nano-channels w/ R. Brusa low-k dielectrics w/ A. Uedono Page 3

Isotopes, reactors, accelerators Production of positrons in weak interactions (mediated by W s) neutron (u d d) ν e e + 27 Al(p,n) 27 Si(β + ν e, 4.2 s) 27 Al W + (u d u) proton Sumitomi Heavy Industries Cyclotron 18 MeV protons, 50 µa beam current Page 4

Isotopes, reactors, accelerators Production of positrons through electromagnetic interactions (photons) Use intense source of photons for pair production Capture-neutron gamma-rays from reactor 113 Cd(n,γ) 114 Cd e - e - e + FRMII Munich KUR Kyoto TU Delft Mc Master γ Bremsstrahlung from electron accelerators (Tsukuba, Dresden/Halle) or high power lasers (Changsha/Wuhan, Osaka/Hyogo, ELI-NP Bucuresti) e - AIST, Tsukuba, Japan ELBE, Dresden Page 5

Positrons from linear accelerators Accelerators can produce intense and pulsed slow positron beams. LINear ACcelerators have some advantage due to their high beam power and time structure. A) normal conducting LINAC (AIST) nelbe photoneutron E ~ 50 MeV source: LPb loop (450 C) I peak ~ 100 ma beam power can take 50 kw power t bunch ~ 1 µs f rep ~ 100 Hz 500 W B) superconducting LINAC (HZDR) E ~ 35 MeV I average ~ 100 µa beam power f rep ~ 1-10 MHz 3500 W in ~ 1cm 3 sophisticated converter designs and heavy shielding needed converter e - e + e + stack of 50 x 100 µm thick W foils Water-cooled electron to bremsstrahlung converter moderator Page 6

Positrons from accelerators sample sample ~ 2.8 GHz / 1 µs / 100 Hz / 10 µa NC-LINAC in bunched mode converter linear storage buncher A buncher B moderator chopper e - e + 1 µs 10 ev 3 ms 5 ns 2 ns 250 ps 38 ns / 26 MHz SC-LINAC in CW mode EPOS facility converter magnetic transport buncher e - moderator chopper Page 7 e + 10 ps 2 kev 2 ns 250 ps

Positrons from accelerators 1.6 ma, 40 MeV (64 kw) CW electron accelerator coherent IR-radiation 3 230 µm THz radiation 100 µm 3 mm Bremsstrahlung 16 MeV Gamma-induced Positrons electrons 34MeV radiation biology detector tests pulsed, mono-energetic positrons 0.2 20 kev SPONSOR: mono-energetic positrons 0.2 30 kev from 22 Na Page 8

The Mono-energetic Positron Source MePS Flux: 1.2 10 6 e + /s on sample max 500 V/cm max 2.5 kv/cm 4 ns chopper 78 MHz buncher sample 20 cm Pb + 3.2 m concrete shielding SC-LINAC beam 30 MeV, 0.1 ma 1.625 MHz repetition rate 615 ns spacing 2 kev magnetic transport system post-accelerator -1.5 +20 kv 30% HPGe detector for DBS BaF 2 detector for PALS Page 9

The Mono-energetic Positron Source MePS DBS (HPGe) PALS (BaF 2, plastic scint.) Page 10

MePS PALS timing resolution Si YSZ Lifetimes of Silicon (218 ps) and Yttria-stabilized Zirkonia can be reproduced, timing resolution is 230 ps with an additional (and stable) lifetime component of 720 ps. Signal to noise ratio: 5 10 5 Page 11

Experiments at MePS porous glasses D. Enke, G. Dornberg (U Leipzig): Porous glasses feature a tunable pore width and adjustable surface properties for membranes, chemo-sensors, drug delivery, optical coatings, etc. glass beads thin film thin film 20 µm Stimulated phase separation in sodium borosilicate glass into silica and an alkali borate phase generates sponge-like porous structures with unknown porosity. H. Uhlig, G. Adouane, C. Bluhm, S. Zieger, R. Krause-Rehberg, D. Enke, J. Porous Materials 23 (2016) 139 Page 12

Experiments at MePS porous glasses Annihilation lifetime experiments reveal depth dependent pore size distributions. 245 5.0 nm 2.6 nm 1.8 nm 1.3 nm 1.0 nm Pore sizes according to modified Tao-Eldrup model by K. Wada, T. Hyodo, JPhysG 443 (2013) 012003. Page 13

Experiments at MePS porous glasses Effect of tempering -> unexpected reduction of porosity. 5.0 nm 2.6 nm 1.8 nm 1.3 nm 1.0 nm Page 14

MePS Si channels R. Brusa, L. Ravelli (U. Trento) :Nano-channeled silicon produced by etching for Ps storage and release for the AEgIS antimatter experiment. Page 15 Courtesy: R. Brusa, Univ. Trento Tune the nano-channel size to optimize the influence of thermalization times (the smaller-the-better) and lowest Ps temperature (the bigger-the-better). Improve Ps output by capping with various materials for Ps reflection. -> Measure Ps lifetimes and yield depending on channel size and capping. R. Brusa, et al., PRL 104 (2010) 243401

MePS Si channels Doppler broadening and annihilation lifetime Capping the nano-channels with TiO 2 changes the S-parameter at the surface. Long lifetimes change showing showing Ps confinement within capped channels. Page 16

MePS Si channels 3γ / 2γ ratio shows the release of Ps into the vacuum and the effective action of the TiO 2 capping layer which reflects Ps back into the channels Region of integration from 4/5 m e (double-compton scattering edge) up to 0.99 m e. Page 17

MePS low-k SiO 2 Nano-porous organo-silicate glasses Depending on the specific treatment processes nano-porous structures can be stabilized or not. A. Uedono et al., Appl. Surf. Science 368 (2016) 272 Page 18

Future work in films In-situ AIDA Gas loading reactor 20 bar 77 900 K e-beam evap. Fe, Co, Ni, Au, Ag, Cr, Pt, Mo Duoplasmotron N, O, H < 30 kev 2 ma Climate chamber Raman spectrometer XPS analysis unit Motivation Hydrogen loading and deloading process, e.g. Hao et al, J. Phys. Chem. Lett. 1 (2010) 2968 Hydrogen storage capacity: decoration of defects, e.g. J. Cizek et al., Phys. Rev. B 79 (2009) 054108 Magnetic properties and open volume defects in Fe60Al40 M.O.Liedke, et al., J. Appl. Phys. 117 (2015) 163908 Page 19

Future work XPS analysis and PALS chamber UHV system at the 6 MV ion beam for H-depth profiling High-voltage cage on top of MePS Page 20

What about bulk materials, fluids, gases? sample sample 38 ns / 26 MHz SC-LINAC in CW mode converter magnetic transport buncher e - moderator chopper e + 10 ps 2 ns 100 ps radiator e - 10 ps γ e + e - 10 ps GiPS The Gamma-induced Positron annihilation Spectroscopy Page 21

Positron production using electronbremsstrahlung Positron beams for material research E I σ e e f t 16 MeV 900 µa 26 MHz 10 ps Annihilation Lifetime Spectroscopy (Coincidence) Doppler Broadening Age-momentum Correlation M. Butterling, et al., Nucl. Instr. Meth. B 269 (2011) 2623 photon beam 2 cm diameter 10 8 cm -2 s -1 Nb foil: 10-3 X 0 studies performed so far: - animal tissue - metals and alloys - (neutron-activated) reactor materials - water, glycerol from 10 C to 100 C Page 22

Positrons: backgnd for nuclear physics exp ts Kapton at 16 MeV electron energy Hard bremsstrahlung produces a huge amount of positrons via pair production inside the target material. High-energy photons act as a volume source of positrons throughout the entire volume. Seite 23

Physics with GiPS: ZnO Learn about either absence of cation vacancies and/or hydrogen passivation. Has already worked nicely for YAG Used chemical vapor assisted phase transport (CVPT) as described in J. Cizek s talk. Sample LT AMOC1 [ps] FWHM, [ps] as grown 176.0±0.2 160.2±0.3 O 2 anneal H 2 &O 2 anneal 174.9±0.1 166.8±0.2 176.1±0.2 159.7±0.3 no source correction necessary no difference in the annihilation lifetimes seen at all! Page 24

Gamma-induced Positron Spectroscopy ~ 1 µs / 100 Hz 38 ns / 26 MHz conventional LINAC mode pulsed RF, highest energy typically pile-up problems F.A. Selim, D.P. Wells, J.F. Harmon, et al. Nucl. Instr. Meth. A 495 (2002) 154 SC-LINAC in CW mode highest average power high yield and low pile-up High resolution lifetime spectrum with signal to noise ratios of better than 10 5 :1 using gamma-gamma coincidence techniques for background reduction. Lifetime spectra are free from artefacts. Long lifetimes reveal atomic defects caused by neutron-induced damage. Can (and how) defects be removed by thermal annealing? Page 25

Physics with GiPS: RPV steel Reactor vessel steel becomes brittle due to neutroninduced defects like open-volume defects. The atomic defects act as seeds for cracks. Collaboration with Reactor Safety Division. Preferential formation of double vacancies Thermal annealing (290 C) not sufficient to remove defects! Page 26

Physics with GiPS: Kapton Annihilation lifetime in Kapton has been under debate for quite some time - > get a measurement without source correction. Kapton applied cuts on Germanium and BaF 2 detector energy signal reduce background from interactions outside the sample consistent single positron lifetime of (381 ± 1) ps two components show larger χ 2 Page 27

Physics with GiPS: Fluids Conventional lifetime measurements: dissolve 22 Na and dispose it afterwards Positrons from bremsstrahlung homogeneously distributed, sharp time stamp Target is temperature-stabilized, continuously circulated, degassed, dry-nitrogen flushed. Kapton tube target fluid Positron Physics Ortho-Positronium (o-ps) in a fluid forms a bubble given by its zero-point energy and the surface tension. o-ps 142 ns e + e - We know estimate the change of the o-ps pick-off annihilation lifetime with temperature in a bubble created by the o-ps itself. p-ps R.A. Ferell, Phys. Rev., 108,167, 1957 S.J. Tao, J. Chem. Phys., 56,5499, 1972 M. Eldrup et al., Chem. Phys., 63,51, 1981 125 ps Page 28

Physics with GiPS: Fluids 2 U r E stationary Schrödinger eqn. 2mPs Rr Rr R0 j kr Ansatz: spherical Bessel fct. l sin kr j0kr 1 st non-trivial solution kr 2 2 2 h E0 zero-point energy 2 2 8mPsr0 4mer0 2 E 4 r fluid e + e - r 0 0 0 4 0 2 2 2m r r a surf E e E 3 0 0 0 8 r 0 2 4.3 A 16m e 4 0 2 m e surf 2 0 c m c 2 1.06 A Page 29

Physics with GiPS: Positron(-ium) Chemistry Experiments with water are in variance with a simple bubble-type model. Extension: chemical reactions between radiolysis products of the slowing-down of the positron Ps chemistry. Courtesy: Maik Butterling, S V.Stepanov Page 30 Radicals are positron scavengers which reduce annihilation lifetimes. Extended bubble model including chemistry [S.V. Stepanov et al., Mat. Sci. Forum 607] and energetics of hydrated e + aq and e - aq (what are the relaxation times of un-/polar media?) Chemistry of radiolysis directly accessible since the probe creates the ionization itself Talk on Wednesday by S. Stepanov