at Slow Positron Facility of Institute of Materials Structure - PowerPoint PPT Presentation

Slow positron applications at Slow Positron Facility of Institute of Materials Structure Science, KEK T. Hyodo 1 , I. Mochizuki 1 , N. Toge 2 , T. Shidara 2 1 Institute of Materials Structure Science, KEK, Tsukuba, 305-0801, Japan 2 Accelerator Laboratory, KEK, Tsukuba, 305-0801, Japan

Outline • Colaborators • Overview of Slow Positron facility • Creation of slow positron • Brightness enhancement • TRHEPD • LEPD • Puls-stretching • Ps - (Ps negative ion) • Ps-TOF

Collaborators • KEK: (IMSS) T. Kosuge, A. Yagishita, A. Ichimiya; (Accelerator) S. Ohsawa, M. Ikeda, A. Shirakawa, K. Furukawa, H. Honma; (Radiation Control) H. Iwase, T. Sanami • JAEA: Y. Fukaya (TRHEPD) • QST: K. Wada, M. Maekawa, A. Kawasuso (TRHEPD, LEPD) • Chiba Univ.: M. Fujinami (LEPD) • Tokyo Univ. of Sci.: Y. Nagashima (Ps - , Ps-TOF), T. Tachibana (Ps - ) • AIST: T. Shirasawa (LEPD), K. Michishio (Ps - ) • Riken: S. Kuma, T. Azuma (Ps - )

Hi High gh Energy gy Ac Accelerator or Rese search O Orga ganiza zation on （ KE KEK ） Tsuku Ts kuba Campus electron-postiron collider ： Super KEKB （ diamter ： circ. １ km 、 electron 7GeV positron 4GeV ） PF ｰ AR （ diameter: circ.120m,electron 6.5GeV ） Main gate Electron-positron linac PF （ diameter: circ.120m, （ circ.400m, electron 7GeV, positron 4GeV ） electron .5GeV ） Photon Factory Electron-positron injector bldg. （ Slow Positron Facility, Linav 5m, electron 50MeV)

Slow Positron Facility, KEK High Intensity 5x10 7 slow e + /s (long pulse mode) variable transport energy （ 0.1-35keV ） Compatible with grounded chamber and sample → high generality Standardized Branching-unit High freedom and expandability Slow-positron production unit SPF-B1 (0.1 - 35 kV) (Ps - ) e + SPF-B2 Gnd. fl. (Ps-TOF) e - Linac SPF-A2 55MeV 600W 50Hz Long pulse mode 1 𝜈 s width, 5x10 7 slow e + /s SPF-A3 SPF-A1 1x10 6 slow e + /s after (TRHEPD) brightness enhancement SPF-A4 Short pulse mode (LEPD) B1 fl. 1-10ns width, 5x10 6 slow e + /s (Stations are not shown.) K. Wada, et al., Eur. Phys. J. D 66, 37 (2012). K. Wada, et al., J. Phys.: Conf. Ser. 443, 012082 (2013).

Plan view of Slow Positron Facility, KEK Bremsstrahlung Electron-positron W foil pair creation TRHEPD 5m transmission- High High-brightness e - energy e - pattern type W moderator positron beam remoderator (50 MeV) (10keV) Electron Linac e + Accelerator Energy-tunable Sample Ta nucleus 4mm MCP + slow positron (15kV) 10kV Screen beam Ta converter (Example of TRHEPD experiment)

Outline • Colaborators • Overview of Slow positron facility • Creation of slow positron • Brightness enhancement • TRHEPD • LEPD • Puls-stretching • Ps - (Ps negative ion) • Ps-TOF

Preparation of monoenergetic slow positrons Positrons do not annihilate with Annihilation an electron quickly. after trapping at a vacancy β ＋ ray or Positrons from pair creation Annihilation from a free Re-emission state after thermalization Good quality Particular metals slow positron beam with negative positron work function Emission energy : absolute value of the W (-3eV) work function Cu (-2eV) Energy width : order of thermal energy Ni (-1eV)

Converter/moderator for slow-positorn production W moderator grid Wehnelt electrode Extraction grid Moderator 2 Converter Moderator 1 Ta converter

Available 4 Stations at Slow Positron Facility, IMSS, KEK (1) Ps negative ion  (Photodetachment) Slow Positron Facility  Energy-tunable Ps beam 55 MeV Dedicated Linac 600 W e- High intensity Electron- Positron Converter(Ta) 10ns pulse Pair creation e+ e- Moderator(W) Slow e+ g (2) Ps time-of-flight (Ps-TOF) Ps － e- Brightness Ps enhancement High intensity e + 10ns pulse Sample (3) (4) Positiron diffraction High intensity High Brightness (3) TRHEPD (4) LEPD  Running | Developing  10

Outline • Colaborators • Overview of Slow positron facility • Creation of slow positron • Brightness enhancement • TRHEPD • LEPD • Puls-stretching • Ps - (Ps negative ion) • Ps-TOF

Brightness enhancemet by reemission of positrons from negative-work-function surface after thermalization I I : Beam intensity, r : Beam radius B  Brightness 2  E : Beam Energy, θ ： Beam divergence 2 Ed Focusing on a remoderator foil. With linac based intense slow positron beam: Let dissipative force (thermalization) Sample orientation by monitoring a TRHEPD break the Leuvile’s theorem. pattern is now possible. Transmission-type remoderator 1hr for a good TRHEPD pattern (thin metal foil with negative 3hrs for a 00-spot rocking curve  1 min for a TRHEPD pattern for positron work function) the rocking curve for an orientation (100 nm W foil) B → B × 10 3 I → I /10 r → r E = 5keV → 3eV Brightness enhancement and TRHEPD chambers at KEK M. Maekawa, K. Wada, et al. , Eur. Phys. J. D 68, 165 (2014) . θ = ~ 50° → ~ 10 °

Outline • Colaborators • Overview of Slow positron facility • Creation of slow positron • Brightness enhancement • TRHEPD • LEPD • Puls-stretching • Ps - (Ps negative ion) • Ps-TOF

Atomic structure analysis by diffraction X-ray diffraction Protein analisys using pattern using a computer synchrotron radiation Positron diffraction pattern usingTRHEP Microscope (imaging method) Diffraction data observes these directly analisys using a computer SSearch for an appropriate structure （ Trial 6 error ） Si(111) (7 × 7) Rocking curve

Status of 3D and 2D structure analysis Characteristics of materials It is widely practiced to  Atomic structure (kinds of atoms and use the methods to their detailed arrangements) recognize the Structure determination independent currigation of a surface from characterization is important. or the crystal symmetry of the surface. 3D materials (crystal of new material, proteins, etc. ) However, precise X-ray diffraction using synchrotron determination of the radiation is the standard method. positions of the atoms is difficult. 2D materials and surfaces No standard method exists. （ Basis and STM, AFM, SXRD, LEED, RHEED accomplishments) Positron Diffraction （ TRHEPD in particular ） is emerging to be a standard techinique. But sufficient intensity of the beam is required, just as the case of X-rays  Use of accelerator for positron production resolves this difficulty.

Origin of the surface sensitivity of electron diffractions LEED (low-energy electron diffraction) Because of low energy No 2𝑒 sin𝜄 = 𝑜𝜇 (Bragg condition ) ex.:Thermal neutron satisfying the condition is not surface sensitive. 10 keV RHEED (reflection high-energy electron diffraction) Because of the grazing angle incidence No sin𝜄 RHEED ∼ sin𝜄 LEED /10 → 𝜇 RHEED ∼ 𝜇 LEED /10 → 𝐹 RHEED ∼ 100𝐹 LEED Inelastic scattering is the origin of the surface sensitivity, common to the electron and the positron diffraction. Just as in Auger electron spectroscopy and photoelectron spectroscopy In addition, an origin unique to positron diffraction exists.

Electrostatic field and electrostatic potential around an atom Electrosotatic potential Electric field around a model atom around a model atom -------- potential due to nucleus -------- field due to nucleus -------- potential due to electrons -------- field due to electrons -------- total potential -------- total field q/r q/r 2 -q/r 2 -q/r Electrostatic potential In every solid

Crystal potential and total reflection of positron 𝐹 o⊥ 𝐹 i⊥ 𝐹 o⊥ > eV e + e V 𝐹 o⊥ < eV e + e - E o⊥ > 0 > − eV 𝐹 i⊥ 𝐹 o⊥ − eV

Glancing angle dependence of the paths of positron and electron and their surface sensitivity Origin of the surface sensitivity common for electron and positron -  inelastic scattering Origin of the surface sensitivity characteristic to positron -  total reflection refraction toward the surface TRHEPD ： θ < 6 ° toral rfl. ： θ c = 2 ゜ - 3 ゜ Positron in the only quantum mechanical particle for which angular range for the total-reflection and the Bragg-diffraction overlap. Data usually include those not satisfying the total reflection condition TRHEPD is the name of the method.

THREPD is an ideal method for topmost surface and immedeate subsurface Penetration depths of e + and e - Positron: TRHEPD 0 0 surface Electron: RHEED e + Penetration depth (Å) 1 Pushed 5 e + atomic layer (unit: Bilayer) Total reflection 2 up 10 3 e - e - θ c 4 surface 15 5 6 20 7 25 E = 10 keV 8 eV 0 = +12 eV ( Si crystal ) 9 Pulled bulk down 30 0 1 2 3 4 Glancing angle: θ ( ° ) Features of TRHEPD 1. Positrons undergo pure or ideal total reflection. The critical angle for total reflection θ C (2 °- 3 ° ) lies in the middle of the 2. TRHEPD measurement region (  unique property of the positron). θ in < θ C : positrons are totally reflected and see the topmost surface only. 3. θ in > θ C : positrons also see the immediate subsurface. 4. Width of interest from the surface is adjustable with varying θ in . 5. 6. No background from the deeper, bulk part at all.