POSITRON AND POSITRONIUM INTERACTIONS WITH CONDENSED MATTER. Paul Coleman University of Bath

Transcription

1 POSITRON AND POSITRONIUM INTERACTIONS WITH CONDENSED MATTER Paul Coleman University of Bath

2 THE FATE OF POSITRONS IN CONDENSED MATTER POSITRON-SURFACE INTERACTIONS positron backscattering BACKSCATTERED FRACTION BACKSCATTERED FRACTION E = 30 kev ATOMIC NUMBER GOLD INCIDENT POSITRON ENERGY (kev)

3 THE FATE OF POSITRONS IN CONDENSED MATTER POSITRON-SURFACE INTERACTIONS secondary electron emission COPPER dn/de (arbitrary scale) 2 kev ELECTRONS IN 2 kev POSITRONS IN ELECTRON ENERGY (ev)

4 THE FATE OF POSITRONS IN CONDENSED MATTER POSITRON-SURFACE INTERACTIONS low-energy positron diffraction θ 2 θ 1

5 THE FATE OF POSITRONS IN CONDENSED MATTER POSITRON-SURFACE INTERACTIONS Auger electron emission

6 THE FATE OF POSITRONS IN CONDENSED MATTER POSITRON-SURFACE INTERACTIONS positron surface trapping, re-emission, positronium formation thermal or epithermal positrons COUNTS (arb. units) eV 200eV 500eV 3000eV ENERGY (ev)

7 THE FATE OF POSITRONS IN CONDENSED MATTER POSITRON-SOLID INTERACTIONS positron implantation depth distributions at thermalisation (after ~ps): mono-energetic e + 2(z/z 02 )exp(-z 2 /z 02 ) β + α exp(-αz) P(z) = 2(z/z 02 )exp(-z 2 /z 02 ) where z o = (40/ρ)E 1.6 with density ρ in gcm -3 and E in kev P(z) = α exp(-αz), with α (cm) 16ρE m -1.4 (ρ = density in gcm -3, E m = maximum beta positron energy in kev)

8 THE FATE OF POSITRONS IN CONDENSED MATTER POSITRON-SOLID INTERACTIONS in-flight annihilation 2.65MeV e + s in Au

9 THE FATE OF POSITRONS IN CONDENSED MATTER POSITRON-SOLID INTERACTIONS positron diffusion and annihilation free annihilation defect trapping

10 THE FATE OF POSITRONS IN CONDENSED MATTER POSITRON-SOLID INTERACTIONS positronium formation insulators, nanoporous materials Ps (3γ) 2γ ** see later talks in this session **

11 RADIOACTIVE ISOTOPES eg 22 Na POSITRON SOURCES Relatively long half-life (2.6 years) Relatively short biological half-life (3 days) MeV γ can be used as a start signal The perennial problem not enough positrons

12 RADIOACTIVE ISOTOPES eg 22 Na POSITRON SOURCES Relatively long half-life (2.6 years) Relatively short biological half-life (3 days) MeV γ can be used as a start signal LINEAR ACCELERATORS eg KEK, Japan ~ 10 2 MeV electrons bremstrahhlung pair production RESEARCH REACTORS eg NEPOMUC, Munich n capture gamma emission pair production

13 POSITRON SPECTROSCOPIES ρ(p) = probability that a pair of annihilation gamma rays has total (ie ~ e - ) momentum p ρ(p x,p y ) = ρ(p) dp z 2-D ANGULAR CORRELATION ρ(p z ) = ρ(p) dp x dp y DOPPLER BROADENING SPECTROSCOPY λ = ρ(p) dpx dpy dpz LIFETIME SPECTROSCOPY

14 ANGULAR CORRELATION OF ANNIHILATION RADIATION γ 2 γ 2 θ 0 θ 2 θ 0 θ 1 γ 1 γ 1 C of M FRAME LAB FRAME δθ p t /mc (~ 5-20 mrad) where p t is the component of momentum of the annihilating pair in a direction transverse to the gamma emission electron momentum densities Bristol 2D-ACAR lab e + 2D-ACAR quartz 2D Fermi surface

15 POSITRON LIFETIME SPECTROSCOPY I( t) = I i exp( λ t) i i γ electron densities e + positronium (Ps): e + -e - bound state -formed in insulators - para-ps lives 125 ps in vacuum - ortho-ps lives 142 ns in vacuum - typical Ps lifetime in solids ~ ns

16 POSITRON LIFETIME MEASUREMENT POSITRONS POSITRONIUM Metals & alloys Semiconductors Polymers Mesoporous materials LIFETIME (ns)

17 DOPPLER BROADENING SPECTROSCOPY Doppler shift in the photon energy = ± cp z /2 If p z is from a 5eV electron cp z /2 = 1130 ev (~ the resolution of a high-purity Ge detector) e + Doppler broadening around 511keV is measured Good for rapid, low-resolution measurements of e - momenta vs time, temperature etc. Ge PRE-AMP AMPLIFIER BIASED AMP LN2 CRYO- STAT DIGITAL STABILISER ADC MCA MEMORY

18 DOPPLER BROADENING SPECTROSCOPY PARAMETERS: Sharpness parameter S = C/A Wing parameter W = (W 1 +W 2 )/A where A = total area under photopeak COUNT RATE (ARB UNITS) C W 1 W GAMMA ENERGY (kev)

19 WHAT CAN BE STUDIED? OPEN-VOLUME POINT DEFECTS VACANCIES, VACANCY CLUSTERS, DISLOCATIONS, GRAIN BOUNDARIES, INNER/OUTER SURFACES sensitive to ~ 1 vacancy per 10 7 atoms TRANSITION LIMITED TRAPPING: High C, deep but narrow traps: K = νc DIFFUSION LIMITED TRAPPING: Low C, large traps: K = 4π r DC NEUTRAL VACANCY: no detrapping, τ > τ bulk, less Doppler broadening: no temperature dependence NEGATIVE VACANCY: no detrapping, τ > τ bulk, less Doppler broadening: stronger trap at lower temperatures SHALLOW TRAP (eg NEGATIVE ION): detrapping below 300K, τ ~ τ bulk, little Doppler broadening

20 SENSITIVITY TO OPEN-VOLUME POINT DEFECTS DEFECT SIZE DEFECT CONCENTRATION RESOLVED DEFECT SIZE (Angstrom) 10 8 STM/AFM OM TEM X-RAY SCATTERING POSITRON SPECTROSCOPY ns DEPTH (Angstrom) DEFECT CONCENTRATION (appm) 10 5 STM/AFM OM TEM X-RAY SCATTERING POSITRON SPECTROSCOPY ns DEPTH (Angstrom)

21 OPEN-VOLUME POINT DEFECTS: EXTRACTING CONCENTRATIONS τ 1 τ 2 τ 3 COUNT RATE S GAMMA ENERGY (kev) κ N( t) = I λ exp( λ t) d i i i - for two states - bulk or defect: Id = µ νcd = (λb - I b i λ d ) S = i f i S i for two states - bulk or defect: f D = S S D - S - S B B = νc νc + λ B ν is the specific trapping rate

22 CONCENTRATION OPEN-VOLUME POINT DEFECTS SIZE ν is the specific trapping rate ν depends on charge state of the defect τ and S are related to open volume of defect S/S Si V V VACANCY CONCENTRATION (cm -3 ) V + S Parameter for Vn Si Vacancy cluster size

23 CHEMICAL SPECIFICITY - ANNIHILATION LINESHAPE ANALYSIS 1.8 COUNTS SPECTRUM RATIO GAMMA ENERGY (kev) annihilation lines for Si Ge Si 0.7 Ge 0.3 Si 0.9 Ge 0.1 Si 0.98 Ge GAMMA ENERGY (kev) spectrum ratios: Ge/Si Si 0.7 Ge 0.3 /Si = 30% of Ge/Si Si 0.9 Ge 0.1 /Si = 10% of Ge/Si Si 0.98 Ge 0.02 /Si = 2% of Ge/Si information on chemical environment and affinity

24 POSITRON MODERATION NEGATIVE WORK FUNCTION BETA SPECTRUM (0 540 kev, PEAK 180 kev) RE-EMITTED POSITRONS (0 3eV, PEAK 1.5eV) NUMBER OF POSITRONS PER ev BETA POSITRONS FROM 22 Na 3eV POSITRONS FROM MODERATOR POSITRON ENERGY (kev) (LOG SCALE) EMISSION FROM TUNGSTEN alternatives include moderation by solid rare gases

25 LAB-BASED POSITRON BEAMS Magnetic-transport positron beam system Implantation Profiles P(E,z) E=0.5 kev E=1.0 kev E=1.5 kev E=2.0 kev Si Depth (z) / nm

26 LAB-BASED POSITRON BEAMS Magnetic-transport positron beam system 1.05 FZ Si implanted with 2MeV Si + ions at 10 n cm -2 (n = 0, 11, 12, 13, 14, 15) NORMALIZED S PARAMETER virgin INCIDENT POSITRON ENERGY (kev) depth sensitivity also: TRAP-BASED BEAMS PULSED BEAMS

27 EXAMPLES OF POSITRON BEAM STUDIES OF SOLIDS 1. SILICON NANOCRYSTALS in SiO 2 MATRIX 1.02 Si-rich SiO 2 ANNEALED AT 1100ºC 1.00 INTERFACE TRAPPING S PARAMETER s 5s 10s 100s 1h as-imp INCIDENT POSITRON ENERGY (kev) POSITRON RESPONSE

28 EXAMPLES OF POSITRON BEAM STUDIES OF SOLIDS 1. SILICON NANOCRYSTALS in SiO 2 MATRIX ERBIUM and nsi ILLUMINATION BY BLUE LEDs 3-5nm clusters Er and Si EELS responses coincident TEM Si Er 1.02 S PARAMETER WITH Er NO Er INCIDENT POSITRON ENERGY (kev) Er in interfaces MINIMUM S PARAMETER (AT 3 kev) TOTAL LED POWER (W)

29 EXAMPLES OF POSITRON BEAM STUDIES OF SOLIDS 2. NANOPORES in ICE CLOSED nm PORES: Ps DECAY TO 3γ and Ps LIFETIME DEPEND ON PORE SIZE Ps (3γ) 2γ INTERCONNECTED PORES: ESCAPE POSSIBLE, 3γ FRACTION INCREASED, VACUUM LIFETIME Ps FRACTION (ARB UNITS) Ps FRACTION grown at 120k 10-4torr 20min grown at 120k 10-7torr Ps FRACTION at 120K at 150K at 170K T (K) INCIDENT POSITRON ENERGY (kev) INCIDENT POSITRON ENERGY (kev) PORE COLLAPSE DEPENDENCE ON GROWTH RATE DURING SUBLIMATION see poster for more information

30 POSITRON BEAMS: APPLICATIONS IN ASTROPHYSICS DUST in the ISM backscattered free/ trapped e + Ps o-ps N. Guessoum, P. Jean & W. Gillard Applied Surface Science 252 (2006) 3352

31 POSITRON BEAMS: APPLICATIONS IN ASTROPHYSICS DUST in the ISM DOPPLER BROADENING CHEMICAL ANALYSIS DOPPLER BROADENING STRUCTURAL ANALYSIS Ps FORMATION AND DECAY PORES AND STRUCTURAL ANALYSIS IN-FLIGHT ANNIHILATION ON MODEL SYSTEMS

32 POSITRON BEAMS: APPLICATIONS IN ASTROPHYSICS

33 Thanks to past and present members of the Bath positron group - and to you for your attention