POSITRON AND POSITRONIUM INTERACTIONS WITH CONDENSED MATTER Paul Coleman University of Bath
THE FATE OF POSITRONS IN CONDENSED MATTER POSITRON-SURFACE INTERACTIONS positron backscattering BACKSCATTERED FRACTION BACKSCATTERED FRACTION 40 35 30 25 E = 30 kev 20 15 10 5 0 0 20 40 60 80 ATOMIC NUMBER 40 35 30 GOLD 25 20 15 10 5 0 0 10 20 30 40 50 INCIDENT POSITRON ENERGY (kev)
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 100 200 300 400 500 600 ELECTRON ENERGY (ev)
THE FATE OF POSITRONS IN CONDENSED MATTER POSITRON-SURFACE INTERACTIONS low-energy positron diffraction θ 2 θ 1
THE FATE OF POSITRONS IN CONDENSED MATTER POSITRON-SURFACE INTERACTIONS Auger electron emission
THE FATE OF POSITRONS IN CONDENSED MATTER POSITRON-SURFACE INTERACTIONS positron surface trapping, re-emission, positronium formation thermal or epithermal positrons COUNTS (arb. units) 1.0 0.5 0.0 100eV 200eV 500eV 3000eV 0 5 10 ENERGY (ev)
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)
THE FATE OF POSITRONS IN CONDENSED MATTER POSITRON-SOLID INTERACTIONS in-flight annihilation 2.65MeV e + s in Au
THE FATE OF POSITRONS IN CONDENSED MATTER POSITRON-SOLID INTERACTIONS positron diffusion and annihilation free annihilation defect trapping
THE FATE OF POSITRONS IN CONDENSED MATTER POSITRON-SOLID INTERACTIONS positronium formation insulators, nanoporous materials Ps (3γ) 2γ ** see later talks in this session **
RADIOACTIVE ISOTOPES eg 22 Na POSITRON SOURCES Relatively long half-life (2.6 years) Relatively short biological half-life (3 days) 1.274 MeV γ can be used as a start signal The perennial problem not enough positrons
RADIOACTIVE ISOTOPES eg 22 Na POSITRON SOURCES Relatively long half-life (2.6 years) Relatively short biological half-life (3 days) 1.274 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
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
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
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
POSITRON LIFETIME MEASUREMENT POSITRONS POSITRONIUM Metals & alloys Semiconductors Polymers Mesoporous materials 0.1 1 10 100 LIFETIME (ns)
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
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 2 504 506 508 510 512 514 516 518 GAMMA ENERGY (kev)
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
SENSITIVITY TO OPEN-VOLUME POINT DEFECTS DEFECT SIZE DEFECT CONCENTRATION RESOLVED DEFECT SIZE (Angstrom) 10 8 STM/AFM 10 7 10 6 10 5 10 4 10 3 10 2 10 1 10 0 OM TEM X-RAY SCATTERING POSITRON SPECTROSCOPY ns 10 0 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 8 DEPTH (Angstrom) DEFECT CONCENTRATION (appm) 10 5 STM/AFM 10 4 10 3 10 2 10 1 10 0 10-1 OM TEM X-RAY SCATTERING POSITRON SPECTROSCOPY ns 10 0 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 8 DEPTH (Angstrom)
OPEN-VOLUME POINT DEFECTS: EXTRACTING CONCENTRATIONS τ 1 τ 2 τ 3 COUNT RATE S 504 506 508 510 512 514 516 518 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
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 1.04 1.03 1.02 1.01 1.00 V 2-10 14 10 15 10 16 10 17 10 18 10 19 10 20 V VACANCY CONCENTRATION (cm -3 ) V + S Parameter for Vn 1.14 1.12 1.10 1.08 1.06 1.04 1.02 1.00 Si 0 5 10 15 20 Vacancy cluster size
CHEMICAL SPECIFICITY - ANNIHILATION LINESHAPE ANALYSIS 1.8 COUNTS SPECTRUM RATIO 1.6 1.4 1.2 1.0 506 508 510 512 514 516 GAMMA ENERGY (kev) annihilation lines for Si Ge Si 0.7 Ge 0.3 Si 0.9 Ge 0.1 Si 0.98 Ge 0.02 0.8 512 514 516 518 520 522 524 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
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 0.0001 0.001 0.01 0.1 1 10 100 1000 POSITRON ENERGY (kev) (LOG SCALE) EMISSION FROM TUNGSTEN alternatives include moderation by solid rare gases
LAB-BASED POSITRON BEAMS Magnetic-transport positron beam system Implantation Profiles P(E,z) 0.15 0.12 0.09 0.06 0.03 0.00 E=0.5 kev E=1.0 kev E=1.5 kev E=2.0 kev Si 0 20 40 60 80 100 120 140 Depth (z) / nm
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 1.04 1.03 1.02 1.01 1.00 0.99 0.98 0.97 11 12 13 14 15 virgin 0.96 0 5 10 15 20 25 30 INCIDENT POSITRON ENERGY (kev) depth sensitivity also: TRAP-BASED BEAMS PULSED BEAMS
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 0.98 0.96 0.94 1s 5s 10s 100s 1h as-imp 0.92 5 10 15 20 25 30 INCIDENT POSITRON ENERGY (kev) POSITRON RESPONSE
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 1.00 0.98 0.96 0.94 0.92 WITH Er NO Er 5 10 15 20 25 30 INCIDENT POSITRON ENERGY (kev) Er in interfaces MINIMUM S PARAMETER (AT 3 kev) 0.961 0.960 0.959 0.958 0.957 0.956 0.955 0 5 10 15 20 25 30 35 40 45 50 TOTAL LED POWER (W)
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) 0.5 0.4 0.3 0.2 0.1 Ps FRACTION 0.35 0.30 0.25 0.20 0.15 0.10 0.05 grown at 120k 10-4torr 20min grown at 120k 10-7torr Ps FRACTION 0.8 0.6 0.4 0.2 at 120K at 150K at 170K 0.0 40 60 80 100 120 140 160 T (K) 0.00 0 2 4 6 8 10 INCIDENT POSITRON ENERGY (kev) 0.0 0 2 4 6 8 10 INCIDENT POSITRON ENERGY (kev) PORE COLLAPSE DEPENDENCE ON GROWTH RATE DURING SUBLIMATION see poster for more information
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
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
POSITRON BEAMS: APPLICATIONS IN ASTROPHYSICS
Thanks to past and present members of the Bath positron group - and to you for your attention