Development status of the positron lifetime beam MePS and the first lifetime measurements of porous ultra-low-k dielectrics with MePS Institut für Physik, Martin-Luther-Universität Halle-Wittenberg
Table of Contents 1. Positron lifetime beam system MePS 2. Simulation with Simion 8.1 3. Measurement of low-k-dielectric films 4. Outlook
Overview MePS
Electron beam parameters repetition frequency 26/2 n MHz pulse width 5 ps 40 kw, 1 ma cw for materials with short positron lifetime metals, semiconductors for materials with long positron lifetimes porous materials
Labor and cave 111
Beamline cave
Beam adjustment electron beam from the cave to the lab over 20 parameters not really successful electron beam from the lab to the cave needs a full magnetic guidance field camera inside the beamline adjusting the field in 20 cm steps successful
Counts Platinum moderator 0 800 600 400-10 0 10 20 30 40 50 60 70 Time [min] Platinum moderator with a thickness of 125 µm fast electrons induce defects moderator annealing during operation not successful count rate decreasing from 800 to 150 counts/s during operation with 9 µa Beam current the count rate decreasing after power-on of the electron beam if the electron beam is power-off the moderator is rebounding
Tungsten moderator tungsten moderator with a thickness of µm and 8 meshes of 20 µm thickness etched, flashed 8000 counts/s with 38µA Beam current 20 times higher than platinum count rate not decreasing
Einzellens einzellens with mesh 92% open area thickness of the lens is 50 µm upper part
Counts/s Counts First spectra What is the reason for these spectra? 0 10 10 1 0 5 10 15 20 1 0 10 20 30 40 50 60 70 80
Counts Counts Counts Disturbance of the time structure 10 Moderator position -1300 mm D 1 0 10 20 30 40 50 60 70 80 with magnetic field C 10 10 1 0 10 20 30 40 50 60 70 80 with lense focus 1 mm 1 0 10 20 30 40 50 60 70 80 with lense focus 290 mm
counts/sec Counts [1/s] Counts [1/s] Test with retarding field analyzer 400 removing fast positrons by the use of a double aperture 350 300 250 200 600 500 without aperture with aperture 150 50 400 300 0 1980 2010 2040 2070 2 200 0 0 0 2000 3000 4000 Voltage [V] 160 140 120 80 60 n2300v p0v 40 the lens has no influence on the positron energy 20 0 1950 1980 2010 2040 2070 2 Voltage [V]
Counts counts/s Time structure by using retarding electric field signals above 2070 ev has no influence on the time structure electron beam current has also no influence on the time structure 0 VG0 VG1995 VG2025 VG2040 VG2070 250 8 µa 5.8 µa 200 3 µa 1 µa 150 50 10 5 10 15 20 25 30 0 1925 1950 1975 2000 2025 2050 2075 2 U [V]
Counts/s Time structure with buncher 00 FWHM 650 ps decreasing resolution time from 2.5 ns to 0.65 ns 0 0 10 20 30 40 50 60 70 80
Bend tube against reflected positrons in spectra in other positron beam systems E x B filter we use a bend tube easy to adjust one coil energy range of reflected positrons unimportant
Counts counts Backscattered positrons accelerator on, without bent tube accelerator on, with bent tube 00 copper 0 00 0 copper 10 10 1-200 -150 - -50 0 50 150 1 0 200 300 400
Summary of MePS count rate is sufficient expected count rate of 52 000 counts per second with 10 kw beam power no disturbance from reflected positrons measurement of long positron lifetimes are possible with 650 ps resolution time improving the time resolution is necessary
Time-dependent Simulation with Simion 8.1 innovations in Simion 8.1 better working surface works with 64 bit simple to import electrodes as stl-files from CAD program simulate coils lua programming language time-dependent simulation
Time-dependent Simulation with Simion 8 1.19 ns time of flight through the chopper plates the pulse is not rectangular field outside of the chopper has a influence of the positrons
Field Field Time-dependent Simulation with Simion 8 electric field from chopper electric field from buncher 2 1 dedx dedy dedz 0,0-0,2 0-1 -0,4-0,6-0,8 dedx dedy dedz -2 7950 8000 8050 8 8150 8200 Position z [mm] -1,0 8500 8510 8520 8530 8540 Position y [mm]
Counts Counts/s Time-dependent Simulation with Simion 8 simulation: structuring background by the use of a buncher measured spectra 00 FWHM 650 ps 10 1 0 0.0 76.9 153.8 230.7 307.6 384.5 461.4 538.3 615.2 0 10 20 30 40 50 60 70 80
Counts Counts Counts Chopper and Buncher 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0 8000 mm Gauss Chopper V 3.14 ns FWHM -4-3 -2-1 0 1 2 3 1400 1200 0 800 600 400 200-1,0-0,8-0,6-0,4-0,2 0,0 1/2 Gauss Chopper V 1.54 ns FWHM 33 ps FWHM 60 50 15 ps FWHM 40 30 20 1/12 10 0-1,0-0,8-0,6-0,4-0,2 0,0
Counts Counts Assumption: positrons starts with an energy spread of 25 ev FWHM Two buncher without chopper 250 200 200 150 150 50 Buncher 1 200V first slit after 0 mm 50 0-0,1 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 Time[ns] Buncher 300V FWHM 32 ps 0 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 Buncher V FWHM 20 ps
Summary of the simulation we can only use the retarding field measurement as initial conditions simulation gives good information about the time structure two buncher work as well as a chopper and a buncher no background of slow positrons
Production of low-k films 800 nm (1) Low-k Dielectrics. http://fcs.itc.it/
Positronium formation in materials without free electrons polymers, glass, liquids, gases, 1 S 0 3 S 1
Counts P "P (z,e) z,e " Lifetime spectra of the low-k dielectric layers 0.0020 lifetime spectra from 5 samples measured by the same positron energy of 5 kev 0.0015 0.0010 0.0005 00 Positron energy = 5 kev 0.0000 0 200 400 600 800 0 1200 1400 "z in nm" 0 10 1 40 60 80 120 140
Outlook the positron beam works for long lifetimes a chopper or a second buncher is required to measure shorter lifetimes strip line kicker perpendicular to the beam line buncher at the beginning of the beam line both