Development status of the positron lifetime beam MePS and the first lifetime measurements of porous ultra-low-k dielectrics with MePS

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1 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

2 Table of Contents 1. Positron lifetime beam system MePS 2. Simulation with Simion Measurement of low-k-dielectric films 4. Outlook

3 Overview MePS

4 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

5 Labor and cave 111

6 Beamline cave

7 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

count rate decreasing from 800 to 150 counts/s during operation with 9 µa Beam current the count rate

8 Counts Platinum moderator 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

9 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

10 Einzellens einzellens with mesh 92% open area thickness of the lens is 50 µm upper part

11 Counts/s Counts First spectra What is the reason for these spectra?

12 Counts Counts Counts Disturbance of the time structure 10 Moderator position mm D with magnetic field C with lense focus 1 mm with lense focus 290 mm

13 counts/sec Counts [1/s] Counts [1/s] Test with retarding field analyzer 400 removing fast positrons by the use of a double aperture without aperture with aperture Voltage [V] n2300v p0v 40 the lens has no influence on the positron energy Voltage [V]

14 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 VG µa 5.8 µa µa 1 µa U [V]

15 Counts/s Time structure with buncher 00 FWHM 650 ps decreasing resolution time from 2.5 ns to 0.65 ns

16 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

17 Counts counts Backscattered positrons accelerator on, without bent tube accelerator on, with bent tube 00 copper copper

18 Summary of MePS count rate is sufficient expected count rate of 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

19 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

20 Time-dependent Simulation with Simion ns time of flight through the chopper plates the pulse is not rectangular field outside of the chopper has a influence of the positrons

21 Field Field Time-dependent Simulation with Simion 8 electric field from chopper electric field from buncher 2 1 dedx dedy dedz 0,0-0, ,4-0,6-0,8 dedx dedy dedz Position z [mm] -1, Position y [mm]

22 Counts Counts/s Time-dependent Simulation with Simion 8 simulation: structuring background by the use of a buncher measured spectra 00 FWHM 650 ps

23 Counts Counts Counts Chopper and Buncher mm Gauss Chopper V 3.14 ns FWHM ,0-0,8-0,6-0,4-0,2 0,0 1/2 Gauss Chopper V 1.54 ns FWHM 33 ps FWHM ps FWHM / ,0-0,8-0,6-0,4-0,2 0,0

24 Counts Counts Assumption: positrons starts with an energy spread of 25 ev FWHM Two buncher without chopper Buncher 1 200V first slit after 0 mm ,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

25 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

26 Production of low-k films 800 nm (1) Low-k Dielectrics.

27 Positronium formation in materials without free electrons polymers, glass, liquids, gases, 1 S 0 3 S 1

28 Counts P "P (z,e) z,e " Lifetime spectra of the low-k dielectric layers lifetime spectra from 5 samples measured by the same positron energy of 5 kev Positron energy = 5 kev "z in nm"

29 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

The MePS System at Helmholtz-Zentrum Dresden-Rossendorf and its special Capability for Positronium Lifetime Spectroscopy

The MePS System at Helmholtz-Zentrum Dresden-Rossendorf and its special Capability for Positronium Lifetime Spectroscopy R. Krause-Rehberg and many colleagues of Univ. Halle and HZDR Martin-Luther University