positron source EPOS - general concept - timing system - digital lifetime measurement

Transcription

1 The pulsed high-brightness positron source EPOS R. Krause-Rehberg 1, G. Brauer 2, A. Krille 1, M. Jungmann 1, S. Sachert 1, A. Rogov 2, K. Nowak 2 1 Martin-Luther-University Halle, Germany 2 Research Center Rossendorf, Germany ELBE radiation source in Rossendorf EPOS project - general concept - timing system - digital lifetime measurement Martin-Luther-Universität RK Halle & FZ Rossendorf R

2 The EPOS positron source at Research Center Rossendorf Main experiment in Rossendorf: Radiation source ELBE = Electron Linac with high Brilliance and low Emittance Primary electron beam (40 MeV x 1 ma = 40 kw) for 5 experiments Main experiment: tunable IR free-electron Laser Very interesting time structure: cw-mode of short bunches electron bunches Martin-Luther-Universität Halle Halle & FZ Rossendorf

3 EPOS = ELBE Positron Source Intense bunched beam of mono-energetic positrons All relevant positron techniques for materials research (positron lifetime, Coincidence Doppler broadening, AMOC) EPOS is external facility of Martin-Luther-University Halle at Research Center Rossendorf User-dedicated facility Remote controlled via internet Financing by University Halle, Land Sachsen-Anhalt and European Community Martin-Luther-Universität Halle Halle & FZ Rossendorf

4 Martin-Luther-Universität Halle & FZ Rossendorf Ground plan of the ELBE hall

5 Positron Lab X-ray Lab Positron Lab 111c 3.2 m concrete screening of Cave 111b (location of e + converter)

6 EPOS scheme Martin-Luther-Universität Halle & FZ Rossendorf

7 Simulation of Positron distribution W target 0.1 mm foils primary beam Martin-Luther-Universität Halle & FZ Rossendorf

8 Directly water-cooled Electron-Positron Converter stack of 50 pieces W-foils 0,1 mm separated by 0,1 mm -> 13,5 l water at 1,5 bar foils cut by IR-laser in our workshop electron beam cooling water Martin-Luther-Universität Halle & FZ Rossendorf

9 IR-Laser-beam cutting of W-foils

10 Simulation of expected γ and n dose Additional Screening in the cave by lead blocks and heavy concrete Martin-Luther-Universität Halle & FZ Rossendorf

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12 Stability test of beam-dump stand beam dump made from 4N-Al (no alloy)

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15 Position of electron-positron converter cable-tunnel coverage

16 Primary electron beam line entrance into our cave 111b

17 Simulation of Energy deposition total power of electron beam 40 kw Al beam dump 21 kw (made of 4N-purity) W target 14 kw primary beam Martin-Luther-Universität Halle & FZ Rossendorf

18 Water-cooling system ready for use (beam dump; W-Converter, and two Be-windows) consists of 2 independent water circulations with heat exchanger

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20 Simulation of positron extraction simulation done by EGUN area of 20 mm diameter at moderator is used and squeezed to about 2 mm Martin-Luther-Universität Halle & FZ Rossendorf

21 Magnetic Beam Guidance Magnetic guidance field of 75 Gauss 45 coils but only 5 different currents thus only 5 Power supplies maximum change 6 G gradient < 0,11 G/mm in addition: 30 steering coils along the tubes with different (computer-driven) currents Martin-Luther-Universität Halle & FZ Rossendorf

22 Positron Lab 211c Martin-Luther-Universität Halle & FZ Rossendorf

23 Monto-Carlo Simulations of Bunching System problem: time focus shifts with different acceleration voltage especially difficult for low positron energies (< 1keV) Simulation of bunching by POSBUNCH C++ code; author: Dr. V. Bondarenko Martin-Luther-Universität Halle & FZ Rossendorf

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25 Simulation of Buncher Voltages Both buncher RF-voltage amplitudes and the drift path energy must be adjusted for each beam energy for optimum time resolution 40 Accelerator voltage [kv] Time standard deviation [ps] Time standard deviation at sample with different voltages at 1. buncher and adjusted 2. Buncher 1. Buncher: 50 V 100 V 140 V Martin-Luther-Universität Halle & FZ Rossendorf Implantation energy [kev]

26 Second timing mode needed for long lifetimes question: How long repetition time to measure a lifetime of 142 ns? we MC-simulated a 3-component spectrum with τ 1 =150 ps, τ 2 = 2 ns, τ 3 = 140 ns we simulated different repetition times from ns spectra were analyzed and compared to the original one Counts Simulation parameters statistic: 10 7 FWHM: 0,2 ns background: 0,04% canalwidth: 0,1 ns Repetition time 616 ns 308 ns 154 ns 77 ns MC-simulated spectrum lifetimes: 0,15 ns 2 ns 140 ns intensity: 40 % 55 % 5 % Time (ns)

27 Second timing mode needed for long lifetimes Result: repetition time must be at least 8-times longer than longest lifetime to be measured 77ns-system: up to 10 ns only Thus: EPOS needs a 616nstiming mode (616 = 8 x 77) ELBE electron beam allows such a mode already: 7 of 8 electron bunches are suppressed Intensity of long compoent (%) Long lifetime component (ns) 5,5 5,0 4,5 4,0 3,5 3,0 2,5 2,0 1, simulated value simulated value I 3 τ Repetition time (ns)

28 Chopper System problem: beam energy of 2 kev is relatively high for a chopper original idea: triggerable rectangular pulse (2 ns width; >100V) very difficult to obtain the required amplitude new: 3 superimposed sinusoidal voltages beam fit through aperture with voltage of U < 80 V for the simulated voltage U < 80 V corresponds to Δt = 2 ns in 616 ns-mode we will use additional slow chopper Voltage (V) ns Superposition of 3 harmonic oscillations 13, 26, and 39 MHz (5000 V each) additionally overlayed with DC = V ,00E+000 2,00E-008 4,00E-008 6,00E-008 t in s

29 Detector system 3 experiments: lifetime spectroscopy (8 BaF 2 detectors); Doppler coincidence (2 Ge detectors), and AMOC (1 Ge and 1 BaF 2 detector) digital detection system: - lifetime: almost nothing to adjust; time scale exactly the same for all detectors; easy realization of coincidence - Doppler: better energy resolution and pile-up rejection expected - pulse-shape discrimination improves spectra quality Martin-Luther-Universität Martin-Luther-Universität Halle Halle & FZ Rossendorf

30 Lifetime detector system Linux cluster (10 CPUs) calculating power can be increased by adding more PCs to the Control PC Martin-Luther-Universität Halle & FZ Rossendorf

31 Trigger & Coincidence Circuit CPLD...Complex Programmable Logic Device

32 Trigger & Coincidence Circuit

33 digitally measured lifetime spectrum (usual 22 Na setup) time interpolation by polynoms (can be improved) 60 Co resolution = 170 ps (FWHM) Martin-Luther-Universität Halle & FZ Rossendorf