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Tests of Fermi Chopper Components in McStas

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Tests of Fermi Chopper Components in McStas

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  1. Bei dieser Präsentation wird sicher eine Diskussion mit dem Publikum entstehen, die zu Aktionsschritten führt. Verwenden Sie PowerPoint, um diese Aktionsschritte während Ihrer Präsentation festzuhalten. • Klicken Sie in der Bild-schirmpräsentation auf die rechte Maustaste. • Wählen Sie 'Besprechungs-notizen'. • Wählen Sie die Registerkarte 'Aktionsschritte'. • Geben Sie die zur Sprache kommenden Aktionsschritte ein. • Klicken Sie auf OK, um dieses Feld zu schließen. Hierdurch wird automatisch eine Folie mit Aktions-schritten am Ende Ihrer Präsentation erstellt, auf der Ihre Notizen erscheinen. McStas Tests of Fermi Chopper Components in McStas Klaus Lieutenant K. Lieutenant Fermi Choppers in McStas

  2. Test by R Peacock and others • Tests presented on MsStas user meeting in Feb 2005: • Chopper_Fermi (A. Garrett) • triangular shape instead of top-hat shape for 1 slit • FermiChopper (M. Pöhlmann) • TOF problems • (transmission 90 %) • Fermi_Chopper_Straight (B. Roure) • wrong intensity • wrong shape K. Lieutenant Fermi Choppers in McStas

  3. New tests • Test method: • Black box test • Compared to analytical functions • Marseguerra and Pauli • Peters • own calculations • Components and treatment before tests: • Chopper_Fermi (A. Garrett) • change: function that creates triangular shape removed • FermiChopper (M. Pöhlmann) • ‘set zero time’ only as an option • ‘efficiency’ introduced as a parameter that can be set • Vitess_ChopperFermi (G. Zsigmond) • converted from VITESS • Fermi_Chopper_Straight (B. Roure) not treated K. Lieutenant Fermi Choppers in McStas

  4. Config. 1: Point source, no diverg. • Configuration 1 (nearly as tested by R. Peacock et al.): • Source • 0.01 x 0.01 mm² • Energy 1, 10, 100, 1000 meV (λ=0.286, 0.9045, 2.86, 9.045 Å) • no divergence • starting time: 0 … 10 ms (= 2 T) • Chopper • straight, r=200 Hz • 1 slit: 30 mm wide, 100 mm long, 80 mm high • min. diameter: 104.40 mm • initial phase: 0° and adjusted to reach 0° position, when first neutrons arrive at centre K. Lieutenant Fermi Choppers in McStas

  5. Check 1a: time structure behind FC Pulses at mon. 2 for E = 1 meV and phase = 0 Check 1a (TOF, pulse length, intensity) Result for all components • TOF correct • pulse length correct • pulse repetition correct • intensity correct Pulses at mon. 2; phase adjusted for first neutrons Analytical calculations: K. Lieutenant Fermi Choppers in McStas

  6. Check 1b: blade in the centre Idea • If a blade is in the centre of the beam, it should block the beam (of the point source) completely Simulation • 3 blades of 2 mm (and 6 channels of 6 mm) use Result • Zsigmond: transmission 0, perfect K. Lieutenant Fermi Choppers in McStas

  7. Check 1c: ‘Zero-time’ option • Idea: • Chopper creates one pulse by setting the time close to zeroThis is a (virtual) source for a TOF instrument • Simulation: • 10 meV (2.860 Å) • Expected: • 1 pulse of 0.3884 ms length and rel. intensity 4 (-0.1508 to 0.2376 ms) • First results: • Poehlmann: 2 pulses 0.232 ms and 0.156 ms of rel. intensity 4 - failed • Zsigmond : 2 pulses 0.156 ms and 0.232 ms of rel. intensity 4 - failed • After corrections: • 1 pulse of 0.388 ms length for both components - correct K. Lieutenant Fermi Choppers in McStas

  8. Config. 2: area source, no diverg. Configuration 2: • Source • 20 x 60 mm² (W x H) • Wavelengths 0.5, 4, 7.5, 11 … 21.5 Å (E=327.2, 5.113, 1.454, 0.6761, 0.3891, 0.2525, 0.177 meV) • no divergence • starting time: 0 … 1.6667 ms (= 0.5 T) • Chopper • straight, r=300 Hz • 20 slits: 0.9 mm wide, 19.1 mm long, 60 mm high • walls : 0.1 mm wide • min. diameter: 27.73 mm • initial phase: ~ 30° Realization: • Garrett : 18 mm width of Fermi Chopper • Poehlmann: 22 channels of 0.9 mm, efficiency 0.9 • Zsigmond : direct (total width 20.1 mm) K. Lieutenant Fermi Choppers in McStas

  9. Check 2a: pulse shape, TOF Pulse shape for a fixed wavelength (4 Å) Pulse shape for a fixed wavelength (4 Å)rotation in negative direction (starting at -30)°: Results: • Garrett : failed - taken out of consideration • Poehlmann and Zsigmond (for both directions of rotation) • time-of-flight correct • transmission correct • shape of pulse correct Analytical function calculated according to Marseguerra and Pauli using Excel K. Lieutenant Fermi Choppers in McStas

  10. Check 2a: different wavelengths Pulse shape for a fixed wavelength of 0.5 Å Pulse shape for a fixed wavelength of 11 Å Short wavelengths: • shape well represented • Poehlmann: Walls partly transparent - bug • Zsigmond : Still perfect absorption assumed Long wavelengths: • Poehlmann: width too large • Zsigmond : practically perfect K. Lieutenant Fermi Choppers in McStas

  11. VITESS Fermi-Ch transm. vs gate positions No difference V2.6 V2.5.1 overestimation K. Lieutenant Fermi Choppers in McStas

  12. Check 2b: large shadowing cylinder Configuration 2b: • same as configuration 2a except • diameter 100 mm Source: 4 Å, no divergence Results: • Pöhlmann: slightly too high transmission for t < tcentre. • Zsigmond: no change to config. 2a. Influence of cylinder is not treated Analytical function calculated as product of transmission of inner and outer channel(s) K. Lieutenant Fermi Choppers in McStas

  13. Check 2c: curved Fermi Chopper Configuration 2c: • same as configuration 2a except: • blades curved for 4 Å (r=262.3 mm) Source: 1.5, 4, 7.5 Å, no divergence Result: • Zsigmond: practically perfect Analytical function calculated according to Marseguerra and Pauli using Excel K. Lieutenant Fermi Choppers in McStas

  14. Check 2d: prototype neutron (Pöhlmann) Comparison: simulations using different speeds of prototype neutron • Configuration 2d: • same as configuration 2a except: • prototype neutron with a velocity of 989 m/s (4 Å) in a simulation of 0.5 Å neutrons • Result: • Pöhlmann: noise is suppressed, rest seems to be identical K. Lieutenant Fermi Choppers in McStas

  15. Config. 3: area source, divergent Configuration 3: • Source • 90 x 60 mm² (W x H) • wavelengths 1, 4, 10 Å (E=81.80, 5.113, 0.8180 meV) • hor. divergence: 1° or 2°, no vertical divergence • starting time: 0 … 4 ms (= 0.667 T) or 0 ms (pulse) • Chopper • straight, r=166.67 Hz • 40 slits: 0.5 mm wide, 20 mm long, 60 mm high • no walls • min. diameter: 28.28 mm • initial phase: 0° or adapted that pulse passes centre in position ‘fully open’ (t’=0) K. Lieutenant Fermi Choppers in McStas

  16. Check 3: divergent case Comparison of analytical calculations and simulation: Divergence distribution behind chopper for pulse of incoming divergence 2.0° arriving at t’=0 Comparison: pulse width and height in divergent (2°) and non-divergent case Pulse shape • principal change correct • maximal pulse width correct • ratio of integral over pulse correct for 1 and 4 Å ‘Escape angle’ (divergence behind chopper) • correct for Zsigmond component • overestimated for 10 Å in Pöhlmann component K. Lieutenant Fermi Choppers in McStas

  17. Summary • Chopper_Fermi (Garrett): do not use it • FermiChopper (Pöhlmann): • for straight FC with absorbing and reflecting (not tested here) walls • good, if missing blades width can be treated otherwise • Vitess_ChopperFermi: • for straight and curved Fermi choppers with absorbing walls • good, if there is sufficient computer power and the shadowing cylinder is not important K. Lieutenant Fermi Choppers in McStas

  18. Validation • Validation • Chopper_Fermi (Garrett): removed from McStas package • FermiChopper (Pöhlmann): validated (though not perfect for long wavelengths) • Vitess_ChopperFermi: validated • This research project has been supported by the European Commission under the 6th Framework Programme through the Key Action: Strengthening the European Research Area, Research Infrastructures. Contract n°: RII3-CT-2003-505925 Thank you for your attention ! Literature: R. Peacock et al., internal report at ILL M. Marseguerra, G. Pauli, Nucl. Instr. Meth. 4 (1959) 140-150. J. Peters, Nucl. Instr. Meth. Phys. Res. (2005) K. Lieutenant Fermi Choppers in McStas

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