Annual Report 2011 / 2012 - E21 - Technische UniversitÃ¤t MÃ¼nchen

More documents

Recommendations

Info

44 E21 Annual Report 2011/2012 Tests of Modulated Intensity Small Angle Scattering in Time of Flight Mode Georg Brandl 2, 1 , Jyotsana Lal 3 , Lee Robertson 4 , Markus Bleuel 5 , Peter Böni 1 1 Physik-Department E21, Technische Universität München, D-85748 Garching, Germany. 2 Forschungsneutronenquelle Heinz Maier-Leibnitz (FRM II), Technische Universität München, D-85748 Garching, Germany. 3 Argonne National Laboratory, Materials Science Division, Argonne, IL 60439, USA. 4 Oak Ridge National Laboratory, Oak Ridge, TN 37831-6477, USA. 5 Technical University of Delft, 2629JB Delft, Netherlands. Introduction MISANS, MIEZE (Modulation of Intensity with Zero Effort) in the Small Angle Neutron Scattering (SANS) geometry is a rather new technique to probe quasi-elastic scattering with extremely high energy resolution. The method is well understood [4] and efforts are under way [1, 2] to establish the technique as a standard tool for measurements of slow dynamics. The general trend of new neutron sources to be accelerator driven and thus to provide pulsed neutron beams raises the question how MISANS will perform in a pulsed mode. In earlier experiments the feasibility of MISANS on pulsed sources was demonstrated [5], however these tests were at relative low MIEZE frequencies. The goal of this experiment was to show that a MIEZE can be set up easily at a new beamline and works well in the time-of-flight mode with samples. Therefore a compact turn-key MISANS setup from the FRM II in Munich was installed at the HFIR in Oak Ridge, Tennessee, USA, at the beamline CG-1D [3]. Setup of the Experiment The used setup is shown here: Two sets of MIEZE coils inside µ-metal shielding [1] are placed between two polarizers (P). The chopper (Ch) provided a pulsed beam with a frequency of 40 Hz and a pulse length of 0.14 ms. The frequencies in the RF coils ranged between 46 and 149 kHz and matched the Larmor frequency of the static fields. The sample (S) was placed inside a cryomagnet. The detector (D) was a fast micro-channel plate area detector. Results Ch A 1 P 1 C 1 Box C 2 P 2 A 2 A 3 S D n f 1 f 2 (99 kHz) (124 kHz) B f ch (40 Hz) L S = 1.04 m 0.61 m L 1 = 0.9 m L 2 = 3.56 m L 0 = 5.07 m The next figure shows the result of a first test, a MIEZE modulation on the time-of-flight spectrum with two different frequencies. The absorption edges of aluminum and silicon are clearly visible as dips in the spectra and were used to calibrate the time-of-flight to wavelength conversion. The RF frequencies were 46/69 kHz and 99/149 kHz, respectively, with distances L 1 = 0.9 m and L 2 = 1.8 m to fulfill the MIEZE condition. The maximum contrast in our tests was about 50 %. Intensity (a.u.) MIEZE contrast C 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 ∆f = 23 kHz ∆f = 50 kHz ∆f = 23 kHz ∆f = 50 kHz 1 2 3 4 5 6 7 8 λ (Å) The last figure shows a measurements of neutrons scattered by the helical magnetic order in a MnSi sample placed inside the cryostat at zero field. MIEZE contrast C, S(q,τ) λ (Å) 3.5 4 4.5 5 5.5 6 6.5 1.6 1.4 S(q,τ) C at T = 27.55 K 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.6 C at T = 2 K 0.4 0.2 0.0 5 10 15 20 25 30 35 40 45 50 Spin-echo time τ (ps) The blue curve shows the contrast measured in the neutron beam scattered by the MnSi sample at T = 27.55 K at the Bragg peak at q = 0.035 Å −1 for different spin-echo times, corresponding to different wavelengths in the incoming beam. When compared with the green curve, which is the contrast at T = 2 K, the black S(τ) shows no decay in the intermediate scattering function as expected [1]. References [1] R. Georgii, G. Brandl, N. Arend, W. Häußler, et al., Appl. Phys. Lett. 98, 073505 (2011). [2] G. Brandl, R. Georgii, W. Häußler, S. Mühlbauer, and P. Böni, Nucl. Inst. Meth. A 654, 394 (2011). [3] G. Brandl, J. Lal, J. Carpenter, L. Crow, L. Robertson, R. Georgii, P. Böni, and M. Bleuel, Nucl. Inst. Meth. A 667, 1 (2012). [4] R. Gähler, R. Golub, and T. Keller, Physica B 156-157, 653 (1989). [5] M. Bleuel et al., Physica B 371, 297 (2006).
Chapter 5. Instrument Development 45 The Instrument Design of KOMPASS – the New, Polarized TAS at FRM II Alexander Grünwald 1 , Stefan Giemsa 1 , Peter Böni 2 , Markus Braden 1 1 II. Physikalisches Institut der Universität zu Köln, D-50937 Köln, Germany 2 Physik-Department E21, Technische Universität München, D-85748 Garching, Germany. Figure 1: Design drawing of the KOMPASS (KOeln Münchner auf Polarisations-Analyse Spezialisiertes Spektrometer) and the instrument specific neutron guide, which will be located at the end of the guide NL1 in the neutron guide hall west at the FRM II. The design of KOMPASS – the new, cold triple-axis spectrometer (to-be) at the FRM II – has been finalized. Based on previous calculations [1, 2, 3], the neutron guide between NREX+ (the next upstream instrument) and KOMPASS is parabolically focusing in the scattering plane and hosts three serial multi-channel V-cavities, which will provide a permanent high-polarized incident neutron beam with polarization rates of P > 99 % (simulated). Motorized exchangeable parabolic and straight guide front ends allow for an optimized energy- and Q-resolution, respectively. The different guide front ends together with the variable double-focusing monochromator and analyzer thus provide a high flux with superior energy- and Q-resolution over a large dynamic range at a small sample volume. For measurements with high energy-resolution the parabolic guide front end can be used at the expense of a slightly reduced Q-resolution. In contrast, for measurements with high Q-resolution, or for the investigation of steep dispersion relations, the straight guide front end is available. The new designed, compact sample table, with inside driving mechanisms for the linear tables and the goniometer, has more space for sample environment; e.g. spherical neutron polarimetry with a mini MuPAD and a 3rd generation ILL system CryoPAD. Figure 3: Picture of the sample table and design drawings of the inside driving mechanisms. Higher order wavelength contaminations can be suppressed by an optional velocity selector in front of the monochromator. As a result, KOMPASS will be ideally suited to investigate all types of weak magnetic ordering and magnetic excitations. In particular complex chiral magnetic structures, multiferroic and magneto-electric materials, high T C - superconductors and quantum critical fluctuations are wide areas for potential applications. This project is supported by the German Federal Ministry of Education and Research (BMBF) by project 05KN7PK1 & 05K10PK1. Figure 2: Picture of the variable double-focusing monochromator mechanics, showing 19 × 13 crystal carriers and magnetic yokes at its top and bottom to close the magnetic flux lines of the permanent magnets at the rear side.(All rights reserved by SwissNeutronics.) Permanent magnetic guide fields throughout the entire beam path preserve the high degree of neutron polarization. References [1] E21 Annual Report 2009/2010, Technische Universität München. [2] A.C. Komarek and P. Böni and M. Braden, Nuclear Instruments and Methods in Physics Research A 647, 63 (2011). [3] M. Janoschek and P. Böni and M. Braden, Nuclear Instruments and Methods in Physics Research A 613, 119 (2010).
Page 1 and 2: Physics Department Research Area St
Page 3 and 4: Annual Report 2011 / 2012 of the Re
Page 5 and 6: Contents v Contents Preface 1 1 Mag
Page 7 and 8: Preface 1 Preface In the name of al
Page 9 and 10: Chapter 1. Magnetism and Supercondu
Page 23 and 24: counts / std. mon. [arb. units] log
Page 25 and 26: counts / stand. mon. counts / stand
Page 27 and 28: Chapter 2. Nuclear and Fundamental
Page 29 and 30: Chapter 2. Nuclear and Fundamental
Page 31 and 32: Chapter 3. Positron Physics 25 Chap
Page 33 and 34: Chapter 3. Positron Physics 27 A Pr
Page 35 and 36: Chapter 3. Positron Physics 29 The
Page 37 and 38: Chapter 3. Positron Physics 31 Quan
Page 39 and 40: Chapter 4. Radiography and Tomograp
Page 47 and 48: Chapter 5. Instrument Development 4
Page 49: Chapter 5. Instrument Development 4
Page 53 and 54: WENN NICHT ANDERS DEFINIERT: BEMASS
Page 55 and 56: Chapter 5. Instrument Development 4
Page 57 and 58: Chapter 6. Activities 51 Chapter 6
Page 59 and 60: Chapter 6. Activities 53 Lectures,
Page 61 and 62: Chapter 6. Activities 55 Michael Wa
Page 63 and 64: Chapter 6. Activities 57 [18] A. Ne
Page 65 and 66: Chapter 6. Activities 59 [58] R. W
Page 67 and 68: Chapter 6. Activities 61 [20] A. Ba
Page 69 and 70: Chapter 6. Activities 63 [63] C. Hu
Page 71 and 72: Chapter 6. Activities 65 [107] C. P
Page 73 and 74: Chapter 6. Activities 67 [152] M. W
Page 75 and 76: Chapter 6. Activities 69 Seminar
Page 77 and 78: Chapter 6. Activities 71 Accomplish
Page 79 and 80: Chapter 6. Activities 73 E21 Member
Page 81 and 82: Chapter 6. Activities 75 Alumni Nam

Annual Report 2011 / 2012 - E21 - Technische UniversitÃ¤t MÃ¼nchen

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?