The longitudinal neutron resonant spin echo spectrometer RESEDA C. Franza,b,∗, O. Soltwedela,b,e, C. Fuchsb, S. S¨aubertb,a, F. Haslbecka,d, A. Wendla, J. K. Jochumb,c, P. B¨onia, C. Pfleiderera aPhysik Department, Technische Universit¨atM¨unchen,D-85748 Garching, Germany bHeinz Maier-Leibnitz Zentrum (MLZ), Technische Universit¨atM¨unchen,D-85748 Garching, Germany cBayerisches Geoinstitut, Universit¨atBayreuth, D-95440 Bayreuth, Germany dInstitute for Advanced Study, Technische Universit¨atM¨unchen,D-85748 Garching, Germany ePhysik Department, Technische Universit¨atDarmstadt, D-64287 Darmstadt, Germany Abstract The instrumental layout and technical realisation of the neutron resonant spin echo (NRSE) spectrometer RESEDA at the Heinz Maier-Leibnitz Zentrum (MLZ) in Garching, Germany, is presented. RESEDA is based on a longitudinal field configuration, boosting both dynamic range and maximum resolution of the spectrometer compared to the conventional transverse layout. The resonant spin echo technique enables the realisation of two complementary techniques: A longitudinal NRSE (LNRSE) option comparable to the classical neutron spin echo (NSE) method for highest energy resolution and large momentum transfers as well as a Modulation of Intensity with Zero Effort (MIEZE) option for depolarising samples or sample environments such as high magnetic fields, and strong incoherent scattering samples. With their outstanding dynamic range, exceeding nominally seven orders of magnitude, both options cover new fields for ultra-high resolution neutron spectroscopy in hard and soft condensed matter systems. In this paper the concept of RESEDA as well as the technical realisation along with reference measurements are reported. Keywords: neutron spectroscopy, neutron spin echo, NRSE, MIEZE 1. Introduction One existing NRSE spectrometer for quasi-elastic scattering is located at LLB, France [7]. Neutron Resonance Spin Echo (NRSE) [1] is The longitudinal NRSE (LNRSE) technique [8] a Larmor labelling technique similar to Neutron overcomes the limitations of conventional trans- Spin Echo (NSE) [2] that was developed to study verse NRSE. First, the longitudinal rf flippers are amongst other things slow dynamics in liquid so- by design self-correcting, which increases the high- lutions, polymers, and spin glasses. In NRSE, est resolution significantly. Second, the field sub- traditionally employed in a transverse geometry traction method [9] allows to reach infinitesimally (TNRSE, magnetic field perpendicular to the neu- small Fourier times, leading to a unique dynamic tron flight path), the large solenoids generating the range, nominally exceeding seven orders of mag- precession fields in NSE are replaced by pairs of nitude. Furthermore the LNRSE method allows much shorter resonant spin flippers (RSF). However the use of Modulation of Intensity with Zero Effort due to the lack of sufficient manufacturing accu- (MIEZE) [10, 11], an NRSE variant that is insen- racy, TNRSE always stayed behind NSE by two or- sitive to depolarising conditions such as large mag- ders of magnitude in resolution. But the technique netic fields at the sample position and depolarising proved to be very successful in combination with arXiv:2103.12836v1 [physics.ins-det] 23 Mar 2021 (e.g. ferromagnetic) samples. For strongly inco- triple-axes spectrometry to resolve narrow phonon herent scattering samples MIEZE does not show linewidths [3, 4, 5] and for Larmor diffraction [6]. the 2/3 background arising from spin flip scat- tering in contrast to conventional NSE (NRSE). Essentially, MIEZE is a high-resolution spin-echo ∗Corresponding author. +49 89 289 14760 Email address: [email protected] (C. TOF method using only the primary spectrometer Franz) arm for beam preparation [12]. Typical applica- Preprint submitted to Elsevier March 25, 2021 tions comprise so far the investigation of quantum typically used for non-magnetic samples and with- phase transitions, ferromagnetic materials, super- out depolarising sample environment when high- conductors, skyrmions and hydrogenous samples. est resolution and/or large scattering angles are The MIEZE method is being actively developed needed. Typical experiments include dynamics in at various places around the world, including the bulk or confinement of macro molecules [16] or liq- Reactor Institute Delft, the ISIS neutron source uids [17] and glass transitions in polymers [18]. For [13] and JSNS at J-PARC [14, 15]. Currently only magnetic samples it is possible to separate nuclear BL06 at the J-PARC Materials and Life Science and magnetic scattering by switching the π-flipper Experimental Facility (MLF) and RESEDA are in off. user operation with RESEDA being the only beam- In this mode two symmetric spectrometer arms line worldwide based on longitudinal resonant spin are used, with a π-flipper before the sample posi- flips. In this paper we present the instrumental tion to reverse the precession direction. All four concept as well as the technical implementation of resonant coils are operated at the same frequency, the LNRSE and MIEZE technique at the instru- with two coils per resonating circuit. To avoid fre- ment RESEDA at the Heinz Maier-Leibnitz Zen- quency drifts between the spectrometer arms, fre- trum (MLZ). Further we show the thorough char- quency generators are coupled on a 10 MHz clock acterisation of both measurement options, includ- signal. Small amplitude mismatches in the rf-coil- ing resolution measurements and compare them to current, caused by manufacturing limitations, are analytical calculations as well as neutron flux cal- compensated by built-in attenuators in the capac- culations using Monte-Carlo simulations. itance boxes. The NRSE signal is identical to an NSE echo with the envelope being the Fourier trans- form of the incident wavelength band and the os- 2. Instrument concept cillation frequency proportional to the mean wave- length (cf. Fig 1 (a)). During regular experimental The aim of the REsonance Spin Echo for Diverse operation only n points of the spin echo signal are Applications (RESEDA) spectrometer is to provide recorded, with n typically ranging from 4 to 19. very high resolution neutron spectroscopy for vari- These are then fitted to yield the polarisation. The ous applications in hard and soft condensed matter. Fourier time can be calculated as Therefore RESEDA has been built to offer an ex- 2 tremely high dynamic range in time resolution, a 2γmN 3 τNRSE = λ B · L (1) broad band from small (SANS) to medium scat- ~2 tering angles as well as the ability to use a large with m the neutron mass, λ the neutron wave- variety of sample environments. To fulfill these N length and the product of the static magnetic field requirements, an NRSE and a MIEZE option are B and rf-flipper distance L. Note the factor two implemented at RESEDA. Both share the primary compared to NSE due to the resonant spin flip [1]. spectrometer arm but make use of individual sec- Fig. 2 shows the calculated resolution τ of ondary spectrometer arms. A simultaneous oper- NRSE the spectrometer color coded in ns depending on ation is not possible, however switching from one the neutron wavelength λ in A˚ and the field in- mode to the other only requires a few minutes. tegral B · L in 10−3 Tm. For short Fourier times the frequency is kept at 35 kHz due to the Bloch- 2.1. LNRSE Siegert shift [19] which determines the probability of a successful π-flip as the ratio of the static and rf- The LNRSE option of RESEDA offers a resolu- field strengths. In this regime the field subtraction tion comparable to standard NSE spectrometers, method is used. Ramping up the magnetic field in outperforming transverse NRSE by two orders of the field subtraction coils (NSE coils) the field in- magnitude in maximum Fourier time. The scat- tegral experienced by the neutrons may be reduced tered beam size is restricted compared to NSE due and short Fourier times are reached. to the use of the resonant flipper coils, limiting the detector size and hence solid angle coverage to 5.08 cm (2 inch) in diameter. The advantage over 2.2. LMIEZE NSE is the increased dynamic range towards small In contrast to LNRSE, MIEZE is essentially a Fourier times. At RESEDA the LNRSE option is high resolution TOF method, which can be used for 2 (a) 10 τ = 1.81ns 8 6 cts (a.u.) 4 2 P = 76.0% RESEDA 0.90 0.95 1.00 1.05 1.10 1.15 1.20 scanning current (A) (b) 0.8 (c) fMIEZE=1464Hz 30 0.6 0.4 20 MV Contrast 0.2 (a.u.) cts 10 A C = 75.0% 0.0 1.5 1.6 1.7 1.8 1.9 0 85 171 256 342 427 512 598 683 scanning current (A) time (μs) Figure 1: Spin echo group in NRSE and MIEZE, respectively. (a) Spin echo group at a moderate Fourier time of 1.81 ns in classical LNRSE mode. The intensity exhibits oscillations as a function of the current in the subtraction coil and the envelope is defined through the selector wavelength band. (b) The contrast of the MIEZE signal as a function of current in the subtraction coil. Here too, the shape of the MIEZE group is determined by the wavelength band. (c) Sinoidal intensity oscillations in MIEZE are in real time. The contrast shown in (b) is the ratio between the amplitude A and the mean value MV. 3 field subtraction 16 100 ns 102 14 10 ns 101 1 ns 100 12 100 ps 10−1 (ns) 10 10 ps 10−2 MIEZE τ 1 ps −3 Wavelength (Å) Wavelength 10 8 100 fs 10−4 6 10 fs 10−5 10−6 10−5 10−4 10−3 10−2 10−1 100 f2 − f1 (MHz) Figure 2: Color coded RESEDA NRSE resolution depending Figure 3: Color coded MIEZE resolution available at on neutron wavelength and the field integral defined by the RESEDA as a function of neutron wavelength and differ- resonant flipper frequency and subtraction field.