Neutron spin-echo spectrometer at JRR-3M

PHYSICAI ELSEVIER

Physica B 213&214 (1995) 863-865

Neutron spin-echo spectrometer at JRR-3M Takayoshi Takeda a'*, Shigehiro Komura a, Hideki Seto a, Michihiro Nagai a, Hideki Kobayashi a, Eiji Yokoi a, Tooru Ebisawa b, Seiji Tasaki b, Claude M.E. Zeyen c, Yuji Ito d, Shiro Takahashi e, Hideki Yoshizawa e aFacul~' of Integrated Arts and Sciences, Hiroshima University, Higashihiroshima 739 Japan bResearch Reactor Institute, Kyoto University. Noda, Kumatori-cho, Sen-nan-gun, Osaka-fu 590-04, Japan lnstitut Laue-Langevin, BPI 56, [:-38042, Grenoble Cedex 9, France d Faculty of Liberal Arts and Education, Yamanashi University, 4-4-37 Takeda, Kofu 400, Japan Institute for Solid State Physics, University of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo 106, Japan

Abstract

We have designed and have been constructing at the C2 2 cold neutron guide port of JRR-3M, JAERI, a neutron spin echo spectrometer (NSE). It is equipped with two optimal-field-shape coils for neutron spin precession, a positionsensitive detector,oa converging polarizer and a wide-area analyzer. The dynamic range of scattering vector Q covers from 0.005 A ~ to 0.2 A - ~ and that of energy E from 10 neV to 30 ~teV. In a performance test of the precession coils, the NSE signal amplitude PNSEdecreased slightly for the neutron beam cross-section of 306 mm as the Fourier time t increases up to 25 ns. We have carried out NSE experiments on complex fluid systems and obtained their diffusion coefficient Deff.

1. Introduction

The neutron spin-echo (NSE) method proposed by Mezei provides an extremely high energy resolution in analyzing small energy changes on scattering via the phase shift in the Larmor precession of each neutron spin in a magnetic field [1]. The intermediate correlation function I(Q, t) of the investigated system is observed directly in the NSE experiment, where Q is the scattering vector. The Fourier time t is given as t = (2 [7nllANm~/h 3) ,~3D, where 2 is the neutron wavelength and D the magnetic field integral. We have designed a NSE spectrometer in order to study a mesoscopic spatial structure of the order of 1 100 nm combined with a nanosecond temporal structure of the order of 0.1 lOOns. This corresponds to the dynamical behavior of large assemblies made of medium-sized molecules or assemblies of large *Corresponding author.

molecules and also to the dynamical critical behavior of condensed matter [2]. The NSE spectrometer shares the C2 2 cold guide port of JRR-3M, a monochromater, a neutron guide, a sample table and a dance floor with an other spectrometer, the neutron spectral-modulation spectrometer (NSM) [3]. In addition to the parts shared with NSM, two precession coils and tables for a polarizer, precession coils, an analyzer and a detector have been constructed. The precession coils were constructed by T O K I N Co. Some remaining parts are still being constructed. We have carried out a test experiment of NSE using the parts already constructed and the complimentary parts brought from another NSE spectrometer which we had previously constructed at KUR [4]. In this paper, we present the design of the NSE spectrometer and the performance of the precession coil. Finally, we report the result of experiments on complex fluid systems as a demonstration of the NSE spectrometer.

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T. Takeda et al. /' Physica B 2 1 3 & 2 1 4 (1995) 863 865

2. Design of the spectrometer The details of the design of the NSE spectrometer were presented in a previous paper [5], but some of the design parameters are revised in the present paper. The dynamic range in Q covers from 0.005 A t to 0.2 A - 1 and that in energy E from 10 neV to 30 laeV. The spectrometer consists of the following components: a monochromator, a polarizer, several magnetic guides, four sets of re/2 and correction coils, two precession coils with spiral correction coils, a set of ~ and correction coils, two symmetry coils, a sample table with several correction coils, an analyzer, a detector, etc. The distance between the monochromator and polarizer is about 10 m, that between the polarizer and the sample table is about 3.5 m, that between the sample table and the analyzer 3.5 3.9 m, and that between the analyzer and the detector 3-3.5 m. The polarizer (and analyzer) is an assembly of magnetic supermirrors of Soller slit type. Each mirror is a sheet of silicon wafers with alternating layers of C o - F e alloy and V metal evaporated on the wafer. The polarizing mirrors are stacked together in such a way that the neutrons reflected by the polarizer converge at the analyzer position in order to improve the Q resolution without sacrificing intensity. Scattered neutrons diverging from the sample are reflected by the analyzer with a beam cross-section of 60 w x 150 h m m z and detected by the position-sensitive detector (PSD) which specifies different divergent neutron paths. The detector is an assembly of 16 1D-PSDs of 1/2 in in diameter. Each 1 D - P S D is set vertically. The long distance between the analyzer and the detector reduces the neutron noise without sacrificing available intensity using PSDs. For the precession coil, we have adopted the optimal field shape coil (OFS coil) as proposed by Zeyen [6], which has a field distribution B=(z) along the axial coordinate z over a length L such that, Bz(z) = Bo cos 2 (~z/L). Since the relative field-integral inhomogeneity t / = AD/D does not couple with the solenoid diameter in the O F S coil, we could reduce the coil diameter in the O F S coil. Decreasing the diameter of the coil reduces the stray field. The O F S coil of this spectrometer consists of 20 coaxial subcoils having different length and diameters stacked together. The maximum field integral Dm of the coil is 0 . 2 2 T m . The magnetic length L is 2.6m. The inhomogeneity t/ proportional to r 2 is corrected using Mezei-type Fresnel spiral coils. The deviation of D from ideal axial symmetry caused by the wide conductor wires and the small diameter of the coils is a very serious problem. The inhomogeneity from the deviation cannot be corrected by the spiral coil. The winding of the conductor wires and the interconnection between the subcoils were designed in such a way as to compensate for

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Fig. 1. Fourier time dependence of the NSE signal amplitude PNSE. 20M = 7.16°, 2 = 8.45 A. When the spiral coils are operated, PNSEincreases as shown by an arrow for W = 30'~mm and t = 25 ns. Inset: NSE signal intensity as a function of the symmetry-coil current.

this deviation as far as possible, t / f o r 40 ~ m m neutron beam must be reduced below 2 x 1 0 - 6 .

3. Performance test of the precession coil Fig. 1 shows an example of the performance test of the precession coils. The N S E signal amplitude PNSE is given as PNSE = ]A/B], where A, the maximum amplitude of the NSE signal intensity, and B were obtained using a least-squares fit of the NSE signal intensity I to the following equation, I = A e x p [ -- a(i -- io)2]cos[b(i - io)] + B, where i is the symmetry coil current. PNSE decreases slightly for neutron beam cross-section W = 10* mm, as t increases up to 25 ns. PNSE decreases for W = 20 ~ m m as t increases. Though PNSE decreases faster for W = 30 ~ m m than for 20 ~ mm, PNSE increases again when the spiral coils are operated, as shown by an arrow in Fig. 1 for W = 30 ~ mm and t = 25 ns. These results show that ~/can be well corrected even for W = 30 ~ mm, by means of the spiral coils.

4. Slow dynamics in complex fluid systems We have started to study slow dynamics in complex fluid systems such as microemulsions and biological membranes. Fig. 2 shows an example of the N S E experiments on a microemulsion system where heavy water droplets coated with an A O T (sodium di(2-ethythexyl)

T. Takeda et al. / Physica B 213&214 (1995) 863 865 .

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We are greatly indebted to Prof. Y. Fujii at ISSP, University of Tokyo, J a p a n for his supports of the construction of the N S E spectrometer a n d to Prof. Y. Yamaguchi at I M R , T o h o k u University, Sendai, J a p a n for valuable advice regarding the design of the N S E spectrometer.

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Fig. 2. Fourier time dependence of the NSE signal amplitude PNSE in the DzO n-decane AOT system. Q = 0.1 A ~, T = 25°C. Inset: NSE signal intensity as a function of the symmetry-coil current. Q = 0.1 A - ~, t = 8.54 ns, T = 25 'C. Preset time 120s.

sulfa succinate) layer are dispersed in n-decane. The volume fraction of D 2 0 , A O T a n d n-decane is 0.12, 0.20 a n d 0.68, respectively. PNSE shown in Fig. 2 is corrected by PNSE of silica gel (reference for elastic scattering) for polarization a n d then normalized using flipping ratio d a t a of the sample. This PNSE c o r r e s p o n d s to the normalized intermediate correlation function

References [1] F. Mezei (ed.), Neutron Spin Echo, Lecture Notes in Phys. 128 (Springer, Berlin, 1980). [2] S. Komura, T. Takeda and H. Seto, JAERI-M 93228(JAERI CONF2) (1993) 953. I-3] Y. lto, A.I. Goldman, C.S. Majkrzak, L. Passell, M.A. Kelley and D.G. Dimmler, ISSP Report 2063 (1988). [4] S. Komura, T. Takeda, T. Miyazaki, M. Saga and S. Ueno, Nucl. Instr. and Meth. (A) 267 (1988) 425. [5] T. Takeda, S. Komura, H. Seto, M. Nagai, H. Kobayashi, E+ Yokoi, T. Ebisawa and S. Tasaki, JAERI-M 93-228 (JAERI-CONF2) (1993) 959. 1-6] C.M.E. Zeyen, R. Hartmann and L. van de Klundert, IEEE Trans. on Magnets 24 (2) (1988).