Physica B 311 (2002) 102–105

Development of new neutron spin echo spectrometerbased on neutron spin interferometry

S. Tasakia,*, T. Ebisawaa, M. Hinoa, T. Kawaia, D. Yamazakia, N. Achiwab,R. Maruyamac, S. Kawakamic

aResearch Reactor Institute Kyoto University, 1010 Kumatori-cho, Ossaka, 590-0494 JapanbFaculty of Science, Kyushu University, Japan

cFaculty of Engineering, Kyoto University, Japan

Abstract

A multilayer spin splitter (MSS) generates the phase difference between the two neutron spin states, one of which is

parallel to the direction of the magnetic field and the other is anti-parallel to it. Since the phase difference is equivalent

to the Larmor precession angle, MSS enables us to construct a new type of the neutron spin echo (NSE) spectrometer.

The new NSE spectrometer has properties; (1) the size of the spectrometer is small compared with a conventional NSE

spectrometer, since the phase shift originates from the neutron flight path difference through the gap layer of the MSS,

(2) the neutron spin echo time is proportional to the neutron wavelength. The feasibility of the new NSE spectrometer

to the pulsed neutron source and the renewal cold neutron beam line, C3-1-2 at the JRR-3M reactor in the Japan

Atomic Energy Research Institute (JAERI ) are reported. r 2002 Elsevier Science B.V. All rights reserved.

Keywords: Neutron spin echo; Multilayer spin splitter

1. Introduction

The multilayer spin splitter (MSS) is a neutronoptical device consisting of a non-magnetic multi-layer, a gap layer and a magnetic multilayerdeposited in series on the silicon wafer as shownin Fig. 1. The top magnetic mirror reflects one spinstate while the bottom non-magnetic mirrorreflects the other spin state and the gap layergenerates the path length difference between thetwo spin states. Its function is equivalent to theLarmor precession magnet in the conventional

spin echo instrument [1]. A neutron polarized inthe plane perpendicular to the magnetic field isregarded quantum-mechanically as the coherentlysuperposed state of the two spin states, one ofwhich has the spin direction parallel to that of themagnetic field and the other is anti-parallel to it.When such a polarized neutron is incident on aMSS, two neutron spin components are reflectedseparately and the relative phase shift is createdthrough the gap layer of an order of 1 mmthickness. The relative phase shift is equivalent tothe Lamor precession in a magnetic field andcalled the quantum precession of the neutron spin.The quantum precession, i.e. the phase shift by theMSS is produced by the path difference throughthe gap layer and thus it allow us to construct the

*Corresponding author. Tel.: +81-724-51-2340; fax: +81-

724-51-2620.

E-mail address: tasaki@rri.kyoto-u.ac.jp (S. Tasaki).

0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 1 1 1 7 - 6

small size of the NSE instrument around 1m inlength.The basic idea for the new spin echo spectro-

meter is that the Larmor precession magnet isreplaced by the MSS.The new NSE spectrometer utilizes four MSSs

as shown in Fig. 2 [3]. In order to avoid polariza-tion reduction due to the beam divergence, the(++) arrangement of MSS is adopted.In this set-up, the spin echo time tNSE is given

by [2]

tNSE ¼4D sin y

v; ð1Þ

where D; y and v are the thickness of the gap layer,an incident angle of neutron on the MSS anda neutron velocity [3]. The Eq. (1) shows thatthe thicker the gap layer and the larger the incidentangle, the energy resolution becomes higher.However it is difficult to fabricate the MSSwith a thick gap layer because it requires theoverall uniformity over the surface of the gap layer

and the very small interface roughness betweenlayers. The other problem is that the thick layerdeposited on the silicon substrate is easy to peeloff. In the recent investigation, [4] the MSSfabricated by the vacuum evaporation methodhad the homogeneity of the gap layer estimated tobe 66.2 (A. It should be lowered less than 50 (A toconstruct a NSE spectrometer using MSSs. Wehave a plan to fabricate the MSS with the ion-sputtering method.The MSS–NSE spectrometer has advantages as

follows; (1) the very weak magnetic field isrequired to work the magnetic mirror of theMSS and the spin flip efficiency is also high, (2)‘the number of precessions’ is independent of themagnetic field integral. These advantages enablesus to construct the small size of the MSS–NSEspectrometer o1m� 1m� 1m. The short lengthreduces the frame overlap region in a time of flightspectrum and for this reason the MSS–NSEspectrometer is suitable for the pulsed neutronsource.

Fig. 1. Structure of MSS.

Fig. 2. Structure of spin echo spectrometer using MSS.

S. Tasaki et al. / Physica B 311 (2002) 102–105 103

In this report, we discuss the characteristics ofthe MSS–NSE spectrometer applied to a pulsedneutron source. A renewal cold neutron beam lineat JRR-3M in JAERI is also presented.

2. Applicability to the pulsed neutron sources

In order to apply the MSS–NSE spectrometer tothe pulsed neutron source, the following technicalproblems should be solved.

1. Polarizer and analyzer mirrors should be super-mirrors. The Permalloy (PA)/Ge multilayerwith a stacking thickness above 8000 (A isrequired. This problem is expected to beresolved by using the ion-sputtering methodfor the fabrication.

2. Both magnetic and non-magnetic mirrors ofMSS should be supermirrors applicable to thepulsed neutron with a wide wavelength range.

3. The spin flippers should be also applicable tothe pulsed neutron beam with a very widewavelength range and the time-of-flight mea-surement.

For the MSS–NSE spectrometer, the spin echotime tNSE is represented by the following equation:

tNSE ¼4D sin y

v; ð2Þ

where v is the velocity of the neutron, y theneutron incident angle on the MSS. The significantdifference from the other spin echo methods is thatthe spin echo time is inversely proportional to v;not v3:Assuming that the quasi-elastic scattering spec-

trum from the sample obeys the Gaussian dis-tribution with standard deviation s; thepolarization PNSE of the spin echo signal isrepresented with tNSE;

PNSEpexp �t2NSEs

_2

� �: ð3Þ

A polarizability of NSE signal simulated for theMSS–NSE spectrometer as a function of neutronwavelength is shown in Fig. 3. Here we assume thegap layer thickness of the MSS to be 10 mm, aneutron incident angle 1.11, and s are 100 and

1000 neV. The MSS–NSE spectrometer with a gaplayer of 10 mm could distinguish the quasi-elasticscattering involved in the energy of severalhundreds neV. Since the tNSE is inversely propor-tional to v; the MSS–NSE spectrometer has itsadvantages that it can give detailed information onthe quasi-elastic scattering.

3. A renewal cold neutron beam line at JRR-3M

reactor

We have renewed the C3-1-2 beam line at JRR-3M reactor in Japan Atomic Energy ResearchInstitute (JAERI) for developing the new type ofthe spectrometer. In the previous arrangement ofthe C3-1-2 beam line, a quadruple monochroma-tor system (QUAD) was installed and 12.6 (A-monochromatized neutron beam was extracted forneutron reflectometry and spin interferometry [6].QUAD has been replaced by a bender system to

get three beam lines with more intense beam. Weare installing; (1) a neutron refractometer with anintense magnetic field, (2) a new type of theneutron spin echo instrument, and (3) a neutronreflectometer/a neutron spin interferometer. Thedetail is reported elsewhere [7].The schematic view of the neutron bender is

shown in Fig. 4. The neutron beam from the maincold neutron guide is devided into two parts, each

Fig. 3. Polarization of NSE signal for MSS–NSE as a function

of neutron wavelength.

S. Tasaki et al. / Physica B 311 (2002) 102–105104

of which has a 10mm� 50mm beam cross section.One of beams is reflected four times by the 2.1 Q-supermirror led to the neutron refractometer. Theneutron wavelength for this beam is longer than9.2 (A. The beam intensity is about 4� 105 n/cm2 s.The other beam is incident on the bender at theincident angle slightly larger than that of theformer beam and reflected six times by the 2.1 Q-supermirror. The neutron wavelength of this beamis longer than 9.8 (A. The beam intensity is about2� 105 n/cm2 s. Near the end of the second beamline, a triple monochromator system is inserted totake out the monochromatized beam. The mono-chromators are multilayer mirrors with a d-spacing of 134 (A and the number of layers of 100which was fabricated with the vacuum evaporationinstrument in the KUR [5]. The wavelength and itsresolution of the monochromatized beam is12.5 (A75.5%. Typical beam intensity is 1000 cpswith 1/1000 rad beam divergence.Each supermirror element of the bender is

concaved to focus the reflected beam on the center

of the next element [8]. These concaved elementsallow the bender to accept the beam with a largedivergence angle.We are now constructing the neutron reflecto-

meter and the test bench for polarized neutrons atthe monochromatic beam line to perform tests ofoptical elements in the spectrometers, and the spininterferometory system at the second beam line.

Acknowledgements

This work is supported by the inter-universityprogram for common use KURRI and JAERIfacility, and financially supported by a Grant-in-Aid for Scientific Research from the JapaneseMinistry of Education, Science and Culture (No.11480124, No. 08404014, No. 10440122).

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Fig. 4. A new neutron bender installed at C3-1-2 port of JRR-

3M reactor.

S. Tasaki et al. / Physica B 311 (2002) 102–105 105