The long-wavelength neutron spin-echo spectrometer IN15 at the Institut Laue-Langevin

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Physica B 241 243 (1998) 164 165

The long-wavelength neutron spin-echo spectrometer IN15 at the Institut Laue-Langevin P. S c h l e g e r a'*, B. A l e f e l d b, J.F. B a r t h e l e m y a, G . E h l e r s c, B. F a r a g o a, P. G i r a u d a, C. H a y e s a, A. K o l l m a r b, C. L a r t i g u e "'d, F. M e z e i c, D . R i c h t e r b Institut Laue-Langevin. BP 156. 38042 Grenoble Cedex 9, France b lnstitut [~ir Festk6rpetJbrschung, Forschungszentrum Jiilich, 52428 Jiilich, Germany Hahn-Meitner Institut, Glienicker Str. 100, 14109 Berlin, Germanv Laboratoire dE" Spectrombtrie Physique, BP 87, 38402 St Martin d'Heres Cedex, France

Abstract

A new ultra-high resolution neutron spin-echo spectrometer at the ILL (Grenoble) extends the time range for measurements of the intermediate scattering function S(Q,tl far beyond what has previously been possible. The spectrometer design is traditional, utilizing long neutron wavelengths (8 25 A) and large, homogeneous precession coils to reach Fourier times approaching the microsecond. The standard operational mode is currently functional, providing a Fourier time range of 0.03 180ns, and a momentum transfer between 0.01 and 0.2~, 1. The other two operational modes (neutron optical focusing and time of flightt are also discussed. ~ 1998 Elsevier Science B.V. All rights reserved.

Keywords." Neutron spin echo; Instrumentation

Neutron spin-echo (NSE) spectroscopy is an extremely effective technique for achieving very high energy resolution in a scattering experiment [1]. It is typically used to examine slow dynamical processes in condensed matter, directly measuring the intermediate scattering function S(Q,t) as a function of momentum transfer Q and Fourier time t. Until now, the upper limit in t had been about 50 ns. The new spin-echo spectrometer IN15, jointly developed by the HMI (Berlin), ILL (Grenoble), and KFA (Jfilich), is a "classical" NSE spectro*Corresponding author. [email protected].

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meter [1] specifically designed to reach very long Fourier times. To date, the instrument has been successfully tuned to reach t = 400 ns, experiments have been conducted out to 180ns, but the design allows one, in principle, to reach times as high as 900 ns. Such an extended time range opens up the possibility to investigate relaxation processes that were previously not measurable using existing instruments. As is illustrated in Ref. [1], the Fourier time at which S(Q,t) is measured for a particular instrument setting is linear in the magnetic precession field strength, and cubic in the neutron wavelength. Thus, an essential ingredient for reaching long Fourier times is to use the longest neutron

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wavelengths possible. IN15 is situated on a cold source poroviding a workable range between 2 = 8 and 25 A (Fig. 1 inset). However, the inevitable loss of intensity at long wavelengths still needs to be compensated by other design components which maximizes the accepted neutron divergence: IN15 uses a 3 2 × 3 2 c m 2 position-sensitive detector (PSD) with a polarization analyzer covering the full PSD and a 4 m long toroidal neutron-focusing mirror which focuses the divergent beam from the velocity selector to a point on the PSD [2, 3]. At high Q-resolution, this results in a large increase in intensity over the use of collimators (about a factor of 50 for a 6 Q - - 10-3 ~-1). With the focusing mirror, the smallest attainable Q is about 2 x 10 3 ~ - 1 (limited by the PSD, not the mirror itself), and closes the gap to light scattering. IN15 is equipped with a chopper system which will test the feasibility of spin echo in time-of-flight (TOF) mode. In this mode, the instrument tuning and data acquisition become significantly more complex, and are still under development. The principle is to use a broad-band (A2/2 _~ 50%) neutron pulse to simultaneously measure at a large range of Fourier times, and to use the neutron time of flight to resolve at the PSD the instantaneous energy of the pulse to within about 2%. This requires, however, a synchronous operation of flipper currents further upstream to correctly flip all the neutrons within the pulse. A successful operation of this mode would (a) enable the simultaneous measurement in a larger Fourier time range, (b) provide a better Q-resolution for the same average flux, and (c) pave the way towards implementing the spin-echo technique on pulsed neutron sources. Fig. 1 shows the normalized echo amplitude for an elastic scatterer with and without the focusing mirror. Above 15,&, it becomes favorable to insert the focusing mirror which functions for 2 ~> 17.5 A. The gradual drop of the echo amplitude is primarily due to the remaining magnetic field inhomogeneities, and the four so-called "Fresnel'" spiral coils placed near the ends of the precession magnets will be replaced by six new "Fresnels" in order to improve the echo amplitude. Currently, the upper limit for the scattering angle is about 25 ° . Above this point, stray fields near the

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Fig. I. Normalized echo anaplitude for an elastic (graphite) scatterer versus Fourier time. The circles represent the spin-echo "'resolution"for the neutron guide version at 15A (A2/2 = 15%) and 35 mm diameter sample cross section (Q -~ 0.06 A ~). The squares represent the resolution for the mirror version at 17.5A under the same conditions. These curves are Q independent up to 0.2,A, ~. The inset shows the incident flux on the sample for the normal version with A2/2 - 15%. sample position become significant and appropriate correction coils will be built in order to increase the accessible Q range. I N 15 started to be used regularly for experiments in November 1996. At the time of writing, it has been successfully used to conduct measurements out to 180ns in diverse fields such as the study of reptation dynamics in polymer systems, tube deformations in strained polymer networks, and magnetic critical scattering in Tb. The ability to conduct three-dimensional polarization analysis at long neutron wavelengths was used to examine the domain structure in amorphous ferromagnetic thin films and ferromagnetic correlations in a CuMn spin glass. Since February 1997, the ILL is accepting propositions for user experiments on INI 5.

References

[1] F. Mezei (Ed.), Neutron spin echo, Lecture Notes in Physics, vol. 128, Springer, Berlin, 1980. [2] C. Hayes et al., J. Phys. Soc. Japan 65 (Suppl. A) (1996) 312. [3] B. Alefeld et al., Physica B 234-236 (1997) 1052.