Neutron optical experiments at the IBR-2 pulsed reactor

Physica B 151 (1988) 108-112 North-Holland, Amsterdam

N E U T R O N OPTICAL EXPERIMENTS AT THE IBR-2 PULSED R E A C T O R

Yu.A. A L E X A N D R O V , B. C H A L U P A a, F. E I C H H O R N ~, J. K U L D A , P. L U K A S a, T.A. M A C H E K H I N A , R. MICHALEC% P. M I K U L A a, L.N. SEDL,/~KOVA and M. VRi~NA Joint Institute for Nuclear Research, Dubna, H.P.O. Box 79, Moscow, USSR aNuclear Physics Institute, Czechoslovak Academy of Sciences, Re~, Czechoslovakia bCentral Institute for Nuclear Research, Rossendorf, Dresden, German Dem. Rep.

At JINR, Dubna, a new DIFRAN facility has been installed on a neutron guide at the IBR-2 pulsed reactor. Results of preliminary experiments including investigations of neutron diffraction on bent perfect crystals, wavelength-dependent Pendell6sung oscillations, anomalous transmission in InSb single crystals and the first successful test of a new three-plate monolithic interferometer are reported.

1. Introduction

The availability of high intensity pulsed neutron sources in connection with the time-of-flight technique opens new possibilities in the study of neutron optical phenomena such as to work at short wavelengths of the order of 10 -2 nm and to measure simultaneously several orders of reflection at a fixed experimental geometry. At LNP JINR, Dubna, a new D I F R A N facility has been installed on a neutron guide at the IBR-2 pulsed reactor designed to investiagate dynamical diffraction on perfect and elastically deformed crystals, or systems of crystals, including neutron interferometry over a large range of neutron wavelengths at several orders of reflection. The aim of this paper is to report about the main characteristic features and results of preliminary experiments performed on it with perfect and deformed single crystals and the first successful test of a new three-plate monolithic interferometer planned to be installed on the D I F R A N facility in near future.

2. DIFRAN facility

The D I F R A N facility is a two-axis difractometer installed on a tangent channel No. 1 at the pulsed reactor IBR-2 in JINR, Dubna. The reac-

tor operates at the repetition rate of 5 pulses/s and the pulse width of 270 Ixs [1]. The schematic view of this instrument is shown in fig. 1. A pulsed beam of neutrons formed in the moderator [2] covers a flight path of 28.5 m in the vacuum neutron guide [3] equipped with a set of collimators [4] and then falls on the sample [8] installed on the goniometer GKS 100 [10] on the first axis [6] of the diffractometer. The mean density of thermal neutron flux at the sample position is 1.9 x 106 n/cm2s at a mean reactor power of 2MW. The horizontal and vertical divergences of the neutron beam formed by the collimator system are 7' and 15', respectively. On a bench [17] that can be rotated about the first axis in the range of 5-120 ° a second sample table [7] is installed with the goniometer GKS 40 [11]. Two 3He detectors [14, 15], each 9cm in diameter are used for the detection of scattered beams. The instrument is equipped with an SM-3 computer to control the experiments and collect TOF data on the scattered neutrons. For the time-of-flight measurements a channel width of 32 ixs is used. The wavelength distribution of incident neutron flux was determined from the TOF spectra of neutrons scattered on the polycrystalline vanadium sample. Fig. 2 displays the effective neutron density of the incident beam obtained from the measured data corrected for absorption and

0378-4363/88/$03.50 (~ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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Fig. 1. Schematicview of the diffractometerDIFRAN. multiple neutron scattering. A relatively high intensity of epithermal neutrons is observed in the neutron spectrum favorable for diffraction experiments at short wavelengths which are hard to perform at conventional stationary reactors.

3. Pendeli6sung effect A typical dynamical scattering phenomenon arising from the interference of the wave fields of

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the incident and diffracted beams in a perfect crystal is the Pendell6sung effect- periodic dependence of the reflected intensity on the thickness of the sample or the neutron wavelength in the Laue case [2-4]. According to the dynamical theory of diffraction [5] the integral intensity of the diffracted beam in the symmetric Laue case can be expressed as 2A

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where A = 2 d h N F h T tg 0a, d h is the interplanar spacing, N the number of elementary ceils in 3 1 c m , F h the structure factor including the scattering length and the Debye-Waller factor, T the sample thickness, and 0B the Bragg angle. The pulsed neutron beam together with the TOF technique enables one to measure such intensity oscillations simultaneously for several orders of the Bragg reflection. The measured period and phase of oscillations help to evaluate the scattering parameters as a function of the scattering vector. That may be useful for the precise determination of the scattering length and the Debye-Waller factor. In our experiment we used a perfect silicon plate 500 ~m thick, set in the symmetric Laue diffraction arrangement on the (220) lattice

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Y u . A . Alexandrov et al. / Neutron optical experiments at 1BR-2 pulsed reactor

plane. The integrated intensity of reflections (220), (440), and (660) was measured over the range of Bragg angles from 30 ° to 55 ° . The results normalized to the neutron spectrum are shown in fig. 3. Full lines are the theoretical curves obtained from (1) using the well-known scattering parameters of silicon. It shows a very good agreement between theory and experiment. Although from our experimental data the parameter A for the reflection (220) can be evaluated with a precision of 10 -3, for higher orders the precision is much worse. The situation may be improved by using a thicker crystal. But in that case serious problems arise due to thermal diffuse scattering increasing rapidly with the order of reflection. This can be seen in fig. 4, which displays the T O F spectra of reflected neutrons on thin ( T - - 0.5 mm) and thick ( T = 50 mm) single crystals.

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4. Experiments with deformed crystals

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is a characteristic value of the bending radius R dependence on the scattering parameters. Fig. 5 shows the dependence of the integrated intensity on bending radius R for the reflections (220) and (440) on a silicon plate 6 mm thick, at 0B = 28.5 °. Although the intensity saturation especially for

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high order reflections in accordance with (2) limits the gain in reflectivity, suitably bent crystals may be successfully employed as multiwave monochromators particularly in the nondispersive arrangements with perfect or deformed crystals. Another situation arises in the symmetric Laue case, when the condition u. h = 0 is fulfilled. No change in the intensity of the reflected beam is produced by bending as can be seen in fig. 6.

111

The measurement has been performed at 0a = 29 ° 58'. This is the exact setting for simultaneous three-beam refection on (222) and (153)/(131) (see fig. 7). The bending of the crystal by enlarging the reflectivity of planes (153) and (131) produces a very strong "unweganregung" effect. Neutrons being successfully double-diffracted on the planes (153) and (131) simulate the diffraction by the forbidden plane (222), fig. 6. The quadratic dependence of the intensity on the bending radius and a small divergence of the beam are the typical features of the dispersive double-reflection system [7, 8]. The neutron diffraction by absorbing perfect crystals exhibits the same specific dynamical effects such as the anomalous transmission and the dependence of the diffracted neutron intensity for a deformed crystal on the sign of the product h . u [9]. In fig. 8 are shown the results of

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Fig. 7. Schematic sketch of the bending device and the diffraction conditions in Si crystal at 0222= 29 ° 56'.

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Y u . A . Alexandrov et al. / Neutron optical experiments at IBR-2 pulsed reactor

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measurements of the intensity of neutrons diffracted by (111) and (333) planes of InSb single crystal in the symmetric Laue case as a function of the temperature gradient. An asymmetry of the curves is seen which is in accordance with the theory [9].

5. Neutron interferometer

The pulsed neutron beam together with the T O F method open new possibilities for neutron interferometry, especially for studies of the energy-depending phenomena. For such investigations we plan in the near future to install at D I F R A N a three-plate monolithic neutron interferometer L L L manufactured by Tesla Ro~nov.

Fig. 9. Experimental interference curve.

Our interferometer was tested at CINR, Rossendorf, on a double-axis diffractometer containing a heavy granite block placed on air cushions with the monochromator and the interferometer installed on it. As the monochromator we used a slightly deformed Si single crystal in a nondispersive arrangement with respect to the interferometer. The diffraction was performed on the (220) lattice planes for the neutron wavelength of 0.097 mm. A plane-parallel Si wafer, 5.2 mm thick, was used as a shifter. The intensity modulation measurement was carried out in the forward direction, where a higher value of the contrast is expected. Fig. 9 shows our first successfully obtained interference dependence. The value of the contrast is about 15%. We believe that by eliminating residual influences of torsion vibrations of the granite block and acoustic noise in the reactor hall the value of the contrast can be further improved.

References [1] V.D. Ananiev et al., Proc. Inst. Phys. Conf. Bristol and London (1983), Ser. No. 64, sect. 9, p. 497. [2] D. Sippel et al., Phys. Lett. 14 (1965) 174. [3] C.G. Shull, Phys. Rev. Lett. 21 (1968) 1585. [4] V.A. Somenkov et al., Solid State Commun. 25 (1978) 593. [5] Z.G. Pinsker, Dynamical Scattering of X-rays in Crystals (Springer, Berlin, 1978). [6] J. Kulda and P. Mikula, J. Appl. Cryst. 16 (1983) 498. [7] P. Mikula et al., Phys. Stat. Sol. (a) 60 (1980) 549 [8] M. Vr~ina et al., Acta Cryst. A 37 (1981) 459 [9] P. L u k ~ and J. Kulda, Phys. Stat. Sol. (a) 102 (1987) K57.