ELSEVIER Physica B 213&214119951 898 900

PHYSICA;

Focussed beam reflectometer for solid and liquid surfaces

F. Mezei*, R. Golub, F. Klose, H. Toews

Hahn-Meimer-lnstitut/BENSC, Glienieker Str. 100. D-14109 Berlin, Germany

Abstract

Neutron reflectometry from liquid surfaces implies reflection in a vertical plane at variable angles and/or variable wavelengths. The TOF wavelength-scan method conventionally used for this kind of geometry offers many technical advantages, but it requires an end position on a beam tube/guide delivering a white beam. Due to space restrictions the reflectometer V6 at BENSC had to be installed using a crystal monochromator. In order to allow variable angle reflection on a horizontal sample surface, a 10-cm high curved monochromator crystal assembly is used. The polarized monochromatic beam is focussed in the vertical plane to the sample at some 3 m distance (angular range of grazing incidence ~ 1.9, i.e. a q range of 0 0.09 A- 1 at 4.75 A wavelength). The instrument can be used in different modes of operation: (la) Fixed collimated narrow beam: the beam is defined by two 0.1 1 mm wide horizontal slits between monochromator and sample. (lb) Fixed collimated, wide beam: for small area samples the slit close to the sample is opened up, so that the beam width is determined by the projection of the sample surface. (2) Collimated beam with the beam direction scanned via moving slits: for liquid surfaces. (3) Focussed, fan-like incoming beam and simultaneous observation of the specular reflections with multidetector bank. Modes (la), (lb) and (2) allow for the investigation of diffuse scattering effects by the use of the multidetector, too.

In recent years neutron reftectometry has become ar- guably the fastest growing field in neutron scattering. After the pioneering work of Gian Felcher [1] the time- of-flight (TOF) method has become the standard ap- proach. The great advantages of this method, which is based on scanning the wavelength at a fixed incident angle, is that there is no change of beam geometry in- volved in the scan and consequently the non-orientable liquid surfaces can also be investigated without any change of the experimental procedure. The TOF tech- nique is ideally suited for pulsed neutron sources, but it can also be advantageously implemented on continuous sources using a disc chopper [2]. Note in passing that this is just another example of the benefits of using TOF for neutron wavelength definition on continuous sources

*Corresponding author.

instead of crystal monochromators. The full capacities of the TOF approach in general are far from having fully been exploited or even having been fully explored.

The reflectometer at the BER II reactor at the Berlin Neutron Scattering Centre to be described here, however, is of the alternative crystal-monochromator, fixed- wavelength type. The reason for this choice was entirely related to the limited beam availability. At the time the decision was made to extend the instrument park at BENSC with a neutron reflectometer, the only remain- ing beam position was a place for a monochromator crystal and no end of guide position. Nevertheless, the design goals included maximum flexibility in order to encompass all applications of neutron reflectometry in- cluding liquid surface studies. Thus we had to face the problem of strongly variable beam geometries during the scans and actually a whole set of different geometries has been adopted in order to provide for the various types of

0921-4526/95/$09.50 c 1995 Elsevier Science B.V. All rights reserved SSDI 0921-4526(95)0(1317-7

F. Mezei et al. / Physica B 213&214 (1995) 898 900 899

experimental conditions. As a result we obtained flux and signal/noise ratios competitive or superior to TOF reflec- tometers, but the ultimate reliability of variable geometry scans has not yet been fully established for all variants in a sufficient number of experiments. The instrument was designed in 1985 86 and went into operation with a pro- visional beam line providing some 10% of the final flux simultaneously with the start up of the HMI reactor in 1991. The final beam-splitter polarizing guide NL4 was installed in early 1994 [4] and instrument operation restarted, this time with the designed full flux, in February 1994.

The beam impinging on a monochromator assembly consisting of 5 strips of 2 x 10cm 1.5 ° grade Union- Carbide pyrolitic graphite crystals has a cross-section of 6 x 10 cm and is polarized for wavelengths beyond 4 A by the beam splitter polarizer guide described in another contribution to this conference [4]. The monochromator assembly includes a remotely controlled curvature ad- justment, so that the 10-cm high beam is focussed in the vertical plane onto the sample at some 3 m distance from the monochromator. The sample is situated level with the bottom of the beam and thus the impinging beam direc- tion can be chosen in a range of 0--2 °, cf. Fig. 1. At ,;~ ± 5 the beam monochromaticity is about 3%. Second-order contamination is reduced by Be filter.

The incoming beam geometry is defined by two adjust- able computer-controlled horizontal slits between mono- chromator and sample. Three basic geometries are used the first one in two different variants: (1) Fixed, collimated incoming beam with

(a) narrow sample slit, (b) broad sample slit;

(2) Variable-angle collimated incoming beam; (3) Uncollimated focussed incoming beam.

In mode (1) the grazing angle on the solid sample surface is scanned by turning the sample goniometer

isototizin 9 Is~o~ ~irl~ r e o c ~ , Ref,ectometer V6 ~erm.r,~, ~ys~e~, ~/ hahn-kled e Inslirul Berlin

/

=L :: ..... : '21 _

~e cedGera

Fig. 1. General layout of the reflectometer V6 at HMI. The polarization-analysis option is only available for the collimated specular beam, not for the whole detector bank.

table. If the sample is rather long (10 cm or more), a nar- row 0.2 0.5 mm wide slit in front of the sample is used, so that the sample intercepts a constant beam cross-section beyond 0.2 ° grazing angle: variant (a). For small samples, typically 1 2 cm long and wide, a broad sample slit is used, so that the incoming beam area and collimation are determined by the projected dimension of the sample. This implies, for example at an incident angle of l ° and a 2-cm long sample, a 0.35 mm intercepted beam height at the sample which assures a good collimation of the beam. For reasonably flat samples (which can readily be tested by observing a laser-light spot from a laser beam directed through the same slits as the neutron in order to help with sample alignment) the intercepted beam area is proportional to the grazing incident angle of the neutron beam, i.e. at higher angles, where the reflectivity is small, we have a higher incoming intensity. Normalization of

I meosurement geometries

solid somples

j j f J J ~

{quid somptes sht syslem 1 s~it system 2

monochrome for

"open slits" geometry

Fig. 2. Various operational modes of the reflectometer (from above) : (1) fixed collimated incoming beam with narrow or broad slit before the sample, (2) adjustable direction collimated beam for liquid surfaces, (3) focussed incoming beam without collimation for simultaneous observation of specular reflections.

900 F. Mezei et al. / Physica B 213&214 (1995) 898 900

100 : ~ . ] . . . . .

~ [30 A La / 30 A FeJ x 33 J ~ 40 A Cr (buffer layer) I

_ x\,,< .......... ...._./ ! %'- 2' ~"

10.t ',.o_..,~,_,_,. / down\ .-, k

10 -s ~/"~" 0.0 014 0!8 1 1 2 ' 1 ! 6 210 214

0 [o1 Fig. 3. Sample polarized-neutron reflection profile [4] deter- mined by the collimated, wide-sample slit mode of operation.

countstnour IO ~

10 5

10 ~

3

F

0 i

i i

o ~ .

0 o

QxIO:/A ~ ] 0 J , J , ± ~ - ~ , , , I J , J _ _ ~ ~ ± ! ~ . = I , , ; L I , , , , , .

-10 ~l ~ 4 2 0 2 4 6 g 10

Fig. 4. Sample diffuse-scattering spectrum Iopen symbols) com- pared to the straight beam profile (filled symbols) observed by simultaneously using several detectors of the bank.

the measured counting rates to beam area was found to work reliably, e.g. constant reflectivity is obtained in the total reflection regime.

In mode (2), the incoming beam collimated slits are moved in the vertical direction so that the sample is illuminated with a beam of arbitrarily chosen direction within the 0 2 range with respect to the horizontal, as determined by the height of the monochromator and the monochromator-sample distance. The incoming beam intensity depends on the area of the monochromator momentari ly looked at, e.g. it shows minima at the gap between two monochromator strips. The slit width can be kept constant during vertical displacement to the precision of the used linear encoders, i.e. about 0.02 mm. The beam intensity variations and reproducibility during the beam direction scan can be checked by observing the direct beam without a sample. This kind of operation is appropriate for liquid surfaces.

In mode (3) all beam directions between horizontal and 2 .0 descending angle impinge simultaneously on the sample. The impact angle is thus defined by the position of detection of the reflected neutron, assuming specular reflection. A multidetector, for the time being consisting of 50 x 5 mm wide (elliptical) individual detectors, pro- vides a resolution of 0.08 ° at the largest sample-detector distance (about 3 m). An improved 0.02 ° angular resolu- tion can be achieved using a Cd-grid in front of the detectors with 1 mm slits at distances corresponding to that of the centres of the detectors. This method can only be used with samples for which the non-specular reflec- tion i.e. diffuse scattering is negligible. This means surfaces with very little roughness, such as liquids, or multilayer structures where the specular reflectivity in the angular range of interest remains substantial (some 10% or above). Figs. 3 and 4 show sample spectra for polariz- ed neutron (specular) reflectometry and determination of diffuse scattering next to the specular reflection on a high quality surface, respectively.

References

[1] G.P. Felcher, K.E. Gray, R.T, Kampwirth and M.B. Brodky, Physica B 136 (1986) 59.

[2] B. Farnoux, Rapport d'Aetivit6, Laboratoire Leon Brillouin, CEA (1987 88).

[3] Th. Krist, C. Pappas, R Golub and F. Mezei, Physica B 213&214 (1995) 939.

[4] F. Klose et. al., to be published.