- Email: [email protected]
Conceptional design of a flexible neutron reflectometer at ‘SINQ’
PHYSICA[ ELSEVIER
Physica B 221 (1996) 507 513
Conceptional design of a flexible neutron reflectometer at 'SINQ' D. Clemens Laboratorium J'ur Neutronenstreuung, ETH Ziirich & Paul Scherrer lnstitut, CH-5232 Villigen PSI. Switzerland
Abstract
A new neutron reflectometer will be installed on a cold neutron guide at the quasi-continuous SINQ spallation neutron source. This paper describes the concept of this flexible instrument and thereby gives a first presentation to the user community. The reflectometer works in the white beam time-of-flight mode with a vertical scattering geometry. If needed, it can be altered into a two-axis reflectometer using a multilayer monochromator. In order to adapt the angular and temporal resolution to the demands of the experiment, it allows for the variation of the temporal width of the entrance window of the chopper, the preset width of the time channels of the detector, and the width of the collimation slits. Additionally, one can adjust the illumination of the sample by setting the sample table and the detector to an appropriate distance. Two geometrically separated beam paths hit the tilting axis of the sample table, to which they are guided by means of deflection mirrors. This allows for the simultaneous data acquisition in adjoining q-ranges. In a horizontal sample position, reflectometry with liquid samples becomes practicable, too. For polarized neutron reflectometry, the deflection unit can be equipped with remanent supermirrors. These polarizing FeCoV Ti multilayers make it superfluous to use a spin flipper. A I T electromagnet installed on the sample manipulation table and polarization analyzers complete the polarized reflectometry setup. Alternatively, an x y-detector and single detectors will be available. The neutron reflectometer at SINQ/PSI can be set up to a low resolution as well as a high resolution instrument that uses either unpolarized or polarized neutrons and accesses a wide q-range. It will be operational by 1996/1997 as a users' instrument.
1. Introduction
The physics and properties in thin films and multilayers as well as the processes in and along lipid membranes cannot be understood without a deep insight into the structure and dynamics of surfaces and interfaces. In the past years, X-ray and neutron reflectometry [-1] have been established as nondestructive microscopic probes for the investigation of stratified structures and hidden interfaces. It can be noticed that there is an increasing demand for access to neutron reflectometers by scientists from a variety of research fields. Consequently, the scheduled beam-time at neutron scattering centers providing
reflectometers to a user community is heavily overbooked. Therefore, the purpose of a newly constructed instrument at the Swiss spallation neutron source SINQ [2] is to supply users from Switzerland and abroad with a modern and precise facility, which is flexible enough to meet most of the requirements that are connected with their specific reflection experiment. Neutrons have unique interactions with matter. Their scattering is dominantly with the potential of the nuclei and with atomic magnetic moments. Specular neutron reflection probes the atomic and magnetic scattering length densities P~c = Pat (b _+ p) (Pat is the atom density, b the coherent scattering length, and p the magnetic
0921-4526/96/$15.00 ( 1996 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 5 1 0 0 9 7 2 - 8
508
D. Clemens/Phvsica B 221 (1996)507 513
scattering length) perpendicular to the sample surface. It can be very sensitive to electrochemical processes at interfaces. Surface and interface magnetism can be studied, as well. Moreover, the contrast variation method adds topics in structural biology to this list, for example processes in membranes, and improves the applicability on polymer science. In the framework of a new theoretical approach [3] the diffuse reflectivity can be analyzed. A wide spectrum for future studies will profit from this unique probe. Growth, wetting, absorption and adhesion, (inter-)diffusion, corrosion, nanocrystals, surface magnetism, magnetic excitations, thin film superconductivity, and dynamical processes taking place at the interface should be mentioned as research fields in which this method will be of increasing importance [4]. Many of these are relevant for industrial applications. The basis of this concept for a Swiss neutron reflectometer is to allow experiments in the majority of the domains mentioned above without being restricted to large solid samples. Therefore, the instrumental resolution will be adaptable to the demands of the individual problem. It must be emphasized that this concept is subject to internal discussions in order to find a solution that harmonizes with the financial and scientific situation at the PSI and in Switzerland. It noticeable that with the existing TOPSI test spectrometer a two-axis diffractometer will be run at SINQ, which has been used mainly as a reflectometer at the decommisioned SAPHIR reactor. This instrument will be moved to a beam port in the guide hall. TOPSI is usable with unpolarized neutrons in a horizontal scattering geometry. The take-off angles at the HOP-graphite monochromator can be set to supply wavelengths 2 between 0.16 and 0.64 nm. With this 0-2~-reflectometer an appropriate but less flexible complement to the proposed TOF reflectometer exists already.
2. Instrumental considerations
After the decommisioning of the SAPH1R research reactor in January 1994, the SINQ [2, 5], which is under construction at the Paul Scherrer Institut (PSI) will be the only source for research with neutrons in Switzerland for the near future. Though it will produce neutrons by spallation, SINQ has to be considered as a continuous source, because the acceleator has duty cycles of 51 MHz. Seven neutron guides, mainly equipped with Ni Ti-based supermirrors [6, 7], accept neutrons that are moderated in a D2-cold source. The end position of one guide (1RNR17) is solely assigned to the neutron reflectometer which shall be constructed as part of the first instrumentation program.
1.0 E+9
1.0 E+8
1RNR17
1.0 E+7
1.0 E+6
~)
1
2
3
4
5
6
7
8
9
10 l i
12 13 14 15
wavelength[~]
Fig. I. Comparison of the flux at the cold neutron guides HI 5 (ILL} and at 1RNR17 (for a proton current of 1 mA impinging on a Pb target, calculated by P. Allenspach). The guide has a cross-section of 50 x 50 mm 2. The guide walls reflect up to two times the critical angle of natural nickel (~9m = mOc (Ni), m = 2) with a reflectivity of 86% and more. The guide geometry leads to a nominal cut off wavelength 2c = 0.19 nm. A ray tracing calculation which takes into account the geometry of the cold source as well as the guide [8] shows that the cold flux (per mA proton current) at IRNR17 will be rather high. A comparison to values measured for H15 at the I.L.L. in Grenoble is given in Fig. 1. The two typical set-ups for neutron reflectometers are denoted as white beam time-of-flight (TOF) mode and fixed wavelength or two-axis mode. The former is usually implemented in reflectometers at spallation sources ]-9-11] and measures the reflectivity in a fixed geometry as a function of neutron velocity (R =R(v)) or wavelength (R = R(2)), respectively, whereas the latter mode is typical for instruments at continuous neutron sources [12, 13] and measures the reflectivity as a function of the angle of incidence R = R(O0. The fixed geometry in the white beam TOE mode offers advantages over the fixed wavelength mode. They are closely connected to the resolution terms. The resolution in q for a TOF reflectometric experiment is given by (Aq) 2
(At,oh) 2 q- (Atpu,) 2
q2
t2
(A0) 2
9~ '
where A0 is the geometrical resolution for the incident beam direction, Atpul is the pulse width, and AGh is the time channel window for neutrons passing the distance L from the pulse source (spallation target or chopper) to the detector in a time t. Each of the contributions can be chosen to give almost equal values. The difference in Atpul/t for neutrons of different velocity can be balanced by adjusting the time channels appropriately, in order to get a constant At/t. A8//8 is constant, due to the fixed geometry. In the two-axis mode the resolution for
509
D. Clemens/ Physica B 221 (1996)507 513
specular reflection is given by
1.0 E+7 ~ - n e u t r o n rate on sample[n/s]
(Aq) 2 q2
-
(A2) 2 (Au0)2 -+ }2 ~2 '
1 0 E+6
-u.- signal [n/16500s]
1.0 E+5 nm^-2)
l 0 E+4,
where A,;. is dominated by the mosaicity of the monochromator, and A0/'0, again, depends on the geometry. Hence, for fixed collimation slit widths the relative angular resolution changes during the experiment, but can be held constant by using programmable slits. This gives a constant 'footprint' on the sample as it is guaranteed in the T O F mode in any case. For T O F the resolution terms do not change during the scan. They are easy to match, independently. In the fixed wavelength mode the matching of the resolution terms depends on the mosaicity of the monochromator. This is coupled to the beam collimation and therefore the flexibility in the adjustment of the instrumental resolution is restricted [9]. It is pleasing that preliminary results can be acquired in the T O F mode due to the fact that every measured burst contains information about the reflectivity. F o r a chosen resolution the instrument can be adapted to the area of interest on the sample by changing the length of the primary and secondary arm. In most cases, this is not possible for very low angles of incidence 3~. T O F reflectometry usually operates at ~9~~ 0 . 5 or higher. Therefore, and contrary to the fixed wavelength mode, its geometry is fully adaptable to the experimental needs. Predetermined by the measurement pinciple there are no or only few sample movements required except for the initial orientation, This minimizes possible positioning errors. Despite the conveniences of T O F reflectometry, the substantial drawback of the lacking beam brightness for an instrument installed on a continuous source has to be considered. For that reason, a full reflection experiment has been simulated, based on flux calculations for the guide 1RNR17. Taken the reflection spectrum for the surface of an average material with a scattering length density p~¢=2.5x10 4nm-2, (p~(Si)_~2.1xl0 4 nm z, p,,JNi) ~ 9.4 x 10 -4 nm 2), 1 calculated the performance of a T O F reflectometer for Aq/q = 2%, 6.5% and 10% (at q = 0.5 nm -~) with negligible At¢h/t and under ideal geometrical conditions. The results for the medium resolution simulation are presented in Fig. 2. It shows the reflection spectrum of the material, the neutron rate at the sample as it is predetermined by the source and the installations in front of the sample and in a third graph the resulting signal at the detector. The graph for the signal that one would detect in a measuring time of 16 500 s can be translated into a plot showing the time one has to wait until 10 counts are run up in the detector. Fig. 3 compiles corresponding plots for three differently chosen q-resolutions.
1.0 E+3 ' 1.0 E+2 ,
1.0 E+I , 1.0 E+0 030
0.00
Fig. 0.7, nm nm
0.60
0.90
] 20
1.50
18 0
2. Simulated TOF reflection experiment at 1RNR17: 0~ = 10m flight path (A0i/0 = 4.6%; Aq/q = 5.8% at q = 0.25 ~; Aq/q - 6.5% at q - 0.5 nm-~; Aq/q = 8% at q = 1.0 ' ). /
1.0 E + 6 -7.
1.0 E + 5
t~
1.0 E ÷ 4
,
.//,~
E 1.0 E + 3
A~'/'O=
1.0 E + 2
1 0 E-1 i.0E-2 0.0
0.2
0.4
I
i
I
I
I
0.6
0.8
1.0
1.2
1.4
1.6
q into "~]
Fig. 3, Calculated measurement time, needed for a signal of 10 counts at q in a simulated TOF reflection experiment at IRNR17: ~9~= 0.7 , flight path L = 10m.
In this ideal case, the reflection spectrum can be measured in less than half an hour at only one preset angle up to q = 1 nm-~. For the presented example of a p~c = 2.5 × 10 4 nm 2 material this corresponds to a reflectivity range extending to R = 10 5. The comparison with a similar calculation gives a factor 10 of gain in time for a 0-20-experiment, with the optimum wavelength (). = 0.4 nm) picked out of the source spectrum and a m o n o c h r o m a t o r with 100% reflectivity. In the range above q = 1 n m - 1 the graph implies that the presumed duration of the experiment is enormously long if one wants to attain a dynamic range of six decades. In a real T O F reflection experiment, it is not feasible to record the spectrum over the whole q-range at only one angular setting, as it has been done for this simulation. Instead of doing so, running a second measurement at higher 0~ increases the accessible q-range without leading to intolerable recording times.
510
D. Clemens / Physiea B 221 (1996) 50~513
multidetector
chopper deflecting mirrors
sample
deflecting mirrors (polarizers)
(analyzers)+.,,
Ilil I~
I I~ . . . . . . . . . . . .
I
.............
,
- ' - ~
~
,
- -
,I--ii, I
o,
"
_
~
,~'~
v,
t
...... m.
]
I1
| ]l ~.~!~~
neutron guide
-+
-t-J,.,,,~,.t . . . . . . . - , t , ~ . l } - - ~ - -
~ 1 ~
,,,
io+
frame overlap mirrors
"
h - - - ~ . -
-
"r
'Ill I
,...ito,-+
+
~
m := collimationslit
¢1--~1=~Ibk movements al the element
~
I L Fig. 4. Sketch of the overall instrumental layout of the proposed SINQ refiectometer. Components for the generation of magnetic fields and positioning aids are omitted.
An approach that has been pursued in this concept is to make the two measurements simultaneously by leading a second beam onto the same spot of a horizontally oriented surface, but under a higher Oi. This is done by means of deflecting mirrors (see Fig. 4). The collimation slits can be set to give the same angular resolution and beam footprint for both beam paths. In the case of spin polarized reflectometry the deflecting mirrors can be substituted by supermirror polarizers. With a second deflecting mirror positioned in the lower beam path a horizontal sample geometry can be achieved. This is an important option for measurements on liquids and can be helpful in the operation of sample environments that have a low flexibility or a large mass. Reflected neutrons originating from either the lower or the higher beam path will reach the detection plane of an x-y-detector at distinct heights. Thus, an independent, parallel signal acquisition can be achieved. Specular and diffuse scattered neutrons may be separated, but attention has to be paid to gravitational effects on the neutrons for which corrections have to be made in the data analysis. Optionally, a spin analyzer can be installed on the secondary arm of the instrument. Usually, a measurement on a solid sample does not require a horizontally orientated sample surface, and if no spin polarized reflectometry shall be performed, only a nonpolarizing deflecting mirror in the upper beam path is needed. The instrument must allow for easy exchange of these mirrors and an exact mirror positioning. At this point, the neutron optical coatings developed at the 'Laboratorium ffir Neutronenstreuung' are capable to fulfil the needs of such a reflectometer. Exceeding the requirements for a T O F reflectometer with a wide application field, the flexibility of the setup
mirrorswitching
npolari2fing
mirror
Fig. 5. Model for the principal arrangement of a polarizer and a second mirror in the deflecting unit.
can be exploited to take advantage of the 0-20-mode. It is possible to move the chopper out of the beam and to replace the deflecting mirrors by a multilayer monochromator. The layer sequence of it has to be specially designed to reflect a wavelength band according to the resolution requirements. Prototype monochromators of this type have been recently produced and tested by our group.
3. Description of the instrument components 3.1. Chopper
If one evaluates the calculated spectrum of the neutron guide in Fig. 1, 2%im -~ 0.1 nm to 2 = 1.3 nm can be considered as a useful wavelength band. The real spectrum may look differently for short wavelengths, because of their sensitivity to geometrical deviations from the ideal curvature in the guide. Nevertheless, compared to a conventional Ni coated guide an increased share of short wavelength neutrons can be expected. On our continuous
D. Clemens/Physica B 221 (1996) 507 513 source the chopper defines the origin of a white beam neutron burst. To afford for the flexibility of the instrument one can adjust the rotation frequency and the width of the gating window. The frequency of the chopper has to be adapted to the length of the flight path, which can be varied in order to illuminate the sample optimally at a chosen resolution and take-off angle. Thus, to prevent a second neutron burst from interference with the foregoing, the chopper frequency v can be chosen between 20 and 100 Hz. Depending on the length of the instrument, i.e. the chopper~zletector distance, and the wanted time resolution opening times top of tens to hundreds of microseconds have to be realized. With the width wsj of the chopper window which is situated at a radius r~ to the axis of the chopper disk and the divergence ~ = c~(2) of the transmitted beam, the opening or burst time of the device is given by
t°P
4~v ~ /'sl
at full width at half maximum [-14]. This is valid for a single window in the disk. In principle, a second window can be put into a diametrical position allowing to rotate at half speed. W~l shall be varied in the range of l 20 mm to get an optimal adaption of Atpujt to the other resolution terms. A method to provide the desired variable chopper window is to rotate two identical chopper disks face to face with an adjustable phase delay.
3.2. Frame overlap mirrors' Frame overlap mirrors are put into the primary arm in order to remove the slowest portion of the neutrons in the burst which might be reached by neutrons of the succeeding burst at the locus of the detector. Undesired neutrons with 2 > 1.3 nm can easily be eliminated from the beam when they are reflected from a supermirror. In our group we produce routinely Ni-Ti-based supermirrors that give a reflectivity of up to 93% at m = 2, depending on the substrate [15]. The supermirrors can be sputtered on the two faces of a silicon wafer, so that the device transmits only 0.5% at Om. As the frame overlap mirrors shall cover a maximum beam height of 20 mm their length would exceed up to 0.5 m.
3.3. Deflecting mirrors/'polarizers An important part for the desired flexibility of the instrument is the cradle that holds the deflecting mirrors. They lead the two beams onto the same spot on the sample. At this installation, several options for the setup of the reflectometer can be chosen: (i) deflecting mirrors in both beams for samples that have to be oriented
511
horizontally on the sample table, (it) a deflecting mirror in only one beam, so that tilted samples can be measured simultaneously at two inclination angles, (iii) a multilayer monochromator with which the reflectometer can be used as 3-20-instrument (chopper removed), and (iv) if no deflecting mirror is in use, one of the beams can be used on tilted samples, the other beam may serve for the direct beam measurement. It is obvious, that polarizing mirrors can be used in each of the three first options, too. As already stated, the double beam path will provide the opportunity to perform simultaneous measurements for two inclination angles and to measure samples with horizontally oriented surfaces. High performance Ni-Tisupermirrors [16], reaching cut-off angles ,9¢ = mOc (Ni) from m = 2 up to m = 4, will be used to reflect neutrons onto the sample under the chosen angle of inclination. A single segment for the lower beam and a set of three segments for the upper beam are planned for this purpose. The upper unit works via reflections at each segment mirror. Both units will have a total length of 1 m. Additionally, to the rotation the cradles must be individually movable in height. Because the total length of the reflectometer can be changed and with it the sample position, and taken into account that the three-segment unit cannot be lifted higher than to the top of the guide, the unit has to be movable along the beam direction, too. Beginning with the deflection elements, all further instrument parts are movable on an optical bench. Using this deflection unit one can attain an angle of incidence 3~ = 4 , which is especially useful for experiments on liquids. Parallel to a first set of mirrors each cradle holds a second set of mirrors. This second set holds thin film polarizers. They can be moved into the working position by a simple shift of the cradle perpendicular to the scattering plane. Just recently we reported the development of polarizing mirrors consisting of a combination of Feo.soCoo.48Vo.oz with T i : N [17]. Their magnetic layers can be saturated in B~x~ ~ 20 roT. Furthermore, this magnetization stays remanent even in small oppositely oriented magnetic fields ( ~ 5 roT). At this stage, they perform with a flipping ratio Rf~p >~ 40. Consequently, there is no need for a flipper coil in the polarized neutron option. The other spin eigcnstate can be reflected just by saturating the polarizer in the opposite direction and setting the field back to the value of the guide field. The needed saturation field can be provided with a simple yoke equipped with electromagnet coils. Such magnets have already been tested successfully in the range of interest [18]. A device similar to the described deflection unit serves as a mount for an analyzer mirror. In the primary arm, a guidance of vertical supermirror segments reduce the losses due to the horizontal divergence of the beam.
512
D. Clemens/Physics B 221 (1996) 50~513
3.4. Collimation slits'
Also running on the optical benches, apertures inserted into the two beams are used for the angular collimation. Vertically, they shall cover a range of 0.05-20 mm with an accuracy of 0.01 mm. The proximity of the two flight paths especially at a place near to the sample ( ~ 250 mm) requires a sophisticated solution for the problem of moving the aperture edges in the neighboring beam paths. 6Li is the first choice as absorber material on the blades because it gives no contribution to the radiative background. 3.5. Sample table and sample encironment
An easy handling and exchange of the specimen can be afforded for by using special mounting plates that may be inserted onto a basis support. Proposed exchange plates are: (i) A simple flat plane with some bore-holes to fix solid samples with clamps, etc. (ii) A variable sample holder for glasses, wafers, etc., where the sample is pressed against the bottom side of a clamp. It also shall allow transmission measurements. Therefore 10 mm space has to be left open underneath the body of the sample. (iii) A plate, onto which samples can be suck by exhausting the air underneath it. Similar mounts are used in the semiconductor industry. (iv) A sample exchanger of one of these types or a combination of them. Its construction shall be made in a way, that a number of samples of a maximum size of 70× 120 mm z can be mounted and can be automatically brought into a position, that was previously stored as a result of the alignment procedure, and (v) a trough for the reception of liquids (70 x 200 x 5 ram3). Additional sample holders and compact sample environments can be produced later according to the demands of the users. Parallel to the rotation axis of the co-circle a maximum magnetic field of B = 1 T shall be applicable to wide samples (up to 150 ram). The purchase of a superconducting magnet by our laboratory, reaching B = 2 T with a high homogeneity over 25 ram, is currently under consideration. The alignment of the sample will be facilitated by a laser beam, fed in parallel to the neutron beam path at a position in front of the deflection mirrors. Furthermore, the tilting angle at the sample can be controlled by a precision inclinometer which the experimenter may attach to the sample holder. 3.6. Detectors
Two kinds of detectors are considered for the reflectometer. A position sensitive x-y-detector and alternatively a set of two (four) single detectors. The single detectors serve as standard detectors for routine specular
reflectometric measurements using both of the beams. Their number increases to four if an analyzer mirror is in operation, that divides the beam into two analyzed shares by reflecting one spin state and transmitting the other (Feo.~9Coo.ll-Si-supermirror [19]). For neutron detection in TOF-reflectometry a low background and a high thermal neutron sensitivity, combined with a good localization of the detection event is required. In cooperation with a research group at the ILL our lab is developing 3He microstrip detectors. With its sensitive area of 172 x 190 mm 2 it can cover the reflected beam under almost all measurement conditions [20]. A comparable but smaller device on the basis of a CCD camera coupled to a neutron sensitive luminescence film is commercially available but may cause problems with the accessible dynamic range and the time for readout. The concept of the ILL/PSI x y-detector is the ideal solution to guarantee for alignment control with neutrons, accumulating data coming from diffuse reflectivity, very good spacial resolution ( ~ 2 mm), spacial separation but simultaneous acquisition of signals coming from the two beams, low detector costs, high detection efficiency, high signal to noise ratio, and a large dynamic range. Suitable electronics are under development at PSI. 3. Z Optical bench
As the backbone for the intended flexibility of the reflectometer and basis for a high stability of the installation this part has to fulfil certain requirements. It shall have a very accurate slide track, effectively suppress vibrations, and it should be easy to process it. Blocks of synthetic granite are considered to match these items. The optical bench extends to a length of 11 m beginning at the first collimation slits. When the instrument is extended to 6 m and more, air scattering gives a substantial smear out of the signal. A way to overcome this is to pack the whole instrumentation from the frame overlap mirrors onwards into a vessel that is filled with a weak scattering atmosphere. 3.8. Software
The software requirements for this instrument are somewhat different from those useful for other instruments at SINQ. For the control of the instrument and the data treatment new software must be developed or existing software from other centers may be adapted, the agreement of the authors implied. 4. Conclusion and outlook The concept of a new S1NQ reflectometer promises to provide a modern instrument flexible enough to meet the
D. (\lemens/ Ptwsica B 221 (1996) 507 513
needs of a large spectrum of polarized a n d n o n p o l a r i z e d n e u t r o n reflectometry experiments. It will be available to Swiss a n d i n t e r n a t i o n a l users. The operation is in timeof-flight mode but it offers a n option to be converted into a m o n o c h r o m a t i c b e a m reflectometer using thin film multilayers as m o n o c h r o m a t o r s . The flexibility is achieved by means of an optical bench on which the sample m a n i p u l a t i o n table, the detector, the apertures a n d mirrors can be set independently. Individual requirements to resolution a n d sample illumination can be matched. Recent i m p r o v e m e n t s in the field of polarizing mirrors have been i m p l e m e n t e d a n d m a k e the spin flipper superfluous. The installation of an I L L / P S I x--ymicrostrip detector promises to fit well with the needs of diffuse n e u t r o n reflectometry. F u r t h e r i n s t r u m e n t a l developments at the S I N Q reflectometer should preferably flow into the enlargement of available sample environments, i.e. a cryostat, a furnace, a deposition c h a m b e r for in situ studies of growing metallic films.
References [1] See for example T.P. Russell, Mater. Sci. Rep. 5 (1990) 171. [2] G. Bauer and J.F. Crawford, The Spallation Neutron Source SINQ (information booklet), Paul Scherrer Institut, Information/PR, CH 5232 Villigen PSI {1992). [3] S.K. Sinha, E.B. Sirota, S. Garoff and H.B. Stanley. Phys. Rev. B 38 {1988) 2297. [4] G.P. Felcher, M. Fitzsimmons, E. Fullerton, W. Hamilton, Ch. Majkrzak, S. Parkin and S.K. Sinha, Surfaces and Interfaces in Technology and Science at a High Power Spallation Neutron Source, Workshop Proc., Argonne National Laboratory (1993). This promotion report for the construction of instruments at a considered 1 MW US spallation source PSS gives an excellent overview concerning the problems, that can be studied with neutron reflectometry and compares this method to other microscopic probes for surface and interface studies.
513
[5] Y. Tanaka led.), Technology of Targets and Moderators for Medium to High Power Spallation Neutron Sources, PS1-Proc. 92-03 (1992), Villigen, ISSN 1019-6447. [6] F. Mezei, Comm. Phys. 1 (1976) 81: F. Mezei and P. Dagleish, Comm. Phys. 2 11977) 41. [7] O. Elsenhans, P. B6ni, H.P. Friedli, H. Grimmer, P. Buffat, K. Leifer and I. Anderson, SPlE-Proc. Neutron Optical Devices and Appl. 1738 {1992) 130. [8] P. Allenspach, Laboratorium fiir Neutronenstreuung ETH Ziirich & Paul Scherrer Institut, CH 5232 Villigen PSI. private communication. [9] G.P. Felcher, R.O. Hilleke, R.K. Crawford, J. Haumann, R. Kleb and G. Ostrowski, Rev. Sci. Instrum. 58 t 1987) 609. [10] J. Penfold, R.C. Ward and W.G. Williams, J. Phys. E 20 (1987) 1411. [11] A. Karim, B.H. Arendt, R. Goyette, Y.Y. Huang, R. Kleb and G.P. Felcher, Physica B 173 {1991) 17. [12] M. Stamm, S. Hiittenbach and G. Reiter, Physica B 173 (1991) 11. [13] H. Toews, Ph.D. Thesis (German), Techn. Univ. Berlin (1993); T. Robertson (ed.), HM1 Report (BENSC) HMI-B 493, Habn-Meitner-lnstitut Berlin {1993). [14] P.A. Egelstaff (ed.), Thermal Neutron Scattering (Academic Press, London, 1965}. [15] P. B6ni, H.P. Friedli and R.E. Lechner, PSI Annual Report Annex F3A (1993) 145. [16] P. B6ni, D. Clemens, H.P. Friedli, H. Grimmer and H. Van Swygenhoven, Lab. fiir Neutronenstreuung Annual Progress Report LNS-175 (1994). [17] D. Clemens, P. B6ni, H.P. Friedli, R. G6ttel, C. Fermon, H. Grimmer, H. Van Swygenhoven, J. Archer, F. Klose, T. Krist, F. Mezei, P. Thomas and H. Toews, Physica B 213&214 11995) 942. [18] P. B6ni, private communication [19] T. Krist, C. Pappas, T. Keller and F. Mezei, Physica B 213&214 (1995) 939. [20] J. Schefer, P. Geltenbort, A. Oed, M. Koch, A. lsacson, N. Schlumpf and R. Thut, in: Proc. Int. Workshop on Microstrip Detectors, Legnaro, Italy 11994).
Physica B 221 (1996) 507 513
Conceptional design of a flexible neutron reflectometer at 'SINQ' D. Clemens Laboratorium J'ur Neutronenstreuung, ETH Ziirich & Paul Scherrer lnstitut, CH-5232 Villigen PSI. Switzerland
Abstract
A new neutron reflectometer will be installed on a cold neutron guide at the quasi-continuous SINQ spallation neutron source. This paper describes the concept of this flexible instrument and thereby gives a first presentation to the user community. The reflectometer works in the white beam time-of-flight mode with a vertical scattering geometry. If needed, it can be altered into a two-axis reflectometer using a multilayer monochromator. In order to adapt the angular and temporal resolution to the demands of the experiment, it allows for the variation of the temporal width of the entrance window of the chopper, the preset width of the time channels of the detector, and the width of the collimation slits. Additionally, one can adjust the illumination of the sample by setting the sample table and the detector to an appropriate distance. Two geometrically separated beam paths hit the tilting axis of the sample table, to which they are guided by means of deflection mirrors. This allows for the simultaneous data acquisition in adjoining q-ranges. In a horizontal sample position, reflectometry with liquid samples becomes practicable, too. For polarized neutron reflectometry, the deflection unit can be equipped with remanent supermirrors. These polarizing FeCoV Ti multilayers make it superfluous to use a spin flipper. A I T electromagnet installed on the sample manipulation table and polarization analyzers complete the polarized reflectometry setup. Alternatively, an x y-detector and single detectors will be available. The neutron reflectometer at SINQ/PSI can be set up to a low resolution as well as a high resolution instrument that uses either unpolarized or polarized neutrons and accesses a wide q-range. It will be operational by 1996/1997 as a users' instrument.
1. Introduction
The physics and properties in thin films and multilayers as well as the processes in and along lipid membranes cannot be understood without a deep insight into the structure and dynamics of surfaces and interfaces. In the past years, X-ray and neutron reflectometry [-1] have been established as nondestructive microscopic probes for the investigation of stratified structures and hidden interfaces. It can be noticed that there is an increasing demand for access to neutron reflectometers by scientists from a variety of research fields. Consequently, the scheduled beam-time at neutron scattering centers providing
reflectometers to a user community is heavily overbooked. Therefore, the purpose of a newly constructed instrument at the Swiss spallation neutron source SINQ [2] is to supply users from Switzerland and abroad with a modern and precise facility, which is flexible enough to meet most of the requirements that are connected with their specific reflection experiment. Neutrons have unique interactions with matter. Their scattering is dominantly with the potential of the nuclei and with atomic magnetic moments. Specular neutron reflection probes the atomic and magnetic scattering length densities P~c = Pat (b _+ p) (Pat is the atom density, b the coherent scattering length, and p the magnetic
0921-4526/96/$15.00 ( 1996 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 5 1 0 0 9 7 2 - 8
508
D. Clemens/Phvsica B 221 (1996)507 513
scattering length) perpendicular to the sample surface. It can be very sensitive to electrochemical processes at interfaces. Surface and interface magnetism can be studied, as well. Moreover, the contrast variation method adds topics in structural biology to this list, for example processes in membranes, and improves the applicability on polymer science. In the framework of a new theoretical approach [3] the diffuse reflectivity can be analyzed. A wide spectrum for future studies will profit from this unique probe. Growth, wetting, absorption and adhesion, (inter-)diffusion, corrosion, nanocrystals, surface magnetism, magnetic excitations, thin film superconductivity, and dynamical processes taking place at the interface should be mentioned as research fields in which this method will be of increasing importance [4]. Many of these are relevant for industrial applications. The basis of this concept for a Swiss neutron reflectometer is to allow experiments in the majority of the domains mentioned above without being restricted to large solid samples. Therefore, the instrumental resolution will be adaptable to the demands of the individual problem. It must be emphasized that this concept is subject to internal discussions in order to find a solution that harmonizes with the financial and scientific situation at the PSI and in Switzerland. It noticeable that with the existing TOPSI test spectrometer a two-axis diffractometer will be run at SINQ, which has been used mainly as a reflectometer at the decommisioned SAPHIR reactor. This instrument will be moved to a beam port in the guide hall. TOPSI is usable with unpolarized neutrons in a horizontal scattering geometry. The take-off angles at the HOP-graphite monochromator can be set to supply wavelengths 2 between 0.16 and 0.64 nm. With this 0-2~-reflectometer an appropriate but less flexible complement to the proposed TOF reflectometer exists already.
2. Instrumental considerations
After the decommisioning of the SAPH1R research reactor in January 1994, the SINQ [2, 5], which is under construction at the Paul Scherrer Institut (PSI) will be the only source for research with neutrons in Switzerland for the near future. Though it will produce neutrons by spallation, SINQ has to be considered as a continuous source, because the acceleator has duty cycles of 51 MHz. Seven neutron guides, mainly equipped with Ni Ti-based supermirrors [6, 7], accept neutrons that are moderated in a D2-cold source. The end position of one guide (1RNR17) is solely assigned to the neutron reflectometer which shall be constructed as part of the first instrumentation program.
1.0 E+9
1.0 E+8
1RNR17
1.0 E+7
1.0 E+6
~)
1
2
3
4
5
6
7
8
9
10 l i
12 13 14 15
wavelength[~]
Fig. I. Comparison of the flux at the cold neutron guides HI 5 (ILL} and at 1RNR17 (for a proton current of 1 mA impinging on a Pb target, calculated by P. Allenspach). The guide has a cross-section of 50 x 50 mm 2. The guide walls reflect up to two times the critical angle of natural nickel (~9m = mOc (Ni), m = 2) with a reflectivity of 86% and more. The guide geometry leads to a nominal cut off wavelength 2c = 0.19 nm. A ray tracing calculation which takes into account the geometry of the cold source as well as the guide [8] shows that the cold flux (per mA proton current) at IRNR17 will be rather high. A comparison to values measured for H15 at the I.L.L. in Grenoble is given in Fig. 1. The two typical set-ups for neutron reflectometers are denoted as white beam time-of-flight (TOF) mode and fixed wavelength or two-axis mode. The former is usually implemented in reflectometers at spallation sources ]-9-11] and measures the reflectivity in a fixed geometry as a function of neutron velocity (R =R(v)) or wavelength (R = R(2)), respectively, whereas the latter mode is typical for instruments at continuous neutron sources [12, 13] and measures the reflectivity as a function of the angle of incidence R = R(O0. The fixed geometry in the white beam TOE mode offers advantages over the fixed wavelength mode. They are closely connected to the resolution terms. The resolution in q for a TOF reflectometric experiment is given by (Aq) 2
(At,oh) 2 q- (Atpu,) 2
q2
t2
(A0) 2
9~ '
where A0 is the geometrical resolution for the incident beam direction, Atpul is the pulse width, and AGh is the time channel window for neutrons passing the distance L from the pulse source (spallation target or chopper) to the detector in a time t. Each of the contributions can be chosen to give almost equal values. The difference in Atpul/t for neutrons of different velocity can be balanced by adjusting the time channels appropriately, in order to get a constant At/t. A8//8 is constant, due to the fixed geometry. In the two-axis mode the resolution for
509
D. Clemens/ Physica B 221 (1996)507 513
specular reflection is given by
1.0 E+7 ~ - n e u t r o n rate on sample[n/s]
(Aq) 2 q2
-
(A2) 2 (Au0)2 -+ }2 ~2 '
1 0 E+6
-u.- signal [n/16500s]
1.0 E+5 nm^-2)
l 0 E+4,
where A,;. is dominated by the mosaicity of the monochromator, and A0/'0, again, depends on the geometry. Hence, for fixed collimation slit widths the relative angular resolution changes during the experiment, but can be held constant by using programmable slits. This gives a constant 'footprint' on the sample as it is guaranteed in the T O F mode in any case. For T O F the resolution terms do not change during the scan. They are easy to match, independently. In the fixed wavelength mode the matching of the resolution terms depends on the mosaicity of the monochromator. This is coupled to the beam collimation and therefore the flexibility in the adjustment of the instrumental resolution is restricted [9]. It is pleasing that preliminary results can be acquired in the T O F mode due to the fact that every measured burst contains information about the reflectivity. F o r a chosen resolution the instrument can be adapted to the area of interest on the sample by changing the length of the primary and secondary arm. In most cases, this is not possible for very low angles of incidence 3~. T O F reflectometry usually operates at ~9~~ 0 . 5 or higher. Therefore, and contrary to the fixed wavelength mode, its geometry is fully adaptable to the experimental needs. Predetermined by the measurement pinciple there are no or only few sample movements required except for the initial orientation, This minimizes possible positioning errors. Despite the conveniences of T O F reflectometry, the substantial drawback of the lacking beam brightness for an instrument installed on a continuous source has to be considered. For that reason, a full reflection experiment has been simulated, based on flux calculations for the guide 1RNR17. Taken the reflection spectrum for the surface of an average material with a scattering length density p~¢=2.5x10 4nm-2, (p~(Si)_~2.1xl0 4 nm z, p,,JNi) ~ 9.4 x 10 -4 nm 2), 1 calculated the performance of a T O F reflectometer for Aq/q = 2%, 6.5% and 10% (at q = 0.5 nm -~) with negligible At¢h/t and under ideal geometrical conditions. The results for the medium resolution simulation are presented in Fig. 2. It shows the reflection spectrum of the material, the neutron rate at the sample as it is predetermined by the source and the installations in front of the sample and in a third graph the resulting signal at the detector. The graph for the signal that one would detect in a measuring time of 16 500 s can be translated into a plot showing the time one has to wait until 10 counts are run up in the detector. Fig. 3 compiles corresponding plots for three differently chosen q-resolutions.
1.0 E+3 ' 1.0 E+2 ,
1.0 E+I , 1.0 E+0 030
0.00
Fig. 0.7, nm nm
0.60
0.90
] 20
1.50
18 0
2. Simulated TOF reflection experiment at 1RNR17: 0~ = 10m flight path (A0i/0 = 4.6%; Aq/q = 5.8% at q = 0.25 ~; Aq/q - 6.5% at q - 0.5 nm-~; Aq/q = 8% at q = 1.0 ' ). /
1.0 E + 6 -7.
1.0 E + 5
t~
1.0 E ÷ 4
,
.//,~
E 1.0 E + 3
A~'/'O=
1.0 E + 2
1 0 E-1 i.0E-2 0.0
0.2
0.4
I
i
I
I
I
0.6
0.8
1.0
1.2
1.4
1.6
q into "~]
Fig. 3, Calculated measurement time, needed for a signal of 10 counts at q in a simulated TOF reflection experiment at IRNR17: ~9~= 0.7 , flight path L = 10m.
In this ideal case, the reflection spectrum can be measured in less than half an hour at only one preset angle up to q = 1 nm-~. For the presented example of a p~c = 2.5 × 10 4 nm 2 material this corresponds to a reflectivity range extending to R = 10 5. The comparison with a similar calculation gives a factor 10 of gain in time for a 0-20-experiment, with the optimum wavelength (). = 0.4 nm) picked out of the source spectrum and a m o n o c h r o m a t o r with 100% reflectivity. In the range above q = 1 n m - 1 the graph implies that the presumed duration of the experiment is enormously long if one wants to attain a dynamic range of six decades. In a real T O F reflection experiment, it is not feasible to record the spectrum over the whole q-range at only one angular setting, as it has been done for this simulation. Instead of doing so, running a second measurement at higher 0~ increases the accessible q-range without leading to intolerable recording times.
510
D. Clemens / Physiea B 221 (1996) 50~513
multidetector
chopper deflecting mirrors
sample
deflecting mirrors (polarizers)
(analyzers)+.,,
Ilil I~
I I~ . . . . . . . . . . . .
I
.............
,
- ' - ~
~
,
- -
,I--ii, I
o,
"
_
~
,~'~
v,
t
...... m.
]
I1
| ]l ~.~!~~
neutron guide
-+
-t-J,.,,,~,.t . . . . . . . - , t , ~ . l } - - ~ - -
~ 1 ~
,,,
io+
frame overlap mirrors
"
h - - - ~ . -
-
"r
'Ill I
,...ito,-+
+
~
m := collimationslit
¢1--~1=~Ibk movements al the element
~
I L Fig. 4. Sketch of the overall instrumental layout of the proposed SINQ refiectometer. Components for the generation of magnetic fields and positioning aids are omitted.
An approach that has been pursued in this concept is to make the two measurements simultaneously by leading a second beam onto the same spot of a horizontally oriented surface, but under a higher Oi. This is done by means of deflecting mirrors (see Fig. 4). The collimation slits can be set to give the same angular resolution and beam footprint for both beam paths. In the case of spin polarized reflectometry the deflecting mirrors can be substituted by supermirror polarizers. With a second deflecting mirror positioned in the lower beam path a horizontal sample geometry can be achieved. This is an important option for measurements on liquids and can be helpful in the operation of sample environments that have a low flexibility or a large mass. Reflected neutrons originating from either the lower or the higher beam path will reach the detection plane of an x-y-detector at distinct heights. Thus, an independent, parallel signal acquisition can be achieved. Specular and diffuse scattered neutrons may be separated, but attention has to be paid to gravitational effects on the neutrons for which corrections have to be made in the data analysis. Optionally, a spin analyzer can be installed on the secondary arm of the instrument. Usually, a measurement on a solid sample does not require a horizontally orientated sample surface, and if no spin polarized reflectometry shall be performed, only a nonpolarizing deflecting mirror in the upper beam path is needed. The instrument must allow for easy exchange of these mirrors and an exact mirror positioning. At this point, the neutron optical coatings developed at the 'Laboratorium ffir Neutronenstreuung' are capable to fulfil the needs of such a reflectometer. Exceeding the requirements for a T O F reflectometer with a wide application field, the flexibility of the setup
mirrorswitching
npolari2fing
mirror
Fig. 5. Model for the principal arrangement of a polarizer and a second mirror in the deflecting unit.
can be exploited to take advantage of the 0-20-mode. It is possible to move the chopper out of the beam and to replace the deflecting mirrors by a multilayer monochromator. The layer sequence of it has to be specially designed to reflect a wavelength band according to the resolution requirements. Prototype monochromators of this type have been recently produced and tested by our group.
3. Description of the instrument components 3.1. Chopper
If one evaluates the calculated spectrum of the neutron guide in Fig. 1, 2%im -~ 0.1 nm to 2 = 1.3 nm can be considered as a useful wavelength band. The real spectrum may look differently for short wavelengths, because of their sensitivity to geometrical deviations from the ideal curvature in the guide. Nevertheless, compared to a conventional Ni coated guide an increased share of short wavelength neutrons can be expected. On our continuous
D. Clemens/Physica B 221 (1996) 507 513 source the chopper defines the origin of a white beam neutron burst. To afford for the flexibility of the instrument one can adjust the rotation frequency and the width of the gating window. The frequency of the chopper has to be adapted to the length of the flight path, which can be varied in order to illuminate the sample optimally at a chosen resolution and take-off angle. Thus, to prevent a second neutron burst from interference with the foregoing, the chopper frequency v can be chosen between 20 and 100 Hz. Depending on the length of the instrument, i.e. the chopper~zletector distance, and the wanted time resolution opening times top of tens to hundreds of microseconds have to be realized. With the width wsj of the chopper window which is situated at a radius r~ to the axis of the chopper disk and the divergence ~ = c~(2) of the transmitted beam, the opening or burst time of the device is given by
t°P
4~v ~ /'sl
at full width at half maximum [-14]. This is valid for a single window in the disk. In principle, a second window can be put into a diametrical position allowing to rotate at half speed. W~l shall be varied in the range of l 20 mm to get an optimal adaption of Atpujt to the other resolution terms. A method to provide the desired variable chopper window is to rotate two identical chopper disks face to face with an adjustable phase delay.
3.2. Frame overlap mirrors' Frame overlap mirrors are put into the primary arm in order to remove the slowest portion of the neutrons in the burst which might be reached by neutrons of the succeeding burst at the locus of the detector. Undesired neutrons with 2 > 1.3 nm can easily be eliminated from the beam when they are reflected from a supermirror. In our group we produce routinely Ni-Ti-based supermirrors that give a reflectivity of up to 93% at m = 2, depending on the substrate [15]. The supermirrors can be sputtered on the two faces of a silicon wafer, so that the device transmits only 0.5% at Om. As the frame overlap mirrors shall cover a maximum beam height of 20 mm their length would exceed up to 0.5 m.
3.3. Deflecting mirrors/'polarizers An important part for the desired flexibility of the instrument is the cradle that holds the deflecting mirrors. They lead the two beams onto the same spot on the sample. At this installation, several options for the setup of the reflectometer can be chosen: (i) deflecting mirrors in both beams for samples that have to be oriented
511
horizontally on the sample table, (it) a deflecting mirror in only one beam, so that tilted samples can be measured simultaneously at two inclination angles, (iii) a multilayer monochromator with which the reflectometer can be used as 3-20-instrument (chopper removed), and (iv) if no deflecting mirror is in use, one of the beams can be used on tilted samples, the other beam may serve for the direct beam measurement. It is obvious, that polarizing mirrors can be used in each of the three first options, too. As already stated, the double beam path will provide the opportunity to perform simultaneous measurements for two inclination angles and to measure samples with horizontally oriented surfaces. High performance Ni-Tisupermirrors [16], reaching cut-off angles ,9¢ = mOc (Ni) from m = 2 up to m = 4, will be used to reflect neutrons onto the sample under the chosen angle of inclination. A single segment for the lower beam and a set of three segments for the upper beam are planned for this purpose. The upper unit works via reflections at each segment mirror. Both units will have a total length of 1 m. Additionally, to the rotation the cradles must be individually movable in height. Because the total length of the reflectometer can be changed and with it the sample position, and taken into account that the three-segment unit cannot be lifted higher than to the top of the guide, the unit has to be movable along the beam direction, too. Beginning with the deflection elements, all further instrument parts are movable on an optical bench. Using this deflection unit one can attain an angle of incidence 3~ = 4 , which is especially useful for experiments on liquids. Parallel to a first set of mirrors each cradle holds a second set of mirrors. This second set holds thin film polarizers. They can be moved into the working position by a simple shift of the cradle perpendicular to the scattering plane. Just recently we reported the development of polarizing mirrors consisting of a combination of Feo.soCoo.48Vo.oz with T i : N [17]. Their magnetic layers can be saturated in B~x~ ~ 20 roT. Furthermore, this magnetization stays remanent even in small oppositely oriented magnetic fields ( ~ 5 roT). At this stage, they perform with a flipping ratio Rf~p >~ 40. Consequently, there is no need for a flipper coil in the polarized neutron option. The other spin eigcnstate can be reflected just by saturating the polarizer in the opposite direction and setting the field back to the value of the guide field. The needed saturation field can be provided with a simple yoke equipped with electromagnet coils. Such magnets have already been tested successfully in the range of interest [18]. A device similar to the described deflection unit serves as a mount for an analyzer mirror. In the primary arm, a guidance of vertical supermirror segments reduce the losses due to the horizontal divergence of the beam.
512
D. Clemens/Physics B 221 (1996) 50~513
3.4. Collimation slits'
Also running on the optical benches, apertures inserted into the two beams are used for the angular collimation. Vertically, they shall cover a range of 0.05-20 mm with an accuracy of 0.01 mm. The proximity of the two flight paths especially at a place near to the sample ( ~ 250 mm) requires a sophisticated solution for the problem of moving the aperture edges in the neighboring beam paths. 6Li is the first choice as absorber material on the blades because it gives no contribution to the radiative background. 3.5. Sample table and sample encironment
An easy handling and exchange of the specimen can be afforded for by using special mounting plates that may be inserted onto a basis support. Proposed exchange plates are: (i) A simple flat plane with some bore-holes to fix solid samples with clamps, etc. (ii) A variable sample holder for glasses, wafers, etc., where the sample is pressed against the bottom side of a clamp. It also shall allow transmission measurements. Therefore 10 mm space has to be left open underneath the body of the sample. (iii) A plate, onto which samples can be suck by exhausting the air underneath it. Similar mounts are used in the semiconductor industry. (iv) A sample exchanger of one of these types or a combination of them. Its construction shall be made in a way, that a number of samples of a maximum size of 70× 120 mm z can be mounted and can be automatically brought into a position, that was previously stored as a result of the alignment procedure, and (v) a trough for the reception of liquids (70 x 200 x 5 ram3). Additional sample holders and compact sample environments can be produced later according to the demands of the users. Parallel to the rotation axis of the co-circle a maximum magnetic field of B = 1 T shall be applicable to wide samples (up to 150 ram). The purchase of a superconducting magnet by our laboratory, reaching B = 2 T with a high homogeneity over 25 ram, is currently under consideration. The alignment of the sample will be facilitated by a laser beam, fed in parallel to the neutron beam path at a position in front of the deflection mirrors. Furthermore, the tilting angle at the sample can be controlled by a precision inclinometer which the experimenter may attach to the sample holder. 3.6. Detectors
Two kinds of detectors are considered for the reflectometer. A position sensitive x-y-detector and alternatively a set of two (four) single detectors. The single detectors serve as standard detectors for routine specular
reflectometric measurements using both of the beams. Their number increases to four if an analyzer mirror is in operation, that divides the beam into two analyzed shares by reflecting one spin state and transmitting the other (Feo.~9Coo.ll-Si-supermirror [19]). For neutron detection in TOF-reflectometry a low background and a high thermal neutron sensitivity, combined with a good localization of the detection event is required. In cooperation with a research group at the ILL our lab is developing 3He microstrip detectors. With its sensitive area of 172 x 190 mm 2 it can cover the reflected beam under almost all measurement conditions [20]. A comparable but smaller device on the basis of a CCD camera coupled to a neutron sensitive luminescence film is commercially available but may cause problems with the accessible dynamic range and the time for readout. The concept of the ILL/PSI x y-detector is the ideal solution to guarantee for alignment control with neutrons, accumulating data coming from diffuse reflectivity, very good spacial resolution ( ~ 2 mm), spacial separation but simultaneous acquisition of signals coming from the two beams, low detector costs, high detection efficiency, high signal to noise ratio, and a large dynamic range. Suitable electronics are under development at PSI. 3. Z Optical bench
As the backbone for the intended flexibility of the reflectometer and basis for a high stability of the installation this part has to fulfil certain requirements. It shall have a very accurate slide track, effectively suppress vibrations, and it should be easy to process it. Blocks of synthetic granite are considered to match these items. The optical bench extends to a length of 11 m beginning at the first collimation slits. When the instrument is extended to 6 m and more, air scattering gives a substantial smear out of the signal. A way to overcome this is to pack the whole instrumentation from the frame overlap mirrors onwards into a vessel that is filled with a weak scattering atmosphere. 3.8. Software
The software requirements for this instrument are somewhat different from those useful for other instruments at SINQ. For the control of the instrument and the data treatment new software must be developed or existing software from other centers may be adapted, the agreement of the authors implied. 4. Conclusion and outlook The concept of a new S1NQ reflectometer promises to provide a modern instrument flexible enough to meet the
D. (\lemens/ Ptwsica B 221 (1996) 507 513
needs of a large spectrum of polarized a n d n o n p o l a r i z e d n e u t r o n reflectometry experiments. It will be available to Swiss a n d i n t e r n a t i o n a l users. The operation is in timeof-flight mode but it offers a n option to be converted into a m o n o c h r o m a t i c b e a m reflectometer using thin film multilayers as m o n o c h r o m a t o r s . The flexibility is achieved by means of an optical bench on which the sample m a n i p u l a t i o n table, the detector, the apertures a n d mirrors can be set independently. Individual requirements to resolution a n d sample illumination can be matched. Recent i m p r o v e m e n t s in the field of polarizing mirrors have been i m p l e m e n t e d a n d m a k e the spin flipper superfluous. The installation of an I L L / P S I x--ymicrostrip detector promises to fit well with the needs of diffuse n e u t r o n reflectometry. F u r t h e r i n s t r u m e n t a l developments at the S I N Q reflectometer should preferably flow into the enlargement of available sample environments, i.e. a cryostat, a furnace, a deposition c h a m b e r for in situ studies of growing metallic films.
References [1] See for example T.P. Russell, Mater. Sci. Rep. 5 (1990) 171. [2] G. Bauer and J.F. Crawford, The Spallation Neutron Source SINQ (information booklet), Paul Scherrer Institut, Information/PR, CH 5232 Villigen PSI {1992). [3] S.K. Sinha, E.B. Sirota, S. Garoff and H.B. Stanley. Phys. Rev. B 38 {1988) 2297. [4] G.P. Felcher, M. Fitzsimmons, E. Fullerton, W. Hamilton, Ch. Majkrzak, S. Parkin and S.K. Sinha, Surfaces and Interfaces in Technology and Science at a High Power Spallation Neutron Source, Workshop Proc., Argonne National Laboratory (1993). This promotion report for the construction of instruments at a considered 1 MW US spallation source PSS gives an excellent overview concerning the problems, that can be studied with neutron reflectometry and compares this method to other microscopic probes for surface and interface studies.
513
[5] Y. Tanaka led.), Technology of Targets and Moderators for Medium to High Power Spallation Neutron Sources, PS1-Proc. 92-03 (1992), Villigen, ISSN 1019-6447. [6] F. Mezei, Comm. Phys. 1 (1976) 81: F. Mezei and P. Dagleish, Comm. Phys. 2 11977) 41. [7] O. Elsenhans, P. B6ni, H.P. Friedli, H. Grimmer, P. Buffat, K. Leifer and I. Anderson, SPlE-Proc. Neutron Optical Devices and Appl. 1738 {1992) 130. [8] P. Allenspach, Laboratorium fiir Neutronenstreuung ETH Ziirich & Paul Scherrer Institut, CH 5232 Villigen PSI. private communication. [9] G.P. Felcher, R.O. Hilleke, R.K. Crawford, J. Haumann, R. Kleb and G. Ostrowski, Rev. Sci. Instrum. 58 t 1987) 609. [10] J. Penfold, R.C. Ward and W.G. Williams, J. Phys. E 20 (1987) 1411. [11] A. Karim, B.H. Arendt, R. Goyette, Y.Y. Huang, R. Kleb and G.P. Felcher, Physica B 173 {1991) 17. [12] M. Stamm, S. Hiittenbach and G. Reiter, Physica B 173 (1991) 11. [13] H. Toews, Ph.D. Thesis (German), Techn. Univ. Berlin (1993); T. Robertson (ed.), HM1 Report (BENSC) HMI-B 493, Habn-Meitner-lnstitut Berlin {1993). [14] P.A. Egelstaff (ed.), Thermal Neutron Scattering (Academic Press, London, 1965}. [15] P. B6ni, H.P. Friedli and R.E. Lechner, PSI Annual Report Annex F3A (1993) 145. [16] P. B6ni, D. Clemens, H.P. Friedli, H. Grimmer and H. Van Swygenhoven, Lab. fiir Neutronenstreuung Annual Progress Report LNS-175 (1994). [17] D. Clemens, P. B6ni, H.P. Friedli, R. G6ttel, C. Fermon, H. Grimmer, H. Van Swygenhoven, J. Archer, F. Klose, T. Krist, F. Mezei, P. Thomas and H. Toews, Physica B 213&214 11995) 942. [18] P. B6ni, private communication [19] T. Krist, C. Pappas, T. Keller and F. Mezei, Physica B 213&214 (1995) 939. [20] J. Schefer, P. Geltenbort, A. Oed, M. Koch, A. lsacson, N. Schlumpf and R. Thut, in: Proc. Int. Workshop on Microstrip Detectors, Legnaro, Italy 11994).