Three new instruments are currently in varying degrees of development/construction at IPNS. One of these, the Glass, Liquid, and Amorphous Materials Diffractometer (GLAD) is the subject of a separate paper in these Proceedings, and so will not be discussed further here. The other two, a second neutron reflectometer (POSY II) and a second small-angle diffractometer (SAD II) are described briefly below. 5 refs., 1 fig., 1 tab

Crawford, R. K.

Epperson, J. E.

Felcher, G. P.

Kleb, R.

Thiyagarajan, P.

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CONF-b.iO]82-- 15DE89 007382September 29, 1988New Instruments at IPNS: POSY II and SAD IIR. K. Crawford, G. P. Felcher, R. Kleb, J. E. Epperson, and P. ThiyagarajanArgonne National LaboratoryPaper prepared for ICANS Xheld at Los AlamosOctober 3-7, 1988Work supported by U.S. Department of Energy, BES, contract No. W-31-109-ENG-38New Instruments at IPNS: POSY II and SAD IIR. K. Crawford, G. P. Felcher, R. Kleb, J. E. Epperson, and P. ThiyagarajanArgonne National LaboratoryIntroductionThree new instruments are currently in varying degrees ofdevelopment/construction at IPNS. One of these, the Glass, Liquid, andAmorphous Materials Diffractometer (GLAD) is the subject of a separate paperin these Proceedings, and so will not be discussed further here. The othertwo, a second neutron reflectometer (POSY II) and a second small-anglediffractometer (SAD II) are described briefly below.POSY IIThe polarized neutron reflectometer (POSY) has been operating at IPNS since.21984. Recently it was shown that by utilizing the large difference inscattering from H and D to measure chemical profiles, rather than the magneticprofiles for which POSY was designed, such a neutron reflectometer couldmeasure chemical density profiles with resolution unmatched by othertechniques. Users in the polymer science community indicated a real need forsuch an instrument - they could provide enough problems to saturate the timeavailable at POSY. Typical problems include interdiffusion of two similarpolymers; concentration gradients for polymers in solutions both at the surfaceand at solid boundaries; and the mixing of chemically different polymers, orpolymeric chains with a head and a tail, such as block, copolymers; all over alength scale (1 nm) not easily probed by other techniques.Work supported by U.S. Department of Energy, BES, contract No. W-31-109-ENG-38To help satisfy such a greatly expanded user community, and since POSY hadbeen conceived and optimized for magnetic studies and, consequently, is notbest suited for polymer work, it was decided to split the neutron beam at beamline C2, which serviced only POSY, for use by both POSY and a new instrument(POSY II) dedicated totally to polymer research. Figure 1 shows thearrangement of the two instruments on the two beams brought out through thesingle bulk shielding penetration. (Beam penetrations in the IPNS bulkshielding are 30-35 cm wide, allowing these two narrow beams to achieve anangular separation of 3.7° within the same penetration.) This constructioneffort, with half of its cost being financed by IBM, is well along, and POSY IIand the rebuilt POSY are scheduled for commissioning in Fall 1988. Table Igives some parameters of the two instruments.In the new arrangement the original POSY will be restructured, implementingmodifications previously tested. The sample will not view the direct beam,since this will be doubly reflected by a pair of polarizing supermirrors.These mirrors are made of layers of cobalt and titanium of progressivethickness on an anti-reflecting backing of gadolinium, and were prepared by Dr.0. Scharpf at Grenoble on accurately polished borosilicate glass. The pair ofmirrors are part of a newly developed "resonant spin flipper" (Fig. 1) toreject neutrons whose wavelength does not correspond to to their nominal time-of-flight, thus sharpening the resolution and improving the background. Thisdevice consists of a supermirror spin polarizer, a Drabkin flipper resonantlyscanned to flip spins only of the correct wavelength for the current flighttime, and a supermirror spin analyzer to reject the unflipped spins. POSYwill still use its original microchannel-plate detector. Appropriately placedcollimators (not shown in the Figure) in the external portion of the incidentbeam will allow its tailoring to the size of the sample. Samples of magnetic2interest are expected to exhibit surfaces of the order of 1-2 cm .The design of POSY II reflects the requirements for polymer work. Thesamples used here have surfaces of several tens of cm . The reflectivity willbe measured over a vide range of momentum transfers. Because of the increasedsurface size, slightly relaxed collimation, and absence of .polarizationanalyzers, POSY II should have data rates several times those at POSY.Particular attention has been given to making this instrument easy tooperate by external users. Thus, for instance, the counter consists of alinear-position-sensitive detector fixed in a preselected location. Thiscounter has an active area of 5 cm x 20 cm and a one-dimensional positionresolution (along the long axis) of 2 mm (Ordela, Inc., Oak Ridge). Thedetector is of the gas-proportional type, filled with a He-CF^ mixture, givingan efficiency of 81% at 3 A.For this instrument as well, the sample will not be in the direct beam fromthe source. However, it was desired to eliminate the transmission filter usedin the past (beryllium cooled by liquid nitrogen) to avoid the small-anglescattering from the sintered beryllium and to extend the dynamical range ofneutrons to span the maximum neutron flux from the cold moderator (~ 3 A).This is accomplished by deflecting the long-wavelength portion of the incidentbeam to the sample position by a stacked sst of mirrors ("soller mirrors").Each of the mirrors will consist of a disk of silicon 10 cm in diameter and 0.4COmm thick. Both faces are covered with 800 A of Ni. The whole soller mirrorconsists of 24 such disks, enclosed between flat reference plates, and set atan angle with the incident beam to ensure total reflection for X = 3 A.The portions of the POSY and POSY II flight paths vithin the collimation inthe bulk shielding will be evacuated, while most portions of the externalflight paths for both instruments will be He-filled. Both instruments will beequipped with heavy-duty sample goniometers capable of supporting a variety ofsample mountings and environments.SAD IIThe Small Angle Diffractometer (SAD) has been operated at IPNS since 1982,and has been in essentially its current configuration since 1984. Thisinstrument has been severely oversubscribed for several years, with proposalsexceeding operating time by a ZactOL of two to three. Because of this, andbecause experience led us to believe we could now build a significantlyimproved instrument, consideration of a second SANS instrument at IPNS wasbegun in early 1986. Development of the new instrument (currently designatedas SAD II) is underway, and the assessment of the final parameters should becompleted in 1989. Many of the features of the new instrument will be, atleast initially , similar to those of SAD. However the new instrument isintended to have a much better detector system and more varied collimationoptions, leading to greater Q range, and is also intended to have a moreconvenient sample environment than does the present SAD.In particular, the instrument will occupy the C3 beamline, where it willview the same solid CH (or liquid H~) moderator viewed by SAD. The moderator-to-sample distance will be 7.5 m, as in SAD, since this is roughly the minimumdistance which allows room for filters, shielding, interchangeable collimation,etc. The instrument will be, at least initially, a filtered-direct -beaminstrument in which the moderator, sample, and detector are in a direct line.A filter (MgO as at SAD, or oriented single-crystal sapphire, probably at 77 K.)in the incident beamline will reduce the fast neutron flux by roughly twoorders of magnitude, while having a transmission of greater than 50% for theneutrons of interest (- I-1 A), and a second filter of oriented single-crystalbismuth will be used to attenuate the gamma rays from the source if necessary.An alternate option, a "beam-bender" which would "bend" the trajectories of thelong-wavelength neutrons (greater than - 1.5 A) away from the main beam of fastneutrons and gamma rays, was considered, but does not appear to permittransmission to wavelengths as short as those allowed by the filters. Theapertures which define the beam from the moderator to the face of the bulk-shielding monolith for the initial filtered-direct-beam instrument have nowbeen installed, and the necessary modifications have been made in the beam gateto accommodate the filter(s).A two-dimensional position-sensitive gas-proportional-counter detector ofnominally 40 cm x 40 cm active area with 4 mm x 4 mm resolution (Ordela, Inc.,Oak Ridge) was purchased for this instrument and was received in Spring, 1988.This detector is filled with He and CF to provide the desired positionresolution and neutron efficiency (61% at 2 A) while having a low gammasensitivity. (For comparison, the SAD detector has an active area of 17 cm x17 cm and a resolution of - 4 mm x 4 mm.) The position and time encodingelectronics for this detector, as well as the rest of the data acquisitionsystem for this instrument, are similar to those on SAD and have already beenacquired. Data acquisition will initially utilize a PDP-11/24 computer, which6will be replaced with a microVax when the instrument development is furtheralong.The remainder of the development of the new instrument is expected toproceed in stages. Temporary beam-transport, collimation, and sample-environment components have been fabricated and will be installed in Fall,1988. These will permit detailed testing of the new detector, and will alsoserve as a testbed for the development of other instrument components. Ofprimary importance in this regard are shielding to reduce the background tosatisfactory levels, the filters discussed above, and the angular collimationsystem for the incident beam. Design and construction of the final instrumentwill depend in part on the outcome of the development and testing efforts withthis temporary version.To achieve the necessary angular collimation in the incident beam whileutilizing the full source size (-10 cm diameter) and a reasonable sample size(- 1 cm diameter) requires the use of converging multiple-aperture collimation.If the collimation channels are all focused to the same point on the detector,4.then a large sample size will not affect Q . or the Q-resolution, even if thenunsample-to-detector distance is short. In SAD such focusing multiple-aperturecollimation is done vith crossed converging soller collimators which define -400 converging beam channels with essentially no "dead" space between them.This entire collimator system occupies only a distance of - 60 cm along theincident flight path, while providing angular collimation of 0.003 radiansFWHM. This system has worked quite well, producing resolutions in goodagreement with the calculated values, and the collimators presently in use haveproduced a cleanly collimated beam. These collimators, and improvements neededin such soller collimator systems, are discussed elsewhere in theseProceedings. One of the uses of the initirl temporary flight path will be totest (and develop if necessary) satisfactory collimators to provide eventighter resolution than this.The practical Q . with the present set of collimators is ~ 0.005 A~ .minVith the detector spatial resolution noted above, Q . and Q-resolution down to_1 min- 0.002 A can be achieved with the 1.5 m sample-to-detector distance simplyby installing different incident beam collimators with the desired angulardivergence in each channel. If satisfactory collimators can be produced,several such collimators will be provided in a "cassette" arrangement to permiteasv changes in resolution and Q . . A further decrease in Q . and resolution° m m minwill require moving the detector further from the sample, with a Q-resolutionof - 0.001 A' achievable with this detector resolution and at a sample-to-detector distance of 3-3.5 m, if the incident beam collimation can be made tomatch. (With the moderator and path lengths being considered, the source pulsewidth is too short to contribute appreciably to the resolution for the casesconsidered here.) The count rate will fall at least as fast as the square ofthe resolution, so it will not be desirable to try to push the resolution thisfar for most cases. Since the multiple-aperture collimation must focus on thedetector a different collimator is needed for each sample-to-detector distance,so there is no point in having a continuously variable sample-to-detectordistance in this instrument.With a filter in the beam, the minimum usable wavelength will be ~ 1 A.With the detector at 1.5 m from the sample and centered on the beam, this willyield usable intensity at a Q of - 0.6 A~ . With the detector operated withJ max vthe beam near the edge of its active area, this could be increased to Q ~ 1_« maxA , at some cost in intensity at the smaller Q values. However in some casesit is desirable to have Q even larger than this, and in most cases it isdesirable to be able to include measurements over a wide range of wavelengthsin the high-Q data, so the ability to cover even higher scattering angles willbe incorporated into the instrument. This will be done either by providing ameans of rotating the detector about the sample position or by supplementingthe main two-dimensional detector by an array of linear position-sensitivedetectors (LPSDs) at higher angles, using the technology for position- andtime-encoding of such detectors which has already been developed for GLAD.Roughly 40 LPSDs at 1.5 m from the sample would extend continuous angularcoverage out to a scattering angle of 30° and a Q - 1.6 A~ .maxEven without the new enriched-uranium target just installed at IPNS in1988, the new instrument would have a much higher data rate at high Q than doesthe present SAD (up to a factor of 10 increase at the highest Q presentlyobtained on the SAD), and a Q _ more than four times that of the present SAD.Under the same conditions, at low Q the new instrument would have a data ratecomparable to that of the present SAD at the same resolution. By tighteningthe collimation, or by changing both the collimation and detector position,8Q . down to 0.001-0.002 A~ should be achievable, with a corresponding datam mrate reduction. However the factor of three increase in source intensityexpected from the enriched-uranium carget should mean that the time requiredfor such higher-resolution measurements will not bs unreasonable in many cases.Source-sample distance (cm)Sample-detector distance (cm)Detector horizontal range (cm)Detector resolution (mm)Wave-vector range (A )Wave-vector resolution (A~ FWHM)2Beam size at moderator (cm )2Beam size at bulk, shield exit (cm )9Table020I.0-0.6.54POSY831602.50.5- 0..003x 10x 507.3.8020POSY II623178202.0 - 0.25-0.003.6 x 10.3.84 x 6.410REFERENCES1. R. K. Crawford, D. L. Price, J. R. Haumann, R. Kleb, D. G. Montague, J.M. Carpenter, S. Susman, and R. J. Dejus, these Proceedings.2. G. P. Felcher, R. 0. Hilleke, R. K. Crawford, J. Haumann, R. Kleb, andG. Ostrovski. Rev. Sci. Instrum. 58, 609-619 (1987).3. T. P. Russell, A. Karim, A. Mansour, and G. P. Felcher, Macromolecules21, 1890 (1988).4. A. C. Nunes. Nucl. Instrum. Methods 119, 291-293 (1974).5. R. K. Crawford, J. E. Epperson, and P. Thiyagarajan, these Proceedings.11FIGURE CAPTIONSFig 1. The reflectometer complex at the IPNS beam line C2. The heavy-dutysample goniometer on POSY can accommodate an electromagnet, a cryostat,or an ultrahigh vacuum chamber, while that on POSY II is a highprecision Huber goniometer to support a temperature-controlled polymersample cell.RESONANTSPIN-FLIPPERNON-ADIABATICSPIN-FLIPPERMICRDCHANNEL-PLATEPD^ITION-SENSITIVEDE7ECTDRPDLARIZINGMIRRORSAMPLEGONIDMETERSAMPLEGDNIDMETERBULKSHIELDING VARIABLECDLLIMATDRGASPOSITION-SENSITIVEDETECTORPDSYPDSY I IDNE METER

New Instruments at IPNS: POSY II and SAD II

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New Instruments at IPNS: POSY II and SAD II

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