4 pags., 6 figs. -- QENS/WINS 2014 - 11th International Conference on Quasielastic Neutron Scattering and 6th International Workshop on Inelastic Neutron SpectrometersA cooled and mechanically retractable beryllium filter has been installed and commissioned on the low-energy OSIRIS spectrometer at ISIS. This instrument development extends the energy-transfer range of the spectrometer up to ca. 20 meV (∼ 5 THz), leading to an excellent resolution at THz frequencies and substantial gains in detected flux relative to existing capabilities on the neighbouring IRIS spectrometer. Herein, we provide a concise account of this new capability for high-resolution neutron spectroscopy in the THz domain, as well as outline a number of ongoing and potential scientific opportunities in condensed-matter physics, chemistry, and materials science.The authors gratefully acknowledge the UK Science &
Technology Facilities Council for financial support and access to
beam time at ISIS

Bermejo, Francisco Javier

Crawford, J.

Demmel, F.

Fernández-Alonso, Félix

García-Sakai, Victoria

Maxwell, D.

McPhail, D.

Mukhopadyay, S.

Pokhilchuk, K.

Skipper, N.T.

Telling, M.T.F.

English

4 pags., 6 figs. -- QENS/WINS 2014 - 11th International Conference on Quasielastic Neutron Scattering and 6th International Workshop on Inelastic Neutron SpectrometersA cooled and mechanically retractable beryllium filter has been installed and commissioned on the low-energy OSIRIS spectrometer at ISIS. This instrument development extends the energy-transfer range of the spectrometer up to ca. 20 meV (∼ 5 THz), leading to an excellent resolution at THz frequencies and substantial gains in detected flux relative to existing capabilities on the neighbouring IRIS spectrometer. Herein, we provide a concise account of this new capability for high-resolution neutron spectroscopy in the THz domain, as well as outline a number of ongoing and potential scientific opportunities in condensed-matter physics, chemistry, and materials science.The authors gratefully acknowledge the UK Science &
Technology Facilities Council for financial support and access to
beam time at ISIS

Mukhopadhyay, S.

Digital.CSIC

EPJ Web of Conferences 83, 03003 (2015)DOI: 10.1051/epjconf/20158303003c© Owned by the authors, published by EDP Sciences, 2015Opening the terahertz window on the OSIRIS spectrometerF. Demmel1,a, D. McPhail1, J. Crawford1, D. Maxwell1, K. Pokhilchuk1, V. Garcia-Sakai1, S. Mukhopadyay1,M.T.F. Telling1, F.J. Bermejo2, N.T. Skipper3 and F. Fernandez-Alonso1,3,b1SIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, UK2Instituto de Estructura de la Materia IEM-CSIC, Consejo Superior de Investigaciones Cientı́ficas, Serrano 123, Madrid28006, Spain3Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UKAbstract. A cooled and mechanically retractable beryllium filter has been installed and commissioned on the low-energyOSIRIS spectrometer at ISIS. This instrument development extends the energy-transfer range of the spectrometer up to ca.20 meV (∼ 5 THz), leading to an excellent resolution at THz frequencies and substantial gains in detected flux relative toexisting capabilities on the neighbouring IRIS spectrometer. Herein, we provide a concise account of this new capability for high-resolution neutron spectroscopy in the THz domain, as well as outline a number of ongoing and potential scientific opportunitiesin condensed-matter physics, chemistry, and materials science.1. The reasons whyOSIRIS is an indirect-geometry neutron spectrometerwith long-wavelength diffraction capabilities [1]. Thesecondary spectrometer sits at ca. 34 m from a 25 Kliquid-hydrogen (H2) moderator on ISIS Target Station I.Optimised for low-energy, high-resolution inelasticneutron-scattering studies, OSIRIS uses a broad bandof incident wavelengths which are Bragg-scattered froma crystal-analyser array following interaction with thesample under investigation. Owing to the pulsed nature ofthe source, time-of-flight methods are used to determineenergy transfer at a particular scattering angle.As the last of first-generation instruments built atISIS, OSIRIS has been operational since 1997. Aschematic diagram of the instrument is shown in Fig. 1.In the early days, its primary use was as a long-d-spacing diffractometer, exploiting its dedicated, 960-pixelbackscattering detector bank with a resolution as highas d/d = 2.5 × 10−3 and a dynamic range extendingup to 20 Å. In 2003, the instrument was upgradedwith a cooled pyrolytic graphite (PG) analyser bank,thereby paving the way for low-energy inelastic andquasielastic neutron-scattering studies [3]. Its primaryspectrometer profited from early advances in neutron-guide technology in the 1990s, with an m = 2 guideextending across the majority of its primary length, andterminated by an m = 3.5 focusing section immediatelyprior to the sample. In spectroscopic mode, OSIRIS usesPG crystals cooled to 10 K so as to define two primaryfinal energies using PG002 (Ef = 1.84 meV) and PG004(Ef = 7.38 meV) Bragg reflections. The spectral resolutiona Corresponding author: F. Demmel (franz.demmel@stfc.ac.uk)b and F. Fernandwz-Alonso(felix.fernandez-alonso@stfc.ac.uk)of these two alternative settings amounts to 25 and 99 µeVfor PG002 and PG004, respectively. Recent Monte Carlosimulations of the instrument have also provided importantinsights into its neutronic performance, including thevarious contributions to the resolution function and lineshape [2]. As on similar neutron spectrometers like IRISat ISIS [4] or, more recently, BASIS at SNS [5] or DNAat JPARC [6], a tight spectral resolution in the µeVdomain comes as a result of several factors, including:a relatively long flight path (>30 m) at a short-pulsespallation source; tight incident neutron pulses from a coldneutron moderator (which, as in the case of DNA, it mayrequire the additional use of a pulse-shaping chopper in theprimary spectrometer); a low-energy (few meV) analyserfor spectral analysis as afforded by materials like PG orsilicon; and an analyser geometry close to back-scattering(>85◦).In addition to simultaneous high-resolution diffractioncapabilities, the high spectral resolution and wide dynamicrange afforded by this family of inelastic neutron-scattering instruments provide a number of opportunitiesfor low-energy spectroscopy. To illustrate the above, Fig. 2shows inelastic neutron-scattering spectra centred at the0→1 rotational transition of the H2 molecule at ca.15 meV. On a steady reactor source, this spectral featureis readily accessible using thermal incident wavelengths,leading to a spectral resolution around 1 meV [7]. Asshown in the figure, an improvement by a factor ofaround three may be attained on broadband crystal-analyser spectrometers like TOSCA at ISIS [8]. IRIS hasalready demonstrated a much tighter spectral response, asshown by a resolution-limited feature of ca. 100 µeV at15 meV in the same figure [9]. At present, going beyondthese capabilities requires the use of inelastic-spin-echotechniques, well within the µeV domain as illustrated inFig. 2 and Ref. [7]. The bottom panel of Fig. 2 alsoThis is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0, which permits unrestricted use, distribution,and reproduction in any medium, provided the original work is properly cited.Article available at http://www.epj-conferences.org or http://dx.doi.org/10.1051/epjconf/20158303003EPJ Web of ConferencesFigure 1. Schematic diagram of the OSIRIS spectrometer. Forfurther details, see the main text and Ref. [2].Figure 2. Top: inelastic neutron-scattering spectra of the firstrotational line of bulk p-H2. Bottom: rotational spectra of solidp-H2 in the absence (red dots) and presence of o-H2 (black dots).In both cases, positive energy transfer denotes neutron-energyloss. For further details, see the main text.illustrates the additional information content associatedwith the use of a near-backscattering spectrometer likeIRIS or OSIRIS for inelastic studies. In the particular caseof p-H2 in its ground rotational level, interactions with o-H2 (J=1) molecules lead to an overall redshift and distinctbroadening of this transition, phenomena associated withsubtle (yet at the same time observable) intermolecularinteractions in the solid hydrogens.On an instrument like IRIS, clean access to energytransfers up to 15–20 meV has been possible owing to theuse of a beryllium (Be) filter in the secondary spectrometerto suppress Bragg reflections beyond the lowest order,namely, PG002. Although this rather beneficial feature hasbeen available on IRIS for some time now, high-resolutioninelastic-neutron scattering studies well beyond ∼1 THz(1 meV = 0.24180 THz) have been severely limited byFigure 3. Top: schematic drawing of the OSIRIS secondaryspectrometer highlighting in green the neutron paths through theBe filter after scattering by the sample. Bottom: photograph ofthe actual Be-filter assembly inside the OSIRIS tank, showingthe radial arrangement of the Be wedges as described in themain text. The Be blocks are enclosed within a thermal shieldand additional multilayer insulation. Two copper legs link the topof the filter assembly to closed-cycled refrigerator (CCR) headslocated outside the vacuum tank. In this photograph, the radialcollimator can be seen behind the filter.intrinsic flux limitations on this day-one ISIS instrument.Reasons include the use of a natural-nickel guide on theprimary spectrometer, as well as a relatively small PGanalyser bank.To circumvent the above limitations, a new Be filterhas been designed, constructed, and installed on theneighbouring OSIRIS spectrometer. In this contribution,we summarise its main technical characteristics andperformance.2. The new beryllium filterIn the new Be filter on OSIRIS, scattered neutrons from thesample are transmitted through wedged, polycrystallineBe blocks, leading to the suppression of PG00X (X > 2)reflections prior to neutron reflection on the PG analyser,and subsequent detection using an array of forty-two 3Hetubes.As shown in more detail in Fig. 3, the filter has thesame angular mapping as that of the radial collimator onthe instrument, with a total of forty-two independent Beslats decoupled by 1 mm sheets of cadmium (Cd). Thedesign criteria for the OSIRIS Be filter have capitalisedfrom prior experience with similar devices on IRIS andTOSCA at ISIS. Given a total cross section for Be of7.63 barn and a particle density of 1.24 × 1023 cm−3 [10],03003-p.2QENS/WINS 2014low-energy neutrons have a mean-free path of ca. 1 cmin the material. As this cross section is dominated bycoherent scattering within the crystalline lattice above anenergy cutoff of ca. 5 meV, a suitable absorbing material(in this case Cd) is required to minimise detrimentalsecondary backgrounds reaching other elements withinthe instrument and, ultimately, the detector array. In ourparticular case, this condition has been met by surroundingeach individual Be slat with thin Cd sheets of the samelength and height as the former. The overall length of theBe slats were chosen so as to be in excess of ten neutronmean-free paths in Be, a limit that could not be surpassedowing to spatial constraints on the instrument.The filter has been placed before the radial collimatorand analyser bank to minimise direct line-of-sight bythe neutron-detector array, which remains positionedimmediately below the filter when operational. In addition,the presence of the radial collimator between sampleand PG analyser also serves to reduce the levels ofunwanted neutrons reaching the detector bank. To increasethe transmission of low-energy neutrons through Be, thefilter has to be cooled [11]. Two independent CCRsenable the cooling of the entire unit to temperaturesbelow 40 K, as measured by a set of 8 independentsensors directly attached to the filter assembly. The CCRheads are mounted outside the vacuum tank and areconnected to the filter through copper legs. A motorised,computer-driven stage can bring the filter into positionfor PG002 operation, or out of position if the lower-energy-resolution PG004 option is required to access awider momentum-transfer range. The filter is retractablein vacuo, thereby enabling changes in spectroscopicsettings on the instrument without further intervention.3. PerformanceThe performance of the new OSIRIS Be filter has beenassessed using a cylindrical vanadium (V) standard atroom temperature. The recorded intensity at elastic energytransfers with the filter in place (hereafter “filter-in”)is reduced by about 40% compared to the “filter-out”measurement. This reduction in intensity is the result ofa number of factors, including: the overall transmission ofBe, the presence of Cd sheets between the Be wedges, anda reduced angular acceptance of the PG array in the verticaldirection in the filter-in position. The latter one arises fromdesign constraints on the instrument which limit the lengthof the wedges as well as the lowest-possible position ofthe filter to avoid direct contact with the neutron-detectorarray. Figure 4 shows angle-averaged inelastic neutron datawith and without the filter obtained at 16 Hz (one third ofthe repetition rate of ISIS Target Station I).These data demonstrate the suppression of both PG004and PG006 reflections with the filter, thereby enlargingthe energy-transfer window of the instrument to ca.20 meV (∼5 THz). The new OSIRIS Be filter thereforeoffers comparable transmission characteristics as thoseon IRIS. The inset in Fig. 4 shows the suppression ofthe PG004 reflection in more detail. From integratedintensities, one can deduce that 99% of PG004 neutrons aresuppressed. The remaining fraction arises from mechanicalFigure 4. Angle-averaged V spectra on OSIRIS, demonstratingthe suppression of higher-order PG00X (X > 2) Bragg reflectionswith the filter-in configuration. For clarity, the inset shows thesedata around the 004 reflection.Figure 5. V spectra with and without filter (PG002 reflection).The logarithmic intensity scale serves to demonstrate a sensibleincrease in signal-to-background ratios in the quasielastic regionusing the new filter.imperfections in the filter assembly, in particular betweenBe wedges and Cd sheets.A closer look at the quasielastic region around thePG002 Bragg reflection is presented in Fig. 5. These datashow that the filter leads to a reduction of backgroundlevels by a factor of 4–5. Taking into account theconcomitant reduction in detected flux for the filter-insetting, the new OSIRIS Be filter therefore leads to a netimprovement in signal-to-background ratios of a factor of3 in this energy-transfer range relative to IRIS.In parallel with the above developments, extensiveneutron-transport simulations of the OSIRIS primary andsecondary spectrometers have been performed using theMcStas code [2]. These simulations provide a necessarystarting point to assess in detail the new spectroscopiccapabilities of the instrument, as summarised in Fig. 6.For the PG002 reflection, OSIRIS has a slightly relaxedresolution relative to IRIS around the elastic line (25 µeVvs. 17.5 µeV). These differences become less significantat higher energy transfers, where the contributionfrom the primary spectrometer to the total spectralresolution (virtually identical in both cases) dominates.The predictions shown in Fig. 6 are not only consistentwith previous results on IRIS but at the same time evincea superb spectral resolution up to 20 meV in neutron-energy loss. On OSIRIS, this excellent energy resolution03003-p.3EPJ Web of ConferencesFigure 6. Energy resolution of OSIRIS as a function of energytransfer obtained from Monte Carlo simulations. For furtherdetails, see the text.is accompanied by a much higher flux at these high energytransfers compared to IRIS owing to the use of an m = 2supermirror guide in the primary spectrometer. Higher m-numbers could also be used in the future to enhance thesecapabilities further.4. OutlookThe new Be filter on OSIRIS opens up new possibilitiesin quasielastic studies due to a higher sensitivity ofthe spectrometer around the elastic line. Furthermore,this instrument development provides new opportunitiesin high-resolution inelastic spectroscopy with access tospectral features up to 20 meV combined with a high flux.Recent examples which have already explored the lowerend of this spectral window include the investigation ofexcitation lifetimes and line shapes in CeCoIn5 [12] and inthe gapped quantum magnet copper nitrate [13]. Furtherdevelopments in the horizon on the instrument includeachieving a tighter spectral resolution. This objective couldbe readily achieved with a second silicon analyser bank, tobe housed on the vacant side of the spectrometer.The authors gratefully acknowledge the UK Science &Technology Facilities Council for financial support and access tobeam time at ISIS.References[1] www.isis.stfc.ac.uk/instruments/osiris/(last accessed 22 September 2014)[2] K. Pokhilchuk, Rutherford Appleton LaboratoryTechnical Report RAL-TR-2013-008 (Chilton,2013, ISSN 1358-6254). Report available at:epubs.stfc.ac.uk/work/65985 (last accessed 22September 2014)[3] M.T.F. Telling and K.H. Andersen, Phys. Chem.Chem. Phys. 7, 1255 (2005)[4] C.J. Carlile and M.A. Adams, Physica B 182, 431(1992)[5] E. Mamontov and K.W. Herwig, Rev. Sci. Instrum.82, 85109 (2011)[6] M. Arai et al, J Phys Soc Jpn 82 SA024 (2013)[7] F. Fernandez-Alonso et al., Phys. Rev. B 86, 144524(2012)[8] wwwisis2.isis.rl.ac.uk/INSdatabase/ (lastaccessed 22 September 2014)[9] F. Fernandez-Alonso et al., Phys. Rev. Lett. 98,215503 (2007)[10] J. Dawidowski et al., in Neutron Scattering –Fundamentals, F. Fernandez-Alonso and D.L. Priceeds. (Academic Press, New York, 2013), Appendix[11] A.K. Freund, Nucl. Instr. Meth. 213, 495 (1983)[12] C. Stock et al., Phys. Rev. Lett. 109, 167207 (2012)[13] D.A. Tennant et al., Phys. Rev. B 85, 014402(2012)03003-p.4

Opening the terahertz window on the OSIRIS spectrometer

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Opening the terahertz window on the OSIRIS spectrometer

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