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A Time-Of-Flight Backscattering Spectrometer at the Spallation Neutron Source, BASIS E

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A Time-Of-Flight Backscattering Spectrometer at the Spallation Neutron Source, BASIS E A time-of-flight backscattering spectrometer at the Spallation Neutron Source, BASIS E. Mamontov and K. W. Herwig Citation: Review of Scientific Instruments 82, 085109 (2011); doi: 10.1063/1.3626214 View online: http://dx.doi.org/10.1063/1.3626214 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/82/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in A radial collimator for a time-of-flight neutron spectrometer Rev. Sci. Instrum. 85, 085101 (2014); 10.1063/1.4891302 Spin exchange optical pumping based polarized 3He filling station for the Hybrid Spectrometer at the Spallation Neutron Source Rev. Sci. Instrum. 84, 065108 (2013); 10.1063/1.4809942 The new cold neutron chopper spectrometer at the Spallation Neutron Source: Design and performance Rev. Sci. Instrum. 82, 085108 (2011); 10.1063/1.3626935 Spallation neutron source (SNS) AIP Conf. Proc. 613, 15 (2002); 10.1063/1.1472000 Time-of-flight neutron spectrometer for JT-60U Rev. Sci. Instrum. 72, 828 (2001); 10.1063/1.1323240 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 129.21.35.191 On: Sun, 21 Dec 2014 10:04:05 REVIEW OF SCIENTIFIC INSTRUMENTS 82, 085109 (2011) A time-of-flight backscattering spectrometer at the Spallation Neutron Source, BASIS E. Mamontova) and K. W. Herwig Neutron Scattering Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA (Received 1 June 2011; accepted 28 July 2011; published online 25 August 2011) We describe the design and current performance of the backscattering silicon spectrometer (BASIS), a time-of-flight backscattering spectrometer built at the spallation neutron source (SNS) of the Oak Ridge National Laboratory (ORNL). BASIS is the first silicon-based backscattering spectrometer installed at a spallation neutron source. In addition to high intensity, it offers a high-energy resolution of about 3.5 μeV and a large and variable energy transfer range. These ensure an excellent overlap with the dynamic ranges accessible at other inelastic spectrometers at the SNS. © 2011 American Institute of Physics. [doi:10.1063/1.3626214] I. INTRODUCTION tering geometry when θ is near 90◦ minimizes the spread in the measured E Most backscattering spectrometers have The time-of-flight neutron scattering spectrometers uti- f. been built at neutron research reactors that provide continu- lize either direct scattering geometry, when the initial energy ous beam, where some kind of a monochromator is needed of the neutrons incident on the sample, E , is selected by a i to set up the E for the neutrons incident on the sample. The monochromator or choppers, or inverted scattering geometry, i variation of the E is then achieved by means of either vary- when the final energy of the neutrons scattered by the sample, i ing the temperature of the monochromator with respect to E , is selected by the analyzer crystals. The technique known f that of the analyzer, thereby changing the lattice constants as backscattering1, 2 is the limiting case of the inverted scat- of the monochromator and the E ,3 or Doppler-moving the tering geometry that utilizes energy analyzer crystals with a i monochromator.4 The latter approach requires much shorter fixed Bragg reflection angle at or near 90◦. This geometry measurement times and is usually adopted, even though the minimizes the energy resolution of the analyzer according to former approach allows measurements over much larger dy- the following expression: namic range not limited by the range of the Doppler-driven δd monochromators. In either case, the spread in E is deter- δE = 2E + cot θδθ , (1) i f f d mined by the monochromator, and is typically similar to the spread in the Ef. This can yield a sub-μeV overall energy res- which can be easily obtained by differentiating Bragg’s law. olution at a reactor-based backscattering spectrometer, such δ Here d/d is the spread of the analyzer lattice constant, and as IN10 and IN16 at the ILL, France,5 HFBS at the NCNR, θ is the Bragg angle; the second terms in the bracket is min- USA,6 and SPHERES at the FRM-II, JCNS, Germany.7 At the θ ◦ imized when approaches 90 (thus the term “backscatter- same time, these backscattering spectrometers suffer from the ing”). Neutron backscattering spectrometry is characterized somewhat limited dynamic range achievable with Doppler- by a high energy resolution, often on the energy scale of driven monochromators (presently, some ±30–35 μeV at μ eV. The scattering at such low energy transfers probes dy- best). These spectrometers use silicon analyzer crystals and namic processes on the nano-second time scale. In typical monochromators, where the δd/d is controlled by introducing neutron time-of-flight experiments, much faster processes on elastic deformation of the crystals. The perfect backscattering the pico-second time scale are usually probed, whereas neu- geometry (θ = 90◦) employed at these spectrometers requires tron spin-echo can be used to probe much slower processes detector positioning in the horizontal plane behind the sample on the tens and hundreds of nano-seconds time scale. Neu- (that is, the neutrons pass through the sample twice on their tron backscattering can thus bridge the gap between the time way to the detectors). scales probed in neutron time-of-flight and spin-echo experi- The time-of-flight (TOF) backscattering spectrometers ments. The precision with which the neutron energy transfer, (TOF-BSS) typically built at spallation neutron sources do E = Ei – Ef, can be determined (that is, the overall energy not use monochromators; instead, the neutron’s Ei is deter- resolution) depends on accurate knowledge of both the ini- mined from the overall time-of-flight between the neutron tial energy, Ei, and the final energy, Ef. The precision of the source and the detectors. The dynamic range is limited only latter is determined mainly by the analyzer crystals type and by the frame overlap, and is determined by the combina- arrangement according to Eq. (1); utilization of the backscat- tion of the source repetition rate, neutron flight path dis- tances, and choice of the bandwidth-limiting choppers. En- ergy transfers of several hundred μeV are easily achievable. a) Author to whom correspondence should be addressed. Electronic mail: [email protected]. Telephone: 1-865-5745109. FAX: 1-865-574- The energy resolution, however, is coarser compared to that 6080. of the reactor-based backscattering spectrometers. To the first 0034-6748/2011/82(8)/085109/10/$30.0082, 085109-1 © 2011 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 129.21.35.191 On: Sun, 21 Dec 2014 10:04:05 085109-2 E. Mamontov and K. W. Herwig Rev. Sci. Instrum. 82, 085109 (2011) approximation, it can be written as eration, also has mica analyzer crystals; however, its PG ana- lyzers have been used much more often. For the main working 2 2 δd δTOF reflection, PG (002), δd/d = 0.002. With the moderator pulse δE = 2E + cot θδθ + . (2) d TOF width δTOF ≈ 120 μs, a match between the resolution con- tributions from the primary and secondary spectrometers was The problem is that it is difficult to reduce the time-of- achieved at a near-backscattering Bragg angle of 87.5◦. With flight term, which decreases with the increasing flight path the analyzers spatially extended in 2000,11 the current resolu- distances, below certain, economically feasible limit. It is of- tion using the PG (002) reflection is 17.5 μeV. The graphite ten more practical to intentionally coarsen the other term in analyzers of the IRIS need to be cryogenically cooled in or- order to match the contribution from the time-of-flight term, der to suppress thermal diffuse scattering from graphite. The thus increasing the signal intensity without degrading the res- OSIRIS spectrometer, built at the ISIS more recently12 and olution much. While a TOF-BSS built on spallation neutron conceptually similar to the IRIS, has a significantly higher sources profitably utilizes pulsed structure of the incident neu- flux and somewhat coarser energy resolution of 24.5 μeV tron beam, its design should find a reasonable balance be- with the PG(002) reflection. tween the energy resolution, dynamic range, signal intensity, With the mica-based TOF-BSS suffering from the low in- accessible Q-range, etc. Unfortunately, these parameters of tensity (and a very limited Q-range) and the PG-based TOF- merit are often in direct conflict with one another. For in- BSS having the energy resolution well above 15 μeV, there stance, high energy resolution is achieved at the expense of has been a need for a TOF-BSS with a higher energy reso- signal intensity, longer neutron flight paths improve the reso- lution, approaching that of the reactor-based backscattering lution but adversely affect the dynamic range, Q-range suffers spectrometers, yet with sufficiently high scattering signal. A when analyzer crystals with large d-spacing are used in an at- new TOF-BSS, BASIS, constructed at the spallation neutron tempt to improve the resolution, etc. In practice, it is the pri- source at Oak Ridge National Laboratory, USA, entered the mary flight path (moderator to sample) that defines the term general user program in the late 2007. This is the first real- δTOF/TOF, because the moderator-to-sample distance is typ- ization of a silicon-based TOF-BSS (hence the name BASIS, ically much longer compared to the sample-to-analyzer and which is an acronym for Backscattering Silicon Spectrome- analyzer-to-detector distances.
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