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date: May 23, 2003

to: Helen Kerch, DOE

from: Steve Shapiro, Igor Zaliznyak

subject: **Answer to the referee reports on HYSPEC Proposal**

We are pleased that two referees judged the HYSPEC proposal to be an
excellent one, and have expressed their strong support for the
construction of the instrument. While the negative response of the third
referee is certainly disappointing, we believe that the referee's
concerns resulted mainly from some deficiencies in the text and the
layout of the proposal, but not in its essence, nor in the instrument's
conceptual design. Below, we present arguments which address in detail
the issues raised by the third referee, and which support our
conclusion.

1.  The introductory remark, that the proposal *"presents a less
    detailed description of the instrument..."* has raised our concern
    of a possibility that somehow, by an error or omission, the proposal
    was not available to the referee in its entirety. While in the main
    text of the proposal we indeed included only a brief, 10-page
    overview of the instrument's conceptual design and main operational
    characteristics, a very detailed, 15-page formal description of the
    instrument was attached in Appendix C, "HYSPEC Top Level
    Specifications" (TLS). It is referred to in the Introduction, and in
    the Section 3 of the main document.

2.  On the technical side, the main concern of the referee is that
    HYSPEC *"has not been optimized to take into account the other
    instruments in the SNS..."*, and, specifically, that he sees *"the
    reason for* (HYSPEC) *long* (secondary) *flight path is to obtain
    the resolution that matches ARCS, CNCS and SEQUOIA"*. Referee
    further suggests that *"optimizing the instrument for its strength
    -- intensity -- would also result in ... savings"*, and proposes to
    reduce the HYSPEC secondary flight-path to 2 -- 2.5 m,
    simultaneously increasing the detector coverage, *ie* the surveyed
    momentum space. To the referee's opinion, this could result in a
    very significant cost savings without serious impact on the
    instrument performance.

> These suggestions raise a number of vital issues in the conceptual
> design of HYSPEC and, to that matter, of any **thermal-neutron**
> spectrometer **for single crystal studies**, which, perhaps, were
> lacking clear exposition in the text of the proposal. In fact, there
> are several very important reasons for which the secondary neutron
> flight-path in such spectrometer has to be long, and in the case of
> HYSPEC can not be shorter than 4 m. In short, these are,

i.  a spectrometer performs best if the energy resolution of the
    analyzer and the monochromator are matched; the shorter is the
    analyzer flight-path, the more out of balance and less optimized is
    the spectrometer for the energy transfers of interest

ii. reducing the length of the secondary flight-path at any (coarse or
    fine) fixed analyzer resolution requires proportional decrease in
    the sample illumination time and, therefore, results in a
    proportionally less efficient use of the long neutron pulse supplied
    by the coupled H~2~ moderator

iii. it follows from the translational invariance of the crystalline
     material that an increase in the reciprocal space surveyed by the
     instrument beyond one Brillouin zone (BZ) does not result in the
     proportional increase in instrument throughput

iv. efficient separation in the detector of up and down neutron
    polarization in the supermirror-bender type transmission
    polarization analysis setup previewed for HYSPEC requires that the
    secondary flight-path is at least 4 m long

> It is essentially the throughput (intensity) optimization, which is
> achieved by matching the instrument with the moderator that requires
> long secondary flight-path in the HYSPEC design. A 4.5 m long analyzer
> is roughly matched with the FWHM of the coupled H~2~ moderator pulse
> for the coarse, but acceptable, ΔE/E resolution of about 10% for E~i~=
> \[10,60\] meV. Relaxing the resolution beyond 10% is unacceptable for
> most experiments, while reducing the secondary flight-path within the
> resolution constraint requires shorter sample illumination time, so
> that less neutrons provided by the moderator are used. In fact, in a
> recent development the SNS Instrument Team in close interaction with
> HYSPEC IDT has proposed a new SNS floor layout, where HYSPEC is
> located on beamline 14B and fits within the SNS building even with 4.5
> m long secondary flight-path.
>
> A spectrometer that is designed to operate in the cold neutron range
> could have shorter secondary flight path, because the time of flight
> of the scattered cold (slow) neutrons in the analyzer is longer and
> allows for the longer sample illumination times. Such is the case of
> FOCUS at PSI cited by the referee, which is optimized at E~i~=5 meV
> and has 2.5 m long analyzer. To match the FOCUS elastic resolution at
> E~i~=50 meV with similar sample illumination time, a thermal neutron
> spectrometer would require 25 m long secondary flight-path. While
> sufficiently long secondary flight-path in HYSPEC can, in principle,
> be used to achieve a higher energy resolution at a cost of lower
> intensity, this is not the regime that the instrument was designed
> for. This is illustrated by the performance comparison for HYSPEC,
> CNCS, ARCS and SEQUOIA by Granroth and Abernaty, that the referee had
> referred to. Contrary to the referee's impression, though, HYSPEC in
> its current design is not capable of operating at high resolutions
> those other instruments are optimized for. To achieve such resolution,
> HYSPEC setup was deliberately changed by Granroth and Abernathy, by
> reducing the opening in the disc chopper. This has a detrimental
> impact on the intensity, and is not previewed in the HYSPEC design,
> although, even with this handicap, HYSPEC noticeably outperforms the
> other instruments in the 10 - 30 meV thermal neutron range, where it
> is best optimized.
>
> Finally, we comment on the referee remarks that neither the immediate
> polarized beam capability nor the *"path that detectors cut through
> Q-E space"*, which *"are the same for all instruments"* make HYSPEC
> unique among the SNS instruments.

i.  While we are ready to share the referees enthusiasm on the
    perspectives of ^3^He filter-polarizers, and do allow for an upgrade
    to one of those once they become available in the HYSPEC design, we
    can not rely entirely on their future availability. In fact, ^3^He
    polarizers with large angular acceptance which are suitable for use
    in the instruments such as HYSPEC probably will not be operational
    within a decade or more, which means that for at least first 5-10
    years of SNS operation HYSPEC is bound to be the only polarized beam
    inelastic spectrometer there. In addition, the maintenance and
    operational costs of ^3^He filter-polarizers are likely to be high,
    in which case their eventual availability will also depend on an
    additional substantial funding. HYSPEC polarized beam setup is
    essentially maintenance free.

ii. The phase space probed by HYSPEC for each fixed incident energy E~i~
    is indeed the same as measured by the corresponding detectors in any
    other inelastic instrument at SNS, a E~i~ = *const* scan. It is
    well-known, though, that such measurement has a significant handicap
    that the momentum space probed shrinks rapidly with the increasing
    energy transfer E; to cover the same amount of reciprocal space at
    higher E one has to increase E~i~. HYSPEC's intended mode of
    operation is such that a number of measurements with different E~i~
    are performed which cover roughly one Brillouin zone (but always the
    same one), for extended range of energy transfers. This is achieved
    by rotating the sample and properly positioning the detector bank,
    so that the measurement is focused on the same region of the
    reciprocal space for all E~i~. A similar mode of operation is
    possible for other TOF spectrometers (although, it is severely
    restricted for the Fermi-chopper instruments, where only few
    discrete E~i~ are possible). Provided the sample is properly aligned
    as incident energy changes, at each E~i~ the Brillouin zone of
    interest would be measured by different parts of a large detector
    bank. In HYSPEC, a smaller detector bank is used with the high
    efficiency, as it follows the movement of the sample momentum space
    with changing E~i~, and always probes the same part of it, which is
    of interest to the experimenter.

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3.  While we are pleased that referee acknowledges the strong scientific
    case presented in the HYSPEC proposal, we find his reservations that
    *"the focus of the IDT is quite narrow"* and *"there is very little
    participation of the general university research community"*
    surprising. Indeed, the HYSPEC IDT was organized by scientists at
    Brookhaven National Laboratory, and therefore has a very significant
    BNL representation. And while the situation when *"the great bulk of
    the members are at the existing neutron sources at the national
    labs"* does not seem un-appropriate for an IDT that builds a neutron
    spectrometer at a DOE laboratory, we note that HYSPEC IDT also
    includes participants from almost all American universities that
    have neutron scattering program in condensed matter. In fact, they
    account for half of the non-BNL membership of the HYSPEC IDT. In
    addition to the U. Missouri and Iowa University mentioned by the
    referee, MIT, Stanford, Rutgers, and McMaster University of Canada
    are represented. The real issue here is the shrinking support for
    the neutron scattering effort at the US universities. Princeton
    group previously lead by Bernhard Keimer is inexistent, formerly
    mighty MIT neutron scattering program is now reduced to one
    tenure-track professor and a postdoc, Martin Greven's group at
    Stanford shifts focus to X-ray scattering, Rutgers group of Valery
    Kiryukhin, who studied in neutron scattering with Keimer and
    Birgeneau, has now only a rudimentary neutron program. The
    scientific effort previewed in the HYSPEC budget is essentially
    aimed at addressing this problem, and it is gratifying that the
    referee acknowledges its importance. The funding will be used to
    revive the neutron scattering efforts at the participating
    universities, in particular, by hiring both postdocs and graduate
    students to work on the HYSPEC-related scientific projects. We also
    envision that a neutron scattering program in condensed matter could
    be conceived in the Physics Department of our own SUNY at Stony
    Brook.

> Addressing the referee suggestion to open the IDT to "*a much broader
> range of participation"* we note that HYSPEC IDT membership policy is
> quite open, it does not require any qualifications, only approval by
> the Executive Committee. In fact, since the IDT was established it has
> grown by a number of members, and nobody was refused the
> participation.

4.  We conclude by addressing the referee comments on the HYSPEC
    estimated budget. For us, it is a painful issue because it was
    prepared with a very limited funding and without the engineering
    support appropriate for the project of this scale. The budget is
    essentially based on the good faith estimates that we were able to
    make at the time. For the few components which are common with other
    instruments at the SNS we obtained the cost estimates from the SNS
    Instrument Systems. These costs are reasonably well defined, and we
    are working closely with the SNS to update them as information of
    the new procurements become available. However, more than half of
    the construction costs are HYSPEC-specific and can not be simply
    copied from other spectrometers. Realistic budget estimate requires
    drafting a detailed specifications for each of the components based
    on the HYSPEC TLS, and obtaining the cost estimates from the
    manufacturers. This implies significant engineering effort, which we
    believe could be completed within 6 to 12 month from the time it is
    funded.

> To answer the most disappointing remark of the referee, that *"the
> budget for the instrument seems excessive"*, we note that main savings
> (\$3-4 M) that the referee proposed result from reducing the
> spectrometer secondary flight-path from 4.5 to 2 m. As we discussed in
> detail above, this would have a detrimental and untolerable impact on
> the instrument performance. More than two-fold loss of flux-on-sample
> at any given resolution is apparent, and many other disadvantages
> follow. Also, a wide 4 cm guide used by HYSPEC does not allow
> employing a single disc chopper in place of the counter-rotating pair,
> thus eliminating another source of savings identified by the referee.
>
> In fact, for proper comparison with the funded instruments, such as
> ARCS and CNCS, one has to look at the HYSPEC FY 2003 estimated
> construction cost, which is k\$14,750 (there was an unfortunate error
> of \$1.2M in inflating the costs to FY2008, that was correctly noticed
> by the referee). Taking into account the higher level of mechanical
> complexity of the Hybrid Spectrometer, and also the fact that this
> price tag includes the polarized beam option which will add up to \$1M
> or more to the cost of the other instruments, we believe that the
> budget does not look excessive.

With best regards,

Igor Zaliznyak

Steve Shapiro

**Appendix**. More on HYSPEC design considerations.

There is currently no much experience in designing and building neutron
spectrometers optimized for single-crystal studies on the pulsed
spallation sources, MAPS at ISIS being essentially the first attempt of
this kind. HYSPEC is a novel design, which appears to offer significant
new advantages for these particular studies. Naturally, it also
incorporates the well-known guiding principles of the design of a
time-of-flight (TOF) spectrometer on a pulsed neutron source, which are
tested by time. Below, we discuss in more detail some of the
considerations that shaped the HYSPEC design, in connection with the
concerns raised by the referee.

1.  HYSPEC is optimized for studies of the dynamic properties of
    crystalline materials in the thermal neutron range \[10,60\] meV.
    Translational invariance is a fundamental physical property that
    sets crystals apart from other materials, such as glasses and
    liquids. It has important consequences for the design of a single
    crystal inelastic spectrometer. The most immediate result of the
    translational invariance (lattice periodicity) is that the momentum
    in the crystal is replaced by the quasi-momentum, which is defined
    only within the Brillouin zone (BZ). The physical properties are
    periodic in the momentum space, and, while covering large part of
    the reciprocal space is undoubtedly useful ( *eg* for structure
    refinement or background characterization), the response in a single
    Brillouin zone essentially provides the bulk of the information on
    the dynamics of the crystal. In other words, while a spectrometer
    that is capable of measuring the phonon dispersion throughout 8
    Brilluoin zones does yield more information compared to the one that
    covers one BZ, the gain is far from the naively expected 8-fold
    increase in throughput. As a result, the historical common wisdom,
    that efficiency of the TOF spectrometer grows proportionally to the
    analyzer angular acceptance, and that a loss in the flux on sample
    can be easily compensated by the increase in a surveyed momentum
    space, does not hold. A reliable measurement requires that a data
    with a satisfactory signal to background ratio are collected in at
    least one Brillouin zone. In fact, because neutron scattering
    cross-section varies strongly with Q, from one Brillouin zone to
    another, combining the data from different BZ is not straightforward
    and, in many cases the increase in the surveyed momentum space does
    not increase the data collection rate at all. This is in contrast
    with the disordered systems, such as liquids or glasses, where the
    collected information is not periodic in the momentum space, and,
    therefore, grows roughly in proportion with the increase in
    reciprocal space surveyed by the spectrometer. Because
    non-crystalline systems were for decades the primary object of
    studies with the TOF spectrometers, it became accepted in their
    design that the solid angle covered by the detectors is maximized at
    the expense of the secondary flight-path, which is made as short as
    possible, while consistent with the resolution that the instrument
    is intended for.

> Here is how the important consideration formulated above applies to
> HYSPEC. Even for a typical metal, with the lattice constant as short
> as about 3.1415, and the Brillouin zone as large as 2 Å^-1^, the
> analyzer acceptance of 60 degrees covers, in elastic scattering, about
> 1.3 BZ for E~i~ = 15 meV neutrons. Because the data collection rate
> cannot be efficiently increased by increasing the detector angular
> coverage once more than one Brillouin zone is surveyed, the only way
> to optimize the instrument is to use as large fraction of neutrons
> produced in the moderator pulse, as possible. This implies that the
> sample illumination time, τ, is increased, at least to match the full
> width at half maximum (FWHM) of the neutron pulse prodced by the
> moderator. For a given analyzer energy resolution, the increase in τ
> requires that the secondary flight-path is increased proportionally.
> Because the pulse FWHM of the coupled hydrogen moderator is rather
> long, up to 100 μs (it decreases with the increasing incident neutron
> energy, but so does the time of flight in the analyzer), matching it
> with the pulse on sample requires a secondary flight-path of 4-5 m for
> energy resolution of 5% to 10%. In fact, the counter-rotating disc
> choppers in the design of HYSPEC do not allow for τ \< 40 μs which
> would be required to achieve the high energy resolutions comparable to
> CNCS or SEQUOIA.

2.  For the optimum performance of an instrument energy resolutions of
    the primary and the secondary spectrometer (monochromator and
    analyzer) have to be matched for some useful value of energy
    transfer E. This condition requires that the time of flight in the
    monochromator for neutrons with energy E~i~ and in the analyzer for
    neutrons with energy E~i~--E are roughly equal. If the monochromator
    is twice longer than the analyzer, the instrument is balanced for
    the energy transfer of roughly half of the incident energy. This is
    close to the optimum setup, and is implemented in the case of MAPS
    at ISIS. Of the spectrometers currently previewed for the SNS, the
    shortest, 17.5 m primary flight-path is planned for SEQUOIA.
    Combined with the 6 m long analyzer it makes this instrument
    balanced within the useful energy transfer range, at E ≈ 2/3E~i~.
    For an instrument at 20 or more meters from the moderator, such as
    HYSPEC, the analyzer is bound to be appreciably shorter than the
    monochromator, and the spectrometer is not balanced for most energy
    transfers. However, the longer is the secondary flight-path
    (analyzer), the less out of balance, and therefore better optimized,
    the instrument is. Because analyzer controls the energy resolution
    for low E under such conditions, the longer secondary flight-path
    also leads to better elastic energy resolution. Our goal in
    developing the HYSPEC design, though, was to optimize the instrument
    by balancing the setup as much as possible, and not to achieve high
    elastic energy resolution, as referee suggested.

3.  Additional restriction on the length of the secondary flight-path is
    imposed by the efficient polarization analysis scheme based on the
    transmission polarizers, envisaged for HYSPEC. Because spatial
    separation of neutrons with up and down polarizations in the
    detector is required in order to measure them simultaneously, thus
    doubling the instrument efficiency, the secondary flight-path should
    not be shorter than about 4 m. It is important to mention that issue
    of the floor space that is required to accommodate the HYSPEC long
    secondary flight path was successfully resolved with the instrument
    relocation to the beamline 14B, and therefore no additional expense
    required by going to the outside building is envisaged anymore.

> While we are ready to share the referees enthusiasm on the
> perspectives of ^3^He filter-polarizers, and do allow for an upgrade
> to one of those once they become available in the HYSPEC design, we
> can not rely entirely on their future availability. In fact, ^3^He
> polarizers with large angular acceptance, suitable for use in the
> instruments such as HYSPEC, probably will not be available within a
> decade or more, which means that for at least first 5 to 10 years of
> SNS operation HYSPEC is bound to be the only polarized beam inelastic
> spectrometer there.

4.  Finally, referee notes that "*the path that the detectors cut
    through Q-E space are the same for all the instruments*", and that
    because of smaller detector coverage HYSPEC "*appears to be less
    flexible*...". The issue of the detector coverage was addressed
    above; HYSPEC allows more than a whole Brillouin zone of a
    crystalline material to be surveyed at once. More importantly, the
    remark of the referee is based on the assumption that HYSPEC is
    operated in exactly the same way as the other inelastic
    spectrometers, *ie* essentially collecting data in the constant-E~i~
    scans. This, however, is not the intended mode of operation for the
    hybrid spectrometer! Actually, the constant-E~i~ mode of operation
    is notoriously inefficient, which is why all or almost all inelastic
    studies with triple-axis spectrometers are made with the fixed final
    energy. Nevertheless, a direct geometry TOF spectrometer is bound to
    perform a scan with fixed incident energy, and analyze E~f~. The
    main disadvantage is that the momentum space surveyed by the
    analyzer shrinks at higher energy transfers, and the instrument
    throughput decreases very fast, where it is most needed! HYSPEC
    offers a unique opportunity to perform a measurement, where the
    incident energy is varied in steps, much like in a 3-axis
    spectrometer, so that the volume in the phase space surveyed by the
    instrument is matched with the scattering intensity. For example, to
    characterize a spin-wave dispersion with bandwidth 40 meV in a
    typical HYSPEC experiment, one would perform a number of
    measurements, a "scan", with E~i~ = 5,10,15,20,30,40,60 meV,
    spending different amount of time at each incident energy, as
    required to obtain similar statistical error on the data for each
    E~i~. By rotating the sample, and properly positioning the detector
    bank, the measurement will always concentrate on the same region of
    reciprocal space. The combined dataset will result in a high-quality
    measurement, whose resolution and statistical error will depend on
    the energy transfer much less than those of the equivalent
    traditional, constant-E~i~ data.