Physics Department

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Building 510B

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Upton, NY 11973-5000

Phone 631 344-3822

Fax 631 344-2918­

shapiro@bnl.gov

Managed by Brookhaven Science Associates\
for the U.S. Department of Energy

www.bnl.gov

November 13, 2023

Dr. Helen Kerch

Division of Materials Sciences and Engineering

Office of Basic Energy Sciences

Department of Energy

Washington, DC 20585

Dear Dr Kerch:

Thank you for the reviewers' comments on the IDT proposal to build the
HYSPEC instrument at the Spallation Neutron Source.

We are obviously pleased that two referees judged the proposal to be an
excellent one and have expressed their strong support for the
construction of this instrument. Reviewer #3 particularly emphasized our
goal of providing a tried and tested method of performing inelastic,
fully-polarized neutron beam experiments. We were also honored that
referee #4 stated that HYSPEC *"...could well set the standard for
future designs"*

It was sobering that referee #5's comments were negative. This referee,
however, seemed to be unaware that the design objectives of the HYSPEC
IDT differ in a number of important respects from those of the other
IDTs developing inelastic scattering instruments for the SNS and from
those designing instruments for earlier generations of spallation
sources. Here we shall address the issues he/she raised in an
abbreviated form: we discuss them in detail in the attached reply to the
referee, which you might wish to transmit to the referee.

UNIQUENESS OF HYSPEC

We disagree with the referee's claim that the "*instrument has not been
optimized to take into account the other instruments in the SNS
instrument suite."* In fact we believe the opposite is true and our
constant goal was to design an inelastic instrument that was unique and
complementary to the other inelastic instruments. Our success in this
respect was confirmed by the independent comparison of Ganroth and
Abernathy who stated in their report that *"HYSPEC bridges the cold and
thermal to epithermal energy ranges. It ... maximize\[s\] the flux on
sample. However, this extra flux is at the expense of the Q~y~
\[vertical\] resolution and ω resolution."*

SHORTENED FLIGHT PATH

The referee mistakenly reasoned that the long analyzer flight path is
used to obtain a resolution that matched the other direct geometry
instruments. There are two reasons for the longer flight path: (i) to
match the sample burst width to the source burst width at an acceptable
(5-10%) energy resolution, (ii) to better match the incident and
scattered flight paths. Both are important for optimal performance.
HYSPEC requires substantial space perpendicular to the incident beam
path and in order not to interfere with the neighboring beam lines there
was no alternative but to extend further out onto the experimental floor
than would otherwise be necessary. We have worked closely with the SNS
Instrument Team and have recently found a satisfactory solution to the
floor space problem by placing HYSPEC on beam line 14B inside the
experimental hall. It is also important to note that in the polarized
beam mode of operation the 4.5m flight path is needed to measure
simultaneously the spin-up and spin-down intensities\-\-- a factor of
two saving in measuring time. If the secondary flight path were made
shorter, as the referee suggests, the two beams could not be separated.

The reviewer also referred to the FOCUS instrument at PSI, which is
similar in concept to HYSPEC. It should be noted that FOCUS is designed
for the cold neutron regime (E~i~≤ 5.0meV). If one wanted the same
resolution at E~i~=50meV, FOCUS's secondary flight path would have to be
25 m long!

POLARIZED BEAM OPTIONS

The reviewer seems convinced that ^3^He neutron polarizers will be
available very soon. Most experts don't share this optimism. We
consulted Tom Gentile of NIST, an expert working on ^3^He polarization
development, who recommended that we proceed with the conventional
methods we propose. Moreover, ^3^He polarizers are very sensitive to
field gradients and this would limit the use of magnetic fields in the
sample area.

INSTRUMENT DEVELOPMENT TEAM:

The IDT is inclusive and includes most of the world's experts in single
crystal neutron inelastic scattering, regardless of affiliation. Since
many of these people are located at National Laboratories it is
reasonable that their expertise should be called upon to design and use
this instrument. It should be noted, however, that the HYSPEC IDT also
includes participants from nearly all American universities that have
neutron scattering programs in Condensed Matter Physics and they account
for half of the non-BNL membership of the IDT. We continue to welcome
new members from universities, or other institutions, who wish to
participate, and have added 6 new members since we formed

BUDGET:

Because of the lack of resources, the HYSPEC budget was prepared without
the normal engineering input that is necessary for a project of this
magnitude and complexity. For items that are common to other instruments
(choppers, guides, shielding, etc.), reliable cost estimates were
obtained from the SNS Instrument Team. For other unique pieces of HYSPEC
the costs were determined as best we could without detailed engineering
support. More realistic estimates will be one of the first tasks of this
project when funding becomes available and will involve drafting
detailed specifications for each of the components based on the HYSPEC
top level design specifications. It is expected and indeed welcomed that
these cost estimates would be thoroughly reviewed by an independent
review team.

Each of the issues raised by the referee is addressed in the detailed
comments attached. It is important to stress that the referee's
suggestion to save \$3-4M by shortening the analyzer flight paths would
have a disastrous and unacceptable impact on performance. The resulting
instrument would be completely different from the one approved by EFAC.

Attached is a detailed response to the technical issues raised by
referee #5. We would be very willing to answer any other questions that
you may have.

Sincerely,

Steve Shapiro and Igor Zaliznyak

# Reply to Referee #5

In his introductory remarks referee \# 5 mentions that the proposal
*"presents a less detailed description of the instrument..."*. The
description of the instrument's conceptual design and main operational
characteristics in the main document was deliberately limited to a
relatively brief, 10-page overview simply to avoid burdening the reader
with an over-abundance of detail. A very detailed,
component-by-component, 15-page description of the instrument was,
however, provided in Appendix C, "HYSPEC Top Level Specifications"
(TLS). Reference is made to this appendix in the Introduction as well as
in Section 3 of the main document.

1.  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"*. He further
    suggests that *"optimizing the instrument for its strength --
    intensity -- would also result in ... savings"*, and proposes to
    reduce HYSPEC's secondary flight-path to 2 -- 2.5 m while
    simultaneously increasing the detector coverage; *i.e.* the momentum
    space surveyed by the instrument. This, he believes, would result in
    a significant reduction in cost without seriously impacting on
    instrument performance.

> His suggestions raise a number of basic issues concerning the
> conceptual design of HYSPEC, and, for that matter, any
> **thermal-neutron** spectrometer **for single crystal studies.** There
> are several very important reasons why the secondary neutron
> flight-path in such spectrometers has to be relatively long, and, in
> the case of HYSPEC, cannot be shorter than 4 m without substantially
> degrading its performance. Briefly stated, these are:

i.  A spectrometer performs optimally when the energy resolutions of the
    analyzer and the monochromator are matched; the shorter the analyzer
    flight-path, the more out of balance and less optimized the
    spectrometer becomes for the energy transfers of interest.

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

iii. It follows from the translational invariance of crystalline
     materials that an increase in the reciprocal space surveyed by the
     instrument beyond one Brillouin zone (BZ) does not lead to a
     proportional increase in the instrument throughput.

iv. Complete spatial separation of spin-up and spin-down neutrons in the
    detector bank increases polarization analysis efficiency. When
    supermirror-bender type polarization analyzers of the type currently
    envisioned for HYSPEC are employed, such separation requires a
    secondary flight-path at least 4 m long.

> It is essentially optimization of the throughput (flux-on-sample) that
> is achieved by matching the instrument and moderator characteristics.
> It is this optimization that requires the 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 a relatively coarse
> (but nonetheless acceptable) ∆E/E resolution of about 10% for E~i~=
> \[10,60\] meV. Relaxing the resolution beyond 10% is an unacceptable
> compromise for most experiments; reducing the secondary flight-path
> while maintaining the 10% resolution constraint would require an
> equivalent shortening of the sample illumination time so that a
> smaller fraction of the neutrons available in the moderator pulse are
> used. It should also be noted that floor space for the analyzer within
> the experimental hall is no longer at issue. Recently the SNS
> Instrument Team - in close collaboration with the HYSPEC IDT - has
> developed a new floor plan in which HYSPEC is located on beamline 14B
> and - even with a 4.5 m secondary flight path -- the instrument is
> still entirely within the SNS experimental building.
>
> A spectrometer that is designed to operate in the sub-thermal rather
> than thermal neutron range could, of course, have a shorter secondary
> flight path because the time of flight of the scattered (slow)
> neutrons in the analyzer is longer and permits correspondingly longer
> sample illumination times. This is the case in the FOCUS spectrometer
> at PSI cited by the referee which is optimized at E~i~=5 meV and has
> 2.5 m long analyzer. We note, however, that to match the FOCUS elastic
> resolution at E~i~=50 meV with a similar sample illumination time, a
> thermal neutron spectrometer would have to have a 25 m long secondary
> flight-path! We also note that while a long secondary flight-path in
> HYSPEC could, in principle, be used to achieve higher energy
> resolution at a cost of intensity, this is not the regime for which
> the instrument was designed. Clear evidence of this can be seen in the
> comparison of the performance of the HYSPEC, CNCS, ARCS and SEQUOIA
> instruments made by Granroth and Abernathy, which is cited by the
> referee. But contrary to his impression, HYSPEC as it is currently
> designed could not operate efficiently at the high resolutions for
> which the other instruments are optimized. To achieve such resolution
> in their simulations, Granroth and Abernathy had to reduce the disk
> chopper slot widths to unrealistically small values, which (not
> surprisingly) had a substantial impact on intensity. Nonetheless, even
> in this very unrealistic configuration HYSPEC noticeably outperforms
> the other instruments in the 10 - 30 meV thermal neutron range for
> which it is optimized.
>
> We now comment on the referee remarks that neither HYSPEC'S
> polarization analysis capability nor the *"path that detectors cut
> through Q-E space"*, which *"are the same for all instruments"* make
> it unique among the SNS instruments.

i.  We share the referee's enthusiasm for the potential of ^3^He
    filter-polarizers and are completely open-minded about eventually
    upgrading to ^3^He-based polarization analysis when filters of
    appropriate design and acceptable performance become available. But
    we would be less than responsible if we based the HYSPEC design on
    their future availability. Current projections suggest that ^3^He
    polarizers with the large angular acceptance needed for instruments
    like HYSPEC are not likely to appear on a time scale of less than a
    decade which means that for at least the first 5-10 years of SNS
    operation HYSPEC will be the only inelastic spectrometer with a
    polarization-analysis capability. Furthermore, ^3^He
    filter-polarizers are extremely sensitive to magnetic field
    gradients and their maintenance and operating costs are likely to be
    high. Their eventual application will therefore require the
    development and installation of appropriate magnetic shielding (far
    from trivial) as well as a substantial increase in operating funds.
    The currently envisioned HYSPEC polarized beam arrangement, on the
    other hand, is insensitive to stray magnetic fields and essentially
    maintenance free.

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i.  As the referee notes, the phase space probed by HYSPEC for any given
    E~i~ is the same as that measured by the corresponding detectors in
    any other SNS inelastic instrument; i.e. in an E~i~ = *const* scan.
    But it is well-known that such measurements suffer from the fact
    that the momentum space probed shrinks rapidly with increasing
    energy transfer. To cover the same amount of reciprocal space at
    higher energy transfers, it is necessary to increase the incident
    energy. HYSPEC's preferred mode of operation is to cover roughly one
    Brillouin zone (but always the same one), for an extended range of
    energy transfers by making a series of measurements with varying
    E~i~'s. This requires nothing more than rotating the sample and
    positioning the detector bank so that the measurement is focused on
    the same region of the reciprocal space for all E~i~'s. A similar
    mode of operation is possible, at least in principle, for
    conventional TOF spectrometers (although, severely restricted in
    practice for the Fermi-chopper instruments where a limited number of
    discrete E~i~'s are available). In such instruments, assuming the
    sample is properly aligned as the incident energy changes, the
    Brillouin zone of interest would be measured by different parts of a
    large detector bank for each E~i~. In HYSPEC on the other hand, a
    much smaller detector bank is used but with very high efficiency: it
    follows the movement of the sample in reciprocal space as E~i~
    changes and always probes the same part of it, which is actually
    what is of most interest to the experimenter.

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1.  While we are pleased that referee acknowledges the strong scientific
    case presented in the HYSPEC proposal, we find his concern that
    *"the focus of the IDT is quite narrow"* and *"there is very little
    participation of the general university research community"*
    surprising. It is of course true that the HYSPEC IDT was initiated
    by scientists at Brookhaven National Laboratory and therefore has a
    very significant BNL representation. The implication that *"the
    great bulk of the members are at the existing neutron sources at the
    national labs"* is somehow inappropriate for an IDT that is building
    a neutron spectrometer at a DOE laboratory seems to us out of place.
    Moreover we note that the HYSPEC IDT also includes participants from
    almost all American universities that have neutron scattering
    programs in condensed matter. In fact, they account for roughly half
    of the non-BNL membership of the HYSPEC IDT. In addition to the
    University of Missouri and Iowa State University mentioned by the
    referee, MIT, Stanford, Rutgers, and Canada's McMaster University
    are represented. The real issue here is the shrinking support for
    neutron scattering programs at US universities. To cite a few
    examples: the Princeton group previously lead by Bernhard Keimer no
    longer exists, the MIT neutron scattering program is now reduced to
    one tenure-track professor and a postdoc, Martin Greven's group at
    Stanford has shifted its focus to X-ray scattering, the Rutgers
    group headed by Valery Kiryukhin, who studied neutron scattering
    with Keimer and Birgeneau, currently has hardly any neutron
    component. The scientific effort outlined in the HYSPEC budget is,
    in fact, aimed at addressing this problem and it is gratifying that
    the referee has made it a point to emphasize its importance. The
    funding requested is intended to be used to revive the neutron
    scattering efforts at the participating universities by supporting
    the hiring of both postdocs and graduate students to work on
    HYSPEC-generated scientific projects. We also envision that a
    neutron scattering program in condensed matter could be developed in
    the Physics Department at SUNY at Stony Brook, our partner
    university.

> Addressing the referee suggestion to open the IDT to "*a much broader
> range of participation"* we note that HYSPEC IDT membership policy is
> completely open. It does not require any particular set of
> qualifications or institutional affiliation, only approval by the
> Executive Committee. In fact, since the IDT was established it has
> grown by a number of members. No interested person has been refused
> admission.

2.  We conclude by addressing the referee comments on the preliminary
    budget estimate in the HYSPEC proposal. This is a difficult issue
    for us because the estimate was prepared with a very limited funding
    and without any of the engineering support needed for a project of
    this complexity and scale. Parts of it were, of necessity, based on
    the best estimates we were able to make at the time. For those
    components that are common to other instruments at the SNS we
    obtained cost estimates from the SNS Instrument Systems. These costs
    are reasonably well defined, and it is our intention to work closely
    with the SNS to update them as information from new procurements
    becomes available. More than half of the construction costs are,
    however, HYSPEC-specific and cannot be reliably estimated by
    referring to other SNS spectrometer construction programs. Realistic
    budget estimates for these components require the development of
    detailed specifications and using them to obtain reliable cost
    estimates from manufacturers. This involves a significant
    engineering effort. It is one we believe could be completed within
    12 months once funding becomes available. Once we arrive at a final
    budget we certainly expect that an independent team of experts would
    review it.

> In response to the referee's comments that *"the budget for the
> instrument seems excessive"*, we note that the main savings (\$3-4 M)
> that the referee proposed come from reducing the spectrometer
> secondary flight-path from 4.5 to 2 m. As we emphasized above, this
> would have an extremely detrimental impact on instrument performance.
> A more than two-fold loss of flux-on-sample at any given resolution
> would be inevitable and many other disadvantages would follow. Also
> the referee's suggestion to replace the counter-rotating disk chopper
> pair with a single disk chopper is simply not realistic as a cost
> saving measure; a single disk chopper in a 4 cm wide guide simply
> cannot provide the burst width that is needed for optimum operation.
>
> Along the same line, we note that employing a curved rather than
> straight guide as the referee suggests is one of the options we plan
> to pursue. But it is not obvious that curving the guide will turn out
> to be a either a viable option or lead to a significant cost
> reduction. Curving the guide reduces transmission at higher neutron
> energies in proportion to the radius of curvature. What needs to be
> kept in mind is that because HYSPEC is specifically designed for small
> samples it is of crucial importance to minimize beam-related
> background. Only a careful Monte Carlo based analysis will show if the
> small amount of curvature that could be introduced without appreciably
> degrading instrument performance at higher neutron energies would be
> sufficient in a relatively short (20 m) guide to eliminate the T~0~
> chopper.
>
> To make a balanced comparison with funded instruments such as, for
> example, ARCS and CNCS, one should look at the HYSPEC FY 2003
> estimated construction cost, which is \$14,750k. Taking into account
> the higher level of mechanical complexity of HYSPEC and also the fact
> that it has a polarized beam option (which would add more than \$1M to
> the cost of the other instruments), the preliminary budget does not
> look excessive.

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

There is currently very little experience in designing and building
neutron spectrometers optimized for single-crystal studies on pulsed
spallation sources. MAPS at ISIS is essentially the first attempt of
this kind. HYSPEC is a more innovative design which, we believe, offers
significant advantages for this particular class of studies. But it is
important to point out that it incorporates all of the time-tested
principles for the design of a time-of-flight (TOF) spectrometers on
pulsed neutron sources that are relevant. 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 (lattice periodicity) is a fundamental
    physical property that sets crystalline materials apart from
    non-crystalline materials such as glasses and liquids. This
    invariance has important consequences for the design of a single
    crystal inelastic spectrometer. The most immediate is that momentum
    is replaced by quasi-momentum, which is defined only within the
    Brillouin zone (BZ). The physical properties of such materials are
    periodic in the momentum space and, although covering large part of
    the reciprocal space is useful (*e.g.* for structure refinement or
    background characterization), the response in a single Brillouin
    zone contains essentially all of 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, say, eight
    Brilluoin zones does yield more information than one that covers one
    BZ, the gain in information is far from commensurate with the
    eight-fold increase in throughput. As a result, the common wisdom
    that the efficiency of the TOF spectrometer grows in proportion to
    its analyzer angular acceptance and that a loss in the flux on
    sample can be compensated by the increase in surveyed momentum space
    simply doesn't hold. A reliable measurement requires that data with
    a satisfactory signal-to-background ratio be collected in at least
    one Brillouin zone. In fact, because neutron scattering
    cross-sections varies strongly with Q from one Brillouin zone to
    another, combining the data from different zones is far from
    straightforward and, in many cases, the increase in surveyed
    momentum space doesn't increase the data collection rate at all.
    This is in stark contrast with disordered systems, such as liquids
    or glasses, where the scattering is not periodic in momentum space
    and the information content grows roughly in proportion to the
    increase in reciprocal space surveyed. Because non-crystalline
    systems were the primary object of study with the TOF spectrometers
    for many decades, it was customary to configure them so that the
    solid angle covered by the detectors was maximized at the expense of
    the secondary flight-path, which was made as short as possible
    consistent with the desired instrument resolution.

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1.  Here is how these considerations apply to HYSPEC. Even for a typical
    metal, say one with a lattice constant as short as 3.1415 Å and a
    Brillouin zone as large as 2 Å^-1^, an analyzer acceptance of 60
    degrees covers, in elastic scattering, about 1.3 Brillouin zones for
    E~i~ = 15 meV neutrons. Because the data collection rate cannot be
    efficiently increased by increasing the detector angular coverage
    (i.e. by surveying more than one Brillouin zone), 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 to at least match the full
    width at half maximum (FWHM) of the neutron pulse produced by the
    moderator. For a given analyzer energy resolution, the increase in τ
    requires that the secondary flight-path be increased proportionally.
    Because the pulse FWHM of the coupled hydrogen moderator is rather
    long, up to 100 μs (it decreases with 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 resolutions in the 5 to 10 percent range. In fact, the
    counter-rotating disc choppers in the design of HYSPEC cannot be
    rotated fast enough to obtain values of τ \< 40 μs, which would be
    required to achieve energy resolutions comparable to CNCS or
    SEQUOIA.

2.  It is a well-known fact that for optimum performance of an
    instrument, the energy resolutions of the primary and the secondary
    spectrometers (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 flight path is twice that of the
    analyzer, the instrument is balanced for energy transfers of roughly
    half the incident energy. This is close to an optimum setup and is
    implemented in the case of MAPS at ISIS. Of the spectrometers
    currently proposed for the SNS, the shortest (17.5 m) primary
    flight-path is planned for SEQUOIA. This, in combination with
    SEQUOIA's 6 m long analyzer means that it is balanced when the
    energy transfer range is roughly 2/3E~i~. As a practical matter, for
    an instrument 20 or more meters from the moderator, such as HYSPEC,
    the analyzer flight path has to be appreciably shorter than the
    monochromator flight path and the spectrometer is not balanced for
    most energy transfers. However, the longer the secondary flight-path
    (analyzer), the less out of balance - and therefore better
    optimized - the instrument will be. Because under such conditions
    the analyzer controls the energy resolution for small energy
    transfers, a longer secondary flight-path also leads to better
    elastic energy resolution. Our goal in developing the HYSPEC design
    was to optimize the instrument by balancing the set-up as much as
    possible rather than to achieve high elastic energy resolution, as
    referee appears to believe.

3.  Apart from balancing considerations, an additional restriction on
    the length of the secondary flight-path is imposed by the
    polarization analysis scheme based on transmission polarizers
    envisaged for HYSPEC. Because spatial separation of neutrons with up
    and down polarizations in the detector is required to measure them
    simultaneously (thus doubling the polarization analysis efficiency),
    the secondary flight-path cannot be much shorter than 4.5 m. In this
    regard it is important to mention that the issue of having enough
    floor space to accommodate the relatively long HYSPEC secondary
    flight path was successfully resolved when the instrument was
    relocated on beamline 14B. None of the additional expenses
    associated with locating part of the instrument outside the SNS
    experimental building are therefore envisioned.

> Further with respect to polarization analysis, while we share the
> referee's enthusiasm for the potential of ^3^He filter-polarizers and
> have made provision in the HYSPEC design to install a ^3^He
> filter-polarizer in the analyzer at such time as they become
> available, it would be less than responsible to design the instrument
> assuming their availability within the instrument construction time
> frame. In fact it was made clear at a recent conference on neutron
> polarization methods that ^3^He polarizers with the nuclear
> polarization and large angular acceptance required for use in
> instruments such as HYSPEC are not likely to be available within a
> decade or more. This means that for at least the first 5 to 10 years
> of SNS operation HYSPEC will be the only inelastic spectrometer with a
> polarization analysis capability at the facility.

4.  Finally, the referee notes that "*the path that the detectors cut
    through Q-E space are the same for all the instruments*", and that
    because of its smaller detector coverage HYSPEC "*appears to be less
    flexible*...". The issue of detector coverage was addressed above:
    what is important is that HYSPEC allows more than a whole Brillouin
    zone of a crystalline material to be surveyed at one time. Even more
    to the point, the referee's comment assumes that HYSPEC will be
    operated in the same way as the other inelastic TOF spectrometers;
    i.e. that the data will be collected in constant-E~i~ scans. This is
    not the intended mode of operation. In fact 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 fixed final energy). Nevertheless, a conventional direct
    geometry TOF spectrometer is more or less constrained to perform its
    scans with fixed E~i~ and analyze E~f~. The main disadvantage of
    this mode is that the momentum space surveyed by the analyzer
    shrinks at higher energy transfers, and the instrument throughput
    decreases very fast in the region where it is most needed! HYSPEC
    offers a unique opportunity to perform measurements where E~i~ is
    varied in steps - much like in a triple-axis spectrometer - so that
    the volume in the phase space surveyed 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 sequence of measurements; scans with E~i~ =
    5,10,15,20,30,40,60 meV, spending the different amounts of time at
    each incident energy needed to obtain the same statistical
    uncertainty in 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
    data set will thus result in a high-quality measurement with
    resolution and statistical uncertainties that will be much less
    dependent on the energy transfer than for scans with fixed E~i~.