![](media/image1.png){width="2.1458333333333335in"
height="0.8958333333333334in"}managed by Brookhaven Science Associates\
for the U.S. Department of Energy

www.bnl.gov

Building 510B

P.O. Box 5000

Upton, NY 11973-5000

Phone 631 344-3761

Fax 631 344-2739

zaliznyak@bnl.gov

HYSPEC Top Level Specifications

This document defines "top-level" requirements for all components of the
hybrid spectrometer, HYSPEC, planned for installation on beamline 15
(BL15) of the Spallation Neutron Source (SNS) that is now under
construction at Oak Ridge National Laboratory. All HYSPEC components,
whether designed by the Instrument Development Team (IDT), members of
the SNS staff, or by third party manufacturers, are expected to comply
with these requirements. It becomes effective only if signed by all of
the parties whose names appear below. Changes and additions (or
deletions) can be initiated by any of the signatories (or their
replacements) at which time a revised document must be prepared with all
modified sections identified as such. They become effective only after
all parties sign the revised document. Whenever revisions are made, the
earlier document is to be retained as part of the IDT record.

Principal Investigator: S. Shapiro ................................,
Date .........\...

Principal Investigator: I. Zaliznyak ................................,
Date .........\...

Project Scientist: TBA ................................, Date
.........\...

SNS Instrument Systems: K. Crawford ................................,
Date .........\...

SNS EFD Head: I. Anderson ................................, Date
.........\...

SNS Director: T. Mason ................................, Date
.........\...

Requirements for HYSPEC.[^1]^1)^

1.  **Instrument footprint and placement on the floor.**

> HYSPEC shall be placed at the end position of the SNS beam-line
> illuminated by a coupled, supercritical H~2~ moderator. Sufficient
> floor space must be available at this position to accommodate the
> instrument's secondary spectrometer for all incident energies and
> sample scattering angles specified in sections 3 and 5 below. The
> greater part of the instrument footprint will be a 6.3 m radius
> semi-circle, on one side of the beam-line or the other. It will be
> centered at the monochromator position, at a distance L~SM~ of about
> 20 to 25 m from the moderator face; nominal moderator-to-monochromator
> distance L~SM~ = 20000 mm is used in this document. It is assumed that
> this semi-circular footprint shall be entirely inside the SNS
> experimental hall. A combined "get-lost" pipe and beam-stop -- if they
> prove to be necessary -- may extend outside the building.
>
> It is currently envisioned that HYSPEC will be located on BL15, behind
> a shorter instrument served by an ambient water moderator on BL16A.
> Should SNS Instrument Team judge that spectrometer footprint cannot be
> accommodated on BL15 in accordance with the above requirements a
> mutually acceptable alternative shall be negotiated between the HYSPEC
> IDT and the SNS. The possible alternatives, in the IDT order of
> preference, are: (i) HYSPEC reassignment to another beam-line served
> by a coupled, super-critical H~2~ moderator, such as BL14B; (ii)
> slight modification of the instrument design by reducing the secondary
> flight-path: the energy resolution is relaxed proportionally, but the
> footprint is reduced also; this, however, may also impact the
> polarization sensitivity; (iii) complete re-design of the instrument
> secondary spectrometer so that the longest secondary flight-path
> available for each scattering angle is used, as a result the footprint
> becomes elongated and "pear-shaped"; (iv) extending the spectrometer
> primary flight-path to a building outside the SNS experimental hall;
> at a given analyzer resolution this reduces the incident neutron flux
> in proportion to the length of the primary spectrometer, and, while
> being more expensive, is somewhat equivalent to reducing the secondary
> flight-path.

2.  **Primary flight-path system**

    1.  **Neutron guide system**

> The incident neutron guide system is to be composed of three
> super-mirror guide sections, **G1-G3,** all with top and bottom
> coatings with critical angles of 3θ^Ni^~c~ and with side coatings with
> the largest practical critical angle larger than 3θ^Ni^~c~. **G1** is
> to be integrated into the primary shutter. So located, its upstream
> end with an internal aperture of *w~0~* x *h~0~* = 40 mm x 128 mm will
> be at a distance L~0~ = 2000 mm from the front face of the coupled,
> supercritical H~2~ moderator. Gaps of the smallest possible size are
> to be provided in the guide system to accommodate shutter, choppers
> and (if necessary) a beam filter assembly. **G2** and **G3**, the
> sections external to the biological shield, are to be steel-jacketed.
> All guide sections shall be 40 mm wide; their internal vertical
> profiles will be chosen to optimize the neutron current illuminating
> the 240 mm tall, vertically-focusing monochromator crystal that will
> be located downstream of **G3** at a distance of L~SM~ = 20000 mm from
> the front face of the moderator. Optimization is to be achieved at a
> reference incident neutron energy E~i~ = 15 meV. Existing Monte-Carlo
> simulations indicate that the optimum guide dimensions are:

  --------------- --------------- ------------------- -------------------
  Guide section   Length          Entrance size, *w*  Exit size, *w* x
                                  x *h*               *h*

  **G1**          5000 mm         40 mm x 128 mm      40 mm x 150 mm

  **G2**          11000 mm        40 mm x 150 mm      40 mm x 150 mm

  **G3**          2000 mm         40 mm x 150 mm      40 mm x 180 mm
  --------------- --------------- ------------------- -------------------

> These dimensions will be further refined as design work proceeds,
> however, and might, at a later stage, differ slightly from the above
> values.
>
> The guide support system should be designed to allow easy alignment of
> the individual guide segments (both vertically and laterally) with
> positional precision better than 0.5 mm and angular precision better
> than 1 minute of arc.
>
> To the extent feasible, the primary flight path sections should be
> either evacuated or filled with ^4^He gas to minimize air scattering
> losses.

2.  **Beam line shielding**

> Beam line shielding shall consist of a stationary bottom piece
> carrying the guide support and alignment systems and removable modular
> pieces on the sides and top. The removable shielding is to be designed
> so as to: (i) minimize the space immediately surrounding the guide
> system (as specified in section 2.1) that is not filled with shielding
> material; (ii) minimize the total thickness and weight of the modular
> shielding and (iii) provide rapid and convenient access to the
> choppers and (if installed) filters for maintenance and, if necessary,
> for their removal. Shielding shall be composite and contain materials
> needed both to slow down and absorb neutrons and to absorb the prompt
> gamma radiation. Radiation dose rates and background outside the beam
> line shielding shall be as low as reasonably achievable (ALARA); less
> than 5 mR/hr and in compliance with the limits established by the SNS.

3.  **Shutter**

> The primary beam shutter for the instrument shall be the SNS shutter
> inside the biological shield. When closed it must reduce the radiation
> incident on the choppers and monochromator crystal sufficiently to
> allow them to be removed for repairs while the SNS is operating at
> full power and also reduce the radiation level at the sample position
> to less than 5 mR/hr. The **G1** section of guide is to be built into
> the "open" channel of the shutter, as specified in section 2.1.
> Shutter position shall be controlled by a single switch interlocked to
> form a radiation exclusion zone details of which are to be specified
> by SNS. A clearly visible sign will show the shutter position; this
> information shall also be available to the instrument control
> computer. Shutter opening and closing times are to be less than 30
> sec.

4.  **Filters**

    If MCNP-X based neutronics calculations indicate that single crystal
    beam filters can significantly reduce fast neutron beam
    contamination, a three-position filter exchanger is to be positioned
    immediately downstream of the in-shield shutter. The filters are to
    be large enough in their lateral dimensions to intercept the full
    beam as specified in section 2.1. Each filter shall have the same
    supermirror coatings and shielding as the **G1** guide so that
    neutrons must either pass through the filter or be absorbed in the
    surrounding shielding. A refrigerator system should be provided to
    cool the filters to 77 K (or colder) and maintain them at that
    temperature even when fully illuminated by the neutron beam. Filter
    transmission better that 60% for neutrons throughout the energy
    range 5\<E~i~\<90 meV is defined as being acceptable. Filter changes
    are to be effectuated from the instrument control computer. The
    filter choices shall be:

    1.  **None:** An open channel with lateral dimensions that
        accommodate the full beam as specified in 2.1.

    2.  **Sapphire:** Single crystal sapphire grade B4 or better with a
        beam path length of 100 mm (available from Crystal Systems
        Inc.). The orientation of the single crystal material is to be
        uniform throughout the filter to within 10 degrees. However, the
        average crystal orientation with respect to the beam direction
        is unimportant and should be chosen to minimize cost.

    3.  **PG:** Pyrolytic graphite grade ZYH with a total beam path
        length of 100 mm (available from Advanced Ceramics). The c-axes
        of the individual crystal plates must be aligned to within 2
        degrees of the local beam axis.

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3.  **Monochromating system**

    1.  **General principle**

> Incident neutron energy selection in HYSPEC shall be defined by the
> combined effects of time-of-flight (TOF) along the primary flight-path
> and Bragg reflection from the monochromator crystal. TOF energy
> selection relies on restricting the burst time τ~0~; i.e. the time
> interval during which the sample is illuminated by the incident
> neutron beam. A counter-rotating disc chopper pair, **T~2~,**
> positioned as close as possible to the upstream face of the
> monochromator crystal is to be used to define burst times τ~0~ \> 50
> μs. An additional "rotating collimator"-type Fermi- chopper, **T~3~,**
> operating at high rotation speeds shall be used in those cases where
> shorter burst times are needed. It is to be placed at the downstream
> end of the converging supermirror guide **G4** between the
> monochromator crystal and the sample, as described in section 3.5.
>
> Crystal monochromator with variable vertical curvature shall serve to
> vertically focus the 240 mm tall neutron beam delivered by the guide
> system onto a sample placed in the beam center at a distance L~MS~
> from the monochromator rotation axis. The crystals will also serve as
> pulse-shapers, cutting off much of the unwanted higher-energy tail
> from the spectral distribution of neutrons produced by the moderator.
> Pyrolytic graphite crystals will be used for most unpolarized beam
> applications although in certain cases where very low incident neutron
> energies are needed fluorinated synthetic mica crystals may be
> employed. For polarized beam studies, a vertically-focusing Heusler
> alloy crystal monochromator will be used to provide polarized
> monochromatic neutrons incident on the sample. The monochromator
> crystals shall be mounted in fixtures such that they can be remotely
> aligned and also remotely rotated around a vertical axis to define the
> "take-off" angles needed to Bragg reflect neutrons with energies in
> the nominal range 2.5\<E~i~\<100 meV.
>
> A sample position arm rotating around the monochromating crystal shall
> connect the monochromator axis and the sample stage with its alignment
> and rotation axes. The arm shall hold a converging super-mirror guide,
> a Fermi chopper, a collimator, an incident beam monitor, beam defining
> slits, beam attenuators (including a beam shutter), and a polarized
> beam equipment such as a guide field and flipper for polarized
> applications. It shall be rigidly attached to the rotating exit
> aperture in the monochromator crystal shielding in such a way that the
> sample is always at the center of the monochromatic neutron beam
> reflected by the crystal. The converging super-mirror guide,
> Fermi-chopper and collimator are all to be contained within the
> monochromator shielding. Distances from the monochromator to the
> sample, L~MS~, and to the upstream counter-rotating chopper pair,
> L~T1M~, should be minimized while maintaining all other
> specifications. In normal operation the nominal value for L~MS~ is
> assumed to be 1800 mm.

2.  **Disk choppers**

> Three disk choppers designated **T~0~**, **T~1~** and
> **T~2\ ~**rotating about their horizontal axes at a multiple or
> sub-multiple of the source frequency and kept very precisely in phase
> with the source shall be installed in the primary flight-path. All
> should fit as tightly within the gaps between guide sections as
> possible, housed in SNS approved, blast-proof housings, and should
> have neutron-transparent windows which, as the chopper rotate, provide
> time-dependent and uniform vertical illumination of the full
> downstream guide apertures as specified in section 2.1. Beam
> transmission should be at least 98% in the chopper "open" positions
> for neutrons with energies in the range 2.5\<E~i~\<100 meV; a minimum
> beam attenuation of 10^-5^ in the chopper "closed" position is defined
> as being acceptable. To the extent feasible, chopper modules shall be
> based on the standard designs developed by the SNS Instrument Systems
> Group. Because choppers are significant sources of scattering,
> additional modular shielding is to be placed around the beam-line at
> each chopper position to reduce radiation leakage to acceptable levels
> as specified in section 2.2.

1.  **T~0~ chopper:** A chopper with a highest rotation rate of 120 Hz
    and a lowest rotation rate 30 Hz made of a material that efficiently
    blocks fast neutrons and gamma-rays from the moderator is to be
    placed as close as possible to the moderator shielding face or
    possibly inside it. This chopper must block prompt fast neutrons and
    gamma rays but be open to neutrons in the energy range
    2.5\<E~i~\<100 meV. Also its "open" channel should be
    supermirror-coated and of the same lateral dimensions specified in
    section 2.1 for the surrounding guide so that the downstream guide
    is fully illuminated when the chopper is open.

2.  **T~1~, the order**-**suppressing and frame-overlap chopper:** This
    chopper is to be a 300 mm radius, single rotor disc chopper with a
    highest rotation rate of 240 Hz and a lowest rate of 30 Hz. It shall
    be located either between guide sections **G1** and **G2**, or in a
    gap in **G2**. The chopper should be designed to remove all unwanted
    higher-order neutrons that could otherwise pass through the **T~2~**
    chopper (when operating at high rotation speeds) and be reflected by
    the monochromator crystal. In cases where frame overlap
    contamination becomes a problem either this chopper or the **T~0~**
    chopper (or both) should be capable of being operated at half the
    source frequency (30 Hz) to block alternate pulses from the
    moderator.

3.  **T~2~, the energy-defining chopper:** This is a double-rotor
    chopper composed of a pair of the counter-rotating 300 mm radius
    disk choppers with a highest operational rotation rate of 360 Hz. It
    should be placed as close as practical to the front face of the
    monochromator crystal, either between guide sections **G2** and
    **G3** or in a gap in guide **G3**.

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3.  **Monochromator crystal assemblies**

> HYSPEC shall have two or more vertically focusing, segmented crystal
> monochromators mounted on a remotely-controlled exchanger; one shall
> be of pyrolytic graphite (PG) and one of Heusler alloy. Optional
> additional monochromator crystals based on intercalated PG, synthetic
> mica, silicon or germanium will be specified at a later stage. Their
> design is to be based on standard GMI-type segmented frames with a
> single-motor-driven mechanism to vary vertical curvature. Individual
> crystal segments shall be attached to aluminum alloy plates of the
> minimum practical thickness using aluminum screws or pins. All
> monochromator crystal assemblies should be of dimensions *w~m~* x
> *h~m~* = 240 mm x 240 mm so as to accommodate the full size of the
> neutron beam exiting guide **G3** for all scattering angles 20º \<
> 2θ~m~ \< 120º. All parts of the monochromator frame and its support
> and crystal exchange mechanisms and such other components as a
> magnetic yoke to provide the field needed to saturate the Heusler
> crystals are to be covered with a neutron-absorbing material such as
> B~4~C and/or LiF. It should be possible to remove either or both
> monochromating crystal assemblies from the instrument for service
> while the SNS is in full power operation. A web camera with
> appropriate lighting shall be provided to enable remote viewing of the
> monochromators for diagnostic purposes when they are moved.

1.  **PG monochromator:** shall be assembled of 3x60 = 180 individual
    rectangular plates of ZYA grade pyrolytic graphite (from Advanced
    Ceramics or another vendor) with an isotropic intrinsic (x-ray)
    mosaic spread with a full width at half maximum (FWHM) of 25(5)
    minutes of arc. The plates shall be 1 mm thick and have lateral
    dimensions *w* x *h* = 80 mm x 12 mm. The plates shall be attached
    to the holder in sets of three and stacked with aluminum spacers to
    slightly offset the angles between them, so that the effective
    horizontal mosaic FWHM of the array is 60(6) minutes. Voids between
    the adjacent plates shall not exceed 0.5 mm.

2.  **Heusler alloy monochromator:** An array of precisely aligned
    Cu~2~MnAl crystals (well annealed so that the (111) Bragg
    reflectivity approaches that expected for ideally imperfect
    crystals) is to be mounted on a standard frame as described in
    section 3.1. The Mn moments in the crystals are to be fully aligned
    by an external magnetic field produced by permanent magnets
    co-mounted with the frame. A polarization efficiency better than 95%
    and reflectivity better than 50% for polarized neutrons at a
    reference energy E~i~ = 15 meV are defined as being acceptable. This
    monochromator assembly is to be procured from a vendor such as ILL
    or ORNL with a demonstrated ability to produce Heusler crystals of
    good performance.

3.  **Optional additional monochromators:** A set of
    potassium-intercalated PG or synthetic mica crystals, whichever is
    more practical, is to be mounted on a standard GMI frame for very
    low-neutron-energy applications. Also to be considered as a possible
    future option is a high-wave-vector-resolution and/or perfect
    crystal focusing arrangement based on silicon or germanium crystals.

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4.  **Monochromator exchanger**

> This device shall enable computer-controlled remote selection of one
> or the other of two crystal monochromator assemblies. Monochromator
> exchange is to take less than 1 minute. During typical operation
> exchange might occur as often as once a week. The exchanger should be
> designed so that either of the monochromators can be removed from the
> incident beam for service while the SNS is operating at full power.

5.  **Monochromator-to-sample supermirror guide**

    A vertically-tapered, vertically-converging supermirror guide,
    **G4,** with a supermirror coating of the largest practical critical
    angle larger than 3θ^Ni^~c~ shall be located on the sample arm
    between the monochromator and sample. It should extend from as close
    to the monochromator as possible to within 400 mm of the sample. It
    shall be 40 mm wide; the inside height as a function of the
    distance, *x*, from the sample shall be given by

    $h(x) = h_{s} + \left( h_{m} - h_{s} \right)\ \frac{x}{L_{\text{MS}}}$,

> where *h~s~=*40 mm is the nominal sample height, *h~m~*= 240 mm is the
> monochromator height and *L~MS~* = 1800 mm is the nominal
> monochromator to sample distance. **G4** shall be split into three
> sections, **G4a**, **G4b**, and **G4c**, all mounted on the sample arm
> as described in section 3.1. A computer-controlled exchanger should
> permit remote replacement of the 150 mm long middle section **G4b** by
> a Fermi-chopper module as specified in section 3.6. Additionally there
> should be a computer-controlled exchanger permitting remote
> replacement of the 150 mm long **G4c** section (the closest to the
> sample) by one of a set of Soller collimators as specified in section
> 3.7. The guide sections shall be either evacuated or filled with ^4^He
> gas, wherever this will significantly reduce beam air scattering.
>
> Each guide segment shall be centered and aligned with respect to a
> horizontal reference line connecting the sample rotation axis to the
> monochromator rotation axis. Centering should be accurate to within
> 0.5 mm and alignment accurate to better than 1 minute of arc. All
> guide segments inside the monochromator drum are to be tightly
> embedded in the shielding. Materials that are illuminated by the
> monochromator and visible from the sample position should have the
> smallest possible incoherent scattering cross section.

6.  **Fermi-chopper**

> For high resolution applications, a computer-controlled exchanger
> shall replace guide section **G4b** with a module of matching size
> containing a Fermi-chopper, **T~3~**, designed to reduce the sample
> illumination times to 10 μs \< τ~0~ \< 50 μs. Because this chopper is
> not intended to be energy-selective it should have short, straight
> slots and rotate about a vertical axis in phase with the source at up
> to 480 Hz (8 times the source frequency). The outer diameter of the
> rotor shall be 120 mm or less, and the height of the slotted channel
> should match the exit height of the **G4a** section of guide.

7.  **Collimators**

> When necessary, a computer-controlled exchanger should make it
> possible to replace the guide section **G4c** nearest the sample with
> one of a set of four Soller collimators of identical external
> dimensions. The four collimators should define effective horizontal
> beam divergences of 20\', 40\', 60\', and 120\' respectively.
> Collimator blade thicknesses should be 0.1 mm or less. If feasible,
> the top and bottom surfaces of the collimators should have the same
> super-mirror coatings as guide **G4c**.

8.  **Monochromator shielding**

> The monochromator shall be surrounded by a neutron and gamma-ray
> absorbing composite shielding with a movable beam-port opening that is
> mechanically connected to the sample arm and rotates with it. To the
> extent practical it should be constructed of non-magnetic materials
> that have minimal long term neutron activation cross sections. The
> range of take-off angles accessible through the beam-port should be
> -5° to 120°. It should take less than 2 min. to change the take-off
> angle from one extreme to the other. The combined monochromator
> crystal and beam-port rotation should define the monochromator crystal
> take-off angle to ± 0.02^o^ or less. Whether the shielding mechanical
> design is to be based on a drum, pie-wedge section or other concept is
> yet to be determined.
>
> The beam-port opening in the shield should be of internal dimensions
> that accommodate the converging guide **G4a-c**, the Fermi-chopper and
> the collimators. The shield design should also provide for
> incorporation of the remote exchange mechanisms. Shielding must be
> fitted tightly around all neutron optics elements attached to the
> sample arm as specified in sections 3.5-3.7 (with radiation steps
> where necessary) so that neutrons from the monochromator either pass
> through these elements or are absorbed in the shielding. The inner
> dimension of the shield should be such as to accommodate the
> monochromator crystal exchange mechanism with two monochromator
> crystal assemblies. Radiation protection requirements will determine
> the outer radius of the shield. It must contain neutron and gamma ray
> absorbing material sufficient to reduce the radiation rate and the
> background to acceptable levels: i.e. the radiation dose rate, and
> background outside the shielding and outside of monochromatic beam
> should be no more than 5 mrem/hr at a distance 300 mm from the neutron
> beam and in compliance with all SNS radiation protection requirements
> and ALARA. Shield thickness should not exceed 1400 mm, so that a
> clearance of at least 400 mm between the shield and the sample axis is
> provided. Internal shield design should be such that the amount of the
> material inside the shield exposed to the incident neutron beam is
> minimized; all shielding around the monochromator that can be viewed
> from the sample position must be recessed in such a way that it is not
> directly illuminated by the incident neutron beam. A convenient access
> to the monochromators and other beam-line neutron optic elements for
> servicing and replacement must be provided.
>
> The location and characteristics of the beam dump or "get lost" pipe
> (whichever is applicable) must be chosen to minimize its contribution
> to the detector count rate. In addition, an interlocked radiation
> exclusion zone around the instrument is to be defined in collaboration
> with the SNS operations staff.

9.  **Beam optics between collimator and sample**

> A neutron optical bench shall be permanently attached to the sample
> arm and/or to the beam-port outside the shielding and aligned to
> better than 1 minute of arc with respect to a horizontal reference
> line connecting the sample rotation axis to the monochromator rotation
> axis as described in section 3.5. All components listed below are to
> be mounted on this bench downstream of the **G4** super-mirror guide.
> Components 3.9.1-3.9.3 shall be semi-permanently mounted; their
> combined thickness must not exceed 80 mm.

1.  A beam monitor with a sensitive area larger than the exit aperture
    of the guide **G4c** and with a wavelength-proportional sensitivity
    that is no greater than 10^-5^ at 15 meV.

2.  A four position beam attenuator controlled from the main instrument
    control computer and capable of introducing three different planar
    objects into the beam. Two of the positions should provide 10 times
    and 100 times attenuation of the beam respectively at 15 meV. These
    two attenuators shall be permanently installed in the exchanger. The
    third position is to be slot that can hold a plate of width 40 mm,
    height 100 mm and between 1 mm and 10 mm thick. When selected by the
    attenuator exchanger this plate shall be positioned in the center of
    the beam to within 1 mm. The fourth position shall be a local beam
    shutter.

3.  A computer-controlled, variable-opening, thermal neutron aperture
    covered with layer of LiF on the side facing the monochromator
    crystal. The aperture must be centered in the beam to within 0.5 mm
    and should have four independent degrees of freedom controlling the
    right, left, top and bottom opening with a positioning accuracy of
    better than 0.5 mm. The design should permit the width and height of
    the aperture to be varied from fully closed to the full width and
    height of the beam as defined by the exit aperture of the guide
    **G4c** as specified in section 3.5.

4.  A computer-controlled aluminum flipper coil, not more than 20 mm
    thick, designed to rotate the neutron polarization in the incident
    beam by 0°-180°.

5.  Auxiliary neutron optics components (of a total length not exceeding
    150 mm) required for applications to be specified in the future.

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4.  **Sample stage**

> The sample stage shall be rigidly attached to the sample arm and
> supported on the floor in such a way that it can move with the arm
> without tilting as it rotates to beam position defined by the
> monochromator crystal take-off angle. The positional accuracy of the
> sample stage should correspond to the same 0.02° precision as for the
> combined monochromator crystal and sample arm setting angle, as
> specified in section 3.8. Sample area shall be surrounded with
> interlocked shielding that reduces radiation levels to 5 mrem/hr or
> less and meets all SNS radiation protection requirements.

1.  **Space and location**

> The space available for sample stage positioning must allow
> monochromator crystal take-off angles covering the range
> -5°\<2θ\<120°. Space for the power supplies, pumps and other auxiliary
> equipment required for both polarized beam operation and special
> sample environments must also be provided on the sample floor. The
> distance from the sample rotation axis to the monochromator crystal
> rotation axis shall be variable within restricted limits which are to
> be defined. It should be no greater than necessary to accommodate a
> Fermi-chopper, a Soller collimator, a neutron beam monitor, and
> polarized-beam equipment (such as a flipper) as specified in sections
> 3.5-3.9. A monochromator-to-sample distance *L~MS~* = 1800 mm is
> defined as nominal.

2.  **Degrees of freedom provided.**

    1.  Sample rotation 0-360^o^ with an accuracy of 0.005 degrees.

    2.  Tilt of sample table ±12^o^ about two mutually perpendicular
        horizontal axes. Tilt accuracy must be better than 0.1^o^ for
        loads in the range specified in section 4.3; the effective
        rotation axes must lie within 20 mm of beam center height.

    3.  Vertical translation of ±40 mm above and below the beam center
        with positional accuracy better than 1 mm.

    4.  Horizontal translation of ±20 mm along two mutually
        perpendicular horizontal directions parallel to the sample tilt
        axes, both with positional accuracy better than 0.5 mm.

3.  **Dimensions and load capacity.**

> The sample stage mounting surface shall be at least 152 mm below the
> beam center. Its on-axis load capacity shall be \>500 kg. A (maximum)
> horizontal torque of 5·10^2^ N·m should result in less than a 0.1^o^
> tilt of the sample rotation axis from the vertical.

4.  **Materials requirement**

> Sample table shall be made of non-magnetic materials. The force on any
> of its elements in a magnetic field gradient of 10 Gauss/mm should not
> exceed 1 N.

5.  **Guide field**

> A computer-controlled coils capable of generating either a vertical or
> a horizontal magnetic field larger than 10 Gauss throughout the
> neutron beam path around sample are to be provided. Coils should be
> mountable on the sample table and be large enough to accommodate a
> standard sample environment (ie provide a horizontal clearance of at
> least 400 mm in diameter around the sample axis).

5.  **Analyzer-Detector system**

    1.  **General principle**

> Analysis of the scattered neutron energy will be by time-of-flight in
> a secondary flight-path of length *L~SD~* = 4500 ± 4 mm as measured
> from the sample axis to the individual detector axis. The secondary
> flight-path shall include a variable vertical aperture after the
> sample, shielding, a gas-tight or vacuum-tight flight vessel
> containing either a radial collimator or a transmission polarizer
> assembly and a detector array as specified below. The secondary
> flight-path (analyzer) vessel shall be mobile, and designed to move on
> the floor in such a way that: (i) for all monochromator crystal
> take-off angles the axis of the cylindrical detector array remains
> vertical and coincides with the sample axis to an accuracy of better
> than 2 mm (ii) the detector bank can be sequentially moved (as
> specified by the experimenter) to those positions where the range of
> sample scattering angles accepted by detectors is best matched to the
> measurement.

2.  **Detector bank**

> The detector bank shall consist of a cylindrical array of identical,
> vertically mounted, one-dimensional, position-sensitive, tube-type
> ^3^He detectors centered on the sample axis and symmetric about the
> horizontal scattering plane. The angular size of the individual
> detector pixels (as seen from the sample position) shall not exceed 20
> minutes of arc in either the vertical or horizontal direction. Each
> detector shall cover a scattering angle of at least ± 7.5° in the
> vertical direction; the horizontal angular coverage of the entire
> detector array will be at least 60°. Detectors shall have partial
> pressure of ^3^He gas and thickness to achieve 90% detection
> efficiency for 15 meV neutrons.
>
> An array of 190 or more 64-pixel, position-sensitive detectors 20 mm
> to 25 mm in diameter and 1200 mm to 1300 mm long satisfying the above
> specifications are to be purchased from an established detector
> manufacturer. They shall be mounted with their axes vertical to within
> 5 minutes of arc and positioned with an accuracy of 2 mm on a 4500 mm
> radius circle in the horizontal plane. This is also the accuracy to
> which the detectors are to be co-centered at the nominal beam-sample
> height. Voids between adjacent detector tubes are not to exceed 1 mm.
>
> The detector array shall be fixed inside the mobile analyzer vessel.
> The vessel design will allow the detector array rotation around an
> axis concentric with the sample axis to within 2 mm with the accuracy
> of angular positioning better than 0.02°. Detector voltage supplies,
> preamplifiers and primary acquisition electronics shall be mounted on
> the analyzer vessel and move with it to reduce the number of cables
> connecting the mobile detector array to the stationary part of the
> data acquisition system to 1 or 2 high-speed, optical or other,
> communication lines.

3.  **Analyzer vessel**

> Except for the initial 400 mm downstream of the sample axis, the path
> of the neutrons scattered by the sample shall be contained within a
> mobile secondary flight-path (analyzer) vessel which is to be either
> evacuated or ^4^He/Ar gas-filled, to minimize beam loss and background
> from air scattering. The auxiliary equipment needed to either maintain
> vacuum or gas pressure shall be permanently mounted on the vessel and
> shall move with it. Either a radial collimator or a transmission
> polarization analysis assembly shall be positioned in the scattered
> beam inside the flight vessel, with its upstream face at a nominal
> distance of 400 mm from the sample axis, as specified in sections
> 5.6-5.7. The vessel shall be equipped with a computer-controlled
> automated exchanger that selects either one of the four radial
> collimators or one of the two polarization analysis assemblies.

1.  **Materials:** The analyzer vessel shall be made of high-strength
    aluminum alloy or similar non-magnetic material with a low neutron
    activation cross section. Its inner walls and all parts of the
    collimator or polarization analysis assembly that are visible to the
    detector array should be covered with a neutron-absorbing material
    such as B~4~C or LiF. The scattered beam entrance window on its
    upstream face should be made of high neutron transparency, high
    strength aluminum alloy and be of the minimal thickness necessary to
    support either vacuum or a small over-pressure of gas within the
    vessel.

2.  **Dimensions:** The size and the shape of the inner and outer
    cylindrical walls of the flight vessel shall be designed to (i)
    allow either a one of the radial collimators or one of the
    polarization analysis assemblies be positioned with its upstream
    face 400 mm from the sample; (ii) provide space for the automatic
    exchanger and the collimators and polarization analysis assemblies
    that are not in use, and (iii) accommodate the detector array which
    is to be rigidly attached the vessel to the vessel outer wall
    inside. A removable port(s) are to be provided for convenient access
    to the automatic exchanger and the detector array. The flat walls on
    the sides, top and bottom of the vessel are to have inner dimensions
    which restrict the area of sample illumination to the size of the
    detector array. For example, the inner height, *h(x)*, of the vessel
    should vary with distance, *x*, from the sample axis in accord with
    the expression

> $h(x) = h_{s} + \left( h_{d} - h_{s} \right)\ \frac{x}{L_{\text{SD}}}$,
>
> where *h~s~=*40 mm is the nominal sample height, *h~d~*= 1280 mm the
> nominal detector height and *L~SD~* = 4500 mm the sample to detector
> distance.

3.  **Position:** The analyzer vessel shall move on the floor following
    the sample arm so that the axis of the cylindrical detector array
    coincides with the sample axis with a positioning accuracy of better
    than 2 mm, as described in section 5.1. The accuracy of the detector
    bank angular positioning with respect to the axis of the beam
    incident on the sample should be better than 0.02° and should be
    reproducible after 1000 analyzer vessel movements. If not restricted
    by the incident guide and associated shielding, sample scattering
    angle extreme positions of -5° and +120° should be accessible to at
    least one detector for all monochromator take-off angles. It should
    take less than 2 minutes to move the analyzer vessel with detector
    bank between the extreme angular positions. Vessel support and
    moving systems shall be chosen to minimize the cost of construction,
    operation and maintenance while at the same time meeting all of the
    above-listed specifications.

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4.  **Variable vertical aperture after sample**

Upstream of the secondary flight-path shielding (but as close to the
sample as practical) there shall be a computer-controlled slit capable
of restricting the height of the beam scattered by the sample from
*h*~min~= 0 mm to *h*~max~= 100 mm. This slit should be centered to
within 1 mm of the beam-center height and have top and bottom apertures
that are independently motorized and controlled. It can either be an
independently mounted device made of 40 mm thick B~4~C-rich material and
covered with LiF on the side facing the sample or a mobile extension of
the analyzer shielding that can be positioned as close to the sample as
needed.

5.  **Shielding**

Shielding made of high neutron cross-section materials shall be placed
around the analyzer vessel. This shielding shall either be a stationary
structure enclosing all possible analyzer positions or be attached to
the mobile analyzer vessel itself; the choice is to be made on the basis
of practicality and cost-efficiency. In either case, the shielding inner
surface shall be covered with a thermal-neutron-absorbing material such
as cadmium or LiF. A background count rate of 1 count/min/detector when
the SNS is operating but both the beamline shutter and the shielding
entrance window are closed is defined as being acceptable. The interior
height of the stationary shielding must be sufficient to contain the
analyzer vessel and all auxiliary equipment mounted on it, as specified
in section 5.3. Also, its top must be modular and allow for easy access
to the analyzer vessel for maintenance and, if necessary, replacement of
the vessel and/or its components. If MCNP-X neutronics calculations
establish the necessity of a "get-lost" pipe, it should be incorporated
into the shield design.

1.  Dimensions and position: The shielding should start as close as
    possible to the variable vertical aperture after the sample
    specified in 5.4. The distance from the sample axis to the upstream
    face of the shielding shall be such that the analyzer vessel can be
    positioned with the upstream face of the radial collimator or
    polarization analysis assembly (whichever is in place) 400 mm from
    the sample axis. This shielding should have a 100 mm tall scattered
    beam entrance window and a computer-controlled horizontal aperture
    of width and position that can be varied so that a selected number
    of detectors (from a minimum of 1 to a maximum of all detectors in
    the array) are visible from the sample position. It should be so
    designed that - if not restricted by the incident guide and its
    associated shielding - extreme sample scattering angles of -5° and
    +120° are accessible to at least one detector at all monochromator
    take-off angles as specified in section 5.3.3. A sufficient
    clearance downstream the flight vessel should be provided to permit
    easy assembly and maintenance of the detectors and detector
    electronics.

2.  **Beam stop:** In some positions of the analyzer vessel the axis of
    the primary beam could intersect the detector array. It will
    therefore be necessary to have either a primary beam stop or a
    "get-lost" pipe to prevent the primary beam from either directly
    impinging on the detector array or creating an unacceptable
    background of primary beam neutrons scattered into the detector
    array. A beam stop should be designed to produce as little neutron
    and hard gamma radiation as possible; its exterior width should be
    minimized and its position chosen so that it cannot scatter neutrons
    into any of the active detection channels.

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6.  **Radial collimators after sample**

    Immediately behind the upstream face of the analyzer vessel there
    shall be a four-position collimator exchanger capable of accurately
    positioning any one of four radial collimators or, alternatively,
    one of two polarization analyzer assemblies in the scattered beam.
    In all positions the collimator axes should be concentric with the
    sample axis to better than 2 mm. Changes from one collimator or
    polarization analyzer to another are to be effectuated from a
    control computer and should take less than 30 seconds. All
    collimators are to be embedded in high boron content neutron
    shielding material in such a way that neutrons either pass through
    the window of the collimator or are absorbed. In addition, the
    lateral dimensions of their entrance windows should match the
    lateral dimensions of the entrance window of the flight vessel and
    their internal dimensions should match the dimensions of the
    scattered beam illuminating the detectors in the same way as the
    inside surfaces of the analyzer vessel match the scattered beam
    dimensions, as specified in section 5.3.2. The spacing between
    blades is to be determined by the expression

$d = \alpha\ell$,

> where $\ell =$`<!-- -->`{=html}200 mm is the length of the collimator
> blades and *α* is the effective beam divergence. Horizontal divergence
> angles *α *for the four collimators shall be 20\', 40\', 60\' and 120'
> respectively. Blade thicknesses should be 0.1 mm or less.
>
> All the radial collimators are to be positioned with the sample
> rotation axis as their focal point to within 2 mm and should be
> parallel to one another to within 0.01 degree. A line passing through
> the central blades of the collimators should also pass through the
> sample rotation axis to within 2 mm.

7.  **Broadband transmission polarizers.**

    It shall be possible to place either of two broadband, multi-channel
    supermirror transmission polarization analysis assemblies in the
    same position as the radial collimators (i.e. immediately behind the
    upstream wall of the analyzer vessel, at 400 mm from the sample
    axis) using the same computer-controlled collimator exchange
    mechanism. One of the two polarization analysis assemblies is to be
    configured for optimum performance at 15 meV; the other at 7.5 meV.
    It should also be possible to substitute the polarization analysis
    assemblies for radial collimators in the exchanger. Changing from
    one combination of polarization analyzers and collimators to another
    will be done manually and should not take more than 30 minutes to
    complete. Like the collimators, the polarization analyzers are to be
    embedded in a boron-rich neutron shielding material so that neutrons
    either pass through the polarizing windows or are absorbed.

    1.  **Polarization analysis assembly:** Each polarization analysis
        assembly is to consist of 19 identical channels mounted in a
        common magnetic frame supplying a permanent magnetic field of at
        least 300 Gauss; i.e. a field sufficient to magnetically
        saturate the polarization-sensitive supermirror coatings in all
        channels. Each channel is to be 20 mm in over-all width and have
        a height profile that matches the scattered beam height as
        specified in section 5.3.2. The height should not, however,
        exceed 200 ± 10 mm at its downstream end. The channels should be
        equally spaced on the circumference of a circle centered at the
        sample position with an accuracy of 2 mm, and co-leveled with
        the sample height with the same accuracy. Tight fitting,
        wedge-shaped neutron absorbing spacers are to be placed between
        channels to assure that all neutrons incident on the detectors
        pass through and not between the channels. Sample scattered
        neutrons passing through the channels will be split into two
        separate beams of opposite polarization. The channels should be
        designed so that the oppositely polarized beams from each
        channel are fully separated at the detector position but are not
        overlapped by beams from adjacent channels for scattered neutron
        energies 3\< E~f~\<30 meV.

    2.  **Individual polarization analysis channel:** Each channel shall
        consist of a 20 mm wide and 150 mm long collimator immediately
        followed by a stack of 20 mm wide, polarization-sensitive,
        supermirror-coated, thin silicon wafers 50 mm long and tall
        enough to accept the full scattered beam height as specified in
        section 5.7. Aluminum spacers are to be used to bend the silicon
        wafer stacks to a horizontal cylindrical curvature that
        optimizes the polarization analysis efficiency of the channel at
        the nominal neutron energy for which it is designed. The
        collimator and polarization analyzer are to be placed in a rigid
        frame and co-aligned so as to achieve optimum beam transmission
        and polarization analysis efficiency at the nominal neutron
        energy. A choice of 10\', 20\' or 40\' collimation should be
        provided for each channel. The critical angle of the supermirror
        coating for one neutron polarization direction should be at
        least 3θ^Ni^~c~ (or larger if possible) and for the other it
        should be no larger than θ^Ni^~c~ (or smaller if possible).
        Over-all, the transmission of the polarized collimated beam
        through the channel is to be better than 90% for energies
        3\<E~f~\< 30 meV. The center-line of every polarization analyzer
        collimator should pass through the sample rotation axis to
        within 2 mm.

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6.  **Instrument Control and Data Acquisition**

> Three independent computer systems are to be supplied with the
> instrument. To the extent possible, standard software developed by the
> SNS staff is to be utilized.

1.  **Instrument Control System.** An Instrument Control Program (ICP)
    running on a dedicated computer shall control the status and
    operation of all instrument systems as specified in this document,
    including any automated sample environments needed for particular
    measurements. The ICP shall communicate with the data acquisition
    program (section 6.2) to provide information about the instrument
    configuration and to synchronize and up-date their mutual status
    during the course of the experiment. The ICP shall also communicate
    with the experiment planning and data analysis program (section 6.3)
    to make it possible for the experimenter to pre-select optimum
    instrument configurations and input parameters for the planned
    measurement.

2.  **Data Acquisition System (DAS).** There shall be a separate,
    dedicated computer with an advanced high-capacity storage system
    that is able to communicate with the detector electronics and
    collect, pre-process and store the data from the individual
    detectors. Data is to be stored in files using the HDF-type
    (hierarchical data) format common to all SNS instruments. Along with
    the data, the files should contain headers with a complete
    description of the instrument configuration and instrument
    parameters provided by the ICP.

3.  **Experiment Planning and Data Analysis Program (EPDAP).** HYSPEC
    shall have a dedicated and integrated software package, EPDAP, to
    plan measurements so as to take optimal advantage of the wide-angle,
    multi-channel TOF analyzing system and also to analyze the data. The
    EPDAP should run on a separate, dedicated computer equipped with an
    extensive set of the peripheral devices for data transfer and
    visualization. It should be able to communicate with the ICP to
    export/import the instrument parameters and with the DAS to read the
    data for analysis and visualization.

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7.  **System Wide Requirements**

    1.  **Hard and soft limits.** All instrument degrees of freedom must
        be equipped with soft and/or hardware limits that prevent
        collisions but allow a full range of physically achievable
        positions. Sample rotation is a special case where hardware
        limits must be variable to allow for the different constraints
        imposed by different sample environments.

    2.  **Automated calibration and alignment.** There should be an
        automated detector calibration procedure and an automated
        alignment protocol for all degrees of freedom of the instrument.

    3.  **Permanent electrical wiring.** The number of cables should be
        minimized; wherever possible optical communication lines are to
        be used. All wiring is to be permanently installed and is to
        comply with all applicable industry and SNS standards.

    4.  **Crane.** There must be an overhead crane capable of lifting
        weights of more than 1 metric ton such as, for example, major
        secondary flight-path instrument components. It should also be
        capable of lifting heavy (and more than 1500 mm tall) sample
        environment devices - such as, for example, cryomagnets - and
        mounting them on the sample table. Crane coverage should extend
        over the monochromator shielding, the sample table and the
        secondary flight-path systems.

    5.  **Radiation Safety Exclusion zone.** In accordance with the "as
        low as reasonably achievable" radiation safety requirements, the
        instrument is to be surrounded by an interlocked exclusion zone
        within which personnel will not be permitted when the primary
        shutter is open and the beam is on. Details are to be specified
        by SNS Health Physics.

[^1]: ^(1)^ Metric units have been used throughout this document. Where
    standard stock sizes of materials differ from those specifiied, the
    closest four-digit decimal inch value is to apply.