ICANS-XVII

17^th^ Meeting of the International Collaboration on Advanced Neutron
Sources

April 25-29, 2005

Santa Fe, New Mexico

Optimization of the HYSPEC design using Monte Carlo simulations

V. J. Ghosh^1^, M. E. Hagen^2^, W.J. Leonhardt^1^, I. Zaliznyak^1^, S.
M. Shapiro^1^, and L. Passell^1^

^1^*Brookhaven National Laboratory, PO Box 5000, Upton, NY, 11973 USA*

^2^SNS, Oak Ridge National Laboratory

Abstract

*HYSPEC is a direct geometry spectrometer to be installed at the SNS
\[1\] on beamline 14B where it will view a cryogenic coupled hydrogen
moderator. The 'hybrid' design combines time-of-flight spectroscopy with
focusing Bragg optics to provide a high monochromatic flux on small
single crystal samples, with a very low background at an extended
detector bank. The instrument is optimized for an incident energy range
of 3-90meV. It will have a medium energy resolution (2%-10%) and will
provide a flux on sample of the order of 10^6^-10^7^ neutrons/s-cm^2^.
The spectrometer will be located in a satellite building outside the SNS
experimental hall at the end of a 35m curved supermirror guide. A
straight-slotted Fermi chopper will be used to monochromate the neutron
beam and to determine the burst width. The 15cm high, 4cm wide beam will
be focused onto a 2cm by 2cm area at the sample position using Bragg
reflection from one of two crystal arrays. For unpolarized neutron
studies these will be Highly Oriented Pyrolitic graphite crystals while
for polarized neutron studies these will be replaced with Heusler alloy
crystals. These focusing crystal arrays will be placed in a drum shield
similar to those used for triple axis spectrometers. Hyspec will have a
movable detector bank housing 160 position sensitive detectors. This
detector bank will pivot about the sample axis. It will have a radius of
4.5m, a horizontal range of 60º, and a vertical range of ±7.5º. In order
to reduce background at the detector bank both a curved guide and a T0
chopper will be used. A bank of 20 supermirror bender polarization
analyzers \[2\] will be used to spatially separate the polarized
neutrons in the scattered beam so that both scattered neutron spin
states can be measured simultaneously. The results of Monte Carlo
simulations performed to optimize the instrument design will be
discussed.*

1\. Introduction.

HYSPEC (HYbrid SPECtrometer) is a direct geometry spectrometer to be
installed at the SNS \[1\]. Its primary mission is to perform inelastic
neutron scattering experiments on small single crystal samples. The
'hybrid' design combines time-of-flight spectroscopy with focusing Bragg
optics to provide a high monochromatic flux on these small single
crystal samples. The instrument is also designed to provide a very low
background at an extended detector bank. The instrument is optimized for
an incident energy range of 3-90meV. It will have a medium energy
resolution (2%-10%) and will provide a monochromatic flux on sample of
the order of 10^6^-10^7^ neutrons/s-cm^2^.

HYSPEC will be installed on beamline 14B where it will view a cryogenic
supercritical coupled hydrogen moderator. This moderator delivers more
neutrons in the 5-50meV range than any of the other SNS moderators\[3\].
The experimental area of the instrument (the energy defining choppers,
the focusing monochromator crystal array, the sample stage and the
detector bank) will be located in a satellite building outside the SNS
experimental hall at the end of a 35m curved supermirror guide. A
straight-slotted Fermi chopper will be used to monochromate the neutron
beam and to determine the burst width. The 15cm high, 4cm wide beam will
be focused onto a 2cm by 2cm area at the sample position using Bragg
reflection from one of two focusing crystal arrays. For unpolarized
neutron studies these will be Highly Oriented Pyrolitic graphite
crystals while for polarized neutron studies these will be replaced with
Heusler alloy crystals. These focusing crystal arrays will be placed in
a drum shield similar to those used for triple axis spectrometers.
Hyspec will have a movable detector bank housing 160 position sensitive
detectors. This detector bank will pivot about the sample axis. It will
have a nominal radius of 4.5m, a horizontal range of 60º, and a vertical
range of ±7.5º. In order to reduce background at the detector bank both
a curved guide and a T0 chopper will be used. A bank of 20 supermirror
bender polarization analyzers \[2\] will be used to spatially separate
the polarized neutrons in the scattered beam so that both scattered
neutron spin states can be measured simultaneously.

One of the primary objectives in designing HYSPEC has been to maximize
the signal to background ratio in the detectors. This objective can be
accomplished by doing two sets of simulations, one set to maximize the
flux on sample and the second set to minimize the beam-related
background. The first set of simulations was performed using the
McSTAS\[4\] neutron ray-tracing package. The second set of simulations
relating to shielding and background will be performed using MCNPX
\[5\]. The HYSPEC shielding has to be designed so that the biological
dose rate is less than 0.25mrem/hr for areas with unlimited access. The
beam-related background goal for HYSPEC is to have less than 1 count per
2.5cm detector per minute.

The layout of the instrument is described in Section 2. Design features
that reduce beam-related background are highlighted in Section 3, and
instrument performance is discussed in Section 4.

2\. Description of HYSPEC.

![](media/image1.jpeg){width="4.191666666666666in"
height="5.490972222222222in"}

*Figure 1: Schematic layout of HYSPEC with the monochromator and
detector bank in a separate building outside the instrument hall*

Figure 1 is a schematic layout of HYSPEC with the experimental area of
the instrument (the energy defining choppers, the focusing monochromator
crystal array, the sample stage and the detector bank) located in a
separate building outside the SNS experimental hall. HYSPEC will be
installed on beamline 14B where it will view a 10cm by 12cm cryogenic
supercritical coupled hydrogen moderator. This moderator delivers more
neutrons in the 5-50meV range than any of the other SNS moderators
\[3\]. The moderator-monochromator distance is expected to be around
37m. Sections of the primary guide are curved so that the monochromator
is out of the line of sight of the moderator. The spatial offset at the
monochromator is expected to be about 16cm.

As illustrated in Figure 1 the major components of HYSPEC are the guides
that transport the neutrons from the moderator to the monochromator,
Box-A, Box-B, the drum shield surrounding the focusing monochromator
crystal array, the sample stage and the detector bank. Box-A and box-B
are virtual boxes containing choppers, motors and pieces of guides.
Box-A contains the T0 and T1A (frame overlap) choppers, and Box-B
contains the T1B (order suppressor) and T2 (wavelength defining)
choppers. Detailed descriptions all these components can be found in
reference \[6\] and references therein. The schematic layout of Box-A
and Box-B are shown in Figs 2 and 3, and that of the HYSPEC guides in
Figures 4 and 5.

![](media/image2.wmf)

*Figure 2: Box-A containing the T0 and T1A choppers*

![](media/image3.wmf)

*Figure 3: Box-B containing the T1B and T2 choppers.*

The primary function of the guides is to transport the neutrons from the
moderator to the monochromator. Guides G1a and G1b extend through the
primary shutter, the biological shield and the chopper archway. The
guide is 4m long, 4cm wide and it expands in height from \~13.2 to 15cm.
Guides G2, G3 and G4 are 4cm wide and 15cm high. The guide coating is
m=3 supermirror. Guide section G2 is curved with a radius of curvature
of 2542m. This radius of curvature was chosen to obtain a 16cm offset (4
times the guide width) at the monochromator. This offset completely
removes the monochromator from the line of sight of the moderator,
thereby reducing the fast neutron flux by a factor of the order of
10^-5^ . The curved guide, along with the T0 chopper, significantly
reduces the background at the detector bank.

*Figure 4: Schematic layout of HYSPEC upstream of the monochromator*

  -----------------------------------------------------------------------
  ![](media/image1.wmf)

  -----------------------------------------------------------------------

*Figure 5: Arrangement of the 1.75m straight guide + Box B & drum
shield*

The primary function of the four choppers is to define the energy and
time profile of the neutron pulse incident on the sample. The secondary
function is to reduce the background at the detector bank. The T0
chopper is a "dumbbell-shaped" rotor made of inconel which can be
rotated at 30 or 60 Hz. The T0 chopper removes a significant fraction of
the fast neutrons and the initial gamma rays from the neutron beam. The
T1A (frame overlap) and T1B (order suppressor) choppers are single-slot
single-disk choppers which will rotate at 30 or 60Hz. They will be made
of boron-loaded material. Their function is to remove contributions due
to frame overlap and order contamination from the neutron beam that
reaches the focusing crystals. The function of the T2 chopper is to
select the burst width (time and energy) of the incident neutrons. This
chopper is envisioned as a vertical axis rotor with a Fermi-type,
straight-slotted slit package insert. The rotation rates are expected to
be integral multiples of the source frequency ranging from 60 to 480Hz.
Two different slit package inserts are under consideration: one with
aluminum beam channels and Gd metal channel defining absorbers, the
other with supermirror-coated single crystal silicon beam channels and
Gd metal channel defining absorbers. The slit package is expected to be
4.2cm wide, 15cm high and 1cm long, and has been optimized to provide a
time burst of 55µs at 180 Hz.

A focusing crystal array will be placed downstream of the T2 chopper
inside a drum shield. A double-focussing array of ZYB grade HOPG (highly
oriented pyrolytic graphite) 30cm wide and 20.4cm high will be used to
produce the non-polarized neutron beam. A Heusler Cu2MnAl crystal array
will be used to produce a polarized neutron incident beam. The crystal
array will be surrounded by a conventional drum shield. The angular
rotation range of the drum shield will be 15º \< 2θ~M~ \< 90º , where
2θ~M~ is the deflection angle of the beam. This corresponds to an
initial energy range of 3.6-90meV. At this time it is estimated that the
internal diameter of the drum shield will be 60cm, and the outer
diameter will be 180cm. MCNPX simulations are being done to determine
the dimensions and composition of the drum shield.

The focusing crystal-sample distance will be allowed to vary from 1.4 to
1.8m. The sample and the sample environment (a CCR, cryostat, furnace or
cryomagnet) will be carried on a sample stage that will move on air pads
across the dance floor. Immediately above the air pads will be a
motorized and encoded rotational stage that will be used to rotate and
position the detector bank. On top of this will be another rotational
stage to rotate the sample, and on top of that a goniometer to translate
and tilt the sample.

![](media/image2.jpeg){width="4.335416666666666in"
height="3.0868055555555554in"}

*Figure 6: A schematic 3d rendering of the detector vessel.*

A schematic of the HYSPEC detector vessel is shown in Figure 6. The
detector vessel will rotate about an axis through the center of the
sample stage. The detector vessel will carry 20 120cm long LPSD 8-packs
at a nominal distance of 4.5m. The horizontal angular acceptance of the
detector bank will be 60º , and the vertical angular acceptance of the
detector bank will be ±7.5º . The vessel will be filled with argon gas
at \~1 atmosphere pressure. The front window of the detector vessel will
be 70cm from the sample, and will be made of 0.05mm aluminum foil. The
detector vessel walls will be shielded both internally and externally.

For experiments that do not require polarization analysis a radial
collimator will be mounted between the sample and the front face of the
detector vessel. For polarization analysis experiments the radial
collimator will be replaced by an array of 20 supermirror bender
transmission polarizers. These are shown in Figure 7 and described in
detail in reference \[2\]. The choice of 20 polarizer benders matches
the 20 "8-packs" of detectors and the 20 segments in the coarse
collimator.

  ---------------------------------------------------- -----------------------------------------
  ![](media/image3.jpeg){width="2.782638888888889in"   ![](media/image4.jpeg){width="3.1625in"
  height="2.4375in"}                                   height="2.44375in"}

  ---------------------------------------------------- -----------------------------------------

Figure 7: Arrangement of the solid state collimator and bender polarizer
(left). Array of 20 polarizer benders with magnets, yoke, and motors
(right).

3\. Plan to minimize beam-related background

In order to minimize the beam related background at the HYSPEC detectors
we have to reduce the fluence of both fast and unwanted thermal neutrons
at the focusing crystal. The selected thermal neutrons are Bragg
scattered down the exit tube and onto the sample. The unwanted neutrons
are scattered (non-Bragg scattering) into the drum shield as well as
down the exit tube. Neutrons can get moderated or absorbed as they pass
through the drum shield, also gamma rays can be created due to neutron
absorption. Both neutrons and gamma rays will contribute to the
biological dose rate and have to be absorbed in the drum shield. The
drum shield will be designed so that the biological dose rate
requirements are satisfied.

HYSPEC will use both a T0 chopper and a curved guide. The curvature of
the guide is such that the focusing crystal is displaced 16cm with
respect to the moderator. This displacement is 4 times the width of the
guide and it will remove the crystal array completely out of the line of
sight of the moderator. Preliminary simulations have shown that this
reduces the fast neutron flux to about 10^-5^ times the flux for a
similar straight guide. In addition to the T0 chopper there are three
other choppers in the lineup -- the T1A, T1B and the T2 chopper. The T1A
and T1B choppers will be boron-loaded so that they can absorb both
thermal and fast neutrons. The T2 Fermi chopper will have gadolinium in
the slit package to absorb thermal neutrons. The choppers and the curved
guide will reduce the flux of neutrons reaching the crystal array.

The He3 detectors in the detector vessel detect only low-energy
neutrons. The absorption curve for the HYSPEC detectors is shown in
Figure 8. Neutrons of energies upto 1keV can easily be prevented from
entering the detector vessel by putting appropriate shielding outside
the detector vessel. Gadolinium on the inside of the detector bank can
absorb the thermal neutrons scattered by the argon in the detector
vessel.

![](media/image5.wmf)

*Figure 8: The absorption curve for the HYSPEC detectors.*

4\. Monte Carlo simulations of HYSPEC performance.

Simulations to optimize the instrument performance are documented in
references \[7\] and \[8\]. The impact of the guide curvature on the
thermal neutrons is illustrated in Figure 9. The reduction in flux at
90meV is almost a factor of 2 when the displacement at the crystal array
is 16cm or 4 times the guide width. Despite this loss of flux curving
the guide is considered an advantage because it results in an
attenuation of fast neutron flux by almost 10^-5^ . This allows us to
reduce the thickness of the drum shield and to move the sample position
closer to the crystal.

![](media/image6.wmf)

*Figure 9: Effect of the guide curvature on the neutron fluence at
Box-B.*

The wavelength-defining T2 chopper is placed at 35.5m from the
moderator. The slit package of the Fermi chopper has been optimized to
give a burst time of \~55µs at a rotation frequency of 180Hz.
Transmission through the chopper can be changed by changing the rotation
frequency of the chopper, transmission increases as the rotation
frequency decreases. These results are plotted in Figure 10.

![](media/image7.wmf)

*Figure 10: Fluence of neutrons passing through the Fermi chopper at
different energies for rotation frequencies of 60,180, and 360Hz.*

The fluence in a 2cm by 2cm area at the sample position is plotted in
Figure 11 for three different configurations of the focusing crystal
array plus a "baseline" case. The "baseline" is a unfocussed (flat)
crystal array using the ZYA grade (24' mosaic) of HOPG. The other three
configurations use ZYB grade (48' mosaic) of HOPG in the flat, only
vertically focused, and both horizontally and vertically focused arrays.
Vertical focusing has a huge impact, increasing the fluence by a factor
of \~6.

![](media/image8.wmf)

*Figure 11: The fluence of neutrons at a 2cm by 2cm area at the sample
position.*

The rotation frequency of the Fermi chopper also determines the burst
time at the sample and the incident energy width. The burst times
(immediately downstream of the Fermi chopper) and the incident energy
widths are tabulated in Table 1 for rotation rates of 60, 180, and 360
Hz.

*Table 1: Burst times and incident energy widths for rotation
frequencies of 180, 60, and 360Hz.*

  ---------- --------- ---------- --------- ---------- --------- ----------
  Energy     180Hz                60Hz                 360Hz     
  (meV)                                                          

             ΔT (μs)   ΔE~i~      ΔT (μs)   ΔE~i~      ΔT (μs)   ΔE~i~
                       (meV)                (meV)                (meV)

  3.6        56        0.032      172       0.047      29        0.032

  5.0        55        0.046      169       0.072      29        0.044

  15         55        0.166      167       0.331      27        0.132

  30         55        0.308      166       0.820      27        0.200

  60         55        0.800      162       2.053      26        0.413

  90         51        1.325      162       3.399      26        0.701
  ---------- --------- ---------- --------- ---------- --------- ----------

An estimate of the energy resolution of the instrument for elastic
scattering can be obtained from the burst times at the sample position.
ΔE~F~ /E~F~= 2\*ΔT~S~ /T, where ΔT~S~ is the time burst width at the
sample and T is the time taken to traverse the distance between the
sample and the detector bank.

*Table 2: Energy resolution of the instrument for elastic scattering.
The rotation rate of the Fermi chopper is 180, 60, and 360Hz.*

  ------------------- ------------------ --------------- -----------------
  Energy (meV)        ΔE~F~ /E~F~=                       
                      2\*ΔT~S~ /T                        

                      180Hz              60Hz            360Hz

  3.6                 0.025              0.078           0.013

  5.0                 0.026              0.081           0.014

  15                  0.045              0.138           0.022

  30                  0.063              0.193           0.032

  60                  0.091              0.265           0.043

  90                  0.103              0.327           0.053
  ------------------- ------------------ --------------- -----------------

The energy resolution of the instrument for inelastic scattering is
plotted in Figure 12.

![](media/image9.wmf)

Figure 12: FWHM resolution widths for the Fermi chopper running at 180
Hz for various energies.

The scaled energy transfer E/E~I~ = (E~F~-E~I~)/E~I~, and the scaled
energy resolution is ΔE/E~I~. The energy resolution for zero energy
transfer or elastic scattering is dominated by the energy resolution of
the secondary spectrometer. The energy resolution at large energy loss
is due to the energy resolution of the primary spectrometer ΔE~I~/E~I~ .

Summary.

As these simulations show HYSPEC will deliver a high monochromatic flux
on small samples for an incident energy range of 3-90meV. The instrument
will have a moderate energy resolution (2%-10%) for a Fermi chopper
rotation rate of 180 Hz. The flux on sample can be increased by
decreasing the rotation rate of the Fermi chopper, thereby increasing
the burst width at the sample. However, this gain comes at the expense
of the energy resolution. The energy resolution of the instrument can be
improved by increasing the rotation rate of the Fermi chopper. This will
decrease the burst width and flux on sample. This versatility in
instrument performance will allow the users to customize the mode of
operation for individual samples.

HYSPEC will also have a large signal to background ratio. The long
curved guide, the system of choppers, the design of the drum shield and
detector vessel will combine to reduce the beam-related background.

A tested, maintenance-free polarization analysis capability will allow
polarized neutron experiments.

Acknowledgements.

We would like to thank members of the SNS instrument team for their
assistance. We would also like to acknowledge the assistance of the SNS
shielding group with the MCNPX simulations, G. Granroth for sending us
his codes, and L. Daemen and P.A. Seeger for assistance with the NISP
code.

Work at BNL is supported by the US DOE under contract No.
DE-AC02-98CH10886

References.

\[1\] S. M. Shapiro, I. Zaliznyak, L. Passell, V. Ghosh, ICANS-XVI, May
2003, proceedings (2003).

\[2\] Igor Zaliznyak, V. Ghosh, S. M. Shapiro, L. Passell, Physica B
**356** 150-155 (2005).

\[3\] E. Iverson, MCNP data for BL5 coupled H2 moderator obtained from
<http://www.sns.gov/users/instrument_systems/components/moderator/HPTS-sct.shtml>
as of 31st July 2002.

\[4\] User and Programmers Guide to the Neutron Ray-Tracing Package
McStas, version 1.8, P.Willendrup, E. Farhi, K. Lefmann, P.O. Astrand,
M. Hagen and K. Nielsen, Riso-R-1416(EN), Riso National Laboratory,
Roskilde, Denmark, May 2004

\[5\] MCNPX 2.4.0, Radiation Safety Information Computer Center, Oak
Ridge National Laboratory, September 2002

\[6\] M.E. Hagen and W.J. Leonhardt, Design Criteria Document for the
Hybrid Spectrometer (HYSPEC), Hyspec-00-DC00D1-R0001, March 2005

\[7\] V.J. Ghosh, McStas Simulations for HYSPEC Guide Configuration
Optimization, HYSPEC Technical Note 0009 (February 2005)

\[8\] M.E. Hagen, McStas Results for HYSPEC focusing crystals, HYSPEC
Technical Note 0012 (March 2005)