Draft includes modifications/suggestions by I. Zaliznyak

**[Proposed Instrument design simulations]{.underline}**

**[Vinita J. Ghosh]{.underline}**

**[June 2003]{.underline}**

One of the primary objectives in designing HYSPEC has been to maximize
the signal to background ratio. This objective will be accomplished by
doing two sets of simulations, one set to maximize the flux on sample,
the second set to minimize the beam-related background. The flux on
sample, energy and Q resolution simulations will be performed using
MCSTAS, the beam-related background and shielding simulations using
MCNPX.

Now that HYSPEC has been assigned a beamline, and we know that HYSPEC
will stay in the target building we can include the detailed
beam-related information into our model before we proceed with the
(final?) design optimization. The following features can be included:

-   The tilt of the moderator surface with respect to the beam
    direction.

-   The fact that there will be no guide in the core vessel insert

-   The guide will start at the upstream end of the shutter and will be
    10cm wide and 12cm high. The total guide length will be 20-25m.

Apart from these changes the model is essentially similar to the one we
have used so far. I have put together a list of BL14B-related
information that I need to include in the design and shielding models. I
would like to send this list to Tom Fornek so he can tell me whom to
contact for this kind of information -- I am sure I will need more
information later.

The following simulations need to be performed to optimize the design of
the instrument.

I [Simulations for optimizing the guide configuration]{.underline}.

1.  First optimize the guide height using a guide with a uniform width
    of 4cm

-   The guide expands continuously from 12cm to 15-20cm.

-   The guide has two expanding funnels as in the current model

2.  After the vertical profile is optimized we optimize the horizontal
    profile. The two possible options are

-   The guide converges continuously from 10 to 4cm

-   The guide converges discontinuously, in the form of two funnels.

3.  Next curve the optimized guide. Neither NISP nor MCSTAS includes
    algorithms for expanding or converging curved guides. However, the
    curved guide can be simulated as a collection of segmented straight
    guides (which can expand or converge) set at appropriate angles with
    respect to each other.

II [Simulation to measure the flux on sample, E and Q resolution of the
instrument.]{.underline}

Energy and Q-resolution of the instrument will be studied for the
optimized guide configuration with 3θc supermirror coatings.

III [Guide coating]{.underline}

The guide optimization will be performed using 3θc supermirror coatings
on all sides of the guides. Using the optimized guide configuration,
check how much improvement is obtained in the flux on sample for θc \>
3. For HERMES it was found that the flux on sample improved by going to
higher values of θc but the Q- resolution of the instrument deteriorated
by going to higher values of θc. The correlation between guide coating
and the Q-resolution will have to be studied.

IV [Positions of the various choppers]{.underline}

We need to know how many choppers will be required and how they should
function. At the very least we will need a t0 chopper, a
wavelength-defining chopper and a frame overlap chopper.

-   The ideal positions of the different choppers have to be determined.
    They will depend on the moderator-monochromator distance.

-   Preliminary results show that the T0 chopper will have to rotate at
    120Hz when we have a 10cm wide beam, 60Hz if the guide width is 4cm.
    A tungsten chopper of thickness between 20-30cm will be ideal, if it
    can be mechanically stable.

-   We still have to determine whether the same chopper can be used as
    both a frame-overlap and as an order-suppressor chopper.

-   We need to make sure that no stray wavelengths leak through when we
    have a Fermi-like chopper in the lineup.

V [Design and operation of the rotating collimator or Fermi-like
chopper]{.underline}.

The motivation of these simulations is to see whether

-   The straight slot Fermi chopper can be used instead of a double disc
    chopper to select the wavelength as well as to control the burst
    width at the sample.

-   Whether this chopper can be used in addition to a double disk
    chopper to reduce the pulse width. For both these simulations the
    time and energy profiles downstream of the chopper have to be
    monitored.

Some simulations have been performed using G. Granroth's algorithm for a
straight-slot Fermi chopper. Preliminary results suggest that there may
be an error in the code. The fwhm of the time profile is about half of
the estimated value. The results have been sent to Garret so he can look
at the code.

[Some other significant results are]{.underline}

-   The fwhm of the time profile appears to be independent of the
    selected energy[.]{.underline}

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-   The time pulse downstream of this chopper is not triangular, but it
    has long time tails. The long tail can be cut off by using a
    60minute collimator immediately downstream of the chopper.

-   The straight-slotted Fermi chopper does not couple well with a
    double disk chopper. ??? Fermi-chopper would only be used to provide
    a time pulse of 10-20 microseconds, the time structure of the 40-80
    microseconds pulse provided by the counter-rotating disc chopper is
    essentially irrelevant then

-   The fwhm of the energy profile (after reflection from the
    monochromator - on sample?) appears to be larger than that for the
    disk chopper when both choppers have the same fwhm for the time
    profile.

Once we are satisfied that the algorithm is correct, we can proceed with
the chopper-related set of simulations.

VI [Polarization analysis]{.underline}.

The bender analyzer configuration has to be optimized. We need to know
the value of the secondary flight path, the two different energies for
which we want to optimize the configuration, and what are the best
supermirror coatings that are commercially available. Majkrzak used
coatings with m=0.6 for one polarization component and m=3 for the other
in 1993. Have there been significant improvements in this technology
since?

VII [No-chopper operation off a crystal monochromator]{.underline}.

One of the interesting possibilities provided by HYSPEC design is an
operation without choppers by using the wavelength and time-frame
selection by the crystal monochromator. The main problem of such
operation is that the sample illumination time which is essentially
determined by the moderator pulse length, is quite long for
thermal-neutron energies. However, for higher Ei = 80-100 meV, the
moderator pulse FWHM becomes \~60-80 microseconds, which is not much
longer than that provided by the counter-rotating disc chopper. In
addition, a long pulse is OK for large energy transfers. Because at high
Ei the beam collimation becomes quite tight, one may expect that an
appropriately bent perfect single-crystal silicon monochromator could
perform very well. It has a low integral reflectivity, but this comes
because of a very tight energy-resolution, which may appear just
appropriate at high Ei. In addition, it may give better geometrical
focus. This configuration has a potential to offer several important
advantages:

-   better geometrical focusing for very small samples with perfect
    crystal monochromator

-   no-chopper operation can be employed to start test-running HYSPEC
    before the funds for procuring and installing expensive chopper
    components become available in the funding profile, and, therefore,
    before the construction is complete (hopefully, right after SNS
    beams become operational)

Apart from the above, this mode of operation is interesting in itself as
a promising novel design concept for a TOF instrument.

VII [Analysis software development]{.underline}

Programs to extract E and Q resolution from the MC data, which can be
stored as N(x,y,t) or as I(t) and J(x,y). This software will eventually
be used to analyze HYSPEC data which will be stored as N(x,y,t).

Let us leave the claim for software development to Brent Fultz for now.
As far as our needs are concerned we are very well served by the Origin,
or few lines in Matlab.

**Shielding simulations**

In order to minimize the beam-related background we currently plan on
looking at:

1.  Five different guide configurations and their impact on beam-related
    background: (i) a straight guide 4cm wide, (ii) a curved guide 4cm
    wide, (iii) a straight guide which converges from 10 to 4cm, (iv) a
    straight guide which converges from 10 to 4cm in 16m, followed by a
    2m straight section, and (v) a curved guide which converges from 10
    to 4cm

2.  Background with/without a T0 chopper. Determine the material and
    dimensions.

3.  Background with/without a single crystal sapphire filter. Study its
    effectiveness in increasing signal/background.

4.  Study two different beamstop options. (i) an in-shield beamstop,
    and (ii) a get-lost pipe and a beamstop placed outside the detector
    cave.

5.  Design the drum shield to minimize the beam related background.

6.  Design the primary guide shielding to minimize beam related
    background.

7.  Study neutron leakage through the different choppers, to determine
    the ideal composition and coating of disc choppers.

8.  Find the position of the wavelength -- defining chopper. This
    chopper should be placed as close as possible to the monochromator
    to minimize the burst width at the sample. However, the chopper
    blade and the accompanying housing will act as neutron scatterers
    and will thus increase the background. We need to find the 'best'
    location, and reinforce the shielding to minimize its negative
    impact.

We will need to calculate the biological dose rates and the He3 detector
response for HYSPEC with the above configurations. It will be assumed
that when HYSPEC is operating, we can take credit for the T0 chopper in
calculating the dose rates and the He3 detector response function. For
the design of the beamstop outside the detector cave we will assume that
the T0 chopper block is not in the beam, and that the full beam will be
dumped on the external beamstop.

[What we have learnt so far]{.underline}

1.  According to E. Iverson's moderator leakage spectra roughly 6% of
    all neutrons are thermal (\< 101meV) neutrons and the rest are
    epithermals. The epithermal neutrons range in energy from 101meV to
    1000MeV. We do not have an emission spectra for the gammas, so only
    the contribution of the photons produced by the neutrons can be
    calculated.

2.  The sapphire filter will help to improve the signal-to-background
    ratio. The transmission of thermal neutrons through 10cm of sapphire
    is 80%, whereas the transmission of epithermals through sapphire is
    less than 10%.

3.  The epithermal neutrons can be very effectively stopped by a T0
    chopper made of tungsten or inconel. The neutron flux at the end of
    a 18m guide (starting 2m from the moderator) is reduced by 2-3
    orders of magnitude by 20cm of tungsten or 30cm of inconel.

4.  A T0 chopper placed 6m from the moderator has to rotate at least at
    120Hz to effectively block all neutrons of energy greater than
    500meV.

[Current simulations are aimed at designing an in-shield
beamstop]{.underline}.

The model includes the guide configuration which will provide the
highest flux on sample. It is also the configuration that is most
challenging from the shielding point of view:

-   The moderator surface is perpendicular to the beam direction \>
    moderator is really irrelevant for the fast neutrons with E\>0.5eV,
    and may be neglected in the first approximation it if this
    simplifies the things

-   There is no guide in the core vessel insert. However, we will
    include a steel insert (1m long) that will protect the guide
    substrate.

-   The guide starts at 2m from the moderator. It converges continuously
    in the horizontal plane from 10 to 4cm, and expands in the vertical
    plane from 12 to 18cm. The guide has 1cm Si substrate and a 8cm
    steel jacket. An air-filled gap of 2mm surrounds the steel jacket.

-   A T0 chopper block consisting of 20cm of tungsten or 30 cm of
    inconel completely covers the guide plus substrate.

-   Simulations will be performed with/without the sapphire filter which
    will be placed downstream of the chopper block

-   A PG crystal with an Al frame is placed at 20.20m and makes a 10deg
    angle to the beam. This angle corresponds to the 60meV position of
    the crystal.

We would like to see whether we can design a beamstop which is small
enough to fit in the drum shield and can still provide low enough dose
rates and He3 detector countrates.

Two sets of simulations will be required. One will assume that the
incident neutrons have energies greater than 400meV, and that the T0
chopper block is in the beam. The other will assume that only thermal
neutrons (\< 400meV) are included in the beam and that the T0 chopper is
no longer in the beam. The energy range of the thermal neutrons may be
restricted further to simulate the fact that only a small energy range
will be allowed to go through by the wavelength-defining chopper.