2005 EXECUTIVE COMMITTEE REVIEW OF HYSPEC

On April 1, 2005, the HYSPEC executive committee, Rob McQueeney and Jim
Rhyne, was treated to a very informative review of design progress on
this instrument that is part of the SING project for the SNS.

Much of the information presented was drawn from the Design Criteria
Document (March 2005) that we found not only very useful, but also to be
a highly comprehensive and competently written summary of all aspects of
the instrument. It is clear that a lot of thought and insight has gone
into the design specifications for HYSPEC on the part of the SNS
instrument team, Mark Hagen and William Leonhardt, who were assisted
significantly by scientific input from Steve Shapiro, Larry Passell, and
Igor Zaliznyak and the calculational expertise of Vinita Ghosh.

In our meeting, Mark Hagen presented an outstanding overview of the
design issues of HYSPEC that gave us a clear picture of the
design/construction timeline for HYSPEC as well as the major technical
hurdles. This was supplemented by discussions of the design layouts by
Bill Leonhardt, and polarization analysis techniques by Vinita Ghosh and
chopper selection criteria by Larry Passell. Since the last IDT meeting,
a firm decision has been made to place the T1b (order suppression) disk
chopper, T2 (Slotted Fermi) chopper, sample chamber, and
analyzer/detector tank in a building external to the SNS target
building. This is clearly a vast improvement for access to the machine,
sample handling, lower background levels, and potential stray magnetic
field interference.

It is obvious that the design team has considered many of the technical
issues and solutions well in advance and had ready answers to our
questions. From the discussions, three issues did emerge (1) the choice
of straight vs curved chopper elements for the Fermi choppers, (2)
appropriate access to the HYSPEC external building for ancilliary
equipment such as cryo-magnets, dilution refrigerators, etc., and (3)
the magnitude of undesirable scattering from the Ar that is planned to
fill the analyzer/detector tank.

On the chopper issue (1) the committee was quickly convinced by Larry
that the choice was appropriate mainly due to the very short (10mm)
length of the chopper element. Generally curved chopper blades are
preferable due to the fact that they present the same opening to the
neutron as it traverses the chopper element. However, in HYSPEC the 10
mm blade length means that the difference between straight and the more
expensive curved path element is insignificant.

On item (2) Bill conceived of a creative solution and even showed us
drawings prepared over lunch! The primary storage/staging area for
environmental equipment at SNS is to be the mezzanine level, and Bill
showed that a small platform built in the HYSPEC building at mezzanine
level would be accessible by the HYSPEC crane with sufficient hook
height and could effectively serve as a launching area for cryogenic and
other equipment. This is deemed to be a good solution and the required
penetration thought the Target building outside wall is not a major
problem.

Item (3) is the only unresolved issue identified by the committee. The
issue is that if a Bragg peak is in reflecting geometry during an
inelastic measurement, then the Ar gas will scatter about 2% of this
intensity into a 4π solid angle. Some of these neutrons will reach the
detector and appear to represent inelastic scattered events or at best
an increased incoherent background. The question is what is the relative
magnitude of the true inelastic events compared to the spurious
scattering arising from the Ar? The obvious solution is to evacuate the
tank; however this would add significantly to the cost of the detector
tank and to the complexity of the air pad structure required to move the
tank. An attempt will be made by Vinita to calculate these relative
count rates from MCNP and also contact will be made with John Copley at
NIST who has build the Disk Chopper Spectrometer at the NCNR with an Ar
filled flight path. Note: In subsequent discussions with John Copley, he
certainly acknowledged the possibility of Bragg peak intensity being
scattered by the Ar and appearing as spurious inelastic events, but said
that it has **not** been a problem on the DCS (4 m flight path). He did
say, however, that **scattering cross-talk between detector tubes (not
limited to adjacent tubes) is quite noticeable and he recommended
strongly that absorbing barriers (e.g., Cd or B materials) be placed
between each detector tube to avoid this problem.**

HYSPEC is the only instrument in the current SNS suite that will have
full neutron polarization capability, and thus it has a considerable
responsibility as well as opportunity. Two methods for polarizing the
incident neutron beam have been evaluated -- a Heusler (Cu~2~MnAl)
focusing crystal array and a ^3^He cell combined with the pyrolitic
graphite monochromator that will be used for non-polarized experiments.
As currently configured, HYSPEC will use the Heusler array due to its
proven performance, lack of a laser system for maintaining the
polarization, and insensitivity to stray magnetic fields. In the opinion
of this reviewer, this was a very prudent choice.

For full four cross-section polarized beam studies a polarization
analyzer is also required. For this two distinct methods have again been
extensively studied -- a supermirror bender transmission polarizer with
a solid state collimator and a ^3^He transmission cell analyzer. The
supermirror bender will produced two beams displaced by a small angle
each with opposite polarization. Both these beams can then be detected
by the LPSD modules. With a single setting, the analyzer will have
band-width of ≈ 10 meV and the center of this band can be tuned
continuously over the range ≈ 5 meV up to ≈ 20 meV. Again because of the
same arguments given above, the HYSPEC team decided to proceed with the
supermirror bender; however the analyzer assembly is designed with
sufficient flexibility that it could be later reconfigured to accept a
^3^He analyzer. In addition to a required performance gain to achieve
comparability with the supermirror bender, the ^3^He analyzer would
require a polarized ^3^He production facility at SNS, which cost
(estimated to be \$1M) is far outside the budget of HYSPEC. Thus the
optimal choice is clearly to use the supermirror bender in the baseline
instrument.

In summary, this reviewer has concluded that the SNS/BNL instrumentation
team has developed a carefully conceived plan for HYSPEC, and is to be
highly complimented for the scientific and engineering insight inherent
in the design. This instrument with its polarization capacity and
excellent versatility of reaching a wide range of energies (particularly
emphasizing low E) and wave vectors promises to be one of the truly
world-class unique instruments at the SNS.