**Deterioration of the calculated HYSPEC performance (non-polarized
neutron beam) due to several "real world" decisions**

**Vinita J. Ghosh**

**May 15, 2006**

1.  HYSPEC moderator-monochromator distance increased from 25 to 37m
    (straight guides) to accommodate the NSE instrument in the target
    building.

![](media/image1.wmf)

  ---------------- ----------------- ----------------- ------------------
  Ei(meV)          LMM=25            LMM=35            LMM=40

  3.6              2.2%              2.2%              2.1%

  15               4.5%              4.3%              4.2%

  30               6.4%              6.0%              5.9%

  60               9.0%              8.2%              8.2%

  90               10.4%             10.1%             9.8%
  ---------------- ----------------- ----------------- ------------------

Flux on sample and energy resolution (ref 1): Flux on sample decreases
from 1x10^7^ to 6x10^6^ (n/cm^2^.s) for 15meV, and from 1.5e^6^ to
8x10^5^ (n/cm^2^.s) for 60meV.

2.  The HYSPEC guide was curved to reduce background at the detector
    bank. The guide curvature was designed to get an offset of 16cm (4
    times the guide width) at the focusing crystal (ref 2).

  ---------------- ----------------- ----------------- ------------------
  Nominal Energy   Fluence(n/s) for  Fluence(n/s) for  Ratio of fluence
  Ei (meV)         offset=0cm        offset=16cm       at the end of the
                                                       curved guide

  3.6              5.27e9            5.26e9            \~1

  5.0              6.24e9            6.24e9            \~1

  15.0             4.84e9            4.75e9            0.98

  30.0             9.60e8            9.00e8            0.94

  60.0             2.98e8            2.40e8            0.81

  90.0             1.86e8            1.06e8            0.57
  ---------------- ----------------- ----------------- ------------------

Ref 2, table 4. Intensity as a function of offset for L~MM~=37m.

3.  Coating of the inner surface of the curved guide to be m=1. This
    will result in a significant reduction in flux on sample at the
    lower energies (ref 2,3).

![](media/image2.wmf)

Ref 3, Fig 5b.

4.  Chopper Box A containing choppers T0 and T1 moved from 6.5 to 8.5m
    to allow for better crane coverage and it was tacitly assumed that
    the curved guide would start at 9.4m. This reduces the length of the
    curved guide from 24.8 to 23.3m, and the radius of curvature of the
    guide G2 has to changed from 2542 to 2323m to get an offset of 16cm
    at the focusing crystal. There is a third (better) option available.
    The curved guide should start at 6.32m, immediately downstream of
    the 4m expanding section. The curved guide is now longer (26.38m)
    and the radius of curvature can be increased to 2884m. However, a
    gap is required in the curved guide to accommodate the choppers T0
    and T1A. Even after allowing for this gap there is an improvement in
    flux compared to the 9.4m option. The McStas results were written up
    in ref 3.

![](media/image3.wmf)

**Fig 4b. Ratio of the fluence at the end of guide G3 for the three
different starting positions of the curved guide: G2start_7.9m,
G2start_9.4m and G2start_6.32m with respect to G2start_7.9m.**

5.  Width of the PSI PG focusing crystal array will be 260mm NOT the 300
    mm required to completely cover the 40mm wide guide at 90meV. This
    260mm crystal array will completely cover the 40mm wide guide only
    upto 76.8 meV. At 90meV the neutron beam intensity will decrease by
    a factor of 26/30.

6.  A straight slotted Fermi chopper will be used as the wavelength
    selector instead of a counter-rotating double disk chopper. The slot
    length of the AlGd slit package is expected to be 10mm. The
    transmission through 10mm of Al can be estimated from the figure
    below, and it is typically 85%.

> ![](media/image4.wmf){width="3.9277777777777776in"
> height="4.615277777777778in"}

**Summary/conclusion**

Each of the above decisions appear to be very reasonable and the
deterioration in performance due to each change in design is relatively
small, but the total impact on the performance of HYSPEC (namely on the
flux on sample) is substantial. Let R(1) represent the reduction in the
flux on sample due to decision 1, and R(NP) the total reduction in flux
due to all 6 decisions.

  ------------ --------- -------- -------- -------- ------- ------- ---------
  Energy       R(1)      R(2)     R(3)     R(4)     R(5)    R(6)    R(NP)

  15meV        0.6       \~1      0.72     0.92     \~1     0.83    0.33

  90meV        0.53      0.57     \~1      0.88     0.87    0.87    0.20
  ------------ --------- -------- -------- -------- ------- ------- ---------

The HYSPEC flux on sample for a nonpolarized neutron beam will be lower
than that of the CNCS, ARCS and SEQUOIA at all incident energies (see
comparison in ref 4).

![](media/image5.wmf){width="6.0in" height="5.022916666666666in"}

**Should we take steps to recover the performance of HYSPEC before it is
too late?**

What can we do to improve the performance of HYSPEC?

Some of the decisions listed above are non-negotiable. This includes
going outside the target hall(1), an offset of 16cm at the focusing
crystal(2) and the use of a straight slotted Fermi chopper with a AlGd
slit package(6). We can change the coating of the inner surface of the
curved guide from m=1 to m=2 (3). However, this involves additional
expense. We can change the length, Rc and starting position of the
curved guide (4) at no additional cost. We can ask PSI to increase the
width of the PG monochromator from 260 to 300 mm (5). This should not
entail a huge amount of re-engineering and they may respond favorably to
our request.

  ------------ --------- -------- -------- -------- ------- ------- ---------
  Energy       R(1)      R(2)     R(3)     R(4)     R(5)    R(6)    R(NP)

  15meV        0.6       \~1      0.95     0.95     \~1     0.83    0.45

  90meV        0.53      0.57     \~1      1.14     \~1     0.87    0.30
  ------------ --------- -------- -------- -------- ------- ------- ---------

These three changes should result in some improvement, though not as
much as I had hoped for. At the very least HYSPEC will have a flux on
sample as good as the CNCS.

**References:**

\[1\] V.J. Ghosh presentation at the HYSPEC IDT meeting (Sept.9, 2004

A comparison of the options for the location of the HYSPEC instrument,
V.J. Ghosh, M.E. Hagen, L. Passell, S.M. Shapiro and I. Zaliznyak,
(April13, 2004)

\[2\] McStas simulations for guide configuration optimization, HYSPEC
Technical Note 0009, V.J. Ghosh and M. Hagen, (February 2005).

\[3\] More McStas simulations for the HYSPEC guides. Vinita J. Ghosh,
(September 2005).

\[4\] HYSPEC: A crystal time-of-flight hybrid spectrometer for the SNS,
S.M. Shapiro, I. Zaliznyak, L. Passell and V. Ghosh, ICANS-XVI (May
12-15, 2003).