Picutre of velocity selector installed in the reactor beam, with the
side and top
shielding removed.
The 'box' on the right contains the computer controlled
variable
apertures, and a 3 cm thick PG filter can be remotely moved into or our
of the beam depending on the experimental configuration required.
Neutron flux
ratio for 2nd order wavelength vs. primary wavelength.
There is no significant difference by
replacing the 25′ collimation with open -50′ collimation.
With the velocity selector and vertically focused
monochromator there is still some λ/2 in the beam. For most
situations there is no need to correct the inelastic scattering, but the
above data are provided if needed. Corrections for
λ/3, λ/4, ... are negligible. Please note that
the monitor correction factor provided in DAVE is much
larger and is not to be used if the velocity selector is employed.
If the monochromator is vertically flat, there are
no corrections needed.
Silicon Diffraction pattern for 14.7 meV (λ = 2.359 Å)
under three different conditions. The broad distribution of scattering
is from the glass container holding the Si powder:
1) no PG
filter in the reactor beam (blue);
2) with the velocity selector
translated into the reactor beam (orange),
3) Using the PG filter in the
reactor beam (green). At this energy the PG filter works better than the
velocity selector.
At higher energies (e.g. 28 meV, 30.5 meV, 35 meV)
where the PG filter can be used,
the velocity selector performance is superior to the PG filter as indicaed
below for Si powder data taken at 28 meV. This is because for PG the
transmission of the primary wavelength is substantially reduced and the
rejection of higher orders is not as good.
.
Diffraction scan at 14.7 meV using a Ge(111) single crystal at the sample
position. The
greatly enhanced sensitvity compared to the Si powder reveals contaminations from aluminum at the
sample position. Employing the velocity selector reduces or elimanates higher-order wavelength contaminations,
and also greatly reduces the overall background.
Here are the very first inelastic data, taken
with and without the velocity selector for comparison. The sample was
a 0.15g
BiFeO3 single crystal. Söller collimators employed were
open-80′-s-80′-120′ and a 58
mm thick PG filter after the sample. Data are
normalized to counts/minute. For the 3.5 meV data the
background was reduced by 46% while the signal was lowered by 31% with the
velocity selector. For
the 7 meV data a large contamination is
evident due to the Al(111) peak from the sample holder in
λ/2,
which was completely
removed by the velocity selector. The dashed line is an estimate of the
actual intensity of the magnon peak.
..
Detailed Information:
The figure above shows scans of the velocity selector rotation speed
for a few incident energies. The
top is for PG (002) and the bottom is for PG (004). The solid black curves
are Gaussian fits, with the parameters in the legends
Monitor rate comparison of PG (002) monochromator through the usable Ei
range.
Monitor counts
versus either energy (left) or wavelength (right) plotted for the velocity
selector at various fixed speeds, chosen to correspond to the energies at
14.7, 28, 30.5, and 41 meV where PG filters are commonly used..
|
Sample
|
Ei (meV)
|
Full size beam (Aperture: W: 86* H: 160)
|
|
|
Transmission for λ (%)
|
Transmission for λ/2 (%)
|
|
|
Si
|
11.5
|
62.6
|
6.34
|
|
|
14.7
|
65.7
|
7.71
|
|
|
20
|
69.5
|
7.17
|
|
|
25
|
69.2
|
6.05
|
|
|
28
|
|
|
|
|
30.5
|
69.8
|
6.23
|
|
|
41
|
68.5
|
8.35
|
|
|
45
|
67.8
|
-
|
|
|
50
|
67.7
|
-
|
|
|
55
|
67.1
|
-
|
|
|
Ge
|
41
|
|
7.41
|
|
|
50
|
|
7.40
|
|
|
55
|
|
7.75
|
|
Table of
transmission for primary and 2nd order wavelength neutrons using PG(002).
For high energy (Ei > 41 meV), the 2nd order peaks from Si powder are
too small to give a reliable fit, and we used the data from the Ge single Crystal
to determint the transmission for 2nd order.
For the velocity selector using PG (004)
as the primary wavelength, the contaminations are (mostly) from PG (002) and
PG (006), which correspond to 2* λ
and 2/3* λ. For Ei = 40, for
example, these correspond to 10 meV and 90 meV neutrons, respectively.
Then the transmission for λ was measured to be 63.6 %, while the
transmissions for 2* λ and 2/3 * λ are 2.29 % and 29.1 %, respectively.
The flux ratios of 2 * λ and 2/3 * λ compare to λ are then 1.1
% and 12 %.
When Ei = 46 meV, the above numbers change to;
transmission for λ: 64.6 %, 2* λ: 1.16 %, and 2/3 * λ: 20.2 %.
Flux ratio of 2* λ: 1.13 %, 2/3* λ: 6.77 %
Vertically Flat Monochromator (PG002)
Almost all data collected on BT-7 are taken with a
vertically focused monochromator (Vf sagittal). However, rejection of
λ/2 is much better with restricted vertical divergence. The
figure below compares transmission measurements
of the primary (λ) and the 2nd order (λ/2) neutrons with
different vertical focusing modes (flat vs sagittal) using PG(002).
The results demonstrate that restricting the angular divergence in
the vertical diretion greatly reduces the 2nd order neutron (λ/2)
transmissions (at the expense of a reduction in the primary intensity) to
less than 0.5 % for both Ei = 14.7 and 28 meV ,compared to the vertical
sagittal focus mode (~ 7 %). The
transmission for primary neutrons (λ) is also improved somewhat, from ~ 65 %
(sagittal) to ~77 % (flat).
Figure. Velocity
selector transmission measurements using Si powder at Ei =
14.7 meV (top) and 28 meV (bottom), showing the effect of restricting the
vertical divergence on the performance of the velocity selector.
The transmission rate for different modes are listed below.
|
Ei (meV)
(open-10´-80´-DD)
|
Vertical mode
|
Transmission for λ(%)
|
Transmission for λ/2(%)
|
|
14.7
|
Sagittal (focus)
|
64.7
|
7.5
|
|
Flat
|
77.4
|
0.39
|
|
28
|
Sagittal
|
65.7
|
5.8
|
|
Flat
|
77.4
|
< 0.3
|
