DS/CS­EF/06
May 3rd, 2004
Ladi3@H112 guide simulation:
effect of a double curved guide
From: E. Farhi
to:
R. Gaehler, P. Timmins, W. Press, C. Vettier
cc:
C. Carlile, M. Johnson, I. Sutton, D. Bazzoli
On the request of C. Vettier and R. Gaehler, a model of the future H112 cold guide upper
channel has been simulated. This project aims at positioning the LADI instrument on
'ILL7
2nd floor'
, at one meter up from the present guide system, and in between the H22
and H23 thermal guides. The H112 model is curved both horizontally (Rh=1.5 km) and
vertically (Rv=2 km), and fully coated with m=2 super mirror.
Substantial flux gains (factor 2 to 4) will be obtained, compared to H142. Moreover, the
beam will be naturally pre­focussed, without loosing on divergence. Such pre­focussing,
resulting from the guide curvature, may also be envisaged for the horizontal
reflectometer (which would require a vertically curved guide).
The H112 Guide description
The H112 guide project is still under design process. Anyway, the geometrical constrains
are quite strong, as it has to fit in the middle of the H22 and H23 thermal guides, and be
bent enough to come at a usable upper plane (say 1 meter up). Today, the envisaged geometry is a double bent guide (1.5 km horizontally, 2 km
vertically), made of 1 meter elements, along about 80 meters [2]. The coating would be
full m=2 super mirror (including in­pile part). Section would be 12h x 6w (in cm2),
splitted into 3h x 6w and 8h x 6w after 30 meters.
The simulated guide model contains the elements listed in Table 1. Aluminium windows
and gaps are in the model. The coating is assumed to produce a constant SM92
reflectivity curve (given by the mean value in the decreasing sloppy region, with
Rm=1=0.99 and Rm=2=0.84). Actually, super mirrors in production today may reach much
better specifications (Rm=2=0.92), which would bring even more flux as the current
simulation results. Anyway, using a lower reflectivity is a way to take into account
imperfections such as the guide misalignments, the coating and substrate
inhomogeneities, the waviness, etc. The VCS specifications are taken from [1]. The Figure 1 shows a view of the model. The
guide alignment is considered to be perfect.
The simulation package is McStas [3] version 1.8. Similar simulations where carried out
in 2001, and are reported in [4,6].
Element
Detail of model
Specifications
Source
Vertical Cold Source [1]
22h x 14w [cm2]
Pink Carter
Focusing, m=2 coating
w=68­>60 mm, h=12 cm,
L=3.17 m
Lead Shutter
m=2
L=0.228 m
H112­2
Polygonal, 6 elements, m=2
L=6.1 m, w=60 mm, h=120
mm, ρ=1.5 km
Vacuum space
For the V.S
L=0.2 m
H112­3, H112­4, H112­5
Polygonal, 20 elements, m=2
L= 20.5 m, w=60 mm, h=120
mm, ρ h=1.5 km, ρ v=2.0 km
Vacuum space
For the V.T.E. Guide splitting L=0.33 m
H112­6, H112­7, H112­8,
H112­9, H112­10
Polygonal, 50 elements, m=2
End of guide: Ladi3 position
(Sample is at 3 m)
L=50 m, w=60 mm, h=30 mm,
ρ h=1.5 km, ρ v=2.0 km
Table 1: H112 guide geometry [2].
H112 Ladi3 Rv=2.0 Rh=1.5 m=2
135 elements
11 monitors
includes gaps Source (VCS) 0
Ladi3 position y/[m]
20
40 Guide splitting 1
0.5
60
0
­1.5 ­1 ­0.5
x/[m]
z/[m]
0 80
Figure 1: Geometric view of the H112 guide model, as seen from the IN12 side of ILL7 building.
Simulated neutron flux
The H112 end of guide monitors provide flux estimates that may be compared with
previous simulated results [4]. We find (see Table 2) about the same capture and
integrated flux as in the previous H112 simulation carried out for the EDM experiment.
Indeed, the main differences are in the total length (was 100 meters in previous
simulation) and the curvature radius (was 2.7 km horizontal in previous simulation). The
geometry is also slightly different. This is why the spectral distribution is a little different
(see Figure 2), providing more cold neutrons for this project, as the curvature is more
pronounced than any other cold guide. Φc [n/s/cm2]
Guide
Φ [n/s/cm2]
dφ/dλ [n/s/cm2/Å] at λ ∼ 5 Å (gold foil)
28.1 109
6.3 109
8.9 108
H112 Rv=2 km 26.6 109
5.7 109
8.2 108
H113 (PF1b)
3.7 109
4.5 108
H112 (EDM)
28.1 109 (Rh=2.7 km) 7.3 109
1.1 109
H142 (LADI)
6.2 109 (S­curved) (Measured: 3 107 at λ ∼
3.5 Å ±10 %)
H112 Rv=0
15.2 109
(Measured: 16 109)
1.6 109
Table 2: Flux estimates for H112 guide model, as well as a few previous results [4]
When looking at H113 (somewhat similar geometry as the current H112 project) and
H142 (present LADI position), we estimate gain factors of about 1.7 and 4 in capture
fluxes respectively. The flux measurement for H142 is given when using a 20%
bandwidth filter, which transmission efficiency is not included in the simulation.
Upgrading the in­pile part to m=2 for both H112 and H113 will probably bring these two
guide fluxes to similar values (increasing the H113 one).
12
x 10
8
H112 Ladi3 Rv=0 Rh=1.5 m=2
H112 Ladi3 Rv=2.0 Rh=1.5 m=2
H112 EDM Rv=0 Rh=2.7 m=2
H113 PF1b Rv=0 Rh=4.0 m=1.2/2
H142 Ladi3 Rv=0 Rh=+/­2.7 m=1
Intensity Φ [n/s/cm
2
/Angs]
10
8
6
4
2
0
0
5
10 λ [Angs] 15
20
Figure 2: Flux as a function of the wavelength at the H112 Ladi3 position, compared with
previous results [4]. The vertical Rv and horizontal Rh guide curvature, as well as the coating m
are indicated.
Monitors positioned along the guides provide an estimate of the flux as a function of the
distance to the moderator (see Figure 3), which is related to the neutron losses. Close to the moderator, the H112 flux is higher than for the H113 and H142 guides,
which use m=1.2 Ni58 coating at the beginning. The in­pile part upgrade brings more
neutrons.
Then the H112 flux loss is comparable to the H15, which has quite a few gaps for the
instruments, and H142, which is a long continuous S­curved guide without major gaps
until IN12. In these latter guides, many neutrons are lost in the in­pile part, and the
remaining neutrons are kept efficiently to the end of the guide.
The H113 guide has a ballistic shape (and no major gap), which reduces the flux loss.
A few capture flux (gold foil) measurements are also presented in Figure 3 (at the end of
the guides), and found in good agreement with simulations.
in­pile
m=2
11
Capture flux Φc [n/s/cm 2]
10
10
H15
H15 measured
H142
H142 measured
H113
H113 measured
H112 Rv=0
H112 Rv=2.0 km
splitting m=2 ballistic guide m=1.2/2 10
m=1.2 20
40
60
Distance from VCS [m]
80
100
Figure 3: Flux along the H112 guide, compared with previous results [4]. For H112, the vertical Rv
guide curvature is indicated. Rh is 1.5 km, and m=2.
Simulated spatial and divergence distributions
In order to estimate the beam distributions at the LADI instrument, we have positioned
monitors just at the end of the guide. The instrument sample position itself should come
after a 3 m beam tube, with filters, slits and shields.
0.015
[H112 Rv=0] PSD at the end of the guide
[H112 Rv=2 km] PSD at the end of the guide
0.015
0.01
0.005
0.005
y [m]
0.01
0
­0.005
­0.005
­0.01
­0.01
­0.015
­0.03
­0.02
­0.01
0
x [m]
0.01
0.02
0
­0.015
­0.03
­0.02
­0.01
0
x [m]
0.01
0.02
Figure 4: Neutron beam spatial distribution (PSD) at the exit of the H112 curved guide, at λ=5 Å.
For both plots, the horizontal curvature is Rh=1.5 km. The plot on the right has an additional
Rv=2 km vertical curvature. Dimensions are in meters. Intensity scale goes from the white (low)
to black (high). Coating is m=2.
The analytical models [5,6] for curved guide do predict a shifting of the beam after a
curved guide. This shift is removed when adding a long enough straight section
downstream. Looking at the Figure 4 – left plot, this effect is indeed observed as
expected: the beam intensity on the outer curvature side is 70 % higher than that of the
inner side.
In the case of a horizontal and vertical curved guide, the shift occurs in both directions
as presented in Figure 4 – right plot, and finally the beam is gathered in one corner of the
guide, producing a natural focusing which might be of great interest for Laue
diffractometers such as LADI. The ratio of the maximum to the minimum intensity in the
guide section is about 2. Other instruments such as horizontal reflectometers may also
benefit from vertically curved guide which would produce well defined pre­focussed
horizontal beams.
[H112] guide end divergence (
α on X, β on Y)
1.4
∆α Rv=0
∆β Rv=0
∆α Rv=2
∆β Rv=2
Divergence half width [deg]
1.2
1
0.8
0.6
0.4
0.2
0
0
5
10
λ [Angs]
15
20
Figure 5: Neutron beam divergence half width ∆α (on X) and ∆β (on Y) at the end of H112 as a
function of the wavelength at the end of the H112 guide, with a vertical curvature Rv of 0 and 2
km. Coating is m=2.
The divergence (see Figure 5) is close to the well known linear law for long guides. The
vertical curvature has a small effect on the vertical divergence, which is lowered at small
wavelengths (for λ < 5 Å ).
Intensity ratio Min/Max on PSD [a.u.]
1
[H112] Spatial anisotropy at 80 m
0.8
0.6
Rv=0
Rv=2 km
0.4
0.2
5
10
λ [Angs]
15
20
Figure 6: Gravitation effect: spatial vertical anisotropy measured as the ratio of the top over the
bottom intensity of the projection of the PSD over the vertical axis, at the end of the H112 guide
with Rh=1.5 km, Rv=0 and 2 km, m=2.
In order to estimate the effect of the gravitation at the end of the H112 guide, at 80
meters from the moderator, we have plotted on Figure 6 the ratio of the top over the
bottom intensity extracted from the PSD projection on the vertical axis. When the
vertical curvature Rv is zero (in­plane), the anisotropy becomes sensible only for λ > 7­10
Å. For 20 Å cold neutrons, the gravitation stacks about 14 % more flux on the bottom part
than on the upper part of the beam.
Concerning the H112 model with vertical curvature, the effects are more pronounced.
For low wavelengths, only garland reflections may occur in the guide, and the neutrons
are stacked to the lower part. Then, reaching the guide critical wavelength (which is
around 1.9 Å), the anisotropy is optimal (with still 10 % more neutrons on the bottom
part), and becomes slowly worse for cold neutrons, reaching 25 % vertical anisotropy.
This anisotropy will control the size and position of the slits for optimal beam flux.
Conclusions It is clear that high flux gains (factor 2 to 4 compared to H142) will be reached with this
new H112 guide. Moreover, the curved guide pre­focusing will be useful to the LADI
instrument. Anyway, as this is a super mirror guide, the divergence will be doubled
compared to the actual H142 one, and the background level may be higher, even if the
short curvature radius is expected to be very efficient for hot and thermal neutron
filtering.
Even if these flux gains might be overestimated (guide is perfectly aligned, glass quality is
constant and reflectivity is high), we believe that the present H113 flux will be easily
reached.
References
[1] P. Ageron, Nucl. Instr. Meth., A284 (1989) 197.
[2] ILL Drawing AP­05­01­002 IJ Sutton '
Guide H112 Avant projet Implantation Ladi III
Option B'
[3] McStas, <http://www.ill.fr/tas/mcstas>
[4] ILL Internal Report DS­CS­EF/72/01 – October 26, 2001.
<http://www.ill.fr/tas/mcstas/instruments/ill/h112/Report.pdf>
[5] D.F.R. Mildner, Nucl. Inst. Meth. In Phys. Research, A 290 (1990) 189.
[6] ILL Internal Report DS­CS­EF – February 16, 2001.
<http://www.ill.fr/tas/mcstas/instruments/ill/h15/R010216.ps.gz>