The system of neutron optics for the diffractometer E PSILON and SKAT
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Transcript of The system of neutron optics for the diffractometer E PSILON and SKAT
The system of neutron optics for the diffractometer EPSILON and
SKAT
K. Walther
A. Bulkin
A. Frischbutter
V. Kudryashov
Ch. Scheffzük
F. Schilling
Thanks to:
♦V. Zhuravlov♦A. Sirotin♦E. Shabalin♦I. Natkaniec♦S. Manoshin♦S. Kulikov♦A. Belushkin♦A. Balagurov
Historical review(1):
♦1983 - reactor IBR-2 became critically♦1983 – the diffractometer NSWR, built in the
workshop of the TU Dresden, was mounted at beamline 7
♦1983 - first plans for a long flight path for an inelastic spectrometer with inverted geometry
♦1984 - idea for an additional multipurpose- instrument:
○quasi-elastic scattering
○high resolution diffraction
○texture investigation
Historical review(2):
♦1985 - sputtering of glass with Cr/Ni and production of the glass segments for both guides in the workshop of the FLNP
♦1986 - commissioning of the instrument NSWR at the long flight path
○for the multi-purpose instrument it was realised only the texture unit
♦1995 - early variant of EPSILON
♦1996 - replacement of NSWR by SKAT♦2007 - begin of the reconstruction of the
former neutron guide 7A
What do we expect from a good diffractometer?
♦Good spectral resolution♦Good spatial resolution♦High intensity♦Low background♦Good sample environment
In-pile improvements for high intensity:
♦ In order to reduce the attenuation due to absorption and scattering at this side should be built in an evacuated tube
♦ Start of the neutron guides as near as possible to the chopper
♦ built in of a guide splitter
Reduction of the background with a background chopper
♦Pulse reactor IBR works as a low power steady state reactor (≈ 100 kW)
♦Every 200 ms a high power neutron pulse is generated (≈ 1000 MW)
Background chopper(1)
♦Flight path: 102≤l1+l2≤110 m
♦resulting velocity for travelling within 200 ms 510≤v≤550 m/s
♦cut-off wavelength due to frame overlapping:
λc-o =7.75Å and 7.19Å, respectively
♦opening in the chopper disk (dist. = 5.5m) 19.4° and 18°, respectively
♦opening of the chopper 10.8 ms and 10 ms, respectively (for a beam of zero width)
Background chopper(2)
Calculations of the transmission function of the background chopper were done for:
♦“Standard” (old) mounting: chopper axis is below the entrance windows of the guides
○edges radial○edges parallel
♦“alternative” mounting: chopper axis is at the same height but aside the entrance windows○edges radial○edges parallel
Guide splitter(1)
♦All guides start closer to the moderator♦The three guides do not “disturb” each other♦at the end of the guide splitter 3 separate
neutron guides will start ♦all guides are evacuated.
Lambda-chopper(1)
♦The 7th beam line will have partial sight on the cold source
♦There is a need to have neutrons beyond the frame overlapping, both for EPSILON and SKAT
due to the large unit cells of some minerals♦Every second power pulse could be eliminated
by an additional chopper♦The back ground chopper should provide this
extended wavelength range too
Lambda-chopper(2)
Lambda-chopper(3)
Lambda-chopper(4)
Neutron guide and some fundamental mathematics
♦Neutrons are classical mechanical particles○They have a mass mo
○They have a velocity v
○They have a momentum p
Neutron guide and some fundamental mathematics(2)
♦Neutrons are waves○They have a wave length
The wave length λ is connected with the flight time t byt
llm
h
210
1
Neutron guide and some fundamental mathematics(3)
In time-of-flight diffraction patterns you see peaks in a distribution of the number of diffracted neutrons (intensity) over the wavelength for a given scattering angle ϑ.
According to BRAGG‘s law
sin2 d we get
tllm
hd
sin
11
2
1
210
Spectral Resolution
cott
t
d
d
Spectral resolution can be improved by increasing the flight path!
Neutron guides♦Classical neutron guides are based on the
phenomena „total reflection“ like fibre optics
♦Small total reflection angles in the order of minutes of arc
♦Only few minerals/elements are of interest
♦Angle of total reflection is proportional to the wavelength
♦Angle of total reflection of Ni with natural abundance of isotopes defines the value m=1.0 which corresponds αtot=0.1°
♦Ni58 corresponds m=1.2
Neutron guides(2)
♦Neutron guides are built of well aligned glass sections
♦The surface of the glass should have nearly no roughness
♦Boron glass meets the requirements♦Neutron guides for thermal neutrons are
bent in the most cases
Neutron guides(2)
♦Neutron guides are built of well aligned glass sections
♦The surface of the glass should have nearly no roughness
♦Boron glass meets the requirements♦Neutron guides for thermal neutrons are
bent in the most cases
Neutron guides(3)♦The neutron guides for EPSILON and SKAT are
built up of sections 1 m long♦The sections are 95 mm high and 50 mm wide♦The first part (within the splitter) is straight♦The next part about 80 m length is bent; radius
of curvature:13 400 m♦Bent neutron guides transport only neutrons
with a wavelength greater the critical wavelenght, which depends on the width and the radius of bending.The critical wavelength is 1.58 Å
♦In order to homogenize the flux across the section the last part are straight
Outlook and conclusion
♦After realization the new neutron guides and the chopper system there are created two high level diffractometers for long wavelength diffraction
Acknowledgment
- the BMBF Germany,
- FLNP Dubna
- Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences
- Karlsruhe Institute for Technology
for support