The system of neutron optics for the diffractometer E PSILON 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): ♦ reactor IBR-2 became critically ♦1983 – the diffractometer NSWR, built in the workshop of the TU Dresden, was mounted at beamline 7 ♦ first plans for a long flight path for an inelastic spectrometer with inverted geometry ♦ idea for an additional multipurpose- instrument: ○quasi-elastic scattering ○high resolution diffraction ○texture investigation

Historical review(2): ♦ sputtering of glass with Cr/Ni and productionof the glass segments for both guides in the workshop of the FLNP ♦ commissioning of the instrument NSWR at the long flight path ○for the multi-purpose instrument it was realised only the texture unit ♦ early variant of E PSILON ♦ replacement of NSWR by SKAT ♦ 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≤l 1 +l 2 ≤110 m ♦resulting velocity for travelling within 200 ms 510≤v≤550 m/s ♦cut-off wavelength due to frame overlapping: λ c-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 7 th beam line will have partial sight on the cold source ♦There is a need to have neutrons beyond the frame overlapping, both for E PSILON 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 m o ○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 by

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 B RAGG ‘s law we get

Spectral Resolution 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 α tot =0.1° ♦Ni 58 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 E PSILON 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: 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