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