Neutron Optics

The neutron optics group within the Working Group EM-AMCT specialises in research and development work in the area of neutron optics and produces neutron optical components for in-house demands. For this purpose we operate a sputter unit and the mirror test beamline V14.

Prominent examples for innovative components are presented below.

In order to allow for the commercialisation of the research results of the group in 2000 a spin-off company, NOB Neutron Optics Berlin GmbH was formed, and technology transfer and license contracts between this company and the HZB were concluded. The development work will be continued by NOB from end of 2017 on.

 

Polarising Fe-Si neutron supermirrors

These systems consist of a few hundred thin silicon and iron layers with thicknesses ranging from 50 Å to 800 Å, sputtered onto glass or silicon substrates. The layers reflect the spin component of a neutron beam which is parallel to a magnetic field, and transmit the anti-parallel component. Presently values of up to m=4 can be reached.

 

Beam splitter and cavity

If polarising supermirrors are sputtered onto a silicon substrate and put at a small angle into a neutron beam both, the reflected and the transmitted beam component can be used.

Inside a so-called “cavity“ the coated substrates are put into a neutron guide at an angle to the side walls, and here the transmitted component is used, while the reflected component is absorbed in the guide walls. [1], [2], [10]

 

Solid state components

The development of neutron optical components in which neutrons are guided inside thin plates of single crystal silicon (silicon wafers) marked a major progress in the design and production of polarising benders and collimators. Advantages of these solid state components are easier mechanical handling and much smaller dimensions. The space required for polarisation along the beam could be reduced from 9 m to 7.5 cm for a beam width of 60 mm. [3], [4]

 

Polarising Co-Cu neutron supermirrors

We were the first to produce Co-Cu neutron supermirrors, which reflect the spin-down (anti parallel) component of a neutron beam, while all other polarising supermirrors reflect the spin-up component. [5]

 

New collimator design

Collimators are also made of silicon wafers by applying an absorbing layer to the silicon and stacking several wafers next to each other in the neutron beam. At different incoming angles, this system shows the well-known triangle transmission function of traditional collimators. However, as for the benders, length and weight of these collimators are much smaller.

The use of silicon wafers allows coating the walls of the channels first with a reflecting layer and then with an absorbing layer. If the total reflection angle of the first layer is as large as half the width of the maximum collimator angle, a square transmission function with the width of the base of the triangle is obtained and the number of the transmitted neutrons is doubled. [6]

 

New polarizing bender design 

We have produced polarizing solid state benders which, as in the classical set up, reflect neutrons with one spin component and absorb those with opposite spin. Built without absorbing layers such a bender transmits both spin states at different angles, thus working as a spin splitter, for which there is no analogue in the classical bender with glass walls.  

With the benders produced so far, we achieved a transmission of 65% with a polarisation up to 95% for neutrons with a wavelength of 5 Å. In benders neutrons of any wavelength are transmitted with high intensity and polarised above a critical wavelength. Below this critical wavelength the neutrons are transmitted with decreasing intensity, but are still polarised. [7]

 

Two-dimensional large angle spin analysers

For the spin analysis of a neutron beam in two dimensions we built a solid state system [8] and several multiple cavities [9].

In all of the analyzers described above polarization values of 95% were achieved.

 

S-Bender

To increase the polarization, S-shaped solid state benders were built. Since there are two reflection processes which polarize the beam a much higher polarization can be reached. A further advantage is that the average beam direction is kept. Thus it can be inserted into an instrument without the need to realign it when switching from unpolarized to polarized mode. Finally, due to the fact that the garland reflections from the first part of the bender are not reflected in the second part, neutrons with wavelengths below a certain threshold are not transmitted.

One of these benders was designed for wavelengths above 3.5 Å and it showed a transmission of 65% and a polarization of 98% at 4.4 Å. [11]

 

Focusing systems

In recent years two different systems which allow focusing neutron beams have been developed.

The first system uses reflection and es thus achromatic. The neutrons are guided through bent silicon wafers of different length coated with mirrors on both sides. Thus the entire neutron beam fed through a neutron guide is focused. A focal width of 2 mm and an intensity gain of a factor of 5.6 compared to the intensity at this place without the lens have been obtained. [12]

The second system uses refraction in prisms and is thus chromatic. The prisms are arranged in lines of different lengths to focus a parallel beam or to image a source. By imaging a slit a gain of a factor of 7.9 compared to the intensity at this place without the lens was achieved within the FWHM of 0.35 mm.

 

Energy analysis

A system of prisms of the same length deflects neutrons of different wavelengths into different angles, thus allowing for the analysis of the neutrons’ energy. With such a system a wavelength band from 2 Å – 8 Å could be analyzed with a resolution of 5% at 6.7 Å. [13], [14]

Literature

[1] Th. Krist, C. Pappas, Th. Keller, F. Mezei: The polarizing beam splitter guide at BENSC, Physica B 213 & 214 (1995) 939-941.

[2] T. Keller, T. Krist, A. Danzig, U. Keiderling, F. Mezei, A. Wiedenmann: The polarized neutron small angle scattering instrument at BENSC Berlin, Nucl. Instrum. Methods Phys. Res. A 451 (2000) 474-479.

[3] Th. Krist, S. J. Kennedy, T. J. Hicks, F. Mezei: New compact neutron polarizer, Physica B 241-243 (1998) 82-85.

[4] Th. Krist: Solid state and conventional neutron optical elements, Nucl. Instrum. Methods Phys. Res. A 529  (2004) 50-53.

[5] Th. Krist, J. Hoffmann, P. Schubert-Bischoff, F. Mezei: Inversely polarizing Co-Cu neutron supermirrors, Physica B 241-243 (1998) 86-88.

[6] Th. Krist and  F. Mezei: High performance, short solid state collimators with reflecting walls, Nucl. Instrum. Methods Phys. Res. A 450 (2000) 389-390.

[7] Th. Krist, J. Peters, H. M. Shimizu, J. Suzuki, T. Oku: Transmission bender for polarizing neutrons, Physica B  356 (2005) 197–200.

[8] Th. Krist, H. Fritzsche, F. Mezei : Large-angle neutron polarisation analyser, Appl. Phys. A 74 [Suppl.] (2002) s221-s223.

[9] Péter Falus, Alexei Vorobiev, Thomas Krist: Test of a two-dimensional neutron spin analyser, Physica B 385-386  (2006) 1149-1151.

[10] Th. Krist, C. Pappas, A. Teichert, C. Fehr, D. Clemens, E. Steichele and F. Mezei: New polarizing guide for neutron wavelengths above 2.5 Å, J. Phys.: Conf. Ser. 251 (2010) 012081.

[11] Th. Krist, F. Rucker, G. Brandl, R. Georgii: High performance, large cross section S-bender for neutron polarization, Nucl. Instrum. Methods Phys. Res. A  698 (2013) 94-97.

[12] R. Bartmann, N. Behr, A. Hilger and Th. Krist: New solid state lens for reflective neutron focusing, Nucl. Instrum. Methods Phys. Res. A 634 (2011) S104–S107.

[13] J. Schulz, F. Ott, Ch. Hülsen, Th. Krist: Neutron energy analysis by silicon prisms, Nucl. Instrum. Methods Phys. Res. A729 (2013) 334-337.

[14] J. Schulz, F. Ott, Th. Krist: An improved prism energy analyzer for neutrons, Nucl. Instrum. Methods Phys. Res. A744 (2014) 69-72.