Neutron Optics

We specialise in research and development work in the area of neutron optics and produce neutron optical components for in-house demands. For this purpose we operate a sputter unit and the mirror test beamline V 14.

In order to allow the commercialisation of the research results of the group, a spin off company, NOB Neutron Optics Berlin GmbH, was formed, and technology transfer contracts between this company and the HZB were concluded.

The most important components are presented below.

Polarising Fe-Si neutron super mirrors

These systems consist of a few hundred thin silicon and iron layers, the thickness of which ranges from 5nm to 80 nm, 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.

Polarising Co-Cu neutron super mirror

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

Beam splitter and cavity

If polarising super mirrors are sputtered on a silicon substrate and put 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, and here the transmitted component is used, while the reflected component is absorbed.

Solid state components

The development of neutron optical components inside which neutrons are guided by thin plates of single crystal silicon (silicon wafer) marked a major progress for 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 7.5cm.

New bender design

We have produced benders based both on classical physics, in which channels defined by reflective glasses deflect neutrons with one spin component and absorb those with opposite spin, and so-called spin glasses, for which there is no classical analogue. These latter benders transmit both components at different angles. With the benders produced so far, we can achieve a transmission of 65% with a polarisation of more than 95% for neutrons with a wavelength of 0.5 nm. In benders neutrons of any wavelength are polarised above a critical wavelength. Below this critical wavelength the neutrons are transmitted with decreasing intensity, but are not polarised.

New collimator design

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

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

Focussing systems

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

In the first system, 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 focussed. A focal length of 2mm and an intensity gain of a factor of 5 have been obtained.

In the second system prisms are arranged in lines of different lengths to focus a parallel beam.