Neutron Optical Devices of Pnpi

Presently different optical devices are used practically at all modern neutron sources to form neutron beams. The basic event in forming neutron optics at PNPI was creation of highly efficient polarizing mirror on the glass substrate in 1972 [1,2]. It had a polarizing Ñî 60 Fe 40 coating and an antireflective Ti 85 Gd 15 underlayer. The coatings were prepared by vacuum thermal evaporation of materials from tungsten spirals in magnetic field. The composition of the polarizing coating and that of the antireflective underlayer was found experimentally. Changing the composition of the layers, reflection of spin-down neutrons was minimized. The working field was 500 Oe, the critical angle of reflection of spin-up neutrons was α c + = 1.75 mrad⋅A-1 , and the angle α-,at which the polarizing efficiency falls down to 0.8 was α-≅ 0.73 mrad⋅A-1 i.e. α c + /α-≅ 2.4. On the basis of this mirror a 5-meter-long polarizing neutron guide with a 8.5×60 mm 2 cross section was built [3]. This neutron guide was installed at channel 6 of the WWR-M reactor more than 20 years ago and is still under use. The average flux at the exit of the neutron guide is 1.5⋅10 7 cm-2 s-1 for the 15 MW power of the reactor, the maximum of the outgoing beam being at λ = 2.2 A and the polarization averaged over spectrum being no worse than 0.96. A multichannel polarizing neutron guide made of 0.2 mm glass substrates was fabricated and tested in 1977 [4]. The tests showed promising features of the use of such a device as compact polarizers for beams of neutrons with wavelengths λ ≥ 2.2 A, rid of γ-rays and fast neutrons. The limitations of the wavelength was owing to comparatively low critical angle of the monolayer Ñî 60 Fe 40 coating and comparatively high waviness of the glass, which did not allow to bend channels into perfect circumference. The development of multichannel polarizers is connected with the appearance of supermirrors, i.e. multilayered aperiodic structures with a critical angle exceeding that of Ñî 60 Fe 40 in two times. So, the neutron guide group of Department of Development of Physical Equipment Neutron Research Laboratory (later it became Neutron Optics Division of Neutron Research Department) commenced creation of polarizing and non-polarizing multilayered coatings. To meet this aim, a new evaporation machine was purchased from Leybold-Heraeus for vacuum thermal evaporation of multilayered, periodic …