Transmission bender for polarizing neutrons

Abstract

A system of a polarizing solid state bender and a collimator was built and tested. The bender consists of a stack of thin silicon wafers coated with polarizing FeCo–Si supermirrors. The stack is deflected in the middle by one wafer thickness. Inserted in an unpolarized neutron beam this device works as a spin splitter by transmitting the spin down component without any change in its flight path and reflecting the spin up component. With a collimator behind the bender it can be selected which one of the spin components is transmitted. In this way a transmission polarizer for neutrons with wavelengths above 4 Å was built which has a total length of less than 10 cm.

Introduction

Nowadays, in many cases neutron beams are polarized using supermirrors. The angular interval which can be polarized by supermirrors is limited by their critical angle ${\Theta }_{\mathrm{c}}$ which is in the order of one degree. To polarize a beam with a width of several centimeter either a supermirror is needed with a length of several meter, as described in Ref. [1], or a system with parallel channels. These channels can be straight with the supermirrors at an angle to beam [2] or curved as in a bender. There are two types of benders. In one of them the neutrons fly in air and one spin component is reflected from supermirrors on bent glass walls [3], in the other one they fly in bent silicon wafers coated with supermirrors [4]. In the first set up the unwanted spin component is necessarily absorbed in the glass wall. The second set up allows to transmit both components which are distinguished by different angles and one spin state can be selected by putting a collimator behind the bender at the angle of the beam which should be used. In particular, the non deflected component can be chosen thus giving a polarizing device which polarizes a beam without changing the flight path of the neutrons.