Development of a new Fourier chopper with laser pickup system for neutron beam modulation in correlation RTOF diffractometry

For the further development of the RTOF (reverse time-of-flight) method, a unique Fourier chopper with a fundamentally improved design and enhanced technical specifications has been developed and put into operation at the FSD Fourier diffractometer on the IBR-2 pulsed reactor (FLNP JINR, Dubna, Russia). The chopper consists of a multi-slit rotor–stator system housed in a dedicated vacuum chamber and mounted on a precision translation platform, enabling rapid switching between TOF (high intensity) and RTOF (high resolution) operational modes.

The new chopper introduces advanced capabilities that provide promising potential for further improvement of the RTOF method. In particular, it implements a set of six different frequency window functions, which enable detailed studies of diffraction peak profiles and residual intensity oscillations caused by the finite value of the maximum neutron beam modulation frequency. Additionally, the device supports measurements of individual Fourier harmonics forming the diffraction peak at various constant chopper speeds across the entire accessible TOF-scale range.

An innovative feature of the chopper is the presence of 1024 physical radial slits (each ∼0.7 mm wide), precisely cut into both the stator plate and rotor disc, which reduces neutron absorption and scattering by the chopper material. Furthermore, this work demonstrates for the first time the feasibility of generating pickup signals using a 1550 nm infrared laser beam passing through the slits in both the rotor and stator (Laser Pickup System – LPS). This approach enables direct measurement of the actual transmission function of the chopper, precisely matching the neutron beam modulation profile. As a backup pickup signal source, a standard incremental optical encoder, mounted on the motor shaft, is also used. Experimental results show that high-resolution RTOF diffraction spectra measured using the LPS system exhibit a high signal-to-noise ratio and are comparable to those obtained with the optical encoder.