Using Monte Carlo ray tracing simulations to model the quantum harmonic oscillator modes observed in uranium nitride

Abstract

Recently an extended series of equally spaced vibrational modes was observed in uranium nitride (UN) by performing neutron spectroscopy measurements using the ARCS and SEQUOIA time-of-flight chopper spectrometers [A. A. Aczel et al., Nat. Commun. 3, 1124]. These modes are well described by three-dimensional isotropic quantum harmonic oscillator (QHO) behavior of the nitrogen atoms, but there are additional contributions to the scattering that complicate the measured response. In an effort to better characterize the observed neutron scattering spectrum of UN, we have performed Monte Carlo ray tracing simulations of the ARCS and SEQUOIA experiments with various sample kernels, accounting for nitrogen QHO scattering, contributions that arise from the acoustic portion of the partial phonon density of states, and multiple scattering. These simulations demonstrate that the U and N motions can be treated independently, and show that multiple scattering contributes an approximate Q-independent background to the spectrum at the oscillator mode positions. Temperature-dependent studies of the lowest few oscillator modes have also been made with SEQUOIA, and our simulations indicate that the T dependence of the scattering from these modes is strongly influenced by the uranium lattice.

Recent developments of MCViNE and its applications at SNS

“…MCViNE simulations of powder and single crystal samples measured at direct-geometry TOF spectrometers (DGS) have shown excellent agreement with experimental datasets, including signals from vibrational and magnetic origins [1] as well as signals from multiple-scattering [5]. The predictive power of MCViNE makes it useful in designing new instruments.…”

Light atom quantum oscillations in UC and US

“…This simulation is similar to the one reported in Ref. [23] for UN and includes various sample kernels accounting for elastic scattering, QHO scattering (from carbon atoms), acoustic phonon scattering (single and multiphonon), and all forms of multiple scattering arising from the processes described above. As discussed earlier, the multiphonon contribution from the optic phonons is equivalent to QHO scattering.…”

“…This approach worked exceptionally well to describe the neutron scattering spectra observed for UN [23]. The ability to model more realistic instrument and sample configurations is ever increasing [37,38].…”

Accelerating Data Acquisition, Reduction, and Analysis at the Spallation Neutron Source

“…[19], [20] The Bragg peak, indicated by the pink arrow to the right of the figure, was used to determine alignment of the single crystal to the beam. A region of interest (ROI) of detector pixels was selected around this peak using the Mantid Instrument View features.…”