Quantification in wavelength-resolved neutron imaging: exemplified using polyethylene to mimic hydrogen in stainless...

We present a systematic and improved quantitative methodology for analysing wavelength-resolved neutron transmission spectra in metallic alloys. The approach is demonstrated using data acquired on both the IMAT beamline at ISIS and the RADEN beamline at J-PARC on Super Duplex stainless steel samples partially covered with thin polyethylene layers to mimic small controlled hydrogen concentrations. Initially, we employed Monte Carlo particle transport simulations to assess the effect of forward-scattered neutrons on the recorded transmission signal as a function of sample-to-detector distance and neutron beam size. Guided by this simulation framework, we define an experimental configuration and data processing workflow that enables the reliable quantification of very small transmission differences, reaching the sub-percent regime. An NCrystal-based least-squares fitting routine is developed to resolve multiple microstructural parameters simultaneously, including unit-cell parameters, phase fractions, element composition and extinction-related parameters. The model quantitatively reproduces the experimental transmission spectra and determines polyethylene layer thicknesses with micrometre-level precision. This work establishes a robust microstructure-sensitive framework for quantitative neutron transmission analysis. The approach enables the quantification of trace elements that produce measurable contrasts in neutron transmission, exemplified here for hydrogen in steel with a demonstrated detection limit of 10 wt p.p.m. The presented workflow provides a broadly applicable framework for improving quantitative neutron imaging across diverse materials and applications.