Frame overlap Bragg edge imaging

Abstract

Neutron Bragg edge imaging enables spatially resolved studies of crystalline features through the exploitation and analysis of Bragg edges in the transmission spectra recorded in each pixel of an imaging detector. Studies with high spectral resolutions, as is required e.g. for high-resolution strain mapping, and with large wavelength ranges have been largely reserved to pulsed neutron sources. This is due to the fact, that the efficiency for high wavelength resolution measurements is significantly higher at short pulse sources. At continuous sources a large fraction of the available neutrons must be sacrificed in order to achieve high wavelength resolution for a relevant bandwidth e.g. through a chopper system. Here we introduce a pulse overlap transmission imaging technique, which is suited to increase the available flux of high wavelength resolution time-of-flight neutron Bragg edge imaging at continuous neutron sources about an order of magnitude. Proof-of-principle measurements utilizing a chopper with a fourfold repeated random slit distribution of eight slits were performed at a thermal neutron beamline. It is demonstrated, that disentanglement of the overlapping pulses is achieved with the correlation theorem for signal processing. Thus, the Bragg edge pattern can be reconstructed from the strongly overlapping Bragg edge spectra recorded and the results demonstrate the feasibility of the technique.

Prospects of spectroscopic neutron imaging: optimizing experimental setups in battery electrolyte research

“…2b and c), which becomes essential when applying state-of-the-art methods such as the frame overlap in Bragg-edge imaging. 24 Nevertheless, modifying these parameters requires the use of a different chopper disk, as they cannot be modified independently. We are currently developing a multi-layer chopper disk, in which the duty cycle and number of windows can be determined by the angular positions of layers.…”

The FALCON double-detector Laue diffractometer add-on for grain mapping at POLDI

“…Fig. 6 shows a histogram of all wavelengths of the assigned spots, as calculated from the indexing process, plotted in comparison with the measured time-of-flight neutron imaging (Busi et al, 2020) wavelength spectrum of POLDI, showing reasonable resemblance.…”

Bragg edge imaging characterization of multi-material laser powder-bed fusion specimens

“…The Bragg edge imaging measurements were performed at the BOA beamline [7] developed Frame Overlap Bragg edge Imaging (FOBI) technique [8] using a TPX3CAM detector system for event-based neutron detection [9]. The FOBI technique is well suited for continuous neutron sources and uses a multiple slit chopper system to enable Bragg edge imaging with a higher duty cycle compared to a conventional single slit chopper disk.…”

“…The slit width was 1.8 mm and the rotation frequency 25 Hz, resulting in a wavelength resolution of ~1.1% Δλ/λ at 1Å. The FOBI technique [8] was utilized to disentangle the pulse overlap produced by the multiple slits. The detector system was equipped with a 100 𝜇m 6 LiF:ZnS scintillator positioned in the direct beam at 4.8 m distance to the chopper.…”

“…For each measurement, the samples were exposed for 20 hs in radiography mode, preceded by an open beam exposure of 10 hs and a pure iron powder measurement of 6 hs for calibration. The data were analyzed pixel by pixel employing the FOBI retrieval method and the signal processing algorithms presented in [8], with the corresponding chopper time delay scheme. After the FOBI retrieval, the time-of-flight to neutron wavelength calibration was performed by fitting the (110), ( 200) and (311) Bragg edges of a BCC iron powder standard with well-known lattice parameters, employing a non-linear function for the edge profile [10].…”