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.
Highlights
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
We present a novel approach for ToF Bragg edge transmission imaging for continuous neutron sources, which is suited to increase the Fig. of Merit (FOM) for such studies at these sources by about an order of magnitude
The Frame Overlap Bragg edge Imaging (FOBI) data is measured with the POLDI chopper at 2000 rpm and the results of corresponding FOBI calculations are based on reference spectra measured at POLDI with the conventional chopper, in order to simulate FOBI results
Summary
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. 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. ToF imaging at continuous sources has been demonstrated using chopper systems[7,16], high-resolution broadband measurements are realized more efficiently at pulsed spallation sources At the latter, no additional flux penalty is implied in achieving the required time structure of the beam. We provide a formal description of the method for the retrieval of absolute wavelength spectra in analogy to methods for imaging techniques using coded sources and we present proof-of-principle results from simulations and experiments
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