Hardware, methodology and applications of 2⁺D backscatter Mössbauer spectroscopy with simultaneous x-ray and γ-ray detection

Abstract

A unique method is presented for the acquisition and analysis of 57Fe backscatter Mössbauer spectra with simultaneous detection of the resonant 14.4 keV γ-rays and characteristic 6.4 keV x-rays, using a Multi-Parameter Analyser (MPA) constructed on the basis of commercial Analogue to Digital Converters (ADCs) and custom-built high-speed digital latches. The system allows for the simultaneous registration of Doppler-modulated velocities and photon energies, with up to 4096 and 8192 digital channels respectively. This arrangement is in contrast to most related systems, which detect at a single energy window per detector. Samples of arbitrary atomic structure, morphology and surface topography can be studied without altering the setup or the analysis procedure, provided that the samples are at least micrometre sized. The hardware and software that are used to acquire data with minimal dead time are described and the custom and self-contained methods for post-measurement energy discrimination, background correction and velocity-axis folding are discussed. The data are fit using a general Hamiltonian model for the nuclear energy levels of 57Fe and a quantum mechanical description of the angular momentum coupling is utilised, with consideration of the crystalline and chemical disorder of the sample under examination. Three examples of distinct magnetic systems, with thicknesses ranging from 5 µm to 6 mm, that were studied using this method are presented, these are: an amorphous CoFeB-based ribbon with ultra-soft coercivity for high-frequency applications, magnetically hard Nd-Fe-B thick films on Si substrates, examined in both pre- and post-annealed states, and a sample from the nickel-rich iron meteorite NWA 6259 that contains the atomically ordered, elevated coercivity, L10 phase, tetrataenite.
The wide applicability and usefulness of this method is thus demonstrated on three distinct sample morphologies that required little to no surface preparation prior to examination.

Accelerating Nature: Induced Atomic Order in Equiatomic FeNi

“…This type of data acquisition collects both backscattered 𝛾-rays and conversion X-rays and is well-suited to the investigation of a variety of samples, including thick samples which cannot be characterized in the conventional transmission geometry, relatively thin samples of large lateral area (e.g., thick films on a wafer) and samples with features that render the surface condition unrepresentative of the bulk interior region. More details on the digital electronics and software enabling this type of data acquisition are provided in the work of Borisov et al [57] and O'Brien et al [58] Optimal Mössbauer results were achieved with the use of lowfluorescent-background Perspex sample holders in combination with sufficiently thick (≈5 cm) primary beam Howitzer collimators. While a Mn absorption filter was incorporated to suppress the non-resonant fluorescent background arising from samples' high Ni content, it was found to provide no substantial improvement of the signal-to-noise ratio.…”

“…The resulting matrices were averaged into two specific energy bands, corresponding to back-scattered gamma photons and conversion X-rays, using optimal energy discrimination, as detailed in Ref. [58] The spectral normalization was done using the non-resonant background of photons detected with energies above both resonant absorption lines, and very efficiently corrects for the variation of stearic angle of pick-up as a function of sample-to-source distance. No further processing was needed -, i.e.…”

“…More details on the digital electronics and software enabling this type of data acquisition are provided in the work of Borisov et al. [ 57 ] and O'Brien et al [ 58 ]…”