ID18 offers new possibilities for Synchrotron Mössbauer Source spectroscopy

Synchrotron Mössbauer source (SMS) spectroscopy at beamline ID18 combines the advantages of energy-domain Mössbauer spectroscopy with the high brilliance of synchrotron radiation. Unlike radioactive sources, SMS provides a beam several microns in size. Therefore, studies of small samples, which take weeks with a radioactive source, can be done in minutes with SMS, and annual scientific programmes can be completed within a matter of days.

The key element of SMS is an iron-borate crystal nuclear Bragg monochromator. By varying the crystal temperature and the angle of incidence, the energy resolution can be improved at the expense of intensity, and vice versa. Until recently, SMS was operated in standard conditions, with a source width of about two natural linewidths and an intensity of ~2×10⁴ γ-quanta/s, in a reasonable compromise between linewidth and intensity. Recent developments now allow users to improve the resolution using a narrower linewidth or to increase the intensity by an order of magnitude.  

The gain in flexibility was enabled by studies of SMS parameters for various crystal temperatures close to the Néel temperature T_N = 75.20 ⁰C and incidence angles close to Bragg reflection.  Figure 1 shows the master plot of all experimental data in the width-intensity representation. 

Fig. 1:  The master plot of all experimental data in the linewidth-intensity representation. The solid lines connect the data points measured for various angles at indicated temperatures. The envelope (blue dashed line) shows the maximum available intensity at the specified effective width of the source. The red solid vertical line shows the natural width Γ₀.

The envelope defines the optimal operation conditions – the maximum intensity at specified linewidth. If an intensity of ~10³ γ-quanta/s is tolerable, the energy resolution can be improved by decreasing the source width down to the natural linewidth Γ₀ = 0.097 mm/s = 4.66 neV. This is a good choice for poorly resolved spectra, like those of non-magnetic minerals at high pressure or iron-bearing inclusions in diamonds. In the opposite extreme, the intensity can be raised by an order of magnitude, up to 10⁵ γ-quanta/s. This is useful for fast operando studies of Li-ion batteries or for glasses with broad resonance lines.

This new development significantly enlarges the output of the Synchrotron Mössbauer Source at the ESRF. Firstly, any experiment now can be precisely customised for each specific need, permitting users to either measure more samples and/or to improve signal-to-noise ratio in each measurement.  Furthermore, the gain in flexibility facilitates frontier studies in researching residual magnetism or quadrupole splitting in challenging systems such as ε-iron. Finally, it also enables fast operando studies with a collection time of about a minute per spectrum, required, for example, in electrochemistry applications. 

Principal publication and authors
Synchrotron Mössbauer source: trade-off between intensity and linewidth, S. Yaroslavtsev & A. Chumakov, J. Synchrotron Radiat. Accepted (2022).