Wombat, the high-intensity diffractometer in operation at the Australian Centre for Neutron Scattering

Wombat is the high-intensity neutron diffractometer in operation at the Australian Centre for Neutron Scattering. While Wombat is primarily used as a high-speed powder diffractometer, the high-performance area detector allows both texture characterization and single-crystal measurements. The instrument can be configured over a large range of operational parameters, which are characterized in this contribution to aid experimental planning. Wombat is particularly optimized for the study of materials in situ and in operando using the wide range of sample environments available at the centre. Over 17 years of operation, Wombat has been used to explore a broad range of materials, including novel hydrogen-storage materials, negative-thermal-expansion materials, cryogenic minerals, piezoelectrics, high-performance battery anodes and cathodes, high-strength alloys, multiferroics, superconductors, and novel magnetic materials. This paper will highlight the capacity of the instrument, recent comprehensive characterization measurements and how the instrument has been utilized by our user community to date.

1. Introduction

The high-intensity diffractometer, otherwise known as `Wombat', at the Australian Centre for Neutron Scattering (ACNS, which was known as the Bragg Institute between 2005 and 2016) has been operating a user programme since 2008 as part of the Australian Nuclear Science and Technology Organisation (ANSTO). Over the past 17 years, data from the instrument have contributed to over 400 reports, theses and journal publications. In the context of new and upgraded neutron diffraction instruments in the Asia/Oceania region (He et al., 2023; Xu et al., 2019; Nambu et al., 2024), we explore here the unique flexibility and capability of Wombat, hoping that this will inspire the neutron community to use the instrument in new and innovative ways.

The instrument is accessible to the Australian and international neutron user community through proposal rounds that are run twice a year, with deadlines usually falling on or around 15 March (for instrument time nominally July–December) and 15 September (for instrument time nominally January–June). Success rates of proposals are usually about 50%, with 200 instrument days available for user operations each year.

Wombat is inherently flexible, allowing for a large range of diffraction characteristics. This is achieved through the three available monochromators, continuous variation in the instrument take-off angle, and a large-area detector which couples with a large sample area for mounting of a wide range of types of samples and sample environments. Below we outline the instrument layout, discuss processes for data collection and reduction, and chart the range of instrument configurations possible. The resolution functions of representative configurations have been characterized to aid experimental planning. Additionally, a number of case studies that showcase the large range of science applications that can be achieved with the instrument will be presented, as well as a overview of the instrument's user community.

The instrument was originally described (Studer et al., 2006) during the construction phase. It is timely now to update the community on the progress that the instrument has made and provide up-to-date configuration and performance information from the instrument as it presently operates.

2. Instrument layout

The Wombat instrument is located on Thermal Guide 1 from the Open Pool Australian Lightwater (OPAL) reactor, upstream from the high-resolution powder diffraction instrument (known as Echidna) (Avdeev & Hester, 2018). A schematic of the instrument is provided in Fig. 1, which shows how a thermal neutron guide of internal height 50 cm delivers neutrons into a chamber housing the monochromator. The chamber has been designed with lead shutters that are pneumatically lifted as the take-off angle of the instrument is changed.

Figure 1 Schematics of the Wombat instrument. Both diagrams show the instrument configured with a take-off angle of 90° and a detector start angle of ∼14°, which is the most common configuration of these components. The upper panel shows the `walk-in' view, displaying the relative height of the components, while the bottom panel gives a birds' eye view of the instrument layout with the beam path marked in purple. The scale marker is approximate

The flight path from the monochromator to the sample table has sufficient space to include ³He-based neutron polarizers and polarization analysers (Klose et al., 2014), to enable full polarization analysis for magnetism research on the instrument. Filters may also be inserted to exclude harmonic contamination incident from the monochromators. Most commonly used is a highly oriented pyrolitic graphite (HOPG) filter that is regularly paired with the focusing Ge monochromator in the 90° take-off position oriented to the 113 reflection (2.41 Å). The instrument also has a cooled beryllium filter, which is used in conjunction with the focusing graphite monochromator for wavelengths above 4 Å. A tapered snout with an adjustable rectangular exit slit is normally placed immediately before the sample position. Typical exit beam dimensions from this snout are 10 mm (h ) × 20 mm (v), with a fully open vertical aperture of around 60 mm.

The detector table houses the electronics and high-voltage amplifiers for the instrument and moves independently around the sample stage to adjust the accessible range of diffraction angles, 2θ. The lowest and highest accessible 2θ angles are ∼12° and ∼150°, with a span of 120° available at any specific detector position. The sample stage and detector table move together (via a series of air-pads) when the take-off angle of the instrument is changed. The take-off angle is continuously variable between 60° and 120°, but this range is restricted to 75–105° when the cooled beryllium filter is in place.

3. Data collection and reduction

3.1. Detector

The key component of the Wombat instrument is the detector, a curved position-sensitive detector constructed by Brookhaven National Laboratory. The detector itself is nearly identical to that designed first for the PCS instrument at Los Alamos Neutron Science Center (Chen & Unkefer, 2017), which was later moved to the WAND² instrument at the High Flux Isotope Reactor at Oak Ridge National Laboratory (Frontzek et al., 2018). It consists of eight 15° curved panels with a radius of curvature of 728 mm and a height of 200 mm, placed 728 mm from the sample position. These are constructed such that there is no gap between them, resulting in 120° continuous coverage. In standard settings, the detector provides a 968 (h) × 128 (v) pixel image, corresponding to a pixel size of 0.125° (h) × 1.6 mm (v). The horizontal spatial resolution may be reconfigured to 0.01625° if required. The two-dimensional detector means that samples throughout the continuum of crystallite orientational order can be studied, ranging from well randomized powders through to textured samples and single crystals. On Wombat, this detector is used in conjunction with a radial collimator constructed by JJ X-ray, which has a 0.5° pitch, excluding parasitic scatter from a radius >50 mm from the instrument centre.

Instrument control and data collection is undertaken with the SINQ instrument control software (SICS; Heer et al., 1997), which is usually accessed through the GumTree interface (Lam et al., 2006).

3.2. Data reduction

For many years, data reduction at the instrument was carried out using the Large Array Manipulation Program (LAMP) (Richard et al., 1996), and macro routines are still available for use with this program. Since 2020 there has been a transition to data reduction implemented using Python code within the GumTree framework (Lam et al., 2006). The Python code is freely available (Hester, 2025).

A typical raw dataset from Wombat contains one or more frames of data, representing counts accumulated across the detector in some time period. The data reduction steps described below are those most commonly used when deriving one-dimensional datasets suitable for input into typical powder diffraction analysis software; other types of measurements typically require alternative approaches to data reduction, for example using Int3D (Katcho et al., 2021) for single-crystal measurements.

3.2.1. Normalization

Beam monitor counts are used to place each frame of data onto a uniform scale, as the incident intensity from the reactor source is likely to fluctuate. Multiple datasets may be treated simultaneously to place them on the same scale.

3.2.2. Efficiency correction

Each pixel on the detector has a variable response to an incident neutron. This is accounted for using a pre-calculated efficiency correction map. To accommodate the gradual changes in the response pattern of the detector, new efficiency maps are generated every few months by measuring a flood field from a vanadium rod and then subtracting a background measured without the rod. In order to account for the presence of weak coherent peaks from vanadium, the detector angle stth is stepped by 0.125° over a range of about 12° during both rod and background measurements, which typically takes an entire day. After subtraction, any pixels judged by a configurable automatic algorithm to contain peak contributions are removed from the final calculation. The response for each retained pixel is averaged, and the final efficiency map is used to calculate a correction factor for each pixel, which is applied to measured data at this data reduction step.

3.2.3. Vertical integration

The final step in data reduction is summation of all pixels at each nominal 2θ position to produce a 1D diffraction pattern suitable for input into powder diffraction modelling software. Prior to this summation, users may optionally remap all pixels to their `true' 2θ position, which is termed `straightening' in the present paper. This procedure invalidates the asymmetry model of Finger, Cox and Jephcoat (Finger et al., 1994) and may therefore complicate diffraction pattern peak-shape modelling, as the calculation of true 2θ assumes a parallel beam and point scatterer, which is rarely the case in most experiments. Nevertheless, in practice the resulting peaks are often sufficiently well modelled using standard pseudo-Voigt profile parameters.

3.2.4. Gain re-refinement

Where the sample contains strong incoherent scatterers, the efficiency correction described above is often insufficient for the high background. In such cases, several frames of data can be collected by stepping the detector by the pixel spacing (typically 0.125°), resulting in multiple measurements of the same signal at each angle after vertical summation. Scale factors for each column of vertical pixels are then refined using the procedure of Ford & Rollett (1968). Wombat data reduction uses the same underlying code as Echidna (Avdeev & Hester, 2018), which should be referred to for details of the mathematics involved.

The final 1D diffraction pattern is one output of this refinement. If straightening has been requested (see §3.2.3), the final 1D diffraction pattern is instead obtained by using the refined scale factors to rescale the 2D frames before straightening and vertical summation.

3.2.5. File output

Data may be output in two variants of three-column ASCII format suitable for GSAS-II (Toby & Von Dreele, 2013) and TOPAS (Coelho, 2018), as well as a pdCIF file (Toby, 2006) containing comprehensive metadata, including locally defined data names capturing all data reduction parameter settings. It is generally possible to exactly reproduce data reduction using the contents of the pdCIF file.

4. Instrument configurations and characterizations

To assist with understanding published Wombat studies and experimental planning, each of the three monochromators used on Wombat has been characterized with appropriate standard materials and resolution curves determined for a variety of take-off angles.

4.1. Resolution function fitting

Data were measured from standard materials contained in 6 mm-diameter vanadium cans and reduced as described above with no straightening applied. Raw datasets and reduced data are available on Zenodo (Studer et al., 2025; Maynard-Casely et al., 2025; Wang et al., 2025).

Data were then analysed within the TOPAS5 refinement suite (Coelho, 2018). Resolution functions were fitted with the following procedure: First the wavelength, sample displacement and zero offset error for a given monochromator reflection and take-off angle were determined from a refinement of LaB₆ standard reference material 660b, supplied by the National Institute of Standards and Technology, USA (Black et al., 2011), which has been isotopically enriched with an 11B precursor. For these refinements, a Chebyshev five-term background and scale factor were also refined. No atomic positions were refined, but the isotropic displacements of both lanthanum and boron atoms were.

Once established, the wavelength and zero offset error values were used to undertake Pawley refinements of patterns of Na₂Ca₃Al₂F₁₄ (NAC, sourced from Laboratoire des Oxydes et Fluorures, Faculté des Sciences Universitie du Maine, France) collected at each configuration. Although NAC is not an official standard reference material, it has been used as a line profile standard (Courbion & Ferey, 1988). Given Wombat's broad range of operating wavelengths (1–6 Å) it was chosen as a consistent standard material to characterize peak shape and asymmetry across all configurations. In addition, the sample of NAC is mixed with CaF₂, which was also accounted for in the refinements. For all of the configurations, the peak shape was fitted with a Gaussian peak-width variation using a simple three-parameter UVW model derived from the Caglioti equation (Caglioti et al., 1958):

where θ is the Bragg angle, that is, half of the diffraction angle 2θ. No crystal size parameter was added, but the lattice parameter was allowed to refine. An overview of the Δd/d resolution curves calculated from the fitted U, V and W parameters for the 113 reflection of the focusing Ge monochromator, 113 reflection of the flat Ge monochromator and 002 reflection for the focusing graphite monochromator across the range of take-off angles is plotted in Fig. 2.

Figure 2 Δd/d resolution curves calculated from the peak shape functions fitted for the 113 reflection of the focusing Ge monochromator, 113 reflection of the flat Ge monochromator and 002 reflection for the focusing graphite (PG) monochromator across the range of take-off angles.

The large aperture of the position-sensitive detector (over 200 mm of height) means that asymmetry is prominent in low-angle peaks. The approach to fitting the asymmetry is detailed in the next sections.

4.1.1. Focusing Ge monochromator

The `workhorse' monochromator of the instrument is a 300 mm vertically focusing monochromator, which consists of 23 germanium crystal plates that each have a length of 70 mm cut along the [115] direction, with the [10] direction vertical. As [10] is vertical and the Ge space group is cubic, the 〈hhn〉 directions lie in the horizontal plane, allowing hhn reflections to be rotated into reflecting position. The focus of the monochromator can be changed, but the optimum between intensity and divergence is ∼0.3° curvature. Hence this was fixed throughout the collection of the data from the standards. Peak shapes were fitted for the two main reflections used, 113 and 115, over the range of take-off angles detailed in Table 1. The majority of the configurations described in Table 1 could be fitted without any additional asymmetry corrections. The exception was at the higher take-off angle for the 115 reflection, where the asymmetry becomes more pronounced due to peaks appearing at lower 2θ angles. For this configuration the Finger et al. (1994) model was used with the `filament' length (S) parameter set at 20 mm to reflect the incident neutron slit size and the receiving slit length (D) parameter set at 200 mm to reflect the size of the detector.

Table 1
Fitted U, V and W parameters, along with wavelengths (determined from LaB₆ refinements), refined from data of a NAC sample for the two main reflections used of the focusing Ge monochromator, across a range of take-off angles

As described in the text, fitting of the 115 reflection at 120° required additional modelling for the peak asymmetry (S and D). Flux relative to Ge 115 at 90° is indicative only; the asterisk (*) indicates pyrolitic graphite filter inserted.

Configuration	λ (Å)	Relative flux	U	V	W	S	D	wRp (%)
113 reflection 90°	2.417	2.5*	0.965 (14)	−0.98 (4)	0.534 (18)	–	–	3.953
115 reflection 90°	1.543	1.0	0.88 (3)	−1.15 (5)	0.63 (2)	–	–	3.354
113 reflection 60°	1.706	2.1	1.63 (6)	−1.00 (8)	0.39 (2)	–	–	3.564
115 reflection 60°	1.089	0.3	1.8 (5)	−1.5 (3)	0.53 (5)	–	–	3.120
113 reflection 120°	2.958	1.8	0.43 (2)	−0.70 (5)	0.64 (2)	–	–	4.834
115 reflection 120°	1.890	1.0	0.50 (2)	−1.09 (5)	0.92 (2)	20	200	3.022

4.1.2. Flat Ge monochromator

In addition to the two monochromators originally described (Studer et al., 2006), Wombat has routinely used a single cut flat (335) germanium monochromator, a heritage component from the MRPD instrument previously operated on the HiFAR reactor (Howard & Kennedy, 1994). The use of this monochromator has proven advantageous for experiments requiring higher flux than that available on Echidna, but with more resolution than is possible with the focusing Ge monochromator. As such it is now used in ∼10% of Wombat experiments. Three reflections are routinely used from this monochromator: 113, 115 and 224. The peak-shape parameters of these reflections across a range of take-off angles are given in Table 2. The asymmetric contribution to peak shape is more pronounced due to the narrower instrumental resolution, so the Finger et al. model was used, with the sample length set at 20 mm and receiving slit length set at 200 mm as described in the previous section.

Table 2
Fitted U, V and W parameters, along with wavelengths (determined from LaB₆ refinements), refined from data of a NAC sample for the three reflections used regularly from the flat Ge monochromator, across a range of take-off angles

In each fit the Finger et al. asymmetry model was applied with S = 20 mm and D = 200 mm. Flux relative to Ge 115 at 90° is indicative only.

Configuration	λ (Å)	Relative flux	U	V	W	wRp (%)
113 reflection 90°	2.422	0.3	0.48 (4)	−0.50 (7)	0.267 (3)	4.938
115 reflection 90°	1.544	0.2	0.38 (1)	−0.38 (1)	0.235 (7)	3.803
224 reflection 90°	1.638	0.3	0.904 (2)	−1.07 (1)	0.47 (2)	3.313
113 reflection 60°	1.713	0.3	0.70 (4)	−0.49 (7)	0.25 (1)	4.683
115 reflection 60°	1.093	0.1	0.6 (6)	−0.5 (5)	0.2 (4)	4.646
224 reflection 60°	1.159	0.2	1.17 (5)	−0.96 (7)	0.36 (1)	3.337
113 reflection 120°	2.962	0.3	0.368 (19)	−0.58 (4)	0.38 (2)	6.592
115 reflection 120°	1.891	0.2	0.290 (17)	−0.53 (3)	0.37 (1)	5.117
224 reflection 120°	2.005	0.3	0.374 (7)	−0.700 (7)	0.474 (4)	5.538

4.1.3. Focusing graphite monochromator

A HOPG monochromator, constructed identically to the focusing germanium monochromator and cut to the 002 reflection, may also be used on Wombat. One use originally envisioned for this monochromator was a `very' high flux 2.4 Å option at a take-off angle of 42°. In practice this has been rarely used, as the flux with the focusing Ge monochromator 113 reflection at 90° has been sufficient for all rapid experiments to date. However, the graphite monochromator has proven useful in providing a longer wavelength, particularly for magnetic studies, and is used in ∼5% of Wombat experiments. To characterize these configurations, which would have few incident peaks from LaB₆, NAC was used to refine lattice parameters as well as subsequent line profiles. These measurements were taken in tandem with a cooled beryllium filter, which limits the accessible take-off angles. The Cagliotti peak shape parameters fitted for the 002 reflection over the range of accessible take-off angles are detailed in Table 3. As for the focusing Ge monochromator, it was found that a satisfactory peak shape could be fitted from all the configurations without the need for an additional asymmetry model.

Table 3
Fitted U, V and W parameters, along with wavelengths, refined from NAC sample data for the 002 reflection of the focusing graphite monochromator, across a range of take-off angles

Flux relative to Ge 115 at 90° is indicative only.

Configuration	λ (Å)	Relative flux	U	V	W	wRp (%)
002 reflection 75°	4.109 (3)	13	2.92 (2)	−2.39 (9)	1.21 (2)	2.085
002 reflection 90°	4.779 (1)	11	2.0 (1)	−1.7 (2)	1.06 (8)	1.813
002 reflection 105°	5.349 (2)	9.0	1.6 (1)	−1.9 (2)	1.4 (1)	2.219

4.2. Sample environment configurations

Wombat's generously sized sample stage can host a huge variety of sample environments, spanning the breadth and depth of many phase diagrams. The families of sample environment equipment include the following:

(1) Cold temperature 4 ≲ T ≲ 300 K (cryostats, cryofurnaces),

  (i) including also cold T < 1 K (dilution insert and a one-shot ³He insert that was previously available).

(2) Warm temperature 273 ≲ T ≲ 773 K (temperature baths, low-temperature heaters and furnaces);

  (i) to span the temperature range from cold to warm 4 < T < 750 K, a `full range' sample stick can be used in the cryofurnaces.

(3) Hot temperature 373 ≲ T ≲ 1473 K (high-temperature vacuum furnace).

(4) Magnetic field, electrical field and charge variation (cryomagnets ≤11 T, electric field generation ≤10 kV, battery tester, potentiostat galvanostat).

(5) Gas handling (gas dosing, gas condensing, gas sampling, vapour delivery).

(6) Pressure/shear (pressurized sample cans, Paris–Edinburgh press ≤10 GPa, rheometer).

The popularity of various sample environment configurations can be gauged from the bar chart shown in Fig. 3. Users are also able to supply their own sample environment, subject to safety and feasibility requirements; for example, an induction furnace has been developed by a group from Newcastle University, details of which are reported by Merz et al. (2023).

Figure 3 Bar chart illustrating the use of different pieces of sample environment equipment on Wombat during user experiments. Multiple pieces of sample environment equipment may be used during a single experiment, which is why the total sample environment equipment uses charted (1222) exceeds the number of experiments (805) listed in Section 6.

Additional instrument accessories such as the Eulerian cradle, polarization capability and a robotic sample changer further extend the flexibility of the instrument. The Eulerian cradle pairs particularly well with Wombat's large-area detector, enabling studies of texture and single-crystal samples (further discussed in Section 5.4). While most sample environments listed in Fig. 3 are best suited to powder samples, some of the cryofurnaces and the cryomagnet have a sample-probe rotation ω axis, which facilitates aligned single-crystal work as will be described in Section 5.4.

5. Wombat 'on the run': examples of experiments on the instrument

Like other high-flux neutron diffractometers, Wombat has been used predominately for in situ studies, and as presented in Section 4.2 the majority of these have been in variable-temperature conditions. The highly configurable nature of the instrument means that multiparameter experiments can also be undertaken. These have included pairing of magnetic-field variation with temperature (e.g. Heinze et al., 2019), gas dosing with temperature (e.g. Auckett et al., 2018) and a novel high-pressure high-electric-field experiment (Yamane et al., 2019). High flux additionally affords the study of small samples, such as those recovered from high-pressure high-temperature synthesis; Wombat regularly measures multi-phase samples as small as 50 mg, such as those presented by Brown et al. (2024).

Selected novel experimental setups and approaches on the instrument have been developed to the point that they are a regular offering as part of the Wombat user programme. These are described in the following sections.

5.1. Rapid data collection

One application envisaged when designing Wombat was rapid real-time acquisition, a result of combining the high-flux source with the high-count-rate capacity of the detector. Wombat has come to specialize in two types of rapid data collection experiments: stroboscopic measurements, where the triggering of the collection is synchronized to external sources, and `one-shot' experiments, where rapid data acquisition is triggered to monitor real-time changes to a sample.

An example of a stroboscopic measurement is the synchronization of the detector collections with a pulse generator to perform high-frequency cyclical measurements, initially of piezoelectric materials (Pramanick et al., 2010). With this setup 250 Hz measurements with multiple time bins are routine. This approach has been extended to the study of multiferroic materials at realistic operating frequencies (Hinterstein et al., 2023).

One-shot experiments have been undertaken on Wombat to elucidate the response of material to rapid heating. Collecting with continuous acquisition means that the data can be segregated down to 50 ms, with no deadtime between frames. This has been used to investigate the response of materials on timescales ranging from 2 s per pattern (Tegg et al., 2021) (Fig. 4) to 0.05 s per pattern (Merz et al., 2023).

Figure 4 Neutron diffraction patterns collected in situ as reagents mixed for Na0.7WO3 are dropped into a preheated furnace, set to various temperatures. The colourmap limits are identical across (c–e) but are different from those in (a, b) due to the different acquisition conditions. These patterns are shown with the SiO2 background subtracted. Reprinted with permission from Tegg et al. (2021). Copyright 2021 American Chemical Society.

5.2. Hydrogenous powder diffraction collections

The common approach when studying hydrogenous materials with neutron powder diffraction is to substitute the hydrogen atoms with deuterium, lowering the background arising from the large incoherent scattering length of hydrogen atoms. This approach has been successful, but there will be some cases where deuteration is not feasible. The high flux of the Wombat instrument affords an opportunity to work around this and to undertake diffraction studies directly from hydrogenous materials. For example, a study of solid methane on the Wombat instrument (Maynard-Casely et al., 2020) allowed lattice parameters to be measured between 4 and 82 K and the thermal expansion fitted. Data collection from these high-hydrogen-content materials is improved by scanning the detector over a range of angles and running gain re-refinement as detailed in Section 3.2.4, with an example presented in Fig. 5. This has been in turn applied to a range of small molecular systems with a number of applications (Klapproth et al., 2019; Choi et al., 2020; Maynard-Casely et al., 2022; Chen et al., 2023).

Figure 5 Detector image from a recent data collection of C3H8 propane on Wombat, collected at λ = 2.422 Å and processed using the gain refinement strategy explained in the text. This illustrates that despite the low symmetry (P21/n space group) and high hydrogen content the Bragg peaks can be seen over the high incoherent background.

5.3. In operando collections

In contrast to in situ measurements, some experiments are intended to capture the behaviour of materials within real-world technology. An example of this is the response of battery materials, where an accurate understanding of component function cannot be fully gained using diffraction data of electrode powders or batteries held at particular states of charge. Hence in operando experimental methods have been developed: time-resolved measurements of materials undergoing change while within a functioning device (Peterson et al., 2024).

The first in operando diffraction experiment of a lithium ion battery was conducted on Wombat, with data acquired every 5 min during battery cycling (Sharma et al., 2010). This work showed that the phase evolution of the battery electrodes could be studied while the battery was being used. All diffraction data represent a time-averaged measurement, necessitating a consideration of the distribution of states for the material undergoing structural change in the analysis and interpretation of any particular dataset. Following the 2010 study, in operando diffraction studies of batteries have increased in number and evolved to enable a more detailed understanding of electrode function, necessary to inform improvements to performance as well as economic and environmental sustainability (Liang et al., 2020). An important factor for in operando battery measurements is the time in which useful (Rietveld quality) data can be obtained; in operando data of a commercial battery have been collected on Wombat with a 10 s time resolution (Liang et al., 2020) (Fig. 6).

Figure 6 (a) The first 10 s dataset for an in operando series of diffraction data of a battery (b) Contour plot of data with intensity in colour (legend shown right). (c) and (d) Results of Gaussian peak fitting of a negative (c) and positive (d) electrode reflection. Reproduced with permission from Liang et al. (2020); all data collected at λ = 2.4168 (1) Å.

The in operando approach had also been applied at Wombat to study adsorbent materials used in the separation and storage of gases (Peterson et al., 2017). The in operando conditions allow study of gas incorporation and interaction within an adsorbent material in an environment that is representative of the real-world application.

5.4. Non-powder data collections

Though Wombat was originally designated as a high-intensity powder diffractometer, its large-area position-sensitive detector has proven advantageous for volumetric surveying of reciprocal space for both single-crystal and texture diffraction experiments. The volume of accessible reciprocal space can be extended when an Eulerian cradle is mounted on the sample stage, allowing sample axis angles ω, χ and ϕ equivalent to the Eulerian angles to be measured. The cradle can be used both to undertake measurements and to manipulate crystal orientation for subsequent study within other sample environments. Comparing our Eulerian cradle four-circle geometry with that of Busing & Levy (1967), using the Cartesian coordinate system defined in their paper, our geometry has all angles (2θ, ω, χ, ϕ) as right-handed rotations about their respective axes, whereas the system of Busing & Levy has 2θ, ω, ϕ as left-handed and χ as right-handed.

At ACNS, measurement of large volumes of reciprocal space for single crystals is usually carried out using the Laue diffractometer Koala (Piltz, 2018), due to the relative speed of the Laue technique. For the majority of single-crystal measurements on Wombat, only a small region of reciprocal space is studied, with the sample aligned and mounted so that the normals to the lattice planes of interest coincide with the scattering plane of the instrument. The data acquisition is performed through a step-scan in the azimuthal angle (ω), for which the rotation axis is perpendicular to the horizontal scattering plane. The typical increment of ω used in Wombat measurements is 0.1° or 0.2°. As Wombat's detector can receive out-of-plane scattering with a span of ±7.8°, a sample with a large out-of-plane lattice spacing allows a survey of reciprocal space that simultaneously covers multiple parallel reciprocal lattice planes. Changes in this volume of reciprocal space can be efficiently monitored as a function of a sample environment variable, such as temperature or pressure. This experimental setup is regularly used to obtain data from single crystals as a function of magnetic-field strength, with the field direction perpendicular to the scattering plane, as illustrated in Fig. 7.

Figure 7 Plotted from data presented by Gao et al. (2024), charting the response of a single crystal of the triangular-lattice magnet HoPdAl4Ge2 to different magnetic field strengths, collected at λ = 2.41 Å. Interested readers are referred to that paper for the interpretation of these results.

Such a setup is advantageous in studying spin-flip, spin-flop or other metastable phase transitions in magnetic materials (Gao et al., 2024). Recently, the Int3D software package (Katcho et al., 2021) has been deployed on the instrument to enhance the analysis of single-crystal diffraction data collected on Wombat, allowing diffraction spots to be indexed and their integrated intensities extracted.

The vertical extent of the detector also allows for the rapid mapping of orientation space for texture analysis, in contrast to point-detector techniques that scan single peaks in succession. This has been undertaken with both single-peak fitting and whole-profile methodologies (Simons et al., 2014; Xu & Liss, 2021). Orientational effects are significant in multiferroic materials, and Wombat has hosted in situ experiments to understand distributions in the static (Lu et al., 2016) and dynamic (Hinterstein et al., 2023) structural response to electric fields.

6. The Wombat user community

Our Wombat instrument is part of a vast global ecosystem of central facility instruments, which serve a diverse user community with far-reaching impacts, as outlined in the neutron research landscape surveys of Velichko et al. (2025) and Shukla et al. (2022). As such, it is good to understand how an individual instrument is contributing to this ecosystem and how support is being maintained over the timescale of user operations. In this section, all analysis and discussion is founded on data from the merit-based access proposal rounds (single-experiment and programme proposals) and director's discretionary beamtime for the period 2008–2023. In 2024 there was a long shutdown (∼6 months) of the OPAL reactor, so we discount user statistics from 2024 as the availability of Wombat was much lower than usual. From 2008 to 2023 Wombat was utilized in 805 experiments across 670 proposals. In addition to this, a number of commercial experiments have taken place, where a commercial client paid for access to the instrument. Wombat instrument scientists and other ANSTO staff are not included in the user demographic data.

The Wombat user community has grown to encompass 1031 individuals who have collectively made 2290 visits to the instrument. Fig. 8 shows how much Wombat was used each year. The instrument was commissioned in 2008. Thereafter the use of Wombat increased dramatically before reaching a period of steady-state operation during 2010–2019: ∼200 user programme days, ∼50 accepted proposals and ∼140 individual users per annum. ACNS aims to provide 200 days of user access to each instrument annually. This is underpinned by a reliable neutron source, the multi-purpose OPAL research reactor, which is at power ∼300 days per year.

Figure 8 Annual Wombat instrument use 2008–2023, including number of individual users, number of user visits, number of accepted proposals and number of user programme days. The target of 200 days for the user programme is indicated by the red dashed line.

The COVID-19 pandemic during 2020–2021 disrupted usual programming for Wombat, as it did for everyone else, with restrictions on working on-site and travel for users. Since then, normal operations have resumed, although total user visits are slightly down. A fully remote user programme is not offered by the ACNS; however Wombat's sibling, Echidna, offers a mail-in service for short proposals run in limited sample environment conditions.

The general trend indicated by Fig. 8 is for there to be two or three visiting users and around four days allocated to each proposal. Further analysis of accepted proposals (Fig. 9 and Table 4) shows that four days is indeed typical for normal proposals and director's discretionary beamtime, which usually involve one single experiment on the instrument. The amount of time awarded to director's discretionary beamtime is consistent with the aim that no more than 10% of user programme days should be allocated via this route. Programme proposals tend to entail multiple experiments on Wombat and other instruments, and usually involve more days of beamtime overall (typically five days per experiment).

Table 4
Table of duration of experiments associated with different proposal types

Proposal type	No. of proposals	No. of experiments	Mean duration of experiments (days)	Standard deviation of duration of experiments (days)
Normal	561	679	3.6	1.8
Director's discretionary	64	67	3.6	1.6
Programme	45	59	4.9	2.2

Figure 9 Duration of experiments on Wombat. This histogram presents data associated with normal, director's discretionary beamtime and programme proposals.

The contribution of Wombat to the wider neutron scattering community is made plain in Fig. 10, which displays the career stages or positions of Wombat users. The vast majority of Wombat users hail from universities, with the instrument supporting the work of 329 academics, 151 post-docs and 304 PhD students from 2008 to 2023. PhD students seem more likely to attend experiments in person, relative to accepted proposals, than other groups. The distribution of users at all career stages, from undergraduate student right through to retired scientist, highlights Wombat's contribution to a thriving research ecosystem based around central facilities such as neutron and synchrotron beamlines.

Figure 10 The Wombat user community according to position/organization. Here an accepted proposal is counted if it features at least one user from that category. Hence the number of proposals on this chart exceeds the total number of unique proposals for 2008–2023. As people progress through their careers, their position changes and so some individual users appear in more than one category in this chart. Industrial users are those who have accessed the instrument through the merit-based proposal rounds. Commercial clients who have paid for access to the instrument are not included. ANSTO staff are also not included.

Ultimately, central facility instruments are usually judged on their peer-reviewed output. Wombat experiments have resulted in 402 peer-reviewed publications from 2008 to 2024 inclusive. Fig. 11 charts this across the years of operation, showing a consistency in output of between 20 and 30 papers a year. This demonstrates that the instrument has achieved a sustainable output, which reflects the accessibility and quality of the data for preparing publications against the inherent experimental challenges presented by most of our user experiments.

Figure 11 Annual publications featuring results from Wombat.

7. Summary and the future of Wombat

Our Wombat instrument is named after an Australian marsupial and metaphorically refers to the broader `fatter' diffraction peaks and the speed of the instrument, as wombats can run at a `high intensity' of nearly 40 km h⁻¹ (de Kruijff & Miolin, 2024) (over short distances). The Wombat instrument is remarkably configurable and able to undertake unique and sometimes pioneering experiments. The instrument supports a healthy and diverse user community, and we are much indebted to our users who return and challenge the instrument setups year on year. The Wombat team are always seeking to support new experiments and user teams and encourage those who are intrigued by the capability of the instrument to get in contact.

As detailed in Section 5, the instrument team support a wide range of scientific applications and stand ready to use the instrument in support of new scientific directions. This undertaking will be supported in the long term by the construction of a new detector for the instrument. With the aim to reproduce the success of the original BNL detector, the new detector will be built in-house at ACNS, an exciting development for detector science in Australia. We hope to be commissioning this new detector in 2028, and that with this the Wombat instrument will continue to provide excellent neutron scattering results for many years.