Instrument alignment and calibration

For residual stress measurement, the calibration and alignment of the instrument is important and the principles are similar for both angular dispersive and TOF techniques [3, 31]. It can be quite a time consuming procedure, and methods facilitating much faster alignment and calibration have been developed over the years. It is an area where automatization can also be implemented. In the future, it is envisioned that automatization, coupled with simultaneous simulation of the instrument, will allow a quick alignment and calibration suitable for the particular measurement about to take place.

The following sections provide an overview of the steps to be taken for the alignment and calibration of a residual stress diffractometer, irrespective of whether these processes are automated or not.

3.5.2. Calibration

Calibration of the instrument in relation to neutron diffraction strain measurement usually means determining the wavelength of the incident neutron beam and the angular response of the neutron detector accurately.

At a monochromatic source, the detector angular response and the wavelength are normally calibrated using a powder sample with a known and well defined lattice parameter, typically silicon, ceria, alumina or calcium fluoride.

These materials are strong neutron scatterers with little intrinsic peak broadening.

Repeated measurement on a reference sample is a method of quantifying the stability of the instrument settings and can be compared to the estimated fitting uncertainties. At TOF instruments, the same process is applied, but here the flight time recording of the detector banks needs to be calibrated, instead of the angular response.

At a TOF source it is additionally necessary to calibrate the detector efficiency as a function of wavelength. This is done using isotropic neutron scatterers with very weak or no diffraction peaks of their own, such as vanadium.

Another component of an instrument that should be calibrated from time to time is the movement of the sample positioning stage. In view of the importance of location in strain measurements, it is clearly necessary that the positioning equipment accurately executes the movements requested by the operator. Positioning tables tend to be mechanically quite stable so that frequent recalibration is not necessary. Care should be taken with older installations where spindles and bearings could be worn out.

Residual strain determination by neutron diffraction in most cases is based on relative measurements in accordance with

sin 0,

for TOF installations. Hereby θ is the observed Bragg angle and a is the lattice parameter obtained after profile refinement of the TOF spectrum.

As a consequence, it is also possible to obtain strain measurements of acceptable quality from instruments where the detector angular response and wavelength have not been calibrated. Doing this introduces an additional uncertainty in the measurement that needs to be considered correctly in the uncertainty analysis. A small error can have significant impact on the uncertainty of the strain measurement [32]. Therefore the detector position should be known to within ±1°, ideally to ±0.1°, especially if measuring at 2θ angles much lower than 90°. The measurement of lattice spacings in accordance with Bragg’s law (Eq. (1) in Section 2.2.1) is absolute and must therefore be done on a fully calibrated instrument. Lattice spacings obtained from non-calibrated instruments should never be cited.

3.5.3. Alignment

Most components of an instrument need to be calibrated and aligned, some more often than others. This is especially important when an instrument is set up

for the first time and at the start of neutron beamtime cycles. Alignment should also be checked after accidental collisions of components during measurement (especially the neutron optics).

3.5.3.1. Sampling volume

In accordance with Ref. [3], it is good practice to position the centroid of the sampling volume at the reference point, i.e. the centre of rotation of the specimen table. This allows for easier changing of the measurement direction by simply rotating the specimen without any need for specimen realignment.

The alignment of the sampling volume with the centre of rotation is facilitated through readjustment of the beam defining apertures, either slits or radial collimators as described above. This type of alignment occurs on a regular basis as the sampling volume is often changed when a measurement series on a new specimen starts.

It is normally possible to position the centroid of the sampling volume to within 0.1 from the centre of rotation of the specimen table. Although this is a good accuracy to aim for, it can often be improved upon if needed. The level of accuracy required depends on the type of measurement being performed. The highest positioning accuracy is needed in the case of large strain gradients and where measurements are made close to surfaces. Several methods can be used to align the sample table and the neutron optics, such as using neutron intensity scans, optical instruments and neutron cameras.

3.5.3.2. Neutron monochromator, beam defining optics

Occasionally, in particular when monochromator settings are changed, it is necessary to realign the monochromator and collimators that may be placed in the incident beam, in order to obtain the highest possible neutron flux or the desired level of beam focusing at the measurement position.

3.5.3.3. Specimen positioning table

For specimen positioning, it is important that the movements of the specimen table are as intended. The horizontal movements should indeed be horizontal and the vertical should be vertical. The movement axes should be orientated orthogonally with respect to one other and their angle with the direction of the incident beam should be known. As the specimen table set-ups are mostly mechanically quite rigid, a realignment of the specimen table is not normally frequently performed.

3.6. MEASUREMENT PROCEDURES