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High-quality slabs for ESRF nano- beamlines

The characterisation of samples at the nanometre scale plays a central role in fundamental and applied sciences. Synchrotron-based X-ray techniques are pivotal to nanometre-scale science. The construction of nano-beamlines requires the implementation of extremely stable environments to minimise disturbances to experiments; the construction of a specific high-quality slab is one of its key elements.

In 2008, the ESRF launched the Upgrade Programme. One of the main objectives of this 10-year project was to build eight new beamlines with unique capabilities, including several nano-beamlines. The success of nanoscience at the ESRF relies on a programme of enabling technologies centred on small beam production and stability, including the design of a High Quality concrete Slab (HQS).

The production of nano-sized beams requires the construction of long beamlines (up to 220 m). Long beamlines can be compared to microscopes, where the size of the incident X-ray spot on the experimental specimen determines its resolution. One of the aims of long beamlines is to focus the X-ray beam to the smallest possible size (up to 20 nm), requiring long focusing distances. Extended experimental halls and satellite buildings with extremely stable environments are, therefore, required to meet the

needs in terms of total length, available space, limited floor deformations, vibratory and thermal stability (∆T ≤ 0.5°C in the experimental hall and ∆T ≤ 0.1°C in the experimental hutches). Particularly important is the HQS that hosts the nano-beamlines; its geometrical and vibratory stability are key aspects for their performance.

The HQS design efforts were concentrated on limiting the deformations and vibrations transmitted to the sample or associated support structures to avoid disturbances to the experiments. The vibratory and deformational behaviour of the slab depends on many parameters (thickness, size, shape, rigidity, materials, links to other structures, etc.). The factors that can induce the deformations and vibrations are also numerous (temperature, humidity, loads, soil behaviour, traffic, external work, equipment, staff, contiguous buildings, etc.).

To reduce vibratory issues, contact with vibratory sources must be avoided and cracking of the concrete minimised, so the slabs behave as a monolith. Regarding the geometrical stability, one of the main phenomena considered during the HQS design was the curl of the slab edges caused by differential temperature and humidity between its surface and its base. The curling occurs mainly at the expansion joints but also at random cracks. The curling effect is greater at the edges of the slab, decreasing to almost zero at the slab centre (Figure 150). Theoretical maximum movements were defined during the design phase of the

Fig. 150: Model of deflection of a 160 x 50 x 0.3 m slab, on a soft base (200 MPa), due to daily air temperature variations (±1°C).