Although the magnetic field of these sensors is inhomogeneous, it is possible to measure NMR parameters such as proton density, relaxation times, self-diffusion coefficient, and even to collect correlation maps [24] of unmovable and precious artifacts and monuments belonging to the cultural heritage. Because of the inhomogeneous field [25], the signal (FID) decays very quickly and cannot be directly detected. Therefore the signal must be recovered as an echo. Moreover, the inhomogeneous field is a further source of relaxation which makes the transverse relaxation time shorter than that measured in a homogeneous field.Figure 1a shows a palm size NMR sensor. It consists of a U-shaped magnet obtained using two anti-parallel permanent magnets mounted on an iron yoke with the rf coil positioned in the gap of the magnet.
The magnetic field is external to the device, enabling large objects to be studied without any sampling. Different probe heads, each tuned to the proper frequency are used to obtain different depths of measurement. A further development of unilateral NMR devices are sensors that can scan depths up to 2.5 cm, producing depth profiles with micrometric spatial resolution [26]. These devices generate an inhomogeneous magnetic field with a uniform gradient to resolve the near surface structure of arbitrarily large samples. To improve gradient uniformity, the device works at a fixed depth from the sensor, where high depth resolution can be achieved. The position of the excited slice inside the sample can be varied by displacing the sensor using a high-precision lift that repositions the magnet with respect to the sample.
Figure 1b shows a device consisting of a permanent magnet mounted on a precision lift. Application of these devices has opened a number of new possibilities also in the field of cultural heritage [27�C34].Figure 1.(a) Unilateral NMR sensor by Bruker Cilengitide Biospin; (b) NMR sensor with a uniform gradient to resolve the near surface structure of arbitrarily large samples, the sensor is placed on a lift that allows one to move the magnetic field inside the object to be analyzed …In the following we report three cases illustrating the potential of NMR sensors in cultural heritage.2.?Quantitative Moisture Distribution Mapping in an Ancient Wall PaintingWater is a major cause of decay to building masonry in cultural heritage sites [35].
Determining the course and distribution of water through the wall is a fundamental step in conservation work and is particularly true for wall paintings. In fact wall paintings are constituted of materials having an open porosity resulting in an easy accessibility of liquids and gases such as salt solutions, atmospheric pollutants, dampness, and solutions of material used for conservation treatments.