Pore-size evaluation by single-sided nuclear magnetic resonance measurements: Compensation of water self-diffusion effect on transverse relaxation

摘要

The determination of penetration depth and distribution of water at surfaces is essential to knowledge of the state of conservation of Cultural Heritage items and materials, such as frescoes, stone, brick, wood, and paper. Water can penetrate the surface of an object, coming from either an external or an internal source, and in general the moisture content of the surface region is the cause of various decay phenomena such as microfractures and disintegration. The nuclear magnetic resonance (NMR) approach can be very powerful for the evaluation of the state of fine arts materials. Not only the water saturation and/or the porosity of the material can be evaluated but also information on material pore-size distributions can be obtained by monitoring the distributions of relaxation times of the transverse (T_2) and longitudinal (T_1) components of the ~1H magnetization of the trapped water. The drawback is that generally the sample does not fit into standard NMR magnets, and for in situ application, single-sided NMR devices have to be used. Therefore, the standard methods to get NMR parameters are not always valid, and some alternative procedures have to be performed. For example, in strongly inhomogeneous magnetic fields due to the geometrical features of single-sided NMR devices, the transverse relaxation is greatly influenced by the molecular self-diffusion even at the shortest interpulse time available for a Carr-Purcell-Meiboom-Gill (CPMG) sequence. In this paper we show how the dephasing effect due to molecular self-diffusion can be corrected by using the "constant echo time method." We report an attempt to recover the corrected T_2 distribution in well-characterized porous materials saturated with water, with data acquired in the highly inhomogeneous magnetic field of a single-sided NMR device. The results are discussed and compared with those acquired on the same samples in the highly homogeneous magnetic field of a traditional NMR instrument.