In this study, we investigated the potential and limitations of using joint X-ray
and time-of-flight (TOF) neutron imaging for mapping the 3-dimensional
organic carbon distribution in soil. This approach is viable because neutron
and X-ray beams have complementary attenuation properties. Soil minerals
consist to a large part of silicon and aluminium, and elements that are relatively
translucent to neutrons but attenuate X-rays. In contrast, attenuation of
neutrons is strong for hydrogen, which is abundant in soil organic matter
(SOM), while hydrogen barely attenuates X-rays. In theory, TOF neutron
imaging does further more allow the imaging of Bragg edges, which correspond
to d-spacings in minerals. This could help to distinguish between SOM
and clay minerals, the mineral group in soil that is most strongly associated
with hydrogen atoms. We collected TOF neutron image data at the IMAT
beamline at the ISIS facility and synchrotron X-ray image data at the I12
beamline at the Diamond Light source, both located within the Rutherford
Appleton Laboratory, Harwell, UK. The white beam (the full energy spectrum)
neutron image clearly showed variations in neutron attenuation within soil
aggregates at approximately constant X-ray attenuations. This indicates a constant
bulk density with varying organic matter and/or clay content. Unfortunately,
the combination of TOF neutron and X-ray imaging was not suited to
allow for a distinction between SOM and clay minerals at the voxel scale.
While such a distinction is possible in theory, it is prevented by technical limitations.
One of the main reasons is that the neutron frequencies available at
modern neutron sources are too large to capture the main d-spacings of clay
minerals. As a result, inference to voxel scale SOM concentrations is presently
not feasible. Future improved neutron sources and advanced detector designs
will eventually overcome the technical problems encountered here. On the
positive side, combined X-ray and TOF neutron imaging demonstrated abilities
to identify quartz grains and to distinguish between plastics and plant seeds.