Investigation into the use of x-rays to determine moisture content in vegetation

This report describes an investigation into the use of some radiometric techniques which could be implemented in portable instrumentation to non-invasively measure the moisture content of vegetation. The primary focus was on the method using differential attenuation of dual x-ray beams which has successfully been used for other applications. While this was found to be theoretically feasible, it was concluded that the high precision demanded of the measurements meant that the application would be impractical for the light loadings of samples. Three other methods in combination with x-ray attenuation were therefore trialled as possible alternatives. Direct determination of moisture using the enhancement in dielectric permittivity of the biomass was tested through capacitance measurements. Physical difficulties preclude accurate use of this non-destructively. A theoretical study was made of the combination of beta particle and x-ray transmission. This is considerably less demanding in measurement precision than dual x-ray beams, but it is difficult to obtain suitable radioisotopic sources and detectors to implement the technique. A series of measurements were made using the combination of Compton scatter with the attenuation of gamma rays. As with the dual x-ray beam technique, it was found that the slight differential between water and biomass for gamma ray interactions was insufficient to warrant confidence in the application of this method in a portable instrument for light sample loadings. The most promising solution is to use a radiometric method to determine sample loading, and a non-radiometric technique to measure the moisture content directly. (author) 7 refs., 12 figs