Bremsstrahlung source for density logging -- Lab study of influence of beam energy

An electron linac has been configured as the X-ray source in a laboratory version of a formation-density tool. The interface between the ''tool'' section, containing the source and the detectors, and the radiation-scattering ''formation'' is planar. The electrons have energies of 1.5, 2.3, or 3.0 MeV and are focused onto a tungsten-rich target. The target charge per linac pulse is measured. The x-ray beam is collimated to enter the scattering material at a 45 degree angle. NaI detectors, in line with the source and at 10, 15, 20, and 25 in. spacings, produce signals proportional to the total energy deposited during the linac pulse. The radiation incident onto the detectors is filtered to effectively remove the influence of the scattering material's photoelectric cross-section. Measurements have quantified the expected increase in a detector's signal with beam energy, as well as the accompanying decrease in density sensitivity. Source-energy stability requirements are an outcome of the study. Tool standoff from the formation, simulated with Plexiglas sheets, also has been investigated