7. Gauge Theory
This chapter covers the theory of operation for nuclear moisture
density gauges.
Gamma and neutron radiation and their interaction with
matter is a complex topic and difficult to cover in sufficient
details in this manual. The discussion below will be limited to the
engineering application of these radioisotope sources and their
operations in the field.
Density Measurement
Density measurement in the gauge is accomplished by using a
Cesium-137 (Cs-137) radioactive isotope and two Geiger
Mueller (GM) detectors. Density measurements are usually
accomplished in two different modes, backscatter (BS) and
direct transmission. In the backscatter mode the source and
the detector are in the same plane. In direct transmission, a
hole is formed in the material and the source rod is inserted at a
desired depth, between 2 and 12 inches (50 to 300 mm).
Cs-137 has a maximum energy of 0.662 Mev. Photons from the
source penetrate the test material and are scattered back to
the GM detector or absorbed by the material by Compton
scattering and photoelectric absorption. In the density range of
interest, the number of photons detected by the GM tubes is
inversely proportional to the material density. For example, a
given gauge at a density of 110 PCF (1760 kg/m3) will show
1500 counts; whereas, the same gauge will show a count of 700
at a density of 160 PCF (2560 kg/m3); therefore, the higher the
counts, the lower the density.
To Next Page