|A nuclear densometer is a field instrument used in geotechnical engineering to determine the
density of a compacted material. Also known as a soil density gauge, the device uses the
interaction of gamma radiation with matter to measure density, either through direct
transmission or the "backscatter" method. The device determines the density of material by
counting the number of photons emitted by a radioactive source (cesium-137) that are read by
the detector tubes in the gauge base. A 60-second time interval is typically used for the
A nuclear densometer is used on a compacted base to establish its percentage of compaction.
Before field tests are performed, the technician performs a calibration on the gauge which
records the 'standard count' of the machine. Standard counts are the amount of radiation
released by the two nuclear sources inside the machine, with no loss or leakage. This allows
the machine to compare the amount of radiation released to the amount of radiation received.
With the use of a 3/4" diameter rod a hole is created in the compacted base by hammering the
rod into the base to produce a hole that the densometer's probe can be inserted into. The
densometer is placed on top of the hole, and then the probe is inserted into the hole by
unlocking the handle at the top of the probe. One source produces radiation that interacts with
the atoms in the soil, and is then compared to the standard count, to calculate the density. The
other source interacts with hydrogen atoms to calculate the percentage of water in the soil.
In direct transmission mode, the source extends through the base of the gauge into a predrilled
hole, positioning the source at the desired depth. The testing procedure is analogous to
burying a known quantity of radioactive material at a specific depth, and then using a Geiger
counter at the ground surface to measure how effectively the soil's density blocks the
penetration of gamma radiation through the soil. As the soil's density increases, less radiation
can pass through it, owing to dispersion from collisions with electrons in the soil being tested.
Since the soil's moisture level is partly responsible for its in-place density, the gauge also
contains a neutron moisture gauge consisting of an americium/beryllium high-energy neutron
source and a thermal neutron detector. The high-energy neutrons are slowed when they collide
with hydrogen atoms, and the detector then counts the "slowed" neutrons. This count is
proportional to the soil's water content, since the hydrogen in this water (H2O) is responsible
for almost all the hydrogen found in most soils. The gauge calculates the moisture content,
subtracts it from the soil's in-place (wet) density, and reports the soil's dry density.