page_title: Nuclear densitometry
section_title: Ground compaction
hierarchical_section_title: Nuclear densitometry / Uses / Ground compaction
caption_attribution_description: English: Asphalt Density Meter being used in Washington, DC.
context_page_description: Nuclear densitometry is a technique used in civil construction and the petroleum industry, as well as for mining and archaeology purposes, to measure the density and inner structure of the test material. The processes uses a nuclear density gauge, which consists of a radiation source that emits particles and a sensor that counts the received particles that are either reflected by the test material or pass through it. By calculating the percentage of particles that return to the sensor, the gauge can be calibrated to measure the density.
  In geotechnical engineering, a nuclear densometer or soil density gauge is a field instrument used to determine the density of a compacted material. 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 that are read by the detector tubes in the gauge base. A 60-second time interval is typically used for the counting period.
context_section_description: 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.
caption: Asphalt Density Gauge
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