Editorial Feature

An Introduction to Nuclear Density Gauges

Download PDF Copy

Sep 23 2014

A nuclear density gauge is a testing device used to measure the density of a test material with the help of nuclear radiation. It includes a small radiation source for discharging nuclear particles and a sensor for evaluating the nuclear particles.

The source of radiation varies based on the application. For instance, a gamma emitter is used to measure the density of shallow bodies. Gamma radiation is produced by different isotopes such as ¹³⁷Ce and ²²⁶Ra. ²²⁶Ra is generally used to measure objects that are about 300 metres deep. An effective radiation source such as ²⁴¹Am is also employed for measuring asphalt content, moisture and density of deep surface objects.

The nuclear density gauge can also be used to make liquid density measurements. It is usually calibrated with liquid and gas which have known densities. The device provides quick and accurate results compared to density measurements obtained from other cores.

Operation of Nuclear Density Gauges

Nuclear density gauges determine the number of nuclear particles reaching the sensor following reflection from the test material. There are two modes of operation of nuclear density gauges:

Direct transmission mode – In this mode, the gauge is inserted with a retractable rod via a pre-drilled hole. The rod is introduced deep into a mat provided within the gauge. The radiation emitted from the source strikes the test material and gets scattered off the sensor due to loss of energy. The interaction between radiation and material is high if the test material is thick. Therefore, the number of particles detected by the sensor is inversely related to the density of the test material.

Backscatter mode – The retractable rod is inserted into the gauge such that the rod is in level with the sensor. The radiation emitted from the source interacts with the test material, loses its energy and gets scattered towards the sensor. With thick test materials, more amount of radiation is scattered. This means the number of particles detected by the sensor is proportional to the material density.