Nuclear density gauge

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A nuclear density gauge is a tool used for construction, mining and archeology purposes and is essentially a variation of an X-ray machine. At the basics, it consists of a radiation source that emits a directed beam of radioactive particles and a sensor that counts received particles that are reflected by the test material or pass through it. By calculating the percentage of particles that return to the sensor the gauge can then draw conclusions on the density and inner structure of the test materials.

Different variants are used for different purposes. For analysis of very shallow shallow objects such as roads or walls, a beta particle emitter such as ⁶⁰Cobalt or ¹³⁷Cesium is used. Those particles are effective in analyzing the top 10 centimeters with a rather high accuracy. For deeper penetration a source like ²²⁶Radium is used and gamma rays that penetrate up to 100 meters can help find underground caves or just locations with lower density that would make tunnel construction hazardous.

Another variant is to use a strong alpha emitter like ²²⁶Radium to produce neutron radiation and then measure the energy of returning neutrons. As hydrogen characteristically slows down neutrons the sensor can in this way calculate the density of hydrogen - which is usually mostly found in water - and find pockets of underground water or humidity up to a depth of several meters.