Cosmic rays - Nature’s x-rays
The Earth is subject to many particles travelling from space into its atmosphere. These particles are formed of high energy protons and other ions and can collide with the atmosphere, producing other types of subatomic particles that rain downwards to the Earth’s surface. The particles raining down are termed cosmic rays, a catch-all term for any particles produced in this natural process.
At the Earth’s surface, the natural cosmic ray particle flux is dominated by muons, a type of particle that can be considered as a heavier electron. Cosmic ray muons are highly penetrating and is the main particle flux used in Geoptic’s imaging technique and technology.
imaging with cosmic ray muons
Many people are familiar with medical X-ray imaging, which is analogous to cosmic ray muon radiography. The technique utilises an X-ray source, an object or person to image, and a detector. X-rays from the source are emitted through the region-of-interest to the detector. When a high density interior region is present, fewer X-rays will reach the detector, as a greater fraction are stopped within, and vice versa.
There are several ways of imaging using cosmic ray muons. One very useful approach is to place muon tracking detectors viewing an engineering structure or the tunnel overburden.
Muon tracking detectors measure the muon rate along many lines-of-sight through the structure or region-of-interest. The rate of muons measured by the detector system is related to the thickness and the average material density along the system’s line-of-sight. By taking several different detector system views, it is possible to apply advanced reconstruction algorithms to produce a 3D dataset of the internal density.