Generation of muons from cosmic rays [Credit: https://physicsopenlab.org/wp-content/uploads/2016/01/muoncascade.jpg]

Atmospheric muons

“Who ordered that?” was the remark made by the famous Nobel laureate(for the discovery of nuclear magnetic resonance) Isidor Isaac Rabi when muons were discovered. Scientists were looking for a meson predicted by another Nobel laureate (for his prediction of the existence of mesons)Hideki Yukawa. A particle, identified in a 1936 experiment on cosmic rays by Carl Anderson (yet another Nobel Prize winnerfor discovering positron) and Seth Neddermeyer, was tantalizingly close to the expected meson but not quite. Detailed analysis made it amply clear that a new elementary particle had been discovered that was almost like an electron, but with a mass roughly 200 times. They are produced in copious numbers when primary cosmic rays interact with the upper atmosphere of our planet, around 15kms above the surface of the earth. Because of their relativistic speed, a large number of them reach the surface of the earth despite the at rest lifetime of 2.2 microsecond. After reaching the surface, they penetrate and continue to travel till several kilometers inside the earth because of their enormous energy. It is interesting to note that thecosmic rays (mostly nuclei of hydrogen, some helium and very small trace of other elements) start their journey from all around the universe,right from its farthest corners, to our neighborhood stars and galaxies.

How can these muons be used?

Muons that reach the earth surface has a wide range of variation in momentum, starting from hundreds of MeV/c going right up to TeV/c.

They lose relatively small amount of energy when passing through intervening media because they do not have strong nuclear interactions and hardly produce electromagnetic cascades (till half of a TeV) and cause very few ionizations (after all, most of them belong to the group that is designated as minimum ionizing particles). Those reaching the earth surface are mostly in the range of 1 – 10 GeV and are known to penetrate almost a kilometer of rock. This power of penetration of the atmospheric muons has attracted lot of attention during recent times, because of the possibility of creating three-dimensional images of large objects, just as created using X-rays while carrying out a Computerized Tomography (CT) scan. For muon imaging, the idea is quite similar – while penetrating an object, a muon may get completely absorbed, or it can get scattered by the object material. The amount of absorption, or scattering gives us an idea of the object in the path of the muon and its material properties. If enough data is available, a tomographic reconstruction can yield even a 3D reconstruction of the object, just as obtained in a CT scan. If successful this approach can serve as a major Non Destructive Evaluation (NDE) technique that can be useful in many spheres of science and technology. The major obstacle seems to be the fact that the muon flux on the earth surface is not very high, around 100 Hz per m², which translates toonly about 1 muon per minute across an area of 1 cm². This number can turn out to be pitifully low, especially when we are in a hurry to scan the object.

Muon imaging – initial attempts

The work on creating images using the atmospheric muons started during the middle of the last century when the British physicist Eric George made an attempt to measure the overburden of a mine in Australia using atmospheric muons. This was tried during 1955 when the technology of particle detectors was at its infancy. As a result, the work was carried out using only Geiger counters fixed on a rail that moved around the mine. The observed count rate of the counter was found to have a clear correspondence with the thickness of the overburden. The next major attempt was made by the Nobel laureate Luiz Alvarez when his team tried to image the internals of the Chephren’s pyramid in order to check whether an unknown upper chamber on top of the Belzoni chamberexist within the pyramid. This time, the detector used was spark chambers and the result was a discouraging negative. It also became obvious that the detector technology and analysis algorithmsneeded further improvement.

Early Geiger counter made by Hans Geiger, [Credit Science Museum London / Science and Society Picture Library 1932.Uploaded by Mrjohncummings, CC BY-SA 2.0 https://commons.wikimedia.org/w/index.php?curid=28024312]

The equipment in place in the Belzoni Chamber under the pyramid. [Reproduced from Luiz W. Alvarez et al paper in Science 167 (3919)]

Muography comes of age

The particle detectors had their own evolution due to demands driven by other areas of science and technology, especially fundamental science and medical imaging. Important inventions such as the multiwire proportional chamber (Nobel prize for GeorgesCharpak in 1992), time projection chamber (TPC), plastic scintillators in conjunction with silicon photo multipliers (silicon photo multipliers are lot less expensive and power hungry than the traditional photo multipliers) are some of the prominent detectors that can be mentioned to indicate the feverous activity in this field. Similarly, computational power increased exponentially and sophisticated mathematical tools were developed for carrying out data analysis and image processing. As a result, what started as faltering steps by few individuals during the middle of the 20^th century, turned out to be an active area of research by the end of the same century. What makes the area of muon imaging, also known as muography these days, even more exciting are the facts that the muons are freely available, has no radiation concerns and represent a perfectly sustainable development.

Working principle of a TPC (by O. Schäfer) [Reproduced from https://www.lctpc.org/e8/e57671]

Various applications of muography

Since early years of the twenty first century, absorption, as well as scattering muography are being employed to reconstruct images of various objects of interest. The silicon photo multipliers are often used for absorption muographywhere angular resolution is not crucial. The interior of volcanoes in various countries, notably in Japan, Italy and France, have been imaged using this technique.The scattering muography, on the other hand, needs excellent angular resolution to identify the amount of scattering the muon suffers while traversing a given object. The gaseous ionization detectors are often the detectors of choice in this case. This approach has been used for cargo inspection, nuclear storage inspection etc. Some of the interesting and news breaking applications in recent times have been the ScanPyramidsproject where the great Khufu pyramid was scanned for unknown cavities within (a positive result obtained this time around) and monitoring of radioactivity after the Fukushima Daichinuclear crisisin Japan due to an earthquake and tsunami in 2011.

ScanPyramids experimental setup. Four Micromegas based detectors are shown collecting data of muons passing through the great pyramid. [Reproduced from https://www.caen.it/news/scanpyramids-project/]

Muon imaging setup for Fukushima Daiichi Unit 2.Credit: LA-UR-15-24802

Outlook

This has been a truly exciting field in terms of research and development. What is more, it is now being considered mature enough to be considered as a marketable technology. Several companies have started operating that work on providing muon imaging solutions. Some of these early torch bearers are Decision Sciences (US),Ideon (Canada),Lingacom (Israel),Lynekos (Scotland), MuonSolutions (Finland), Muon Systems (Spain). It is really heartening to see that research in particle physics and related fields has once again led to a safe and sustainable, yet effective, technology within a very brief period of time.

Since the field is still growing rapidly, it is possible to get associated with it from an R&D point of view. This will usually require expertise in particle physics experiments, knowledge about different detection systems and ability to handle sophisticated instruments. In addition, analysis of reasonably big data, machine learning algorithms, image processing techniques, tomography are key know-hows related to this emerging and exciting field.

Once someone gets trained in these areas, it is possible to find job opportunities in academia, research institutes and laboratories dealing with similar technologies. It is also possible to get involved in industries that work on imaging technologies (including muography), data analytics, IT, instrumentation and so on.