Physicists Capture First Optical Images of Muon Beams

A muon is a particle similar to an electron, with an electric charge of e− for a negative muon and e+ for a positive muon; however, its mass is 207 times that of an electron. A muon has a mean life-time of 2.2 μs and a negative muon decays to one electron and two types of neutrinos, while the positive muon decays to one positron and two types of neutrinos. Since these characteristics are quite different from familiar radiations such as X-rays, electrons, protons, or carbon ions, new results for such applications for quality assessment, research, or radiation therapy may be obtained by the optical imaging of muons. For the high-energy cosmic muons, they are used for the radiography of huge subjects such as a volcano dome, an ancient pyramid or a nuclear reactor, which is impossible to image with other methods. “We developed a new imaging technique that shows promise for quality assessment and research of muon beams, and should be of benefit for muon radiotherapy in the future,” said Dr. Seiichi Yamamoto of Nagoya University and colleagues. “The technique depends on a phenomenon that occurs when charged particles travel through transparent media, like water.” “Water slows light down relative to high-energy particles. Particles moving faster than light cause something similar to the sonic boom we hear when a jet plane breaks through the sound barrier.” “In the case of the particles, an ‘optical boom,’ called the Cherenkov effect, causes a brief flash.” The physicists imaged this effect with a charge-coupled device camera when a muon beam was directed through water or a plastic scintillator block. “The technique allowed us to image muons and the positrons that form when muons decay,” they said. “This helped us measure the beam’s range through the water or plastic scintillator, and the deviation of its momentum, as well as clarify the direction of positron movement.” “The system is compact, low cost and easy to use, showing promise as a tool for quality assessment in muon beam facilities,” Dr. Yamamoto added. A paper on the findings was published in the journal Scientific Reports. _____ S. Yamamoto et al. 2020. Optical imaging of muons. Sci Rep 10, 20790; doi: 10.1038/s41598-020-76652-8