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Cosmic ray muon has strong penetrating power and no ionizing radiation hazards, which makes it an ideal probe for detecting special nuclear materials (SNMs). In this article, a high spatial resolution muon tomography system based on Micromegas detectors is proposed to optimize the imaging time and quality. The proposed system includes eight Micromegas detectors based on the thermal bonding technique and a scalable readout system. In addition, a multiplexing method based on position encoding is developed to reduce the number of electronics channels by order of magnitude. The spatial resolution of the proposed system with encoding readout can reach a value of hundred micrometers. Finally, a tomography test is performed, and test results show that this proposed system can image 2-cm objects and distinguish different materials. [ABSTRACT FROM AUTHOR]

Optical imaging of muon beams is a promising method for range estimations. However, our previous optical imaging method could only measure 2-D projection images. To measure the beam ranges and widths at any position of the muon beam, 3-D beam images are desired. For this purpose, we developed an optical imaging system using a silver-activated zinc sulfide (ZnS(Ag)) sheet combined with a mirror and a cooled charge-coupled device (CCD) camera. The ZnS(Ag) sheet was set in a black box and irradiated by a positive muon beam at the Japan Proton Accelerator Research Complex (J-PARC). Acrylic plates were used to absorb the muon beam. The measured optical images with different thicknesses of the acrylic plates were stacked and interpolated to create a 3-D optical image, and then the depth and lateral profiles were evaluated. From the depth profile derived from the 3-D image, the Bragg peak position could be estimated. The lateral profiles at the Bragg peak positions could also be derived. We confirmed that 3-D optical imaging was possible using the developed system with a ZnS(Ag) sheet. The system is promising for measuring muon beam distribution, conducting research on muons, and developing future muon radiotherapy. [ABSTRACT FROM AUTHOR]

We present a field-programmable gate array (FPGA)-based technique for the on-line identification of highly ionizing particles in a liquid xenon (LXe) detector. The method was developed and successfully exploited to select $\alpha $ particles emitted by 241Am sources submerged in LXe in an overwhelming, mostly beam-related, $\gamma $ -ray background. After revising the main features of xenon and other liquid noble gases as ultra-violet (UV) scintillating media, we describe the algorithm idea and its firmware implementation. We then present the results in terms of efficiency and background suppression for the real-time $\alpha $ -particle tagging and the limits of the MEG trigger configuration. Finally, we show that in MEG II we are going to overcome the main issues and further improve the performances. [ABSTRACT FROM AUTHOR]

We study, by optical spectroscopy, the properties of high-current electric arcs that form between opening Cu contacts. The presence of a current that increases linearly with a rate of about 14 kA/ms leads to the separation of the contacts and to the formation of a bridge of molten metal that eventually ruptures. We show that after the molten bridge rupture, it takes about 40 $\mu \text{s}$ for the initial arc to be at local thermal equilibrium. This long thermalization time suggests that the plasma density during that time range is low, which we confirm using a simple modeling of the energy transfer between electrons and heavy particles. We explain these low densities by the fact that the molten bridge rupture leads to the formation of a shock wave. The initial dense plasma rapidly expands and a region with a pressure below the background pressure forms between the two contacts, leading to low plasma densities and long thermalization times. [ABSTRACT FROM AUTHOR]