Search

Cosmic-ray muon imaging is a new non-destructive technique that uses highly penetrating natural muon to reconstruct objects with high atomic number (Z) or subjects hidden in something by applying muon scattering or transmission technique, respectively. Up till now, muography mainly focused on objects with high Z or large volume. But few studies have been reported on objects at small scale with low or medium Z in China. A method that aims to reconstruct 3D image for objects at small scale as well as low or medium Z has been brought out in this paper, which uses the amount of muonic secondary particles and coinciding muon trajectories in different directions to reconstruct objects. A simulation about how muonic secondary particles generate and their performances based on Geant4 toolkit was carried out. The results of this simulation show that the secondary particles are mainly composed of secondary electrons and gamma photons. And as the thickness of the objects increases along one certain direction, the amount of secondary particles increases approximately linearly. Therefore, the amount of them carries the shape and location information of objects. Furthermore, the energy of muonic secondary particles is mainly beyond 01 MeV, so secondary particles can be distinguished with the background noise in the experimental environment. After selecting coincide muon trajectory through injected cosmicray muon and its secondary particles coincide detection technique, the 3D images of three 3 cm×3 cm×3 cm cubes made up from different materials (iron, water, and lead) were reconstructed based on limitedangle computed tomography algorithm. It can be seen from the imaging results that the reconstructed image of iron has best quality. Both water and lead have some voids in their reconstructed images, but the voids of water is apparently less than those of lead. Thus, this imaging method is fit for imaging small scale objects with low or medium Z and the imaging effect of objects at the small scale with medium Z is especially good. A reconstruction of complex objects which is defined as “USC” letter model has also been made. The reconstructed image can match with the size and location of the geometry model in Geant4 and can also distinguish the intervals of 08 cm in the letter “S”. A much better imaging accuracy will be achieved by optimizing the mathematical model of the algorithm. This method extends the application area of muon tomography.

BackgroundThe barrel-sampled electromagnetic calorimeter (Ecal) is an important part of the Multi-Purpose Detector (MPD) in Nuclotron-based Ion Collider fAcility (NICA) that is being built in Russia. It is primarily used to detect energy, time, and position information of electrons and photons in the energy domain from 10 MeV to a few GeV. MPD-Ecal is comprised of 2 400 modules with 16 towers per module. Each tower is made up of alternating layers of 211 scintillator sheets and 210 lead sheets, as well as 16 wave-length shift fibers.PurposeThis study aims to evaluate the performance, such as energy resolution, time resolution, and coordinate resolution, etc., of the Ecal by simulation.MethodsThe Geant4 software was employed to simulate single-energy electron incident on Ecal to examine the effects of several parameters on the performance of Ecal. Influences of the position of the particle incidence point, the number and thickness of the scintillator and lead layers, the polish of the optical fibre end-face, and the energy and type of incident particles on the energy deposition and resolution, time distribution and time resolution, and coordinate resolution were investigated in details. Finally, the time resolution of a single tower was simulated using the natural cosmic ray package, and the tower's expected time resolution in the cosmic ray test was obtained.ResultsAs the electron incidence position moves from the edge to the center of the module, the energy deposition within the scintillator rises from 718 MeV to 758 MeV. With a limited tower length of 415.5 mm, increasing the number of scintillator layers decreases the energy resolution of the module, improves the time resolution of the tower, and worsens the coordinate resolution of the Ecal. Taking into account the performance of the Ecal gauge, the optimal number of scintillator layers in the tower is 211. SiPM detects 42% more photoelectrons at a polish of 0.6 when the fiber end is coated with a reflective material than when it is not. As the polish of the fiber end-face increases, so does the number of photoelectrons detected by the SiPM, and the time resolution of the tower improves. When the fibre end-face polish is 1.0, the time resolution of the tower is less than 103 ps while time resolution of the tower (with 211 layers) in the cosmic ray is 185 ps.ConclusionsThe time and coordinate resolutions of Ecal improve with increasing electron energy under the same circumstances.