Development of low noise cosmic ray muon detector for imaging density structure of Usu Volcano, Hokkaido, Japan

We are developing low noise cosmic ray muon detector to image a density structure of Usu Volcano, Hokkaido, Japan by muon radiography. Intensity of cosmic ray muon penetrating through the object is expressed as a function of the product of muon path length and density along muon path. And, the intensity of penetrating muon steeply decreases if muon path length becomes longer or density along muon path becomes larger. The detector that we are developing is called hodoscope that consists of multiple Position Sensitive Detectors (PSDs). A PSD has NxM grids consisting of N vertically aligned Scintillation Counters (SC: a plastic scintillator attached to a photo multiplier tube) and M horizontally aligned SCs. We can identify a muon path direction with two or more PSDs by connecting muon-detecting points in each PSD. But, Usu Volcano is so large that the intensity of penetrating muon becomes lower, and then noise rate becomes higher: the count of penetrating cosmic ray muon is estimated to be a few counts per month with the detector of which has the cross-section area of one square meter and the solid angle of 0.01 steradian. The noise is defined as a particle other than the muon penetrating the observed object such as electrons, photons, vertically arriving muons and so on. If noise rate becomes higher, the measured intensity of penetrating muon becomes higher than the theoretical intensity of that. Then we get a wrong result as if there were matter of lower density relative to real. So we need to develop a low noise detector. The ElectroMagnetic (EM) shower that consists of many electrons and photons is thought to be one of noise. When EM shower reaches the detector, each PSD detects arriving particles and detecting points are sometimes connected by a straight line. In that case, we cannot discriminate the penetrating muon from EM shower, and we count it as a muon event. This results noise. In order to discriminate the noise event, the use of more PSDs for our detector is expected to reduce possibility of the event which EM shower makes a straight track. Thus we would be able to a real penetrating muon track or a fake track formed by EM shower. In this presentation, we show that we can reduce more fake tracks formed by EM shower with the use of more PSDs, combining Cosmos^{[1]}, a versatile Monte Carlo simulaiton code for propagatio of cosmic rays in the atmosphere and near earth environment, and Geant4^{[2]}, a toolkit for the simulation of the passage of particles through matter. References [1] K. Kasahara http://cosmos.n.kanagawa-u.ac.jp/cosmosHome/index.html [2] Agostinelli et al., Nucl. Instr. Meth. Phys. Res. A, vol. 506-3, pp. 250-303, 2003.